Journal of Aerospace Engineering

An analytical solution for the impact response is obtained for the central impact of mass on a simply supported laminated composite plate under prestresses based on the Fourier series expansion and Laplace transform technique. A linearized version of the elastoplastic contact law proposed was used in the analytical formulation to consider permanent indentation during the impact. Permanent indentation including damage effects was included in the elastoplastic contact law. The effects of initial stresses on the contact force, plate center displacement, as well as strain time histories are presented. It is shown that higher initial stresses increase the maximum value of the contact force but reduce the plate central displacement. Effects of impactor velocity, mass, interlaminar shear strength of the laminates, and plate thickness on the contact force and dynamic response of the plate under tensile prestresses are also discussed.

Different from conventional metal foams, sintered metallic hollow sphere (MHS) material contains a certain volume fraction of enclosed pore space inside the spheres, as well as the interstitial porosity between the sintered neighbors, and this mixed open/closed-cell characteristic offers low density, high stiffness, and good energy absorption capacity. It is a new type of prospective light weight material for automotive and aerospace industry. In this study, the mechanical properties, especially the dynamic behaviors of the MHS materials are examined experimentally. Basic geometrical measurements reveal that the two types of MHS specimens we used were composed of randomly packed hollow spheres with uniform outer radius (0.9 and $1.5\mathrm{mm}$ ) and wall thickness $0.05\mathrm{mm}$ (relative density $<6\%$ ). A very long, plateau length (up to 67% nominal strain) with almost constant stress was found in quasi-static tests, indicating that they are appropriate for energy absorption applications. Dynamic tests were performed by using a modified split Hopkinson pressure bar (SHPB), and significant dynamic strength enhancements were observed. Moreover, based on the experimental observations the relevant collapse and dynamic enhancement mechanisms are discussed, whereas the localization phenomena in dynamic crushing process were revealed by using the particle image velocimetry correlation method.

A full-scale blast test was conducted on a structure representing a mailroom, constructed with unreinforced masonry walls. The four walls were retrofitted with different quantities of glass fiber-reinforced polymers (GFRP) on the outside face to increase their resistance to the blast load. In addition, shotcrete was added to the inside face of the two long walls. The objective of this test was to validate a method of analysis that can be used to design effective retrofit techniques to contain blast loads. A blast load was produced by the detonation of a $0.91\mathrm{kg}$ $\left(2\mathrm{lb}\right)$ equivalent TNT charge placed near the center of the room. Instrumentation on individual walls monitored the blast pressure and the consequent displacement and velocity of the walls. Although the walls sustained extensive internal damage and plastic deformation, the retrofit was able to withstand the blast load. It was observed through the postmortem analysis of the test that the stiffness of the walls is completely lost at an early stage and only membrane action of the GFRP provides structural resistance.

Both the crushing and indentation behaviors of sandwich materials are important aspects in failure analysis and energy absorption. In this paper, the core crushing strength of honeycomb materials from the experiment is briefly introduced, and the indentation of sandwich structures is studied in detail. Using the beam on elastic-plastic foundation models, theoretical formulations for predicting indentation behavior of sandwich materials are proposed, from which the mechanical response of elastic-plastic sandwich beams are obtained. Three stages of failure are clearly depicted by the global stiffness changes in the load versus displacement curve of elastic-plastic beams. The models are compared with the available experimental data and numerical simulation, and relatively close agreements are achieved. The compliance and compliance gradient derived form the indentation models are then incorporated with the equation of motion of the projectile to study impact response of elastic-plastic beams, and the impact energy dissipation due to the plasticity of elastic-plastic sandwich beam is uniquely recovered from the derived damping ratio. The beam on elastic-plastic foundation models proposed can be used to predict the indentation behavior of honeycomb sandwich materials, and the unique incorporation of the compliance and compliance gradient of the elastic-plastic sandwich beams in the standard mass-spring model can be utilized to characterize the effect of core compression and plasticity in the impact process.

In this paper a review of impact mechanics and high-energy absorbing materials is presented. We review different theoretical models (rigid-body dynamics, elastic, shock, and plastic wave propagation, and nonclassical or nonlocal models) and computational methods (finite-element, finite-difference, and mesh-free methods) used in impact mechanics. Some recent developments in numerical simulation of impact (e.g., peridynamics) and new design concepts proposed as high energy absorbing materials (lattice and truss structures, hybrid sandwich composites, metal foams, magnetorheological fluids, porous shape memory alloys) are discussed. Recent studies on experimental evaluation and constitutive modeling of strain rate-dependent polymer matrix composites are also presented. Impact damage on composite materials in aerospace engineering is discussed along with future research needs. A particular example for the design of a sandwich material as an impact mitigator is given in more detail. This brief review is intended to help the readers in identifying starting points for research in modeling and simulation of impact problems and in designing energy absorbing materials and structures.

