Effect of environment on fracture toughness and debond growth of aluminum/FM (R) 73/boron-epoxy adhesively bonded joints

Tests were conducted to determine the fracture toughness of aluminum/FM(R)7 3/boron-epoxy adhesively bonded joints. Double cantilever beam (DCB) specim ens and end-notched flexure (ENF) specimens were exposed to one of the foll owing environments: hot/wet (71 degreesC (160 degreesF) and >90% RH) condit ions for 5000 h, thermal cycles between 71 degreesC (160 degreesF) and -54 degreesC (-65 degreesF), or an outdoor "rooftop" environment for 10 000 h. The environmentally exposed specimens were tested along with as-received (o r unexposed specimens). The different adherend materials had different coef ficients of thermal expansion (CTE). The mismatch of CTE between adherends caused residual thermal stresses in the adhesive joints at every temperatur e other than the elevated cure temperature. The residual stresses could not be accounted for with closed form solutions. Finite element analysis (FEA) of the specimens was required. Using FEA solutions, it was determined that the fracture toughness of the environmentally exposed specimens was lower than the as-received specimens for every environment tested for both DCB an d ENF specimens.