Impact Behavior and High-Energy Absorbing Materials

Impact behaviors of structures are gaining more research attention due to increasing demand for designing structures under extreme loading. At the same time, effective protection of structures from low-velocity impact and blast loading and the development of various high-energy absorbing materials as well as strengthening and retrofit-hardening techniques for conventional materials and structures are also of contemporary interest.

This special issue on “Impact Behavior and High-Energy Absorbing Materials” is the second part of a two-issue series [July (Qiao and Binienda 2008) and October (the present issue) 2008]. It provides a comprehensive survey of theoretical, computational, and experimental investigations of impact mechanics and high-energy absorbing materials. The issue begins with a review paper, followed by papers on several relevant topics. These range from impact response of prestressed composite laminates to indentation behavior of sandwich structures, dynamic enhancement mechanism of metallic hollow sphere material, full-scale blast test of a masonry structure retrofitted with fiber-reinforced polymer composites, experimental characterization of residual tensile strength of woven graphite epoxy laminates due to low-velocity impact, and concrete confined by a prefabricated composite jacket for potential impact protection.

Qiao, Yang, and Bobaru provide a comprehensive review of the theoretical, computational, and experimental study of impact mechanics and high-energy absorbing materials. 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 are reviewed and discussed. In particular, 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, magneto-rheological-MR-fluids, porous shape memory alloys) are emphasized. Recent studies on experimental evaluation and constitutive modeling of strain-rate-dependent polymer matrix composites, for example, are also presented in some papers in this special issue. The review is intended to help readers in identifying starting points for research in modeling and simulation of impact problems and in designing energy-absorbing-materials and structures.

Using Fourier series expansion and Laplace transform, Zheng and Binienda obtained an analytical solution for the impact response of a simply supported laminated composite plate under prestresses, in which a linearized elastoplastic contact law was adopted when considering permanent indentation during impact. The effects of initial prestresses on the contact force, plate center displacement, and strain-time histories were investigated, demonstrating increased contact force and reduced plate displacement with a high initial prestress. A parametric study was also conducted of impactor velocity, mass, shear strength of the laminates, and plate thickness to the contact force and dynamic response of the plate under tensile prestresses.

Yang and Qiao have characterized the indentation behavior of sandwich structures. Beam-on-elastic-plastic foundation models were developed to predict the indentation behavior of sandwich materials, and three stages of failure were clearly depicted by the global stiffness changes in the load versus displacement curve of elastic-plastic beams. The models were compared with the available experimental data and numerical simulation, and relatively close agreements were achieved. The compliance and compliance gradient derived from the indentation models were 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 was uniquely recovered from the derived damping ratio.

Gao, Yu, and Zhao experimentally characterized the mechanical behaviors of sintered metallic hollow sphere (MHS) material, which offers low density, high stiffness, and good energy absorption capacity. The dynamic behaviors of MHS materials were examined by a modified split Hopkinson pressure bar (SHPB) testing system. In particular, the relevant collapse and dynamic enhancement mechanisms of MHS material were discussed, and the localization phenomena in a dynamic crushing process were revealed by using the particle image velocimetry correlation technique.

Maji, Brown, and Urgessa conducted a full-scale blast test on a structure representing a mailroom constructed with unreinforced masonry walls, of which 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, and shotcrete was added to the inside face of the two long walls. Airblast shock loads and gas pressure loads were considered when designing retrofits to a structure to resist an internal detonation. The measured blast peak pressures compared well with BLASTX. A simple step-by-step design guideline for blast retrofit using FRP composites was provided. Their study validated the method of analysis to design effective retrofit techniques to contain blast loads, and the postmortem analysis of the test indicated that the stiffness of the walls is completely lost at an early stage and only membrane action of the GFRP provides structural resistance.

Mosallam, Slenk, and Kreiner conducted 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. Their study indicated that a significant reduction in tensile strength properties of woven cross-ply [0°/90°] laminates due to low-velocity impact was observed, while in comparison, a moderate strength reduction exhibited in angle-ply woven laminates $[\pm 45°]$ . The identified characteristic of increasing tensile strength in $[\pm 45°]$ specimens helps provide better design of fiber orientation in composite laminates with limited fiber damage.

Uddin, Purdue, and Vaidya experimentally investigated the effects of low-velocity impact loading on high-strength concrete confined by a prefabricated polypropylene jacket and compared the results with similar specimens confined by carbon-fiber-reinforced polymer (CFRP) composites. Their study was intended for developing potential concrete bridge impact protection using thermoplastic prefabricated composite jackets.

Again, on behalf of the Journal of Aerospace Engineering and ASCE, we want to acknowledge the following reviewers for their effort and constructive comments, which make this double-issue, series on “Impact Mechanics and High-Energy Absorption Materials” [the July (Qiao and Binienda 2008) and October (the present one) issues of 2008] such a success.