Civil Engineering Reference
In-Depth Information
Figure 3.1
A typical stress-strain curve for soil.
Figure 3.2
Tangent and secant stiffness moduli.
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 work-
ing 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.
3.2 Choice of parameters for stress and strain
Figure 3.1 shows the characteristics of material behaviour, but axes of stress and strain
are not carefully defined. The choice of axes will depend on the tests carried out to
examine the material behaviour and the parameters required. For metals that are essen-
tially elastic and then plastic the parameters required are Young's modulus
E
, Poisson's
ratio
and the yield and ultimate stresses, and these can be obtained from a simple
uniaxial extension test. For concrete, the required parameters can be obtained from
ν
