A. D. Brown

Numerical modelling of closed-cell aluminium foams under shock loading

The present research numerically investigates shock propagation through closed-cell aluminium foam via flyer-plate impact. The mechanics of foam deformation was elucidated using the finite element (FE) software ABAQUS/explicit. X-ray computed micro-tomography was performed to render a full 3D foam geometry mesh for understanding detailed macrostructural response due to shock propagation. Elastic wave propagation and pore collapse mechanism with time were studied. The free surface velocity of the foam was measured at two different flyer-plate impact velocities to observe the profile of the shock wave with time. Good correlations were observed between experimental data and FE predictions for both test conditions.

Dynamic mechanical response of additive manufactured Ti-6Al-4V

The dynamic mechanical response of additive manufactured (AM) Ti-6Al-4V has been investigated via Split-Hopkinson Pressure Bar (SHPB) and single-stage gas gun plate-impact experiments. Rods of Ti-6Al-4V were manufactured from Arcam and Renishaw electron beam melting (EBM) and laser melting deposition (LMD) powder bed systems, respectively. Horizontal and vertical build directions were produced for plate impacts to determine any effect of build layers on dynamic tensile strength. The as-received microstructures were characterized via optical microscopy and electron backscatter diffraction (EBSD). Johnson-Cook constitutive model parameters were found for the EBM condition via SHPB experiments. Plate impacts producing nominal peak shock stresses near the theoretical tensile strength of the material (3-5 GPa) were conducted to study incipient spall damage conditions as a function of AM material condition.The dynamic mechanical response of additive manufactured (AM) Ti-6Al-4V has been investigated via Split-Hopkinson Pressure Bar (SHPB) and single-stage gas gun plate-impact experiments. Rods of Ti-6Al-4V were manufactured from Arcam and Renishaw electron beam melting (EBM) and laser melting deposition (LMD) powder bed systems, respectively. Horizontal and vertical build directions were produced for plate impacts to determine any effect of build layers on dynamic tensile strength. The as-received microstructures were characterized via optical microscopy and electron backscatter diffraction (EBSD). Johnson-Cook constitutive model parameters were found for the EBM condition via SHPB experiments. Plate impacts producing nominal peak shock stresses near the theoretical tensile strength of the material (3-5 GPa) were conducted to study incipient spall damage conditions as a function of AM material condition.

Dynamic crushing response of closed-cell aluminium foams during shock loading

Understanding the impact response of aluminium foams is essential for assessing their energy absorption capacity under dynamic loading. In this paper, the dynamic compaction behavior of closed-cell aluminium foam (CYMAT ™) has been tested using the plate-impact technique. Post-impacted samples have been examined using optical microscopy to observe the microstructural changes with the objective of elucidating the pore-collapse mechanism.

Effects of chemical composition and test conditions on the dynamic tensile response of Zr-based metallic glasses

The effects of impact velocity and temperature on the dynamic mechanical behavior of two bulk metallic (BMG) alloys with slightly different elemental compositions (Zr55Cu30Ni5Al30 and Zr46Cu38Ag8Al38) have been investigated. Bullet-shaped samples were accelerated by a gas gun to speeds in the 400∼600m/s range and tested at both room temperature and 250°C. The samples impacted steel extrusion dies which subjected the bullets to high strains at relatively high strain-rates. The extruded fragments were subsequently soft recovered by using low density foams and examined by means of optical/scanning electron microscopy and differential scanning calorimetry. It was found that shear banding was the dictating mechanism responsible for the fracture of all BMGs. At room temperature, the Zr55Cu30Ni5Al30 alloy exhibited a higher resistance to fragmentation than the Zr46Cu38Ag8Al38 alloy. At 250°C, significant melting was observed in the recovered fragments of both alloys, which indicates that the BMG glassy structure...

