Instrumented end notched flexure – Crack propagation and process zone monitoring Part II: Data reduction and experimental

A mode II instrumented end notched flexure three point bending (ENF) adhesion test is described. The adhesive joint consists of two aluminium alloy (AW7075-T6) plates bonded with a structural epoxy adhesive (Hysol® EA 9395™). Strain gauges are attached to the outer surface (backface) of the substrates in the lengthwise direction to measure local surface strain during crack propagation. Simultaneously, load/displacement measurements are performed. Two cases were investigated. The first was static: the joint was loaded below the crack propagation threshold. In the second, applied load above the threshold led to crack propagation. The former test confirmed the predicted load transfer mechanism between bonded and unbonded parts of the joint. In the second case, the crack front process zone was revealed in situ in mode II, we believe for the first time. These new results permitted validation of simple or refined analytical/numerical models including those of the cohesive zone. In addition, the backface strain gauge monitoring technique exhibited unexpected mode I contributions, quantitatively evaluated. Finally, R-curves are presented, as estimated with various standard models and compared with that postulated, where the process zone is accounted for.