Unformatted text preview: 9.7 Fracture of Polymers
Strength (psi) 0.16 0.14 0.12
Frequency of fracture ● 249 200 300 400 500 FIGURE 9.16 The frequency distribution of observed fracture strengths for a portland cement. (From W. Weibull, Ing. Vetensk. Akad., Proc. 151, No. 153, 1939.) 0.10 0.08 0.06 0.04 0.02 0 1.5 2.0 2.5 3.0 3.5 Strength (MPa) For compressive stresses, there is no stress ampliﬁcation associated with any existent ﬂaws. For this reason, brittle ceramics display much higher strengths in compression than in tension (on the order of a factor of 10), and they are generally utilized when load conditions are compressive. Also, the fracture strength of a brittle ceramic may be enhanced dramatically by imposing residual compressive stresses at its surface. One way this may be accomplished is by thermal tempering (see Section 14.7). Statistical theories have been developed that in conjunction with experimental data are used to determine the risk of fracture for a given material; a discussion of these is beyond the scope of the present treatment. However, due to the dispersion in the measured fracture strengths of brittle ceramic materials, average values and factors of safety as discussed in Sections 7.19 and 7.20 are not normally employed for design purposes. STATIC FATIGUE (CD-ROM) 9.7 FRACTURE OF POLYMERS The fracture strengths of polymeric materials are low relative to those of metals and ceramics. As a general rule, the mode of fracture in thermosetting polymers is brittle. In simple terms, associated with the fracture process is the formation of cracks at regions where there is a localized stress concentration (i.e., scratches, notches, and sharp ﬂaws). Covalent bonds in the network or crosslinked structure are severed during fracture. 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 ...
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