Page 66 - Ship Construction.DJ Eyres 6Ed

Ch07-H8070.fm Page 55 Wednesday, October 18, 2006 6:53 AM

7
Testing of Materials

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Metals are tested to ensure that their strength, ductility, and toughness are
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suitable for the function they are required to perform.
In comparing the strengths of various metals stresses and strains are
often referred to and require to be defined. Stress is a measure of the ability
of a material to transmit a load, and the intensity of stress in the material,
which is the load per unit area, is often stated. The load per unit area is sim-
ply obtained by dividing the applied load by the cross-sectional area of the
material, e.g. if a tensile load of P kg is applied to a rod having a cross-
2
sectional area of A mm , then the tensile stress in the material of the rod is
2
P/A kg/mm (see Figure 7.1).
Total strain is defined as the total deformation which a body undergoes
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when subjected to an applied load. The strain is the deformation per unit
length or unit volume, e.g. if the tensile load P applied to the rod of original
length l produces an elongation, or extension, of the rod of amount dl, then
the tensile strain to which the material of the rod is subjected is given by the
extension per unit length, i.e.

Extension dl
--------------------------------------- or -----
Original length l

It can be shown that the load on the rod may be increased uniformly
and the resulting extension will also increase uniformly until a certain
load is reached. This indicates that the load is proportional to extension
and hence stress and strain are proportional since the cross-sectional
area and original length of the rod remain constant. For most metals this
direct proportionality holds until what is known as the ‘elastic limit’ is
reached. The metal behaves elastically to this point, the rod for example
returning to its original length if the load is removed before the ‘elastic
limit’ is reached.
If a mild steel bar is placed in a testing machine and the extensions are
recorded for uniformly increasing loads, a graph of load against extension,
or stress against strain may be plotted as in Figure 7.1. This shows the
straight line relationship (i.e. direct proportionality) between stress and
strain up to the elastic limit.