University of Babylon , College of Engineering , Eng. Materials, Maithem H - Rasheed

Materials Testing

Introduction:

Testing of materials are necessary for many reasons, and the

subject of materials testing is very broad one. Some of the

purpose for the testing of materials are:

1. To determine the quality of a material. This may be one

aspect of process control in production plant.

2. To determine such properties as strength, hardness, and

ductility.

3. To check for flaws within a material or in a finished

component.

4. To assess the likely performance of the material in a

particular service condition.

It is obvious that there is not one type of test that will provide all

the necessary information about a material and its performance

capabilities, and there are very many different types of test that

have been devices for use in the assessment of materials. One of

the most widely tests is the tensile test to destruction. In this

type of test a test-piece of standard dimensions is prepared, and

this is then stressed in un axial tension. Other tests that are often

used for the determination of strength data are compression,

torsion, hardness, creep and fatigue tests. With the exception of

hardness tests, these are all test of a destructive nature and they

normally require the preparation of test-pieces to certain

standard dimensions.

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1. Tensile test

The main principle of the tensile test is denotes the resistance of

a material to a tensile load applied axially to a specimen.

There are a several tensile testing machine, as in figure 1 (a)

shows a popular bench-mounted tensile testing machine, whilst

figure 1(b) shows a more sophisticated machine suitable for

University of Babylon , College of Engineering , Eng. Materials, Maithem H - Rasheed

industrial and research laboratories, while in figure 1(c) shows

the schematic drawing of a tensile

– testing apparatus. These machines are capable of performing

compression, shear and bending tests as well as tensile tests.

(b) (a)

(c)

Figure 1. Tensile testing machines.

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It is very important to the tensile test to be considered is the

standard dimensions and profiles are adhered to.

The typical progress of tensile test can be seen in figure 2 .

Figure 2. Typical progress of a tensile test: (1) beginning of test,

no load; (2) uniform elongation and reduction of cross-sectional area; (3)

continued elongation .,maximum load reached; (4) necking begins, load

begins to decrease; and (5) fracture. If pieces are put back together as

in (6), final length can be measured.

Figure 3. Properties of tensile test specimens: (a) cylindrical; (b) flat.

The elongation obtained for a given force depends upon the

length and area of the cross-section of the specimen or

component, since:

elongation = applied force L / E A

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where L = length

A = cross-sectional area

E = elastic modulus

Therefore if the ratio [ L/A ] is kept constant (as it is in a

proportional test piece), and E remains constant for a given

material, then comparisons can be made between elongation and

applied force for specimens of different sizes.

The tensile test experimental results on some

materials:

1- stress-strain curve for an annealed mild steel.

Figure 4. Typical stress-strain curve for annealed mild steel.

From such a curve we can deduce the following information.

1- The material is ductile since there is a long elastic range.

2- The material is fairly rigid since the slope of the initial

elastic range is steep.

3- The limit of proportionality (elastic limit) occurs at about

230 MPa.

4- The upper yield point occurs at about 260 MPa.

5- The lower yield point occurs at about 230 MPa.

6- The ultimate tensile stress (UTS) occurs at about 400Mpa

University of Babylon , College of Engineering , Eng. Materials, Maithem H - Rasheed

2-Stress-strain curve for a grey cast iron.

Figure 5. Typical stress-strain curve of grey cast iron.

From such a curve we can deduce the following information.

1- The material is brittle since there is little plastic

deformation before it fractures.

2- A gain the material is fairly rigid since the slope of the

initial elastic range is steep.

3- It is difficult to determine the point at which the limit of

proportionality occurs, but it is approximately 200 MPa.

4- The ultimate tensile stress (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.

University of Babylon , College of Engineering , Eng. Materials, Maithem H - Rasheed

3- typical stress-strain curve for a wrought light alloy.

Figure 6. Typical stress-strain curve of a light alloy.

From this curve we can deduce the following information:

1- The material has a high level of ductility since it shows a

long plastic range.

2- The material is much less rigid than either low-carbon

steel or cast iron since the slope of the initial plastic range

is much less steep when plotted to the same scale.

