Investigation of hypervelocity impact damage to space station truss tubes

Fifty-eight hypervelocity impact data shots were used to determine the effect graphite-epoxy tube properties, such as ply orientation, elastic modulus, wall thickness, and surface coatings, have on the extent of impact damage. Visible and internal damage areas were measured optically and ultrasonically. Correlations were developed relating impactor and target parameters to damage extent. These experimental results were then applied in a preliminary assessment of the failure rate of Space Station truss tubes from meteoroid and orbital debris impacts. A failure criterion for the truss tubes was developed from an analytical study using a finite element model and on-orbit structural loading conditions.     

Fracture due to damage from projectile impact

In the usual application, the methods of fracture mechanics are used to determine growth rates and critical load levels for sharp-tipped flaws such as fatigue cracks. By way of contrast to this classic application, another way of introducing fracture producing flaws is to expose a structure to impacts from penetrating projectiles. This paper presents recent test data and analytical techniques for predicting the fracture response of tensile panels impacted by small caliber bullets. Panels of 7075-T6, 2024-T81 and 6A1-4V were tested in thicknesses ranging from 0.032 to 0.375 in. A bracketing technique was used to establish threshold values of prestress for catastrophic fracture. For comparison, a number of impact damaged panels were statically tested for residual tensile strength. These results are compared with the impact fracture threshold values and with strength predictions using the plane-stress stress intensity factor for center-notched panels.     

铝双层板结构撞击损伤的板间距效应实验研究

为了研究空间碎片对航天器防护结构的超高速撞击损伤特性,采用二级轻气炮发射球形弹丸,对铝双层板结构进行了超高速撞击实验研究.弹丸直径为3.97 mm,撞击速度分别为(2.58±0.08)km/s、(3.54±0.25)km/s和(4.35±0.11)km/s,板间距为10~100 mm.实验得到了铝双层板结构在不同撞击速度区间的后板损伤模式.结果表明,弹丸撞击速度一定时,后板弹坑分布随前后板间距的不同而不同.前板背面返溅影响区和后板弹坑分布区随板间距的增大而增大,各弹坑分布区扩散角随板间距的增大而减小.     

Hydrodynamic tweezers: impact of design geometry on flow and microparticle trapping

Here we explore the role of microfabricated device geometry on frequency-dependent low Reynolds number steady streaming flow and particle trapping behavior. In our system, flow and particle trapping is induced near an obstruction or cavity located in an otherwise rectilinear oscillating flow of frequency ω and amplitude s in a fluid of kinematic viscosity ν. This work expands prior studies to characterize nine distinct obstruction/cavity geometries. The imaged microeddy flows show that the device geometry affects the eddy number, shape, structure, and strength. Comparison of measured particle trap locations with the computed eddy flow structure shows that particles trap closer to the wall than the eddy core. Trapping strength and location are controlled by the geometry and the oscillation frequency. In most cases, the trapping behavior is linearly proportional to the Stokes layer thickness, δ(AC) ~ O((ν/ω)(1/2)). We show that steady streaming in microfluidic eddies can be a flexible and versatile method for noncontact microparticle trapping, and hence we call this class of devices "hydrodynamic tweezers".     

Stress invariants in an orthotropic damage space

In the classical theory of plasticity, it is often convenient to express failure criteria and/or flow rules in terms of stress invariants (I₁,I₂,I₃) and deviatoric stress invariants (J_2D, J_3D). Within the framework of continuum damage mechanics, the microscopic degradation of materials is reflected through the damage variables that conceive the notion of net-stress (the term net-stress has been used here in place of the term effective-stress since the latter already has an established meaning in Geotechnical Engineering). Any operative yield or failure criterion in the continuum damage mechanics will then need to be expressed in terms of the net-stress. This motivates an attempt to find the equivalent stress invariants in terms of the net-stress. In this paper, the stress invariants in an orthotropic damage space have been successfully derived and presented.     

Low-velocity impact damage in brittle coatings

Based on thin plate/elastic foundation theory and strain energy release rate calculations, this paper presents a lateral crack model to analyse the threshold for and development of impact damage in brittle coatings. Numerical solutions indicate that an increase in coating-to-substrate elastic modulus ratio tends to enhance the coating's resistance to impact damage initiation. The size of the lateral crack in a coating layer is a function of the coating thickness. For comparison with the theoretical prediction, experiments were performed on SiC-coated graphite specimens, as well as on glass microscope slides resting on various kinds of substrates.     

