Damage tolerance of layer-wise hybrid laminates consisting of glass reinforced flexible and rigid epoxy resins

The aim of this investigation is to increase the low velocity impact damage tolerance of a fiber-dominated composite laminate scaled down from that which might be used in a typical rotorcraft blade. The approach taken was to co-cure flexible matrix composites (FMCs) with traditional, rigid (glassy) matrix composites (RMCs) in a technique dubbed layer-wise hybridization (LWH). Small-scale test specimens were thin walled tubes consisting of ±45-deg. and ±2-deg. filament wound layers. Four types of tube were fabricated; one type contained all rigid epoxy layers and the other types each had one FMC layer located at a different location in the laminate. The damage tolerance of the tubes was evaluated by comparing the axial compressive strength of unindented tubes with the strength of tubes that had been subjected to a quasi-static lateral indentation force of 2500 N. The post-indentation compressive strength of the tubes decreased with an increasing area of matrix intraply cracking and delamination, with the all-RMC tubes having the least area of damage and the LWH tube with an inner layer of FMC having the most. However, the type of damage caused by the indentation was related to the percent reduction in strength. The all-RMC tubes experienced the most severe matrix intraply cracking near the indentation and had a greater percent drop in strength than one type of LWH tube that had less matrix cracking and therefore a lower percent reduction in strength. In the cases investigated, replacing an angle-ply RMC layer with a comparable FMC layer resulted in a loss of strength for both unindented and indented tubes. However, on a basis of percent of unindented strength, including an FMC layer was shown to be viable approach to reduce the amount of damage and percent reduction of compressive strength caused by indentation.