In this study, impact craters in water ice are modeled using the hydrodynamic code CTH. In order to capture impact craters in ice an equation of state and a material model are created and validated. The validation of the material model required simulating the Split Pressure Hopkinson Bar (SPHB) experimental apparatus. The SPHB simulation was first compared to experiments completed on Al 6061-T6, then the ice material model was validated. After validation, the cratering simulations modeled known experiments found in the literature. The cratering simulations captured the bulk physical aspects of the experimental craters, and the differences are described. Analysis of the crater simulations showed the damaged volume produced by the projectile was proportional to the projectile’s momentum. Also, the identification of four different stages in the crater development of ice (contact and compression, initial damage progression, crater shaping, and ejected damaged material) are described.

The initiation and propagation of adiabatic shear bands is analyzed numerically for an impulsively loaded thick-walled steel tube. A circumferential V-notch located at the outer surface of the center of the tube provides a stress concentration. The material is modeled as strain hardening, strain-rate hardening and thermal softening. The dynamic loading conditions considered are pure torsion, axial pressure combined with torsion, and internal pressure combined with torsion. Because of the stress concentration, a shear band will first initiate in an element adjoining the notch tip and propagate radially inwards through the thickness of the tube. The speed of propagation and the amount of energy required to drive a shear band through the material are calculated. The effects of the pressure preload and the depth of the notch are studied. Also, the influence of thermal softening is investigated by modeling it after a relation proposed by Zhou et al. <i>[Vita removed July 18, 2008 CK/GMc 2/2/2012]<i> / Master of Science

The optimisation of compressor stage aerofoil and fan blade design remains an important area of titanium alloy research and development for aerospace gas turbines. Such research has important implications for critical and sensitive component integrity and efficiency. In particular, a better understanding of how deformation twinning interacts with microstructural features in titanium alloys is required, because such twinning facilitates plastic deformation at a higher strain rate than dislocations. To investigate this behaviour, commercial purity titanium and the titanium alloy Ti–6Al–4V were subjected to ballistic impact testing at room temperature with a high strain rate of 10³s⁻¹. In addition, a detailed analysis was conducted of three manufacturing processes of Ti–6Al–4V (wt. %) that are likely to cause deformation twinning: metallic shot peening, laser shock peening and deep cold rolling. The results presented in this thesis have furthered the understanding of the role of deformation twinning in the plastic deformation of α-phase titanium. Key findings of the research include the characterisation of deformation twinning types and the conditions that favour certain deformation twinning types. From the analysis of the ballistic testing of commercial purity titanium, the first definitive evidence for the existence of {112‾4} twinning as a rare deformation twinning mode at room temperature in coarse-grained commercial purity titanium is presented. In addition, the ballistic testing results of the Ti–6Al–4V alloy highlighted very different deformation twinning characteristics. Commercial purity titanium deformed plastically by a combination of {101‾2} and {112‾1} tensilve twinning and {112‾4} and {112‾2} compression twinning modes. By contrast, the deformation twinning of Ti–6Al–4V was limited to only the {101‾2} and {112‾1} tensile twinning modes. The two tensile deformation twinning types have very different morphologies in equiaxed fine grained Ti–6Al–4V. {112‾1} deformation twins span multiple grain boundaries and {101‾2} deformation twins reorient entire grains to a twinned orientation. This observation provides evidence for whole grain twinning of equiaxed fine grained Ti–6Al–4V by {101‾2} twinning. Grain boundary interactions between various deformation twinning types and alpha phase grain boundaries in commercial purity titanium and Ti–6Al–4V are reported and analysed. In commercial purity titanium {101‾2} as well as other deformation twinning types were observed interacting across alpha phase boundaries and higher angle alpha phase grain boundaries. The analyses of the manufacturing processes of Ti–6Al–4V highlight the very different dislocation and deformation twinning structures in surfaces processed by these techniques. A notable feature of material processed by laser shock peening is the almost complete absence of deformation twinning, contrasting with the frequent observation of extensive deformation twinning observed in the material processed by metallic shot peening and deep cold rolling. Therefore, the findings suggest that there is a strain rate limit above which deformation twinning is suppressed. The implications of this research are that a better understanding of the conditions that that favour certain deformation twinning types or propagation behaviours will enable more accurate plasticity modelling and better alloy design. This is important for the design and the manufacturing of titanium components and the high strain rate deformation to which titanium components in aerospace gas turbines can be subjected because of bird strike, foreign object debris ingestion or fan blade failures.

