SHEAR TESTING – SR4

SUBMITTED BY ARJUN RADHAKRISHNAN CID 00704997

 

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SUMMARY The test is conducted to estimate the shear strength and shear modulus. Iosipescu and ±45o tensile test are used to calculate in-plane shear modulus and strength. Short beam test is utilized to obtain apparent inter-laminar shear strength of the material. The load, displacement and the strain are monitored. The short beam test yielded reliable inter-laminar shear strength of mean 98.56 MPa. The Iosipescu test was for shear properties in 1-2 plane yielded a mean of 4.62 GPa and 97.978 MPa of shear modulus and shear strength. The ±45o tensile test indicates a close correlation with current literature for both shear modulus and strength. Shear strength obtained from Iosipescu [0]n test is almost double the data from literature, although, the shear modulus is comparable. The short beam test indicates a reliable data for apparent inter-laminar shear stress (ILSS). INTRODUCTION The composite material has a poor shear resistance. The common failure mode in composites is delamination and inter-laminar shear strength is one of the parameters that regulate it. The in-plane shear resistance is poor too and this leads to quick crack growth. The challenge in shear testing for composite specimen lies in achieving a pure shear stress state. All test methods address this by aiming to maximize the shear stress state in the specimen. The inherent specimen defects augments the difficulty in achieving a pure shear stress. Few types of tests for shear strength and modulli are listed in table 1.

 

 

 

       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure  1  Iosipescu  test1

Figure  2  Two-­‐rail  shear  test1  

 

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Figure  3  Three  rail  shear  test1  

 Figure  4  Short  beam  test2  

 Figure  5  Failure  modes  for  Iosipescu  test1  

   

 

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Table  1  Shear  testing  methods1  TEST NAME DESCRIPTION STANDARDS COMMENTS

±45o tensile test A specimen with symmetric layup of ±45o is loaded under tension. Strains are monitored in 0o & 90o. Specimen dimensions varies with standards

BS EN ISO 14,129 ASTM 3518M-94

The shear strength and shear modulus in 1-2 plane is calculated. The ultimate stress is indicative of sum of shear and tensile strengths.

V-notched Iosipescu test

A uni-directional specimen with 2 opposing v-notches at the center is utilized to maximize shear. A Wyoming test fixture, as shown in fig 1, is used to load the specimen. The strains are monitored in 45o and -45o. The fibers can be in transverse or longitudinal direction to the loading direction.

ASTM D 5379 The shear strength and modulus in 1-2 or 2-1 plane is calculated. The failure mode is to be reported. The ASTM provides a list of acceptable failures. The transverse fiber specimens have an unambiguous failure but longitudinal fiber specimen is preferred as it is more representative of the shear strength.

Short beam test A flat or ring specimen of span/thickness of 5 or 4 is utilized depending on the standards. A three point bending method is used to load the specimen as shown in fig 4.

BS EN ISO 14130 ASTM D 2344-84

The test measures the apparent ILSS. The tensile and compressive forces are reduced to minimum by having a small span/thickness ratio.

10o off axis test A specimen with fibers oriented 10o of the loading direction is subjected to tension. A rosette strain gage arrangement is used to monitor strain.

Currently not registered in any of the standards.

The 10o axis fibers maximize the shear state. The in-plane shear strength and modulus can be calculated.

Rail shear test The specimens are designed according to the standards. It can be 2 or 3 rail system, as shown in fig 2 and 3.

ASTM D-4255

Shear strength and modulus can be calculated. Possibility premature failure at bolting joints.

 Figure  6  Failure  modes  in  Short  beam  test2  

     

 

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Table  2  Failure  modes  for  various  testing  methods1  Test name Failure modes

±45o tensile test The failure is mixture of tensile and shearing. Fiber rotation, edge and internal delamination precede fracture. The specimen might take strain even beyond shear failure due to the fiber. According to standards 5% strain is a cap at which the specimen is safely assumed to have failed by shear.

V-notched Iosipescu test (0o) The failure initiated by stress concentration at the notches, leading to cracking at tips. The cracks occur at both the notches. The crack grows axially but is arrested by the longitudinally aligned fibers. Minor interfacial cracks combine to grow the crack until specimen fails completely. The specimen might not split as shearing of finer might not take place. The failure mode will vary with specimen material as shown in fig 5.

