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Tham khảo tài liệu 'plastics product design part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 140 Plastics Engineered Product Design in orthotropic materials with one direction parallel and one at right angles to the fibers. Multilayer plates in which layers of fabric or of roving are laid up parallel or perpendicular to each other are also orthotropic. If the same number of strands or yarns is found in each principal direction balanced construction the strength and elastic properties are the same in those directions but not at intermediate angles if the number of strands or yarns is different in the two principal directions unbalanced construction the strength and elastic properties are different in those directions as well as at all intermediate angles. In the foregoing discussion the direction perpendicular to the plane of the plate has been neglected because the plate is assumed to be thin and the stresses are assumed to be applied in the plane of the plate rather than perpendicular to it. This assumption which considerably simplifies the theory carries through all of the following discussion. It is true of course that properties perpendicular to the plane of the plate are undoubtedly different than in the plane of the plate and in thick plates this difference has to be taken into account particularly when stresses are not planar. For isotropic materials such as mat-reinforced construction if E is the modulus of elasticity in any reference direction the modulus Eị at any angle to this direction is the same and the ratio Eị E is therefore unity. Poisson s ratio V is similarly a constant in all directions and the shearing modulus G E l 1 p . If V for example is 0.3 G E 0.385 at all angles. These relationships are shown in Fig. 2.33. The following familiar relationships between direct stress Ơ and strain E and shearing stress T and strain Y hold e ơ E 2-88 y r G 2-89 A transverse strain contraction or dilation Ey-is caused by Ơ equal to T -V 2-90 For orthotropic materials such as fabric and roving-reinforced construction E and ET are the elastic moduli in
