Micromechanically derived lowest threshold stress intensity factor for macroscopic stress-corrosion cracking in unidirectional GFRP composites

Observation by scanning electron microscopy of a typical fracture surface in glass fiber-reinforced polymer (GFRP) composites shows that the fracture surface of each fiber is characterized by a mirror region surrounded by a hackle region. On the premise that the time-dependent failure of GFRP composites in acid environments is controlled by the initiation and slow growth of a crack from a pre-existing inherent surface flaw in a glass fiber, a micro-mechanical model of stress-corrosion crack growth was constructed. If tougher and more ductile matrices are used, there exists a lowest threshold value of the stress intensity factor. Arguing that the average macroscopic tensile stress over a small distance D in front of the macroscopic stress-corrosion crack tip is shared between the glass fiber and matrix according to a rule of mixtures, the relationship between the axial tensile stress acting on the glass fiber in front of the macroscopic stress-corrosion crack tip and the apparent crack tip stress intensity factor can be obtained.