Impact resistance of polyurea coated aluminium plates subjected to low and high velocity impacts

© 2013 Dr. Pushpa Damith Jayasekara MohottiPublications included in thesis:Mohotti, D., Ali, M., Ngo, T., Lu, J., & Mendis, P. (2014). Strain rate dependent constitutive model for predicting the material behaviour of polyurea under high strain rate tensile loading. International Journal of Materials and Design, 53, 830-837. DOI: 10.1016/j.matdes.2013.07.020Mohotti, D., Ngo, T., & Mendis, P. [2012]. Comparison of two aluminium alloys for their energy absorption capacity under high strain rate dynamic loading. International Journal of Aerospace and Lightweight Structures (In Press).Mohotti, D., Ngo, T., Mendis, P., & Raman, S. N. [2013]. Polyurea coated composite aluminium plates subjected to high velocity projectile impact. Materials and Design, 52, 1-16. DOI: 10.1016/j.matdes.2013.05.060Mohotti, D., Ngo, T., Raman, S. N., & Mendis, P. [2013]. Analytical and numerical investigation of polyurea coated composite aluminium plates subjected to high velocity projectile impact. International Journal of Impact Engineering (Under Review).Mohotti, D., Ali, M., Ngo, T., Lu, J., Mendis, P., & Ruan, D. (2013). Out-of-plane impact resistance of aluminium plates subjected to low velocity impacts. Materials and Design, 50, 413-426. DOI: 10.1016/j.matdes.2013.03.023‘Polyurea coated composite aluminium plates subjected to low velocity impact-experimental and numerical investigations’ published as: Mohotti, D., Ngo, T., Raman, S. N., Ali, M., & Mendis, P. (2014). Plastic deformation of polyurea coated composite aluminium plates subjected to low velocity impact. Materials and Design, 56, 696-713. DOI: 10.1016/j.matdes.2013.11.063With the increasing possibility of structural damage due to natural disasters, collision of vehicles, and blast and ballistic impacts, the demand for protective measures for structures is on the rise. Localised impacts are among one of the most common loading mechanisms that many modern structures undergo during their life span. Over the years, many investigations have been conducted in order to mitigate the structural damages caused by impact actions from different objects such as flying projectiles, debris, hail and collision of vehicles. Due to their unpredictability and complex nature, impact loads are considered one of the most intricate loading mechanisms to mitigate in the field of structural engineering. Engineers have been depending on high strength and high hardness materials to achieve the required resistance against such severe loadings. Better performance was achieved by either increasing the strength of the materials, or by increasing the dimensions of the structural elements. However, in recent years, attention has been tilted towards using lightweight materials in structural components in areas such as the armour industry. Due to their low density, there is comparatively high demand for lightweight materials in many industrial applications. Metal–elastomer composites can be considered as one of the main alternatives in such applications. Over the last decade, the use of elastomers as composite materials to mitigate blast loads has been extensively investigated. However, less attention has been paid to study the use of composites subjected to projectiles impacts. Therefore, in this study, aluminium–polyurea composite (metal–elastomer) has been selected as an alternative structural material to improve the performance of structures under impact loading. This thesis presents experimental, analytical and numerical investigations of aluminium–polyurea composite plates subjected to low and high velocity projectile impacts. The research has been divided into three main parts: (a) an experimental study of material behaviour under different strain rate loadings, (b) experimental, analytical and numerical investigation of aluminium–polyurea composite plates subjected to low velocity impacts, and (c) a comprehensive study of the behaviour of aluminium–polyurea composite plate systems subjected to high velocity projectile impacts. Each section consists of two peer reviewed journal papers that have either been published or are under review in international engineering journals. Over the years, aluminium alloys have been considered as a possible partial substitute for structural steel. Aluminium alloys AA5083-H116 and AA6061-T656 have been investigated for their behaviour under different strain rates, ranging from 10-3/s to 104/s. Aluminium alloy AA5083-H116 was selected as the base material in the formation of the composite in this study. With the advancement of alloying techniques, the manufacturers have been able to produce aluminium alloys with ultimate tensile strengths in the range of 450–750 MPa. Especially in the personal armour industry, aluminium AA5083-H116 is a commonly used alloy due to its considerably high ductility and moderate strength. Strain rate sensitivity and energy absorption capacities were obtained from the experimental data and are discussed in this thesis. In the present study, polyurea has been selected to form a composite with the aluminium alloy. Due to its ability to bond well with materials such as concrete and steel, and its advancement in manufacturing techniques over the last few years, polyurea is considered as one of the prospective materials to be used in composites, especially in the application of impact mitigation. High strain rate behaviour of polyurea under tensile loading has been studied, and a constitutive model to predict high strain rate behaviour of polyurea under different loading conditions has been proposed. This simplified model is capable of predicting strain rate sensitivity of polyurea reasonably well. Low velocity impact test programs on both uncoated and coated aluminium plates have been conducted to investigate the ability of polyurea to resist permanent damage caused by such impacts. An analytical model to predict plastic deformation of aluminium plates subjected to low velocity impacts was also proposed. The same experimental program was extended to investigate the effects of polyurea coating in terms of energy absorption and reduction in permanent structural damage. A comprehensive numerical study has been performed to understand the behaviour of composite plates under the impact of projectiles travelling at low velocities using the advanced finite element code LS-DYNA. One of the main objectives of this study was to investigate the applicability of layered aluminium–polyurea composite plate systems to mitigate ballistic impacts. An experimental study was conducted in order to evaluate the performance of aluminium–polyurea composite plate systems subjected to penetration by NATO standard ammunitions (high velocity impact). A broad study was performed using finite element code LS-DYNA on numerical simulation of the experimental program. The models were validated with the experimental results. An analytical model to predict the residual velocities of the composite plate systems has been proposed and validated with both experimental and numerical results. The results of this research, which have been summarised in six journal papers, have been used to show the applicability of polyurea–aluminium composites in mitigation of low and high velocity projectile impacts. Strain rate sensitivity of the mechanical properties of aluminium and polyurea has been highlighted. A strain rate dependent constitutive material model was proposed based on the well-known Mooney-Rivlin model. Applicability of polyurea or a similar type of elastomer in reducing the residual velocity and acting as a protective shield in composite plate arrangements was highlighted. This finding can be used effectively in armour and many other industries in manufacturing light weight structures. An analytical model to predict the residual velocity of projectiles penetrating through composite plate systems has been proposed. This model can be effectively used to predict the residual velocities of multi-layer composite systems. In addition applicability of polyurea-aluminium composite in mitigation of low velocity impact has been highlighted. Polyurea has shown good ability in energy absorption, and subsequently has reduced the permanent deformation of the composite structure. Therefore, the outcomes of this research study can be used in the future in the protective structures design industry

