Fatigue Crack Propagation in Polymethylmethacrylate; the Effect of Loading Frequency

Abstract

The influence of loading frequency on the cyclic rate of crack propagation a_N, in a glassy thermoplastic (polymethylmethacrylate, p.m.m.a.) in air, at a temperature of 21° ± 1 degC and relative humidity of approximately 50 per cent, has been investigated and linear–elastic fracture mechanics concepts have been used to analyse the results. Tests have been conducted on sheet specimens, fatigue loaded in tension, under conditions of constant maximum and minimum values of stress intensity factor, K, and at three frequency levels, namely 0·1, 5 and 20 Hz. It was found that, under these conditions, an increase in frequency led to a decrease in a_N. Also, the results showed that when K_max achieved levels equal to or greater than 750 lbf/in^3/2, the contribution of ‘dynamic creep’ type damage to the crack growth process became more significant. The lower the frequency, the greater was the effect of K_m, the mean level of K, on a_N. It was also found that for the material under investigation the crack propagation power law index, n in equation a_N = β(λ) ⁿ, where λ = K_max – K_min², had an average value approximately equal to 2·25 and varied only by a small amount with frequency changes.