Fatigue of Plastics

In amorphous polymers it is possible that cracks may develop in the voids which are formed during viscous flow. 

Moulded plastics will also have crack initiation sites created by moulding defects such as weld lines, gates, etc and by filler particles such as pigments, stabilisers, etc. And, of course, stress concentrations caused by sharp geometrical discontinuities will be a major source of fatigue cracks. Fig. 2.72 shows a typical fatigue fracture in which the crack has propagated from a surface flaw. 

Another important aspect of the fatigue of all materials is the statistical nature of the failure process and the scatter which this can cause in the results. In a particular sample of plastic there is a random distribution of microcracks, internal flaws and localised residual stresses. These defects may arise due to structural imperfections (for example, molecular weight variations) or as a result of the fabrication method used for the material. There is no doubt that failure 

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ASTM D671, Test Methodfor Flexural Fatigue of Plastics by Constant Amplitude of Force, Vol. 8.01, ASTM, Philadelphia, Pa., 1993. 

ASTM D671. Flexural fatigue of plastics by constant amplitude of force, 1993. 

Considering static fatigue of plastics, service failures are almost inevitably brittle, whereas failures from short-term tests at higher stresses arc often ductile. Extrapolations from tests giving ductile failures will result in an overestimate of longterm performance. At some point there will be a ductile brittle transition that may be seen as a "knee" in the lifetime curve, beyond which the decline in strength accelerates. The transition is shifted to lower times by... 

Jansen J. Fatigue of plastics, 2006. 

An extensive introduction, Introduction to Fatigue of Plastics and Elastomers, consists of the three chapters. The initial chapter focuses on fatigue, what it is, how it is measured, and how data are presented. The second chapter, Introduction to the Tribology of Plastics and Elastomers, focuses on tribology properties. The field of tribology is extensive so this... 

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