Nanocrystalline materials with grain size less than 100 nm have been receiving much attention because of their unparallel properties compared with their microcrystalline counterparts. Because of its high hardness, nanocrystalline nickel has been used for MEMS. Long term thermomechnical properties and deformation mechanism at both ambient and elevated temperatures need to be evaluated which is vital for reliability of its applications as structural material. In this thesis, nanoindentation creep of nanocrystalline nickel with an as-deposited grain size of 14 nm was characterized at elevated temperatures. The nanoindentation creep rate was observed to scale with temperature and applied load (or stress), and could be expressed by an empirical power-law equation for describing conventional crystalline solids. Creep activation energy was found to be close to that for grain boundary self-diffusion in nickel. The activation volume was also evaluated using a stress relaxation technique. The creep results were compared with those for fine-grained nickel in the literature. Possible mechanisms were discussed in light of the creep rate and temperature ranges. To provide a direct comparison, uniaxial creep tests were conducted on nanocrystalline nickel with an as-deposited grain size of 14 nm at 398 K. It was found that stress exponents under the two test conditions are almost the same, indicating a similar creep mechanism. However, the strain rate measured by nanoindentation creep was about 100 times faster than that by uniaxial creep. The rate difference was discussed in terms of stress states and the appropriate selection of Tabor factor. To further explore the time-dependent plastic behavior, multiple unload-reload tests were conducted on electrodeposited nanocrystalline nickel in both compression and tension. A hysteresis was observed during each unload-reload cycle, indicating irreversible energy dissipation. The dissipated energy was evaluated and the energy dissipation rate was found to increase with the flow stress to the third power and sensitive to the stress state (tension or compression). A mechanistic model based on grain boundary sliding was proposed to describe the unload-reload behavior. Experimental results were found to be in good agreement with the model predictions, suggesting the observed hysteresis was indeed caused by grain boundary sliding.

Random Glass Mat Thermoplastic (GMT) composites are increasingly being used by the automotive industry for manufacturing semi-structural components. The polypropylene based materials are characterized by superior strength, impact resistance and toughness. Since polymers and their composites are inherently viscoelastic, i.e. their mechanical properties are dependent on time and temperature. They creep under constant mechanical loading and the creep rate is accelerated at elevated temperatures. In typical automotive operating conditions, the temperature of the polymer composite part can reach as high as 80°C. Currently, the only known report in the open literature on the creep response of commercially available GMT materials offers data for up to 24 MPa at room temperature. In order to design and use these materials confidently, it is necessary to quantify the creep behaviour of GMT for the range of stresses and temperatures expected in service. The primary objective of this proposed research is to characterize and model the creep behaviour of the GMT composites under thermo-mechanical loads. In addition, tensile testing has been performed to study the variability in mechanical properties. The thermo-physical properties of the polypropylene matrix including crystallinity level, transitions and the variation of the stiffness with temperature have also been determined. In this work, the creep of a long fibre GMT composite has been investigated for a relatively wide range of stresses from 5 to 80 MPa and temperatures from 25 to 90°C. The higher limit for stress is approximately 90% of the nominal tensile strength of the material. A Design of Experiments (ANOVA) statistical method was applied to determine the effects of stress and temperature in the random mat material which is known for wild experimental scatter. Two sets of creep tests were conducted. First, preliminary short-term creep tests consisting of 30 minutes creep followed by recovery were carried out over a wide range of stresses and temperatures. These tests were carried out to determine the linear viscoelastic region of the material. From these tests, the material was found to be linear viscoelastic up-to 20 MPa at room temperature and considerable non-linearities were observed with both stress and temperature. Using Time-Temperature superposition (TTS) a long term master curve for creep compliance for up-to 185 years at room temperature has been obtained. Further, viscoplastic strains were developed in these tests indicating the need for a non-linear viscoelastic viscoplastic constitutive model. The second set of creep tests was performed to develop a general non-linear viscoelastic viscoplastic constitutive model. Long term creep-recovery tests consisting of 1 day creep followed by recovery has been conducted over the stress range between 20 and 70 MPa at four temperatures: 25°C, 40°C, 60°C and 80°C. Findley’s model, which is the reduced form of the Schapery non-linear viscoelastic model, was found to be sufficient to model the viscoelastic behaviour. The viscoplastic strains were modeled using the Zapas and Crissman viscoplastic model. A parameter estimation method which isolates the viscoelastic component from the viscoplastic part of the non-linear model has been developed. The non-linear parameters in the Findley’s non-linear viscoelastic model have been found to be dependent on both stress and temperature and have been modeled as a product of functions of stress and temperature. The viscoplastic behaviour for temperatures up to 40°C was similar indicating similar damage mechanisms. Moreover, the development of viscoplastic strains at 20 and 30 MPa were similar over all the entire temperature range considered implying similar damage mechanisms. It is further recommended that the material should not be used at temperature greater than 60°C at stresses over 50 MPa. To further study the viscoplastic behaviour of continuous fibre glass mat thermoplastic composite at room temperature, multiple creep-recovery experiments of increasing durations between 1 and 24 hours have been conducted on a single specimen. The purpose of these tests was to experimentally and numerically decouple the viscoplastic strains from total creep response. This enabled the characterization of the evolution of viscoplastic strains as a function of time, stress and loading cycles and also to co-relate the development of viscoplastic strains with progression of failure mechanisms such as interfacial debonding and matrix cracking which were captured in-situ. A viscoplastic model developed from partial data analysis, as proposed by Nordin, had excellent agreement with experimental results for all stresses and times considered. Furthermore, the viscoplastic strain development is accelerated with increasing number of cycles at higher stress levels. These tests further validate the technique proposed for numerical separation of viscoplastic strains employed in obtaining the non-linear viscoelastic viscoplastic model parameters. These tests also indicate that the viscoelastic strains during creep are affected by the previous viscoplastic strain history. Finally, the developed comprehensive model has been verified with three test cases. In all cases, the model predictions agreed very well with experimental results.

