Ballistic limit equation for equipment placed behind satellite structure walls

A new ballistic limit equation has been developed for the case of a Whipple shield configuration or a sandwich panel with honeycomb core placed in front of a backwall. This “triple plate” ballistic limit equation considers explicitly the thicknesses, materials and spacings of each of the three plates. The third plate, i.e., the backwall, represents the cover plate or external wall of the equipment that is placed behind the satellite structure wall. The ballistic limit equation has been calibrated with experimental results obtained from hypervelocity impact tests on satellite equipment that was placed behind typical satellite structure walls. The equipment considered were fuel and heat pipes, pressure vessels, electronic boxes, harness, and batteries, all representative of real satellite equipment. The new equation was applied to prove that if the inherent protection capability of satellite equipment against hypervelocity impacts is explicitly considered in a ballistic limit equation, the critical projectile diameters for failure of such equipment are raised considerably compared to the case where equipment is assumed to fail as soon as the structure wall that protects it is perforated.     

Hypervelocity impacts on gas filled pressure vessels

Space debris is a recognized threat for any Space Mission. Risk analyses have identified pressure vessels as the most critical items onboard spacecraft. Impacts of meteoroids or debris on pressure vessels can indeed lead to the catastrophic failure of the vessels and terminate prematurely spacecraft missions. In addition, pressure vessels bursts act as space debris generators. The aim of this work is to define experimentally the limit between simple perforation and catastrophic burst of pressure vessels under hypervelocity impacts. Targets have been selected according to the most immediate ESA needs : thin wall gas filled aluminium pressure vessels. Hypervelocity impact tests have been performed on aluminium vessels at a constant impact velocity (7 km/s) with aluminium projectiles, at a normal trajectory and, for most of the cases, on unshielded vessels. Projectile size and pressure in the vessels were varied to explore the conditions under which different damage mechanisms could be obtained. Results presented range from simple penetration holes and no visible damage on the internal wall opposite to the impact point to global burst of the vessel. While failure was thought to only occur from the side opposite to the impact point, certain combinations of pressure and projectile kinetic energy led to burst from the impact side.     

Propagation of hypervelocity impact fragment clouds in pressure gas

This study investigated the propagation of hypervelocity impact fragment clouds in pressure gas. Fragment clouds were generated through perforation of thin aluminium bumper plates by spherical aluminium projectiles. A thick aluminium backwall plate, placed inside a pressure container at a given distance from the bumper plate, caught the fragments to act as a witness plate for the residual damage potential of the fragments. Crater depth statistics are presented as a function of container pressure. The fragment cloud was photographed by means of an image converter camera. The images showed a strong deformation of the fragment cloud for increased container pressures and were used to extract residual velocities until up to 50 μs after impact. The deceleration of the velocity as a function of time after impact suggested an exponential decay function as the best fit to the curve. Thus, maximum fragment impact velocities on the backwall plate could be extrapolated from the axial cloud velocities. The extrapolated curves were compared with experimental time-of-flight measurements, and proved a good match. Fragment impact velocities and maximum crater depths were used to calculate maximum fragment particle sizes as a function of the container gas pressure.     

Hypervelocity impact testing of pressure vessels to simulate spacecraft failure

A series of hypervelocity impact tests are conducted against thin-walled aluminum pressure vessels to investigate failure mechanisms. The vessels, 0.05 and 0.08 inches thick, are constructed to replicate the material properties of the International Space Station (ISS). The vessels are pressurized to simulate the conditions experienced by the habitable modules of the ISS. A test matrix incorporating shielded thin plates, shielded vessels under no pressure and shielded vessels under internal pressure is developed to take advantage of knowledge gained in earlier tests. Given the design parameters of the ISS coupled with the capabilities of light gas gun testing, it is shown that a catastrophic failure due to unzipping is unlikely.     

Projectile density, impact angle and energy effects on hypervelocity impact damage to carbon fibre/peek composites

This paper explores the effects of projectile density, impact angle and energy on the damage produced by hypervelocity impacts on carbon fibre/PEEK composites. Tests were performed using the light gas gun facilities at the University of Kent at Canterbury, UK, and the NASA Johnson Space Center two-stage light gas gun facilities at Rice University in Houston, Texas. Various density spherical projectiles impacted AS4/PEEK composite laminates at velocities ranging from 2.71 to 7.14 km/s. In addition, a series of tests with constant size aluminum projectiles (1.5 mm in diameter) impacting composite targets at velocities of 3, 4, 5 and 6 km/s was undertaken at incident angles of 0, 30 and 45 degrees. Similar tests were also performed with 2 mm aluminum projectiles impacting at a velocity of approximately 6 km/s. The damage to the composite was shown to be independent of projectile density; however, debris cloud damage patterns varied with particle density. It was also found that the entry crater diameters were more dependent upon the impact velocity and the projectile diameter than the impact angle. The extent of the primary damage on the witness plates for the normal incidence impacts was shown to increase with impact velocity, hence energy. A series of tests exploring the shielding effect on the witness plate showed that a stand-off layer of Nextel fabric was very effective at breaking up the impacting debris cloud, with the level of protection increasing with a non-zero stand-off distance.     

