Creep, Recovery and Stress Relaxation Behaviour

Because of the potential applications of high modulus polyethylene for reinforcement of brittle matrices, it was soon recognized that the creep and recovery behaviour should be studied in some detail. 

In the first of a series of publications. Wilding and Ward compared the creep and recovery behavi rar of comparatively low molecular weight drawn monofilaments (Rigidex 50 Grade, M, = 6,180 = 101,450) of draw ratios 10, 20 and 30 with 

It was foimd that increasing draw ratio in the low molecular weight sample gives a reduction in the creep rate at any given strain, and at all draw ratios the strain rate eventually reaches a constant creep rate. The observation of a final constant creep rate is extremely important, because it suggests that the samples effectively achieve a constant structure where creep is controlled by a defined flow process. From a practical standpoint the existence of a constant creep process is very unsatisfactory because there is no reason to suppose that failure will not eventually occur. 

Wilding and Ward showed that the creep and recovery behaviour of the low molecular weight samples could be represented to a good approximation by the model representation shown in Fig. 35(b), which consists of a Maxwell and Voigt element in series, on the basis that the parameters E, E, r and r), are dependent on the stress level. Data for the creep response of the samples under discussion at a constant applied stress Op were therefore fitted to the equation 

The critical stress can now be seen to be essentially an experimental limitation. The smallest strain reading on our present creep apparatus is 5x 10 . The anticipated plateau strain rate of 10 s for a typical sample with a critical stress of 0.2 GPa would therefore only produce a measurable creep response after 500 days, which is on the limit of the time scale of present creep tests at Leeds University. 

Values for AHeff 30 kcal/mol were obtained, which are in the range for the a-relaxation process in polyethylene, when studied by dynamic mechanical measurements. 

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