This study examined the change in acoustic emission as a function of measurement position of fiber-reinforced composites. Single-edge-notched carbon fiber/epoxy composites were prepared and tested under cyclic loading, with sensors located at specific distances from the end of the notch. Although the Ib-value increased overall, the degree of increase significantly varied with position and acoustic emission frequency. Notably, the proportion of acoustic emission signals for each failure mode varied due to a high attenuation rate at high frequencies, which increased the Ib-value. Accordingly, the high-frequency fiber-failure signals significantly affected the Ib-value. This study focused on the importance of analyzing acoustic emission signals by considering the crack location and frequency-dependent attenuation rate. We concluded that an acoustic emission sensor should be located 20–40 mm from the crack location for woven carbon fiber/epoxy composites.

This study analyzed failure behavior using Ib-values obtained from acoustic emission (AE) signals. Carbon fiber/epoxy specimens were fabricated and tested under tensile loads, during which AE signals were collected. The dominant peak frequency exhibited a specific range according to fracture mode, depending on the fiber structures. Cross-ply specimens, with all fracture modes, were used and analyzed using b- and Ib-values. The b-values decreased over the specimens’ entire lifetime. In contrast, the Ib-values decreased to 60% of the lifetime, and then increased because of the different fracture behaviors of matrix cracking and fiber fracture, demonstrating the usefulness of Ib-values over b-values. Finally, it was confirmed that abnormal conditions could be analyzed more quickly using failure modes classified by Ib-values, rather than using full AE data.

Deterioration of the physical properties of fiber-reinforced composites is inevitable under a high temperature and humidity environment. The resin transfer molding (RTM) process for large composite parts is often accompanied by micropore formation, making the composite more sensitive to water absorption. In this study, carbon fiber-reinforced composite specimens were manufactured using the RTM process at room temperature, and their water absorption and mechanical properties were investigated. The water absorption was saturated after about 40 d, and the absorption ratio was higher at 75 °C than at 50 °C. The tensile strength of water-soaked specimens was decreased by 15%, exhibiting interlaminar delamination. However, plasticization of the cured epoxy resin by water increased interlaminar fracture toughness. These results indicate that water absorption accelerated interlaminar cracking initiated by micropore and interfacial failure in toughened resin, especially under low-temperature curing condition.