A virtual test facility for simulating the dynamic response of materials

The goal of the Caltech Center is to construct a virtual test facility (VTF): a problem solving environment for full 3D parallel simulation of the dynamic response of materials undergoing compression due to shock waves. The objective is to design a software environment that will: facilitate computation in a variety of experiments in which strong shock waves impinge on targets comprising various combinations of materials; compute the target materials' subsequent dynamic response; and validate these computations against experimental data. Successfully constructing such a facility requires modeling of the highest accuracy. We must model at atomistic scales to correctly describe the material properties of the target materials and high explosives; at intermediate (meso) scales to understand the micromechanical response of the target materials; and at continuum scales to capture properly the evolution of macroscopic effects. The article outlines such a test facility. Although it is a very simplified version of the facilities found in a shock-compression laboratory, our VTF includes all the basic features, offering a problem solving environment for validating experiments and facilitating further development of simulation capabilities

Parallel adaptive fluid-structure interaction simulations of explosions impacting on building structures

We pursue a level set approach to couple an Eulerian shock-capturing fluid solver with space–time refinement to an explicit solid dynamics solver for large deformations and fracture. The coupling algorithms considering recursively finer fluid time steps as well as overlapping solver updates are discussed. Our ideas are implemented in the AMROC adaptive fluid solver framework and are used for effective fluid–structure coupling to the general purpose solid dynamics code DYNA3D. Beside simulations verifying the coupled fluid–structure solver and assessing its parallel scalability, the detailed structural analysis of a reinforced concrete column under blast loading and the simulation of a prototypical blast explosion in a realistic multistory building are presented

A virtual test facility for simulating the dynamic response of materials

The goal of the Caltech Center is to construct a virtual test facility (VTF): a problem solving environment for full 3D parallel simulation of the dynamic response of materials undergoing compression due to shock waves. The objective is to design a software environment that will: facilitate computation in a variety of experiments in which strong shock waves impinge on targets comprising various combinations of materials; compute the target materials' subsequent dynamic response; and validate these computations against experimental data. Successfully constructing such a facility requires modeling of the highest accuracy. We must model at atomistic scales to correctly describe the material properties of the target materials and high explosives; at intermediate (meso) scales to understand the micromechanical response of the target materials; and at continuum scales to capture properly the evolution of macroscopic effects. The article outlines such a test facility. Although it is a very simplified version of the facilities found in a shock-compression laboratory, our VTF includes all the basic features, offering a problem solving environment for validating experiments and facilitating further development of simulation capabilities

An animal-to-human scaling law for blast-induced traumatic brain injury risk assessment

Despite recent efforts to understand blast effects on the human brain, there are still no widely accepted injury criteria for humans. Recent animal studies have resulted in important advances in the understanding of brain injury due to intense dynamic loads. However, the applicability of animal brain injury results to humans remains uncertain. Here, we use advanced computational models to derive a scaling law relating blast wave intensity to the mechanical response of brain tissue across species. Detailed simulations of blast effects on the brain are conducted for different mammals using image-based biofidelic models. The intensity of the stress waves computed for different external blast conditions is compared across species. It is found that mass scaling, which successfully estimates blast tolerance of the thorax, fails to capture the brain mechanical response to blast across mammals. Instead, we show that an appropriate scaling variable must account for the mass of protective tissues relative to the brain, as well as their acoustic impedance. Peak stresses transmitted to the brain tissue by the blast are then shown to be a power function of the scaling parameter for a range of blast conditions relevant to TBI. In particular, it is found that human brain vulnerability to blast is higher than for any other mammalian species, which is in distinct contrast to previously proposed scaling laws based on body or brain mass. An application of the scaling law to recent experiments on rabbits furnishes the first physics-based injury estimate for blast-induced TBI in humans.United States. Army Research Office. Institute for Soldier Nanotechnologies (Contract DAAD-19-02-D-0002

Response on reinforced concrete structural elements to ballistic impact and contact detonations

Concrete is a widely studied material with a composite nature. It is used both in civil and military buildings and infrastructures. An issue of great importance is the protection of people from terrorist attacks that target critical infrastructure. Explosions, detonations and/or projectile impacts are some of the most severe actions a concrete structure can face. Experimental analysis is necessary in order to understand and predict the response of a structure to such dynamic and strain rate sensitive conditions. However, as the cost of performing experiments is significant and numerical simulations offer improved blast and impact analysis capabilities, there is an effort to limit experiments to validation purposes. In recent years, many researchers have studied the impact loads transferred to reinforced concrete (RC) structures both through direct projectile impacts or blast waves at both near and far field. The aim of the current study is twofold. First, to investigate contact detonations on this type of material (RC), since literature can provide us with limited information. Secondly, to assess the behaviour of the RC structure under combined ballistic impact and contact detonation of a very specific geometry of projectile (HESH) that exists currently on the market and behaves differently from the normal projectiles that consist of one single material. The author analysed and discussed in depth the response of RC members exposed to contact detonations. More precisely, the effect of the mass of explosive (C4) on pressures, impulses and energy balances. Also, she investigated the kinematic response of RC slabs and the structural role of the reinforcing bars. The driving force of this RC structures. Currently, the majority of studies regarding contact blast are focusing either on innovative types of concrete or normal concrete. However, normal concrete is investigated as a control parameter (to prove the effective resistance of the innovative material) rather than a detailed study on the behaviour of the material. Thereafter, the author analysed the response of a RC wall under the combined effect of kinetic energy (terminal ballistics) and contact detonation caused by the impact of a 90 mm HESH (High Explosive Squash Head) projectile fired from a distance of 70 m. The aim was to investigate the response of the structural member under the superposition of those two actions and analyse the combined effects of the impact velocity and detonation on the response of the structure. The numerical modelling is based on a Multi-Material-Arbitrary-Lagrangian-Eulerian approach (MMALE, using LS-DYNA) using the Winfrith concrete constitutive material model to investigate the dynamic response of the RC members under high strain rate conditions. The efficiency of the proposed numerical modelling is validated with experimental results – based on open-arena testing – and provided by the Royal Military Academy of Belgium. Some of the key findings of this research are that the increase of the amount of the explosive affects the damage failure of the RC members from flexural failure to shear failure. In addition, fitting curves that could be used in design, were proposed, that show the relation between the mass of explosive and the resulting pressures and impulses, within the tested range. In the case of the combined blast and impact scenario, the detonation was found to dominate the structural response of the RC slab

