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.

As one category of energetic materials, impact-initiated reactive materials are able to release a high amount of stored chemical energy under high strain rate impact loading, and are used extensively in civil and military applications. In general, polymers are introduced as binder materials to trap the reactive metal powders inside, and also act as an oxidizing agent for the metal ingredient. Since critical attention has been paid on the metal / metal reaction, only a few types of polymer / reactive metal interactions have been studied in the literature. With the higher requirement of materials resistant to different thermal and mechanical environments, the understanding and characterization of polymer / reactive metal interactions are in great demand. In this study, PTFE (Polytetrafluoroethylene) 7A / Ti (Titanium) composites were studied under high strain rates by utilizing the Taylor impact and SHPB tests. Taylor impact tests with different impact velocities, sample dimensions and sample configurations were conducted on the composite, equipped with a high-speed camera for tracking transient images during the sudden process. SHPB and Instron tests were carried out to obtain the stress vs. strain curves of the composite under a wide range of strain rates, the result of which were also utilized for fitting the constitutive relations of the composite based on the modified Johnson-Cook strength model. Thermal analyses by DTA tests under different flow rates accompanied with XRD identification were conducted to study the reaction mechanism between PTFE 7A and Ti when only heat was provided. Numerical simulations on Taylor impact tests and microstructural deformations were also performed to validate the constitutive model built for the composite system, and to investigate the possible reaction mechanism between two components. The results obtained from the high strain rate tests, thermal analyses and numerical simulations were combined to provide a systematic study on the reaction mechanism between PTFE and Ti in the composite systems, which will be instructive for future energetic studies on other polymer / reactive metal systems.

When producing thermomechanical pulps (TMP), wood chips and fiber material are loaded mechanically in a disc-refiner to separate the fibers and to make them flexible. In the process, much of the energy supplied is transferred to the fiber material through cyclic compression, shear and friction processes. Therefore, compression and friction characteristics are needed in order to gain a better grasp of the forces acting during refining. To this end, in this thesis, the compressive and frictional behaviors of wood were investigated under simulated chip refining conditions (i.e., hot saturated steam, high strain rate compression, and high sliding speed). Two new, custom-designed, experimental setups were developed and used. The equipment used for compression testing was based on the split Hopkinson pressure bar (SHPB) technique and the friction tester was a pin-on-disc type of tribotester (wear rig). Both pieces of equipment allow a testing environment of hot saturated steam.   In the wood–steel friction investigation, the influence of the steam temperature (100-170°C) was of primary interest. The wood species chosen for the friction tests were spruce (Picea abies), pine (Pinus sylvestris, Pinus radiata), and birch (Betula verrucosa). When performing measurements in the lower-temperature region (100-130°C), the friction coefficients registered for the softwoods were generally low and surface properties such as lubrica­tion were suggested to have a great influence on the results; however, in the higher-tempera­ture region (~130 -170°C), the friction coefficients of all investigated wood species were probably determined by bulk properties to a much greater extent. When most of the wood extractives had been removed from the specimens, testing results revealed distinct peaks in friction at similar temperatures, as the internal friction of the different wood species are known to have their maxima at ~110–130°C. One suggested explanation of these friction peaks is that reduced lubrication enabled energy to dissipate into the bulk material, causing particularly high friction at the temperature at which internal damping of the material was greatest. During the friction measurements in the higher-temperature region, the specimens of the different wood species also started to lose fibers (i.e., produce wear debris) at different characteristic temperatures, as indicated by peaks in the coefficient of friction. In refining, the generally lower shives content of pine TMP than of spruce TMP could partly be explained by a lower wear initiation temperature in the pine species.   Wood stiffness is known to decrease with temperature, when measured at low strain rates. The results presented in this thesis can confirm a similar behavior for high strain rate compression. The compressive strain registered during impulsive loading (using a modified split Hopkinson equipment) increased with temperature; because strain rate also increased with temperature. Accordingly, the strain rates should determine the strain magnitudes also in a refiner, since the impulsive loads in a refiner are of similar type. Larger strains would thus be achieved when refining at high temperatures. The results achieved in the compression tests were also considered in relation to refining parameters such as plate clearance and refining intensity, parameters that could be discussed in light of the stress–strain relations derived from the high strain rate measurements. Trials recorded using high-speed photography demonstrated that the wood relaxation was very small in the investigated time frame ~6 ms. As well, in TMP refining the wood material has little time to relax, i.e., ~0.04–0.5 ms in a large single disc refiner. The results presented here are therefore more suitable for comparison with the impulsive loads arising in a refiner than are the results of any earlier study. It can therefore be concluded that the modified SHPB testing technique combined with high-speed photography is well suited for studying the dynamic behavior of wood under conditions like those prevalent in a TMP system.

