In-line monitoring of cryogenic three-point bending test of fiber-reinforced composites using acoustic emission

Remotely in-line monitored acoustic emission (AE) of interlaminar shear strength tests in a 3-point bending test of fiber-reinforced polymer composites (FRP) conducted under cryogenic conditions in liquid nitrogen (LN2) were studied. The AE sensors were reversibly mounted at the outer components of the testing machine to thermally decouple them from the cryogenic area and use AE signal transmission through the testing machine structure as a waveguide. The damage mechanisms and effect of the cryogenic temperature were studied at 296 K room temperature (RT) and in LN2 at 77 K, considering machine and process noises. The correlation of machine data with acoustics and the AE hit analysis revealed matrix cracking as the most frequent damage mechanism under both conditions but with different failure mechanisms. At RT and applying higher loads, the most damage suddenly occurred, and the specimen failed. In LN2, the damage occurred continuously from the beginning of testing. The amount of fiber failure increased, and the AE feature ranges enlarged. This study presents a method by AE for remote monitoring the mechanical response of FRP in cryogenic fluids such as liquid hydrogen. The method provides a new approach to support the more efficient development of FRP materials for storage vessel structures and structural health monitoring systems.