The stiffness of a material largely determines the strains and displacements in structures,
or in the ground, as they are loaded or unloaded. Another term often used in soil
mechanics to describe the relationship between stress and strain is ‘compressibility’,
but this is basically the reciprocal of stiffness. Often there is a marked change in the
gradient of a stress–strain curve at a yield point, as shown in Fig. 3.1. This is associated with a fundamental change in behaviour, often from elastic and recoverable straining
to inelastic and irrecoverable straining.
In simple terms the strength of a material is the largest shear stress that the material
can sustain and it is this which governs the stability or collapse of structures.
Stiffness and strength are quite different things: one governs displacements at working
load and the other governs the maximum loads that a structure can sustain.
Materials may be stiff (i.e. have high stiffness) or soft and they may be strong or
weak and they may have any reasonable combination of stiffness and strength. Steel is
stiff and strong while margarine is soft and weak; blackboard chalk is relatively stiff
and weak while rubber is relatively soft and strong.