Mechanical Engineering Principles

2

Tensile testing

At the end of this chapter you should be

able to:

• describe a tensile test


• recognise from a tensile test the limit of
  proportionality, the elastic limit and the
  yield point


• plot a load/extension graph from given data


• calculate from a load/extension graph, the
  modulus of elasticity, the yield stress,
  the ultimate tensile strength, percentage
  elongation and the percentage reduction
  in area

2.1 The tensile test

Atensile testis one in which a force is applied
to a specimen of a material in increments and
the corresponding extension of the specimen noted.
The process may be continued until the specimen
breaks into two parts and this is called testing
to destruction. The testing is usually carried out
using a universal testing machine that can apply
either tensile or compressive forces to a specimen in
small, accurately measured steps.British Standard
18 gives the standard procedure for such a test. Test
specimens of a material are made to standard shapes
and sizes and two typical test pieces are shown
in Figure 2.1. The results of a tensile test may be
plotted on a load/extension graph and a typical graph
for a mild steel specimen is shown in Figure 2.2.

(i) BetweenA andB is the region in which

Hooke’s law applies and stress is directly

proportional to strain. The gradient ofABis

used when determining Young’s modulus of

elasticity (see Chapter 1).

(ii) PointBis thelimit of proportionalityand

is the point at which stress is no longer

proportional to strain when a further load is

applied.

Gauge length

Thickness

Width

(b)

(a)

Diameter

Figure 2.1

A

G

B

J

E

F

H

CKD

Load

Permanent elongation

(^0) Extension
Figure 2.2
(iii) PointCis theelastic limitand a specimen
loaded to this point will effectively return to
its original length when the load is removed,
i.e. there is negligible permanent extension.
(iv) PointDis called theyield pointand at this
point there is a sudden extension toJ, with
no increase in load. The yield stress of the
material is given by:
yield stress=
load where yield begins
to take place
original cross-sectional area