Optimized damage detection of steel plates from noisy impact test

Model‐based non‐destructive evaluation proceeds measuring the response after an excitation on an accessible area of the structure. The basis for processing this information has been established in recent years as an iterative scheme that minimizes the discrepancy between this experimental measurement and sequence of measurement trials predicted by a numerical model. The unknown damage that minimizes this discrepancy by means of a cost functional is to be found. The damage location and size is quantified and sought by means of a well‐conditioned parametrization. The design of the magnitude to measure, its filtering for reducing noise effects and calibration, as well as the design of the cost functional and parametrization, determines the robustness of the search to combat noise and other uncertainty factors. These are key open issues to improve the sensitivity and identifiability during the information processing. Among them, a filter for the cost functional is proposed in this study for maximal sensitivity to the damage detection of steel plate under the impact loading. This filter is designed by means of a wavelet decomposition together with a selection of the measuring points, and the optimization criterion is built on an estimate of the probability of detection, using genetic algorithms. Numerical examples show that the use of the optimal filter allows to find damage of a magnitude several times smaller. Copyright © 2006 John Wiley & Sons, Ltd.     

Scaling of impact damage in fiber composites from laboratory specimens to structures

Impact damage in fiber composite structures remains of much concern, and is often the limiting factor in establishing allowable strain levels. The complexity of impact damage formation usually dictates that experiments are required, but scaling of results from small laboratory scale specimens to large structures is not well understood. The following paper presents the results of an analytical and experimental investigation intended to develop procedures for prediction of damage formation and subsequent strength loss, with particular emphasis on scaling of results with respect to structure size. The experimental investigation involved both drop-weight and airgun impact on carbon/epoxy plates and cylinders. Five sizes of plates ranging from 50 × 50 × 1·072 mm to 250 × 250 × 5·36 mm, and two sizes of cylinders with diameters of 96·5 and 319 mm, were employed in the experimental program. Delamination was observed to increase with specimen size more than would be expected if stresses controlled the delamination extent, as would be predicted by fracture mechanics. Regions of broken fibers were observed in the impacted specimens, that were best correlated with the applied specimen stress or strains, independent of specimen size. It is seen that knowledge of the failure mechanisms involved is required to predict scaling of damage with confidence.     

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.     

Quasi-static impact damage in confined ceramic tiles

The effect of confinement on the failure mechanisms in dense alumina tiles during the penetration of a projectile was investigated and the role played by lateral mechanical confinement in inhibiting some failure modes was examined.Alumina tiles were placed in a confinement frame which simply and accurately allows high biaxial compressive pre-stresses. The confinement frame is a modular system which enables the application of various boundary conditions on the impacted tile to be generated. Tile samples were supported by semi-infinite support blocks made of steel or aluminum, and were confined by steel wedges.In the tests carried out under ballistic penetration conditions, the confined tiles were impacted by 0.3 cal. NATO AP (armor piercing) projectiles. These contained a hardened steel cone-shaped core with a sharp tip. In this investigation, the effect of reflected stress waves was small or even negligible, since the tile/support impedance mismatch was low, so that quasi-static failure mechanisms predominated. The effect of tile thickness on the damage mechanisms was also examined. Under the described test conditions, the major failure modes were radial cracks, cone crack, and fragmentation of the cone. Confined tiles exhibited reduced damage; damage in the form of radial cracks was reduced, and cone crack and fragmentation of the cone were inhibited. Based on the theory of a plate on an elastic foundation, the quasi-static loads that initiate the radial cracks were formulated. This model assists in understanding the initial damage mechanism.     

Detection of impact damage in CFRP laminates

The initiation and propagation of damage in carbon fibre composites subjected to impact loading has been investigated. High velocity impact tests were conducted on a variety of stacking configurations using a nitrogen operated gas gun. The damage processes were characterised using X-radiography, ultrasonic C-scanning, optical microscopy and the deply technique. The merits and weaknesses of applying such damage detection techniques to the monitoring of impact damage in composites are discussed. The consequence of the various fracture mechanisms on residual tensile strength is also considered.     

