In the July 2014 issue of CW, Dr. Stephen Tsai, professor research emeritus in the Department of Aeronautics & Astronautics at Stanford University, suggested an invariant-based method for accelerating aerospace certification testing. He identified trace (the sum of three components of stiffness, plus that of shear) as the one and only property of composites that can make composites easier to understand, and design and testing of composites simple and straightforward.
Tsai and José Daniel Diniz Melo, a consulting professor at Stanford, have expanded on this topic in their book, “Composite Materials Design and Testing - Unlocking mystery with invariants.” In the book, they further explain that for all carbon/epoxy composites in use today, the stiffness of all laminates is simply a fixed fraction of trace. For example, they say, longitudinal stiffness is 88% trace, and stiffness of [π/4] is 34% of trace — all within an error of 1.5% or less. So, according to Tsai, if one measures the value of trace for each material, they can predict the stiffness of all laminates.
Among Tsai and Melo’s recommendations are homogenization through use of thin plies, continuous as opposed to discrete ply angles, a ranking procedure to reach optimal profile and ply drop, bi-angle instead of unitape building blocks, tapered edges to reduce scrap and edge delamination, and others. Such strategies, they say, may make composites more competitive, and make simultaneous weight and cost savings possible.
Tsai and Melo, among others, are offering a Composites Design Workshop June 20-24 to give participants a solid foundation and skills to master this invariant-based approach. The intensive online course includes 20 hours of sessions via WebEx; live software demonstrations and practice sessions; a free copy of the book referenced above; additional free reference books; a copy of MicMac and iMicMac software packages designed by Tsai; and access to an interactive forum to post questions and share comments. Registration and cost information can be found here: web.stanford.edu/group/composites/Workshop/priceinfo.html.