Delamination Threshold and Loading Effect in Fiber Glass Epoxy Composite

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Abstract
Helicopter rotor heads made of glass fiber-epoxy composite show good strength, particularly in fatigue failures. However, interlaminar defects may always grow under the effect of loading in bending. To predict the development of these plane defects, the delamination threshold and low crack growth rate were studied in double cantilever beam type specimens calibrated by compliance. Experimentally, a delamination threshold exists below which a plane crack does not propagate, either in Mode I or Mode II testing. Fatigue loading may cause the growth of delamination between adjacent plies at a strain energy release rate ΔGth five times lower than the fracture energy Gc. This difference is relatively insignificant by comparison with metals since in numerous alloys the difference can be as high as 100. This demonstrates the importance of determining a delamination threshold to evaluate the damage tolerance of the composite.Complex loadings were introduced into our tests by programming tempory overloads and sequences of loading at two stress levels during cycling. The glass fiber-epoxy composites do not show significant memory effect, but are sensitive to the load ratio R.