Optimization of Energy Utilization for dewatering system in Bogala Graphite Mine, Aruggammana, Sri Lanka

Mining sustainability implies the idea of extracting non-renewable resources from the Earth at maximum extent and minimum environmental impact. Any mine has a certain economic mining depth beyond which the production cost for ton of product will be greater than the income generated due to increasing operational costs. Considerable contribution to operational cost is generated by the energy consumption for dewatering, ventilation and man & material hoisting. Dewatering cost is often considered among the most critical and governing factors that decide the economic mining depth of an underground mine, especially if it is located in a wet climatic zone. Reduction of energy expenditure and cost for dewatering leads to increase of economic mining depth, consequently expanding the resource extraction and ensuring the growing sustainability of the mining industry.  This study focuses on the dewatering system in Bogala graphite mine, a medium-depth underground mine located in the wet region of Sri Lanka. The methodological approach proposed in this work aims to optimize the energy utilization for dewatering and can be adapted to any general underground mine dewatering system. An Energy System Analysis targeted the critical elements of the dewatering system identified during the literature survey and verified by field studies carried out onsite. The main objective of a dewatering system is to drive up the water which accumulates underground to the surface with the use of combination of pumps. Selecting a more effective combination together with the application of more efficient pumps is one potential option for optimizing the dewatering energy consumption. Another option is to control underground water accumulation by suppressing the origin of underground water in the particular mine; however, the economical viability of implementing control measures to suppress water origin and accumulation should be carefully analysed since the cost of such implementation would sometimes be unrecoverable throughout the mine’s life.     This report evaluates several possible engineering applications to control the root-causes of underground water accumulation & recirculation while improving energy efficiency of water conveyance taking into consideration the viability under technical, financial and environmental constraints in order to optimize the energy utilization of the dewatering system in Bogala mines

