Moisture absorption significantly affects the quality and strength of epoxy adhesives containing hardeners. These adhesives must be refrigerated to prevent curing reactions during storage, which leads to the inevitable water buildup. This study aims to elucidate the effect of moisture on the crack resistance of an epoxy adhesive and delineate the underlying mechanisms. The effectiveness of vacuum freeze-drying as a technique for reducing the moisture content of the adhesive was explored to assess its impact on improving adhesive toughness. Tests were conducted on carbon fiber reinforced plastic (CFRP) joints using adhesives with varying moisture contents to evaluate their toughness. The results showed that an increase in the moisture content of the adhesive resulted in weaker bonds and greater variability in its strength. Additionally, removing moisture via vacuum freeze-drying significantly enhanced the reliability of the adhesive. Furthermore, surface morphology analysis revealed that moisture primarily damages the adhesive itself rather than the adhesive-CFRP bond, leading to a shift in the failure mode from bond splitting to internal adhesive breaking.

Lightning damage to carbon fiber reinforced polymer (CFRP) laminates is a complex phenomenon in which thermal, electrical, and mechanical effects on the laminates occur simultaneously over a short period. Although the damage mechanism in CFRP laminates due to lightning strikes has been investigated, few studies have reported lightning damage to adhesively bonded composite structures. In this study, the effects of lightning current on the mechanical properties of adhesively bonded composite joints were evaluated. Electrically conductive adhesives were manufactured by adding an electrically conductive filler and inserting a copper mesh. The shear strength of the adhesively bonded CFRP joints was evaluated before and after the direct application of lightning current. The lightning damage caused to the adhesively bonded joints with high impedance (～40 kΩ) was found to be relatively small, and no significant reduction was found after the lightning strike tests. Conversely, the joints with low impedance (4–20 Ω) were severely damaged due to lightning currents, leading to a decrease in joint strength. The failure mechanisms are discussed herein on the basis of the observation made using high-speed photography and the results of shear strength tests.

Carbon fiber-reinforced plastics (CFRPs) are known for their high specific strength and stiffness; however, interlayer delamination occurs inside CFRPs owing to impact. Previous studies have demonstrated the recovery of the compressive strength in CFRP laminates using polyamide 6 as the matrix through thermal fusion bonding. However, because the actual structure is larger than the hot press, fibers and resin in the carbon fiber-reinforced thermoplastic (CFRTP) laminates may flow out from the heated and compressed areas. To solve this problem, we employed specimens larger than the press section to conduct repairs while cooling the portions exposed outside the press. The laminates were first subjected to indentation tests to introduce delamination, followed by repair under three conditions: 5 min at 306 and 316℃ and 50 min at 316℃. The results indicated successful delamination repair without flow out of the fiber and matrix. In compression tests, the compressive strengths of the repaired specimens were comparable to those of virgin specimens. However, the compressive strength of the intact parts decreased as the repair temperature and time increased. This paper presents a viable repair method for delamination in CFRTP laminates, which can be used for preserving the structural performance of CFRTP components.