THE INTERFACIAL FRACTURE CHARACTERISTICS OF BIMATERIAL AND SANDWICH BLISTER SPECIMENS

The interfacial crack initiation characteristics of bimaterial and sandwich strip blister specimens were compared. Interface cracks were grown along glass/epoxy interfaces and the corresponding loads, normal crack opening displacements (NCOD) and crack front geometries were measured. Finite element analyses were used to compare NCOD, extract fracture parameters and examine near front stress fields. On the small scale of crack extension that could be measured, the bimaterial specimens exhibited resistance curve behavior whereas none could be resolved in the sandwich specimens. The phase angle dependence of toughness in the bimaterial specimens was similar to that obtained in previous blister tests but was much steeper than has otherwise been measured. The toughness of the glass/epoxy interface in the sandwich specimen decreased with decreasing epoxy thickness in spite of the fact that it was never completely spanned by plastic zones.