Materials for engineering, 3rd Edition - (Malestrom)

2
Determination of mechanical properties
2.1 Introduction
Mechanical testing of engineering materials may be carried out for a number
of reasons: The tests may simulate the service conditions of a material, so
that the test results may be used to predict its service performance. Mechanical
testing may also be conducted in order to provide engineering design data,
as well as acceptability, the main purpose of which is to check whether the
material meets the specification.
In the USA, the American Society for Testing Materials (ASTM) publish
standard specifications and methods of testing which are updated every three
years. In the UK, the British Standards Institution (BSI) publish an annual
catalogue of all BSI Standards, and agreed European Standards (EN series).
All of these organizations issue publications relating to the selection of testpieces
and the conducting of mechanical tests. We will consider a number of
these tests in turn.
2.2 The tensile test
The tensile test is widely used for measuring the stiffness, strength and
ductility of a material. The testing machine subjects the test-piece to an axial
elongation and the resultant load on the specimen is measured. Depending
on the nature of the product being tested, the specimen may be round or
rectangular in cross-section, with the region between the grips usually being
of reduced cross–section. The gauge length is marked in this region.
We will consider the response of a ductile metal as an illustration. The
load–elongation data are normally converted to stress and strain:
Stress = Load/Cross-sectional area
Strain = Extension of gauge length/Original gauge length
Figure 2.1 illustrates the behaviour at small strains. The linear part of the
curve may correspond to easily measured elongations in some polymeric
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