THE INTERPRETATION OF DATA FROM THE 4-POINT BEND DELAMINATION TEST TO MEASURE INTERFACIAL FRACTURE-TOUGHNESS

In this paper several points of interest about the four-point bend testing of a laminate specimen are addressed. Crack advance may take place smoothly or in a series of bursts, depending on the variation of G(ic) with position (possibly caused by microstructural inhomogeneities) and on factors dependent on the beam stiffness. Furthermore, the crack advance on either side of the test-piece may take place symmetrically or asymmetrically, depending on interfacial toughness and beam shape variations. Experimental arrangements are described for monitoring and controlling the effects of asymmetrical crack advance. Finally, a methodology is outlined for predicting the effect of residual stresses on the calculation of G(ic). These residual stresses are expected to become at least partially relaxed during crack advance, thus contributing to the energy balance. This procedure involves examination of beam curvatures and their effect on residual deflections observed when the beam is unloaded. These points are illustrated using data from thermally sprayed titanium coatings and powder route SiC/C/SiC sandwich specimens.