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Ductile Fracture of Ti-6Al-4V Titanium Alloy Under Compressive Stress States
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2025-05-07
[PDF-72.97 MB]
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Abstract:A notoriously challenging modeling and simulation problem is the impact physics of jet engine debris striking the engine case during a fan blade-out or rotor-burst event. The multiplicity of potential failure modes resulting from the impact and subsequent penetration of high-speed engine fragments advances the challenge. The ductile fracture model must be robust to account for all potential failure modes. The failure locus, a key ingredient in many state-of-the-art ductile fracture models, is a three-dimensional surface plot of the equivalent plastic strain at fracture as a function of the state of stress, quantified by the stress triaxiality and Lode parameter. Standard mechanical tests are generally used to populate the failure locus, but standard tests can only capture a limited window of stress states. This limited window potentially leaves important regions of the failure locus unpopulated. For instance, a significant body of previous research suggests that fracture will not occur above a stress triaxiality of 0.33 (known as the “cut-off” value). However, recent ballistic impact simulations involving 12.7-mm-thick Ti-6Al-4V titanium alloy plates predict large positive (compressive) triaxialities in the vicinity of the adiabatic shear band. These results not only suggest the potentially unanticipated importance of the positive triaxiality (compressive) region of Lode-triaxiality stress space, but also the need to experimentally revisit previous interpretations of the “cut-off” value of the triaxiality. Toward that end, this thesis investigates the ductile fracture of 6.35-mm-thick and 12.7-mm-thick Ti-6Al-4V titanium plate under compressive stress states. Specifically, our experimental plan is designed to interrogate the quasi-static ductile fracture behavior of Ti-6Al-4V plate under complex, highly triaxial, compression-dominated stress states. To achieve this, our testing program employs a series of cylindrical compression specimens with asymmetric features and/or geometric irregularities including vertical through-holes, 45-degree through-holes, horizontal through holes, a 45-degree slot, and a spherical recess. This test series is intended to complement previous ductile fracture experiments performed on the same lot of 12.7-mm-thick Ti-6Al-4V plate stock. Stereo digital image correlation is used to measure surface displacements and compute full-field strains at and near the fracture location. The experimental results are coupled with numerical simulations in LS-DYNA to calculate the stress state evolution and fracture strain at the site of fracture initiation. Weighted-average triaxialities of 0.61, 0.72, and 0.81 are reported for the specimens with through-holes, placing these experiments among the most compression-dominated in the literature.
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