A stress singularity approach for the prediction of fatigue crack initiation in adhesive bonds. PART 2: Experimental

Using the epoxy-aluminum wedge specimens defined and analyzed in Part 1, we measure the number of cycles required to initiate an interfacial fatigue c rack near the apex, where the stress field is predicted to be singular. The eigenvalue, lambda, and the generalized stress intensity factor, Q, are va ried via the wedge angle, and via the beam deflection, respectively. Crack initiation is detected using a strain gage bonded near the tip of the wedge . Following the methodology developed in Part 1, the fatigue data are then us ed to construct a fatigue initiation criterion characteristic of the bimate rial interface. This criterion is a 3-D surface, with the ordinate represen ting the generalized stress intensity factor and the two horizontal axes re presenting the number of cycles to initiation and the eigenvalue, respectiv ely. Three key assumptions of the model are found to be satisfied in the sp ecimens tested herein: (1) geometric imperfections at the apex are smaller than the singular region, (2) the plastic zones near the apex are also smal ler than the singular region, and (3) the locus of initiation is near inter facial. Finally, a thermomechanical analysis indicates that the residual thermal st resses generated during the fabrication process make a significant contribu tion to the critical stress intensity factor. With high T-g adhesives and u nder unfavorable conditions thigh modulus, high CTE, poor adhesion), we pre dict that the residual stresses alone could be sufficient to cause debond i nitiation.