|Frequency of fracture||0.16||Strength (psi)||9.7 Fracture of Polymers||●||249|
For thermoplastic polymers, both ductile and brittle modes are possible, and many of these materials are capable of experiencing a ductile-to-brittle transition. Factors that favor brittle fracture are a reduction in temperature, an increase in strain rate, the presence of a sharp notch, increased specimen thickness, and, in addition, a modification of the polymer structure (chemical, molecular, and/or
microstructural). Glassy thermoplastics are brittle at relatively low temperatures; as the temperature is raised, they become ductile in the vicinity of their glass transition temperatures and experience plastic yielding prior to fracture. This behav-ior is demonstrated by the stress–strain characteristics of polymethyl methacrylate in Figure 7.24. At 4�C, PMMA is totally brittle, whereas at 60�C it becomes ex-tremely ductile.
One phenomenon that frequently precedes fracture in some glassy thermoplas-tic polymers is crazing. Associated with crazes are regions of very localized yielding, which lead to the formation of small and interconnected microvoids (Figure 9.17a). Fibrillar bridges form between these microvoids wherein molecular chains become oriented. If the applied tensile load is sufficient, these bridges elongate and break, causing the microvoids to grow and coalesce; as the microvoids coalesce, cracks begin to form, as demonstrated in Figure 9.17b. A craze is different from a crack in that it can support a load across its face. Furthermore, this process of craze growth prior to cracking absorbs fracture energy and effectively increases the frac-ture toughness of the polymer. Crazes form at highly stressed regions associated with scratches, flaws, and molecular inhomogeneities; in addition, they propagate perpendicular to the applied tensile stress, and typically are 5 �m or less thick. Figure 9.18 is a photomicrograph in which a craze is shown.