Effect of Moisture Absorption on Mode I and Mode II Interlaminar Fracture Toughness of CFRP Laminates.

Abstract

Static and fatigue double-cantilever beam (DCB) and end-notch flexure (ENF) tests were conducted to determine the effect of the simulated initial delamination on in-terlaminar fracture toughness, Go, and fatigue fracture threshold, Gth. Unidirectional, 24-ply specimens of $2/SP250 glass/epoxy were tested with Kapton inserts at the midplane at one end to simulate an initial delamination. Four insert thicknesses were used: 13, 25, 75, and 130 ixm. Some specimens were also tested with either tension or shear precracks as the initial delamination. To determine Gth, fatigue tests were conducted by cyclically loading specimens until delamination growth was detected. The fatigue fracture threshold was de-fined to be the maximum cyclic strain energy release rate, G .... below which no delamina-tion growth would occur in less than 1 x 106 cycles. For the DCB specimens, consistent values of Mode I fracture toughness, Gk, were mea-sured for specimens with inserts of thickness 75 v,m or thinner, or with shear precracks. The fatigue DCB tests gave similar values of Gl,h for the 13, 25, and 75-1xm specimens. Results for the shear precracked specimens were significantly lower than for specimens without precracks. Results for both the static and fatigue ENF tests showed that measured Gik and Gnth values decreased with decreasing insert thickness, so that no limiting thickness could be determined. Results for specimens with inserts of 75 wm or thicker were significantly higher than the results for precracked specimens or specimens with the 13 or 25-~m inserts.