Propagation, crack overview

Molecular defects as well as the real polymer morphology tend to decrease the strength. Some of these molecular, macromolecular, and morphological defects are illustrated in the schematic overview of Fig. 1.20. The right part shows macromolecular defects, such as chain ends with weak entanglements, overstressed bonds, and others, which initiate the first nanovoids and local stress concentrations. The left part illustrates morphological defects, for example, interlamellar and inter-spherulic defects in semicrystalline polymers, filler particles, and phase-separated particles in blends and composites. If the size of such defects is above a critical size, they initiate crack propagation and fracture. 

Overview of Fracture Mechanics. The objective of fracture mechanics is to establish the maximum section stress that can be applied to a material containing a sharp crack of defined geometry without propagating the crack and, in particular, result in partial or complete fracture... 

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