Bimodularity of interface layer and curing stress coupling effects on mixed mode fracture behaviour of functionally graded tee joint

Adhesive bonded tee joints are being used excessively in civil, military aircraft and ship building industries to join light weight Fiber reinforced Polymeric (FRP) laminates to other metal or composite structures. The mismatch of thermo-mechanical properties between adherend and adhesive bond line induces residual curing stresses even during manufacturing stages resulting in unaccounted failure and fracture behaviour of adhesive joints. The present work investigates the influence of bimodulus behaviour of adhesive and curing stresses on interface delamination fracture behaviour of tee joints by conducting a sequential thermal and geometrically non-linear finite element analysis (FEA) iteratively for evaluating strain energy release rate along the delamination front. The stress dependent elasticity problem of interface fracture has been studied based on the concepts of Modified Crack Closure Integral (MCCI) for different numerical specimens with tension compression modulus ratio R varying from 1 to 5 along the bond line for the respective functionally graded bimodular adhesively bonded tee joint. It is illustrated that the variation of bimodularity index has a strong influence on the interface delamination crack propagation characteristics apart from the other aspects of functionally grading and thermo-elastic anisotropy. The influence of bimodularity in comparison to functionally grading is found to be much more significant on mixed-mode fracture behaviour depicting dissimilar variation of GI, GII and GIII along the interface failure front indicating retardation of the interfacial failure propagation rate over certain zones of adherend.