No description available.

The need for efficient, lightweight armour solutions has never been so great as it is today. Increasing numbers of personnel, both military and civilian are being placed in an expanding variety of life-threatening situations, and we must recognise the responsibility to maximise their combat survivability. One way to help protect these people is to provide them with some form of armour. Advanced polymeric materials are finding an increasing range of industrial and defence applications. These materials have the potential to improve the performance of current armour systems, whilst also reducing their cost and weight. Polymers may be reinforced with the addition of nanofillers such as carbon nanotubes or graphene, to produce nanocomposites, an exciting emerging polymer technology. Nanomaterials have been shown to exhibit extraordinary strength, far higher than that of traditional armour materials. Nanocomposites have the possibility of being remarkable materials, with high strength and light weight. The work detailed in this report is an investigation into the mechanical properties of nanocomposites along with some novel blended polymer composites. Two compressive testing techniques have been used to carry out this investigation. The intermediate strain-rate Optical Drop-Weight, and the high strain-rate Split-Hopkinson Pressure Bar. The latter required some significant modifications in order to optimise it for use with low-density polymers. Ultimately, nanocomposites were found to behave virtually indistinguishably from the monolithic polymer matrices. Yield strengths and energy absorption characteristics remained inside the ordinary experimental scatter. Blended composites, in which a long chain length polymer is combined with a chemically similar polymer with a shorter chain length, proved to be more interesting. Yield strengths of these novel materials were increased over that of either constituent material, although energy absorption remained low.

The purpose of this study is to examine the propagation of waves of finite deformation in rubbers through experiments and analysis. First, attention is focused on the propagation of one-dimensional dispersive waves in strips of latex and nitrile rubber. Tensile wave propagation experiments were conducted at high strain-rates by holding one end fixed and displacing the other end at a constant velocity. A high-speed video camera was used to monitor the motion and to determine the evolution of strain and particle velocity in rubber strips. Analysis of the response through the theory of finite wave propagation indicated a need for an appropriate constitutive model for rubber; by quantitative matching between the experimental observations and analytical predictions, an appropriate instantaneous elastic response for the rubbers was obtained. This matching process suggested that a simple power-law constitutive model was capable of representing the high strain-rate response for both rubbers used. Next, the propagation of one-dimensional shock waves in strips of latex and nitrile rubber is examined. Shock waves have been generated under tensile impact in pre-stretched rubber strips; analysis of the response yields the tensile shock adiabat for rubbers. The propagation of shocks is analyzed by developing an analogy with the theory of detonation. Attention is then focused on the propagation of unloading waves of finite deformation in a rubber specimen analytically and experimentally. A rubber strip stretched to many times its initial length is released at one end and the resulting unloading is examined. Dispersive waves as well as shock waves are observed in these experiments. Quantitative discrepancies between the analytical model and experimental observations are again used to motivate a power-law model. Hysteresis in the response is attributed to strain-induced crystallization and melting phase transitions in natural latex rubber, and to nonequilibrium microstructural deformation in nitrile rubber. Finally, a Kolsky experiment is conducted and analyzed under the framework of dispersive loading and unloading waves utilized in the previous experiments. In this experiment, a phase boundary is introduced separating low and high strain phases of the rubber and is demonstrated to persist as a stationary boundary in latex rubber. / text

