mechanical tests

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 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.
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