The purpose of this investigation was to provide quality dynamic strength properties for a solid grade severe-weather (SW) brick material and to illustrate the need for careful evaluation of the strain-rate effects on geomaterials. A split Hopkinson pressure bar (SHPB) was used to perform a series of tests on specimens from a solid grade SW brick to determine the mechanical response of this material at high strain-rates. Both classical and modified SHPB tests were performed. The results from the classical SHPB tests provided evidence that modifications to the SHPB are necessary when testing geomaterials such as brick. To modify the SHPB, a small copper disk was placed at the impact end of the SHPB incident bar to increase the rise time of the initial pulse. The material response from the modified SHPB tests provided an average compressive strength of 104 MPa, which resulted in a dynamic increase factor of 1.42.

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.

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.

The objective of this research project was to simulate the behavior of armor metals at high strain rates and large strains, using the Johnson-Cook visco-plastic model, while incorporating the formation of adiabatic shear bands. The model was then to be applied to three armor metals, namely maraging steel 300, high hardness armor (HHA), and aluminum alloy 5083-H131; supplied by the Canadian Department of National Defense and tested in compression at the University of Manitoba. The Johnson-Cook model can accurately simulate the behavior of BCC metal (steels) up to a point of thermal instability. Conditions for complete shear failure in the model match closely to conditions at which adiabatic shear bands formed in specimens tested experimentally. The Johnson-Cook model is not quite valid for FCC metals, such as aluminum, where strain rate and temperature effects are dependent on the strain while in the Johnson-Cook model, these parameters are separable.

The objective of this research project was to simulate the behavior of armor metals at high strain rates and large strains, using the Johnson-Cook visco-plastic model, while incorporating the formation of adiabatic shear bands. The model was then to be applied to three armor metals, namely maraging steel 300, high hardness armor (HHA), and aluminum alloy 5083-H131; supplied by the Canadian Department of National Defense and tested in compression at the University of Manitoba. The Johnson-Cook model can accurately simulate the behavior of BCC metal (steels) up to a point of thermal instability. Conditions for complete shear failure in the model match closely to conditions at which adiabatic shear bands formed in specimens tested experimentally. The Johnson-Cook model is not quite valid for FCC metals, such as aluminum, where strain rate and temperature effects are dependent on the strain while in the Johnson-Cook model, these parameters are separable.

A Thesis Submitted Towards the Partial Fulfilment Degree of Master of Technology (M.Tech.) FACULTY OF ENGINEERING MECHANICAL ENGINEERING DEPARTMENT Cape Peninsula University of Technology 2011 / The split Hopkinson pressure bar (SHPB) test is the most commonly used method for determining material properties at high rates of strain. The theory governing the specifics of Hopkinson bar testing has been around for decades; however, it has only been for the last decade or so that significant data processing advancements have been made. It is the intent of this thesis to offer the insight of application of SHPB to determine the compressive dynamic behaviour for welded low carbon steel (mild steel). It also focuses on the tensile behaviour for unheat-treated and heat-treated welded carbon steel. The split Hopkinson Pressure bar apparatus consists of two long slender bars that sandwich a short cylindrical specimen between them. By striking the end of a bar, a compressive stress wave is generated that immediately begins to traverse towards the specimen. Upon arrival at the specimen, the wave partially reflects back towards the impact end. The remainder of the wave transmits through the specimen and into the second bar, causing irreversible plastic deformation in the specimen. It is shown that the reflected and transmitted waves are proportional to the specimen's strain rate and stress, respectively. Specimen strain can be determined by integrating the strain rate. By monitoring the strains in the two bars and the specimen's material, stress-strain properties can be calculated. Several factors influence the accuracy of the results, including the size and type of the data logger, impedance mismatch of the bars with the specimens, the utilization of the appropriate strain gauges and the strain amplifier properties, among others. A particular area of advancement is a new technique to determine the wave's velocity in the specimen with respect to change in medium and mechanical properties, and hence increasing the range of application of SHPB. It is shown that by choosing specimen dimensions based on their impedance, the transmitted stress signal-to-noise ratio can be improved. An in depth discussion of realistic expectations of strain gages is presented, along with closed form solutions validating any claims. The thesis concludes with an analysis of experimental and predicted results. Several recommendations and conclusions are made with regard to the results obtained and areas of improvement are suggested in order to achieve accurate and more meaningful results.

Traumatic brain injury (TBI) is a growing concern among American citizens and globally. This study proposes the use of a novel mechanical testing method for interrupting adult porcine brain tissue while under varying levels of high rate compressive strain to better understand the mechanical response of brain while under TBI inducing conditions. Testing was performed using a polymeric Split Hopkinson Pressure Bar (SHPB) along with customized attachments developed in-house to interrupt tissue samples at strain levels of 15%, 30%, and 40% while being compressed at strain rates of 650, 800, and 900 s-1. Following interruption, the samples were chemically fixed in preparation for histological processing. Microscopy techniques were used to examine the microstructure of the deformed tissue samples and measure the area fraction of their neural constituents. The combination of both the mechanical and microstructural responses of the brain tissue allowed for the development of a structure-property relationship.

No description available.