The aim of this thesis is to design and build a torsional Kolsky bar apparatus for testing cylindrical specimens in torsion at high strain-rates. In addition to well-established designs, this testing apparatus will include a conical mirror combined with a high speed camera that allows time-resolved optical observation of the shear deformation on the surface of the specimen. The basic design of a Kolsky bar consists of a loading bar, input bar, specimen, and output bar. The experiment is conducted by storing torque in the loading bar and then releasing the torque by breaking the clamp and sending a shear wave pulse through the apparatus into the specimen. This shear wave pulse is monitored by strain gages mounted on the input and output bars. Analysis of the strain waves in the input and output bar is used to extract the shear stress - shear strain profile of the specimen. Several experiments were conducted on 6061-O and 1100-O aluminum with wall thicknesses ranging from 0.3 to 1.5 mm. / text

More and more, advanced polymer and composite materials are being applied in engineering situations where a high resistance to loading at high rates of strain, such as by impact or blast deformation, are a vital requirement. Specific examples exist in the fields of defence and sport research and development for personal, and in the case of the former, vehicular, protection. There are obvious advantages to the use of polymer materials for these applications in augmenting the more widely used metals and ceramics, most notably the evident reduction in weight, and it is believed that with suitable nano-reinforcement these materials may exhibit improved combat survivability. The current study concerns the effect that nano-reinforcements in the form of Carbon Black, Titanium Dioxide, Exfoliated Hectorite Nanoclay and Carbon Nanotubes; have upon the high strain rate mechanical properties of structural variants of Polyethylene (Linear Low Density Polyethylene, LLDPE; High Density Polyethylene, HDPE; Ultra-High Molecular Weight Polyethylene, UHMWPE) and blends of UHMWPE and HDPE. The testing samples were manufactured using a novel process developed in the Loughborough University Materials Department, which has produced well-dispersed specimens. The formed nanocomposite samples were studied using an in-house four-bar Split Hopkinson Pressure Bar (SHPB) system for high strain rate performance, instrumented dropweight for intermediate strain rates and a conventional commercial Hounsfield H50KM universal testing machine for quasi-static strain rate compressive tests. The experimental results recorded for un-reinforced materials are used as a reference to allow comparative analysis of any effect the nano-reinforcements or the blending process have upon the structure, performance and properties of the composite material. From the mechanical testing, it was seen that the stress-strain behaviour of Polyethylene is highly strain-rate-dependent, as plots of the average representative yield stress as a function of strain rate show a bilinear relationship when plotted on a logarithmic strain rate scale, with the gradient of the curve rising sharply at around 103s-1. Concerning the addition of the nanofiller materials, it was seen that there was an increase in the flow and yield stresses and the energy absorption characteristics of the resulting composite with the magnitude dependent upon whether it was a pure or blended polymer that was reinforced. Of the aforementioned fillers it was seen that the addition of Carbon Nanotubes in the small concentrations studied resulted in the greatest increase in properties compared to the pure polymers, closely followed by the Carbon Black fillers. Also of note, the un-reinforced blended samples showed significant increases in flow stress, yield stress and energy absorption when compared to the constituent UHMWPE and HDPE polymers. Additionally, a complete set of Differential Scanning Calorimetry and density measurements were made before testing to assess any changes in the properties after reinforcement or blending, and to help in the interpretation of the results from the different mechanical tests.

