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Composite Materials Design and Testing - Unlocking mystery with invariants

News International-French

11 Feb 2015

An innovative invariant based approach developed makes trace of the plane stress stiffness matrix as the heart and soul of composite materials.


In the beginning, there was carbon fiber with high stiffness and strength. We were able to generate all the data and design to build B-787 and A-350. What we do not know even today is the transverse and shear stiffness and strength of the fiber. We have learned to live without them. Authors Tsai and Melo discovered trace as the heart and soul of composites. It is the one and only one property that is needed.  Just like carbon fiber they need to know only the longitudinal properties. They can be obtained from [0] coupons. Their tensile and compressive stiffness and strength are measured without need to know their transverse and shear properties. The mystery of composites being complicated and difficult to understand can be unlocked by this straightforward path of design and testing.  By using trace and trace alone the authors can define the stiffness and strength of all laminates.

It was interesting that the filament-wound pressure vessel designer had found excellent correlation between the hydrostatic burst pressure with the fiber strength. In fact, the relation is used to measure the fiber strength conversion factor. In the same spirit, the authors found a similar correlation between the tensile failure of laminates with the uniaxial tensile failure of [0]. Another practice of General Electric using 4-point bend test of [0] as the most effective (and also the quickest) screening test to predict the structural performance of a given material is again consistent with the recommended procedure of the trace-based theory.

Instead of hundreds of coupons as the building blocks of a strength pyramid, Tsai and Melo see one block that can support an inverted strength pyramid. This block is the uniaxial tensile and compressive stiffness and strength of [0]. The data from this panel stand as the upper bound because there are no defects from lamination process, residual stress, and edge delamination. The authors used “logic and ingenuity” to rate as-built laminates in their lamination efficiency in a master stress-strain template. Templates are easy to build (in Excel) and to compare directly different laminates and different materials. Improvements in materials and processes can be demonstrated by test data in minutes without extensive laminate data that can take days if not weeks. If design allowable is demanded, many materials and processes never have a chance. Trace-based technology makes design and testing of composites no different or difficult from those of metals. One panel of [0] can map out instantly the upper bound stiffness and strength of all as-built laminates. When actual data of as-built laminates become available they will reveal the quality of processing and effects of defects.