Numerical simulations of response of tubular steel beams to close-range explosions

A numerical study of hollow and concrete-filled square tubular steel columns subjected to near-field detonations has been undertaken and validated through the experimental results. The experiments used concrete-filled and hollow square tubular columns (100 x 5 mm SHS Grade C350) made out of cold-formed structural steel hollow sections (SHS) that were simply supported at both ends. High explosives TNT charges were placed above the top surface of the column at two different scaled standoff distances of 0.12 m/kg1/3 and 0.15 m/kg1/3. Failure patterns and permanent mid-span deformations were recorded and compared with the numerical analysis results. Arbitrary Lagrangian-Eulerian (ALE) formulations coupled with fluid-structure interaction (FSI) algorithms that are available in the advanced finite element code LS-DYNA were used in the numerical study. A detailed description of the numerical technique adopted in the study is presented. The models were validated with the experimental results and were used to obtain the failure pattern, permanent plastic deformation, pressure and impulse time histories, stress distribution, and energy absorption of the different configurations of the columns. The performance of hollow and concrete-filled SHS tubes for blast load mitigation was assessed and discussed

Incorporation of shear thickening fluid effects into computational modelling of woven fabrics subjected to impact loading : a review

Soft armour consisting of multi-layered high-performance fabrics are a popular choice for personal protection. Extensive work done in the last few decades suggests that shear thickening fluids improve the impact resistance of woven fabrics. Shear thickening fluid–impregnated fabrics have been proven as an ideal candidate for producing comfortable, high-performance soft body armour. However, the mechanism of defeating a projectile using a shear thickening fluid–impregnated multi-layered fabric is not fully understood and can be considered as a gap in the research done on the improvement of soft armour. Even though considerable progress has been achieved on dry fabrics, limited studies have been performed on shear thickening fluid–impregnated fabrics. The knowledge of simulation of multi-layered fabric armour is not well developed. The complexity in creating the geometry of the yarns, incorporating friction between yarns and initial pre-tension between yarns due to weaving patterns make the numerical modelling a complex process. In addition, the existing knowledge in this area is widely dispersed in the published literature and requires synthesis to enhance the development of shear thickening fluid–impregnated fabrics. Therefore, this article aims to provide a comprehensive review of the current methods of modelling shear thickening fluid–impregnated fabrics with a critical analysis of the techniques used. The review is preceded by an overview of shear thickening behaviour and related mechanisms, followed by a discussion of innovative approaches in numerical modelling of fabrics. A novel state-of-the-art means of modelling shear thickening fluid–impregnated fabrics is proposed in conclusion of the review of current methods. A short case study is also presented using the proposed approach of modelling

An innovative approach of using continuous impedance-graded metallic composite system for attenuation of stress waves

