Scott A. Mullin

Debris cloud dynamics

Abstract The hypervelocity impact of a projectile upon a thin metal plate and subsequent formation of back-surface debris is reviewed. At sufficiently high impact velocities, roughly greater than 3.0 km/s (depending upon the shock impedances of the materials involved), shock formation and interaction dominate and control the overall response of both the projectile and the target plate. We focus upon the importance of shock heating, melting, and vaporization in this application. Because of the complexity of the physical interactions, numerical simulation of such problems is necessary to draw quantitative conclusions. Thus, we also assess the current status of computational modeling of this kind of impact event, specifically addressing recent work bearing on the sensitivity of such modeling to the equations of state and certain numerical issues.

Computer simulation of strain-rate effects in replica scale model penetration experiments

Abstract : A computational study has been performed to quantify the effects of strain rate on replica-model (scaled) experiments of penetration and perforation. The impact of a tungsten-alloy long-rod projectile into an armor steel target at 1.5 km/s was investigated. It was found that over a scale factor of 10, strain-rate effects change the depth of penetration, for semi-infinite targets, and the residual velocity and length of the projectile, for finite- thickness targets, on the order of 5%. Although not modeled explicitly in the present study, the time-dependence of damage was examined. Damage accumulation is a strong function of absolute time, not scaled time. At homologous times, a smaller scale will have less accumulated damage than a larger scale; therefore, the smaller scale will appear stronger, particularly in situations where the details of damage evolution are important.

A hypervelocity fragment launcher based on an inhibited shaped charge

Abstract A hypervelocity fragment launcher based on an inhibited shaped charge was developed, which launches a 0.5–1.0 g aluminum fragment at 11.2±0.2 km/s . Experimental and computational work performed during its development are presented. The launched fragment is characterized by in-flight flash radiography and impact crater examination.

SCALE MODEL EXPERIMENTS WITH CERAMIC LAMINATE TARGETS

Abstract Ballistic impact experiments were performed on ceramic laminate targets at three scale sizes, nominally 1 3 , 1 6 , and 1 12 , to quantify the effects of scale on various responses, in particular, the ballistic limit velocity. The experiments were carefully designed and controlled so that the different scale sizes were high fidelity replicas of each other. A variety of responses, such as residual projectile quantities, hole size, and the extent of bulging, were measured. Some of the measured quantities showed little or no dependence on scale size, whereas other quantities, particularly the ballistic limit velocity, were found to vary with scale size. The percentage difference was quantified, and the results extrapolated to estimate full-scale response from the subscale tests.

Scale modeling of hypervelocity impact

Abstract The significance of different Pi terms which result from a dimensional analysis of the parameters involved in hypervelocity impact is discussed. The consequences of distorting some physical phenomena in models are analyzed, and experimental verification is presented for the use of replica and dissimilar material models.

Experimental impacts above 10 km/s

Abstract In the proceedings of the last symposium, recent work on a technique for launching small projectiles to hypervelocities above 10 km/s using an inhibited shaped charge was presented [1]. In the interim, experiments have been conducted using the inhibited shaped charge to launch aluminum, nickel, and molybdenum projectiles. This paper presents the results of the impact tests, as well as discusses the shaped charge design modifications for the nickel and molybdenum launchers. Radiographs are presented of the impacting projectiles, as are post test photographs of various targets. The data are unique in that they represent low L D projectile impacts into both monolithic blocks and spaced plates at velocities above 10 km/s. The aluminum projectiles are being launched at 11.25±0.20 km/s, the molybdenum projectiles at 11.72±0.10 km/s, and the nickel projectiles at 10.81±0.10 km/s.

