A new test to determine the tensile strength of brittle balls—The notched ball test

A new strength test for ceramic spheres (balls) is presented. A long narrow notch is cut in the equatorial plane of the ball and the ball is then loaded in compression perpendicular to the notch. This causes tensile stresses in the outer surface region of the ball opposite to the notch, which are analysed carefully with finite element (FE) methods. The tensile stress amplitude depends on the bending moment in the notch ligament – given by the applied force – and on details of the notch geometry. The stress state in the highly stressed surface is almost uniaxial showing only a slight influence of Poissonʹs ratio. Numerical solutions for balls with quite different notch geometries are given. th tests have been performed on commercial silicon nitride balls of 5 mm diameter. Two sets of specimens having notches of different length have been tested. Although the typical fracture loads in both sets of data are quite different, the tensile strengths are closely similar. This indicates the validity of the data evaluations. Experimental details are discussed and an analysis of the experimental uncertainties on the test results is made. For balls with 5 mm the uncertainties are estimated to be less than ±3% (of the measured value). For balls having a diameter of 10 mm or more the uncertainties are less than ±1%.