Dong-Kuk Kim

Microstructural study of adiabatic shear bands formed by high-speed impact in a tungsten heavy alloy penetrator

Abstract The microstructures of adiabatic shear bands formed by high-speed impact in a tungsten heavy alloy penetrator were investigated in the present study. The penetrator was highly deformed at high strain rate by high-speed impact, and microstructural observation of the remained penetrator and the debris was conducted after the impact test. Heavily elongated tungsten particles and reaction products such as tungsten oxides were observed on the surface region of the debris presumably due to the local temperature rise occurring upon high-speed impact. A few adiabatic shear bands were observed in the regions near the surface cracks of the remaining penetrator, and their width was wide in comparison to the shear band formed in armor plates. The crack had general trends to propagate along the shear band, but sometimes changed its propagation path along the interfaces between adhering tungsten particles. It was suggested from the microstructural observation of the shear bands that in order to improve the penetration performance of the tungsten heavy alloy, the minimization of the tungsten–tungsten particle interfaces and the optimization of the fabricating process were required.

DYNAMIC CRASHING AND IMPACT ENERGY ABSORPTION OF EXTRUDED ALUMINUM SQUARE TUBES

Abstract The present article aims at investigating the compressive deformation behavior under dynamic loading to evaluate experimentally impact absorption energies of extruded aluminum tubes for space frame design. The dynamic compressive tests were conducted on three different extruded aluminum tube specimens of thicknesses 1.5, 2.0 and 3.0 mm using a high performance servo-hydraulic machine. As the strain rate increased, the size of plastic folds and the distance between folds tended to be non-uniform, but the calculated values of impact absorption energy were nearly constant. The number of plastic folds decreased by increasing the specimen thickness, and the impact absorption energies also decreased considerably as the tube specimens could not absorb energy effectively due to the presence of severe overall bending. To improve the impact absorption energy of the extruded tube for the space frame design, it is recommended to make the thickness of the extruded tube below 2.0 mm for the cross-section in the present study to promote folding formation instead of bending.

Impact energy absorption of 6061 aluminum extruded tubes with different cross-sectional shapes

Abstract The present article aims at evaluating experimentally impact absorption energies of an extruded 6061 aluminum tube for space frame and at investigating the effect of cross-sectional shape on the compressive deformation behavior under dynamic loading. The dynamic compressive test was conducted on extruded aluminum tube specimens, whose shape and thickness/width (or diameter) ratio are different, using a high performance servo-hydraulic machine. Asymmetric folds were formed mainly in the rectangular tube specimens, whereas symmetric folds were formed in the circular tube specimens. However, the fold shape tended to become symmetric as the thickness/width ratio increased, and absorption energy increased linearly within the scope of the present study. To improve impact absorption energy and maximum resistive force against compressive load of extruded aluminum tube for space frame, it is recommended to make the thickness/width ratio large enough and to design the circular cross-sectional shape in a way to promote the formation of symmetric folds instead of asymmetric ones.

Effect of tungsten particle shape on dynamic deformation and fracture behavior of tungsten heavy alloys

The effect of the tungsten particle shape on the dynamic deformation and fracture behavior of tungsten heavy alloys was investigated. Dynamic torsional tests were conducted using a torsional Kolsky bar for five alloys, one of which was fabricated by the double-cycled sintering process, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. The dynamic torsional test results indicated that in the double-sintered tungsten alloy whose tungsten particles were very coarse and irregularly shaped, cleavage fracture occurred in the central area of the gage section with little shear deformation, whereas shear deformation was concentrated in the central area of the gage section in the other alloys. The deformation and fracture behavior of the double-sintered alloy correlated well with the observation of the impacted penetrator specimen and the in situ fracture test results, i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that the cleavage fracture mode would be beneficial for the self-sharpening effect, and, thus, the improvement of the penetration performance of the double-sintered tungsten heavy alloy would be expected.

Microstructural evolution in a strip-cast Ni-base superalloy

Microstructure and mechanical properties of the strip-cast Ni-base superalloy(Hastelloy-X) alloy have been compared with those of a conventionally ingot-cast alloy. As-cast strip shows a fine columnar dendritic structure as a result of a relatively high cooling rate of 102 -104 K/sec during the process. The alloying elements, such as Cr and Fe, are homogeneously distributed in the γ matrix of the as-cast strip. Whereas, Mo is segregated at the center part of the as-cast strip. Mo distribution, however, becomes homogeneous after cold rolling and subsequent heat treatment at 1175°C. Two types of carbide, a Mo-rich M6C type and a Cr-rich M23C6 type, are present in the γ matrix of the as-cast and rolled strips. Rolling and subsequent heat treatment of the strip-cast alloy results in a fine grain structure compared to the ingot-cast alloy. The γ matrix grains in stripcast and ingot-cast alloy are approximately 14 μm and 34 μm in diameter respectively. Superior tensile properties in the strip-cast alloy is due to the fine microstructural evolution during the process.

Effect of size and shape of tungsten particles on penetration performance in tungsten heavy alloys

The effects of the size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys were investigated. Dynamic torsional tests were conducted on seven tungsten alloy specimens, four of which were fabricated by repeated sintering, using a torsional Kolsky bar, and then the test results were comparedvia microstructure, mechanical properties, adiabatic shear banding, and deformation and fracture mode. The size of tungsten particles and their hardness were increased with the increasing of the sintering temperature and time, thereby deteriorating the fracture toughness. The dynamic torsional test results indicated that a cleavage fracture occurred predominantly with little shear deformation in the specimens whose tungsten particles were coarse and irregularly shaped whereas shear deformation was concentrated into the center of the gage section in the conventionally fabricated specimens. The deformation and fracture behavior of the specimens having coarse tungsten particles correlated well with the observation of thein situ fracture test results,i.e., cleavage crack initiation and propagation. These findings suggested that there would be an appropriate tungsten particle size condition in the penetration performance since the cleavage fracture mode would be beneficial for the self-sharpening of tungsten heavy alloys.