Design guide

contents update

release 05//1997

aE N

Design guide

GE Plastics

release 05//1997

C o n t e n t s

1 Genera l In t roduct i on

1 Des i gn Deve l opm ent . . . . . . . . . . . . . . . 9

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.2 Material design factors . . . . . . . . . . . . . . . . . . . . . . . 9

1.3 Development steps . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.3.1 End-use requirements . . . . . . . . . . . . . . . . . . . . . . 9

1.3.2 Preliminary design . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.3 Material selection . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.4 Design modification . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.5 CAD/CAE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.5.1 Flow analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.5.2 Stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.3.6 Prototyping and testing . . . . . . . . . . . . . . . . . . . . . 11

1.3.7 End-use testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2 Des i gn fo r s t i f fness . . . . . . . . . . . . . . . . 13

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2 How to determine stiffness . . . . . . . . . . . . . . . . . . . . 13

2.2.1 Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2.2 Geometry & loading . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.3 Other factors influencing stiffness . . . . . . . . . . . . 14

2.2.4 Calculating the stiffness . . . . . . . . . . . . . . . . . . . . 15

2.2.5 Modal analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2.6 Safety factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3 Increasing part stiffness . . . . . . . . . . . . . . . . . . . . . . 15

2.3.1 Ribs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.2 V-grooves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.3 Corrugation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.4 Optimization of stiffness . . . . . . . . . . . . . . . . . . . . . . 16

2.4.1 Optimization for part weight . . . . . . . . . . . . . . . . . 16

2.4.1.1 Material Selection . . . . . . . . . . . . . . . . . . . . . . . . 16

2.4.1.2 Geometric considerations . . . . . . . . . . . . . . . . . . 16

2.4.2 Optimization for stiffness to cost ratio . . . . . . . . . 16

3 Des i gn fo r s t rength . . . . . . . . . . . . . . . . 19

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2 Material Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2.1 Ultimate strength . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2.2 Yield strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Design gu ide 1 Contents

3.2.3 Strain to failure . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2.4 Proportional limit . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2.5 Material toughness . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2.6 Other measures . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.3 Effects of various factors on strenght . . . . . . . . . . . 20

3.3.1 Other factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.4 Part Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.4.1 Ultimate part strength . . . . . . . . . . . . . . . . . . . . . . 21

3.4.2 Part yield strength . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4.3 Part toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.5 Improving part strength . . . . . . . . . . . . . . . . . . . . . . . 22

3.5.1 Material choice . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.5.2 Geometry optimization . . . . . . . . . . . . . . . . . . . . . . 23

3.6 Design considerations . . . . . . . . . . . . . . . . . . . . . . . . 23

4 Des i gn fo r behav i our over t ime . . 25

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.2 Static time dependent phenomena . . . . . . . . . . . . . . 25

4.2.1 Creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.2.1.1 Creep recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.2.1.2 Creep failure modes . . . . . . . . . . . . . . . . . . . . . . . 26

4.2.1.3 Apparent modulus . . . . . . . . . . . . . . . . . . . . . . . . 26

4.2.2 Stress relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2.3 Design considerations . . . . . . . . . . . . . . . . . . . . . . 28

4.3 Dynamic time dependent phenomena . . . . . . . . . . . 28

4.3.1 Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.3.1.1 Fatigue and endurance limits . . . . . . . . . . . . . . . . 28

4.3.1.2 Factors affecting fatigue . . . . . . . . . . . . . . . . . . . 29

4.3.2 Wear resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.3.2.1 Types of wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.3.2.2 Factors affecting wear . . . . . . . . . . . . . . . . . . . . . 29

5 Des i gn fo r i m pact per formance . . 31

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.2 Common impact testing methods . . . . . . . . . . . . . . . 31

5.2.1 Pendulum methods . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.2.2 Falling weight methods . . . . . . . . . . . . . . . . . . . . . 32

5.3 Design considerations . . . . . . . . . . . . . . . . . . . . . . . . 33

Design gu ide 1 Contents

6 Des i gn fo r appearance . . . . . . . . . . . . 35

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.1.1 General remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.2 Surface defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.2.1 Sink marks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.2.2 Weld lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.2.3 Air traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.2.4 Voids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.2.5 Streaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.2.6 Delamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.2.7 Jetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.2.8 Gate marks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.2.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

7 Des i gn fo r p rec i s i on . . . . . . . . . . . . . . . 39

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.1.1 Mould shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.1.2 Secondary effects . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.2 Shrinkage phenomena . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.2.1 Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.2.2 Packing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

7.2.3 Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

7.3 Materials and shrinkage . . . . . . . . . . . . . . . . . . . . . . . 40

7.3.1 Amourphous materials . . . . . . . . . . . . . . . . . . . . . . 40

7.3.2 Crystalline materials . . . . . . . . . . . . . . . . . . . . . . . 41

7.3.3 Reinforced materials . . . . . . . . . . . . . . . . . . . . . . . 41

7.4 Design related factors . . . . . . . . . . . . . . . . . . . . . . . . 41

7.4.1 Part wall thickness . . . . . . . . . . . . . . . . . . . . . . . . . 41

7.4.2 Ribs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

7.5 Mould related factors . . . . . . . . . . . . . . . . . . . . . . . . . 41

7.5.1 Gate location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

7.5.2 Gate type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.5.3 Gate size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.6 Processing related factors . . . . . . . . . . . . . . . . . . . . . 42

7.6.1 Melt temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.6.2 Mould temperature . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.6.3 Injection time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.6.4 Packing pressure . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.7 Secondary effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.7.1 Thermal expansion . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.7.2 Moisture absorption . . . . . . . . . . . . . . . . . . . . . . . . 42

Design gu ide 1 Contents

7.7.3 Post crystallization . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.7.4 Creep under load . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.8 Simulation techniques . . . . . . . . . . . . . . . . . . . . . . . . 42

7.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

8 Des i gn fo r m ou l dab i l i t y . . . . . . . . . . . 45

8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

8.2 Material issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

8.2.1 Melt flow length . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

8.2.1.1 Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

8.2.1.2 Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . 46

8.2.1.3 Shear properties . . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.2.2 Melt temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.3 Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.3.1 General remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.3.2 Warpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

8.4 Cooling time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

8.5 Design considerations . . . . . . . . . . . . . . . . . . . . . . . . 48

8.5.1 General remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

8.5.2 Nominal wall thickness . . . . . . . . . . . . . . . . . . . . . 48

8.5.3 Projections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

8.5.4 Radii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

8.5.5 Ribs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

8.5.6 Support ribs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

8.5.7 Bosses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

8.5.8 Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

8.5.9 Coring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

8.5.10 Draft angles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

8.5.11 Textures & lettering . . . . . . . . . . . . . . . . . . . . . . . 51

8.5.12 Flow leaders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

8.5.13 Moulded-in-stress . . . . . . . . . . . . . . . . . . . . . . . . . 52

8.5.14 Weld lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

8.6 Processing considerations . . . . . . . . . . . . . . . . . . . . . 52

8.6.1 Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

8.6.2 Gating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

8.7 Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

9 Des i gn fo r recyc l ab i l i t y . . . . . . . . . . . 55

10 Des i gn fo r au tom at i on . . . . . . . . . . . 57

11 Append i x . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Design gu ide 1 Contents

The intention of this guide is toprovide the design and engineeringcommunities with an insight into theconsiderations necessary whendesigning applications in engineeringthermoplastics. Many of theseconsiderations relate to mouldingcriteria, so those involved in the

This design guide differs from most byvirtue of its Designing for concept,helping the reader move quickly to theissue that needs addressing.Consequently, discussion of certainaspects can occur in more than onesection, which demonstrates how

1

In t roduct ionplastics components should alsofind it useful.

Typically, design manuals deal with aspecific resin family, presentingproperties, design criteria, assembly andother related information. GE Plasticsproduct line includes crystalline,amorphous, thermoplastic elastomersand glass mat reinforced polymers.Because of this diversity, this brochurewill concentrate on issues common to allinjection mouldable thermoplasticresins.manufacturing and processing of

chapters is a section showing typicalengineering material performancegraphs for a range of GE Plasticsthermoplastic polymers.

Supplementary publications coveringassembly techniques, polymerprocessing considerations and overviewsof specific resins in the GE Plasticsproduct lines are available on request.In addition, monographs discussingdetailed design studies are releasedperiodically, covering these issues ingreater depth than is possible in thisgeneralized guide.integral the process of designing forplastics is. Following the explanatory3

V I B R A T I O Nperiods may result in materialdegradation.

