Ball-on-ring test in ceramic materials revisited by means of fluorescence piezospectroscopy

Ball-on-ring test is widely used to measure the biaxial strength of brittle materials by often making the approximation that the pressure applied by the ball in the central loading region is uniform. The purpose of the present study is to demonstrate the limits of such approximation by means of piezospectroscopy and to substantiate the spectroscopic findings with calculations based on finite element modeling (FEM). In addition, we shall discuss in some detail the validity of theoretical expressions previously developed by Kirstein and Woolley for radial and tangential bending stresses in the case of a uniformly distributed concentric load applied at the center of the disk loading region. A comparison is also offered between the experimentally retrieved radius of such region and those computed by theoretical models. Errors in estimating the radius of the central loading region, which have led in the past to controversial discussions, have been shown to play a minor role in the analysis of the overall stress field.