Multimodal micron-sized spherical aluminum powders were subjected to the detonation products of a gaseous oxyacetylene mixture. The objective was to analyze the blast enhancement from the combustion of non-uniform-sized aluminum particles. These multimodal aluminum powders consisted of a 50/50 mixture by mass of larger (~30 μm) and smaller (~1-10 μm) particles. Experiments were conducted at the large-scale Virginia Tech Shock Tube Research Facility to measure blast pressure, impulse, and heat release efficiency during combustion in these detonations. These results were compared against oxyacetylene detonations conducted with the addition of unimodal aluminum particles approximately 1, 10, 30, and 95 μm in diameter. These experiments were controlled by maintaining a particle mass concentration of 200 g/m3, a constant volume of air for particle dispersion, and a consistent size for the gaseous oxyacetylene explosive charge of 0.11 m3. This approach ensured that any variations in explosive output were due to the characteristics of the aluminum powder. For unimodal aluminum, the combustion of 1 μm aluminum powder yielded the highest increase in blast pressure, impulse, and heat of combustion efficiency whereas H-95 provided the least amount of blast enhancement. These results showed an inverse relationship where decreasing aluminum particle size resulted in increased blast output, a phenomenon driven by the shorter combustion times of smaller particles. For multimodal aluminum combustion, the performance of these powders exceeded the pressure and impulse performance of their unimodal counterparts. The heat of combustion efficiency—defined as the ratio of energy driving the shock wave to the total energy available—was estimated using a two-part blast scaling methodology. The first step in this process used Sachs' blast scaling laws to infer time-dependent energy release contributing initially to blast pressure and impulse. The second step introduced a new modified Sachs scaling technique to account for late-time energy release contributing solely to blast impulse. This scaling approach addressed the previously neglected impact of delayed aluminum combustion on blast behavior. This two-part scaling approach revealed that the combustion of multimodal aluminum powders in oxyacetylene detonations resulted in 75.1%-85.3% of the available heat of combustion contributing to blast pressure and impulse compared to the 30.8%-74.6% provided by unimodal aluminum powders. These results suggest that the combustion of multimodal aluminum powder results in more powerful and efficient detonations, providing a technique to improve and optimize energetic performance. / Master of Science / Micron-sized spherical aluminum powders serve as additives to enhance the performance of propellants, pyrotechnics, and explosives. Previous laboratory-scale research has shown that aluminum's ignition and combustion characteristics are influenced by particle size, with smaller particles tending to ignite more quickly and release more energy than larger ones. However, little research has been directed at understanding the impact of particle size distribution on aluminum combustion, and whether combining smaller particles with larger ones can enhance the overall combustion reactivity and efficiency. This work investigated the impact of mixed (multimodal) aluminum combustion on the blast pressure, impulse, and overall heat of combustion efficiency of oxyacetylene detonations. To achieve this, the experimental procedure consisted of three testing series: (i) oxyacetylene detonations without aluminum powder; (ii) unimodal aluminum combustion in oxyacetylene detonations; and (iii) multimodal aluminum combustion in oxyacetylene detonations. These blast experiments were conducted using the large-scale Virginia Tech Shock Tube Research Facility. This detonation-driven shock tube maintained a constant aluminum particle mass concentration of 200 g/m3, a constant volume of air for particle dispersion, and a consistent size for the gaseous oxyacetylene explosive charge of 0.11 m3. This experimental design ensured that any variations in explosive output were due to the explosive charge size and particle characteristics of the aluminum powder. Results showed that introducing unimodal aluminum powder into oxyacetylene detonations significantly enhanced blast pressure, impulse, and energy efficiency compared to the control case of pure oxyacetylene. Furthermore, a reduction in the mean particle size of aluminum powder resulted in greater blast output, revealing an inverse relationship where smaller particle sizes led to higher blast performance due to their faster reaction rates. For multimodal aluminum powders, the use of mixed particle sizes produced even greater blast pressure, impulse, and energy efficiency than their unimodal counterparts. These findings indicate that the combustion of multimodal aluminum powder produces more powerful and efficient detonations, providing an approach to enhance and optimize energetic performance.

