Thomas L. Warren

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.

Effects of strain hardening and strain-rate sensitivity on the penetration of aluminum targets with spherical-nosed rods

Abstract We present penetration equations for rigid, spherical-nosed rods that penetrate 6061-T651 aluminum targets. The penetration models use the spherical cavity-expansion approximation and constitutive equations for the target that include strain hardening and strain-rate sensitivity. We obtained closed-form penetration equations for an incompressible target material; however, predictions for a compressible target require the numerical solution of a coupled set of nonlinear ordinary differential equations. Numerical results show the effects of compressibility, strain hardening, and strain-rate sensitivity. We also show that our penetration model requires compressibility, strain hardening, and strain-rate sensitivity to obtain good agreement with previously published depth of penetration data for striking velocities between 300 and 1200 m/s.

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 into low-strength (23 MPa) concrete: target characterization and simulations

A combined experimental, analytical, and computational research and development program investigates the penetration of steel projectiles into low-strength concrete. Laboratory-scale material property tests conducted at the US Army Waterways Experiment Station on the concrete provide the data used in parameter estimation for a geomaterial constitutive model. The experiments and the model are described as well as the procedure used to fit the material model to the experimental data. The model accurately reproduces the data and predicts experimental results not used in the evaluation of model constants. The model, used in conjunction with an explicit transient dynamic finite element code, accurately predicts deceleration and depth of penetration of 3 CRH ogive-nosed 4340 steel penetrators.

Random Cantor set models for the elastic-perfectly plastic contact of rough surfaces

The objective of this study was to formulate discrete and continuous spatial models to describe the elastic-perfectly plastic deformation of two rough surfaces in contact. The two surfaces in contact are assumed to exhibit fractal behavior and are modeled as an effective fractal surface compressed into a smooth rigid substrate. The rough self-affine fractal structure of the effective surface is approximated by a random Cantor set representation embedded in two dimensions. Both of the proposed models admit analytical solutions whether the plastic deformation is volume conserving or not. Presented results illustrate the effects that volume conservation and initial surface structure have on the elastic-perfectly plastic deformation process. The results from the continuous model are compared with the results obtained from the discrete model, and existing experimental load displacement data for the deformation of a bead-blasted steel surface.

Simulations of the Penetration of 6061-T6511 Aluminum Targets by Spherical-Nosed VAR 4340 Steel Projectiles

In certain penetration events it is proposed that the primary mode of deformation of the target can be approximated by known analytical expressions. In the context of an analysis code, this approximation eliminates the need for discretizing the target as well as the need for a contact algorithm. Thus, this method substantially reduces the computer time and memory requirements. In this paper a forcing function which is derived from a spherical-cavity expansion (SCE) analysis has been implemented in a transient dynamic finite element code. This irnplementation is capable of computing the structural and component responses of a projectile due to a three dimensional penetration event. Simulations are presented for 7.1 l-mm-diameter, 74.7-mm-long, spherical-nose, vacuum- arc-remelted (VAR) 4340 steel projectiles that penetrate 6061-T6511 aluminum targets. Final projectile configurations obtained from the simulations are compared with post-test radiographs obtained from the corresponding experiments. It is shown that the simulations accurately predict the permanent projectile deformation for three dimensional loadings due to incident pitch and yaw over a wide range of striking velocities.

Simulations of the penetration of limestone targets by ogive-nose 4340 steel projectiles

In this paper we extend Forrestals semi-empirical penetration method for limestone targets to account for pitch, yaw and projectile deformation. This is achieved using a combined analytical and computational technique we have developed to calculate permanent projectile deformation without erosion. With this technique we use an explicit transient dynamic finite element code to model the projectile, and an analytical forcing function based on the dynamic expansion of a spherical cavity derived from Forrestals depth of penetration equation to represent the target. Additionally, this work extends the forcing function methodology by introducing a successive layering technique to account for the loss of confinement due to entrance cratering effects. Results from simulations are compared with the corresponding experimental results and shown to be in good agreement. Furthermore, it is shown that in many of the events the projectile goes through significantly more deformation than what is observed from its post-test configuration.

A fractal model for the static coefficient of friction at the fiber-matrix interface

Abstract The physical, geometrical, and mechanical properties at the fiber/matrix interface of a fiber-reinforced composite material have a dominant effect on the overall mechanical behavior of these materials. Specifically, the toughening of these materials is largely attributed to the energy dissipation due to the frictional sliding of fibers at their interface with the matrix material. The micromechanisms involved with interfacial failure and sliding are currently not entirely understood, and the failure threshold is generally predicted using macro-scale friction laws which neglect the micromechanical aspects. The objective of this study is to explore the derivation of a macro-scale static coefficient of friction at the interface of a previously debonded fiber based on the micro-scale properties of the contacting surfaces. Presented results illustrate that the macro-scale static coefficient of friction obtained from the proposed micro-scale model is independent of the normal load and is therefore consistent with the classical Amontons-Coulomb phenomenological laws of friction.

Perforation of Aluminum Plates with 7.62 APM2 Bullets

We conducted an experimental analytical study to understand the mechanisms and dominant parameters for 7.62mm APM2 bullets that perforate 6082-T651 aluminum plates at oblique impacts. Tests were conducted with the full bullet and the hard core only to show that the hard core dominates the perforation process. Models show good agreement with measured residual and ballistic-limit velocities.