Analysis of diametral strain in uniaxial tensile and compression testing of round specimens of anisotropic materials

The development of elliptical cross sections in initially round test pieces of anisotropic materials makes it difficult to measure the true strain on a cross section using a transverse extensometer. In this paper, an analysis is given of the diametral strain and strain rate on a round cross section. Based on the experimental observation that static and dynamic microstructural changes of many anisotropic materials was too small to cause a noticeable change in anisotropic factor, R, up to relatively low strain levels, it is assumed that R is independent of strain in this analysis. Analytical results suggest that, when both the imposed axial strain rate and the anisotropic factor R remain constant and R⩾0.5, the change in diametral strain rate is negligible no matter which orientation a transverse strain extensometer is positioned on a cross section. The analysis includes two methods of measuring the diametral strain: (i) measurement of the distance between two diametral-intercept points on the circumference of a cross section; and (ii) measurement of the distance between two parallel tangent lines of the cross section. Based on the analytical and experimental results, a new method has been proposed for obtaining true stress–strain data in uniaxial tensile or compression testing of anisotropic materials with constant or varying anisotropic factor.