Water Absorption Effects on Mode I and II Delamination of Carbon/Epoxy Composites

Ensuring the structural integrity of composite materials throughout their service life is essential. However, composite structures are vulnerable to moisture, which can degrade their interlaminar strength. This study characterizes the delamination behavior of unidirectional carbon/epoxy composites immersed in continuous distilled water at a constant aging temperature of 70 ℃. Specimens were fabricated using hand layup technique and tested at various water absorption levels (M): 0%, 2.2%, 3.8%, and 5.3%, using mode I Double Cantilever Beam (DCB) and mode II End Notched Flexure (ENF) configurations. The results indicate that mode I fracture toughness (GIC) is unaffected by moisture, while mode II fracture toughness (GIIC) shows a significant decrease, with approximately a 50% drop at high moisture level (M = 5.3%). Fitting the experimental data to a residual fracture toughness model revealed a linear relationship between GIIC and M. These findings suggest that shear mode delamination is more susceptible to moisture than normal mode delamination.