This paper presents the results of an experimental study to investigate the effects of impact loading on the residual tensile strength of woven graphite epoxy laminates with a toughened resin system. In this study, both cross-ply, ${[0∕90]}_6$ , and angle-ply, ${[\pm 45]}_6$ , laminate lay-up configurations were studied and compared. Test specimens were subjected to various levels of impact loading, after which tensile pull tests were performed to determine the residual tensile strength properties. The study results demonstrated that impact damage causes a significant reduction in tensile strength properties of woven cross-ply [0°/90°] laminates. For example, cross-ply laminated composite specimens subjected to the lowest impact level, $6.8\mathrm{J}$ , exhibited a 25% decrease in ultimate tensile strength. Angle-ply woven laminates $[\pm 45°]$ , however, exhibited an 18% increase in ultimate tensile strength after being subjected to $6.8\left(\mathrm{J}\right)$ impact. This characteristic of increasing tensile strength in $[\pm 45°]$ specimens is an example of increased fiber reorientation in composite laminates with limited fiber damage.

The research focused on the effects of low velocity impact loading on high-strength concrete confined by a prefabricated polypropylene jacket and comparing the results with similar specimens confined by carbon fiber-reinforced polymer (CFRP) composites. In order to accomplish this, both static and dynamic load tests were performed. Concrete cylinders were used for static loading. Twelve concrete cylinders were prepared for static load testing: three were plain concrete and used as control specimens, three were wrapped with one layer of unidirectional CFRP composites, and six were confined by the polypropylene jacket. The thickness of the polypropylene wrap was machined to different thicknesses; three $3\mathrm{mm}$ and three $6\mathrm{mm}$ . The cylinders were standard $(D\times H)$ $152\mathrm{mm}\times 305\mathrm{mm}$ . Cylinders were loaded to failure in uniaxial compression using a Tinius-Olsen Universal Testing Machine. Impact testing was performed using four $(D\times H)$ $152\mathrm{mm}\times 914\mathrm{mm}$ columns. The columns consisted of one control sample; one CFRP composites wrapped, and two (one of each thickness) wrapped with polypropylene. Impact testing was conducted using an Instron drop-tower testing machine.

The particle size distribution (PSD) of lunar dust, the $<20\mu \mathrm{m}$ portion of the regolith, was determined as an initial step in the study of the possible toxicological effects it may have on the human respiratory and pulmonary systems. Utilizing scanning electron microscopy, PSDs were determined for Apollo 11 (10084) and 17 (70051) dust samples, as well as lunar dust simulant JSC-1Avf. The novel methodology employed is described in detail. All measured PSDs feature a log-normal distribution having a single mode in a range $100–300\mathrm{nm}$ for lunar dust samples, but the lunar simulant has a mode at $\sim 600\mathrm{nm}$ .

The morphology (shape and texture) of dust fractions of five Apollo lunar soils and a lunar dust simulant, JSC-1Avf, was studied using scanning electron microscopy. Shape (aspect ratio and complexity) of particles was described based on the two-dimensional projection images. The distributions of aspect ratio and complexity of particles are reported. It was determined that the Apollo lunar dust particles consist mainly of impact-produced glass, with complicated morphologies, extensive surface areas per grain, and sharp, jagged edges. Importantly, many grains contain elaborate vesicular textures, representing minute agglutinates. Dust simulant JSC-1Avf also has similar shapes as lunar dust, but differs in surface texture and area (smooth and nonvesicular). These data provide information for toxicity studies of lunar dust and for selecting a suitable lunar dust simulant.

This paper presents wind tunnel measurements of pedestrian wind conditions in passages between various configurations of two long narrow perpendicular buildings in open country exposure. The investigated parameters are passage width, building height and wind direction. The measurements were made along the passage centerline. The aim of this paper is to provide more insight in the pedestrian wind conditions in these basic building configurations, to address some contradictory statements reported in the literature and to provide detailed experimental data for computational fluid dynamics (CFD) validation. The results show that the wind speed amplification factors in diverging passages are generally larger than in converging passages. It is also shown that the maximum wind speed amplification factors increase monotonically with decreasing passage width, contrary to some general building design guidelines proposed in the past for such building configurations. Significant issues concerning the use of the experimental data for CFD validation are also discussed.

An artificial swarm system consisting of multiagents is considered in this paper. The agents may interact with each other based on their relative position. Each agent exhibits a repulsion/attraction behavior toward another agent, which mimics some biological swarm systems. The performance of each individual agent is the accumulation of these respective considerations toward other agents. The overall performance of the swarm system is analyzed, which includes uniform boundedness, uniform ultimate boundedness, and convergence. This mimics aggregation and formation in biological systems. The control design for each agent toward achieving the performance is then proposed. The control is a mimic of nature’s strategy in constraining mechanical systems.