Jb502 - Chapter 2

Diploma in Mechanical Engineering (Material)

JB502 DESTRUCTIVE TESTING

www.pis.edu.my

peneraju ilmu sejagat

Chapter

Two (2)

1 pis/yth/jb502/chapter2


CLO 1 : Explain the principle of material testing

and mechanical properties for engineering

material.

CLO 2 : Formulate the testing data and result

acquired for various types of engineering

material.

CLO 3 : Differentiate types of defects and the

factors that influences its. 2

pis/yth/jb502/chapter2

Week : Two-Three (2-3)


2.1 Understand tensile test

2.2 Show the standard specimen/specification for

this test.

2.3 Understand stress -strain & load-elongation

graph for several different types of material.

2.4 State the important data acquired from load-elongation

& stress-strain graph for test & its significance.

2.5 List the types of fracture for several types of

material.

2.6 State the standard used for tensile test.



Testing principle Standard specimen / specification Stress-strain & load-elongation graph Important data from - & p-l graph

Types of fracture Standard for tensile test

TENSILE TEST DEFINITION

Tensile testing, also known as tension testing, is a fundamental materials science test

in which a sample is subjected to uni-axial tension until failure.

The results from the test are commonly used to select a material for an application, for

quality control, and to predict how a material will react under other types of forces.

Properties that are directly measured via a tensile test are ultimate tensile strength,

maximum elongation and reduction in area.

From these measurements the following properties can also be determined: Young's

modulus, Poisson's ratio, yield strength, and strain-hardening characteristics.





PRINCIPLE

The test consists of straining a test piece, by

tensile force, generally to fracture, and

recording the relationship between force and

extension, for the purpose of determining

one or more of the tensile properties (yield

strength, proof strength, tensile strength,

elongation, reduction of area).

The tensile test measures the resistance of

a material to a static or slowly applied

tension force, i.e. the type of loading in

which the two sections of material on either

side of a plane tend to be pulled apart or

elongated.

A test setup is shown in figure

below:

Schematic diagram for

tensile test.






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A typical specimen has a particular diameter and gauge length, is placed in the testing

machine and force (load) is applied.

A strain gage or extensometer is used to measure the amount that the specimen

stretches between the gage marks when the force is applied.

The result of a tensile test are shown in table and figure below as load versus gage

length.

The result of a tensile test result

The load versus gage length.






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STANDARD SPECIMEN (TEST-PIECE) Standard Specimen

A tensile test consists of slowly pulling a sample of material with a tensile load until it breaks.

The test specimen used may have either a circular, square or a rectangular cross section.

The end of tensile specimens are usually enlarged to provide extra area for gripping and to

avoid having the sample break where it is being gripped.

Various Tensile Test Specimens






http://www.google.com.my/url?sa=i&rct=j&q=tensile+test&source=images&cd=&cad=rja&docid=XZwpxwvxmfDnWM&tbnid=7GRIaMHSdCMQ_M:&ved=0CAUQjRw&url=http://en.wikipedia.org/wiki/Tensile_testing&ei=_2zbUZKkMMPPrQeLuoCIDg&bvm=bv.48705608,d.bmk&psig=AFQjCNGxqR2dPSCKb54C5PiZBYeqwrH5DQ&ust=1373421058086748http://en.wikipedia.org/wiki/File:Tensile_testing_on_a_coir_composite.jpghttp://www.google.com.my/url?sa=i&rct=j&q=tensile+test&source=images&cd=&cad=rja&docid=NlpJ7YzDdTCHRM&tbnid=L_j4a_0kulGR5M:&ved=0CAUQjRw&url=http://www.engr.uky.edu/~asme/hpv/&ei=x3HbUdKUIMWHrgeY3ICYDg&bvm=bv.48705608,d.bmk&psig=AFQjCNGxqR2dPSCKb54C5PiZBYeqwrH5DQ&ust=1373421058086748http://www.google.com.my/url?sa=i&rct=j&q=uniaxial+tension+test&source=images&cd=&cad=rja&docid=5WGpGGct3Hz2KM&tbnid=majRobhRV3WmLM:&ved=0CAUQjRw&url=http://www.endolab.org/content_detail_master.asp?sid=21006&Id=&langsel=EN&ei=U3LbUY-QBInrrAfKjIGYDA&bvm=bv.48705608,d.bmk&psig=AFQjCNHiB_DoiTAte6F5aCbxF_vOZykbRA&ust=1373419244615242




The are various types of test-piece holder available.

