What new analysts should know about composites

Recent additions to FEA software make it easier to find stresses and strains in composites. Composites of fiberglass have been around for almost 70 years. Carbon fibers came along about 20 years ago and promised even lighter and stronger composites. The only drawback was accurately predicting their physical properties before building and testing prototypes. Analyzing them once amounted to just running through closed form solutions. Today's FEA does a better job. But before getting into simulations, it's useful to review how the materials are made, and see why accurate analyses have been slow in coming. Aerospace engineers were the first to use carbon and boronfiberreinforced materials. They are a mixture of brittle but strong fibers imbedded in a resin or binder. The resin is more ductile than the fibers and much weaker in tensile strength. Designers soon found they could "tune" composites for high strength-to-weight ratios. Production, however, was expensive and reliability of the finished materials was unproven under longterm loading, environmental damage, and handling. The early promise of radically stronger materials was broken because outside the lab, fiber and resin matrices could not maintain the strength of the fibers. It took a further period of test, analysis, and development before composites were accepted and understood. One thing that has not changed: experimental strength data comes still from testing coupons. These samples have fiber orientations defined relative to a coupon's long axis. Testing loads it on this axis. To show effects of fiber orientation, a series of FE simulations were run on computer models of designs with fibers varying from aligned with the coupon (0°) to right angles to it (90°). Fibers at 0° produce the strongest plies. (The table shows strength properties.) The characteristic , for example, indicates the material good for 145,000 psi when tension is aligned with the