Page 355 - Introduction to Marine Engineering

Engineering materials 327

applications. For measurement purposes a number of terms are used,
with 'stress' and 'strain' being the most common. Stress, or more
correctly "intensity of stress', is the force acting on a unit area of the
material. Strain is the deforming of a material due to stress. When a
force is applied to a material which tends to shorten or compress it, the
stress is termed 'compressive stress'. When the force applied tends to
lengthen the material it is termed 'tensile stress'. When the force tends to
cause the various parts of the material to slide over one another the
stress is termed 'shear stress'.

Tensile test

A tensile test measures a material's strength and ductility. A specially
shaped specimen of standard size is gripped in the jaws of a testing
machine, and a load gradually applied to draw the ends of the specimen
apart such that it is subject to tensile stress. The original test length of
the specimen, LI, is known and for each applied load the new length, L-2,
can be measured. The specimen will be found to have extended by some
small amount, Lg—LI. This deformation, expressed as

extension
original length
is known as the linear strain.
Additional loading of the specimen will produce results which show a
uniform increase of extension until the yield point is reached. Up to the
yield point or elastic limit, the removal of load would have resulted in the
specimen returning to its original size. The stress and strain values for
various loads can be shown on a graph as in Figure 16.1. If testing

Fracture

Strain
Figure 16.1 Stress strain curve