The test piece should generally be made as per standard dimension.

The test pieces should be held by suitable means, for example, wedges, screwed

holder, shouldered holders, etc., at most convenient.


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There are many variations of this test to accommodate the widely differing character of

materials such as metals, elastomers, plastics and glasses.

The tensile test on a metal test piece (BS18:1987) is described below:





GAUGE LENGTH AND PARALLEL LENGTH

Gauge length (Lo) is the length over which the elongation of the specimen is measured.

The minimum parallel length (Lc) is the minimum length over which the specimen must maintain a constant cross-sectional area before the test load is applied.

The lengths Lo, Lc, and L1 and the cross-sectional area (A) are all specified in BS 18.





Cylindrical test specimen are proportional so that the gauge length Lo and the cross-sectional area A maintain a constant relationship.

Therefore such specimens are called proportional test pieces.

The relationship is given by the expression:

Therefore a specimen 5 mm diameter will have a gauge length 25mm

(Lo = 5d =5 x 5mm = 25mm).

The minimum parallel length (Lc) is the minimum length is given by the expression:





ACTIVITY 1:

Determine the original gauge length (Lo) and the minimum parallel length (Lc)

for metals test piece if the diameter of sample is 10 mm.

SOLUTION:

Lo = 5do

= 5(10) mm

= 50 mm

Lc = 5.5do

= 5.5(10) mm

= 55 mm





STRESS VERSUS STRAIN & LOAD VERSUS EXTENSION

GRAPH FOR SEVERAL DIFFERENT TYPES OF MATERIAL

LOAD EXTENSION CURVE

Figure below shows the results get from a typical tensile test on a piece of annealed low-carbon steel.

The load applied to the specimen and the corresponding extension can be plotted in the form of a graph, as shown in figure below:


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From A to B the extension is proportional to the applied load. Also, if the applied load is removed the specimen returns to its original length. Under these relatively lightly loaded conditions the material is showing ELASTIC properties.

From B to C it can be seen from the graph that he metal suddenly extends with no increase in load. If the load is removed at this point the metal will not spring back to its original length and it is said to have taken a PERMANENT SET. This is the YIELD POINT.

The YIELD STRESS is the stress at the yield point; that is, the load at B divided by the original cross-section area of the specimen. Usually, a designer works at 50 per cent of this figure to allow for a FACTOR OF SAFETY.

From C to D extension is no longer proportional to the load, and if the load is removed little or no spring back will occur. Under this relatively greater loads the material is showing PLASTIC properties.





The point D is referred to as the ULTIMATE TENSILE

STRENGTH when referred to load-extension graphs

or ULTIMATE TENSILE STRESS (UTS) when

referred to the stress-strain graphs. This ultimate

tensile stress is calculated by dividing the load at D by

the original cross-sectional area of the specimen.

Although a useful figure for comparison the relative

strengths of materials, it has a little practical value

since engineering equipment is not usually operated

so near to the breaking point.

From D to E the specimen appears to be stretching

under reduced load conditions. In fact the specimen is

thinning out (necking) so that the LOAD PER UNIT

AREA, or stress is actually increasing. The specimen

finally work hardens to such an extent that it breaks at

E.

In practice, values of load and extension are of limited

use since they apply only to one particular size of

specimen and it is more usual to plot the stress-strain

curve.





STRESS-STRAIN CURVE

Figure below shows the stress-strain curve for the low

carbon steel.

Upto the point M Hookes law holds good and this

point is known as LIMIT OF PROPORTIONALITY.

Beyond the point M Hookes law is not obeyed

although the material remains elastic i.e., strain

completely disappears after the removal of load.

At the point N elastic limit is reached. If the material is

loaded or stress upto this point the material will regain

its original shape on the removal of load.

