The fracture of epoxy resins and the effects of rubber inclusions
The deformation and fracture behaviour of a rubber-modified epoxy, and that of-its unmodified counterpart has been studied. The materials have been examined both in 'bulk' and when used as a thin adhesive layer for bonding steel adherends. Values of fracture toughness, the type of crack growth and the resultant fracture surfaces have been studied over a wide range of temperatures, displacement rates and specimen thicknesses. Both systems exhibit essentially the same types of crack growth behaviour. However, the values of fracture toughness for the rubber-modified epoxy were almost always significantly higher than those for the unmodified epoxy. A mechanism based on cavitation of the rubber particles and shear yielding of the matrix is proposed. This accounts for the increased fracture toughness and other alterations in property in the rubber-modified material. The deformation and fracture data are used to calculate values of the crack opening displacement. The rate/temperature dependence of this, together with other correlations, indicate that the extent of blunting at the crack tip governs the toughness of the epoxy materials and controls the type of crack propagation observed. A quantitative expression is presented which describes the variation of fracture toughness with both temperature and rate. Two parameters from this expression are shown to be material constants and to provide a unique failure criterion. The fracture behaviour of the rubber-modified epoxy as an adhesive between mild-steel adherends has also been examined. Values of adhesive fracture energy, type of crack growth and resultant fracture surface appearance have been studied as a function of adhesive bond thickness, joint width and displacement rate. The bond thickness particularly is shown to have a pronounced effect on adhesive fracture energy with a maximum occurring at a specific bond thickness. This relationship is discussed in terms of a plastic zone restriction/adhesive layer constraint model. This enables the bulk properties of the adhesive to be used to semi-quantitatively explain the dependence of Glc upon bond thickness.