Failure Behavior

The design of plastic parts requires the avoidance of failure without overdesign of the part, leading to increased part weight. The type of failure can depend on temperatures, rates, and materials. Some information on material strength can be obtained from simple tensile stress-strain behavior. Materials that fail at rather low elongations (1 percent strain or less) can be considered to have undergone brittle failure.4 Polymers that produce this type of failure include general purpose polystyrene and acrylics. Failure typically starts at a defect where stresses are concentrated. Once a crack is formed, it will grow as a result of stress concentrations at the crack tip. Many amorphous polymers will also exhibit what are called crazes. Crazes appear to look like cracks, but they are load bearing, with fibrils of material bridging the two surfaces as shown in Fig. 1.10. Crazing is a form of yielding and, when present, can enhance the toughness of a material. Ductile failure of polymers is exhibited by yielding of the polymer or slip of the molecular chains past one another. This is most often indicated by a maximum in the tensile stress-strain test or what is termed the yield point. Above this point, the material may exhibit lateral contraction upon further extension, termed necking.5 Molecules in the necked region become oriented and result in increased local stiffness. Material in regions adjacent to the neck are thus preferentially deformed, and the neck region propagates. This process is known as cold-drawing (see Fig. 1.11).

Cold drawing results in elongations of several hundred percent. Under repeated cyclic loading, a material may fail at stresses well below the single-cycle failure stress found in a typical tensile test.6 This process is called fatigue and is usually depicted by plotting the maximum stress versus the number of cycles to failure.

Fatigue tests can be performed under a variety of loading conditions as specified by the service requirements. Thermal effects and the presence or absence of cracks are other variables to be considered when the fatigue life of a material is to be evaluated.

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