Upto the point P strain increase more quickly than

stress and at this point the metal YIELDS. In the mild

steel yielding commences/start immediately and two

points P and Q, the upper and lower yield points

respectively are obtained. On further increasing the

load slightly, the strain increases rapidly till R when

neck is formed. When this point (R) is reached the

deformation or extension continues even with lesser

load and ultimately fracture follows.


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Stress-Strain Curve For Ductile Material And Brittle Material

A typical stress-strain curve for a ductile metal, such as aluminium or copper is shown in

figure below:

The initial linear portion of curve OA is the elastic region within which Hookes law obeyed.

Point A is the elastic limit, defined as the greatest stress that the metal can withstand

without experiencing a permanent strain when the load is removed.

The determination of the elastic limit is dependent on the sensitivity of the strain measuring

instrument.

For this reasons, it is often replaced by the proportional limit point A.

The proportional limit is the stress at which the stress-strain curve deviates form linearity.

The slope of the stress-strain curve in this region is called the modulus of elasticity.





For engineering purposes the limit of usable elastic behaviour is described by the yield

strength, point B.

The yield strength is defined as the stress which will produce a small amount of permanent

deformation, generally s strain, equal to 0.01 or 0.02% of the gauge length of the tensile

specimen.

In figure above, this permanent strain, or offset, is OC.

Plastic deformation begins when the elastic limit is exceeded.

As the plastic deformation of the specimen increase, the metal becomes stronger.

Higher and higher load is required as the strain increase.

Finally, the load reaches a maximum value, as given by the point M.

The maximum load divided by the original cross-section area of the specimen is called the

ultimate tensile strength.

For a ductile metal, the diameter of the specimen begins to increase rapidly beyond

maximum load, so that the load required to continue deformation drops off until the

specimen fracture at point F.





The general behaviour of material under load can be classified as ductile or brittle depending

upon whether or not the material exhibits the ability to undergo plastic deformation.

The tensile stress-strain curve for brittle material is shown in figure below:

A completely brittle material would fracture almost at the elastic limit (a), while a brittle metal,

such as white cast iron, shows a little plasticity before fracture (b).

Figure below shows stress-strain curves for different metals / alloys.






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ACTIVITY 2:

Figure show the stress-strain curve for an annealed low carbon steel. Indicate the following on the curve:

a) Elastic range

b) Plastic range

c) Proportionality limit

d) Elastic limit

e) Upper yield point

f) Lower yield point

g) Tensile strength

h) Fracture strength





TENSILE TEST RESULTS (TENSILE PROPERTIES)

A. TENSILE STRENGTH (ULTIMATE OR MAXIMUM STRENGTH)

It is calculated by dividing the maximum load carried by the specimen during a tension

test by the original cross-sectional area of the specimen.

Tensile strength is widely used design factor, although there is more justification for yield

strength.

load-extension curve

stress-strain curve

P

L

Pmax

Tensile strength =

=

oS

Pmax

=

areationalcrossoriginal

appliedloadmaksimum

sec

max

Tensile strength = max stress on the curve

max





B. PROPORTIONAL LIMIT

It is the maximum stress at which stress remain directly proportional to strain.

The proportional limit is determined from the stress-strain curve by drawing straight

line tangent at the origin and noting the first deviation of the plot from the line.

The proportional limit is great dependence upon the precision available for its

determination.


stress-strain curve

P

L

Pa

Strength at proportional limit =

oS

Pa


italproportionatload

sec

lim

a

Stress at proportional limit = stress on the proportional limit on curve

a






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C. ELASTIC LIMIT

The elastic limit is the maximum stress which the material can withstand without

causing permanent deformation which remains after removal of stress.


stress-strain curve

P

L

Pa

Strength at elastic limit =

oS

Pa '


itelasticatload

sec

lim

'a

Stress at elastic limit = stress on the elastic limit on curve

'a





D. YIELD STRENGTH

The stress at which a material exhibits a specified limiting permanent set.

The yield strength is the load corresponding to a small specified plastic strain divided by the original cross-sectional area of the specimen.

