Emma A. Taylor

Hypervelocity impact on carbon fibre reinforced plastic/aluminium honeycomb: comparison with Whipple bumper shields

Normal and oblique incidence hypervelocity impact tests (velocity range 4-6 km s(-1)) were carried out to determine the ballistic limit of a 1.6 mm carbon fibre reinforced plastic facesheet bonded to 45 mm aluminium honeycomb core, as typically used in Low Earth Orbiting spacecraft. The internal honeycomb damage was determined as a function of the impactor parameters. The ballistic limit data showed a strong dependence with impact angle. The internal honeycomb damage was found to be independent of impact angle for constant impact energy for Theta<50 degrees. An empirically-determined damage equation linking honeycomb damage to impact energy was developed. For the highest impact energy perforating impacts, the debris cone angles for the primary and secondary debris cones were determined. As the impact angle increased, the centre of the damage cones rotated away from the line of flight. The data have been compared with the ballistic limit curve defined by the modified Cour-Palais aluminium Whipple bumper equation and show broad agreement with the equation predictions. A reduced value of the rear facesheet thickness is required to bring the normal and 15 degrees incidence data into agreement with the ballistic limit curve.

Hydrocode modelling of hypervelocity impact on brittle materials: depth of penetration and conchoidal diameter

Summary The Johnson-Holmquist brittle material model has been implemented into the AUTODYN hydrocode and used for Lagrangian simulations of hypervelocity impact of spherical projectiles onto soda-lime glass targets. A second glass model (based on a shock equation of state and the Mohr-Coulomb strength model) has also been used. Hydrocode simulations using these two models were compared with experimental results. At 5 km s−1, the Mohr-Coulomb model under-predicted the depth of penetration, whilst adjustment of the Johnson-Holmquist model bulking parameter was required to match the experimental data to the simulation results. Neither model reproduced the conchoidal diameter; a key measured parameter in the analysis of retrieved solar arrays, so two failure models were used to investigate the tensile failure regime. A principal tensile failure stress model, with crack softening, when used with failure stresses between 100 and 150 MPa and varying bulking parameters, reproduced the conchoidal diameter morphology. Empirically-determined, power-law damage equation predictions for the range 5–15 km s−1 were compared with simulations using both models since no experimental data was available. The powerlaw velocity dependence of the depth of penetration simulations was found to be significantly lower than the 0.67 predicted by the empirically-determined damage equations.

Normal and oblique hypervelocity impacts on carbon fibre/peek composites

A series of normal and oblique hypervelocity impact tests were performed on carbon fibre/PEEK composite specimens using the Light Gas Gun facilities at the University of Kent at Canterbury. The tests were conducted on 16 ply and 24 ply targets, using 1 mm and 2 mm aluminum projectiles, at velocities of approximately 5 km/s, and at impact angles of 0, 30 and 45 degrees. The primary objective of this investigation was to add oblique impact data to the University of Toronto Institute for Aerospace Studies (UTIAS) database on hypervelocity impact damage to composites. The parameters investigated in this study were debris cloud (primary and secondary) dispersion cone angles, (equivalent) entry and exit crater diameters, and front and rear surface delamination damage zones. The results of the analysis showed that the cone angles were not symmetric about the projectile velocity vector, maxima for both angles occurring at a normalized energy of approximately 55 J, followed by an asymptotic decrease. The results obtained for the entry crater diameter and equivalent entry damage diameter were found to be in good agreement with the existing UTIAS database. For a given impact energy, the damage area in the 24 ply targets was found to be nearly twice that of the damage in the 16 ply targets. Results were found to be in good agreement when compared, using an empirical equation, to predictions of impact damage of Al projectiles on aluminum targets (when a scaling factor of 0.9 was applied to the PEEK data to account for the greater damage in composites).

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.

Cost effective honeycomb and multi-layer insulation debris shields for unmanned spacecraft

Ways to improve the tolerance of unmanned spacecraft to hypervelocity impact are presented. Two new honeycomb and multi-layer insulation (MLI) shields were defined: (1) double honeycomb, and (2) enhanced or toughened MLI (with additional Kevlar 310 and/or Betacloth layers). Following hypervelocity impact testing, a new ballistic limit threshold was defined, based on rear facesheet perforation and witness plate damage characteristics. At 12 km/s, the ballistic limit of single honeycomb was 0.58 mm (aluminium sphere), rising to 0.91 mm for double honeycomb, 1.00 mm for double honeycomb with MLI and 1.17 mm for double honeycomb with toughened MLI. A damage equation, based on the modified Cour-Palais equation with ESA constants, was compared with the data and found to be conservative. The impact angle exponent was increased in order to reduce the equation under-prediction for the oblique incidence data. An equivalent rear wall thickness was defined in order to distinguish between shield types above 7 km/s. The spacecraft survivability analysis showed that the double honeycomb and toughened MLI significantly reduced the number of perforating particles over the baseline single honeycomb design. The mass increase of these shields is approximately 1.2 kg/m2 for double honeycomb and 0.8 kg/m2 for toughened MLI.