Asphalt pavement rutting is one of the most commonly observed pavement distresses and is a major safety concern to transportation agencies. Millions of dollars are reportedly spent annually to repair rutted asphalt pavements. Research into improvements of hot-mix asphalt materials, mix designs and methods of pavement evaluation and design, including laboratory and field testing, can provide extended pavement life and significant cost savings in pavement maintenance and rehabilitation. This research describes a method of predicting the behaviour of various asphalt mixes and linking these behaviours to an accelerated performance testing tool and pavement in-situ performance. The elastic, plastic, viscoelastic and viscoplastic components of asphalt mix deformation are also examined for their relevance to asphalt rutting prediction. The finite element method (FEM) allows for analysis of nonlinear viscoplastic behaviour of asphalt mixes. This research determines the critical characteristics of asphalt mixes which control rutting potential and investigates the methods of laboratory testing which can be used to determine these characteristics. The Hamburg Wheel Rut Tester (HWRT) is used in this research for asphalt laboratory accelerated rutting resistance testing and for calibration of material parameters developed in triaxial repeated load creep and creep recovery testing. The rutting resistance criteria used in the HWRT are developed for various traffic loading levels. The results and mix ranking associated with the laboratory testing are compared with the results and mix ranking associated with FEM modeling and new mechanistic-empirical method of pavement design analyses. A good relationship is observed between laboratory measured and analytically predicted performance of asphalt mixes. The result of this research is a practical framework for developing material parameters in laboratory testing which can be used in FEM modeling of accelerated performance testing and pavement in-situ performance.

