Andrew J. Piekutowski

Penetration of 6061-T6511 aluminum targets by ogive-nose steel projectiles with striking velocities between 0.5 and 3.0 km/s

Summary We performed a series of depth-of-penetration experiments using 7.11-mm-diameter, 71.12-mm-long, ogive-nose steel projectiles and 254-mm-diameter, 6061-T6511 aluminum targets. The projectiles were made from vacuum-arc remelted (VAR) 4340 steel (Rc 38) and AerMet 100 steel (Rc 53), had a nominal mass of 0.021 kg, and were launched using a powder gun or a two-stage, light gas gun to striking velocities between 0.5 and 3.0 km/s. Since the tensile yield strength of AerMet 100 (Rc 53) steel is about 1.5 times greater than VAR 4340 (Rc 38) steel, we were able to demonstrate the effect of projectile strength on ballistic performance. Post-test radiographs of the targets showed three different regions of penetrator response as the striking velocity increased: (1) the projectiles remained rigid and visibly undeformed; (2) the projectiles deformed during penetration without nose erosion, deviated from the target centerline, and exited the side of the target or turned severely within the target; and (3) the projectiles eroded during penetration and lost mass. To show the effect of projectile strength, we present depth-of-penetration data as a function of striking velocity for both types of steel projectiles at striking velocities ranging from 0.5 and 3.0 km/s. In addition, we show good agreement between the rigid-projectile penetration data and a cavityexpansion model.

Perforation of aluminum plates with ogive-nose steel rods at normal and oblique impacts

Abstract Perforation experiments were conducted with 26.3 mm thick, 6061-T651 aluminum plates and 12.9 mm diameter, 88.9 mm long, 4340 R c = 44 ogive-nose steel rods. For normal and oblique impacts with striking velocities between 280 and 860 m/s, we measured residual velocities and displayed the perforation process with X-ray photographs. These photographs clearly showed the time-resolved projectile kinematics and permanent deformations. In addition, we developed perforation equations that accurately predict the ballistic limit and residual velocities.

Penetration Experiments with 6061-T6511 Aluminum Targets and Spherical-Nose Steel Projectiles at Striking Velocities Between 0.5 and 3.0 km/s

We conducted depth of penetration experiments with 7.11-mm-diameter, 74.7-mm-long, spherical-nose, 4340 steel projectiles launched into 250-mm-diameter, 6061-T6511 aluminum targets. To show the effect of projectile strength, we used projectiles that had average Rockwell harnesses of R{sub c} = 36.6, 39.5, and 46.2. A powder gun and two-stage, light-gas guns launched the 0.023 kg projectiles at striking velocities between 0.5 and 3.0 km/s. Post-test radiographs of the targets showed three response regions as striking velocities increased: (1) the projectiles remained visibly undeformed, (2) the projectiles permanently deformed without erosion, and (3) the projectiles eroded and lost mass. To show the effect of projectile strength, we compared depth-of-penetration data as a function of striking velocity for spherical-nose rods with three Rockwell harnesses at striking velocities ranging from 0.5 to 3.0 km/s. To show the effect of nose shape, we compared penetration data for the spherical-nose projectiles with previously published data for ogive-nose projectiles.

Characteristics of debris clouds produced by hypervelocity impact of aluminum spheres with thin aluminum plates

Abstract Debris clouds produced by the normal impact of aluminum spheres with aluminum bumper plates are shown to consist of an ejecta veil, an external bubble of debris, and a significant internal structure composed of three distinct elements. Effects of variations in bumper-plate thickness, sphere diameter, and impact velocity on the shape and velocity of the elements of the internal structure are described and compared. Three alloys of bumper material and several diameters of 2017-T4 aluminum spheres, ranging from 6.35 mm to 12.70 mm, were used in the tests described in this paper. Test results were sorted into two sets. In the first set, impact velocity was held constant at 6.7 km/s and the bumper-thickness-to-projectile-diameter ratio, t/D, varied from 0.026 to 0.424. In the second set, t/D ratio was held constant at 0.049 and the impact velocity varied from 3.77 km/s to 7.23 km/s. In both sets of test results, debris-cloud properties are shown to scale with projectile diameter. Characteristics of the front element of the debris-cloud internal structure are shown to be sensitive to changes in t/D ratio and impact velocity. A model for the formation of this front element is presented and used to develop a description of a debris cloud consisting of material in the solid-liquid and/or liquid-vapor phases.

Penetration of confined aluminum nitride targets by tungsten long rods at 1.5–4.5 km/s

Abstract A series of 26 terminal ballistics experiments was performed to measure the penetration of simple confined aluminum nitride targets by a long tungsten rod. Impact velocities ranged from 1.5 to about 4.5 km/s. The experiments were performed in the reverse ballistic mode using a two-stage light-gas gun. Penetrator diameter, D, was 0.762 mm (0.030 in). The length-to-diameter ratio for the penetrator was L D = 20 for nearly all the tests and never less than L D = 15 . Primary instrumentation for these experiments was four independently timed, 450 kV flash X-rays. These X-rays provided four views of the penetrator-target interaction during the penetration event from which the following data were determined: p = penetration depth as a function of time, Lr = remaining length of penetrator as a function of time, as well as final penetration depth, target hole geometry, spatial distribution of the eroded rod material, etc. From these data, u = d p d t = speed of penetration into the target and ν c = d (L−L r ) d t = speed of “consumption” of the long rod were obtained.