1

tConsiderable information is needed bythe design engineer to develop aproduct design from the initial concept.This can be a smooth process if carefulattention is given to each step involved.The designer must know the end-useperformance requirements of theproposed application. To determinewhether a material can meet theserequirements, the designer must be ableto rely on information provided by theraw material supplier, indicating theenvironmental and physical capabilitiesof the material.

1.3 Development steps

1.3.1 Establish end-use requirements

The development of any component1.1 Introduction

Throughout the process of productdesign, both functional and materialaspects must be considered. Functionaldesign factors relate to production andassembly. Material design factorsconcern the performance of a materialin service. This performance, whichincludes strengths, weaknesses andlimitations, is investigated to provide thestarting point of the design process.

1.2 Material design factorsstarts with careful consideration ofanticipated end-use requirements.In general, the lower strengthproperties of polymers compared withmetals and woods require that productsbe designed to utilize a largerpercentage of their available strength.R A T E O F L O A D I N G

A thermoplastic may demonstratedifferent behaviour with changes inloading rate. Therefore, in additionto its magnitude, the rate at whichloading is applied should beinvestigated.

D U R A T I O N O F L O A D I N G

Initial negligible deflectionsresulting from a small load maybecome unacceptably large if theload is maintained.

I M P A C T F O R C E S

Because the application of highloads for short periods of time mayresult in premature failure, thenature of impact forces to which thecomponent will be subjected shouldbe determined.Specific information needed to establishend-use requirements includes:

(a) Anticipated structural requirements

L O A D S

These dictate the stresses a materialwill be subjected to, and they definecomponent deflections.Design dThis induces stress and deflectionchanges. Though these may besmall, component failure may occurthrough constant repetition.

F O R E S E E A B L E M I S U S E

Though structural requirements mayhave been satisfied, and anappropriate design proposed, failurecan still occur as a result of misuse.To establish the anticipated risklevel, therefore, a risk assessment ofthe product in use should be made.

The relevance tree method is idealfor this purpose, providing data tofacilitate assessment of foreseeablemisuse. The results should then beand, if required, contact should bemade with the nearest GE Plasticssales office.

(b) Anticipated environments

T E M P E R A T U R E E X T R E M E S

All materials possess a workingtemperature range. Outside thisrange the component cannotproperly perform its intendedfunction. In addition, the propertiesof the material may vary considerablywithin working limits.

As all thermoplastics are subject toattack by certain chemical agents,the service environment of theproposed component must beestablished.

Outdoor exposure for prolongedweighed against the financial aspectsof the product. This exercise is oftencalled Product Risk Analysis.Additional details are not given heresince each company applies its ownmeasures.

General Electric Plastics is preparedto provide assistance in this mattereve lopmen(c) Assembly and secondary operationsUsually a plastic component is notused in isolation but is just one of anumber of components making upthe end-product. The techniqueused for assembly, such asmechanical fastening, welding andadhesive bonding, needs to beconsidered at the initial design stageto optimize the component for easeof assembly, (and ease of handling inthe case of automated assembly), orease of disassembly for maintenanceand recycling.

9

Secondary operations, such aspainting, printing and hot stamping,

10

Design gu ide 1 Design Deve lopmentDIN Deutsche Industrie frNormungen

BSI British Standards Institute

NF Normes Francais

ASTM American Society for Testingof Materials

R E G U L A T I O N S

UL Underwriters Laboratories

CSA Canadian Standards Association

CEE Publications of the CommitEuropan dElectricit

Factory and building codes.the component

(e) Regulations/standards compliance

Check which standards orregulations apply or can be appliedin the market place to thecomponent, product or appliance,for example:

S T A N D A R D S

IEC/CEE International ElectricalCommittee/Commit EuropandElectricit

ISO/CEN International StandardsOrganization/Commit Europande Normalisationalso have to be given earlyconsideration in order to design thebest surface profile. This meansavoiding for example sink marks andsharp changes in shape to achieve ahigh quality smooth surface.

(d) Cost limits

The following should be established:

Component cost resulting inprofitable sales

Annual volume Economic processing method(s)

with estimated cycle time(s) Tooling cost(s) for selected

processing method(s) The expected service life of Coefficient of friction Thermal conductivity Density Mould shrinkage

(b) Other relevant properties:

Hardness Impact strength Chemical resistance Weathering resistance Abrasion resistance Ductility Flammability Heat deflection temperature Electrical properties

To design a plastic component,information concerning anycombination of material propertiesmay be required. If the data areunavailable, or assistance is neededessentially for component designcalculations:

Elastic limit Tensile strength Modulus vs temperature Poissons ratio Apparent (creep) modulus Fatigue limit Coefficient(s) of thermal

expansionend-use requirements. The comparativeproperty data sheet should be the firstdocument which is consulted. Initialselection may subsequently be refinedby reviewing the time, temperature andenvironment-dependent propertiesrelevant to the particular application.Supplementary data, such as abrasionresistance or ductility, may be neededto confirm the selection.

Material properties can be divided intotwo main categories.

(a) Mechanical properties used1.3.2 Establish a preliminary design

A preliminary concept sketch of theproposed component can help thedesigner to determine which aspects areinflexible, and which can be modified toachieve required performance.The preliminary sketch, therefore,should include both fixed and variabledimensions.

1.3.3 Select the material

A GE Plastics material should beselected that will satisfy initially defined,in interpretation, contact should bemade with the nearest GE Plasticssales office. relation to its neighbouring elements,the total structure, and the physicalconstraints on the system. A largenumber of simultaneous equationsresult, the solution of which isparticularly suited to the repetitivecapabilities of a computer.

1.3.5.1 Flow analysis

Because the performance of aninjection moulded component is largelydependent on the moulding process,consideration of the service conditionsof the component in isolation isinsufficient to ensure a successfulproduct. Simple shapes should not givematerial flow problems to theexperienced tool-maker and moulder.However, larger components ofcomplex geometry often presentdifficulties, for example positioning andmay warrant a computer-aidedapproach.

1.3.5 CAD/CAE

Two particularly relevant computer-aided systems are Flow Analysis of aninjection moulding, and Stress Analysisof a final component. Both generallyuse the Finite Element Method. Thisapproach considers the geometry andphysical properties of the component asa continuum of small manageable parts,or finite elements. Each element of thestructure is individually investigated in(c) Assembly methods, (e.g. snap-fits, adhesives)

(d)Cost of modification and its impact on component and/or project bud-get, (refer to n F IGURE 1).

Strength of materials formulae shouldbe used in conjunction with materialproperty data to calculate necessarydimensions such as wall thickness.(Refer to Chapter 3 Design forstrength). Design calculations of arepetitive or iterative nature, however,1.3.4 Modify the design

If this is necessary, four areas inparticular should be considered:

(a)The specific property balance of theselected grade, (e.g. tensile strength,impact resistance)

(b)Processing limitations, (e.g. wall thickness vs flow lengths)number of gates, runner dimensionsand location of weld lines.

1.3.6 Prototyping and testing

At this point in the procedure,a prototype should be constructed.The prototype and its testing can helpthe designer by:

Establishing confidence in the design by confirming that component require-ments do not exceed design limits.

Developing preliminary product performance information.

Identifying potential problem areasin performance, manufacturing orassembly.

Allowing pre-launch assessment andfeedback from consumer trials.

In order to obtain useful results,particular attention should be paid to

Product performance tests can beconducted on functional prototypes orproduction parts. Since functionalprototypes may be produced using non-production tooling or part modelling,caution must be exercised duringtesting and interpreting the results.The prototype may not behave inexactly the same manner as aproduction component. The initialproduction components should also betested to confirm product performancetesting.

Several specialized techniques,examples of which follow, can be usedfor product performance testing:

Strain gauge analysis Brittle coating analysis Photoelasticity

11

Design gu ide 1 Design Deve lopmentAs with Flow Analysis, the investigationof a simple geometry should not presentany difficulty, since equations which canoften be solved by substitution havebeen derived for many commonlyencountered situations. However, theanalysis of a complex geometry, thoughnot generally suitable for this approach,may be solved by the finite elementmethod. A mathematical model of thecomponent, defining the geometry by x,y and z co-ordinates, is first described,together with the properties of thematerial. The boundary and loadingconditions are then entered and specificoutput requested. Stresses in aparticular area may be of interest forexample.

Numerical values of deflections,diagrams of the distorted structure andOther injection moulding relatedsoftware prediction tools include:warpage, mould cooling, fibreorientation, (for example for glass-filledthermoplastics), moulded-in stressesand many more.

1.3.5.2 Stress analysis

A component in service will besubjected to forces which induce stressesin the material. To ensure that failureresulting from overstressing does notoccur, it is essential that the stresses donot exceed recommended design limits. calculated, in addition to weld linelocations and undesirable conditionssuch as overpacking. Moulding variablesand/or gating positions are subsequent-ly changed, if necessary, in order toachieve an optimum flow pattern.

