Evaluation of the Uniaxiality of the Stress State in C-Ring Fracture Strength Specimens

To develop guidelines for using C-ring specimens for evaluating the fracture strength of ceramics in tubular form, a series of finite-element analysis (FEA) calculations was performed. These calculations focused on the ranges of specimen dimensions and loading distributions required to maintain a uniaxial stress state within the C-ring specimen under diametral loading. Results of the FEA calculations indicated that the degree of uniaxiality of the C-ring stress state could be described completely by a combination of the width-to-thickness ratio (b/t) and the inner radius-to-outer radius ratio (r_i/r_o). It was also found that a reasonably wide range of geometries can be used with accuracy to extract fracture strength design data, provided b/t does not get too large or r_i/r_o too low. Given the potential statistical distribution of flaws in most ceramics, there is also cause for concern when b/t → 0 or r_i/r_o → 1 as this implies a small sample of flaws in the critical tensile stress region. Calculations also indicated that the uniaxiality of the C-ring stress state followed the expected 1/ν dependence so that the behaviors of different materials can be surmised easily from the current results calculated for ν = 0.155. As expected, and in contrast to O-ring specimens, the loading distribution between the C-ring surface and platum do not seem to have any effect on the stress state.