Debris clouds generated by hypervelocity impact of cylindrical projectiles with thin aluminum plates

Abstract Orthogonal, flash x rays were used to observe the debris clouds produced by the hypervelocity impact of cylindrical aluminum projectiles with thin aluminum sheets or bumpers. Three major structural features were observed in the debris clouds--a front cone, a bulbous main debris cloud, and an inner cone. Inclination of the projectile at impact changed the orientation of these features and the severity of damage to the rear wall of a double-sheet structure; projectiles with the greatest inclination produced the most damage. Two experiments, using aluminum and copper as projectile and target or target and projectile, respectively, were performed to determine the source of material in each of the three structural features of the debris clouds. The front cone and main cloud were shown to consist of bumper debris while the inner cone was composed of projectile fragments.

A simple dynamic model for the formation of debris clouds

Abstract A simple model for describing the motion of material in a debris cloud is presented. Motion and distribution of this material are described using three axial velocities, one radial velocity, and the diameter of the projectile. Results of hypervelocity impact tests using copper projectiles and aluminum bumpers are presented. Data from these tests were used to verify several assumptions integral to the development of the model. A method for approximating the pressure-loading history applied to the interior wall of a double sheet structure is also presented.

Fragmentation of a sphere initiated by hypervelocity impact with a thin sheet

Abstract Selected results of tests in which 9.53-mm-diameter, 2017-T4 aluminum spheres impacted 0.25-mm- to 4.80-mm-thick, 6061-T6 aluminum sheets are presented. Impact velocities for these tests ranged from 1.98 km/s to 7.38 km/s. Flash x-rays were used to view the debris clouds produced by the impacts. As impact velocity was increased, failure of the aluminum sphere progressed through the following stages of fracture and fragmentation: (1) formation of a spall failure at its rear surface, (2) development of a detached shell of spall fragments, and (3) complete disintegration of the sphere. The threshold impact velocity for development of the spall failure in the sphere was observed to be a function of the bumper-thickness-to-projectile-diameter ratio ( t D ), and to increase as the t D ratio decreased. When the debris cloud was fully developed, the disintegrated projectile formed its dominant feature--an internal structure, composed of a front, center, and rear element, located at the front of the debris cloud. The front element was small and consisted of finely-divided projectile and bumper material. The bulk of the fragmented projectile was contained in the center element, a disc-like structure made up of a large central fragment surrounded by numerous smaller fragments. A shell of fragments, spalled from the rear of the sphere, formed the rear element. Radiographs of the debris clouds were analyzed to determine the size and size distribution of certain fragments within the cloud. The size of the large fragment was shown to be dependent on impact velocity and t D ratio. The smaller fragments in the center element were several times larger than the fragments in the shell of spall fragments forming the rear element. Detailed analyses of fragments in the shell of spall fragments were made. The analyses indicated their median Martins statistical diameter exhibited an orderly dependence on impact velocity and t D ratio.

Effects of scale on debris cloud properties

Abstract Results of tests using various thicknesses of 6061-T6 aluminum sheet and 6.35-, 9.53-, 12.70-, and 15.88-mm-diameter, 2017-T4 aluminum spheres are described. Impact velocities for these tests ranged from 3.77 to 7.38 km/s. Multiple-exposure, orthogonal-pair, flash radiographs of the debris clouds produced by the impacts were analyzed to provide quantitative data which described the size and velocity of a number of characteristic morphologic features in the debris clouds and the sizes and size distributions of fragments in the structural elements of the debris cloud.The axial and diametral velocities of these morphologic features were shown to be the same, regardless of sphere diameter, when debris clouds produced by impacts with similar bumper-thickness-to-projectile-diameter ratios and impact velocities were compared. As a result, the dimensions of these debris clouds differed only by the differences in the diameters of the spheres that produced them.An analyses of fragment sizes showed that the equivalent diameter of the large projectile fragment along the center line of the debris cloud scaled with projectile diameter; the dimensions of fragments forming the shell of spall fragments at the rear of the debris cloud did not scale with projectile diameter. The large central fragment appeared to originate from near the center of the sphere and was a part of the sphere which remained intact after all processes that worked to reduce the size of the sphere were complete. Formation of spall-shell fragments was a shock-related process which was sensitive to rate effects and other material properties that did not scale.

Penetration dynamics of rods from direct ballistic tests of advanced armor components at 2–3 km/s

Abstract The penetration of semi-infinite steel and spaced-plate armors by continuous and segmented rods has been analyzed and measured by direct ballistic tests, hydrocode calculations, and hydrodynamic models at velocities from 2 to 4 km/s. An empirical equation of rod penetration in semi-infinite steel was formulated from hydrodynamic models of rod impact. Penetrations predicted by the equation agreed well with measured values. Increasing the spacing between segments from one to two diameters increased the penetration significantly (∼20%). Structures to support and align the segments can either increase or decrease the penetration, depending on their design. The relative penetrations of continuous and segmented rods depend on the parameters selected for the comparison: the segmented rod having greater penetration for equal mass and diameter and vice versa for equal mass and length. Tests of segmented rods penetrating spaced-plate armor showed that the armor is defeated by the front segment (or segments) punching a hole in the front plate (or plates) that allows the remaining segmented rod through intact to attack the main armor.