Mode II Interlaminar Crack Growth Process in Polymer Matrix Composites

Abstract

The mode II interlaminar crack growth process in polymer matrix composites is studied by focusing on the underlying microscopic failure mechanisms. A recently developed mode II failure model based on fracture mechanics principles is used to analyze a wide range of failure phenomena. Examined in detail is how hackles or microcracks typifying the failure process are induced or in some cases suppressed. The crack tip stresses controlling the crack growth are found to be highly dependent on fiber arrangement and resin content. Resin microcracking at the crack tip is mode I in nature and can be overcome by resin yielding depending on the resin variables involved. Multiple microcrack formation with regular spacing is controlled by a shear lag effect often influenced by resin plastic deformation for even brittle resins. Such microcrack spacing affects the stress distribution leading to microcrack coalescence and thus crack propagation. From the failure process described, laminate mode II fracture toughness G11C can be quantitatively expressed as a function of resin properties (fracture toughness GIc, yield strength τy, and shear modulus G), reflecting the underlying resin deformation/fracture mechanisms. Fiber arrangement is found to uniquely affect the hackle pattern in a way consistent with the identified failure mechanisms. Temperature and loading rate effects on Guj C are also well interpreted by the identified failure mechanisms. This study further elucidates the crack growth process of mode II interlaminar failure.