Impacts on HST and EuReCa solar arrays compared with LDEF using a new glass-to-aluminium conversion

Abstract In order to compare impact fluxes measured on solar arrays in Low Earth Orbit (LEO) to those on aluminium surfaces, as measured by the Long Duration Exposure Facility (LDEF) for example, a conversion between the conchoidal diameter impact feature ( D co ) (as observed on solar array surfaces) and the ballistic limit in aluminium ( F max) has been developed. We find good agreement between the converted HST and EuReCa solar array fluxes and LDEF fluxes at large sizes ( F max > 30 μ m) where meteoroids dominate. We also find the converted HST and EuReCa fluxes are in broad agreement with the microdebris enhancement ( F max μ m) apparent on LDEF, indicating that the microdebris flux has not changed significantly between the LDEF epoch (1984–1990), and the EuReCa (1992–1993) and HST (1990–1993) epochs.

Hypervelocity impact on brittle materials of semi-infinite thickness: Fracture morphology related to projectile diameter

Hypervelocity impact on brittle materials produces features not observed on ductile targets. Low fracture toughness and high yield strength produce a range of fracture morphologies including cracking, spallation and shatter. For sub-mm diameter projectiles, impact features are characterised by petaloid spallation separated by radial cracks. The conchoidal or spallation diameter is a parameter in current cratering equations. An alternative method for interpreting hypervelocity impacts on glass targets of semi-infinite thickness is tested against impact data produced using the Light Gas Gun (LGG) facility at the University of Kent at Canterbury (UKC), U.K. Spherical projectiles of glass and other materials with diameters 30–300 μm were fired at ∼5 km s−1 at a glass target of semi-infinite thickness. The data is used to test a power law relationship between projectile diameter and crack length. The results of this work are compared with published cratering/spallation equations for brittle materials.

Simulation of hollow shaped charge jet impacts onto aluminium whipple bumpers at 11 km/s

The computational technique of Smoothed Particle Hydrodynamics (as implemented in the hydrocodes AUTODYN-2D and AUTODYN-3D) has been used to simulate the impact of hollow shaped charge jet projectiles onto stuffed Whipple bumper shielding. Due to limited availability of material models, the interim Nextel/Kevlar-Epoxy bumper was modelled as an equivalent thickness of aluminium. Stuffed Whipple bumper shields are used for meteoroid and debris impact protection of the European module of the International Space Station (the Columbus APM). A total of 56 simulations were carried out to investigate the impact processes occurring for shaped charge jet impact. Sensitivity studies were carried out on the influence of projectile shape, pitch, yaw and strength at 11 km/s to determine the range of debris cloud morphologies. The debris cloud structure was shown to be highly dispersed, and no projectile remnant was observed at the centre of the cloud. The mass of an aluminium sphere producing equivalent damage to a shaped charge jet projectile was in the range 1.5 to 1.75 times greater than the mass of the shaped charge jet projectile. Upon loading by the dispersed debris cloud, the interim bumper failed by spallation, producing fragments moving at 2 km/s or less. The fragments distorted the rear wall (pressure wall) of the shield but did not perforate it. The experimental data show rear wall deformation but to a lesser degree. Perforation of the rear wall, observed for one test, was not reproduced by the simulation. Nextel/Kevlar-epoxy material models are required to reproduce correctly the interim bumper failure under debris cloud loading.

Hypervelocity impact on soda lime glass: Damage equations for impactors in the 400–2000 m range

Abstract The results of a Light Gas Gun hypervelocity impact program on 25 mm thick soda-lime (float) glass targets are compared to the values predicted by an empirically determined power law spallation equation (Paul and Berthoud, 1995). Impact velocities were in the vicinity of 5 km s −1 , projectile densities were between 1 and 8 g cm −3 and the projectile diameters, d p were 0.8 to 2 mm respectively, producing spallation diameters between 10 mm and 50 mm. Previously published data in the range d p = 7 – 1000 μ m are also used to assess the validity of the equation. We conclude that D spall is predicted by the empirically determined power law spallation diameters for d p μ m but only to within ±50 %. An alternative equation to describe the response of target material for d p > 400 μ m is presented.

Hypervelocity impact on spacecraft carbon fibre reinforced plastic/aluminium honeycomb

Samples of a spacecraft primary external wall structure, as used in a low earth orbit remote sensing platform, have been tested to determine the response to the hypervelocity impact and ballistic limit (for mm-sized impactors) of the 47 mm thick structure at 5 km/s. A strong dependence of the ballistic limit on projectile density was identified. This programme was carried out using the two-stage light gas gun at the University of Kent at Canterbury. The equivalent diameters of the front and rear holes for each impact were analysed as a function of the impactor parameters. Damage equations derived by other experimenters were compared to the experimental results. X-ray non-destructive testing was used to determine the level of internal honeycomb damage for a sample. The dependence of the witness plate damage (placed behind the target to capture any ejecta from the rear surface) on the impactor parameters was recorded. It was found that the use of ‘equivalent thicknesses’ of aluminium may not be appropriate as a general conversion factor for carbon fibre reinforced plastic (CFRP) facesheets. A simple damage equation is presented, based on the total hole size as a function of the impact energy. The ballistic limit cannot be defined solely in terms of impact energy and shows an additional dependence with projectile density. The amount and type of ejecta produced is a strong function of density and a less strong function of projectile diameter, and its production cannot be linked with the rear hole diameter.