Abstract
Fracture mechanics based considerations were made for investigating the ring-crack formation of ceramic plates contacted with a ceramic sphere under a combined normal and tangential indentation load. The intrnsic tensile strength of the ceramic plate did not dictate the onset of ring-crack formation. The size of pre-existing defects (being less than one micron) that may result in the ring-crack initiation were somwhat smaller than the grain size of the ceramic plate. On the other hand, the stress gradient induced beneath the contact was significantly dependent on the contact angle of normal/tangential combined load; when the contact angle is increased, the mean dimension of defects which lead to ring-crack initiation becomes large, whereas the location of the induced crack is progressively away of the contact circle. The stress gradient induced beneath the contact was taken into account for calculating the stress intensity factor of the pre-existing surface defect. The fracture toughness value thus estimated for the ceramic plates satisfactorily predicted the onset of ring-crack formation of all the ceramic plates examined.
