In the present work a straightforward calibration procedure of ductile damage models is proposed. The direct methodology involves the use of a simple multiaxial specimen, to be tested with a universal testing machine, capable to reproduce different stress states in the material. The specimen geometry was the one proposed by Driemeier et al. [1]. In addition, a numerical-analytical procedure was devised for the identification of material strains to fracture and corresponding stress states, directly from experimental tests. This allowed to overcome the use of Finite Element Analysis and inverse methods usually adopted to retrieve the local parameters representative of the material ductility.
In the last few decades, great effort has been spent on advanced material testing and the development of damage models intended to estimate the ductility and fracture of ductile metals. While most studies focused on static testing are applied at room temperatures only, in this paper, multiaxial tests have been executed to investigate the effects of dynamic action and temperature on the mechanical and fracture behavior of an API X65 steel. To this end, a Split Hopkinson Bar (SHB) facility for dynamic tests, and a uniaxial testing machine equipped with a high-temperature furnace, were used. Numerical simulations of the experiments were setup for calibration and validation purposes. Based on the experimental results, the Johnson–Cook and Zerilli–Armstrong plasticity models were first tuned, resulting in a good experimental–numerical match. Secondly, the triaxiality and Lode angle dependent damage models proposed by Bai–Wierzbicki and Coppola–Cortese were also calibrated. The comparison of the fracture surfaces predicted by the damage models under different loading conditions showed, as expected, an overall significant increase in ductility with temperature; an appreciable increase in ductility was also observed with the increase in strain rate, in the range of low and moderate triaxialities.
The present work summarizes the results of an experimental campaign aimed at assessing the ductility of a wrought 17-4PH steel alloy. A simple specimen reproducing multiaxial stress states through a universal testing machine is selected. A Finite Element Model (FEM) for each test is setup to extract the local values of stress and strain in the most critical point on the onset of failure. A Digital Image Correlation (DIC) technique is employed to assess the strain field estimated via FEM. The collected data are used to analyse the material ductility, calculating the triaxiality and deviatoric parameter at the fracture strain. The proposed tests fall in the range of low triaxialities, which are less investigated in the literature. The results obtained are compared with the prediction of a damage model, previously calibrated on the material through more conventional tests. The prediction accuracy of the damage model was fully confirmed by the outcome of the new tests. Eventually, the possibility of replacing some of the conventional tests used for calibration with the proposed specimen is explored.
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