Plastic deformation and ductile fracture of Ti-6Al-4V plate stock is investigated under multiple loading conditions. The objective of this study is to generate experimental data that can be used for the development and calibration of constitutive and failure models for numerical simulations of dynamic events. Plastic deformation is investigated at various strain rates, orientations, temperatures, and stocks. The stress state dependence of ductile fracture is also investigated.
Uniaxial tension, compression, and pure shear experiments are conducted at strain rates ranging from 1.0E-4 1/s to 8000 1/s. Specimens are fabricated from several sheet and plate stocks with thicknesses of 2.29mm, 3.56mm, 6.35mm, and 12.7mm. Compression and tension tests are conducted with specimens oriented in several different directions. These data show significant strain rate sensitivity in tension, compression and shear. Both plates exhibit anisotropic plastic deformation behavior in tension and compression. The response of each of the plates are significantly different for yield stress, flow stress, hardening, failure, and anisotropic effects.
Ductile fracture testing is conducted at various stress states, which are achieved with mechanical tests on various sample geometries subjected to various loading conditions. Tension tests are conducted on thin flat specimens, wide flat specimens and axisymmetric specimens with varying notch radii. Thin walled tube specimens are subjected to combined axial-torsional loading for additional states of stress. The results show that the stress triaxiality alone is unable to properly capture the failure characteristics of material. Digital image correlation is used to measure surface strains of the specimens. Parallel LS-DYNA simulations are used to determine the stress states and fracture strains. A fracture locus for Ti-6Al-4V is created in the stress triaxiality and Lode parameter stress space giving a more accurate description of the material fracture.
An experimental technique is introduced to measure full field strains using three dimensional digital image correlation at temperatures up to 800C. This test setup has been designed to be a straight forward, repeatable, and accurate method for measuring strains at high temperatures. Design hurdles included thermal gradients of air, speckle pattern adhesion, viewing window image distortion, camera calibration, and infrared light pollution of the camera sensor. For validation, the coefficient of thermal expansion for Ti-6Al-4V up to 800C is measured using the technique and compared to published values. Tests on Ti-6Al-4V were conducted in tension, compression, and torsion (shear). Experimentally measured coefficient of thermal expansion values correlate well with handbook values. The system performs well for each of the tests conducted here and gives substantially more data than standard methods.