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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 interlaminar fracture toughness, Gc, and fatigue fracture threshold, Gth. Unidirectional, 24-ply specimens of S2/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 μm. 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 defined to be the maximum cyclic strain energy release rate, Gmax, below which no delamination growth would occur in less than 1 × 106 cycles.
For the DCB specimens, consistent values of Mode I fracture toughness, GIc, were measured for specimens with inserts of thickness 75 μm or thinner, or with shear precracks. The fatigue DCB tests gave similar values of GIth for the 13, 25, and 75-μm 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 GIIc and GIIth values decreased with decreasing insert thickness, so that no limiting thickness could be determined. Results for specimens with inserts of 75 μm or thicker were significantly higher than the results for precracked specimens or specimens with the 13 or 25-μm inserts.
delamination, ENF, DCB, strain energy release rate, fracture toughness, Mode I, Mode II, fatigue, fatigue fracture threshold
Research engineer, U.S. Army Aerostructures Directorate, NASA Langley Research Center, Hampton, VA
Research scientist, Analytical Services and Materials, Inc., Hampton, VA