Fatigue crack propagation in poly(methyl methacrylate): Effect of molecular weight and internal plasticization

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Abstract
In spite of the importance of fatigue behavior in engineering plastics, relatively few fundamental studies have been made of the effects of polymer structure, molecular weight, composition, and morphology on fatigue crack propagation (FCP). As, part of a broad program for the study of such effects, the role of molecular weight and internal plasticization has been studied in poly(methyl methacrylate) (PMMA) which had been specially prepared and characterized with respect to molecular weight, dynamic mechanical behavior, and, in some cases, stress‐strain response. As expected, values of fracture toughness,Kc, varied considerably as the molecular weight was rai ed, from 0.7 MPa, √matMv= 1.0 × 105to 1.1 atMv, = 4.8 × 106. However, a specific effect of fatigue was noted: over the same range ofKc, values of FCP rate decreased by two orders of magnitude as molecular weight was; increased. It is proposed that this high sensitivity is due to differences in the degree of chain disentanglement effected by the cyclic loading, with consequent differences in the strength of the craze preceding the crack. With PMMA plasticized internally with a low level (10 percent) of n‐butyl acrylate (nBA), the FCP rate andKc, were similar to those of controls, with very high rates shown. At higher nBA levels (up to 30 percent), the sensitivity of FCP rate to stress intensity factor range decreased considerably,Kc, increased by 30 percent and the pre‐exponential constant in the growth rate law increased. Plasticization weakens the polymer but at high degrees leads to enough hysteretic heating to induce local creep and crack blunting.