3- The limit of proportionality is almost impossible to

determine,. For this sample a 0.2 per cent is approximately 500

MPa (the line AB).

It is important to determine the properties of polymeric

materials which are may ranged from highly plastic to the highly

elastic. As in figure 7

the stress-strain curves for polymeric materials have been

classified in to five main groups by Carswell and Nason.

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Figure 7. Typical stress-strain curves for polymers.

2. The compression test

Because of the presence of submicroscopic cracks, brittle

materials are often weak in tension, as tensile stress tends to

propagate those cracks which are oriented perpendicular to the

axis of tension. The tensile strengths they exhibit are low and

usually vary from sample to sample. These same materials can

nevertheless be quite strong in compression. Brittle materials are

chiefly used in compression, where their strengths are much

higher. A schematic diagram of a typical compression test is sho

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Figure 8. Compression test of ductile material.

Figure 9. shows a comparison of the compressive and tensile

strengths of gray cast iron and concrete, both of which are brittle

materials.

Figure 9. Tensile and compressive engineering stress-strain

curves for gray cast iron and Concrete .

University of Babylon , College of Engineering , Eng. Materials, Maithem H - Rasheed

Because the compression test increase the cross-sectional area of

the sample, necking never occurs. Extremely ductile materials

are seldom tested in compression because the sample is

constrained by friction at the points of contact with the plants of

the apparatus.

4. Impact testing (toughness testing) Impact tests consist of striking a suitable specimen with a

controlled blow and measuring the energy absorbed in bending

or breaking the specimen. The energy value indicates the

toughness of the material under test.

Figure 14 shows a typical impact testing machine which has a

hammer that is suspended like a pendulum, a vice for holding

the specimen in the correct position relative to the hammer and a

dial for indicating the energy absorbed in carrying out the test in

joules (J). When the heavy pendulum, released from a known

height, strikes and breaks the sample before it continues its

upward swing. From knowledge of the mass of the pendulum

and the difference between the initial and in figure 15 the

schematic drawing of the impact test machine.

final heights, the energy absorbed in fracture can be calculated,

as shown

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Figure

10. Typical impact testing machine.

Figure 11. Schematic drawing of standard impact-testing apparatus.

University of Babylon , College of Engineering , Eng. Materials, Maithem H - Rasheed

.

Figure 12. Impact loading: (a) a rod of high-carbon (1.0%) steel in the

annealed (soft) condition will bend struck with a hammer (UTS 925

MPa); (b) after hardening and lightly tempering, the same piece steel will

fracture when hit with a hammer despite its UTS having increased to

1285 MPa.

There are several types of the impact tests and the most famous

type is the Izod test.

In the Izod test, a 10mm square, notched specimen is used, it is

preferred to use a specimen that have a more than one or two

and even three notched in the same specimen. The striker of the

pendulum hits the specimen with a kinetic energy of 162.72 J at

a velocity of 3.8m/s.

Figure 13 shows details of the specimen and the manner in

which it is supported

University of Babylon , College of Engineering , Eng. Materials, Maithem H - Rasheed

Figure 13. Izod test (a/I dimensions in millimeters); (a) detail of notch;

(b) section of test piece (at notch); (c) position of strike.

A second type of impact test is the Charpy test. While in the

Izod test the specimen is supported as a cantilever, but in the

Charpy test it is supported as a beam. It is struck with a kinetic

energy of 298.3 J at a velocity of 5m/s.

The Charpy impact test is usually use for testing the toughness

of polymers. Figure 18 shows details of the Charpy tes: manner

in which it is supported.

Figure 14. Charpy test (all dimensions in millimeters).

University of Babylon , College of Engineering , Eng. Materials, Maithem H - Rasheed

The effects of temperature on the materials mechanical

properties:

The temperature of the specimen at the time of making the test

also has an important influence on the test results.

Figure 19 Shows low-carbon steels at refrigerated temperatures,

and hence their unsuitability for use in refrigeration plant and

space vehicles.

Figure 15. Effect of test temperatures on toughness.

. The results of impact tests for several materials are shown

Figure 16. Impact test results for several alloys over a range of testing

temperatures.