V-notched Iosipescu test (90o) The crack grows along the notch tip and occurs at a very low load, as the fracture resistance is low.

Short beam test

Inter-laminar shear failure is expected for glass and carbon fiber reinforced composites. DRC exhibits a tensile through thickness cracking. The failure may occur at the loading points due to localized buckling of fiber. The various modes are depicted in fig 6

OBJECTIVES • The in-plane shear test is to be done using V-notched Iosipescu (0o & 90o) and ±45o

tensile test. • The through thickness shear test is to be done using short beam bending test. • A Wyoming test fixture (as shown in fig 1) is used for V-notched Iosipescu test, A tensile

loading jig is for ±45o tensile test and three point bending fixture (as shown in fig 4) is used for short beam test. • The parameters to be monitored for the three tests are shown in table 3.

Table  3  Parameters  to  be  monitored  Test name Loading Displacement Strain

V-notched Iosipescu ✓ ✓ ✓ ±45o tensile test ✓ ✓ ✓ Short beam test ✓ ✓ ✗

• The properties to be calculated for the three tests are shown in table 4.

 Table  4  Properties  to  be  calculated  

Test name Properties Expected failure mode V-notched

Iosipescu test Shear strength, Shear modulus, Failure strain

Failure at the notch width with no delamination

±45o tensile test Shear strength, Shear modulus, Failure strain

Failure at the gage length with minimized tensile failure

Short beam test Apparent Inter-Laminar shear strength

Inter-laminar failure

• The failure mode of the specimen is to be reported and irregularities reported. • The Stress-Strain is to be plotted for the Iosipescu and ±45o tensile test specimens.

 

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PROCEDURE • Four specimens are prepared using MTM 44-1 HTS prepreg tapes the dimensions are

according to the standards and tabulated in tables 5,6 & 7 • The V-notched Iosipescu specimen is shown in fig 7.

                                                             Figure  7  Iosipescu  test  specimen3

Table  5  ±45o  tensile  test  specimen  dimensions  Specimen Length (mm) End tab length

(mm) Width (mm) Thickness

(mm) 1 96.5 51.5 24.96 1.06 2 96.5 51.7 24.98 1.01 3 98.87 50 24.21 1.05 4 98.93 50 24.87 1.04

Table  6  Iosipescu  test  specimen  dimensions  

Fiber direction

Specimen Length (mm) Width (mm) Notch width (mm)

Thickness (mm)

0o 1 19.91 75.63 11.1 3.52 2 20 75.81 11.4 3.06

90o 1 19.79 75 11.57 3.22 2 20 75 11.33 3.33

Table  7  Short  beam  test  specimen  dimensions  

Specimen Length (mm) Width (mm) Thickness (mm) Span/Thickness

1 19.88 9.96 2.08 4.8  2 19.92 996 2 10  3 19.08 9.97 2.24 4.46  4 19.96 9.97 2.117 4.72  

• The Iosipescu and tensile test specimens are end-tabbed with GRP. • The Iosipescu and tensile test specimen surfaces are cleaned and roughened using an abrasive

cloth. • The strain gauges are attached to those specimens using adhesives.

IOSIPESCU TEST

• The V-notched specimen is loaded on to the Wyoming fixture as shown in fig 1. • The span between the loading rollers is kept at 10mm. • The strain gage outputs are connected to the data acquisition device. • The load is applied at a constant rate of 1mm/min using an Instron at one end of the fixture.

±45O TENSILE TEST

• The ±45O tensile specimen is loaded on to the jig as shown in fig 8. • The strain gage outputs are connected to the data acquisition device. • A tensile load is applied at a constant rate of 1mm/min for specimen 1 and 3 and

5mm/min for specimen 2 and 4 using an Instron testing machine.

 

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SHORT BEAM TEST

• The specimen is loaded on jig that applies a three point bending as shown in fig 4. • The specimen is subjected to a constant displacement rate of 1mm/min. DATA ACQUISITION AND INTEPRETATION • The parameters are monitored as shown in table 3 and the stress and strain are calculated

using equations2,3 1 and 2,respectively. Report the failure modes. τi = Pi/A - eqn 1 γi= |εxi| + |εyi| - eqn 2 Where, Pi- load at ith point A – cross-sectional area εxi, εyi- Strain in directions 45 o and -45o to the fiber

• Plot the stress vs. strain to failure for Iosipescu and tensile test specimens. • The shear strengths and shear chord modulus are calculated using the formulae in table 8

and table 9, respectively. • Discuss the failure modes reported and the quality of the data.