By means of this iterative approach,analyses of different options are possiblebefore commitment to actual toolproduction. Moreover, processingdifficulties are identified and may berectified at the design stage.

Nowadays, flow simulation is not theonly software available to the designer.In the flow analysis process, a computermodel of the component is produced,and initial gating positions selected.The predicted manner in which thematerial will fill the cavity is thenpresented graphically and numerically.Isochronous temperatures andpressures throughout the system areof stress distribution may also contributeto a useful investigation. By this method,areas of unacceptably high stress ordeflection may be identified andsuitably modified.

Construction Commisioningequipment developed can be used forfuture quality control testing.

Specification Part Mould Design Designcertain aspects of the testing:

Proper analysis of component requirements.

Close similarity to the proposedproduct, particularly in critical or suspect performance areas.

Development of realistic simulated use and storage tests

Commitment to the time and effortrequired for testing before product introduction

1.3.7 End-use testing

Tests should be conducted to simulateuse and storage of the component. Itshould be established which tests reflectdefined component requirements, andwhether they can be conducted in thelaboratory or in situ. Frequently, the testCost of modificationEase of modification

Timeprototype is therefore advisable.

n F IGURE 1Influence of modificationto designs

Mould Mould Production Stress analysis by thermal emission Infra-red light banks for radiant heateffect measurement

Environmental chambers for thermalcycling

Life testing under simulated use cond-itions

Accelerated ageing under elevated tem-perature, high humidity, or ultraviolet radiation conditions

Holography

Although computer-aided techniquesallow accurate modelling of a proposeddesign, they should not exclude orreplace finished part testing.The construction of a functional

2.1 Introduction

The stiffness of a part is defined as therelationship between the load and thedeflection of a part. This Chapter willdiscuss what modifications can be madeto a part in order to influence andoptimize stiffness. It will also give some

is assumed to be linear. Especially forthermoplastics, the range in which thestress/strain curve can be estimated witha straight line is limited. For this reason,when a stiffness calculation of a part ismade, it is necessary to check if the

strain curves are heavily dependent ontemperature. It is advised to considerthe stress/strain curve at thetemperature at which the load is appliedfor the calculation of part stiffness.

2

Design fo r s t i f fnessIn general the stiffness of a part isdetermined by its material and itsgeometry.

2.2.1 Material

The most important material propertyfor stiffness is the stress/strain curve.In general, the Youngs modulus, whichis determined from the stress/straincurve, is the best parameter to be usedwhen comparing the stiffness ofmaterials. However, when the Youngsmodulus is used, the stress/strain curve

Stre

ss strguidelines of how the stiffness of a partcan be calculated.

2.2 How to determine the stiffnessA

Y

Y*

sConsider n F IGURE 2 . Suppose that thestiffness of a part is calculated using theYoungs modulus Y. Suppose that averification of the occurring stressresults in the value s. This value isclearly out of the range in which a linearapproximation of the stress/strain curveis justified. In this case it is better torecalculate the stiffness of the part withthe secant modulus Y*.

Furthermore, it is important to considerthe temperature at which the load isapplied. For thermoplastics the stress/

ess strain relation with Youngs modulus Y occurring stresses and strains still allowa linear approach. If this is not the case,it is advised to use a secant modulus forthe stiffness calculation. Strain

measured stress straincharacteristic(a) the material is loaded for along time,

(b)the material is loaded in cycles, (c) the material is loaded during

a very short time.

The phenomena (a) and (b)aretypically known as creep and fatigue.How these effects must be taken intoaccount can be found in Chapter 4Design for behaviour over time.Phenomenon (c) is known as impact.Refer to Chapter 5 Design forimpact.

n F IGURE 2Accounting for materialnonlinearitiesTime also plays a role in thedetermination of the stiffness of a part.It can influence stiffness in one of twoways: 13

that, in reality, the fixing system will

14

Design gu ide 2 Design fo r s t i f fness

I=Aey2

n F IGURE 3A part loaded in tension

n F IGURE 4Cross section of a part

cF F

area A

dA y

n F IGURE 5A rectangular cross-section

dA y

a

b

n F IGURE 6Clamped part loadedat the end

cFsectional area. For example, for arectangular cross-section, with thedimension a and b ( n F IGURE 5),I can be calculated with the formulamentioned above:

(d) Torsional loadingWhen a part is loaded in torsion,the polar moment of inertia and thelength are the importantgeometrical parameters thatdetermine stiffness. The polarmoment of inertia Ip can becalculated with a similar formula to

dA = eay2 dy = 1/3 a y3 d = 1/12 a b3-b_2

-b_2

b_2

b_2n F IGURE 4 , the moment of inertia Iis defined as:

where y is the directionperpendicular to the neutralbending axis and A is the cross-

I=Aey2 dA(b) Compressive loadingWhen a part is loaded undercompression, again the cross-sectional area A and the length ofthe part are the geometricalparameters that determine thestiffness. In this case, buckling of thepart is something that has to beconsidered separately.

(c) Flexural loadingFor flexural loading, the moment ofinertia and the length are the geo-metrical parameters that determinethe stiffness. As can be seen in2.2.2 Geometry and loading

Besides the material, the geometry isalso important for part stiffness. Whichfactors of the geometry are important ismainly determined by the type of load-ing. It should be noted that, in general,a part is loaded in more than one of thefollowing types at the same time.

(a) Tensile loadingFor a part loading in tension,( n F IGURE 3), the cross-sectionalarea A and the length of the part care important.the moment of inertia.modulus of the material. The stiffness ofthis part can be increased by decreasingc, increasing Y and increasing I.Suppose that the part has a rectangularcross-section. Then I is given by 1/12 ab3,( n F IGURE 5). This means that I can beincreased by increasing either a or b,though increasing b has a much largereffect.

2.2.3 Other factors influencingstiffness

Besides the geometry and the material,other important factors have aninfluence on stiffness, such as the typeof loading or the restraints of the part.A load can, for instance, beconcentrated on a point, but it can alsobe a pressure on an area. Differentrestraints that are used in calculationsare clamped or simple supports. NoteEXAMPLE 2A part under bending

For a part of length c and moment ofinertia I, loaded by a force F at the end,( n F IGURE 6), the deflection f at theend can be calculated using:

f = Fc 3/ 3YI

where F is the force and Y the Youngsf = Fc/ yA

where F is the tension force and Y is theYoungs modulus of the material. From this formula it can be seen thatthe stiffness of this part can be increasedby decreasing I, increasing Y orincreasing A.As mentioned earlier, the stiffness ofa construction is determined by the combination of the material and thegeometry. The following two examplesillustrate how the stiffness of a part canbe calculated.

EXAMPLE 1A part under tension

For a part of length c, and constantcross-sectional area A, the deflectionf can be calculated with the formula:always be something in between fullyclamped and simply supported.

of additional material and they do not

15

Design gu ide 2 Design fo r s t i f fnesscomputer system and software must beavailable.

In general, the following guidelines canbe given:

(a)For simple geometries and smalldeflections, hand calculations can bemade.

(b)For more complex geometries with small deflections, linear finiteelement analysis techniques can beused.

(c) Only nonlinear geometrical finiteelement analysis can account forlarge deflections on complexgeometries.

Note that the analysis costs increasedramatically when going from step (a)Taking into account the other factors ismore complicated. The simple formulaecannot be used anymore when theseeffects play an important role. If thepart stiffness has to be determined forthis type of problem, the only possibilityis computer simulation using the finiteelement method. A computer model ofthe part has to be made and evaluatedwhich requires much more time thanhand calculations. Also, an appropriatestress/strain curve of the material isa straight line.

(b)The geometrical effects are assumed to be linear. For large deflectionsthis leads to considerableinaccuracies.

(c) The formulae given above assume a relatively simple shape.

(d)In general, a part is not purely loaded in one mode, but in acombination of modes.

A simple way of accounting for materialnonlinearities is already given above.2.2.4 Calculating the stiffness

Similar formulae to those given aboveare available for different cross-sections,for changing cross-sections, and so on.(Please refer to the Appendix foradditional sources of reference).

The application of formulae is limiteddue to different factors:

(a)The material behaviour is assumed to be linear, which means that the to step (c).all types of material. Plastics are noexception to this rule. Conservativeassumptions should be made whenpossible and worst case loads should beconsidered. It is the responsibility of thedesign engineer to anticipate thatproducts will not always bemanufactured or used as intended orplanned. In some cases, product failureis acceptable under unusual loading, orfor an occasional poorly manufacturedpart. However, there are many cases inwhich safety factors must be considered.

