FABRIC, KINEMATICS AND FINITE STRAIN

In relating fabric to kinematics it is generally assumed that planar and linear fabric elements are orthogonal with finite strain axes. A common exception occurs with respect to shear zones, where it is recognised that shape fabrics are generally steady-state and therefore are orthogonal with neither finite strain axes nor ISA. This is due to the fact that the fabric is continually being destroyed and redeveloped so that it is related to only an increment of the finite strain. In addition shear bands are also recognised as an exception. This problem is likely to be even more acute if deformation is generally triclinic.

Another complication that may result in a lack of correlation between finite strain and fabric stems from partitioning of the deformation at the grain scale. In many noncoaxial situations elongate grains are rotated towards a shear plane and grain boundaries therefore may be aligned preferentially parallel to a shear direction resulting in a weakening of the material due to grain boundary sliding mechanisms. During coaxial flow grain boundaries rotate towards the flattening plane of finite strain and therefore grain boundary sliding ceases to be a deformation mechanism. It follows therefore that if grain boundary sliding is a viable mechanism in a given situation the deformation may become partitioned such that the simple shear component of flow occurs primarily within the grain boundaries and the pure shear component involves intra-crystalline mechanisms within the grains. In this way fabrics may develop that are related to finite strain at the grain scale, but not at the scale of the aggregate, due to grain scale partitioning.

There is obviously a complex interplay between boundary conditions, mechanical properties, the geometrical relationships between the various parameters, kinematics and fabric. We should not expect simple relationships between fabric and kinematics and should be careful therefore in interpreting fabric in terms of finite strain without thought to deformation mechanisms.