Hypervelocity Gouging Research Articles

Vibration and hypervelocity gouging of sliding pairs in electromagnetic railguns

Hypervelocity gouging has been observed over several decades in engineering applications involving rocket sled, electromagnetic railgun and light-gas gun. In this paper, numerical simulations using explicit finite element method were carried out to investigate the gouging mechanism in railguns. Two models were constructed and discussed: one is the non-uniform contact model under magnetic pressure, and the other is the transverse swing impact model. The former shows that, the non-uniform contact under the action of magnetic pressure will not cause gouging, but will form grooves, although the armature slides parallel to the rail at a high speed. The latter shows that, the lateral swing of the armature at a high speed can lead to oblique impacts on the interface, forming pressure cores, and eventually gouging.

The Influence of Material Matching and Rail Irregularities on Hypervelocity Gouging

Gouging is a phenomenon of large non-linear deformations which may lead a permanent damage on a rocket-sled system. To analyze how gouging occurs and tracing of its development with different matching and rail irregularities, the influences of material matching and rail irregularities on hypervelocity gouging are studied by MPM algorithm in this paper. 5 sets of material matching and 5 set of rail irregularities cases have been simulated in this paper. The results show that gouges would be easier to occur with a lower yield strength material or a rail with irregularities. These results can help us find the methods to avoid gouging.

The Study of Hypervelocity Gouging Based on the Material Point Method

Gouging is a large nonlinear deformation phenomenon experienced in rail materials under severe loading conditions such as the tremendous heating caused by frictional effects at the slider/rail interface. The material point method (MPM) algorithm is designed specifically to solve such nonlinear, large deformation problems. The theories and steps of the MPM algorithm are described in this paper, and an investigation of hypervelocity gouging is conducted using the MPM algorithm. Because of frictional heating near the contact region, the effects of temperature on the gouge development are evaluated by several developed thermal environment scenarios. Moreover, the regular patterns of gouging at various impact velocities, horizontal velocities, and friction coefficients are analyzed. The results show that the MPM algorithm can successfully simulate the gouging phenomenon. With the gouge initiation and development, the temperature rapidly increases to the melting temperature, and the yield stress rapidly decreases at the gouge location. The regular patterns of gouging provide a new effective method for antigouging research.

The effect of surface indentations on gouging in railguns

Hypervelocity gouging occurs in high speed sliding systems such as rocket sled test tracks, light gas guns, and electromagnetic railguns. Gouging takes the form of teardrop-shaped craters on the rail surface, and occurs above a threshold speed which is dependent on the slider and rail materials. In this study, gouging of aluminum railgun armatures on flat and indented copper rails was performed to examine the effect of macroscopic surface defects on gouge onset velocity and morphology. Both galling and gouge craters were shown to initiate at existing macroscopic and microscopic defects. Macroscopically flat rail surfaces provide no practical advantage over indented surfaces insofar as gouge onset velocity is concerned. However, the shape of the resulting gouge craters can be significantly affected.

Sliding Instabilities and Hypervelocity Gouging

Hypervelocity gouging occurs in material systems with high sliding speeds and has been observed in rocket sleds, light-gas guns, and railguns. Gouging takes the form of teardrop-shaped craters on the rail surface and usually occurs above a threshold speed that is material-pair dependent. The best predictive tools developed for gouging rely on semiempirical relationships that relate material hardness to shock pressure and hydrocodes that model the fluid-like behavior of materials under high stresses and strain rates. A parallel field of research is explosive welding. In explosive welding, the interface between two sliding surfaces impacted at high speeds undergoes a transition between flat bonded surfaces to a wavy interface. The onset of these interface waves, like hypervelocity gouging, is primarily dependent on the relative sliding speed of the materials, with other operating conditions being second order. Of particular interest is the fact that gouging and explosive welding waves occur at similar speeds for similar material pairs. This paper explores the parallels between these two fields of research and assesses the applicability of the respective predictive tools in determining the onset of gouging/interface waves.

The Effects of Surface Coatings on the Onset of Rail Gouging

In solid-armature railguns, hypervelocity gouging damage can significantly limit the useful life of rails. Most solutions to this problem have involved the use of hard rail materials, which tend to be poor electrical conductors. However, it has also been observed that precoating the conductors of a railgun can significantly delay the onset of gouging. This is usually accomplished by launching several aluminum-alloy armatures on a set of rails below the gouging threshold. When an aluminum-alloy armature is launched at high speeds on copper-alloy rails, a thin (tens of microns) layer of aluminum is deposited on the surface of the rail. Subsequent tests at higher speeds often result in the delay or absence of gouging at speeds where it is normally expected to occur. However, this effect is not particularly robust, since gouging can still occur with sufficient lateral loads. In the experiments reported here, tests were conducted on UNS C15725 (Glidcop Al-25) copper rails to examine the effect of a controlled precoating applied with an electroplating process. Aluminum thicknesses of 2, 5, 25, and 50 μm were tested, providing a well-characterized aluminum layer as opposed to the rough variable-thickness layer typically deposited with an aluminum armature. Results of the experiments and analysis are reported in this paper.

