Fatigue Crack Propagation in Polymethylmethacrylate; the Effect of the Mean Value of Stress Intensity Factor

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Abstract
Polymeric components often contain structural defects which give rise to regions of internal stress concentration. As the engineering application of these materials expands, the need to understand their behaviour under various loading conditions becomes more necessary. This paper reports the results of part of a programme of research undertaken to study fatigue crack propagation phenomena in thermoplastics. A fracture mechanics approach is used and the effects of the mean stress intensity factor, Km, and the range of stress intensity factor, ΔK, on crack propagation phenomena in polymethylmethacrylate are studied. Based on the experimental data available, a relationship of the following form, between the cyclic crack growth rate d(2 a)/d N and the tensile loading levels, has been proposed d(2 a)/d N = β( Kmax2 – Kmin2) n where Kmax and Kmin are the maximum and minimum values of the stress intensity factor in each loading cycle. In tests at room temperature (21°C), in air, at a loading frequency of 5 Hz, n was found to be equal to 2·5; log β equalled —12·37.

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