Table  8  Equations  to  calculate  shear  strength  

Test name Shear strength Terms Iosipescu test3 Fu = Pmax/A

• Fu- Ultimate shear strength • Pmax – maximum load prior to failure • For 0o it has to be noted that the

second load drop is taken as the failure load as the first drop in load is due to notch root crack development.

• A – cross-sectional area at the failure location, usually the notch area.

±45O tensile test4 τ12= Pmax/2A • τ12- Maximum shear stress • Pmax – maximum load at or below 5%

shear strain. • A- cross-sectional area of the gage

zone. Short beam test2 SH= 0.75Pb/bd • SH- apparent inter-laminar shear

strength • Pb- breaking load • b- width of the specimen • d- thickness of the specimen

Table  9  Shear  chord  modulus  formulae  

Test name Shear modulus Strain range Iosipescu test3 Gchord=Δτ/Δγ

Gchord- shear chord modulus of elasticity Δτ- difference in applied shear stress Δγ- difference between the shear points

Start point within the range of 1500-2500 µε End point at 4000±200µε

±45O tensile test4

 

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 Figure  8  ±45o  tensile  testing  

RESULT

Table  9  Results  for  Iosipescu  tests  Fiber angle

Specimen Shear strength (MPa)

Shear modulus

(GPa)

Failure mode and observations

0o 1 95.616 4.2 • A horizontal crack near the bottom notch • The specimen did not split completely • Debonding of end tabs at the static end

2 100.34 4.8 NA* 90o 1 26.29 2.83 • Specimen cracked into two

• The split is not at the notch width as shown in figure 11

2 34.2 3.5 NA* * The specimen testing was not visually studied

Table  10  Results  for  ±45O  tensile  test  Specimen Shear strength

(MPa) Shear modulus (GPa)

Failure mode and observations

1 109.987 3.333 • Specimen failed near the bottom tab at an angle as shown in figure 12.

• Debonding noticed at the end tabs. • A very slight bending was observed

2 120.690 4.333 • Specimen failed near the top at an angle. • Debonding noticed at the end tabs.

3 116.441 3.667 NA* 4 107.590 3.667 NA*

* The specimen testing was not visually studied

 

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 Figure  9  Stress-­‐Strain  plot  for  Iosipescu  specimens

 Figure  10  Stress-­‐Strain  plots  for  tensile  test  specimen  

 

Figure  11  Iosipescu  90  degree  -­‐  specimen  1  

 

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 Figure  12  ±45O  tensile  test  -­‐  specimen  1    

Table  11  Results  for  Short  beam  test  Specimen Apparent ILSS (MPa) Failure mode and observations

1 98.793 A delamination was visible on top plies. The bottom plies indicated no failure. The mode is inter-laminar shear. A slight indentation is visible.

2 99.615 A slight crack was noticed, but unable to define it as delamination. Compressive and inter-laminar failures.

3 100.671 NA* 4 95.313 NA*

* The specimen testing was not visually studied

DISCUSSION In the Iosipescu test for 0o specimens, the stress increases as the strain until there is slight drop as shown in fig 9. The first drop is indicative of the initial crack growth at the notches. The crack initiates at both the notches and grows. The slight drop in stresses can be attributed to formation of multiple cracks and its growth until finally the specimen fails. The rise in stress with no strain increase is due to strain gage failure before the failure load was reached. The trend is similar for both the specimens. The final failure did not result in total splitting of specimen, as fibers were not sheared. The specimen shear strength is twice more than that is presented in the literature. The result variation can be attributed to either of the reasons stated below:

