Evaluation of the uniaxiality of the stress state in O-ring fracture strength specimens

To investigate the experimental implications of using O-ring specimens for evaluating the fracture strength of ceramics in tubular form, a statistically designed 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 O-ring specimen under diametral loading. Results of the FEA calculations indicated that the degree of uniaxiality of the O-ring stress state could be described completely by a combination of the depth-to-thickness ratio (b/t) and the inner radius-to-outer radius ratio (r(i)/r(o),). It was also found that a relatively wide range of geometries can be accurately used to extract fracture strength design data, provided b/t does not get too large nor r(i)/r(o) too close to zero. However, there is cause for concern when b/t-->0 or r(i)/r(o)-->1, as this implies a small sample of material (and flaws) in the critical tensile stress region. Results also indicated that the uniaxiality of the O-ring stress-state followed the expected 1/nu dependence so that the behaviors of different materials can be easily surmised from the current results. On the other hand, load magnitude did not seem to have any effect on the stress-state despite the Boussinesq-type loading, while the distribution of the load over a finite contact area section could.