Experimental Determination of Engineering Elastic Constants

A tensile specimen cut from an angle-ply laminate, made by bonding two unidirectional laminae at angles 0 to the load direction, has deformation characteristics as 

The procedure described above is straightforward in principle. However, in practice, great care must be taken in the test fixture design to assnre that applied loads cause a uniform state of stress in the test specimen. Two types of tests that have been developed for this purpose include (1) the simple tensile test for uniaxial states of stress, (2) the thin-walled tube subjected to combined torsion and internal pressure, for biaxial states of stress. Some theoretical aspects of the simple tensile test are developed in the sample problem which follows. For a more detailed discussion on experimental procedures for characterizing the material properties of composite materials, see Caisson and Pipes and Whitney et al.  

FIGURE 8.17 Influence of end constraints in the testing of anisotropic lamina, (a) Off-axis loading, (b) uniform state of stress, and (c) effect of clamped ends. 

Solving explicitly for the strains a and b, and making use of Equations 8.66 and 8.68, gives the following results in terms of engineering constants  

The apparent Young s modulus E, is, by definition, the ratio oAe-x) (slope of the stress-strain plot). Hence, for an off-axis test specimen subjected to uniaxial stress in the x-direction, we obtain the following equation for the modulus  

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