The objective of this research is the development of a new constitutive model to describe the behavior of cohesive soils under time dependent loading. In the work presented here, the modified SIMPLE DSS model is expanded to account for the effects of strain rate on clays in simple shear conditions. The response of clay soils is highly dependent on the rate of strain for both effective stress path and stress-strain behavior. The undrained shear strength is strongly influenced by strain rate both in monotonic and cyclic simple shear tests. Nevertheless, the few available experimental results cover a very limited range of loading conditions and rates. The existing literature established that the soil response display a unique relationship between shear strength and log scale of strain rate. To include the effects of strain rate, the modified simple effective stress model starts with two assumptions: (1) a specific shear strength corresponds to a specific strain rate in a unique relation; and (2) the effect of strain rate does not change the failure envelope. The proposed model is developed from the original SIMPLE DSS model, based on an effective stress formulation in a reduced stress space, and utilizing concepts related to the framework of bounding surface plasticity. The proposed model evaluationwas carried out comparing model simulations with results of simple shear tests on Boston Blue Clay and San Francisco Young Bay Mud. The model capability is useful especially in strain rate dependent responses for both monotonic and cyclic behavior, including irregular loading and step-changed condition. It was found that undrained shear strength in simple shear is directly related to strain rate effects and the responses in cyclic test show the more rate dependent behavior than those in monotonic test. The proposed model is able to predict the increase in undrained shear strength for higher strain rate.

The objective of this research is the development of a new constitutive model to describe the behavior of cohesive soils under time dependent loading. In the work presented here, the modified SIMPLE DSS model is expanded to account for the effects of strain rate on clays in simple shear conditions. The response of clay soils is highly dependent on the rate of strain for both effective stress path and stress-strain behavior. The undrained shear strength is strongly influenced by strain rate both in monotonic and cyclic simple shear tests. Nevertheless, the few available experimental results cover a very limited range of loading conditions and rates. The existing literature established that the soil response display a unique relationship between shear strength and log scale of strain rate. To include the effects of strain rate, the modified simple effective stress model starts with two assumptions: (1) a specific shear strength corresponds to a specific strain rate in a unique relation; and (2) the effect of strain rate does not change the failure envelope. The proposed model is developed from the original SIMPLE DSS model, based on an effective stress formulation in a reduced stress space, and utilizing concepts related to the framework of bounding surface plasticity. The proposed model evaluationwas carried out comparing model simulations with results of simple shear tests on Boston Blue Clay and San Francisco Young Bay Mud. The model capability is useful especially in strain rate dependent responses for both monotonic and cyclic behavior, including irregular loading and step-changed condition. It was found that undrained shear strength in simple shear is directly related to strain rate effects and the responses in cyclic test show the more rate dependent behavior than those in monotonic test. The proposed model is able to predict the increase in undrained shear strength for higher strain rate.

Poly(methyl methacrylate) (PMMA) and polystyrene (PS), which are fully amorphous polymers, have been extensively studied for over a decade to discover how their mechanical behaviours vary with temperatures and strain rates. In this study, Mechanical tests were carried out at a range of strain rates and temperatures using a Hounsfield H50KM Test Machine wluch provides quasi- static rates (10-4 - 10-3 S-l) and low strain rates (10-2 - 10-1 S-l), and an in-house built Dropweight Machine which provides high strain rates (102 - 103 S-l) Mechanical tests were also performed in a high-speed photographic system, which provides high strain rates (103 S-l), to visualise the deformation of the polymers at a range of temperatures. An aluminium-heating block was built to heat up the samples to the required temperature. Strain limited tests were carried out at a range of strain rates and temperatures. Differential Scanning Calorimetry (DSC) was employed to study the glass transition temperatures and the specific heats of the samples. Dynamic Mechanical Thermal Analysis (DMTA) was adopted to study the transitions in the samples and the change of moduli with temperature densities of samples before and after high strain rate compression at certain strain were measured using a Six Column Density Apparatus The polarising microscope was used to study the orientation of the polymer chains at a range of temperatures, strains and strain rates. Eyring's theory of viscous flow was applied on yield point, 20% and 30% strain to relate the activation energy and volume with strain rate and temperature from the thermodynamic perspective. Temperature rise was calculated for high strain rate data to fit into the isothermal curve for the application of Eyring's theory and to obtain the actual smnple temperature at which the deformation took place. PMMA and PS showed ductile behaviour when tested at quasi-static and low strain rates at temperatures below their ductile-brittle transition temperatures. The densities of samples were not found to increase at different strains. The orientations of polymer chains did not influence the increase at Yield stress at high strain rates. The interpretation of activation energy and volume provided information of how the flows of chains took place at different temperatures and strain rates.

