Session J03: Dynamic Properties of Materials

Dynamic tensile extrusion response of Al2024-T351

Al 2024-T351 is known to exhibit a significant shear effect on ductility. The dynamic tensile extrusion (DTE) test provides the possibility to probe material response under pressure and shear at large strain and high strain rates. In this work DTE tests at different velocity were performed in our single stage, light gas gun facility. The impact velocities were selected to have deformation in the die without extrusion, partial extrusion before fragment formation, and complete extrusion. Deformation and damage have been investigated analyzing with optical microscopy and EBSD analysis the microstructure evolution and fracture in the recovered projectile fragment trapped in the extrusion die. The propensity of the material to fracture along localized shear bands, which limits the overall extrusion ductility, has been observed. The results have been compared with numerical simulation performed with the plasticity-damage self-consistent (PDSC) model.   

Taylor impact test of Al 2024-T351 at elevated temperature

Shear has a significant effect on the fracture strain of Al2024-T351. This effect is particularly evident with a reduced fracture strain in torsion compared with the uniaxial tensile condition. To explain this, ductile damage, and therefore the resulting fracture strain, has been proposed to depend on the stress triaxiality and the third invariant of stress deviator. In this work, the role of temperature on such dependence has been investigated. To this purpose, Taylor rod impact tests at elevated temperature (80°C and 155°C) have been performed at approximately 290 m/s, which is the velocity at which fracture development start to occur at room temperature. These tests showed that, although the overall ductility increases with temperature allowing more rod mushrooming, extended fractures occur along shear planes. Based on these results, the the Plasticity Damage Self Consistent (PDSC) model was revised and extended. Numerical simulation results are presented.   

Multi-scale computational and experimental mechanics for the design of high-performing additively manufactured ceramics

This research follows a hybrid multi-scale computational and experimental approach for the design of AM alumina ceramics with tailored microstructures (e.g., grain sizes and porosity features) and properties (e.g., strength). The strain-rate dependent mechanical properties of the AM alumina ceramics were experimentally tested combined with ultra-high-speed imaging and DIC analysis to capture the in situ failure. Informed and validated by the experimental data, our multi-scale computational framework covers nanoscale (e.g., molecular dynamics simulations and developing machine learning-based inter-atomic potentials), microscale (e.g., EBSD-based polycrystalline RVE modeling), and macroscale (e.g., hybrid FE-DEM modeling of the specimen) simulations. Altogether, this experimentally validated multi-scale numerical modeling enables us to inform on the microstructure-property-performance relationships of AM ceramics which have implications for the design of better-performing materials and structures in a range of applications.   

Fracture Induced by Giant Nonlinear Acoustic Pulses

Using a technique recently developed in our research group to build up propagative strain waves from the linear to the nonlinear regime, we investigate transient fracture dynamics in strontium titanate, in which surface acoustic waves with longitudinal strain amplitudes reaching up to 3 % can be generated — well within the regime of mechanical failure. We identify dislocation lines and cleavage planes in the material from the damage induced by the propagation of high-amplitude acoustic waves. Furthermore, since our technique, based on the superposition of numerous individual laser-generated acoustic waves, can generate giant strain waves without damage to the excitation region, we analyze how fracture evolves as a function of the number of acoustic waves launched into the sample. Finally, we study the anisotropy of fracture dynamics from orientation-dependent measurements and show that counter-propagating strain waves lead to qualitatively different behaviors due to the nonlinearity of the fracture process.   

Multi-fragmentation of various metals under dynamic radial expansion

When a cased explosive charge detonates, its metallic envelop inflates because of the expansion of the detonation products until its multiple fragmentation that generates high kinetic energy debris. In industrial and military applications, the spatial distribution of the fragments and their statistics in terms of number, mass, and velocity are mandatory to assess safety or lethality areas. Since the founding works of N.F. Mott (1940’s), numerous experimental, theoretical and numerical studies have been issued on the fragmentation of cased charges or simplified configurations such as hemispheres, cones, cylinders or rings. The present authors recently contributed to this topic designing the PIDRET set-up (plate-impact-driven ring expansion test) that is able to expand reliably thin metallic rings at strain rates close to 10⁴ s^-1. This work will provide new in-situ and post-mortem observations of ring expansion for various metals. Number of fragments and visible necks will be compared to existing analytical approaches such as linear stability analysis (LSA) and Grady’s model.