Stress Corrosion Crack growth in engineering plastics

Stress Corrosion Cracking (SCC) is the process of brittle crack growth in a normally ductile material exposed to a combination of a corrosive environment and relatively low constant or intermittent stresses. There is a specific atomic level pathway of SCC for each material-environment system. At the same time, there is also a striking commonality of the phenomena in different material-environment systems, when the problem is considered on a continuum level. In this paper we present a mathematical model of SC individual crack growth. A process zone (PZ), occupied by crazing, shear banding and/or other forms of strain localizations, are commonly observed in front of a crack in engineering thermoplastics. The growth of the crack is strongly coupled with the evolution of PZ. Thus, it is convenient to consider a crack with PZ as one system referred to as "Crack Layer" (CL), i.e., a crack with finite, variable thickness. Crack Layer (CL) formalism is employed here modeling of slow stress corrosion crack growth. There are thermodynamic forces X^C and X^PZ associated with crack and PZ evolution respectively. The forces X^C and X^PZ are conventionally expressed as the derivative of Gibbs potential with respect to crack and PZ sizes, and are presented as the difference between the driving and resisting parts. The driving part of X ^C is the elastic energy release rate G₁ due to crack extension into PZ. The resisting part of X^C is the specific fracture energy 2γ of PZ material. The distinction of corrosive environment on CL is a reduction of the resisting part 2γ due to chemical degradation of PZ material. It leads to a noticeable acceleration of an average crack growth rate, reduction of the lifetime as well as a change in the slope in Paris-Erdogan plot of crack growth rate vs. stress intensity factor. A modification of CL formalism that accounts for the presence of aggressive environment and an algorithm for evaluation of stress corrosion CL (SCCL) growth is proposed in this work. Examples of numerical simulation of SCCL are also presented.