Effects of cryogenic environment on strength properties of double-lap composite bonded joints

Experimental and numerical investigations were conducted to determine the effects of a low-temperature environment on the strength properties and fracture modes of double-lap composite bonded joints. The experimental results indicated that the strength of the bonded joints decreases significantly as the temperature decreases. Finite element analysis (FEA) with consideration of the temperature dependence of the material constants was conducted to elucidate the cause of the strength reduction at low temperatures. The energy release rates at the interface between the inner adherend and the adhesive layer were used to determine the contributions of the stiffness change, thermal contractions, and elastoplastic properties of the materials to the strength reduction at cryogenic temperature. The numerical results with decreasing plastic behavior owing to the low-temperature environment revealed that the mode I energy release rate at the interface increased by about 250%, even though the mode II energy release rate decreased by about 50%. By contrast, the change in energy release rate with elastic stiffening and adhesive shrinkage with decreasing temperature waslimited to about 20%. A reduction in the plastic characteristics of adhesive materials is thus a dominant factor in the strength reduction of double-lap composite bonded joints in a low-temperature environment.