Quasi static and intermediate strain rate (5 s-1 and 500 s-1) tests are conducted on various aluminum and steel ASTM E8 subsize tensile specimens to validate a newly developed testing method which combines a previously developed serpentine bar for load monitoring and a newly described high-speed actuator. This new actuator is controlled by a semi-passive piezoelectrically actuated brake system mounted to a standard actuator, which allows for the actuator to produce high loads and quick response times (≈100 µs). Limitations of this experimental method are that tests must be monotonic (tension or compression but not cyclic loading) and strain rate rise times limit this method to the intermediate strain rate regime (below 500 s-1).

This work presents a combined theoretical-experimental study of strain rate behavior in metals. The method is to experimentally calibrate and validate an Internal State Variable (ISV) constitutive model with a wide range of strain rate sensitivity. Therefore a practical apparatus and methodology for performing highly sought-after intermediate strain rate experimentation was created. For the first time in reported literature, the structure-property relations of Rolled Homogeneous Armor is quantified at the microscale and modeled with varying strain rates, temperatures, and stress states to capture plasticity and damage with a single set of constants that includes intermediate strain rates. A rolled homogeneous armor (RHA) was used as a material system to prove the methodology. In doing so, a newly implemented strain rate dependent nucleation parameter for RHA was implemented to transition the dominant damage mechanism from void growth to void nucleation as strain rate increased. The ISVs were utilized in finite element analysis for robust predictability of mechanical performance as well as predictability of microstructural evolution with regards to void size and number distribution. For intermediate strain rate experiments, robust load acquisition was achieved using a novel serpentine transmittal bar that allowed for long stress waves to traverse a short bar system; this system eliminated load- ringing that plagues servo-hydraulic systems. A direct hydraulic loading apparatus was developed to provide uniform strain rates throughout intermediate rate tests to improve on the current limitations of the state-of-the-art. Key recommendations on the advancement of predictive modeling of dynamic materials, as well as performing advanced dynamic experimentation, are elucidated.

The evolution of the physical sciences and engineering has involved a detailed and quantified understanding of the properties of metals. In particular, it is necessary to know how metals deform and the stresses that are involved which, in turn, are affected by the rate at which strain is applied. The nature and layout of this work is outlined. The history of the pressure bar transducer is summarised. The original concept of Hopkinson in 1914 was to use a long metal bar to study the propagation of wave pulses. During the Second World War, Davies refined the instrumentation and studied the shape of such pulses as modified by dispersion. Kolsky in 1949 adapted the technique to investigate the dynamic plasticity of specimens wedged between two instrumented pressure bars. Subsequent workers have used variants of this apparatus to make measurements at strain rates up to 10<SUP>5</SUP> s<SUP>-1</SUP>, whilst others have considered the effects of friction and inertia on the specimen. After an explanation of the particular design requirements, a description is given of the high strain rate apparatus that forms the basis for the research reported in this dissertation. The components that make up the system are described separately and the experimental procedures are outlined. The accuracy of components critical to the experimental technique is investigated. The effects of friction at the specimen interfaces, inertia during deformation and wave dispersion in the pressure bar are discussed. Bar calibration is described. Experiments have been carried out on copper in five different microstructural states at average strain rates of 6 x 10<SUP>4</SUP> s<SUP>-1</SUP> and 5 x 10<SUP>-2</SUP> s<SUP>-1</SUP> and their behaviour compared. The metal has been specially worked to induce anisotropy in the form of texture. Special techniques have been developed to prepare specimens of known orientation from the bulk of the raw material. The results show correlations between the texture severity and the anisotropy of stress-strain properties. A dynamic work hardening effect is observed. There is evidence that the Petch relationship holds at high strain rates. The high strain rate deformation of uranium alloyed with titanium or molybdenum is investigated. Specimens often display evidence of macroscopic localised shear bands whose adiabatic formation is accompanied by a sharp fall in the materials' dynamic strength. Metallographic sections reveal the morphology of these bands and the relative motion of microstructural features during deformation. Results are presented on a eutectoid zinc-22% aluminium alloy in a lamellar and superplastic microstructural state and a gun steel. The high strain rate deformation of titanium-6% aluminium-4% vanadium alloy is compared with uranium-0.75% titanium alloy regarding their tendency to form macroscopic shear bands. The dynamic behaviour of copper is contrasted with that of uranium alloy. In conclusion, the current work is viewed in the context of the historical development of the miniaturised Hopkinson pressure bar. Some comments are made about the application of the technique, and the scope for further research.

