Damage analysis of CFRP plates exposed to cryogenic shock by AE monitoring

The progression of micro-fractures in carbon fiber reinforced composite (CFRP) plates subjected to cryogenic thermal shock was studied by acoustic emission (AE). Thermal shocks were given by putting liquefied nitrogen into a cell mounted on the upper surface of the specimens with different matrix resins. No damage was observed in the specimen with heat-resistant resin, while three hundred AE counts were detected in the specimen with conventional resin, in which many matrix cracks were observed from the small temperature differences in the wall. AE signals were classified into three fractures types, namely, delaminations and matrix and transverse cracks, by waveform characterization. Here, the three AE signal types were simulated by adiabatic thermal expansion with a pulse laser. The numbers of all three types of AE signals linearly increased with time. AEs from the matrix cracks were more frequent than those from delaminations and transverse cracks.

Vertical immersion testing of the CFRP plate into liquefied nitrogen revealed only matrix cracks on the specimen surface at temperatures higher than 228 K. At 213 K, a large number of AE signals due to matrix cracks but few signals due to delaminations and transverse cracks were observed. At 77 K, a large number of AE signals due to the three types of fractures were observed.