Dependence of debris cloud formation on projectile shape

A two‐stage lights‐gas gun has been used to impact thin zinc bumpers by zinc projectiles over the velocity range of 2.4 km/s to 6.7 km/s to determine the propagation characteristics of the impact generated debris. Constant‐mass projectiles in the form of spheres, discs, cylinders, and rods were used in these studies. Radiographic techniques were employed to record the debris cloud generated upon impact and the dynamic formation of the resulting rupture in an aluminum backing plate resulting from the loading of the debris cloud. The characteristics of the debris cloud generated upon impact is found to depend on the projectile shape. The data indicate that the debris front velocity is independent of the shape of the projectile, whereas the debris lateral/radial velocity is strongly dependent on projectile geometry. Spherical impactors generate the most radially dispersed debris cloud while the normal plate impactors result in column‐like debris. It has been observed that the debris generated by the impact o...

Bursting of shielded pressure vessels subject to hypervelocity impact

Abstract During the 30-year lifetime of the Space Station, NASA is concerned that a large piece of orbital debris could strike one of the inhabited or laboratory modules. The modules are basically cylindrical pressure vessels, 4.3 meters in diameter and 9.1 meters long, made of Al 2219-T87. There is a potential for unstable crack growth (“unzipping”) in these pressure vessels if a sufficiently-long crack were formed in the pressure vessel wall. The ragged hole generated when debris strikes an exterior shield and impulsively loads the pressure vessel wall could lead to such a crack. The central concern of this research is quantifying the minimum crack length (critical crack length) to initiate unstable crack growth. This paper reports on a two-part investigation into this problem: 1) fracture experiments and analyses aimed at determining the fracture resistance and critical crack length of the module walls, and 2) examination of impact data to determine the impact conditions that could cause the critical crack length to be exceeded. Al 2219-T87 was found to be a modestly rated sensitive material, exhibiting an increase in both ultimate strength and fracture toughness at high strain rates. The results of the conservative linear elastic fracture mechanics analyses indicate critical cracks at least 22.9 cm in length are required for unzipping (3.17-mm thick wall), and 45.7-cm length (for 4.83-mm thick wall). The dynamic analysis results indicate that the critical crack lengths are even longer, about 48.3 to 61.0 cm in length. Examination of the rather limited experimental database indicates that the dynamic analysis values are more realistic, and that under certain conditions of projectile size, wall stress, and shield design the critical crack length can be exceeded.

Computational simulations of experimental impact data obtained at 7 to 11 km/s with aluminum and zinc

A combined experimental/computational program was conducted to asses the physical characteristics of impacts at speeds above those attainable using conventional light‐gas guns, but within the realm of impact encounters in space, where velocities can range from 7 to 15 km/s. A major goal of the program was to assess the capability of state‐of‐the‐art hydrocodes to reproduce damage and loading characteristics seen in hypervelocity impacts that produce diffuse molten or vaporous debris clouds. In this study the Eulerian hydrocode CTH was used. Comparisons have been made to experiments conducted with aluminium projectiles traveling from 7.4 to 11.4 km/s. A three‐stage light‐gas gun launcher was used to launch 17×1 mm aluminum disks, and an inhibited shaped charge launcher was used to launch 20×5 mm aluminum rods. Additional experiments were conducted at velocites from 3.5 to 6.6 km/s using zinc. In that range, scale modeling analysis predicts that a response similar to aluminum impacting at roughly twice the ...

Verification and validation of a penetration model for the design of a blast containment vessel part I: Validation experiments

Model verification and validation (V&V) provides a mechanism to develop computational models that can be used to make engineering predictions and decisions with quantified confidence. Model V&V procedures are needed to reduce the time, cost and risk associated with component and full-scale testing of products, materials and engineered systems. The Los Alamos National Laboratory Dynamic Experimentation (DynEx) program is designing and validating steel blast containment vessels using limited experiments coupled with computational models. This paper describes the testing program for the validation experiments in support of a verification and validation process for an analytical and computational model used to predict the penetration depth of explosively released fragments into the containment vessel structure. The V&V process is described as well as pre-test analytic modeling and validation experiments. Uncertainties in the experiments that may influence model validation are discussed from an uncertainty quantification perspective since there are inherent and subjective uncertainties in the model that must be correlated with the uncertainties from the experiments.