Abstract
In the present paper, end notched flexure (ENF) tests are conducted for CFRP/CFRP, CFRP/aluminum sheet adhesive bonded specimens at room temperature (296 K), at low temperature (223 K) and at cryogenic temperature (77 K) in order to investigate the mode II adhesive fracture toughnesses. Large difference of the coefficients of thermal expansion (CTE) between the CFRP and the aluminum sheet derives considerable thermal stress at the cryogenic temperature. Therefore, in the ENF tests, the effect of thermal stress must be considered. In the present paper, first the effect of thermal deformation is analytically discussed, and ENF tests are then carried out. The test results reveal that the fracture toughness is much higher at the low temperature than at the room temperature. However, the toughness is significantly lower at the cryogenic temperature than at the low temperature. Moreover, the results show that the crack extends in the adhesive film (cohesive failure) at the room and the low temperatures, whereas it extends at the interface between adhesive film and CFRP (adhesive failure) at the cryogenic. These results imply that the transition of the crack surface caused the degraded adhesive fracture toughness.
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