Page 356 - Introduction to Marine Engineering

328 Engineering materials

continues beyond the yield point the specimen will 'neck' or reduce in
cross section. The load values divided by the original cross section would
give the shape shown. The highest stress value is known as the 'ultimate
tensile stress' (UTS) of the material.
Within the elastic limit, stress is proportional to strain, and therefore
stress
= constant
strain
This constant is known as the 'modulus of elasticity* (E) of the
material. The yield stress is the value of stress at the yield point. Where a
clearly defined yield point is not obtained, a proof stress value is given.
This is obtained by drawing a line parallel to the stress—strain line at a
value of strain, usually 0.1%. The intersection of the two lines is
considered the proof stress (Figure 16.2).

Stress
Fracture

—H h*~0.1% strain Strain
Figure 16.2 Stress strain curve—material without a definite yield point

A 'factor of safety' is often specified for materials where this is the
ratio of ultimate tensile strength to working stress, and is always a value
greater than unity.
UTS
factor of safety =
working stress

Impact test
This test measures the energy absorbed by a material when it is
fractured. There are a number of impact tests available; the Charpy
vee-notch test is usually specified. The test specimen is a square section