Ring-on-ring strength measurements on rectangular glass slides

The ring-on-ring test [1–6] has been standardized and used extensively for strength measurements on glasses and ceramics. Whereas bend bars suffer from flaws at their edges that dominate the strength, the edges of carefully prepared circular disks for ring-on-ring tests almost never fail from their edges. For these specimens, the mechanical defects in the surface determine their strength. Very often, specimens are of interest which are not of a circular shape and yet are tested using the ring-onring configuration. The stress and strain distributions of these specimens can deviate strongly from that of the circular disk for which stress–strain relations are available from literature [1]. This is the case for glass slides of rectangular shape. Figure 1a shows a slide of 25 · 75 mm in dimension, which is loaded by two rings 11 mm and 22 mm in diameter. In order to determine the stress distributions of such specimens, finite element computations were carried out with 1 mm and 1.5 mm slide thickness. The resulting tangential stresses, rt, and radial stresses, rr, on the tensile surface are given in a normalised representation