Modelling impact damage in marine composite panels

In order to establish the survivability of a composite structure under a dynamic load, it is important to be able to predict the damage incurred and the effect of the contact force. The damage caused by low velocity impact is often hidden and for thicker plates the transverse stresses are significant in promoting delamination. In this study, woven vinyl-ester composite plates up to 1.37 m long are numerically modelled with a simple, gradually damaging three-dimensional material model and the results are compared with full-scale tests. The model is based on damage mechanics principles using cyclic test data to obtain modulus reduction with damage. Delamination is modelled with a mixed-mode traction-separation law using cohesive elements. The nonlinear elastic orthotropic material model for the woven plies is written in a VUMAT in Abaqus/Explicit and the effects of varying some of the modelling parameters are briefly discussed through the examination of the contact force.     

Effects of parametric variations of complex targets on damage from projectile impact

The effects of a hypervelocity projectile striking complex targets have been investigated. The targets consisted of metallic and low-density shock attenuating layers and void regions. The major features of the targets were systematically varied to correlate changes in the targets with the projectile's effectiveness in damaging the targets. Two-dimensional numerical simulations were done with the Eulerian computational fluid dynamics program PINON. Projectile effectiveness against the various targets was measured by determining the maximum pressure, pressure integral, P²τ value, and hole size at several locations in the targets.     

Prediction of impact damage region in SMC composites

A failure criterion based on strain energy concepts was proposed for predicting the impact region in an SMC-R50 composite. The finite element method, in conjunction with an experimentally determined contact law, was used to perform the dynamic impact analysis. Comparison of the predicted impact damage boundary with the experimental results showed good agreement.     

锻锤基础的冲击疲劳损伤分析

用损伤力学的概念提出了一种研究由锻锤锻打引起锻锤基础系统的疲劳损伤增长和疲劳寿命估计的方法。该方法通过定义,引入锻锤基础系统中减振垫与基础块的损伤状态寿命因子,能够估计出锻锤基础系统的减振垫和基础块的损伤疲劳寿命上、下限。通过分析发现,长期的重复冲击载荷会引起锻锤基础系统发生疲劳微损伤积累,疲劳损伤积累的宏观损伤又引起锻锤基础的动力响应随宏观损伤发展而增大,动力响应的增大又加剧基础系统的损伤发展。因此,为减少周围环境的振动以及保护周围环境免受锻锤冲击损伤,在锻锤基础系统设计中需考虑吸振和损伤控制。     

Microscopy of impact damage in particulate-filled glass-epoxy laminates

Low-energy impact damage was induced in particulate-filled glass-epoxy laminates by means of a falling weight device, and the damage features were studied by sectioning through the impacted area. The polished sections were studied by optical and scanning electron microscopy to obtain comparative qualitative information about the effects of several different mineral fillers: solid glass beads, hollow glass microspheres, quartz, alumina trihydrate and mica. Approximately 8.5 mm thick laminates, with and without fillers, were impacted at various energies up to 43 J. At the lower impact-energy levels, the damage in the unfilled laminates in most samples was typically transverse matrix cracking, at some distance below the point of impact. Transverse and interlaminar cracking was more extensive in filled than in unfilled laminates. The type and distribution of the damage features are discussed in terms of the properties of the filler particles, and of the stress distributions involved.     

Healing of low-velocity impact damage in vascularised composites

This research has sought to characterise damage formation and self-healing efficiency within vascularised carbon fibre reinforced polymer (CFRP) laminates over a range of low velocity impact energies. Using ultrasonic C-scanning and compression after impact (CAI) analysis, vascularised laminates were shown to conform to the same damage size to residual compression strength relationship established for conventional laminates. The damage tolerance level of the host laminate was carefully determined, an important consideration in selection of the most appropriate vascule spacing for a reliable self-healing system. The healing functionality imparted full recovery of post impact compression strength over the range of impact energies tested (2.5–20 J), via healing of matrix cracking and delamination damage. The successful implementation of this technology could substantially enhance the integrity, reliability and robustness of composite structures, whilst offering benefits through reduced operational costs and extended lifetimes. However, establishing the benefits of such novel systems to existing design criteria is challenging, suggesting that bespoke design tools will be required to fully attain the potential benefits of self-healing technologies.