Adhesive joints in composite laminates—A combined numerical/experimental estimate of critical energy release rates

The characterization of critical energy release rates of adhesive joints in laminated composite structures is a key issue when failure analyses have to be performed. Critical energy release rates, or fracture toughnesses, are known to be dependent on the mode mixing ratio, i.e. the portion of shear loading. It is thus useful to determine a criterion which gives the critical energy release rate as a function of the mode mixing ratio, which is the overall goal of this paper. For this purpose several experiments have been performed, for single mode I, single mode II, and mixed mode I/II loading conditions with pre-defined mode mixing ratios. Unfortunately, most of the experimental outcome cannot be used directly for least squares fitting of suitable fracture toughness criteria due to a couple of reasons, which will be discussed in detail. Hence, a numerical approach based on cohesive interface elements is employed to determine some of the critical energy release rates by fitting against experimental load–deformation curves. This combined numerical/experimental approach yields a useful database of discrete critical energy release rate values. These are utilized to fit suitable criteria which then allow the calculation of critical energy release rates for any given mode mixing ratio. The results are discussed in terms of convergence to the discrete values and physical plausibility, and a simple possibility to include mode III behavior is presented.