Effects of chemical composition on the shock response of Zr-based metallic glasses

The effect chemical composition on the shock response of two bulk metallic glass (BMG) alloys with slightly different elemental compositions (Zr55Cu10Ni5Al30 and Zr46Cu38Ag8Al8) has been investigated. Plate-impact experiments were conducted at a peak compressive stress of ∼10GPa, above the expected elastic limit of these alloys (∼7GPa). Velocity interferometry was used to measure the particle (up) and free surface velocity (FSV) histories. These measurements allowed calculation of the Hugoniot elastic limits and onset stresses of fracture (i.e. spall strength) for each alloy. The soft recovered specimens were characterized by means of optical and electron microscopy. It was found that the Zr55Cu10Ni5Al30 exhibited a higher HEL and spall strength and a smooth fracture surface morphology consisting of dimple-like features. Conversely, the lower spall strength of the Zr46Cu38Ag8Al38 alloy seems to correlate with rougher fracture surface that shows cup-cone features associated with a predominantly brittle dyn...

Effects of temperature and strain rate on the dynamic mechanical behavior of a fine grained Al-Sc alloy

A rod gravity cast aluminum alloy of nominal composition Al-3.0Mg-0.8Sc-0.08Zr has been tested at high strain rates in tension and compression using a Split Hopkinson Pressure Bar. Peak aging heat treatments were performed in an attempt to grow secondary phase Al3Sc particles at grain boundaries to strengthen the alloy under high strain rate loads and at elevated temperatures. Electron backscattering diffraction and energy dispersive x-ray spectroscopy were performed for as cast and peak aged material conditions. Preliminary characterization results indicate negligible changes to the microstructure and second phase particles from the peak aging process performed in this study. Results show that the alloy’s mechanical response to compression and tension at high strain rates is less sensitive at elevated temperatures when compared to a high strength 7010 aluminum alloy, providing further evidence of scandium’s high temperature strengthening effects in aluminum alloys.

Deformation Mechanisms of Closed Cell-Aluminium Foams During Drop Weight Impact

The present study investigates the dynamic deformation mechanisms of closed-cell aluminium foams during low velocity drop weight impact with experimental analyses and finite element (FE) simulation. The evolution of foam collapse was explored with FE simulation using ABAQUS/Explicit. X-ray computed tomography (XRT) based geometry was reconstructed to understand the actual microstructural changes during impact. The experimental stress-strain response was compared with FE simulation with reasonably good agreement observed between FE prediction and experimental data. A vertical cross-sectional XRT slice was analysed at different strains from FE simulation to understand the pore collapse mechanisms.

Mechanical response of a gravity cast Mg-9Al-1Zn-0.2Sc alloy at strain rates from 10-4 to 10 3 /s

A magnesium alloy of nominal composition Mg-9Al-1Zn-0.2Sc was formed into plates by die casting and underwent annealing and T4 condition heat treatments to investigate the mechanical response of varying microstructures at strain rates from 10−4-103/s in tension and compression. Full microstructural characterization was performed using optical microscopy, electron backscatter diffraction and energy dispersive x-ray spectroscopy. Quasi-static and dynamic testing was performed using a universal testing machine and a Split Hopkinson Pressure Bar in conjunction with digital image correlation for strain field mapping. Characterization and mechanical testing indicates that the T4 condition has the highest overall strength due to small equiaxed grains, a decrease in the size of s-Mg17Al12 phase at the grain boundaries, and an increase in the size of scandium intermetallics. Testing indicates an increase in strain hardening for dynamic compression and strain rate dependence in tension; failing suddenly due to casting defects dominating the fracture mechanics.

Methodology for determining spall damage mode preference in shocked FCC Polycrystalline metals from 3D X-ray tomography data

Three-dimensional X-ray tomography (XRT) provides a non-destructive technique to determine the location, size, and shape of spall damage within shock loaded metals. Polycrystalline copper samples of varying thermomechanical histories were shocked via plate impacts at low pressures to ensure incipient spall conditions. Additionally, samples of similar heat-treated microstructures were impacted at various loading rates. All 3D XRT volumetric void data underwent smoothing, thresholding, and volumetric sieves. The full inertia tensor was found for each void, which was used to create best fit ellipsoids correlating shape to damage modes. Density distributions were plotted for the best-fit ellipsoid semi-axes aspect ratios alc and blc, where, a≤b≤c. It was found that >60% of voids in heat-treated samples resembled transgranular damage, whereas >70% of voids in the rolled sample resembled intergranular damage. Preliminary analysis also clearly indicates an increase of void coalescence with decreasing tensile loading stress rates for impacted samples of similar microstructures.