Why not Designing Multidirectional Laminates with In-Plane Strength Design Sheets Applying the UD Criteria of TSAI–WU and Cuntze? Part1: Analytical Foundation

A structural engineer must design to several Design Dimensioning Load cases and in each case to all the activated strength failure modes. As the task here is the in-plane loading of the envisaged multi-stack laminates, composed of transversely isotropic unidirectional UD layers, the 3D Strength Failure Criteria (SFCs) of Tsai–Wu and Cuntze were reduced to the necessary 2D-SFCs. The article addresses an idea of Stephen Tsai [1], in order to get a deeper mechanical feeling for conventional and so-called Double-Double laminates, when designing them to First-Ply Failure (FPF), which includes Fiber Failure (FF) and Inter-Fiber-Failure (IFF) and marks the ‘Onset-of-fracture’. This would enable to reduce the effort for Design Dimensioning regarding optimization and finally for Design Verification including testing. Omni- (principal strain) failure envelopes’ (term from S. Tsai) are derived, which surround a Non-FPF area, for Tsai–Wu and Cuntze, in order to perform the comparison. Five differently stiff CFRPs and one GFRP were examined for both SFCs. Part 1 of the paper provides the analytical foundation for the development of ‘Non-FPF areas’ (denoted as Omni-FPF- envelopes by Tsai). Part 2 of the paper focusses on the application. It outlines the process of creating Non-FPF areas for Cuntze and Tsai–Wu and compares the resulting Non-FPF areas for the examined materials.