Upper yield strength : The value of stress at the moment when the first decrease in first at yield is observed.

Lower yield strength : the lowest value of stress during plastic yielding, ignoring any transient effects.






stress-strain curve

P

L

y1

y2

oSareationalCross

Pypoyieldupperatload

,sec

int, 1

oS

Py1

Upper yield strength, ReH =

lLwer yield strength, ReL = oSareationalCross

Pypoyieldloweratload

,sec

int, 2

y1

y2

Upper yield strength, ReH = Stress at y1

Lower yield strength, ReL = stress ay y2

oS

Py2





E. MODULUS OF ELASTICITY, E

The slope of the initial portion of the stress-strain curve is the modulus of elasticity.

The modulus of elasticity is a measure of the stiffness of the material.

The greater the modulus, the smaller the elastic strain resulting from the application of a given stress.


stress-strain curve

P

L

P

l


lengthgaugexcurveofslope

sec

oS

Lx

l

P

E =

=

Strain

Stress

E =

=





F. PROOF STRESS

For the material s which do not exhibit a well defined yield phenomenon or yield point,

such as cold-worked and heat-treated steels, the yield stress may be replaced by the

word proof-stress.

The proof stress is defined as the stress that produces a specified amount of plastic

strain, such as 0.1 or 0.2%.






stress-strain curve

P

L

Pp P

E

0 A

x% extension of gauge length

X% extension of gauge length =

lengthgaugex

x

100

X% proof stress, Rpx% =

Soareationalcrossoriginal

Pstrengthproofx p

,sec

,%

Rpx% P

E

0 A

x% plastic strain

B

X% plastic strain =

100

x

X% proof stress, = Rpx%





F. RUPTURE STRENGTH

It is determined by dividing the load at the time fracture by the original cross-sectional

area.


stress-strain curve

P

L

Prupture


ruptureatload

secPrupture =

rupture

rupture = Stress at rupture point





G. ELONGATION

Elongation of a specimen after fracture may be determined by placing the parts of the

broken specimen closely together and holding them in place by a vice.

The distance between gauge marks may be measured by means of dividers.

Elongation has considerable engineering significance because it indicates ductility

%100xlengthoriginal

ngthoriginallelengthFinalelongationPercentage





H. REDUCTION OF AREA

After the metal has fractured the percentage reduction in area is calculated by

measuring the test piece diameter at the point of fracture, calculating the cross-

sectional area at this point, and expressing it as a percentage of a original area.

%100xareaoriginal

fractureatareaareaoriginalareareductionPercentage





ACTIVITY 3

A 10 mm x 10 mm square tensile bar obtained from a nickel super-alloy has a 40 mm gauge length. The results of the tensile test are as follows:

Load (N) Gauge length (mm)

0 40.00

43,100 40.10

86,200 40.20

102,000 40.40

104,800 40.80

109,600 41.60

113,800 42.40

121,300 44.00

126,900 46.00

127,600 48.00

113,800 (fracture) 50.20

From the stress-strain curve, calculate:

a. The tensile strength in megapascals.

b. The 0.2% offset yields strength in megapascals

c. The modulus of elasticity in gigapascals

d. The approximate % elongation.





SOLUTION:

Load

(N)

Gauge

length (mm)

l

(mm)

Ao

(mm)

(N/mm)

(mm/mm)

(mm/mm)

(x10)

0 40 0 100 0 0 0

43,100 40.1 0.1 100 431 0.0025 2.5

86,200 40.2 0.2 100 862 0.0050 5

102,000 40.4 0.4 100 1020 0.0100 10

104,800 40.8 0.8 100 1048 0.0200 20

109,600 41.6 1.6 100 1096 0.0400 40

113,800 42.4 2.4 100 1138 0.0600 60

121,300 44 4 100 1213 0.1000 100

126,900 46 6 100 1269 0.1500 150

127,600 48 8 100 1276 0.2000 200

113,800 50.2 10.2 100 1138 0.2550 255





SOLUTION:

a. The tensile strength in magepascals.

