DELAMINATION CRACK-TIP BEHAVIOR AT FAILURE IN COMPOSITE LAMINATES UNDER MODE-I LOADING

A technique is presented for measuring the local crack tip displacemen t fields and the global applied parameters during mode I loading of de laminations in composite laminates up to and including failure. To ach ieve this objective, an instrumented loading set-up has been designed to fit inside the chamber of a scanning electron microscope (SEM). Wit h this technique, the SEM images of loaded specimens are analyzed to o btain the crack opening displacements (GOD) as a function of distance from the crack tip. For mode I tests conducted on AS4/3501-6 unidirect ional material, good agreement is obtained between the Linear Elastic Fracture Mechanics (LEFM) calculations and the measurements, in terms of both general shape and magnitude. The results show that the COD inc reases linearly with the applied displacement up to failure. Thus, for the: case of a brittle material under mode I loading, there is good a greement between the measured and calculated crack tip displacements, and the material behaves in a linear elastic way up to failure. By ext ending the technique to mode II and mixed-mode loading and by testing other tougher materials in the future, we hope to increase our underst anding of mixed-mode failure criteria, which play an important role in the detailed analysis of complex composite structures under realistic loading.