In an automotive crash event, hot stamped, die quenched martensitic structural components have been shown to provide excellent intrusion resistance. These alloys exhibit only limited ductility, however, which may limit the overall impact performance of the component. The introduction of lower strength and more ductile “tailored” properties within some regions of a hot stamped component has the potential to improve impact performance. One approach being applied to achieving such tailored properties is through locally controlling the cooling rate within the stamping die. The primary motivation for the current work is to understand the role of cooling rate on the as-quenched mechanical response of tailored hot stampings, which has required characterization of the high strain rate mechanical behaviour of tailored hot stamped boron steel. The effect of cooling rate and resulting microstructure on the as-quenched mechanical behavior of USIBOR® 1500P boron steel at strain rates between 10-3 and 103 s-1 was investigated. Specimens quenched at rates above the critical cooling rate (~27 °C/s) exhibited a fully martensitic microstructure with a UTS of ~1,450 MPa. Sub-critical cooling rates, in the range 14°C/s to 50 °C/s, resulted in as-quenched microstructures ranging between bainitic to martensitic, respectively. Tension tests revealed that predominantly bainitic material conditions (14 °C/s cooling rate) exhibited a lower UTS of 816 MPa compared to 1,447 MPa for the fully martensitic material condition (50 °C/s cooling rate) with a corresponding increase in elongation from 0.10 to 0.15 for the bainitic condition. The reduction in area was 70% for the bainitic material condition and 58% for the martensitic material conditions which implied that a tailored region consisting of bainite may be a desirable candidate for implementation within a hot stamped component. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the measured flow stress curves were used to develop a strain rate sensitive constitutive model, the “Tailored Crash Model (TCM)”. The TCM accurately reproduced the measured flow stress curves as a function of effective plastic strain, strain rate and Vickers hardness (or area fraction of martensite and bainite). The effect of deformation during quenching and the associated shift in the CCT diagram on the subsequent constitutive response was also examined for this material. Specimens were simultaneously quenched and deformed at various cooling rates to achieve a range of as-quenched microstructures that included ferrite in addition to martensite and bainite. Tensile tests conducted on these specimens at strain rates ranging from 0.003 s-1 to ~80 s-1 revealed that the presence of ferrite resulted in an increase in uniform elongation and n-value which increased overall energy absorption for a given hardness level. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the TCM constitutive model was extended to account for the presence of ferrite. This extended constitutive model, the “Tailored Crash Model II (TCM II)”, has been shown to predict flow stress as a function of effective plastic strain, strain rate and area fraction of martensite, bainite and ferrite. As a validation exercise, uniaxial tension test simulations of specimens extracted from the transition zone of a hot stamped lab-scale B-pillar with tailored properties [1] were performed. The measured hardness distribution along the gauge length of the tensile specimens was used as input for the TCM constitutive model to define the element constitutive response used in the finite element (FE) models. The measured stress versus strain response and strain distribution during loading (measured using digital image correlation) was in excellent agreement with the FE models and thus validated the TCM constitutive model developed in this work. Validation of the TCM II version of the model is left for future work.

