Influence Of Liquid Nitrogen Steeping On Properties Of C/BMI Composite

More attentions have recently been paid to carbon fiber reinforced bismaleimidecomposites(C/BMI) for its excellent properties and potentials in orbital and aerospacevehicles as making structural components, cryogenic fuel tanks and cryogenic fueldelivery lines. Due to the requirement for long-serving in cryogenic environment, theinfluences of low temperature on material are concerned. A study on the long-termcryogenic aging is essential and meaningful.In order to reveal the damage effects and mechanisms of long-term cryogenicaging on C/BMI composite, two unidirectional carbon fiber reinforced BMI composites,T700/6421BMI and T700/5429BMI, were submerged in Liquid Nitrogen (LN2) for uplong to2400hours. Various modern material test and analysis techniques have beenemployed to evaluate the effect of LN2steeping time on properties of the C/BMIcomposite.The results show that the transverse tensile strength and bending strength of theT700/6421composite display a tendency of decreasing firstly then rising with theincreasing steeping time, and the short beam strength exhibits in the contrary. Theimpact energy (Ak) and crack extension energy (Ap) increase in the initial stage andthen remain stable，the initiation energy (Ai) reflects contrary trend. The DMA resultsdirectly imply that the interface bonding becomes weaker with increasing LN2steepingtime, which would destroy the composite integrity and thus degrade the mechanicalproperties. No remarkable evidence of chemical and structure change in the steeped6421BMI has been detected by FTIR.The transverse tensile strength of the T700/5429BMI composite shows a tinyvariation in the initial stage of LN2steeping, and decreases after long-term steeping.While the bending strength exhibits a tendency of decrease firstly then rising withincreasing the steeping time and recovers original state in the end. The short beamstrength displays a tendency of gradually decrease with the increase in steeping time.The DMA results show that the peak height of the T700/5429BMI composite increasesas increasing steeping time, indicating that the internal dissipation of compositeincreases with the increase in LN2steeping time. The above results directly imply thatthe interface bonding becomes weaker after LN2steeping. FTIR analysis indicates thatno remarkable evidence of chemical and structure change occurred in the steeped5429BMI.