COMPRESSION CRACKING OF STEEL BALL-BEARINGS

Quasi-static compression cracking of 3 mm diameter steel ball bearings by uniaxial loading between tungsten carbide flats has been investigated experimentally. It is shown that the cracking occurs only after the bearings have been deformed plastically at the contact zones. Moreover, it is also shown that the cracking occurs not only during loading, but also after complete unloading if the maximum applied load is at least 65-70% of the load required for cracking during loading. The time delay between complete unloading and cracking has been found to be in the range of a few seconds to several hours. The fragments formed by cracking during loading are like orange segments in shape, whereas only two fragments of nearly equal masses are always formed when the cracking occurs after complete unloading. The unloading cracks are thought to form by the residual tensile circumferential stresses in the plastically deformed spheres. Separate experiments on specially made mild steel hemispheres have shown that the displaced material is pushed into the bulk of the sphere parallel to the load axis, and this leads to unloading residual stresses. It is suggested that the cracking during loading is initiated sub-surface at the load axis due to tensile radial stresses, whereas unloading cracks are formed at the surface due to tensile circumferential stresses. Experimental evidence in support of the latter is also presented.