Análise numérica do processo de fratura no ensaio de indentação vickers em uma liga de carboneto de tungstênio com cobalto

Abstract

The Vickers indentation testing has been used for decades for surface hardness evaluation of ceramic, metallic and composite materials. Recently, researchers have developed new techniques based on this test in order to evaluate mechanical properties such as Young's modulus, Poisson coefficient and fracture toughness, to characterise peculiar materials and superficial coatings. However, the indentation testing presents some limitations to toughness determination. To evaluate these difficulties, a numerical analysis was implemented using finite element discrete models to simulate the mechanical behaviour of tungsten carbide with 6% cobalt during the indentation cycle. This material presents a mechanical behaviour between ductile metallic and brittleceramic materials. In the sintered form, the WC-6Co is used to manufacture cutting tools because it has high superficial hardness and mechanical compression strength, despite its low toughness. Three numerical models were implemented to evaluate the Vickers' stress and strain fields with a finite element commercial code. Experimental values of hardness and the indentation load behaviour in the testing cycle were used as parameters to calibrate these numerical models. The conditions for crack nucleation and propagation mechanism during the testing were evaluated using damage criteria. The obtained results of surface hardness were inside the range of experimental values given by the literature. The numerical behaviour of the load versus displacement curve was also similar to those present by the literature. Finally, the analyses of the stress and strain fields and the implemented damage criteria made it possible to identify the critical regions for crack nucleation and growth, such as the region of the direction of the indentator diagonal line.