The primary objectives of this research were to determine the macroscopic properties of polycrystalline OFHC copper for various grain sizes over a wide range of strain rates and temperatures, and to relate the properties to the fundamental microscopic mechanisms which control the deformation characteristics of the material....

With the focus of the automotive industry on decreasing vehicle weight and improving fuel efficiency, aluminum is being used for structural components in automobiles. Given the high strain rates associated with vehicle impact, it is necessary to understand the rate sensitivity of any potential alloy (eg. AA5754) in order to accurately predict deformation behaviour. Furthermore, the magnitude and strain path associated with the residual strains remaining after forming of the component also play a major role in how the material will behave. It has been found that AA5754 sheet exhibits negative rate sensitivity up to a strain rate of 0.1/s, and positive strain rate sensitivity at strain rates between 0.1/s and 1500/s. Increasing the strain rate also has the effect of increasing the yield stress as well as the ductility. When a strain path change is involved between the prestrain stage and subsequent uniaxial loading, it has the effect of reducing the rate sensitivity of the material as well as reducing the overall flow stress. A rate-sensitive adaptation of the Voce material model was successfully implemented in LS-DYNA and used to predict the response of AA5754 sheet in bending for applied strain rates of 0.001/s and 0.1/s. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2008-09-23 20:11:30.829

Failure of metals under impact loading is known to occur through the formation of adiabatic shear bands (ASBs). ASBs appear in materials as evidence of damage, and are known to be sites for material failure. General purpose plasticity models fail to predict the phenomenon of ASB formation. The present research validates and extends a model developed at the University of Manitoba by Feng and Bassim to predict damage due to ASBs. Parameters for the Feng and Bassim model are determined experimentally using a direct impact pressure bar to impact specimens at temperatures of 20-500oC and strain rates of 500-3000/s. A direct impact experiment is simulated in ANSYS using the model and fitted parameters. The results of the simulation show localized temperature rise and predict failure at the same locations as those observed experimentally. Nominal strain to failure is approximately 40-50% for a specimen impacted at 38 kg-m/s.

Failure of metals under impact loading is known to occur through the formation of adiabatic shear bands (ASBs). ASBs appear in materials as evidence of damage, and are known to be sites for material failure. General purpose plasticity models fail to predict the phenomenon of ASB formation. The present research validates and extends a model developed at the University of Manitoba by Feng and Bassim to predict damage due to ASBs. Parameters for the Feng and Bassim model are determined experimentally using a direct impact pressure bar to impact specimens at temperatures of 20-500oC and strain rates of 500-3000/s. A direct impact experiment is simulated in ANSYS using the model and fitted parameters. The results of the simulation show localized temperature rise and predict failure at the same locations as those observed experimentally. Nominal strain to failure is approximately 40-50% for a specimen impacted at 38 kg-m/s.