FRACTURE-TOUGHNESS OF SINTERED SIC CERAMICS - A COMPARISON BETWEEN DIFFERENT METHODS

In the present study the fracture toughness K(lc) is evaluated by three- and four-point bending tests on straight-notched bending bars, by three-point bending tests using chevron-notched specimens, and by the indentation microfracture method. The influence of the crack growth rate on the fracture toughness at room temperature is examined experimentally. As a result, K(lc) tests on chevron-notched specimens are shown to yield a lower value of fracture toughness with a much better reproducibility than tests on straight-notched specimens. Fracture toughness evaluated in indentation microfracture tests is shown to depend on the method of measuring the crack length. Moreover, the load dependence of the apparent fracture toughness and the large number of different formulae available were further difficulties in evaluating valid K(c) values from indentation tests. Three-point bending tests of chevron specimens showed a slight increase of K(lc) with higher crack growth rates at room temperature. The critical strain energy release rate G(lc) was evaluated from bending tests with chevron-notched specimens as an energetical fracture parameter. It was compared with the conventional fracture parameter K(lc).