Summary of Research 2000, Department of Mechanical Engineering

The views expressed in this report are those of the authors and do not reflect the official policy or position of the Department of Defense or U.S. Government.This report contains project summaries of the research projects in the Department of Mechanical Engineering. A list of recent publications is also included, which consists of conference presentations and publications, books, contributions to books, published journal papers, and technical reports. Thesis abstracts of students advised by faculty in the Department are also included

Analytical and Experimental Evaluation of Precast Sandwich Wall Panels Subjected to Blast, Breach, and Ballistic Demands

Due to heightened security concerns federal as well as many public facilities require some level of blast design, whether it be intentional or accidental. In addition, with the increasing cost in utilities and continuous rise in global warming, a movement has begun to streamline the construction process and limit the environmental footprint of every building. In response, the federal government now requires that all government buildings not only be designed for blast loads, but also sustainability.Insulated wall panels are capable of meeting both the blast and sustainable requirements due to the inherit strength of a reinforced concrete slab and the thermal resistance provided from the insulating layer; however, limited experimental testing is available to prove that insulated wall panels are an ideal system for both blast and sustainability. The objective of this research is to develop the tools to design a blast and ballistic resistant insulated wall panel system. As part of this research, experimental tests were conducted on insulated panels to validate models developed to predict panel behavior observed. Using the results of the research an approach was developed to create a 1) Thermally efficient, 2) Blast Resistant, 3) Spall/Breach Resistant and 4) Ballistic Resistant panel.Insulated wall panels are inherently thermally resistive due to the insulating foam located between the two layers of concrete. Parametric studies were performed via analytical calculations to determine the efficiency of the wall system. The calculations indicated that the insulating layer is fundamental to the resistance of the panel; an 8in. solid concrete panel had a thermal resistance of less than 10% of a panel 2in. of insulation sandwiched between two 3in. concrete wythes. Additionally, the parametric study indicated that the shear connectors located between the interior and exterior wythes can have a significant effect on the overall panel thermal resistance due to the thermal bridging phenomenon. Three panels were modeled with identical layout and wythe connectors with identical dimensions but different material: concrete, steel, and low-conductive material. The panel with concrete and steel wythe connectors saw a reduction in thermal resistance compared to the low-conductive material of nearly 78% and 62%respectively. Thus, to decrease the panel resistance while maintaining strength, a strong thermally resistive material must be used as a shear connector.To improve the response to far-field detonations, experimental tests were performed on small solid panels as well as larger insulated panels. Locally unbonding the small solid panels allowed the panel to reach support rotations past the 10° specified by the United States Army Corps of Engineers as the highest threat level while the bonded panels reached less than 5° before softening. Additionally, testing of insulated wall panels revealed that the panel behavior is highly dependent on the shear tie constitutive property and location along the span. A numerical model was created to predict the behavior of an insulated and as a result, a new shear tie was developed to improve the flexural response of the panel while at the same time, decreasing the production cost.To assess the response of insulated wall panels to close-in detonations, experimental tests and numerical models were conducted. The tests revealed that the insulation results in a detriment to panel performance as a panel with 2in. of insulation sandwiched between two 3in. thick concrete wythes breaches the exterior wythe while a 6in. thick solid concrete panel does not breach under the same demand. As the insulating layer thickness is increased, the panel does not breach due to the increased standoff created by the additional thickness. Additionally, the empirical formulas developed by the Unified Facilities Criteria for solid panels were shown to be inaccurate when used for insulated wall panels, while numerical simulations were able to bound the response of an insulated wall panel.To investigate the performance of insulated wall panels to ballistic and fragment demands, a probabilistic method was developed. The method results in the creation of fragility curves allowing a designer to assess the probability of perforation and residual velocity for a given threat at any wall thickness. Additionally, the likelihood of injury occurring to personnel behind the wall panel was assessed by using organ threshold tolerances provided in literature. Using the method developed, engineers can design the thickness of an insulated wall panel to achieve an acceptable probability of occurrence for injury.Finally, all of the material learned through the first four stages were combined to create a comprehensivedesign example. An 8in. thick panel with 2in. of insulation was designed using the newly designed shear tie as well as a ductile shear tie with the same strength, and then subjected to the demands reviewed throughout the research project. The tie system allowed the wall to reach a support rotation of 10° while behaving in a moderate to heavy damage level when subjected to the far-field detonation demand. From the conclusions of the close-in detonation study, the panel is known to breach under the load prescribed. Ballistic fragility curves were developed showing that the panel stops a low threat ballistic with 100% certainty, but under a high ballistic threat the projectile has an 86.5% chance of perforating the wall system. For the fragmenting munition considered in the study, the wall system has a 15.4% chance of causing injury to personnel behind the wall. Finally, by using the new shear tie system developed, the wall system results in a reduction of less than 3% in the total R-value when compared to an insulated panel without thermal bridges due to the low thermal conductivity of the shear tie material