This paper presents an innovative approach of stress attenuation through a continuous impedance-graded material system for high strain-rate events. High energetic dynamic events such as blasts and impact could cause stress waves - in the form of elastic, plastic, and shock - to propagate in a solid material. An impedance-graded composite is created by arranging different metallic alloys in the reducing order of their impedance through the system. Impedance, which is the product of volumetric mass density and wave velocity, is chosen as the function as it plays a governing role in elastic, plastic, and shock waves. An analytical framework to quantify the stress wave propagation through an impedance-graded multimaterial system is developed based on the principles of shock and elastic wave theories. The numerical simulations carried out using nonlinear finite element code, LS-DYNA, were able to capture and quantify the elastic, plastic, and shock waves and their reflections at different interfaces. It was identified that the final transmitted stress wave, which could comprise elastic, plastic, and shock waves, as well as the reflected tensile elastic wave at each material interface, needs to be controlled in order to develop a robust multimaterial system

Behaviour of explosively welded impedance-graded multi-metal composite plates under near-field blast loads

This paper presents a comprehensive analysis of the blast response of functionally graded composite metallic plates, fabricated using explosive welding. Explosive welding is a solid-state welding technique, which has previously shown capabilities in bonding metals that have different chemical and mechanical properties. Impedance, which is the product of volumetric mass density and wave propagation velocity for a given material, was chosen as the function when designing the graded composite plates. The performance of two composite plate configurations, namely, Steel-Titanium-Aluminium and Steel-Brass-Aluminium were compared with a monolithic steel configuration of equal overall plate dimensions. The plates were subjected to highly intensive blast loads produced by detonation of the 250 g cylindrical Composition-B charges at standoff distances between 20-65 mm and spherical 1 kg Nitromethane charges at standoff distances of 200 mm and 250 mm. Detailed numerical models of near-field loading of the monolithic and composite plates were developed using the non-linear finite element analysis software LS-DYNA and validated using the experimental deformation measurements. Experimental evaluation of the impedance-graded explosively- welded composite plates has been carried out for the first time and herein lies the novelty of the work presented in the paper. It was observed that the impedance graded composite plates, which were lighter in density than the monolithic plates, resisted the highly intensive blast loads through their enhanced ductility

Experimental investigation and simplified modeling of response of steel plates subjected to close-in blast loading from spherical liquid explosive charges

Detonations of nitromethane spherical charges have been carried out to study close-in blast loading of steel plates and the effectiveness of several protective solutions. Three types of bare steel plates, namely mild steel, high-strength steel, and stainless steel were subjected to explosive blast loading. Steel plates of the same type with polyurea coating and composite covers were also subjected to localized blast loading. During an explosive field trial, the blast pressures and displacements of steel plates were measured. Additionally, loading of steel plates by the impinging detonation products was captured by high-speed video recordings. This experimental program has produced results which can be used to calibrate numerical models and to refine the simplified models for predicting blast loads and response of structural elements due to close-in detonations. The effectiveness of polyurea coating for enhancing blast protection of steel plated structures is discussed. The engineering-level model for predicting the blast impact impulse of the detonation gases from the charges in close proximity from the target is introduced and validated using the experimental results obtained during the course of the explosive trials

Impact evidence and post-ricochet behaviour of shotgun pellets ricocheting off standard floor tiles

Compared to popular handguns and rifle bullets, quantitative and empirical-based ricochet studies using shotgun pellet ricochets are observed to be far fewer. This empirical study examines the ricochet behaviour and impact evidence when shotgun pellets (Buckshot) ricochet off standard floor tiles, providing a series of novel findings related to the resultant ricochet marks. Among these findings, a novel and statistically significant relationship between the lengths and widths of individual ricochet marks and the shot impact angles is demonstrated, offering useful forensic application. Ricochet mark shapes and morphologies highlighted in this study at different impact angles demonstrate the interactions between the ricocheting spherical pellets and tile surfaces, and the effects of acting frictional forces and degree of energy transfer in the production of impact evidence on the tile surface. Relationships with high statistical significance were also reported between the shot spreads on the tile surfaces and the post-ricochet cardboard witness screens, with shot impact angles. Finally, this work reports on the first documented observations of ’Pinch Points’ and ’Nucleus’ ricochet marks with shotgun pellet ricochets as angle-specific phenomena