From stress-strain curve,

UTS = 1290 N/mm

= 1290 x 106 N/m

= 1290 MPa

b. The 0.2% offset yields strength (proof stress) in megapascals

i. 0.2% plastic strain = 0.2 / 100

= 0.002

= 2 x 10

ii. From stress-strain curve,

0.2% proof stress = 970 N/mm

= 970 x 106 N/m

= 970 MPa





c. The modulus of elasticity (E) in gigapascals

From stress-strain curve,

E = 862 N/mm / 5 x 10

= 172400 N/mm

= 172400 x 106 N/m

= 172400 MPa

= 172.4 GPa


Lo = 40 mm, Lf = 50.2 mm

% elongation = [(50.2 - 40) / (40)] x 100%

= (10.2 / 40) x 100%

= 25.5%





ACTITIVY 4:

A 10 mm x 10 mm square tensile bar obtained from a nickel super-alloy has a 40 mm gauge length. The results of the tensile test are as follows:

Load (N) Gauge length (mm)

0 40.00

43,100 40.10

86,200 40.20

102,000 40.40

104,800 40.80

109,600 41.60

113,800 42.40

121,300 44.00

126,900 46.00

127,600 48.00

113,800 (fracture) 50.20

From the load-extension curve, calculate:

a. The tensile strength in magepascals.

b. The 0.2% offset yields strength in megapascals

c. The modulus of elasticity in gigapascals






THE INTERPRETATION OF TENSILE TEST RESULTS

The interpretation of tensile test data requires skill borne out of experience, the

temperature at which the test is carried out and also the rate which the specimen is

strained.

The tensile modulus and tensile strength decrease as the temperature rises for most

metals and plastics, whereas the ductility increase as the temperature rises.

Table below shows a typical stress-strain curve for:

a) Annealed mild steel

b) Grey cast iron

c) Wrought light alloy

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pis/yth/jb502/chapter2




a) Annealed mild steel

The material is ductile since there is long elastic range.

The material is fairly rigid since the slope of the initial elastic range is steep.

The limit of proportionality (elastic limit) occurs at about 230 MPa.

The upper yield point occurs at about 260 MPa.

The lower yield point occurs at about 230 MPa.

The ultimate tensile stress (UTS) occurs at about 400 MPa.

b) Grey cast iron

The material is brittle since there is little plastic deformation before it fractures.

Again the material is fairly rigid since the slope of the initial elastic range is steep.

It is difficult to determine the point at which the limit of proportionality occurs, but it is approximately 200MPa.

The UTS is the same as the breaking stress for this sample. This indicates negligible reduction in cross-section (necking) and minimal ductility and malleability. It occurs at approximately 250 MPa.

c) Wrought light alloy

The material has a high level of ductility since it shows a long plastic range.

The material is much less rigid than either (a) or (b) since the slope of the initial plastic range is much less steep when plotted to the same scale.

The limit of proportionality is almost impossible to determine, so the proof stress will be specified instead. For this sample a 0.2% proof stress is approximately 500 MPa.





ACTIVITY 5:

Figure shows the stress-strain graph for four materials. Which of the materials is:

a) The most ductile?

b) The most brittle?

c) The strongest?

d) The stiffest?





FRACTURE IN METALS DEFINITION

Fracture is the breaking of a metal to yield/result an irregular

surface.

Fracture is the separation of a solid under stress into 2 or more

parts.

In general metal fractures can be classified as ductile or brittle.

DUCTILE FRACTURE

In ductile fracture, there occurs an appreciable/large plastic deformation prior to failure and the fractured surface give cup and cone appearance (after extensive plastic deformation).