There are numerous reasons why aproduct can have less than its originallyintended properties. Some of thesereasons are beyond the control of thedesigner and include:

Exposure to chemicals which does notnormally occur each of the eigenfrequencies. This typeof analysis is called modal analysis.Modal analysis results are very sensitiveto the type of loads, restraints and theirlocations on the part. Often it is possibleto significantly change the vibrationbehaviour of a plastic part byredesigning the way it is mounted orrestrained.

2.2.6 Safety factors

The use of safety factors in engineeringdesign is common practice with almostloaded with a vibration load with afrequency close to, or equal to, one ofits eigenfrequencies, a potential dangerof part damage exists.

In automotive components, it is oftendesirable to have the lowesteigenfrequency of any part to be abovethe normal operating frequencies of thevehicle. Therefore, although a part maybe stiff enough to meet static loadingrequirements, it may require additionalstiffness to increase theeigenfrequencies.

Finite element analysis can be used topredict the eigenfrequencies of the partas well as the vibration modes shape for2.2.5 Modal Analysis

Vibration resistance is important formany applications such as automotivecomponents. All parts designed inplastic or any other material will haveeigenfrequencies. These eigenfrequen-cies will, amongst other factors, dependupon the stiffness of the part. If a part is Moulding issues such as insufficientdrying or excessive heating of the resin

Tool wear resulting in dimensional changes of the part Ribs are the most commonly usedstiffeners. Processing and toolingdetermine certain requirements for theshape and the thickness of the ribs,(seeChapter 8). For a part under bending,the ribs should run perpendicular to thebending moment. For parts undertorsion, ribs most efficiently increase thestiffness if they are placed diagonally.For pure stiffness reasons, it isimportant to note that high ribs aremore efficient than thick ribs.

2.3.2 V-grooves

V-grooves, ( n F IGURE 7), are often in-corporated into parts where significantincreases in stiffness are necessary, andother requirements permit their use.V-grooves are very efficient stiffenersbecause they do not use large amountsand the load bearing possibilities of apart. This is accomplished by locating asmuch material as possible as far as pos-sible from the neutral axis of the part.This increases the moment of inertia.As can be seen from Example 2 inSection 2.2.2 (d), the moment of inertiaincreases to the third power of thedistance of the material to the neutralaxis, and only linearly with the distancealong the neutral axis. This is aprinciple that has to be kept in mindwhen designing stiffeners into a part.

2.3.1 RibsA safety factor is typically decided uponbased on prior experience with similardesigns. In general, the greater thepotential damage from failure, thegreater the factor of safety that shouldbe used. In many industries, safety codesand test procedures exist, which givestandards and recommendations forsafety factors.

2.3 Increasing part stiffness

Features like ribs and V-grooves can beused efficiently to improve the stiffness The part is used at temperatures above the expected maximum.

The part is used in a manner other than what it was designed for.

The part is assembled poorly or forced,incurring a pre-load

Excessive moulded-in stresses resulting from high moulding pressures, too small gates and too low tool temperatures.require additional cooling time.However, V-grooves often cannot beused because they provide an uneventop and bottom surface.

16

Design gu ide 2 Design fo r s t i f fness

n F IGURE 7V-grooved structure

n F IGURE 8Corrugated structure

M1

M2

M1

M22.4 Optimization of stiffness

Before an optimization study is started,the parameter that must be minimizedmust be determined. In most cases thisis either part weight or part costs.

2.4.1 Optimization for stiffness toweight ratio

2.4.1.1 Material selection

When designing for an optimumstiffness to weight ratio, for exampleaircraft components, often a materialwith a high Youngs modulus and yieldstrength is recommended. Of course thematerial also has to meet otherrequirements like the mouldingcapability, aesthetics, environmentalresistance, and appropriate regulatoryand can even be reduced for bendingmoments along that axis.The corrugation shown in n F IGURE 8 isefficient for bending moment M1 butinefficient for bending moment M2.

Note that for V-grooves and corrugatedstructures, when larger loads areapplied, the stiffness can decrease dueto the fact that the profile becomesflatter, and hence the moment of inertiabecomes smaller. This can be avoided byputting ribs perpendicular on the axisof the corrugated structure.bending moment is directed along theV-groove. The V-grooves given inn F IGURE 7 are efficient for bendingmoment M1, but inefficient for bendingmoment M2.

2.3.3 Corrugation

Corrugation is similar to the V-groove.It also does not use large amounts ofadditional material and does notrequire additional cooling time, but,like the V-groove, often cannot be usedsince it provides an uneven top andbottom surface. A stiffness increase isonly achieved for a bending momentperpendicular to the corrugated axis,As with ribs, V-grooves provideadditional stiffness by increasing theaverage distance of material from theneutral axis of the part. For bendingproblems, V-grooves should be orientedin the direction perpendicular to thebending moment. V-grooves candecrease the bending stiffness when theor standard requirements. Often glass-fibre reinforced materials are selected,together with materials such as Ultem

polyetherimide resin because of theirstrength and stiffness. should be added where it most effective-ly enhances the stiffness of the part.Finite element analysis techniques areparticularly useful for geometrical partweight optimization. Localized areas ofhigh and low stress can be easilyidentified and redesigned. Materialsituated in an area of low stress does notcontribute significantly to the partstiffness. Areas of high stress are areaswhere also large strains occur whichinduce large deflections. An optimumpart design for stiffness, and strength,would result in the most uniform stressdistribution possible, subject to all otherlimitations.

2.4.2 Optimization for stiffness tocost ratio

Designing for an optimum stiffness tocost ratio is often critical in applicationsincreased material stiffness due to glassfibre reinforcement is used.

2.4.1.2 Geometric considerations

It is not possible to give a general ruleof how the geometry of a part should beoptimized, since it heavily depends onthe type of loading and other factors.Furthermore, design limitationsimposed by the material and the processwill limit the designers freedom.In general, it can be stated that materialgives a greater moment of inertia forbending loads.

Note that when a glass fibre-reinforcedmaterial is used, the stiffness is mainlyincreased in the direction, (orientation),of the glass fibres. This means that thedesign engineer must ensure that in thecritical areas the fibres are aligned inthe right direction in order to optimizethe performance of the glass-filledmaterial. The direction of the glass fibresis dependent on the flow direction ofthe material during injection moulding.The gating of the part therefore has tobe designed in such a way that theAnother important factor for partweight optimization is density. It is oftenrecommended to choose a material withthe highest ratio of elastic modulus todensity, which meets all the otherrequirements. A material with a verygood stiffness to density ratio isengineering structural foam.Furthermore, the larger wall thicknessat equivalent weight is a plus, since itsuch as material handling pallets andbuilding and construction components.The situation is very similar to designingfor optimum stiffness to weight, exceptthat the final part weight must be

multiplied by the cost per kg. of thematerial. All of the techniquesmentioned above still apply.

When designing an application forwhich the cost is critical, all costs shouldbe considered, including material costs,processing costs, tooling costs,secondary costs and inventory costs.

EXAMPLE 3Comparison of a ribbed, V-groovedand corrugated structure.

This example shows how different levelsof stiffness can be reached using thesame amount of material. Furthermore,differences between hand calculations,linear finite element calculations, andgeometrically nonlinear finite elementcalculations are shown and discussed.

Suppose that a beam, supported on twosides as shown in n F IGURE 9 is loadedwith a force in the middle. The length isassumed to be 800 mm. Suppose that forthe cross-sectional area a rectangle witha width of 20 mm and a height of 10 mm

ribbed and a corrugated structure havebeen chosen. n TA B L E I summarizessome geometrical properties of thethree different sections. As can be seen,for the load on the beam, which resultsin bending moment around the y-axis,the corrugated beam has the higheststiffness.

For the beams with the three differentcross-sections, a hand calculation and alinear finite element simulation usingshell elements have been performed.For the finite element simulation, themid-lines of the cross-sections aremodelled and the thickness issuperposed on that. The results arepresented in n TA B L E I I . The formulafor the hand calculation to calculate thedeflection f is:

f = Fc 3/ (48YI )

where F is the force, c the length of thebeam, Y the Youngs modulus of thematerial and I the moment of inertia.

17

Design gu ide 2 Design fo r s t i f fness

n F IGURE 10The cross-sections of the beam

V-groove

Ribs

Corrugated

20

210y

x

y

x

y

x

101.41

1.41 10

20

20

n F IGURE 9The restraint and the loadson the beam

c

c F_2

c_2Quantity V-groove Ribs

J (mm4) 348.6 480.6

x-coord c.g. (mm) 10 10

y-coord c.g. (mm) 5.896 6.983

Area (mm2) 52.57 52.57

n TABLE I I

Hand calculation and linear finite eleme

Deflection at 10N V-groove Ribs

Hand calc. (mm) 139 101

Linear f.e.m. (mm) 148 99.6is available.

