Aplicabilidade da metodologia da taxa de alívio de energia de deformação para determinação da tenacidade à fratura de ligas metálicas por indentação instrumentada

Abstract

The determination of fracture toughness of ductile metals through the instrumented spherical indentation technique has been an alternative for characterizing in-service components—where sample removal is not possible—due to its semi-destructive nature and cost-effectiveness. Different mathematical models are presented in the literature as indirect estimates of this toughness. Zhang, Wang and Wang (2019b) introduced the first model based on the Energy Release Rate (ERR) concept of fracture mechanics and the ideas of continuum damage mechanics for assessing fracture toughness. The model determines the value of the J-Integral from the loading–unloading cycles of indentation. Like the other methodologies, the ERR methodology—despite being an indirect measure of fracture toughness—has the strongest basis in fracture mechanics and was the methodology chosen to be studied in detail in this work. During the development of studies to verify the applicability and limitations of the ERR methodology, some issues were observed, and thus alterations were proposed to correct the methodology and broaden its applicability and improve result accuracy. For the study, AISI 4340 steels with different microstructural compositions, AISI 4130M, API X65, X80 and X100 used in the oil and gas sector were selected. Additionally, to understand the model’s limits, the austenitic stainless steel AISI 304 and the aerospace alloy AA7050-T7451 were studied. The results were compared with values extracted from standardized fracture toughness tests, following ASTM E1820 and conducted at the Friction and Wear Technology Laboratory, as well as from the literature. It was observed that the JIIC value calculated by indentation showed a strong dependence on the indenter diameter, and to correct, the fracture toughness value must be multiplied by a factor C = (1/Di)^1,18 depending on the diameter used. Furthermore, to obtain the mode I fracture toughness, JIC, from JIIC, it is necessary to obtain the ratio α 2 = JIIC/JIC, assumed to be constant and equal to 0.35 for all metal alloys, a fact that did not hold true. Through the present work and the literature, values of α^2 and also its relation to the hardness of the studied materials were obtained, with the exception of API X100 steel. Thus, a function for estimating α was proposed: (i) α = f(HV), with R^2 = 0.99 for HSLA steels and (ii) more generally α = f(σy/E), with R^2 = 0.97 for all metal alloys. Applying the proposed corrections made it possible to obtain JIC values calculated from indentation tests with errors less than 12.8%.