Modeling and simulation have played an essential role in understanding the effects of blast waves. However, a broad area of engineering problems, such as vehicle structures, buildings, bridges, or even the human body, can benefit by accurately predicting the response to blasts with little need for test or event data. This thesis reviews fundamental concepts of blast waves and explosives and discusses research in blast scaling. Blast scaling is a method that reduces the computational costs associated with modeling blasts by using empirical data and numerically calculating blast field parameters over time for various types and sizes of explosives. This computational efficiency is critical in studying blast waves' near and far-field effects. This thesis also reviews research to differentiate between free-air blasts and gun muzzle blasts and the progress of modeling the muzzle blast-structure interaction. The main focus of this thesis covers an investigation of different numerical and analytical solutions to a simple aerospace structure subjected to blast pressure. The thesis finally presents a tool that creates finite element loads utilizing muzzle blast scaling methods. This tool reduces modeling complexity and the need for multiple domains such as coupled computational fluid dynamics and finite element models by coupling blast scaling methods to a finite element model. / Master of Science / {Numerical integration methods have helped solve many complex problems in engineering and science due to their ability to solve non-linear equations that describe many phenomena. These methods are beneficial because of how well they lend to programming into a computer, and their impact has grown with the increases in computing power. In this thesis, ``modeling and simulation" refers to the characterization and prediction of an event's outcome through the use of computers and numerical techniques. Modeling and simulation play important roles in studying the effects of blast waves in many areas of engineering research such as aerospace, biomedical, naval, and civil. Their capability to predict the outcome of the interaction of a blast wave to vehicle structures, buildings, bridges, or even the human body while requiring limited experimental data has the chance to benefit a wide area of engineering problems. This thesis reviews fundamental concepts of blast waves, explosives, and research that has applied blast loading in modeling and simulation. This thesis describes the complexity of modeling an axially symmetric blast wave interaction by comparing the numerical and theoretical response blast loaded structure.

<p>Explosions are associated with more than 80% of the casualties in the Iraq and Afghanistan wars. Given the widespread use of thoracic protective armor, the most prevalent injury for combat personnel is blast-related traumatic brain injury (TBI). Almost 20% of veterans returning from duty had one or more clinically confirmed cases of TBI. In the decades of research prior to 2000, neurotrauma was under-recognized as a blast injury and the etiology and pathology of these injuries remains unclear.</p><p>This dissertation used the finite element (FE) method to address many of the biomechanics-based questions related to blast brain injuries. FE modeling is a powerful tool for studying the biomechanical response of a human or animal body to blast loading, particularly because of the many challenges related to experimental work in this field. In this dissertation, novel FE models of the human and ferret head were developed for blast and blunt impact simulation, and the ensuing response of the brain was investigated. The blast conditions simulated in this research were representative of peak overpressures and durations of real-world explosives. In general, intracranial pressures were dependent on the peak pressure of the impinging blast wave, but deviatoric responses in the brain were dependent on both peak pressure and duration. The biomechanical response of the ferret brain model was correlated with in vivo injury data from shock tube experiments. This accomplishment was the first of its kind in the blast neurotrauma field.</p><p>This dissertation made major contributions to the field of blast brain injury and to the understanding of blast neurotrauma. This research determined that blast brain injuries were brain size-dependent. For example, mouse-sized brains were predicted to have approximately 7 times larger brain tissue strains than the human-sized brains for the same blast exposure. This finding has important implications for in vivo injury model design, and a scaling model was developed to relate animal experimental models to humans via scaling blast duration by the fourth-root of the ratio of brain masses. </p><p>This research also determined that blast neurotrauma is correlated to deviatoric metrics of the brain tissue rather than dilatational metrics. In addition, strains in the blasted brain were an order-of-magnitude lower than expected to produce injury with traditional closed-head TBI, but an order-of-magnitude higher in strain rate. The 50th percentile peak principle strain metric of values of 0.6%, 1.8%, and 1.6% corresponded to the 50% risk of mild brain bleeding, moderate brain bleeding, and apnea respectively. These findings imply that the mechanical thresholds for brain tissue are strain-based for primary blast injury, and different from the thresholds associated with blunt impact or concussive brain injury because of strain rate effects.</p><p>The conclusions in this dissertation provide an important guide to the biomechanics community for studying neurotrauma using in vivo, in vitro, and in silico models. Additionally, the injury risk curves developed in this dissertation provide an injury risk metric for improving the effectiveness of personal protective equipment or evaluating neurotrauma from blast.</p> / Dissertation

