Studies of Mode I and Mode II Delamination Using a J-Integral Analysis and In-Situ Observations of Fracture in the SEM

摘要

The mode I and II delamination of multi-directional composite laminates have beenstudied using a J-integral approach. The analytical and experimental formalism to apply the J-integral to delamination have been developed and verified on three different composite material systems with matrix toughness varying from very brittle to very tough. Large deflections and rotations, midplane straining and distributed damage are taken into account in the methodology. The experimental conditions under which a path independent J may be measured have been established. The effect of several variables including specimen geometry, stacking sequence, and material constitutive behavior on the J-Moment-at-the-crack-tip relationship have been determined. Multidirectional composite laminates are both more susceptible to far field damage in the off-axis plies and less stiff than undirectional layups. These two factors cause a greater tendency to geometric and material nonlinearity in delamination testing. The use of a traditional G approach to characterize fracture behavior of such laminates gives artificially high indications of the delamination resistance. (JES)