Cup and cone fracture


http://www.google.com.my/imgres?q=ductile+fractures,+cup+and+cone&um=1&hl=en&safe=active&biw=1117&bih=765&tbm=isch&tbnid=16FPN66q-_fMyM:&imgrefurl=http://www.hsctut.materials.unsw.edu.au/Crack Theory/cracktheory2b.htm&docid=nCgYC3X83XstpM&imgurl=http://www.hsctut.materials.unsw.edu.au/Crack Theory/Images/cup and cone.png&w=295&h=367&ei=u1k0UOHPKMyHrAeT0YGYDg&zoom=1&iact=hc&vpx=266&vpy=116&dur=2732&hovh=250&hovw=201&tx=59&ty=123&sig=105238352186312354484&page=1&tbnh=127&tbnw=102&start=0&ndsp=24&ved=1t:429,r:1,s:0,i:72http://www.google.com.my/imgres?q=stress-strain+for+brittle+materials&um=1&hl=en&safe=active&biw=1117&bih=765&tbm=isch&tbnid=nKqj8-cRasmzgM:&imgrefurl=http://www.ficientdesign.com/2010/06/design-considerations-welding-metallurgy-carbon-steel/&docid=mwW7ZVwXnR3gaM&imgurl=http://www.ficientdesign.com/wp-content/uploads/2010/06/Brittle-Ductile-Stress-Strain1.gif&w=311&h=244&ei=wGA0UKm6E4nIrQeW4IDADQ&zoom=1&iact=hc&vpx=326&vpy=382&dur=785&hovh=195&hovw=248&tx=114&ty=59&sig=105238352186312354484&page=2&tbnh=132&tbnw=168&start=24&ndsp=27&ved=1t:429,r:17,s:24,i:201




The fracture is found to start only after a necked portion shows up on the test piece.

The first formed micro-cracks and cavities grow larger and finally join together to form a

crack in the centre of the necked portion.

The cavity then spreads in a direction inclined (condong) at 45o to the tensile axis.

The size of the cup depends on the relative shear and cleavage (belahan) strength values.

Metal with a high yield strength gives a smaller cup.

The fracture faces are irregular and fibrous (bergentian) in appearance.

Stages in the formation of a cup and cone ductile fracture.





BRITTLE FRACTURE

In brittle fractures, failure of the metal occurs when the fracture crack propagates (menyebar) through the cross-section without an appreciable plastic deformation (very little plastic deformation).

The fracture crack may start form any location where there are stress raisers.

The surface condition of the metal can be critical and makings on it can initiate cracks.

Such a fracture is more likely to occur in metal with poor plasticity and low temperatures.

Brittle fracture


http://www.google.com.my/imgres?q=stress-strain+for+brittle+materials&um=1&hl=en&safe=active&biw=1117&bih=765&tbm=isch&tbnid=nKqj8-cRasmzgM:&imgrefurl=http://www.ficientdesign.com/2010/06/design-considerations-welding-metallurgy-carbon-steel/&docid=mwW7ZVwXnR3gaM&imgurl=http://www.ficientdesign.com/wp-content/uploads/2010/06/Brittle-Ductile-Stress-Strain1.gif&w=311&h=244&ei=wGA0UKm6E4nIrQeW4IDADQ&zoom=1&iact=hc&vpx=326&vpy=382&dur=785&hovh=195&hovw=248&tx=114&ty=59&sig=105238352186312354484&page=2&tbnh=132&tbnw=168&start=24&ndsp=27&ved=1t:429,r:17,s:24,i:201




COMPARISON BETWEEN DUCTILE FRACTURE AND BRITTLE FRACTURE

DUCTILE FRACTURE

Ductile fracture is accompanied with large plastic deformation.

Slow rate of crack propagation.

Ductile fracture is characterised by the formation of cup and cone.

Surface obtained at the fracture is shining.

Failure is on account of shear stress developed at 45o .

BRITTLE FRACTURE

Brittle fracture is one in which the movement of the crack involves very little plastic deformation.

Rapid rate of crack propagation.

Brittle fracture is characterised by separation normal to tensile stress.

Surface obtained at the fracture is dull accompanied with hills and valleys.

Fracture is on account of direct stress.





STANDARD FOR TENSILE TEST

1) MS ISO 6892 : 2002 [Malaysia Standard, Metallic Material Tensile Testing At

Ambient Temperature (ISO 6892 : 1998, IDT)]

2) ASTM E 8M 91 [(Standard Test Methods For Tension Testing Of Metallic Materials

(Metric)]

3) BS 18 : 1987 [British Standard Method For Tensile Testing Of Metals (Including

Aerospace Materials)]


Jb502 - Chapter 2

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