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.
It is feasible to design laminated composites with negative Poisson's ratio. Although the minimum value of negative Poisson's ratio design can be done via numerical approach, it is possible to use the analytical method in this paper to perform the following tasks: (a) evaluate the approximated ply orientation angles to obtain maximum transverse strain cy, (b) determine the necessary conditions of design negative Poisson's ratio, and (c) investigate the influence of laminae properties on the Poisson's ratio of a composite laminate. In this study, it is also found that, in general, the ply orientations for obtaining maximum transverse strain, sy, in a composite laminate are close to 70'/20I%.
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