The effects of reactive metal particles on the microstructure and mechanical properties of epoxy-based composites are investigated in this work. To examine these effects castings of epoxy reinforced with 20-40 vol.% Al and 0-10 vol.% Ni were prepared, while varying the aluminum particle size from 5 to 50 microns and holding the nickel particle size constant at 50 microns. In total eight composite materials were produced, possessing unique microstructures. The microstructure is quantitatively characterized and correlated with the composite constitutive response determined from quasi-static and dynamic compressive loading conditions at strain-rates from 1e-4 to 5e3 /s. Microstructures from each composite and at each strain rate were analyzed to determine the amount of particle strain as a function of bulk strain and strain rate. Using computational simulations of representative microstructures of select composites, the epoxy matrix-metallic particle and particle-particle interactions at the mesoscale under dynamic compressive loading conditions were further examined. From computational simulation data, the stress and strain localization effects were characterized at the mesoscale and the bulk mechanical behavior was decomposed into the individual contributions of the constituent phases. The particle strain and computational analysis provided a greater understanding of the mechanisms associated with particle deformation and stress transfer between phases, and their influence on the overall mechanical response of polymer matrix composites reinforced with metallic particles. The highly heterogeneous composite microstructure and the high contrasting properties of the individual constituents were found to drive localized deformations that are often more pronounced than those in the bulk material. The strain rate behavior of epoxy is shown to cause a strain rate dependent deformation response of reinforcement particle phases that are typically strain rate independent. Additionally, the epoxy matrix strength behavior was found to have a higher dependence on strain rate due to the presence of metal particle fillers. Discrepancies between experimental and simulation mechanical behavior results and these findings indicate a need for epoxy constitutive models to incorporate effects of particle reinforcement on the mechanical behavior.

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.

In an automotive crash event, hot stamped, die quenched martensitic structural components have been shown to provide excellent intrusion resistance. These alloys exhibit only limited ductility, however, which may limit the overall impact performance of the component. The introduction of lower strength and more ductile “tailored” properties within some regions of a hot stamped component has the potential to improve impact performance. One approach being applied to achieving such tailored properties is through locally controlling the cooling rate within the stamping die. The primary motivation for the current work is to understand the role of cooling rate on the as-quenched mechanical response of tailored hot stampings, which has required characterization of the high strain rate mechanical behaviour of tailored hot stamped boron steel. The effect of cooling rate and resulting microstructure on the as-quenched mechanical behavior of USIBOR® 1500P boron steel at strain rates between 10-3 and 103 s-1 was investigated. Specimens quenched at rates above the critical cooling rate (~27 °C/s) exhibited a fully martensitic microstructure with a UTS of ~1,450 MPa. Sub-critical cooling rates, in the range 14°C/s to 50 °C/s, resulted in as-quenched microstructures ranging between bainitic to martensitic, respectively. Tension tests revealed that predominantly bainitic material conditions (14 °C/s cooling rate) exhibited a lower UTS of 816 MPa compared to 1,447 MPa for the fully martensitic material condition (50 °C/s cooling rate) with a corresponding increase in elongation from 0.10 to 0.15 for the bainitic condition. The reduction in area was 70% for the bainitic material condition and 58% for the martensitic material conditions which implied that a tailored region consisting of bainite may be a desirable candidate for implementation within a hot stamped component. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the measured flow stress curves were used to develop a strain rate sensitive constitutive model, the “Tailored Crash Model (TCM)”. The TCM accurately reproduced the measured flow stress curves as a function of effective plastic strain, strain rate and Vickers hardness (or area fraction of martensite and bainite). The effect of deformation during quenching and the associated shift in the CCT diagram on the subsequent constitutive response was also examined for this material. Specimens were simultaneously quenched and deformed at various cooling rates to achieve a range of as-quenched microstructures that included ferrite in addition to martensite and bainite. Tensile tests conducted on these specimens at strain rates ranging from 0.003 s-1 to ~80 s-1 revealed that the presence of ferrite resulted in an increase in uniform elongation and n-value which increased overall energy absorption for a given hardness level. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the TCM constitutive model was extended to account for the presence of ferrite. This extended constitutive model, the “Tailored Crash Model II (TCM II)”, has been shown to predict flow stress as a function of effective plastic strain, strain rate and area fraction of martensite, bainite and ferrite. As a validation exercise, uniaxial tension test simulations of specimens extracted from the transition zone of a hot stamped lab-scale B-pillar with tailored properties [1] were performed. The measured hardness distribution along the gauge length of the tensile specimens was used as input for the TCM constitutive model to define the element constitutive response used in the finite element (FE) models. The measured stress versus strain response and strain distribution during loading (measured using digital image correlation) was in excellent agreement with the FE models and thus validated the TCM constitutive model developed in this work. Validation of the TCM II version of the model is left for future work.