Esta dissertação apresenta um projeto de um dispositivo para levantar dados característicos de materiais submetidos às altas taxas de deformação, dispositivo este que leva o nome do seu idealizador o engenheiro Inglês Sir Bertram Hopkinson. Mais especificamente, esta dissertação está inseparavelmente ligada ao desenvolvimento de um embalado para transporte de elementos radioativos como sendo uma das partes do escopo geral, de um projeto da CAPES em convênio com o Centro de Ciência e Tecnologia de Materiais - CCTM do, Instituto de Pesquisas Energéticas e Nucleares IPEN - CNEN/SP, autarquia associada à Universidade de São Paulo. O desenvolvimento do dispositivo faz parte do escopo de obtenção, e levantamento dos dados necessários para o projeto, e a construção do embalado. Esta dissertação versa sobre a concepção mecânica do dispositivo, importante, dividida em duas partes, dimensionamento das barras, que seriam a barra de impacto, a barra de entrada, e a barra de saída, e o dimensionamento do dispositivo de impacto. O dimensionamento das barras envolve conhecimentos do conceito de ondas elásticas em meios sólidos para que o comprimento das barras seja estimado de forma a servir de guia das ondas elásticas, que provocarão a deformação no corpo de prova, e possibilite a boa leitura dessas ondas para análise dos dados. O dispositivo de impacto, este tem que ser robusto o suficiente para produzir a onda de tensão que deforme o corpo de prova, mas não para deformar plasticamente as barras, que terão que continuar durante todo o teste dentro do regime elástico. / This dissertation presents a design of a device to collect characteristic data of materials submitted to the high strain rates, device that takes the name of its idealizer the English engineer Sir Bertram Hopkinson. More specifically, this dissertation is inseparably linked to the development of a package for the transport of radioactive elements as part of the general scope of a CAPES project in partnership with the Materials Science and Technology Center (CCTM), Nuclear and Energy Research Institute IPEN - CNEN / SP, autarchy associated with the University of São Paulo. The development of the device is part of the scope of procurement, and collection of data required for the design, and the construction of the packaging. This dissertation deals with the mechanical design of the device, important, divided into two parts, dimensioning of the bars, which would be the impact bar, the input and output bars and the design of the impact device. The sizing of the bars involves knowledge of the concept of elastic waves in solid media so that the length of the bars is estimated in order to serve as a guide for the elastic waves, which will cause deformation in the test body, and enable a good reading of these waves for analysis of the data. The impact device has to be robust enough to produce the stress wave that deforms the test body but not to deform the bars plastically, which will have to continue throughout the test within the elastic regime.

Friction stir welding (FSW) and aluminium alloy 2139-T8 are currently being considered for use in future military vehicles. However, stringent regulations on weld integrity under extreme loading conditions limit the adoption of new technologies. Moreover, current finite element (FE) based methods do not give reliable predictions of strain distribution in welds, which makes it difficult to assess the performance of structures. Therefore, an extensive research program was carried out to develop a generalised finite element (FE) based methodology to predict the response of welded structures under complex loading configurations. The methodology enables the complex distribution of mechanical properties arising from welding, which is linked to microstructural variation, to be incorporated into a macro scale structural model. The method is general, and is applicable for any heat treatable aluminium alloy under a range of joining processes. To achieve this, the microstructure of 2139-T8 alloy was characterised at a range of length scales, with particular emphasis on the size and distribution of strengthening Omega precipitates. 2139-T8 was subjected to bead on plate FSW to enable characterisation of the effects of processing on the local microstructure. In addition, kinetic data for 2139-T8 was generated, allowing a simple softening model to be developed; this allowed the post-weld strength distribution to be predicted. The model was also used to recreate bulk specimens of 2139-T8 with equivalent local weld microstructure, which was verified by transmission electron microscopy. Material with equivalent microstructure was used to estimate the local mechanical property distributions across the weld, including the initial yield stress and plastic response; the mechanical properties of 2139-T8 are known to be representative of 2139-T84. From observations of this combined data, a methodology was developed to enable the estimation of the complex mechanical property distributions arising during welding. Furthermore, an automated computer program was written to implement the property distributions into FE based models. The methodology was verified using data generated for 2139-T8 and was used to simulate the response of FSW 2139-T8 loaded in uniaxial tension. The simulations were verified experimentally using digital image correlation (DIC) and the methodology was shown to demonstrate increased accuracy and reliability over existing FE methods, with respect to strain predictions. In addition, the method eliminates the need to calibrate the structural model to a particular loading configuration. Theoretically, the models are insensitive to loading and this property was tested by extending the model to simulate the strain distribution of large scale welded panels subject to explosive blast loading. The simulations were verified against blast tests where FSW 2139-T84 panels were subjected to blast loading from the detonation of plastic explosive. The results indicate that the modelling methodology developed is capable of producing accurate and reliable predictions of strain distribution in welded structures under complex loading configurations.