Determining indentation toughness by incorporating true hardness into fracture mechanics equations

The Lawn-Evans-Marshall (LEM) indentation fracture mechanics model for the well-developed half-penny crack is reexamined theoretically, with a particular emphasis on the physical meaning of the hardness parameter in this model. It is shown that there may be some confusion arising from the use of the hardness definition. A modified indentation toughness equation is then proposed in which a true hardness number, rather than the apparent hardness, is incorporated This modified equation predicts that if is the quantity a(2)/c(3/2), rather than P/c(3/2), that would keep constant in the half-penny crack regime. This prediction is verified by analyzing the previously published indentation data. It is also confirmed that a comparable value of indentation toughness can be deduced with this modified equation. (C) 1999 Elsevier Science Limited. All rights reserved.