Page 23 - Mechanical Behavior of Materials

22 Chapter 1 Introduction

Figure 1.3 Tension test showing brittle and ductile behavior. There is little plastic deformation
for brittle behavior, but a considerable amount for ductile behavior.

and some metals, such as the high-strength steel used to make a file. (Note that the word plastic
is used both as the common name for polymeric materials and in identifying plastic deformation,
which can occur in any type of material.)
Tension tests are often employed to assess the strength and ductility of materials, as illustrated
in Fig. 1.3. Such a test is done by slowly stretching a bar of the material in tension until it breaks
(fractures). The ultimate tensile strength, σ u , which is the highest stress reached before fracture,
is obtained along with the yield strength and the strain at fracture, ε f . The latter is a measure
of ductility and is usually expressed as a percentage, then being called the percent elongation.
Materials having high values of both σ u and ε f aresaidtobe tough, and tough materials are
generally desirable for use in design.
Large plastic deformations virtually always constitute failure. For example, collapse of a steel
bridge or building during an earthquake could occur due to plastic deformation. However, plastic
deformation can be relatively small, but still cause malfunction of a component. For example, in
a rotating shaft, a slight permanent bend results in unbalanced rotation, which in turn may cause
vibration and perhaps early failure of the bearings supporting the shaft.
Buckling is deformation due to compressive stress that causes large changes in alignment of
columns or plates, perhaps to the extent of folding or collapse. Either elastic or plastic deformation,
or a combination of both, can dominate the behavior. Buckling is generally considered in books on
elementary mechanics of materials and structural analysis.

1.2.2 Creep Deformation
Creep is deformation that accumulates with time. Depending on the magnitude of the applied stress
and its duration, the deformation may become so large that a component can no longer perform its