Mode I and Mixed Mode Energy Release Rate Values for Delamination of Graphite/Epoxy Test Specimens

Abstract

This study is part of a larger program directed at developing guidelines for preventing web-to-stiffener debonding in stiffened composite panels. An important ingredient needed to attain this goal is the identification of a criterion to predict debonding. The objective of this work is to evaluate the energy release rate as a criterion for predicting debonding. A combined experimental and finite element modelling approach was used. Comparisons of strains and displacements measured prior to debonding and the corresponding values calculated with the finite element models are presented to illustrate the capability of the model to represent the behavior of the test specimens. Energy release rate values asso ciated with debonding of two types of graphite/epoxy composite test specimens were eval uated. One test specimen was the Mode I double cantilever beam. The other was a mixed mode (Mode I and Mode II) cracked lap shear specimen. Test specimens were loaded in laboratory conditions until the onset of debonding was observed. Critical load levels and the geometric and material properties of the test specimens were input data for the finite element analysis which evaluated the Mode I and Mode II components of the energy release rate at the onset of debond propagation. The tests provided a range of ratios of Mode II to Mode I energy release rates from 0 to 5.0. Data are shown in a graph of G, versus G~ for both types of test specimens. A discussion of the energy release rate as a criterion for debonding is presented.