Random Glass Mat Thermoplastic (GMT) composites are increasingly being used by the automotive industry for manufacturing semi-structural components. The polypropylene based materials are characterized by superior strength, impact resistance and toughness. Since polymers and their composites are inherently viscoelastic, i.e. their mechanical properties are dependent on time and temperature. They creep under constant mechanical loading and the creep rate is accelerated at elevated temperatures. In typical automotive operating conditions, the temperature of the polymer composite part can reach as high as 80°C. Currently, the only known report in the open literature on the creep response of commercially available GMT materials offers data for up to 24 MPa at room temperature. In order to design and use these materials confidently, it is necessary to quantify the creep behaviour of GMT for the range of stresses and temperatures expected in service. The primary objective of this proposed research is to characterize and model the creep behaviour of the GMT composites under thermo-mechanical loads. In addition, tensile testing has been performed to study the variability in mechanical properties. The thermo-physical properties of the polypropylene matrix including crystallinity level, transitions and the variation of the stiffness with temperature have also been determined. In this work, the creep of a long fibre GMT composite has been investigated for a relatively wide range of stresses from 5 to 80 MPa and temperatures from 25 to 90°C. The higher limit for stress is approximately 90% of the nominal tensile strength of the material. A Design of Experiments (ANOVA) statistical method was applied to determine the effects of stress and temperature in the random mat material which is known for wild experimental scatter. Two sets of creep tests were conducted. First, preliminary short-term creep tests consisting of 30 minutes creep followed by recovery were carried out over a wide range of stresses and temperatures. These tests were carried out to determine the linear viscoelastic region of the material. From these tests, the material was found to be linear viscoelastic up-to 20 MPa at room temperature and considerable non-linearities were observed with both stress and temperature. Using Time-Temperature superposition (TTS) a long term master curve for creep compliance for up-to 185 years at room temperature has been obtained. Further, viscoplastic strains were developed in these tests indicating the need for a non-linear viscoelastic viscoplastic constitutive model. The second set of creep tests was performed to develop a general non-linear viscoelastic viscoplastic constitutive model. Long term creep-recovery tests consisting of 1 day creep followed by recovery has been conducted over the stress range between 20 and 70 MPa at four temperatures: 25°C, 40°C, 60°C and 80°C. Findley’s model, which is the reduced form of the Schapery non-linear viscoelastic model, was found to be sufficient to model the viscoelastic behaviour. The viscoplastic strains were modeled using the Zapas and Crissman viscoplastic model. A parameter estimation method which isolates the viscoelastic component from the viscoplastic part of the non-linear model has been developed. The non-linear parameters in the Findley’s non-linear viscoelastic model have been found to be dependent on both stress and temperature and have been modeled as a product of functions of stress and temperature. The viscoplastic behaviour for temperatures up to 40°C was similar indicating similar damage mechanisms. Moreover, the development of viscoplastic strains at 20 and 30 MPa were similar over all the entire temperature range considered implying similar damage mechanisms. It is further recommended that the material should not be used at temperature greater than 60°C at stresses over 50 MPa. To further study the viscoplastic behaviour of continuous fibre glass mat thermoplastic composite at room temperature, multiple creep-recovery experiments of increasing durations between 1 and 24 hours have been conducted on a single specimen. The purpose of these tests was to experimentally and numerically decouple the viscoplastic strains from total creep response. This enabled the characterization of the evolution of viscoplastic strains as a function of time, stress and loading cycles and also to co-relate the development of viscoplastic strains with progression of failure mechanisms such as interfacial debonding and matrix cracking which were captured in-situ. A viscoplastic model developed from partial data analysis, as proposed by Nordin, had excellent agreement with experimental results for all stresses and times considered. Furthermore, the viscoplastic strain development is accelerated with increasing number of cycles at higher stress levels. These tests further validate the technique proposed for numerical separation of viscoplastic strains employed in obtaining the non-linear viscoelastic viscoplastic model parameters. These tests also indicate that the viscoelastic strains during creep are affected by the previous viscoplastic strain history. Finally, the developed comprehensive model has been verified with three test cases. In all cases, the model predictions agreed very well with experimental results.

Nanocrystalline materials are being explored as potential off-chip interconnects materials for next generation microelectronics packaging. Mechanical behavior and deformation mechanisms in nanocrystalline copper and nickel have been explored. Nanostructured copper interconnections exhibit better fatigue life as compared to microcrystalline copper interconnects at a pitch of 100 and #956;m and lower. Nanocrystalline copper is quite stable upto 100 oC whereas nickel is stable even up to 400 oC. Grain boundary (GB) diffusion along with grain rotation and coalescence has been identified as the grain growth mechanism. Ultimate tensile and yield strength of nanocrystalline (nc) Cu and Ni are atleast 5 times higher than microcrystalline counterparts. Considerable amount of plastic deformation has been observed and the fracture is ductile in nature. Fracture surfaces show dimples much larger than grain size and stretching between dimples indicates localized plastic deformation. Activation energies for creep are close to GB diffusion activation energies indicating GB diffusion creep. Creep rupture at 45o to the loading axis and fracture surface shows lot of voiding and ductile kind of fracture. Grain rotation and coalescence along direction of maximum resolved shear stress plays an important role during creep. Grain refinement enhances the endurance limit and hence high cycle fatigue life. However, a deteriorating effect of grain refinement has been observed on low cycle fatigue life. This is because of the ease of crack initiation in nanomaterials. Persistent slip bands (PSBs) at an angle of 45o to loading axis are observed at higher strain ranges (> 1% for nc- Cu) with a width of about 50 nm. No relationship has been observed between PSBs and crack initiation. A non-recrystallization annealing treatment, 100 oC/ 2 hrs for nc- Cu and 250 oC/ 2 hrs for nc- Ni has been shown to improve the LCF life without lowering the strength much. Fatigue crack growth resistance is higher in nc- Cu and Ni compared to their microcrystalline counterparts. This is due to crack deflection at GBs leading to a tortuous crack path. Nanomaterials exhibit higher threshold stress intensity factors and effective threshold stress intensity is proportional to the elastic modulus of the material.