Penetration of HSLA-100 steel with tungsten carbide spheres at striking velocities between 0.8 and 2.5 km/s

A 51 mm thick plate of high-strength low-alloy (HSLA-100) steel was impacted by 6.4 mm diameter tungsten carbide spheres traveling at velocities ranging from 0.8–2.5 km/s. The width and depth of the crater for each impact event are provided in tabulated form and graphed as a function of velocity. The impacts were simulated using an explicit Lagrangian finite element model. A residual stress map over a cross-section through the crater was also measured by the Contour Method for the 2.2 km/s impact. The predominant feature of the stress map was a peak compressive stress of 1100 MPa, which is 1.6 times the yield strength, centered approximately one crater diameter below the crater floor. Residual stresses in the as-received HSLA-100 plate were also measured and were used to evaluate the effect of initial stresses on the model prediction. Good agreement is shown between the numerical simulation of the impact event and the experimental data.     

Micrometeoroid impact on ceramic thin components for interplanetary probe

A new advanced ceramic thruster made of monolithic silicon nitride (Si₃N₄) is under development for the next interplanetary probe of PLANET-C Venus exploration mission in Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA). In order for secure operation of a spacecraft with a ceramic component onboard a real mission, the reliability against micrometeoroid impacts on the ceramic component has to be investigated in addition to the quasi-static mechanical and thermal analyses and verifications. First, the risk probability of the micrometeoroid impacts was evaluated by using an interplanetary flux model, and the risk evaluation in terms of impact energy was proposed by combining the velocity distribution with the flux model. The probability of impacts on the ceramic thruster during the mission was estimated with this model. Second, hypervelocity impact tests were performed with a two-stage light-gas gun. Three types of failure were observed: one was only a crater formed on the impact surface. Another type was crater formation on the front-face and spall fracture on the back-face and in the last type a perforation was formed in addition to cratering and spalling. The samples did not either shatter or breakdown for the impact energies tested in this study. The impact failure morphology observed in this study showed dependency on the plate thicknesses and the projectile kinetic energy. The energy-based risk evaluation together with the series of the hypervelocity impact tests indicated that the silicon nitride ceramic thruster onboard the interplanetary probe would have only a local damage and survive during the mission term.     

Debris cloud distributions at oblique impacts

The purpose of this study is to obtain mass, spray angle and velocity distributions of fragments in debris cloud generated by oblique impacts on an aluminum alloy plate. Hypervelocity impact tests were performed with a two-stage light gas gun at Kyushu Institute of Technology. The impact angles were changed to 0°, 15°, 30°, 45° and 60°. The projectile impacted on the targets at 2 km/s. After the impact, the debris cloud was taken with flash X-rays and an ultra high-speed video camera. The fragments were then captured in a stack of polystyrene sheets. As a result, the projectile was broken up into smaller fragments by oblique impacts with the larger impact angles. Lower velocity fragments dispersed in wider spray angles according to the increase of the impact angles.     

The effects of repeated transverse impact load on the burst pressure of composite pressure vessel

The present experimental study deals with the repeated transverse impact effect on the burst pressure of composite pressure vessels. Filament winding method is used to produce the vessels. Glass fiber reinforced (GFR) vessels are manufactured by using E-glass and epoxy resin. Composite pressure vessel was manufactured from fibers oriented [+55°/−55°/+55°/−55]_2s and the impact energies were chosen as 10, 15, 20, 25, 30 J for empty vessel during the impact tests. In addition, 10, 15, 20, 25 J for water filled conditions at 25 and 70 °C. The transverse impact load was applied in single and three times repeated form. The results show that when the impact load and water temperature increases, the burst pressure decreases.     

Shielding against space debris. A comparison between different shields: The effect of materials on their performances

The space environment requires the Space Station to be shielded against orbital debris. A technological programme undertaken by the European Space Agency has led to a preliminary definition of the shield configuration for the European Attached Pressurized Module. The envisaged shield is a modified Whipple shield. A second bumper is located midway between the first bumper and the backwall.

