Stress distributions in thin bilayer discs subjected to ball-on-ring tests

Ball-on-ring tests have been used extensively to measure the biaxial strength of brittle materials. However, the tests and analyses are limited to materials of uniform properties. An analytical model is developed in the present study to analyze thin bilayer discs subjected to ball-on-ring tests. It is found that closed-form solutions for bilayer discs can be obtained from existing solutions for monolayer discs by replacing the position of the neutral surface and the flexural rigidity of monolayers with those of bilayers. To validate the analytical solutions for bilayers, ballon-ring tests are performed on a thin Al2O3 scale grown on an oxide dispersion-strengthened FeCrAl alloy disc and photo-stimulated luminescence spectroscopy is used to measure the stress distributions in the Al2O3 layer. In this case, the stress-induced peak shift of the R lines emitted by the Al2O3 scale is used to determine the average stress through the scale thickness. Good agreement between predictions and measurements is obtained.