The influence of texture on the fatigue behaviour of copper

This investigation attempts to assess the effect of preferred crystallographic orientation on the fatigue properties of a cubic metal. Quantitative texture analysis is performed using the crystallite orientation distribution function (c.o.d.f.) which enables quantitative predictions of mechanical properties to be made from texture measurements. Copper has been chosen as a model material since its single crystal properties exhibit values which are typical of cubic metals and also since the effects of thermomechanical processing are reasonably well documented.
The fatigue properties are evaluated by producing textured plates of different texture types and severities and machining specimens at specific orientations to the rolling direction of the plate. Fibre textures are also investigated by evaluating the fatigue properties parallel to the long axis of thermomechanically processed rod. Smooth sided cylindrical specimens were tested under fully reversed strain amplitude control to generate strain-life and cyclic stress-strain data. These data were analysed using the parametric approach of Morrow to develop a set of characteristic material parameters. Similarly, fatigue crack propagation studies were conducted on single edge notch specimens machined at specific angles from textured plate. Testing under constant load amplitude control produced data in the form of crack length vs. number of cycles which was analysed to produce crack propagation rate vs. stress intensity amplitude data which may be parametrically expressed by the constants C and m in the Paris equation.
It is shown that the anisotropy of fatigue behaviour may be predicted from texture measurements. The cyclic stress-strain data display anisotropy which is related to the type and severity of the texture. As the texture severity increases, the fatigue data displays greater anisotropy. The anisotropy of fatigue life data is shown to be a function of the product △ơ △Ɛ per cycle which is directly dependent on the cyclic stress strain curve. The anisotropy of fatigue crack data has been shown to be a function of the ability of the texture to influence the crack orientation and hence the crack opening mode. It has been shown that this effect may be predicted from texture data by considering the plastic strain ratio "R".