The work described has been initiated to quantify experimentally the merits of different shields compatible with the APM system requirements. For this technological investigation, two requirements had to be satisfied. The spacing between the front bumper and the backwall had to be limited to 120 mm. The backwall thickness could not be reduced to technological limits as it has structural functions as well. In addition, the long life requirements of the Space Station precludes the use of unproved materials for the external parts of the shield.

Different materials have been tried as second bumper. The effect of the first bumper thickness on the projectile fragmentation has been explored as well. Shields based on Aluminium, Kevlar and Glare have been investigated. Kevlar 29 fabrics impregnated with epoxy resin were used for this work. Glare is a material developed to improve the fatigue strength of metal structures. It is primarily intended for aircraft skin applications. Glare consists of a 60 percent fibre volume adhesive prepreg with high-strength unidirectional or cross-ply R-glass fibres. A variety of lay-up sequences is available ranging from 2/1 (two layers of aluminium alloy sheet bonded by one layer of prepeg) to any number of layers. The 2/1 layers version of the Glare material has been used for this work.

The tests results indicate the performances of materials can change significantly with the impact conditions. Glare shows the best performances in the low velocity regime while Kevlar is very promising in the high velocity regime. It is concluded the use of Kevlar can improve substantially the performances of the APM shield.     

Evaluation and replication of impact damage to bicycle helmets

A group of 72 impacted bicycle helmets were collected, primarily from manufacturers with a crash replacement policy that encourages the return of damaged helmets. Each damaged helmet was thoroughly inspected and measured to determine the construction details and collision damage. Laboratory replication tests were then performed on selected samples using exemplar helmets to determine impact velocity and peak headform aceleration. The predominant impact location was the front left quarter and the replication studies indicate that the majority of impacts took place on flat surfáces from drop heights of 1 meter or less. Overall, it is evident that a large number of bicycle helmet users who have benefited from the use of a bicycle helmet, and future bicycle helmet standards must incorporate the protective requirements of this unique group.     

A study of hypervelocity impact on cryogenic materials

This paper reports a result of hypervelocity impact experiments on cryogenically cooled aluminum alloys and a composite material. Experiments are carried out on a target palate at 122 K. Aluminum spheres at 1.95 km/s in 50 kPa air were impinged against the target plate at cryogenic temperature and the result was compared with room temperature target plates. Hypervelocity impact (HVI) processes were visualized with shadowgraph arrangement and recorded with high-speed video camera and to ensure the temperature dependence we compared HVI tests with metal target plates with AUTODYN 2D and SPH numerical simulations. We found that cryogenic impacts created slight differences of impact damage from room temperature ones, i.e., the shape and averaged diameters of HVI crater holes were less at cryogenic impacts.     

Response of a replicated human heart system to dynamic loading

The response of the human heart and attached major vasculature to rapid acceleration loading was studied by means of a non-pulsatile replica consisting of leotard fabric components. The system was designed to prevent leakage of water intended to simulate the blood present. This model was emplaced inside a rigid thorax and mounted on a moving cart, with sudden arrest produced by barrier impact. Large displacements of the ventricles and other attached vessels and pressures in the heart compartments were measured. Twelve impacts to the front, side, rear and at oblique incidence for a condition corresponding to the end of diastole were executed. The tests indicated that the strains in the right ventricle and the interchamber pressure were significant. Strains in the inferior vena cava were relatively high with lower values for the superior vena cava. Significant extensions were also observed in the brachiocephalic, left common carotid and left subclavian arteries. These findings qualitatively substantiate clinical experience for humans subjected to rapid non-invasive loading. A simple spring-mass system was used to model the dynamic response of the replicated heart unit. Predicted values were found to be in fair agreement with test data.     

Hypervelocity impact damage to composites

This paper presents the results of hypervelocity impact tests conducted on graphite/PEEK laminates. Both flat plate and circular cylinders were tested using aluminum spheres of varying size, travelling at velocities from 2–7 km/s. The experiments were conducted at several facilities using light gas guns. Normal and oblique angle impacts were investigated to determine the effect of impact angle, particle energy and laminate configuration on the material damage and ejecta plumes. Correlations were established between an energy parameter and the impact crater size, spallation damage and debris cone angle. Secondary damage resulting from the debris plume on adjacent composite structures was studied using high-speed photography and witness plates. It was observed that for hypervelocity impacts, the debris plume particles have sufficient energy to penetrate adjacent structures and cause major structural damage as well.     

