Defining Composite Stiﬀness (Through-the-Thickness)
By Rik Heslehurst, PhD
Most composite laminate design analysis is based on the in-plane stiﬀness properties of the individual plies (Ex, Ey and Gxy). However, there are some design areas where through-the- thickness stiﬀness values are needed, such as fastener torque, T-joint peeling loads, composite skinned sandwich structure imbedded tension fittings, and others. Using in-plane stiﬀness properties significantly overestimate the composite laminate performance since Ez << Ex.