Dennis L. Orphal

On the hydrodynamic approximation for long-rod penetration

Steady-state hydrodynamic theory, or variations thereof, has been applied to long-rod penetration since the 1940s. It is generally believed that projectile strength is of little consequence at high velocities, and that hydrodynamic theory is applicable to long-rod penetration when penetration pressures are much greater than the target flow stress. Substantiating this belief is the observation that at approximately 2.5 km/s, for tungsten alloy projectiles into armor steel, normalized penetration (P/L) nominally saturates to the classical hydrodynamic limit of the square root of the ratio of the projectile to target densities. Experimental data herein, however, show penetration velocities and instantaneous penetration efficiencies fall below that expected from hydrodynamic theory, even at impact velocities as high as 4.0 km/s. Numerical simulations, using appropriate strength values, are in excellent agreement with the experimental data. Parametric studies demonstrate that both projectile and target strength have a measurable effect even at such high impact velocities.

The influence of experimental design on depth-of-penetration (DOP) test results and derived ballistic efficiencies

Experimental data for ceramic armor materials from two test methods, small-scale reverse ballistic tests and depth-of-penetration (DOP) tests, are reviewed and compared. Results from reverse ballistic tests can be used to estimate the length of rod erosion in the ceramic tiles of DOP tests. The outcome of a given DOP test can then be predicted by using recently published data bases on RHA penetration to determine the residual penetration into the steel back-up of the DOP test. Results of this methodology, compared to experimental DOP-test results, agree reasonably well for aluminum nitride and silicon carbide, even though scale sizes, impact velocities and experimental procedures varied considerably between investigators. The methodology was then applied to single-valued performance criteria for ceramic armor materials, for example, mass efficiency. This analysis demonstrates that in certain cases, test parameters, like the ratio of penetrator length to ceramic tile thickness, affect test results considerably more than differences between ceramic types. Thus, DOP tests must be properly designed and interpreted in order to assess correctly the ballistic performance of ceramics.

Target damage from highly oblique hypervelocity impacts of steel spheres against thin laminated targets

Target hole sizes and geometries were measured for a series of highly oblique hypervelocity impacts of steel spheres against thin laminated targets. The impact velocity was nominally 4.6 km/s for most of the experiments with a few tests conducted at 7.3 km/s. Impact obliquity ranged from 60° to 80° from the normal to the target plane. Projectiles were stainless steel spheres with masses of 222 g, 25 g, and 1 g. Targets were laminated MX-2600 silica phenolic bonded to a 2024-T3 substrate. Target thickness, t, was varied to give thickness to projectile diameter, d, ratios of t/d = 0.6 and 0.3 for each projectile. CTH Eulerian wavecode calculations of selected tests were performed to improve our understanding of the experimental results.

Ballistic Response of Fabrics: Model and Experiments

Walker [1] developed an analytical model for the dynamic response of fabrics to ballistic impact. In the model the force on the bullet is a function of fabric displacement (h) along the axis of impact and the radius (R) of the fabric deformation or ‘bulge”. Ballistic tests against Zylon™ fabric have been performed to measure h and R as a function of time. The results of these experiments are presented and analyzed in the context of the Walker model.

Failure Wave in DEDF and Soda-Lime Glass During Rod Impact

Investigations of glass by planar, and classical and symmetric Taylor impact experiments reveal that failure wave velocity vF depends on impact velocity, geometry, and type of glass. vF typically increases with impact velocity vP to between cS and cL or to √2cS (shear and longitudinal wave velocity). This paper reports initial results of an investigation of failure waves associated with gold rod impact on high‐density (DEDF) glass and soda‐lime glass. Data are obtained by visualizing simultaneously the failure propagation in the glass with a high‐speed camera and the rod penetration velocity u with flash radiography. Results for DEDF glass are reported for vP between 1.2 and 2.0 km/s, those for soda‐lime glass with vP ≈1.3 km/s. It is shown that vF > u, and that in the case of DEDF glass vF/u decreases from ; 1.38 to 1.13 with increasing vp. In addition, several Taylor tests were performed. For both DEDF and soda‐lime glass the vF‐values, found here as well as vF‐ data reported in the literature, reveal t...

FAILURE KINETICS IN BOROSILICATE GLASS DURING ROD IMPACT

Failure front (FF) and penetration velocity have been measured for long gold rods impacting and penetrating borosilicate (BS) glass. Data are obtained by visualizing simultaneously FF propagation with a high speed camera and rod penetration with flash X‐rays. Results for BS glass are qualitatively similar to those of DEDF (PbO) glass. FF velocity rapidly decreases from an initial value to a lower, approximately constant value. FF velocity increases with impact velocity, vp. The FF velocity remains significantly lower than the shear velocity, even at the highest impact velocity tested, about 2.5 km/s. The ratio of the FF velocity to the rod penetration velocity, vF/u, decreases with increasing vp and appears to be approaching vF/u = 1 asymptotically, as observed previously for DEDF glass. The separation of the FF and the tip of the rod decreases with increasing impact velocity. Importantly, since vF/u⩾1, the gold rod is always penetrating glass behind the FF.

SOME RECENT RESULTS ON THE PROPAGATION OF THE FAILURE FRONT ASSOCIATED WITH ROD PENETRATION OF BOROSILICATE GLASS

Recent results obtained regarding the failure front (FF) associated with rod penetration of borosilicate glass (BS) are reviewed. Key results are: propagation velocity of the FF, vF, decreases rapidly after impact to an approximately constant value; vF increases with rod impact velocity, vP; vF ≪shear wave velocity; vF/u≥1 and decreases with increasing vP, where u is the rod penetration velocity; vF/u appears to asymptotically approach unity with increasing vP; Taylor impact with stress state between 1D strain and stress and an impact stress in the range of the Bernoulli stress for the rod experiments, show vF(Taylor)≫vF(rod); the FF ceases to propagate a short time after the rod fully erodes and thus the stress at the rod‐glass interface goes to zero (this means the FF does not obey the classical wave equation); the cessation of the FF is abrupt, not gradual as would be expected for a diffusion phenomenon; the time delay between full rod erosion and halting of the FF is consistent with a “communication s...