An expression to determine the Vickers indentation fracture toughness obtained by the finite element method on Fe₂B layers

A reverse analysis of the Vickers indentation fracture toughness was carried out to derive a numerical expression for estimating the fracture resistance of the Fe₂B layer. The boride layers were created at the surface of AISI 1018 borided steels by the paste boriding process, in which a 4-mm-thick layer of boron carbide paste was applied to the material surface. The treatment was carried out at temperatures of 1123-1273K with exposure times of 4, 6 and 8h at each temperature. From the set of experimental conditions used in the boriding process, Vickers indentations were performed with an applied load of 1.96N at 25μm from the surface of the borided steels. The crack lengths created from the corners of the Vickers indentation prints were analyzed in the Palmqvist crack regime. The numerical expression of the stress intensity factor (K) of the Fe₂B layer was assisted by both a dimensional analysis and the superposition principle. The analysis considered the residual stress field generated by the indentation load, which was superimposed upon different crack lengths; the stress intensity factor was expressed as a function of the elastic-plastic properties of the layer, the dimensionless crack length, and the location on the Palmqvist crack front. From an extensive numerical analysis, the results of the fracture toughness (K_C) values of the Fe₂B layer were estimated with the finite element method, and then compared with the values obtained by traditional Palmqvist crack models proposed by other researchers.