Models for the Description of Stress Crack Formations

Models on a microscopic level describe the behaviour of the so-called tie molecules which are attributed a special role in stress cracking. These models take into accoimt the effect of molecular stmcture. 

Phenomenological models describe the phenomena of craze and crack formation and crack growth in notched tensile bars in die long-term tensile test. The failure behaviour in the tensile tests is quantitatively parameterised and analysed using fracture-mechanics theories. 

The so-called structure particle model explains craze formation and crack formation by microcracks along the interface of structural units ( particles ) of the morphology. The formation of microcracks is determined by the interface energy between these particles. Microcracks develop if a critical deformation limit, which depends on the interface energy, is exceeded. This model primarily provides a quantitative description of the effect of liquid or gaseous media on stress crack formation. 

The model theories will be briefly discussed in the following on the basis of a few important references. Research work that had chosen the longterm tensile test on notched samples (versions of the NCTL tests) as their test method have mainly been selected since this test plays an important role for geomembranes. A complete and exhaustive overview of the technical literature would go beyond the scope of this book. 

This model concept can be quantified in a simple way (Ramsteiner 1990). The friction force is proportional to the overall length s, with which the molecule is anchored in the crystal lamellae, and the pull-out speed ds/dt. The proportionality factor rj, a kind of intrinsic viscosity, depends on 

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