Creep behavior under sustained loading

The mechanical behavior of plastics on time-dependent applied loading can cause different important effects on materials viscoelasticity. Loads applied for short times and at normal rates (Chapter 2) causes material response that is essentially elastic in character. However, under sustained load plastics, particularly TPs, tend to creep, a factor that is included in the design analysis. 

Recovery is the strain response that occurs upon the removal of a stress or strain. The mechanics of the recovery process are illustrated in Fig. 2-34, using an idealized viscoelastic model. The extent of recovery is a function of the load s duration and time after load or strain release. In the example of recovery behavior shown in Fig. 2-34 for a polycarbonate at 23°C (73°F), samples were held under sustained stress for 1,000 hours, and then the stress was removed for the same amount of time. The creep and recovery strain measured for the duration of the test provided several significant points. 

Consolidation settlement is usually significant in soft to medium stiff clayey soils. Creep settlement occurs most significantly in overconsolidated clays under large sustained loads and can be estimated by using the method developed by Booker and Poulos (1976). In principal, however, long-term settlement can be included in the calculation of ultimate settlement if the design parameters of soil used in the calculation reflect the long-term behavior. 

A good example for the occurrence of unpredicted events is offered by the behavior of polyethylene (PE) pipes — and of other semicrystalline materials — under constant circumferential stress they showed a rapid drop of sustained stresses after long loading times (Fig. 1.5). With PE — as with PVC — an initial time range is observed where the times to failure are only a weak function of stress. Depending on the temperature, the failure mode is either brittle (Fig. 1.1) or ductile (Fig. 1.2 and 1.6). Both materials are also comparable insofar as the thermally activated growth of a creep crack (Fig. 1.3, 1.7, and 1.8) can be held responsible for the fail-... 

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