Ductility is the amount of strain that a material can withstand before fracture. In turn the fracture behavior of plasdcs, especially microscopically brittle plasdcs, is governed by the microscopic mechanisms operadng in a heterogeneous zone at their crack or stress dp because of internal or external forces. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. Fracture is preceded by the formation of a craze zone, which is a wedge shaped region spanned by oriented microfilms. Methods of craze zone measurements include optical emission spectroscopy, diffraction techniques, scanning electron beam microscopy, and transmission electron microscopy.

Fig. 3.6 is an example of the ductile plastic tensile stress-strain curve. This curve identifies behavior so that as the strain increases, stress initially increases approximately proportionately (from point 0 to point A). Point A is called the proportional limit. From point 0 to point B, the behavior of the material is purely elastic/stretches; but beyond point B, the material exhibits an increasing degree of permanent deformation/stretch. Point B is the elastic limit of the material. At point C the material is yielding and so its coordinates are called the yield strain and stress (strength) of the material. Point D relates to the S-S elongation at break/failure. Table 3.2 provides these type data at room temperature for different materials.

Temperature influences the S-S curve. With a decrease in temperature the yield stress and strain usually decreases or the strain rate decreases. Point D corresponds to specimen fracture/failure. It represents the maximum elongation of the material specimen; its coordinates are called the ultimate, or failure strain and stress. As temperature decreases the ultimate elongation usually decreases or the strain rate increases.

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