Creep, and Fatigue of Unoriented Polymers

Polymeric materials are viscoelastic solids. Their propensity to anelastic and plastic deformation is reduced when they are tested at high load rates and/or at low temperatures. The reduced deformability causes a formerly tough or highly elastic polymer to respond in a brittle manner. The impact fracture of natural rubber at liquid nitrogen temperature is convincing evidence of this fact. 

The resistance of polymeric materials to impact loading is of considerable technical importance as illustrated by the following examples  

In view of the technical relevance the impact resistance of polymers has been intensively investigated. The cited references [88—103] may serve as an introduction into the large body of literature. Vincent [88] and Bucknall et al. [89] give a general survey of the impact testing of polymers. Other references concern the molecular aspects [88-96], instrumentation [97—100], and particle impact [101—103]. The extensive literature on the fracture mechanical analysis of impact bending tests will be referenced in Chapter 9. 

In the context of this chapter only the three-point bending test (Charpy) will be discussed. In this test the energy loss A is measured which a pendulum incurs in striking and breaking a prismatic specimen (of thickness D and width B). The observed energy loss comprises basically four different terms  

The simple excess energy measurement of the pendulum does not permit discrimination between these different energy sinks. The separation of the first three terms becomes possible, however, if the bending force P acting between pendulum head and specimen is measured (so-called instrumented impact test). 

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