Effect of Anisotropy of Composites on Mechanical Testing

The long fibre composite exhibits the characteristics of inhomogeneity, anisotropy and inelasticity. If the composite is viscoelastic then the testing procedure demands much more things. However, we will not consider this fact in this study. We will consider the effect of anisotropy on mechanical testing.

The following are the key points in the mechanical testing of long-fibre composites:

1. generation of a uniform stress field in the critical reference volume
2. avoiding the ‘end-effects’
3. attainment of adequate loading levels without damage or failure near the loading points
4. appropriate specimen dimensions related to the scale of structural inhomogeneities
5. tension – shear coupling

The first four considerations are similar to the testing of homogeneous isotropic materials. These considerations give rise to various constraints on specimen dimension, test configurations and machine specifications. However, the fact of heterogeneity imposes more severe constraints and demands more considerations while testing.

In case of composite, the St. Venant’s Principle reflects in more stringent requirement. In anisotropic composites, the region of uniform stress is developed more gradually. It shown that the decay length, is of the order

(8.1)

where   is the maximum dimension of the cross-section. In case of rectangular strips subjected to end tractions

(8.2)

where,   is the distance over which a self equilibrated stress applied at the ends decays to its end value of . In above expressions, the ratio , that is degree of orthotropy, is an important factor. For unidirectional composites this ratio varies between 40 to 50 whereas for an isotropic material this is about 3. Thus, the ratio of respective decay lengths is about 3.5:1.