1. The material is stronger than that is stated in literature 2. The identification of final failure is flawed

The second reason is most probable one, as identifying the ultimate failure from axial splitting is a current problem in Iosipescu testing. Much work has been done regarding identification of the failure load, but its still inconclusive. The strength obtained in [0]n Iosipescu testing can be over 10 % overestimation. The [90]n Iosipescu testing is considered to give underestimation of around 20 %. The results are flawed as the shear gets coupled with transverse tension leading to premature failure. As seen from fig 9, the failure occurs much below the [0]n specimens. The strain increases with stress and the drops as it fails. The variation in trend for specimen 1 can be attributed to errors by the strain gages rather than any phenomenon of academic interest. The stress-strain for ±45O tensile test is plotted in fig 10, but only till the maximum strain recorded. The strain gage maximum stress was 2.1% normal strain. It cannot be verified if the maximum strain went above 5%, hence giving no indication if shear failure occurred even before recorded failure load. The trend is similar for the entire specimens and indicates a transition region. The specimens seem to indicate similar stresses in the linear region, but as the transition occurred it diverted. The mean shear strength is 113.677 MPa that are comparable to the available literature, indicating that the specimen failure can be attributed to shear. The failure occurred at an angle and indicates possible shear assisted by the tensile forces. The fiber rotation must have occurred as the specimen did not explosively fail and

 

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remained intact due to the fibers. The case is similar for both the specimens except for the position of the failure.

Table  12  The  validity  of  the  test  Test name Standards Current test and comment2,3,4

Iosipescu test Both surfaces are to be bonded with strain gages two monitor if twisting of specimen occurs.

Only one side was bonded with specimen, it can only be assumed no twisting is occurring. It should be verified by reliability of data and comparison with literature.

Failure should be at the notch The notch is used to reduce effects of tensile and compressive failure. If, the failure did not occur at the notch it is indicative of effects of tension. The failure for Iosipescu test for [90]n occurred away from the notch so invalid.

Strain gage of minimum resistance 350 Ω.

The strain gage resistance is 120 Ω, but as carbon is conductivity material the heat won’t build and hence effects of temperature on the readings are negligible.

±45O tensile test The strain gage with capacity upto 5% strain is required.

The strain gage failed at around 2.1% strain hence ultimate failure strain could not be measured. The strength maybe an overestimation as it is unsure if the strain crossed 5%.

The strain gage alignment The strain gage on specimen 2 was slightly misaligned. Although the result would not have been affected severely.

Short beam test Width/Span ratio near to 2 The width span ratio was close to 5 for all specimens. This possibly could have affected the properties, as increased width wise shear would have existed. ref

Table  13  Comparison  with  literature5  Test name Shear strength (MPa) Shear modulus (GPa)

Current test Literature Current test Literature Iosipescu [0]n 97.978 50.3 4.5 4.31

±45O tensile test 113.677 115 3.75 4.83 The short beam test is a quality assurance test. As the shear is induces by flexure the shear strength obtained is an “apparent” inter-laminar stresses and cannot be used for design

 

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purposes. The specimen 1 had a visible indentation possibly due to the loading roller, this could have attributed to a lower ILSS. A visible crack is present on top ply of specimen 2 indicating a possible compressive failure. The current test data is reliable as the variation is less with a mean strength of 98.598 MPa.

Table  14  Standard  deviation  of  the  properties  Test name Standard deviation

Shear strength Shear modulus Apparent ILSS ±45O tensile test 5.986 0.419 -

Iosipescu [0]n 3.240 0.424 Short beam test - - 2.321

CONCLUSION The in-plane shear testing done using Iosipescu [0]n , [90]n and ±45O tensile test. The Iosipescu [90]n is poor for obtaining shear properties due to specimen’s premature failure. The Iosipescu [0]n test provides a reliable shear modulus of mean value 4.5 GPa, but the shear strength value indicates an anomaly. The accuracy of identifying of the final failure is essential to the calculation of shear strength. The shear strength and modulus obtained from ±45O tensile test is 113.677 MPa and 3.75 MPa, respectively. The better result for shear modulus is obtained from Iosipescu test while shear strength is from ±45O tensile test. The short beam test yielded an apparent ILSS of 98.598 MPa. The test is suitable for quality assurance as the standard deviation of the test is 2.321. The shear test method is to be chosen from among the various tests based on the requirement, accuracy and cost of testing. REFERENCES

1. Hodgkinson, J. M., Mechanical testing of advanced fibre composites, 2000 pg. 43-73, edition 2000

2. ASTM D-2344, “Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates”, 2000

3. ASTM D 5379, “Standard Test Method for Shear Properties of Composite Materials by the V-Notched Beam Method”, 1998

4. ASTM 3518, “Standard Test Method for In-Plane Shear Response of Polymer Matrix Composite Materials by Tensile Test of a 645° Laminate”, 2001

5. Shear testing - SR4 laboratory handout