The aim of this thesis is to provide microstructural and mechanical characterisation of α-β titanium alloys exposed to a range of thermo-mechanical conditions, in particular under-going high rate deformation at elevated temperatures, representative of the Linear Friction Welding (LFW) manufacturing process. Three α-β titanium alloys provided by Rolls-Royce are studied: Ti-64 blade, disc and Ti-6246 disc. Ti-64 and Ti-6246 show complex deformation behaviour with strain, strain rate and temperature, especially near the transus temperature, where the low temperature α phase is transformed into the high temperature β phase. The microstructure and mechanical properties evolve in an interconnected fashion, and understanding this mutual influence is necessary to better predict the behaviour of these alloys. Characterisation of the mechanical properties was performed through uniaxial compression tests at strain rates from 0.001 to 3000 s^-1, using an Instron screw-driven machine at quasi-static rates, a servo-hydraulic machine at medium rates and a Split-Hopkinson Pressure Bar and a drop-weight tower at high strain rates. The tests were performed over a range of temperatures from room temperature to 1300 °C. The main focus was on high strain rate and high temperature tests, with the development of a gravity driven direct impact Hopkinson bar, referred as a drop-weight system, which is intended to evaluate the mechanical response of metals to high strain rate loading at temperatures up to c. 1300 °C. The design and principles of operation of the system are presented, along with calibration and validation data. Preliminary tests were performed on stock Ti-64, heated at two rates: 1 and 20 °C s^-1. The evolution of the mechanical properties was analysed, focussing on the strain rate, temperature and phases dependencies. Characterisation of the microstructure was realised by performing interrupted compression tests, first at room temperature, three plastic strains, 4%, 10% and 20%, and two different strain rates, 0.001 and 2000 s^-1; then at 4% plastic strain, a strain rate of 2000 s^-1 and three elevated temperatures, 700, 900 and 1100 °C. A better understanding of the microstructure evolution with strain, strain rates and temperature, including the macrotexture and microtexture of the specimens, was obtained using Electron Backscatter Diffraction (EBSD) to characterise the texture of the undeformed and deformed materials. The better understanding of the flow stress and microstructural evolution of both Ti-64 and its individual α and β phases with various strain rates and temperatures is intended to be used in the development of more accurate models representing the behaviour of these alloys. Predicting the microstructure evolution and then the mechanical properties of a material is essential to optimise the final mechanical properties of the alloys when welded by manufacturing processes such as the LFW process.

Above true strain rates of $10^4$ s$^{-1}$ FCC metals exhibit a rapid increase in strength. Understanding of the physical mechanisms behind this strength transition is hindered by the number and interdependence of candidate mechanisms. Broadly, contributions to strength can be split into `instantaneous' effects and the more permanent `structural' ones. In this thesis a series of experiments are presented which are designed to separate the two types of contribution. Chapter 2 outlines the basics of dislocation plasticity, based on the seminal works of Taylor and Orowan. It then progresses on to discuss recent experimental and theoretical work on the understanding of slip as avalanche behaviour. Chapter 3 summarises traditional modelling approaches for instantaneous strength contributions which are routinely applied below $10^4$ s$^{-1}$. It then continues on to outline a number of different approaches which have been adopted to attempt to explain and model the strength transition. Chapter 4 outlines the methods used in the earliest stages of the study: Instron and split Hopkinson pressure bar methods. Both methods are well established, and cover the majority of the range of rates under study. Emphasis is made on minimising experimental sources of error, and subsequently accounting for those which are unavoidable. Finally, the specimen material is introduced and is shown to be fit for purpose. Chapter 5 presents a set of mechanical tests of specimens at strain rates between $10^4-10^5$~s$^{-1}$. The softening of the specimens with increased temperature is observed to increase with strain rate, both in absolute terms and when normalised to the 300 K measurement for each strain rate. The observations are most easily explained if the strength transition is due to an increase in early stage work hardening, however, some anomalous behaviours remain. Chapter 6 introduces a new experimental technique; direct impact Hopkinson pressure bars, required to perform experiments shown to be necessary by the results of Chapter 5. Photon Doppler velocimetry is applied to the projectiles used in experiments, removing one of the most significant flaws of the technique, and creating a more confident basis with which to perform further experimental work. Chapter 7 presents a series of `jump tests' at ambient temperatures. Specimens are deformed at strain rates ranging from $10^{-2}$ to $10^5$~s$^{-1}$ to a fixed strain of 0.1, then reloaded to yield at a strain rate of $10^{-1}$. The yield point at reload is shown to have the same rapid upturn as seen when the specimens were deforming at high rates, providing strong evidence that the increase in strength is due to changes in the underlying dislocation structure, rather than a dynamic effect, as it remains even when the high strain rate is removed. Chapter 8 continues on from the conclusions of Chapter 7. Jump tests are expanded to a variety of temperatures and strains, to provide a more complete characterisation of metal behaviour. No dramatic change in the saturation of work hardening is observed to coincide with the increase in early stage work hardening. Chapter 9 discusses discrepancies between contemporary high rate models and recent developments in the understanding of plasticity being an avalanche process. Potential consequences of incorporating avalanche plasticity into high rate models are explored. Particular attention is paid to Brown's observation that based on quasi static observations of avalanche behaviour, the formation of dislocation avalanches will begin to fail at strain rates of approximately $10^4$ s$^{-1}$. Consequences of the progressive breakdown of avalanche behaviour are discussed with respect to the experimental observations presented in earlier chapters. In Chapter 10, we will discuss the key conclusions of the work. Finally, a number of avenues are proposed for building upon the current work both theoretically and experimentally.