Three possible cross-sections are shownin n F IGURE 10 , each of them using thesame amount of material. A V-groove, a

n TABLE I

Some geometrical properties of the crosCorrugated

668.0

10

4.403

52.57

nt results

Corrugated

72.5

74.0s-section

The deviations between the results fromthe hand calculation and those from thelinear finite element method, (f.e.m.),are due to the fact that for f.e.m. I iscalculated from the thicknesssuperposed on the mid-line of the cross-section,( n F IGURE 11). This means thatthe stiffness is underestimated by thef.e.m. for the V-groove and to a lesserextent for the corrugated section.The stiffness is slightly over-estimatedfor the ribs.

In order to study the effect of a linearf.e.m. calculation versus a nonlinearf.e.m. calculation, geometricallynonlinear f.e.m. simulations have alsobeen performed. The results are shownin n F IGURE 12 . It can be seen that forlarger deflections, there is aconsiderable difference between thelinear and the nonlinear simulation.

18

Design gu ide 2 Design fo r s t i f fness

n F IGURE 12The difference betweena linear and a non-linearfinite element analysis

n F IGURE 11The cross-sections of the beamsin a finite element analysis withshell elements

V-groove

Ribs

Corrugated

orce deflection curve ofhe corrugated beam

Forc

e ( N

)

Deflection (mm)

40

35

30

25

20

15

10

5

00 50 100 150 200 250 300

non-linear

linear

Force deflection curve ofthe ribbed beam

Forc

e ( N

)

Deflection (mm)

40

35

30

25

20

15

10

5

00 50 100 150 200 250 300

non-linear

linear

Force deflection curve ofthe V-groove beamForc

e ( N

)

40

35

30

25

20

15

10

Ft5

00non-linear

linearDeflection (mm)50 100 150 200 250 300

3.1 Introduction

The strength of a part is defined as themaximum load that can be applied to apart without causing part failure undergiven conditions. In order to be able todetermine the strength of a part, firstfailure has to be defined. The rightdefinition of failure depends on the

3.2 Material strength

From a materials point of view, strengthis a stress/strain related propertyinherent in the material. There aremany different stress/strain issues whichcan relate to strength and which mustbe understood in order to design forstrength. A variety of standard tests is

soon nonlinearities occur; a closeobservation of the stress/strain curveshows that actually a proportional partdoes not exist. For larger strains,yielding occurs and the maximum stressis reached. If the strain is increasedfurther, necking will occur and the neckwill propagate through the structureuntil the material fails.

For a glass-filled thermoplastic, the

3

Design fo r s t rengthdefined as breakage of the part. In othermore critical applications, failure mustbe defined as the load at which the firstcrack occurs in the material.

This chapter explains how to obtainstrength measures from standardmaterial data. Different failuredefinitions are discussed and ways ofdetermining the strength of a part arepresented. Furthermore, some generalguidelines to design for strength aregiven.

Stre

ss straiapplication. In some cases a part hasfailed if it shows a certain amount ofpermanent deformation, while in othercases this can be allowed and failure isProportional Limit

yield re

slope = elastic modulus

unfilled thermoplastic

glass-filledthermoplasticengineering design. Other standardtests that are performed to obtainstrength data are flexural testing, sheartesting and compression testing.

Two typical stress/strain curvesobtained from a tensile test are shown inn F IGURE 13 . One curve is from anunfilled thermoplastic and the othercurve is from a filled thermoplastic.For the unfilled thermoplastic, for smallstrains the stress increasesproportionally with the strain however

n at fractureavailable to study the stress/strainbehaviour of a material under variousconditions. In general, the tensile testprovides the most useful information forStrain

maximum stress

necking/propagationgionstress increases faster with an increasingstrain; a glass-filled thermoplastic has ahigher Youngs or elastic modulus. Forlarger strains, nonlinearities occur andthe part fails in brittle mode when thefailure strain is reached. The followingstrength measurements can be obtainedfrom these curves:

n F IGURE 13Typical stress-straincurves for thermoplastics19

fact that, for the glass-filled materialbrittle failure occurs, and for theunfilled material ductile failure occurs.

3.2.4 Proportional limit

The proportional limit of a material isthe point at which the stress/straincurve becomes nonlinear,(see n F IGURE 13). Although theproportional limit of a material is oftenconsidered to be the same as theyielding point, the two are distinctlydifferent. For many conventionalmaterials like metals, the stress/straincurve is so highly linear that the twovalues are very close. For plastics,however, the viscoelastic nature of thematerial can push the yield point of thematerial well out into the nonlinearregion of the stress/strain curve.Also the proportional limit is heavilytemperature- and strain rate-dependent.

3.2.6 Other measures

Other measures that are equallyimportant for strength, but are notdirectly related to the stress/straincurve, are impact strength and fatiguestrength. For impact strength, readersare referred to Chapter 5, Design forimpact performance.

The fatigue strength of a material canbe measured in a number of ways.Typically, it is expressed as the numberof cycles to failure of a standard testspecimen when subjected to tensile orflexural cyclic loading, in accordancewith test standards. It can also beexpressed as the stress which a materialcan withstand up to a given number ofcycles without failing.

3.3 Effects of various factors onthe strength measures

20

Design gu ide 3 Design fo r s t rength

*2 The influence on material toughness orecovery is dependent on thetemperature and the rate at which thesample is tested. The yield point of athermoplastic cannot be determinedfrom a stress/strain curve. For thisreason, thermoplastic yield points aretypically estimated or shown as a yieldrange, (see n F IGURE 13).

3.2.3 Strain to failure

Another material property which isrelated to strength is the percentage ofstrain at which the material fails,(see n F IGURE 13). This can easily beobtained from the stress/strain curve.As with many other plastic materialproperties, the strain to failure is highlydependent on the temperature, thestrain rate and the ductility of thematerial. When the stress/strain curvesof the glass-filled material and thefrom the steel industry, but it alsoapplies to plastics. However, the yieldpoint of thermoplastics is difficult todetermine because of their viscoelasticnature. The yield point for classicalmaterials such as metals can easily bedetermined and tested by loading thesample to a point slightly beyond theproportional limit, (linear portion ofthe stress/strain curve), and thenunloading it and measuring thedeflection. Therefore, with someexperience it is easy to predict.When plastic materials are tested in asimilar manner, what originally appearsto be permanent deformation can berecovered slowly. The amount ofused to obtain strength values from theload-displacement output of non-tensiletype tests. Ultimate strength is afunction of temperature and strain rate.

3.2.2 Yield strength

The classical definition of the yieldstrength of a material is the stress levelwhich will cause a small amount ofpermanent deformation, (standard is0.2 % strain). This definition originates3.2.1 Ultimate strength

The ultimate strength, (tensile, flexural,compressive, or shear), of a material isthe maximum stress level in a sampleprior to failure. The ultimate tensilestrength of a material is the maximumload per unit area that the material willbear before failure. This value can beobtained from the maximum stressvalue on the stress/strain curve (seen F IGURE 13). Simple formulae are oftenunfilled material are compared, thestrain to failure of the glass-filledmaterial is much smaller than that ofthe unfilled material. This is due to then TABLE I I I

Effect of temperature, strain rate and gla

Temperature Str

Ultimate strength

Yield strength

Strain to fracture

Proportional limit

Material toughness *1

*1 A measure of material toughness is theAn increase in temperature decreasesincrease toughness. But an increase oto failure which will decrease the tougtemperature increase on material tougtwo factors described.Note that, strictly speaking, due to theviscoelastic nature of plastics, aproportional part of the stress/straincurve does not exist for these materials.

3.2.5 Material toughness

Material toughness is a useful propertyfor comparing material strengths.As with most material properties, it ishighly dependent on temperature andstrain rate. Toughness is the energyabsorbed by the material. The areaunder the stress/strain curve is equal tothe absorbed energy per unit volume.on the balance of two different effeclarger strain rates due to the increashand, it is decreased due to the lower ss fibres on the strength of materials

ain rate Add glass fillers

stress, strain

*2

area under the stress/strain curve. the maximum strength. This effect willf temperature also decreases the strainhness. Hence, the effect of ahness depends on the balance of the

f an increased strain rate also dependsAs mentioned earlier, temperature andstrain rate heavily influence the strengthmeasures of materials. In addition, theuse of glass fibres alters the strength ofmaterials. n TA B L E I I I summarizes theeffects of temperature increase, strainrate increase and addition of glass fillerson the strength measures.