Analysis and simulation of hypervelocity gouging impacts for a high speed sled test

Abstract An analysis and simulation of the gouging impact phenomenon which occurs at the Holloman Air Force Base High Speed Test Track (HHSTT) during hypervelocity impact testing is presented. Simulations of the sled/rail interactions were conducted using the hydrocode, CTH. These simulations utilize the most accurate and validated material models for the sled shoes (VascoMax 300) and rail (1080 steel) – which were recently developed. Sled shoe impacts with the rail were evaluated using various geometries possible in the field. The conditions leading to hypervelocity gouging were identified, as well as the condition which resulted in rail wear. The CTH simulations match results observed in the field extremely well. Recommendations are made, based on the latest material models and simulations, which should significantly reduce the occurrences of hypervelocity gouging at the HHSTT.

Investigation of a Scaled Hypervelocity Gouging Model and Validation of Material Constitutive Models

In an effort to improve the accuracy and validate the performance of a hypervelocity gouging model developed for the shock-wave physics code CTH, the physical dimensionality of an impact problem at the Holloman Air Force Base High-Speed Test Track is mathematically scaled. The resulting scaled impact problem is sized for laboratory experimental testing and modeled. The contact schemes and approaches to friction available in CTH are summarized. In addition, results of a hypervelocity laboratory gouging experiment are presented. CTH is used to model these experimental tests, and the modeling efforts are validated. These efforts enhance the ability of a previously developed full-scale gouging model to be improved and validated.

Investigation of Damage to Solid-Armature Railguns at Startup

This paper describes work investigating a rail damage mechanism observed in solid-armature railguns at the Institute for Advanced Technology, The University of Texas at Austin. The damage occurs in the starting section of rails and is not associated with high-speed phenomena such as hypervelocity gouging or transition to arcing contact. The damage, which we call grooving, is localized to the region of the rails adjacent to the insulators. In this paper, we describe grooving damage observed in multiple tests using copper rails. In one series, in which we tested up to 20 shots on one pair of copper rails, we obtained grooves on the order of a millimeter deep and several millimeters wide. We present evidence that grooving is caused by liquid erosion and is not a result of plasma heating or mechanical deformation

Further validation of a general approximation for impact penetration depth considering hypervelocity gouging data

A first-order approximation of penetration depth is developed for use in engineering design. A survey of available penetration data is used to construct a one-dimensional approach for estimating the geometry of a crater resulting from high-energy impact. The results are generalized to allow approximations to be made using existing experimental data without the requirement for laboratory testing. This approach for penetration depth approximation is validated using the hypervelocity gouging data from the Holloman High Speed Test Track (HHSTT) and hypervelocity gouging impact tests conducted by the authors.

Structural Dynamic Considerations for a Hydrocode Analysis of Hypervelocity Test Sled Impacts

Numerical analysis of a hypervelocity test sled configuration has been accomplished with structural dynamic analysis and hydrocode analysis for investigating hypervelocity gouging. Previous studies have considered these two aspects of the system primarily apart from each other. It is desired to obtain numerical analysis of hypervelocity gouging that applies directly to a given test sled configuration. An improved analysis may be obtained for this purpose by applying structural dynamic considerations to the hydrocode analysis of slipper impacts on a rail. The primary objective of improving the input model for the hydrocode analysis is to simulate actual field conditions, such as sliding impacts with rail misalignments and on coated rails. The CTH hydrocode input model is adjusted by the addition of an artificial mass to simulate the test sled mass. Three-dimensional impacts are considered in the context of two-dimensional plane strain conditions. Shock reflections are also considered, and the results for sliding impact with a rail misalignment are discussed. The numerical results indicate characteristics seen in field gouges such as jetting and high-temperature effects.

Metallographic Examination and Validation of Thermal Effects in Hypervelocity Gouging

In this work, a gouged section of 1080 railroad rail steel is examined using metallographic techniques to characterize the nature of the damage. The gouging was performed by a rocket sled at Holloman Air Force Base, riding on VascoMax 300 steel shoes at 2.1km∕s. The damaged rail is evaluated in detail to examine material phase changes, shear bands, and heat effects. The results are compared to samples of the virgin material, machined and prepared exactly as they are prior to the Holloman AFB High Speed Test Track (HHSTT) runs. The gouged section was examined using optical microscopy, scanning electron microscope (SEM), and other techniques. The resulting microstructure is presented and compared to the virgin material. Material mixing, shear band formation, and significant thermal damage, consistent with a high energy gouging event, are confirmed. In addition, the material phase change evident in this approach allows us to estimate the thermal conditions present during the formation of the gouge. This thermal history establishes a profile by which related research in gouge simulation can be validated against. A one-dimensional heat conduction model is presented that validates the cooling rates required to generate the presented microstructure.

