Ductile fracture of 2024-T351 aluminum and Ti-6Al-4V under plane strain compression

The compression-dominated or positive (as defined in this paper) triaxiality region of stress space has proven important in numerical simulations of jet engine containment and ballistic impact. Despite this practical relevance, few existing fracture tests are able to interrogate the compression-dominated region of stress space and provide experimental data – specifically, a “point” in three-dimensional stress space defined by triaxiality, Lode parameter, and equivalent plastic strain at fracture – for ductile fracture model calibration. Thus, this paper presents a novel experiment that induces ductile fracture under a state of plane strain compression for two common aerospace alloys, 2024-T351 aluminum and titanium alloy Ti-6Al-4V. The specimen is a cylindrical ring with an axisymmetric semicircular notch cut into the outer surface at the axial midplane. When axially compressed, the thin wall created by the notch induces radial deformation due to circumferential confinement of the material. Thus, the material is strained axially and radially, but not circumferentially, leading to a state of plane strain in the center of the outer surface of the specimen. Three-dimensional 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 failure location. A triaxiality of 0.669 (0.602), Lode parameter of -0.034 (0.040), and equivalent plastic strain at fracture of 0.900 (0.449) are reported for 2024-T351 aluminum (Ti-6Al-4V), placing this ductile fracture experiment among the most compression-dominated in the literature.