abstract: The mechanical behavior of Pb-free solder alloys is important, since they must maintain mechanical integrity under thermomechanical fatigue, creep, and mechanical shock conditions. Mechanical shock, in particular, has become an increasing concern in the electronics industry, since electronic packages can be subjected to mechanical shock by mishandling during manufacture or by accidental dropping. In this study, the mechanical shock behavior of Sn and Sn-Ag-Cu alloys was systematically analyzed over the strain rate range 10-3 - 30 s-1 in bulk samples, and over 10-3 - 12 s-1 on the single solder joint level. More importantly, the influences of solder microstructure and intermetallic compounds (IMC) on mechanical shock resistance were quantified. A thorough microstructural characterization of Sn-rich alloys was conducted using synchrotron x-ray computed tomography. The three-dimensional morphology and distribution of contiguous phases and precipitates was analyzed. A multiscale approach was utilized to characterize Sn-rich phases on the microscale with x-ray tomography and focused ion beam tomography to characterize nanoscale precipitates. A high strain rate servohydraulic test system was developed in conjunction with a modified tensile specimen geometry and a high speed camera for quantifying deformation. The effect of microstructure and applied strain rate on the local strain and strain rate distributions were quantified using digital image correlation. Necking behavior was analyzed using a novel mirror fixture, and the triaxial stresses associated with necking were corrected using a self-consistent method to obtain the true stress-true strain constitutive behavior. Fracture mechanisms were quantified as a function of strain rate. Finally, the relationship between solder microstructure and intermetallic compound layer thickness with the mechanical shock resistance of Sn-3.8Ag-0.7Cu solder joints was characterized. It was found that at low strain rates the dynamic solder joint strength was controlled by the solder microstructure, while at high strain rates it was controlled by the IMC layer. The influences of solder microstructure and IMC layer thickness were then isolated using extended reflow or isothermal aging treatments. It was found that at large IMC layer thicknesses the trend described above does not hold true. The fracture mechanisms associated with the dynamic solder joint strength regimes were analyzed. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2012

Ceramics have been widely used for personnel and vehicle armour because of their desirable properties such as high hardness and low density. However the brittle nature associated with the ceramic materials, i.e. low toughness, reduces their ability to withstand multiple ballistic hits. The present work is focused on ceramic armour materials made from alumina and zirconia toughened alumina (ZTA). The effects of grain size and zirconia phase transformation toughening on the mechanical and high strain rate properties in both materials were investigated in detail. Alumina, 10%, 15% and 20% nano ZTA with 1.5 mol% yttria stabiliser were produced with various grain sizes. The processing of the materials started from suspension preparation, spray freeze drying of the suspension and die pressing to produce homogeneous green bodies with densities above 54%. Then, the green bodies were sintered using conventional single stage and/or two stage sintering to produce the samples with full density and a range of grain sizes (0.5 to 1.5 µm alumina grains and 60 to 300 nm zirconia grains). The effects of the processing conditions on the microstructures were studied and the optimum processing route for each sample was determined. The mechanical properties of the alumina and ZTA samples were investigated, including Vickers hardness, indentation toughness, 4-point bend strength and wear resistance. The results showed that, with an increasing amount of zirconia addition, evident increases of the toughness, strength and wear resistance properties were observed, whilst the hardness reduced slightly correspondingly. The effect of density and grain sizes on the hardness and toughness were studied as well: larger alumina grain size led to a higher hardness and negligible change in toughness, whilst the zirconia grain coarsening enhanced the phase transformation toughening effect and the samples displayed a higher toughness. In addition to the investigation of the mechanical properties, the alumina and nano ZTA samples were subjected to high strain rate testing, including split Hopkinson pressure bar (SHPB) (8-16 m/s) and gas gun impact testing (100-150 m/s). The high strain rate performances were compared in terms of their fracture behaviours, fragmentation process and fragment size distribution. Raman spectroscopy was used to measure the amount of zirconia phase transformation in ZTA samples after the high strain rate testing. The residual stress and dislocation density in alumina grains after testing were quantitatively measured using Cr3+ fluorescence spectroscopy. The results indicated that zirconia phase transformation can reduce the residual stress and dislocation densities in the ZTA samples, resulting in less damage, lower plastic deformation and less crack propagation. In addition, a nano zirconia material with 1.5 mol% yttria stabiliser (1.5YSZ) was subjected to a gas gun impact test with a very high impact speed (142 m/s); a deep projectile penetration was observed, due to the low hardness of the pure zirconia, whilst the sample stayed intact. The result further confirmed that the zirconia phase transformation toughening effect can improve the sample's high strain rate performance.