In this work,a test method was developed to determine the interfacial fracture toughness of the air plasma sprayed (APS) thermal barrier coatings (TBCs) over a wide range of mode mixities.For this mixed-mode test method,the analytical expressions for the energy release rate and stress intensity factors were derived based on the energy theory and the concept of "equivalence".The fidelity of these expressions was affirmed by selected finite element analysis.The experimental results showed that the critical energy release rate increased with the increase of the positive mode mixity,which was mainly due to the increase in contact/friction effect and plastic work dissipation with increasing shear mode loading.Furthermore,an elliptical interfacial failure criterion in terms of the stress intensity factors was proposed.The agreement between the experimental results in the literature and those in our work indicated that our test method and the corresponding analytical solutions can well determine the interfacial fracture toughness of the TBCs over a wide range of mode mixities.
Gigacycle fatigue behaviors of two SNCM439 steels with different tensile strengthes were experimentally studied by rotating bending tests,to investigate the effects of the tensile strength obtained by different heat treatment processes on very high cycle fatigue failure mechanisms.The material with higher tensile strength of 1 710 MPa exhibited typical gigacycle fatigue failure characteristics,whereas one with lower tensile strength of 1 010 MPa showed only traditional fatigue limit during the tests and no gigacycle failure could be found even when the specimen ran up to more than 10 8 cycles.Metallographic and fractographic analysis were carried out by an optical microscope (OM) and scanning electron microscope (SEM).It showed two different crack initiation mechanisms that for the specimen with lower tensile strength the crack prefers surface initiation and for that with higher strength the crack initiates from subsurface inclusions revealed by a fish-eye like microstructure.
