Page 24 - Mechanical Behavior of Materials

Section 1.2 Types of Material Failure 23

Figure 1.4 A tungsten lightbulb filament sagging under its own weight. The deflection
increases with time due to creep and can lead to touching of adjacent coils, which causes
bulb failure.

function. Plastics and low-melting-temperature metals may creep at room temperature, and virtually
any material will creep upon approaching its melting temperature. Creep is thus often an important
problem where high temperature is encountered, as in gas-turbine aircraft engines. Buckling can
occur in a time-dependent manner due to creep deformation.
An example of an application involving creep deformation is the design of tungsten lightbulb
filaments. The situation is illustrated in Fig. 1.4. Sagging of the filament coil between its supports
increases with time due to creep deformation caused by the weight of the filament itself. If too much
deformation occurs, the adjacent turns of the coil touch one another, causing an electrical short and
local overheating, which quickly leads to failure of the filament. The coil geometry and supports are
therefore designed to limit the stresses caused by the weight of the filament, and a special tungsten
alloy that creeps less than pure tungsten is used.

1.2.3 Fracture under Static and Impact Loading
Rapid fracture can occur under loading that does not vary with time or that changes only slowly,
called static loading. If such a fracture is accompanied by little plastic deformation, it is called a
brittle fracture. This is the normal mode of failure of glass and other materials that are resistant to
plastic deformation. If the loading is applied very rapidly, called impact loading, brittle fracture is
more likely to occur.
If a crack or other sharp flaw is present, brittle fracture can occur even in ductile steels or
aluminum alloys, or in other materials that are normally capable of deforming plastically by large
amounts. Such situations are analyzed by the special technology called fracture mechanics, which is
the study of cracks in solids. Resistance to brittle fracture in the presence of a crack is measured by
a material property called the fracture toughness, K Ic , as illustrated in Fig. 1.5. Materials with high