Experimental Science

The Small Particle Hypervelocity Range (SPHR) facility

The High Strain-Rate Testing (HSRT) facility

The experimental campaign of the center will consist of: i) integrated hypervelocity impact tests, and ii) component experiments designed to validate critical models of material behavior identified as ‘predictivity bottlenecks’. The integrated experiments will be carried out at Caltech’s Small Particle Hypervelocity Range (SPHR) facility. The component experiments will be carried out in Caltech’s High Strain-Rate Testing (HSRT) facility. In order to reduce and mitigate risk, the experimental campaign will be incremental and will be phased in time in order of increasing complexity. A number of state-of-the-art time-resolved diagnostics will supply a wealth of time and space-resolved data for validation and uncertainty quantification. The ability to conduct the experimental campaign in-house is necessitated by the high level of coupling and coordination with the computational campaign that is envisioned within the program as an integral part of our QMU methodology. This close-loop tight coupling between experiment and computation is one of the unique strengths and distinguishing characteristics of the Caltech PSAAP Center.

The Small Particle Hypervelocity Range (SPHR) is capable of launching small projectiles in vacuum at speeds ranging from 1-10 km/s depending on particle mass. The projectiles are small spheres of various sizes or cylinders of various aspect ratios (L/D~1-2). The sphere or cylinder diameters range from 0.25-2.5mm. The proposed projectile and target materials are Ta and Fe. These materials will be used in both projectiles and plate targets in pair-wise combinations, e.g. Fe (projectile) on Ta (target) and Ta (projectile) on Fe (target). The projectiles are delivered into a large vacuum chamber in which an appropriate target configuration will be constructed. The vacuum chamber features a large number of access ports in order to allow for various types of externally located diagnostic instruments to scrutinize and measure the physical processes attendant to the impact, penetration and perforation processes and the associated, plasma, ejecta and debris clouds.

In ballistic and hypervelocity impact events, the precise form of the rate-sensitivity law, the convertibility of plastic dissipation to heat, and pressure and temperature dependence of yield strength of metals are known to strongly influence phase transformations including melting and deformation patterns at high rates of deformation, including the extent of flow localization into shear bands and failure. Away from the immediate region of impact, the pressures, temperatures and strain rates induced by hypervelocity impact fall within the range of conventional testing apparatus such as the split Hopkinson (Kolsky) pressure bar. Caltech has world-class facilities in that class for high strain-rate testing and measuring equation of state of materials. Under the Caltech ASC/ASAP Center, those facilities have produced a wealth of high-quality data that have provided the basis for the validation of strength models. Those data include stress-strain relations as a function of temperature and strain-rate, adiabatic heating rates, electron microscopy (SEM, TEM) of microstructural evolution, electron backscattered diffraction patterns, texture maps, and others. These facilities will continue to play a key role in support of the validation campaign under the PSAAP Center.

However, hypervelocity impact also results in extremely high energy densities leading to high pressures and strain rates in the immediate vicinity of the point of impact. These conditions are not generally achievable with the conventional flyer plate shock wave experiments. In order to realize such conditions in a laboratory using impact, a new experimental configuration, the Shock Wave Lens (SWL) configuration, is proposed for development. The experimental configuration consists of impacting a flyer plate on a planar face of a shock wave lens which is in contact with target material of interest (e.g., Fe, Ta). The shock wave lens will be designed based on the knowledge of finite amplitude shock wave propagation in materials and mixtures. Biaxial stress gages placed along the axis of the target will be used to measure the state of stress and thus establish the equations of state under extreme pressures and energy densities. Apart from the stress gages, a VISAR will be employed to measure the particle/interface velocities as a distance of propagation. High density impactors (e.g., Tungsten, GGG) will be used for generating high pressures. These impact and converging shock wave experiments are expected to provide a wealth new data concerning material behavior at high pressures otherwise inaccessible in impact experiments. Due to the converging nature of the shock, it is also expected that this configuration will be useful for the study of phase transformation, melting and hydrodynamic instabilities in metals of interest.