Crack initiation in engineering thermoplastics resulting from chemical degrdation

Stress Corrosion Cracking (SCC) in thermoplastics usually appears as a microcrack network within a layer of degraded polymer adjacent to the surface exposed to combine action of mechanical stress and chemically aggressive environment. The stage of crack initiation is primarily controlled by chemical degradation. Degradation of polymers is usually manifested in a reduction of molecular weight, increase of crystallinity in semicrystalline thermoplastics, increase of material density, a subtle increase in yield strength, and a dramatic reduction of toughness, i.e., specific fracture energy. An increase in material density, i.e., shrinkage of the degraded layer is constrained by adjacent unchanged material. It results in a buildup of tensile stress within the degraded layer and compressive stress in the adjacent unchanged material due to increasing incompatibility between the two. These stresses sum up with manufacturing and service stresses leading to an increase of the strain energy. At a certain level of degradation, a combination of toughness reduction and increase of the stored elastic energy results in fracture initiation (FI). From an energy consideration, SCC starts, when the release of the stored strain energy due to microcrack network formation compensates the required specific fracture energy. Considering a chain of micro-events leading to fracture initiation on macro scale, the probability of FI is evaluated using the formalism of Statistical Fracture Mechanics (SFM). A critical level of degradation is determined as the degradation at which FI takes place with certainty. The kinetics of the degradation process and the critical level of degradation then determine the time of fracture initiation. Experimental observations and a mathematical model of the described above processes are presented in this work.