Effect of specimen geometry on fracture toughness measurement of plasma-sprayed ceramic coatings by the tapered double cantilever beam approach

The measurement of fracture toughness of plasma-sprayed ceramic coatings is usually performed using a standard double cantilever beam approach. The design of specimen geometry using a tapered double cantilever beam (TDCB) approach makes the compliance of specimens be linearly proportional to crack length. Fracture toughness in terms of critical strain energy release rate can be obtained by measuring fracture load only. Three varieties of the TDCB specimens are used to examine the effect of the geometry and size of specimens on the results. These are simple TDCB, modified and long TDCBs. Plasma-sprayed Al₂O₃ coatings are used in test. It was found that linearly proportional relations are experimentally obtained between the compliance of the specimen and crack length for all three varieties of the TDCB samples. Fracture toughness of plasma-sprayed Al₂O₃ coating is independent on the geometry of the test piece used in the study. The TDCB approach is a simple and reliable approach to measure fracture toughness of thermal spray coatings without the measurement of crack length.