3.3.1 Other factors

(a) ChemicalsThe chemical compatibility ofengineering thermoplastics is a complexphenomenon dependent on thematerial, type of chemical, mode andts. Material toughness is increased ated ultimate strength but, on the other

strain to failure.

of what part strength really means.In some cases, stress limits like yieldstrength and ultimate strength are mostimportant, while in other cases yieldstrain and ultimate strain are important.

The meaning of the term strength of apart will depend upon the type ofapplication, the function of the part,loading conditions, restraint conditions,and the performance of competitive orcomparable parts. In this section threemeasures of part strength will bediscussed: ultimate part strength, partyield strength and part toughness.The material parameters that areimportant for part strength depend onboth the measure of part strength andon the type of loading. Two types of

This concept of strength is common inengineering design, especially whenfactors of safety and worst case loadingsare considered.

An example of this type of strengthconcept on a load controlledapplication is the design of load bearingaircraft components. In these designs,loads of many times the expected loadare considered in order to assure safety.Other part strength concepts likepermanent deformations are not ofconcern. In this case, the ultimate partstrength is dependent on the ultimatestrength of the material and on thegeometry of the part.

Another example is a flexible bearingcover in a car. In this case, theapplication is deflection controlled andit is possible that an extreme loadingcondition on the bearing could produce

21

Design gu ide 3 Design fo r s t rengthwhich comprise the material is reduced.This can embrittle the material or causeit to have a reduced modulus, yieldstrength or ultimate strength.

Other moulding parameters such as lowtool temperatures or high injectionpressures can cause a significantincrease in the levels of moulded-instresses. These, in turn, add to thestresses induced in the part by the loadand hence decrease the load bearingcapabilities of the part.

Another important aspect of processingis the occurrence and the location ofweld lines. The material strength on aweld line is significantly lower. Thismeans that it is important to avoid weldlines in areas where large stresses areexpected due to the applied loads.(c) Processing conditionsProcessing conditions can have asignificant effect on the strength of amaterial in the final product. Moisture,small gate sizes, long residence times,(material at melt temperature in thebarrel of the machine), sharp corners orwall transitions, or excessive use ofregrind can all cause degradation of thepolymer. Effectively this means that theaverage length of the molecular chains(b) MoistureMoisture can significantly affect thestrength of engineering thermoplastics,much in the same way as chemicals.The hydrolytic stability of engineeringthermoplastics is a complex issuedependent upon material type, modeand duration of exposure, temperature,and stress level and stress states in thematerial. Some materials like Noryl

PPO resin have an excellent hydrolyticstability, while others, such as Lexan

polycarbonate resin, can have hydrolyticstability problems at elevated tempera-tures. GE Plastics representatives cansupply more specific information. duration of exposure, temperature, andlevels and state of stress present duringexposure. Different types of chemicalscan embrittle a thermoplastic materialor can cause it to become softened.However, it is not possible to classify thegeneral effect of chemicals on materials.For more specific information readersare invited to contact their GE Plasticsrepresentative.3.4 Part Strength

The strength of a plastic part can havedifferent meanings depending on thepart. There are many different conceptsreferred to as the ultimate strength,while the maximum deflection level isreferred to as the ultimate deflection.

Forc

e ( N

)

B

Cloading will be distinguished: oad (or force) controlled applicationsand deflection controlled applications.A load controlled application is definedas an application which must bear a loadof a certain magnitude. A deflectioncontrolled application is an applicationto which a deflection of a certainmagnitude is applied.

3.4.1 Ultimate part strength

Consider the force deflection curve asdepicted in n F IGURE 14 . Suppose thiscurve is the result of a test on an actualplastic part. When the part is loaded topoint A, the part breaks. The force anddeflection level at which the part breakscan be defined as the strength of thepart. The maximum force level is usuallypermant deformationFailure n F IGURE 14Sample load-deflection curveof a part

Aa deflection far beyond the normaloperating range, which can cause thepart to rupture. The ultimate strengthof a part is dependent on the strain tofailure of the material and thegeometry.

In summary, when ultimate strength isconsidered, for load controlledapplications the ultimate strength is thegoverning material parameter, while fordeflection controlled applications thestrain to failure is the governingmaterial parameter. Materials should beselected accordingly.Deflection (mm)

3.4.2 Part yield strength 3.4.3 Part toughness

Part toughness is often considered tobe a good measure of the strength ofthe part, and depends on materialtoughness and geometry. It is measuredby calculating the area under the load-deflection curve from initial load to partfailure. The toughness of a part is anissue for both load and deflectioncontrolled applications, and it is ameasure of the amount of energy thata part can absorb.

3.5 Improving the part strength

Once the strength criterion is decided,the strength of a part can be improved.This can be done by selecting the rightmaterial and/or by optimizing the partgeometry.

3.5.1 Material choice

Based on the strength criterion and theloading type of the application,n TA B L E IV can be used to decide whichmaterial parameter should be increasedin order to improve the strength of thepart. Based on this information theoptimal material can be selected.

22

Design gu ide 3 Design fo r s t rength

n F IGURE 15Strain recovery

n TABLE IV

Governing material properties depenent on failure criterion and load type

ultimatedstrength part yie

Per

cen

t af

ter

reco

very

10

min

.

Applied Strain (%)

100

90

80

70

60

503 4 5 6 7 8 9 10

mineral-filledmodified PPE PC - PBT

PBT

PC PEIdisappear in time.

In order to acquire material data,a laboratory test was developed.Tensile specimens were loaded tovarious strain levels at a rate of 10%/sand then rapidly unloaded, while thestrain recovery was monitored.Since the rate of recovery was very smallafter 10 minutes, the percent recovery,which is defined as the ratio betweenthe residual strain and the appliedstrain, was plotted after this time as afunction of the applied strain.The results are depicted in n F IGURE 15 .As an example, a design rule of 90%recovery is shown in this Figure.It can be seen that in the case of PC amaximum strain of approximately 9.3%will be allowed, compared toapproximately 5.8% for PC-PBT.(or yield strain), of the material and onthe geometry of the part.

A refinement of this failure criterioncould be the allowance of a certainamount of permanent deformation.If this is the case, strain recovery playsan important role. When a material isloaded and afterwards unloaded, (forexample up to point C in n F IGURE 14),the permanent strain will partlya new strength criterion which isreferred to as the part yield strength.An example of a load controlledapplication is a thermoplastic spring.Permanent deformation of the springusually makes it non-functional. In thiscase, the part strength depends on boththe yield strength of the material and onthe geometry of the part. Anotherexample is a plastic snap-fit. A snap-fit isa deflection controlled feature whereover-extension can cause permanentdeflection. This can make the snap-fituseless if it cannot return to engage thestop. In this case, the part strengthdepends on both the yield stress,Suppose that the part, ( n F IGURE 14)is loaded to point B and then unloaded.After a certain time, the part regains itsoriginal shape. If the part is loadedfurther, (for example to point C) andthen unloaded, it does not retain itsoriginal shape and a permanentdeformation will occur.

If at a certain level of permanentdeformation the part ceases to be usefuland must be replaced, then this is againno permanendeformationallowed

load ultimate yield stresscontrolled material

strength

deflection strain to strain at yielcontrolled failureld strength part toughness

t somepermanentdeformationallowed

maximum materialstrain toughnessaccording ton F IGURE 15

d maximum materialstrain toughnessaccording ton F IGURE 15

3.5.2 Geometry optimization

Since the strength of a part depends onmany factors, like loads and restraintconditions, it is very difficult to give

3.6 Design considerations

Some general guidelines for design forstrength are as follows:

(g) Use safety factorsAs with all materials, it is impossible tocontrol all aspects of production anduse. For this reason, conservativeestimates and risk dependent safetyfactors should be applied whenappropriate.

(h) Consider worst caseIn addition to analyzing the expecteduse for a proposed part, a worst casescenario should also be studied.

23

Design gu ide 3 Design fo r s t rengthidentified. In general, hand calculationsare not sufficient here, since the stressand strain levels are usually far beyondthe point at which the linearcalculations are valid. Even the results ofdetailed finite element analyses have tobe interpreted with care. The reason forthis is that the accuracy of especially thestresses and the strains is dependent onthe type of element used and on themesh density. This means that theaccuracy of the stress and strain levels isnot always as high as desired. However,finite element analysis is very well-suitedto identifying areas of the largeststresses and to comparing the effect ofdifferent possible design changes.For deflection controlled applicationsthe situation is different. In this case, acertain deflection is prescribed to thepart and this results in a given strain inthe material. The strength can beincreased by removing material in theareas where the largest strains occur. This will significantly decrease thestiffness of the part, so it must beverified if this is allowed.

