A new criterion for composite failure is proposed based on the Yeh Stratton (Y-S) criterion which is proven to work well for isotropic materials. Since the Y-S criterion requires three yield strengths from simple tension, compression, and torsion tests, the Y-S criterion can be applicable for ductile materials as well as brittle materials with different strengths in tension and compression. Like many other theories of anisotropic materials which were developed from those of isotropic materials, the Y-S criterion is now generalized in this paper to examine the failure of composite materials. One unique feature of the Y-S criterion is in its ability to change its format depending upon what types of stresses are applied and what types of material strengths are considered. Thus, it is shown that the entire closed failure surface by the Y-S criterion is composed of piecewise surfaces. When a material is in compression as an example, the compressive strength should be the dominant critical strength, not the tensile strength. Accordingly, the failure function must be adjusted so that it can accommodate the compressive strength. It is shown that the failure surface of the generalized Y-S criterion will consist of piecewise surfaces depending on types of stresses and strengths of composite materials. The validity of the generalized Y-S criterion is examined with experimental results of off-axis uniaxial strength tests of unidirectional fiber composites, and cross-ply and angle-ply laminated composites. Comparisons were made with other well-known anistropic theories.
In isotropic materials, it is known that values of Poisson's ratio larger than one half are thermodynamically inadmissible, for such values would lead to negative strain energy under certain loads. Although a negative Poisson's ratio is not forbidden by thermodynamics, it is rare in crystalline solids. With the development of modem fiber reinforced composite materials, the effective Poisson's ratio of laminated fiber reinforced composites shows a peculiar behavior as it becomes larger than one half or less than zero. In this article, a study of negative in-plane Poisson's ratio for a general class of randomly-oriented composite laminates is presented. A simple random statistical analysis has been presented. It is demonstrated that composite laminates with negative in-plane Poisson's ratio could be achieved by using the specific values of independent elastic constants E1, E2, G12, and v12 in each lamina. Also, the influence of the lamina material properties to the negative in-plane Poisson's ratio of the composite laminates is presented. The results from this statistical analysis provide a set of general guidelines for designing composite laminates with a special character-the negative in-plane Poisson's ratio.
Composite materials can be regarded as structures, so there is a great opportunity to design a wide range of properties for such kind of material to meet the various application requirements, even a very particular one. A negative Poisson's ratio is an example. The mechanism of negative Poisson's ratio and the basic conditions for obtaining such a property were analyzed briefly, the formulas of optimal angle for designing minus Poisson's ratio were deduced, and the experimental results were given. The designed composite did show negative, but small Poisson's ratio.
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