Scaling numerical models for hypervelocity test sled slipper-rail impacts

Hypervelocity test sled slipper-rail impacts have been simulated numerically using the finite volume hydrocode, ChartD to the Three-Halves (CTH). This study addresses the difficulties of applying CTH model solutions to real test sled runs. Past CTH models using dimensions different than actual test sleds have been used to study phenomenological aspects of the problem. However, quantitative results from the CTH model solution do not apply directly to actual test sled runs due to strain rate effects and time scale differences. The Buckingham Pi Theorem is applied to two potential hypervelocity gouging models. Validity of the invariant products is tested using sample CTH hypervelocity gouging models that are scaled up to simulate dimensions of a real test sled. Real test sled dimensions are desired in order to more closely simulate actual test sled runs. Invariant products developed from application of the Buckingham Pi Theorem can be used as guidelines for determining whether a CTH model is applicable to a test sled with specific dimensions. Strain rate effects are investigated to study whether deviations between scaled CTH models may be reduced by modifying the constitutive model.

Effect of Temperature on the Process of Hypervelocity Gouging

Effect of Temperature on the Process of Hypervelocity Gouging

Effect of Temperature on the Process of Hypervelocity Gouging

The sliding interface of a flyer/target system is subject to immense forces caused by dynamic loads and impact of the flyer with the target. In addition, tremendous heating caused by aerodynamic and frictional effects is produced at the interface. The buildup of the temperature caused by external heat sources is important for an understanding of any impact environment between the flyer and the target. Under these severe loading conditions the material in both the flyer and the target will experience large unstable nonlinear deformations known as gouging. To successfully model the gouging phenomenon, the tangential impact and thermal heating caused by aerodynamic flow and frictional heating must be properly handled. Previous gouging analysis has not accounted for thermal profiles present in the system prior to impact. The hydrocode CTH is employed to solve the gouging problem through the proper modeling of the applicable physics. Results of simulated gouges at elevated temperatures are presented along with a discussion of the mechanics involved and the influence of elevated temperature states.

Experiments to measure gouging threshold velocity for various metals against copper

Hypervelocity gouging is a form of damage that can occur to surfaces in sliding contact at high relative velocity. Gouges, which are in the form of teardrop-shaped craters, have been observed on rocket sled tracks, in light gas gun barrels, and in the bore of railguns. One aspect of gouging that has not been adequately explained is the existence of a minimum velocity (or threshold velocity) for a given material pair below which gouging does not occur. This paper reports a series of experiments to test the hypothesis that the onset of gouging is governed by the hardness of the harder material and by the density and sound speed of both materials. In the tests, samples of various metals were accelerated to 2.2 km/s while in direct sliding contact with CD110 copper rails. The samples were carried in a Lexan polycarbonate forebody, modified to apply normal loads of 40-80 MPa to the sample/rail interface. The portion of the armature directly in line with the samples was cut away to avoid contaminating the gouge track. Visual inspection of the resulting gouges was used to establish a gouging threshold for each metal. The tests were conducted in the 40 mm square bore electromagnetic launcher at the Institute for Advanced Technology (IAT). Metals tested include AISI 1015 steel, silver, molybdenum, pure copper, tungsten, nickel, magnesium, and 7075 aluminum alloy. The results of the experiments show the existence of a straight line fit between hardness of the harder material and the shock pressure for a normal collision at the gouging threshold velocity.

Post-test characterization of the hardness and microstructure of copper rails from a 9 MJ electromagnetic launcher

Following a test series consisting of sixty-seven experiments, the rails from a 90 mm diameter /spl times/10 m long launcher were removed for metallurgical characterization. The rails were machined from OFHC copper extrusions. The measured Brinell hardness values of the rails ranged from HB 60 to HB 86. The starting material had a hardness of HB 86. Observed values increased from the breech up to a distance of approximately 4 m and tended to decrease and level off beyond 4 m. These hardness values are normal for worked OFHC copper, falling between those for fully annealed (HB 34) and cold rolled (HB 91) conditions. The compressive flow strength of the as-received material was determined to be /spl sigma//sub f/=331 /spl epsi//sup 0.12/ MPa. Extensive cratering and hypervelocity gouging damage was observed except in the regions 1 m from the breech end and between 7.5 and 10 m. The microstructure of the as-received rail material was fully dense and equiaxed and the average grain size was 100 /spl mu/m. >