For load controlled applications thearea of maximum stresses and fordeflection controlled applications thearea of maximum strains have to beFor load controlled applications, theamount of load that has to be carried bythe part is known. In the material thisload will be transformed to stresses.Areas of large stresses have to beidentified and material should be addedto the appropriate areas. The goal is tohave the load carried by a maximumamount of material. This will decreasethe occurring stress levels.some general guidelines on how tochange a design in order to improve thestrength of a part. However, somegeneral remarks can be madedependent on the type of loading onthe part. Of course when a design ismodified to improve the strength, thedesign rules that are summarized in thefollowing section have to be respected.material should be located as far awayfrom the neutral axis as possible. Formaximum deflection without yield,material should be concentrated alongthe neutral axis.

(f) Consider processing aspectsProcessing can be critical for thestrength of a part. Recommendationsinclude:

Weld lines should be avoided or located in non-critical areas.

The frozen-in stresses should be as low as possible.

For fibre-reinforced plastics, thedifference in strength in the flow and the cross-flow direction should beconsidered(c) Design for compressive stressAs with many materials, unfilled plasticmaterials tend to be stronger incompression than in tension, as long asbuckling does not become critical.

(d) In tension, design for a uniformcross-sectional areaIn a plastic part which is primarily intension, a uniform cross-sectional areashould be used to obtain a moreuniform stress distribution.

(e) In bending, design for momentof inertiaIn a plastic part which is primarilysubjected to flexural loading, themoment of inertia is critical indetermining the load bearing capability.For maximum load bearing strength,(a) Avoid stress concentratorsStress concentrators can significantlyreduce the strength of a part and shouldbe avoided. These include sharpnotches, or internal corners, sharplyangled wall intersections, large wallthickness transitions and surfaceinterruptions such as holes and inserts.

(b) Avoid uncontrollableloading situationsExamples include pipe threads wherethe designer can have no control overhow much it is tightened, causing hoopstress.

4.2.1 Creep

Under the action of a constant stress,(load), a viscoelastic material undergoesa time dependent increase in straincalled creep or cold-flow. Creep is there-fore the result of increasing strain overtime under a constant load. The creeprate for any material is dependent uponapplied stress, temperature and time.

deformation due to the applied load ona specimen. Up to this point, theresponse is elastic in nature andtherefore the specimen will fully recoverafter the load is removed. However,continued application of the load willresult in a gradual increase indeformation over time to point B; inother words, it creeps. In the flexuraland compressive modes, the specimen is

4

Design fo r behav iorover t ime

stress

ur

deformationdesign.

The biggest problem experienced whenattempting to predict long-term loadingeffects, is the availability of data specificto an application; most data are basedupon standard tests performed uponmaterial specimens. However, thisinformation can be used to give anindication of the performance of acertain design and can be verified byactual physical testing of thethermoplastic components for therequired performance.

4.2 Static time dependentphenomena

Traditional structural calculationscentre around short-term material datain which the material exhibits perfectlyelastic behaviour. However, another4.1 Introduction

When discussing time related behav-iour, two types of phenomena should beconsidered. Static time dependentphenomena, such as creep, are causedby a single long-term loading of anapplication. Dynamic time dependentphenomena, such as fatigue, areproduced by cyclic loading of anapplication. Both types of behaviour areheavily influenced by material choice,operating environment and componentimportant property exhibited bythermoplastics is that of viscoelasticity.This is the simultaneous demonstrationof both viscous flow and elasticdeformation under an applied load overa long time period. Short-term stress/strain behaviour usually occurs withinless than an hour of the load beingapplied and is often consideredinstantaneous. Viscoelastic behaviour,however, may continue throughout theworking life of a component, often overa period of years.time, over a range of loads at a giventemperature. Measurement may betaken in the tensile, flexural orcompressive mode. In the tensile mode,a test specimen is subjected to aconstant tensile stress and the change inlength is measured as a function of time.The resultant stress/strain/time creepdata are normally presented as curves ofstrain versus log time. Point A inn F IGURE 16 illustrates the initial

Stra

in

constantvis

coel

asticCreep behaviour is initially examinedusing plots of strain as a function ofelas

tic

A

0subjected to either a constant bendingor compressive load and the deflectionis measured as a function of time.

The data may also be presented in otherforms to suit particular requirements.Sections taken through the creep curvesat constant times produce isochronousstress/strain curves, whilst isometricstress versus log time can be derivedfrom constant strain.

n F IGURE 16Typical creep behavioof an amorphousthermoplastic,assuming no plastic

B25

Log Time

4.2.1.1 Creep recovery

If the long-term process of creep isinterrupted by removing the load, animmediate but partial elastic recovery inthe deformation of the specimen can beseen, as illustrated in n F IGURE 17 frompoint B to point C. This represents therelease of energy stored elastically bythe material when it was first deformedto point A.

As time progresses, recovery continues;in fact, the shape of the curve frompoint B is almost an inverted copy of theshape up to point B. The amount ofrecovery depends upon the type ofmaterial, the applied stress, the tem-perature and the duration of loading.In some cases, there will eventually betotal recovery of the deformation, as

illustrated by point D in n F IGURE 17 ,whilst in others, a significant amount ofpermanent plastic deformation willoccur, as illustrated by point E.

4.2.1.2 Creep failure modes

There are two failure modesencountered under the action of creep.

(a) Excessive deformation When the deformation of a componentexceeds the allowable limit for thatapplication.

(b) Creep Rupture This may result in either a brittle orductile fracture of the component.Measurement of creep rupture isperformed in the same manner as

creep, except that higher stresses areused and time measured to failure.Results are usually presented as logstress versus log time to failure, as inn F IGURE 18 .

4.2.1.3 Apparent Modulus

Though creep and creep rupture datagive an indication of the long-termbehaviour of a material, for practicaldesign purposes the correspondingreduction in modulus over time is ofgreater value. If a calculation isperformed on a component subjected toa continuous load, and short-termmoduli such as E, (modulus ofelasticity),or G, (shear modulus), are used, theresult is likely to be misleading sinceneither E nor G reflect the effects of

26

Design gu ide 4 Design fo r behav iour over t ime

n F IGURE 17Typical creep recoverydiagram of an amorphousthermoplastic, assuming noplastic deformation

Stra

in

constant stressvisc

oela

stic

visc

oela

stic

elas

tic

elas

tic

permanantdeformation

B

E

A

Cn F IGURE

Typical crewith one c

018ep rapture data,ycle projection S

tres

s

measureLog Time

D

d data

one cycle projectionLog Time to failure

creep. When the stress level andtemperature are known and creepcurves are available at the giventemperature, an apparent or creepmodulus, Eapp. may be calculated usingthe creep curves;

s1Eapp. = ec

Where;s is the calculated stress level. ec is the strain from the creep curve

at a given temperature and time.

The value Eapp. can then be substitutedfor E, or the like, in standard structuraldesign equations. Creep modulus curvesor log creep versus log time at eitherconstant stress or strain are usually de-rived from creep data, see n F IGURE 19 .

4.2.2 Stress relaxation

Unlike creep where the strain increasesover time, stress relaxation is thereduction in stress in a component overtime under a constant strain. The areawhere stress relaxation is of greatestimpact is in component assembly. Thisincludes threaded assembly, inserts,press-fits and snap-fits, although anycomponent undergoing long-termdeformation may be affected to somedegree.

Initially, the material is subjected to astrain e1 which is maintained for a longperiod, that is to e2, (see n F IGURE 20).The immediate response of the materialwhen the strain is applied is an increasein stress from zero to s1. This value istemperature dependent and tends todecrease as a function of time, that is tos2,(see n F IGURE 21).

Stress relaxation data can be generatedby applying a fixed strain to a sampleand measuring the gradual decay ofstress over time. The resultant data canthen be used to generate stressrelaxation curves which are very similarto isometric strain curves.

Stress relaxation data can be used togenerate a relaxation modulus, similarto the creep modulus.However, relaxation data are not ascommonly available as creep data.In such instances, however, it is possibleto approximate the decrease in load dueto stress relaxation by using the creepmodulus, Eapp.

The use of standard moduli, (E or G ),in structural calculations, is inapprop-riate for the long-term structuralperformance of assembled componentsas it does not reflect the effects of stressrelaxation.

