Mechanical Engineering Principles

TENSILE TESTING 19

The yield stress gives an indication of the

ductility of the material (see Chapter 1).

(v) For mild steel, the extension up to the point

Jis some 40 times larger than the extension

up to the pointB.

(vi) Shortly after pointJ, the material strain hard-

ens, where the slope of the load-extension

curve is about 1/50ththe slope of the curve

fromA toB, for materials such as mild

steel.

(vii) Between points D andE extension takes

place over the whole gauge length of the

specimen.

(viii) Point E gives the maximum load which
can be applied to the specimen and is used
to determine theultimate tensile strength
(UTS)of the specimen (often just called the
tensile strength)

UTS=

maximum load

original cross-sectional area

(ix) Between pointsEandF the cross-sectional

area of the specimen decreases, usually about

half way between the ends, and awaistor

neckis formed before fracture.

Percentage reduction in area

=

(original cross-sectional area)

−(final cross-sectional area)

original cross-sectional area

×100%

The percentage reduction in area provides

information about the malleability of the

material (see Chapter 1). The value of stress

at pointF is greater than at pointEsince

although the load on the specimen is decreas-

ing as the extension increases, the cross-

sectional area is also reducing.

(x) At pointFthe specimen fractures.

(xi) DistanceGHis called thepermanent elon-

gationand

permanent elongation

=

increase in length during

test to destruction

original length

×100%

(xii) The pointK is known as theupper yield

point. It occurs for constant load experi-

ments, such as when a hydraulic tensile test-

ing machine is used. It does not occur for

constant stain experiments, such as when a

Hounsfield tensometer is used.

2.2 Worked problems on tensile testing

Problem 1. A tensile test is carried out on a

mild steel specimen. The results are shown

in the following table of values:

Load (kN) 0 10 23 32

Extension (mm) 0 0.023 0.053 0.074

Plot a graph of load against extension, and

from the graph determine (a) the load at an

extension of 0.04 mm, and (b) the extension

corresponding to a load of 28 kN.

The load/extension graph is shown in Figure 2.3.

From the graph:

0 0.01

4

8

12

16

17.2

20

24

28

32

0.02 0.03 0.04 0.05 0.06 0.07 0.08

Load / kN

Extension / mm

Figure 2.3

(a) when the extension is 0.04 mm, the load is

17.2 kN

(b) when the load is 28 kN, the extension is

0.065 mm.

Problem 2. A tensile test is carried out on a

mild steel specimen of gauge length 40 mm

and cross-sectional area 100 mm^2. The

results obtained for the specimen up to its

yield point are given below: