Abstract
The influence of cryogenic environment on mechanical properties of a unidirectional carbon fiber reinforced bismaleimide composite (T700/6421BMI composite) has been investigated in this study. Cryogenic treatment was performed by steeping the T700/6421BMI composite specimens in liquid nitrogen for 0,24,240,480,1200, 1920 and 2400 hours, respectively. After liquid nitrogen steeping treatment, the transverse tension test, three-point bending test, short beam shear test and impact test were carried out to evaluate the effect of steeping time on mechanical properties of the T700/6421BMI composite. The fracture surface morphology of typical specimens was examined by SEM while the chemical variation and interface damage of the composite were characterized by FTIR and DMA. The results indicate that mechanical properties of the T700/6421BMI composite are affected by liquid nitrogen steeping. The bending strength displays a tendency of decreasing firstly then rising with increase in steeping time, and the shear strength exhibits on the contrary. The total impact energy and crack propagation energy of the T700/6421BMI composite increases rapidly in the initial stage and increases slightly as increases LN2 steeping time. It is suggested that the mechanical properties of the T700/6421BMI composite after liquid nitrogen steeping might be dominated by interface state related to steeping time. With the increase in steeping time, the stress concentration in the surface of matrix and fiber induced by coefficient of thermal expansion (CTE) difference increases and might cause interface debonding.