Experimental Design for Negative Triaxialities: Ductile Fracture Under Combined Uniaxial Tension and Hydrostatic Pressure

Many modern continuum-scale approaches for modeling the ductile fracture of metals regard the equivalent plastic strain at fracture as a function of the stress triaxiality and Lode parameter, a pair of invariant-based quantities that together characterize the three-dimensional state of stress at a point. Generally, these ductile fracture models (whether parameterized or tabulated) are calibrated using standard mechanical tests, e.g., notched axisymmetric (round), plane stress (thin), and plane strain (thick) specimens subjected to tensile loading. However, these standard tests are only able to capture a limited window of stress states, leaving potentially important “regions” of the ductile fracture model unpopulated with experimental data. For instance, although previous research has suggested that fracture will not occur below a triaxiality of 0.33 (the “cut-off” value), recent ballistic impact simulations involving 0.5-inch-thick titanium Ti-6Al-4V target plates predicted large negative (compressive) triaxialities in the vicinity of the adiabatic shear band. These results not only suggest the potentially unanticipated importance of the negative 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. As a first step, this paper presents a novel physical interpretation of the Lode parameter = 1 (constant) meridian over a range of triaxialities, spanning positive (tensile) to negative (compressive). Guided by this physical insight, ductile fracture experiments that employ hydrostatic pressure superposed on uniaxial tension are proposed and numerically simulated in LS-DYNA®, with initial efforts focusing on 2024-T351 aluminum. Our numerical simulations provide a promising “virtual” proof-of-concept demonstrating that stress triaxiality can be tuned (at constant Lode parameter) by adjusting the magnitude of the applied pressure, allowing a wide range of stress states to be accessed through a single experimental test setup.