Crack growth in polymethylmethacrylate under low frequency cyclic loading was studied by double torsion method. The torques were varied with time in the triangular wave form. The cycle duration, r, was ranged from 1 to 2000 secs. It was found that there were two qualitatively different modes of crack growth under cyclic loading. They are determined by the value of G, the crack extension force. At high values of G the length increment of the crack per loading cycle is proportional to r. At lower values of G it does not depend on ~-. The transition from one mode to the other is determined by a certain value of G = Gth, the threshold value, characterising a given material and ensuring continuous crack propagation under static loads.
I. Introduction
Cracks growing under periodically varying loads are usually called fatigue cracks (FC). Their growth rate is measured by the length increment of FC per loading c y c l e -A l l A N (l is the crack length and N is the number of fatigue cycles). Specific c h a r a c t e r i s t i c -A l l A N (instead of v = dl/dt) should be introduced because for many materials (high plasticity metals and a number of plastics, such as polycarbonate, polysulphone and nylon 66 [1,2]) the value of AllAN is virtually independent of the cycle duration ~-(and, correspondingly, of frequency f = l/r). Probably, that is why the absence of the dependence of AllAN on r is considered to be a trivial fact and the researchers are not greatly interested in discussing it.
At present many cases of divergence from this peculiarity are known. For instance, the value of AllAN increases with r for many high strength alloys [3]. The similar increase in AllAN is also observed for many p o l y m e r s -p o l y m e t h y lmethacrylate (PMMA), polystyrene and polyvinylchloride [2].
All these facts have led to the conclusion that it is possible to distinguish two groups of materials according to the reaction of FC to changes in the cycle duration: frequency sensitive and frequency insensitive materials [2].
To our opinion, these results suggest not only the difference in the properties of the materials themselves but also the comparative complexity of the processes of FC propagation. Actually, we observed that for one and the same material (a slightly plasticized P M M A containing 4 -6 % of dibutylphthalate) the character of frequency effect on the low frequency FC growth rate depends on the level of m a x i m u m and minimum values of the stress intensity factor during the loading cycle. It was found that under some conditions of loading the value independent of the cycle duration was