27

Design gu ide 4 Design fo r behav iour over t imeA

pp

aren

t M

odu

lus

(GP

a)

2.5

2.0

1.5

1.0

23C

40C

n F IGURE 19Typical CreepModulus curve ofApparant Modulusversus Log Time fora given material atdifferenttemperaturesStra

in

Log Time

0.5100 101 102 103 1

e1 eLog Time (s)04 105 106

Stre

ssn F IGURE 20Graph illustratingconstant strainover time2 s160CLog Time

n F IGURE 21Graph illustratingstress relaxationover time forconstant strain

s2

Fatigue tests are usually conductedunder flexural conditions, thoughtensile and torsional approaches arealso possible. A specimen of material isrepeatedly subjected to a constantdeflection at constant frequency, andthe number of cycles to failure isrecorded. The procedure is thenrepeated over a range of deflections.The test data are usually presented as aplot of log stress versus log cycles; this iscommonly referred to as a S -N curve.

4.3.1.1 Fatigue and endurance limits

The most important features of anS -N curve are;

The greater the applied stress or strain,the fewer the cycles a specimen willsurvive.

The curve gradually approaches a con-stant value, known as the fatigue limit,as illustrated in n F IGURE 2 2 . Below thisvalue of stress or strain failure is veryunlikely. Some materials do not exhibita fatigue limit and for these materials anendurance limit is specified. This is thevalue of stress or strain at a statednumber of cycles beyond which thespecimen will fail, (see n F IGURE 2 3).

28

Design gu ide 4 Design fo r behav iour over t ime

n F IGURE 22Typical fatigue curvefor a material with afatigue limit

n F IGURE 23Typical fatigue curvefor a material with anendurance limit

Stre

ss

Log Time to failure

S-N curve

Fatigue Limit4.3 Dynamic time dependentphenomena

4.3.1 Fatigue

This is the process by which a material isstressed repeatedly or in some definedcyclical manner. The magnitude of theloading is usually of such low value thatfailure would not normally be expectedwere it applied only once. Additionally,the loading and its frequency ofapplication may vary in value.

Structural components subjected tovibration, components subjected torepeated impacts, reciprocatingmechanical components, plastic snap-fitlatches and moulded-in plastic hingesare all examples of applications wherefatigue can play an important factor.be used concerning the extent of theextrapolation in time. It is not recom-mended that the extrapolation exceedmore than one unit of logarithmic timeand a strain/elongation limit of 20%of the yield or ultimate stress/strengthvalue for the material being analyzed.It should be noted that althoughamorphous materials give a single curvewhen interpolating to other tempera-tures, for semi-crystalline materials twoplots are required to create the curve,one above and one below the glasstransition temperature (Tg) of thematerial.methods of interpolating andextrapolating data from short-termbehaviour are necessary.

Several methods for extrapolatingtime/strain data exist, each with theobjective of obtaining the most accuratefit to the actual data, while givingreasonable extrapolation predictions forlonger time periods. From workperformed within GE Plastics, it wasdetermined that for its materials thebest fit for the data appeared to be givenby a quadratic function of log time.However, engineering judgement must4.2.3 Design considerations

Creep is often a critical concern whendesigning a structural part in thermo-plastics. Although data exist for specifictimes, stress levels and temperatures,it is often difficult for engineers totranslate this data into structuralanalyses relevant to their application.Since the process of individual testingover long time periods is not feasible,Cyclic loading can result in mechanicaldeterioration and fracture propagationthrough the material, leading toultimate failure. Stre

ss

Endurance LimitLog Cycles to failure

4.3.1.2 Factors affecting fatigue

S-N curves, obtained under laboratoryconditions, may be regarded as ideal.However, practical conditions usuallynecessitate the use of a modified fatigueor endurance limit, as other factors mayaffect performance. These include:

Type of loading Results of tests performed under flexuremay not be directly applicable tocomponents subjected to axial ortorsional loading.

Size of component

29

Design gu ide 4 Design fo r behav iour over t ime

n F IGURE 24Microscopic view ofsurfaces in contact

ear bond plane of motion

normal reactionbetween surfacesTherefore, it is essential that tests areperformed on actual mouldedcomponents under actual end-useoperating conditions, in order todetermine the true fatigue enduranceof that component.Loading frequency At high frequencies, an increase intemperature may be experienced ifinadequate provision for heatdissipation exists. This may lead todeterioration in fatigue properties.

Amplitude The onset of fatigue failure may also beaccelerated at low frequency underconditions of high amplitude.

Fatigue testing provides only anindication as to a given plastic materialsrelative ability to survive fatigue.A deterioration in fatigue properties isusually experienced with increasingphysical dimensions.

Mean stress Amplitudes about a tensile mean stressreduce fatigue and endurance limits,whereas an increase occurs when themean stress is compressive.bearing materials. In the case of plastics,this is strongly dependent on themodulus of elasticity, (E-modulus), ofthe materials being used.

Optimally moulded surfaces of Valox

PBT resin components have a relativesmoothness which makes them ideallysuited to low friction, high wearresistance applications over a widerange of operating conditions.In general, Valox resins exhibit very lowcoefficients of static and dynamicfriction against metals and also againstsimilar Valox resins. Other semi-crystalline resins generally exhibit anappropriate wear resistance againstmetals but invariably may not besuitable for applications where they bearagainst themselves. It is important to remember that theoperating environment of the end-usethe endurance limit of the material.

4.3.2.2 Factors affecting wear

The wear resistance of a thermoplastic isdependent on the environmentalconditions of the end-use application.Temperature, surface contamination,(chemicals, grease, etc.), surfacestructure and contouring, all have aninfluence on wear resistance. Changesin the wear process occur with thedegree of surface roughness of the4.3.2.1 Types of wear

Adhesive wear Surface shearing, deformation andremoval of material at the points ofadhesion.

Abrasive wear Surfaces of different hardness incontact, with grooves being ploughed inthe softer material, the presence of anabrasive medium and erosion abrasiondue to particles impacting the surfaces.

Fatigue Surface fatigue failure as a result ofsurface or sub-surface stress exceeding

w4.3.2 Wear resistance

Wear can be described as the reductionof a bearing surface due to the loss ofmolecular adhesion as a result ofmechanical friction, (see n F IGURE 24).There is no apparent consistentrelationship between friction and wear;friction may be high and wear low, andvice versa.application should be taken intoconsideration when using frictionalvalues for engineering design purposes.

5.2 Common impact testing methods

A number of tests are commonly used toprovide insight into the response ofplastics to impact loadings. Two generictypes of testing exist, pendulum andfalling weight methods. It is importantto note that none of these traditionaltest methods results in real, geometryindependent material data which can beused in engineering design calculations.

They can be useful in applications forquality control and initial materialcomparisons, but even in this latter roledifferent tests will often rank materialsin a different order. Typical traces forboth ductile and brittle response to aninstrumented impact measurementsystem are illustrated in n F IGURE 26 .

5

Design fo r impactper formance

n F IGURE 25Graph illustratingeffect of tempera-ture upon impactresponse

Imp

act

valu

e

Ductile/brittle transition

Ductile behaviour Type of material Wall thickness Geometric shape of component Size of component Operating temperature Rate of loading Stress state induced by loading

For ductile polymers, the load at whichyielding occurs in a component isaffected by the operating environmentas illustrated by the last three factorsdetailed above. Of even moresignificance to design is the fact that,under the appropriate circumstances,the impact behaviour of a ductilepolymer will undergo a transition fromductile and forgiving in its response, tobrittle and catastrophic.

Usually this change in behaviour isdescribed in terms of a transition5.1 Introduction

Impact strength can be described as theability of a material to withstand animpulsive loading. By definition, anybody in motion possesses kinetic energyand, when this motion is stopped, theenergy must be dissipated.

There are several factors whichdetermine the ability of a plasticcomponent to absorb impact energy:temperature above which the failure ismore ductile in nature, and belowwhich it is more brittle, as illustrated inn F IGURE 25 . However, both rate ofloading and stress state have an effecton this transition temperature. Brittle behaviourForc

e

Forc

e

Time

Ductile polymer

Energie absorbed by specimenTemperature31

n F IGURE 26Typical force/timecurves for ductileand brittle materialresponses, asproduced by aninstrumentedsystem

Time

Brittle polymer

5.2.1 Pendulum methods

In this form of testing, a specimen ofmaterial is struck by a pendulum underdefined conditions, and the energyrequired for fracture is measured.It should be noted that the results ofthese tests are highly dependent uponthe geometry of the test specimen andthat the results do not define actualmaterial properties.

(a) Charpy impact In this test, a notched specimen issimply supported. When released,the pendulum strikes the specimencentrally, resulting in fractureThe energy absorbed by the specimenis displayed by a system activated by themotion of the pendulum,(see n F IGURE 27).

(b) Izod impact This is similar to Charpy thoughdiffering more in notch geometry andmethod of specimen support. In thistest, the specimen is mounted vertically,impact consequently occurring distantto the notch. The test can also be

(c) Tensile test This test u