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Materials for engineering, 3rd Edition - (Malestrom)

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2<br />

Determination of mechanical properties<br />

2.1 Introduction<br />

Mechanical testing of <strong>engineering</strong> materials may be carried out <strong>for</strong> a number<br />

of reasons: The tests may simulate the service conditions of a material, so<br />

that the test results may be used to predict its service per<strong>for</strong>mance. Mechanical<br />

testing may also be conducted in order to provide <strong>engineering</strong> design data,<br />

as well as acceptability, the main purpose of which is to check whether the<br />

material meets the specification.<br />

In the USA, the American Society <strong>for</strong> Testing <strong>Materials</strong> (ASTM) publish<br />

standard specifications and methods of testing which are updated every three<br />

years. In the UK, the British Standards Institution (BSI) publish an annual<br />

catalogue of all BSI Standards, and agreed European Standards (EN series).<br />

All of these organizations issue publications relating to the selection of testpieces<br />

and the conducting of mechanical tests. We will consider a number of<br />

these tests in turn.<br />

2.2 The tensile test<br />

The tensile test is widely used <strong>for</strong> measuring the stiffness, strength and<br />

ductility of a material. The testing machine subjects the test-piece to an axial<br />

elongation and the resultant load on the specimen is measured. Depending<br />

on the nature of the product being tested, the specimen may be round or<br />

rectangular in cross-section, with the region between the grips usually being<br />

of reduced cross–section. The gauge length is marked in this region.<br />

We will consider the response of a ductile metal as an illustration. The<br />

load–elongation data are normally converted to stress and strain:<br />

Stress = Load/Cross-sectional area<br />

Strain = Extension of gauge length/Original gauge length<br />

Figure 2.1 illustrates the behaviour at small strains. The linear part of the<br />

curve may correspond to easily measured elongations in some polymeric<br />

37

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