On the optimization of tapered composite laminates in preliminary structural design

Thickness tapering is widely employed for tailoring the specific stiffness and strength of thin-walled structural elements subjected to spatially distributed loads. Alloy plates and shells are easily tapered by machining. On the contrary the thickness tapering of composite laminates requires terminating, i.e. “dropping-off”, plies; thus, while machining alloys allows a continuous thickness variation, the tapering of composite laminates is normally performed in discrete steps. Ply drop-offs have been shown to produce stress concentrations within the laminate [1-3]; those might be severe enough to outweigh any improvement in specific stiffness and strength which is supposed to be gained by tapering the composite structural element. The stress raiser effects of ply drop-offs depend on several design variables: those include primarily the terminated ply orientation and position within the laminate, the overall laminate stacking sequence and nature of the applied load. In principle the preliminary design of tapered laminates would require comparing a large number of candidate configurations, for which the overall thickness, position and stacking sequence of terminated plies are varied [4]; those comparative studies aim at identifying ply drop-off arrangements which avoid the onset and growth of delaminations from the terminated layers under service loading. It is easy to appreciate that the design search space comprising suitable dropping-off configurations can be vast even for relatively thin laminates; thus the optimal thickness tapering of composite structures can represent a major challenge.