<p>The focus of this study is on evaluating the performance of nine, one-third scale, arching, reinforced masonry (RM) walls subjected to blast loads and three, one-third scale arching, RM walls subjected to out-of-plane static airbag loads. These RM walls were supported on four sides to enforce two-way arching allowing the ability to monitor individual response to varying levels of blast loads and standoff distances. The uniformity of the blast pressure and impulse were ensured by a specifically designed test enclosure, diminishing the wrap-around and clearing effects, allowing valuable data to be documented. The damage levels noted, ranged from Superficial to Blowout compared directly to the CSA S850-12 performance limits. The outcome of this experiment demonstrates the beneficial effect of two-way arching on the flexural behaviour of RM walls under impulsive loading. The use of two-way arching RM walls significantly reduces structural damage and increases out-of-plane resistance, which in turn enhances the overall structural integrity and building preservation. Further, when subjected to blast, two-way arching RM walls considerably reduces debris and their dispersal, thus increasing public safety and minimizing hazard levels. When using the experimental test data results to calibrate finite element models (FEM), more analytical data points can be obtained and therefore getting a larger range of scaled distances and trials. The validation of the LS-DYNA model can be used as an alternative to the costly experimental data, as the information collected concluded that the FEM gave damage patterns and failure modes that were comparable with experimental results. The test data collected provides a better understanding of RM wall response to blast loads and to the ongoing Masonry Blast Performance Database (MBPD) project at McMaster University. The generated MBPD will in turn contribute to masonry design clauses in the future editions of the recently introduced Canadian Standards CSA S850-12 “Design and Assessment of Buildings Subjected to Blast Loads”.</p> / Master of Applied Science (MASc)

The current thesis focuses on estimating the damage levels and evaluating the out-of-plane behavior of fully-grouted reinforced masonry (RM) structural walls under blast loading, a load that they are typically not designed to resist. Twelve third-scale RM walls were constructed and tested under free-field blast tests. Three different reinforcement ratios and three different charge weights have been used on the walls, with scaled distances down to 1.7 m/kg1/3 and two different boundary conditions, to evaluate the walls’ performances. In general, the results show that the walls are capable of withstanding substantial blast load levels with different extents of damage depending on their vertical reinforcement ratio and scaled distance. It worth mention that the current definitions of damage states, specified in ASCE/SEI 59-11 (ASCE 2011) and CAN/CSA S850-12 (CSA 2012) standards, involve global response limits such as the component support rotations that are relatively simple to calculate. However, these quantitative damage state descriptors can be less relevant for cost–benefit analysis. Moreover, the reported experimental results showed that the use of quantitative versus qualitative damage descriptors specified by North American blast standards [ASCE 59-11 (ASCE 2011) and CSA S850-12 (CSA 2012)] can result in inconstancies in terms of damage state categorization. Therefore, revised damage states that are more suitable for a cost–benefit analysis, including repair technique and building downtime, were presented. These damage states are currently considered more meaningful and have been used to quantify the post-earthquake performance of buildings. In addition, a nonlinear single-degree-of-freedom (SDOF) model is developed to predict the out-of-plane behavior of RM structural walls under blast loading. The proposed SDOF model is first verified using quasi-static and free-field blast tests and then subsequently used to extend the results of the reported experimental test results with different design parameters such as threat level, reinforcement ratio, available block width, wall height, and material characteristics. In general, brittle behavior was observed in the walls with a reinforcement ratio higher than 0.6%. This is attributed to the fact that seismically detailed structural masonry walls designed to respond in a ductile manner under in-plane loads might develop brittle failure under out-of-plane loads because of their reduced reinforcement moment arm. In addition, increased ductility can be achieved by using two reinforcement layers instead of a single layer, even if the reinforcement ratio is reduced. Also, it is recommended to consider the use of larger concrete masonry blocks for the construction of RM structural walls that are expected to experience blast loads in order to reduce the slenderness ratio and for the placement of two reinforcement layers. Finally, a probabilistic risk assessment (PRA) framework is proposed in order to develop design basis threat (DBT) fragility curves for reinforced concrete block shear wall buildings, which can be utilized to meet different probabilities of failure targets. To illustrate the proposed methodology, an application is presented involving a medium–rise reinforced masonry building, under different DBT levels. The DBT fragility curves are obtained via Monte Carlo sampling of the random variables and are used to infer the locations, within the building premises, that are most suitable for the erection of barriers for blast hardening. / Thesis / Doctor of Philosophy (PhD)