Analysis of formation of the Odessa crater

The impact of the meteorite that formed the Odessa crater is examined in this numerical study. Extensive information collected from excavation of the site has made it possible to use the Odessa crater as a code validation test, to the extent that a calculated impact can produce equivalent cavity dimensions and stratification. In addition, the calculations can be used to provide a better estimate of velocity and trajectory of the meteorite at impact. The initial set of simulations performed in this study suggests that original estimates of the impact conditions may not have been sufficient to produce a crater of the size and shape known from the Odessa site. Supplemental analysis was performed and suggested that the meteor impacted at a much larger obliquity angle and may have been much larger than originally speculated. Details of the analysis leading to these observations are presented.     

Fluid fracture interaction in pressurized reinforced concrete vessels

The water penetration and flow through cracks in reinforced concrete structures are investigated experimentally and numerically. First, wedge-splitting tests of reinforced concrete under strain control were performed. These specimens were subjected to a mechanical load, as well as to an internal hydrostatic pressure. Pressure along the propagating crack and flow rates were measured. Then, nonlinear fracture mechanics-based finite element simulations were performed. From this study, it was determined that the rebars on the outer side of the wall in concrete containment vessels are more effective in preventing leakage than are the inner ones. Furthermore, it was once again determined that the presence of a hydrostatic pressure reduces not only the fracture energy of concrete but also the bond between reinforcement and concrete. Finally, two analytical models are proposed.     

Bursting of shielded pressure vessels subject to hypervelocity impact

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.     

Highy oblique impacts into thick and thin targets

Hypervelocity impact (HVI) tests have been conducted at the JSC Hypervelocity Impact Test Facility (HIT-F) with aluminum projectiles impacting semi-infinite (thick) and thin aluminum plates (with plate thickness to projectile diameter ratios of 6.4 and 0.14, respectively) at impact angles ranging from normal to the plate (0°) to highly oblique (88°). The targets were impacted by solid homogeneous aluminum spheres from 1 mm to 3.6 mm diameter. Results of the HVI tests were not unusual up to 65°, where impact damage is characterized as smooth craters and holes that become progressively elliptical and distended along the projectile flight path. Above 65° angles, however, a transition occurs to an irregularly shaped hole in thin materials and rough bottomed crater in thick targets. Above 80°, multiple damage sites in the targets were formed with the damage areas separated by variable distances of undamaged target surface. Analytical and numerical simulations of the impact process at oblique angles above 65° demonstrates that shock compression and release of the projectile into multiple fragments occurs before the projectile fully engages the target. The resulting projectile fragments are then responsible for the multiple impact sites observed on the targets.     

Effect of distance from the support on the penetration mechanism of clamped circular polycarbonate armor plates

A 1.91-mm thick circular polycarbonate plate of 115 mm diameter was impacted by a spherical steel projectile of 6.98 mm diameter at its center. Subsequent impacts were made at 10, 20, 30, 40, and 50 mm radii of the plate. Dent dimensions for the damaged plate were measured using optical microscope. For a constant projectile velocity of 138 m s⁻¹ which was below the perforation limit of the plate under investigation, a maximum thickness reduction close to the edge support was observed. The experimental work was modeled into explicit finite-element analysis program LSDYNA for simulations. LSDYNA was able to predict the dent depth and reduction in plate thickness at impact points precisely. In this research, the effect of the impact location distance from the supports on the damage mechanism of circular polycarbonate armor plates is investigated. The target plate was subjected to constant velocity projectile impacts starting at the plate midpoint and varying the impact distance from midpoint towards the clamped edge. Failure of plate is predicted close to the constrained boundary under uniform conditions.     

Environmental effect on fatigue life of glass–epoxy composite pipes subjected to impact loading

The main objective of this experimental study was to investigate the effects of seawater and impact loading on the fatigue life of glass–epoxy composite pipes under cyclic internal pressure. The pipes were produced by filament winding technique. Composite specimens were immersed in seawater for periods of 3, 6, and 9 months. After the impact tests are carried out at three different energy levels (5, 7.5, and 10 J), fatigue tests were conducted on the specimens. It is seen from results that fatigue life changes according to both impact energy and seawater immersion time. Fatigue life of non-impacted specimen is greater than the impacted one. Fatigue life increases in the impacted specimens up to 3 months and reaches generally maximum value. After that it decreases with increase in seawater immersion time. During the fatigue tests, fatigue damage types named perspiration, leakage, and eruption were observed.