Microstructural characterization of low -velocity and hypervelocity impact crater cross-sections in OFHC copper targets

Stella Chavez Quinones, University of Texas at El Paso

Abstract

This dissertation study involves the first systematic microstructural analysis of a series of impact craters. Optical and transmission electron microscopy techniques were used to characterize low velocity and hypervelocity impact crater cross-sections in OFHC copper targets. The impact craters were formed by 3.17 mm 1100 aluminum spheres at impact velocities between 1 and 7 km/s. Measurements of the corresponding geometrical ratios, etc. were consistent with previous studies in copper, and showed a convergence of penetration-to-crater diameter ratio (p/D$\sb{\rm c}$) to an average value of 0.4 at hypervelocities ($\rm u\sb{o} > 5$ km/s). Impact pressure estimates based on Bernoulli pressure P$\sb{\rm B}$ calculations indicated that shock pressures between 1 and 25 GPa were reached for the series of impacts. Consequently, the presence of deformation twins was expected for impact velocities greater than 6 km/s (where the steady-state or Bernoulli pressure was 20 GPa). These estimates were pressure minimums since the Hugoniot instantaneous shock pressures values at the impact surface were many times these calculations (up to 140 GPa).^ Several microstructural zones were observed along the impact axis and each zone position and width was observed to change with impact velocity. The zones consisted of recrystallized grains at the crater wall, extensive plastic deformation, and microbands. In place of expected deformation twins, microbands were observed in connection with all the impact craters. The presence of microbands rather than deformation twins was unexpected, and may be due to the strain or stored energy of deformation involved in the crater formation process, or specific deformation features of cratering, i.e. a spherical shock wave in contrast to a plane-wave shock.^ Vickers microhardness measurements were taken along the impact axis and for the material surrounding the crater wall cross-section. Vickers microhardness and microstructural variations along the impact axis provide evidence on how the impact pressure levels decrease away from the crater floor and into the target material. The maximum hardness seems to be associated with the presence of microbands within highly deformed regions surrounding the crater floor. The microband region extends further into the target material with increase in impact velocity. Maps of the hardness values surrounding the crater walls resulted in a series of profiles which illustrate the pressure effects associated with the shock wave, and distinguishes the low velocity and hypervelocity cratering phenomena. In addition, maps of microstructural regimes combined with the microhardness maps will demonstrate for the first time the progressive variations of material response beyond the crater wall. Finally, the experimental hardness mapping data was compared to computer simulations pertaining to the series of impacts produced in this study. Crater geometry along with estimated residual yield stress and hardness values were compared to the experimental data in order to simulate craters at hypervelocities beyond those attainable in the laboratory ($>$7 km/s), and more applicable to low-Earth or related orbital environments. In addition, the experimental hardness maps allowed for the first, systematic calibration attempts to be made for 2-dimensional modeling software utilizing constitutive equation-based computations. (Abstract shortened by UMI.) ^

Subject Area

Engineering, Metallurgy|Engineering, Materials Science

Recommended Citation

Quinones, Stella Chavez, "Microstructural characterization of low -velocity and hypervelocity impact crater cross-sections in OFHC copper targets" (1996). ETD Collection for University of Texas, El Paso. AAI9718115.
http://digitalcommons.utep.edu/dissertations/AAI9718115

Share

COinS