The creep models of Sn/3.0Ag/0.5Cu solder material under tensile and shear loads are investigated in this study. The creep test results for Sn/3.0Ag/0.5Cu solder material with four operating temperatures, i.e. 120o, 135 o, 150 o and 165 oC are presented. The experimental results reveal that different creep equations are derived for the Sn/3.0Ag/0.5Cu solder material under tensile and shear loadings. The creep parameters, i.e. stress exponent, material constant and activation energy are curve fitted for the tensile and shear loading tests. The concept of failure toughness of solder ball joints is proposed and studied. The effects of high temperature aging and the thermal cycling loading on the failure toughness of different solder materials and ball sizes have also been explored. The difference between failure toughness values of traditional Sn/37Pb eutectic solder ball joints and the lead free Sn/3.0Ag/0.5Cu solder are compared and discussed. The results simulated from finite element method and experiment measurements under the ball shear test (BST) have been compared and studied. The variation stress, strain distributions and failure toughness during the ball shear testing are studied. The fracture behaviors of different ball joints under the high temperature aging and thermal cycles testing are examined and studied. The ball shear test results measured for the same size Sn/37Pb and Sn/3.0Ag/0.5Cu solder ball joints reveal different load-displacement variations. The relative ductility results are measured for the joint of Sn/37Pb solder ball. However, a high peak load and larger deformation are measured for Sn/3.0Ag/0.5Cu solder ball joints. Based on the definition of failure toughness proposed in this study, the higher failure toughness values are observed for the same size lead free Sn/3.0Ag/0.5Cu solder joints. The variation of failure toughness of different ball joints reveals that the high temperature aging and thermal cyclic loading reduce the failure toughness significantly. However, the measured failure toughness values indicate that the Sn/3.0Ag/0.5Cu solder joints have better ductility for the joints undergoing the high temperature aging and the thermal cycle loadings. Based on the measured results, the better reliability for the Sn/3.0Ag/0.5Cu ball joints is expected, due to the aging and cycling load testing.

The interaction between the operating speed and the creep behavior of mini plastic fan has investigated in this study. The thermal-elastic-creep coupling model in Marc finite element method package are employed to simulate the stress distribution and creep deformation of a plastic fan operated in different operating temperature are simulated in this study. Results indicate that operating temperature affect the creep deformation significantly for a plastic fan or impeller. A comparison between the simulated data and measured data of PA66+ GF30 plastic fan was provided. A good agreement has been observed in this study. A comparison between the creep deformation of PET+GF30 and PBT+GF30 fan sets has also presented. Results indicate that PA66+GF30 plastic fan has a much better creep resistance a high temperature operating.

Nanoindentation has been widely used for measuring mechanical behavior of nanocrystalline (nc) metals that cannot be measured by tensile and compressive test. The hardness and elastic modulus are usually obtained by Oliver and Pharr method. However, this may not be true for materials showing viscoelastic characteristics. This study aims at clarifying the effect of testing parameters, especially loading rate and holding time, on the hardness and elastic modulus of a nanocrystalline Fe-51Ni coating obtained in nanoindentation tests as the material exhibits anelastic and creep characteristics. An analytical method based on the correspondence principle for linear viscoelasticity was developed. The holding displacement-time data obtained in indentation creep tests at a high loading rate of 20 mN/s were analyzed and material parameters related to the elastic, anelastic and creep characteristic were derived using a model containing one Maxwell unit and two Kelvin units. The anelastic deformation thus contains at least two relaxation processes having relaxation times of 0.37 s and 6.8 s, respectively and the creep deformation is described by a viscosity value of 4.2x104 GPa.s for the alloy in an as-deposited state. Moreover, electrodeposited (ED) Ni was analyzed by electron backscatter diffraction. Results indicated that the ED Ni exhibits a bimodal distribution of grain size. The grains having sizes larger than 2 £gm shows a strong fiber texture of <100>//ND, whereas the small grains (<2 £gm) are mainly randomly oriented.

Recovered core samples from the San Andreas Fault Observatory at Depth (SAFOD), located near Parkfield, CA, offer a unique opportunity to study the products of faulting and to learn about the mechanisms of slip at 3 km depth. Casing deformation reflects active creep along two strands of the San Andreas Fault (SAF) at SAFOD. The two fault strands are referred to as the Southwest Deforming Zone (SDZ) at 3194 m measured depth (MD) and the Central Deforming Zone (CDZ) at 3301 m MD. The SDZ and CDZ contain remarkably similar gouge layers, both of which consist of a clay-bearing, ultrafine grain matrix containing survivor clasts of sandstone and serpentinite. The two gouges have sharp boundary contacts with the adjacent rocks. We have used X-ray Computed Tomography (XCT) imaging, at two different sampling resolutions, to investigate the mesoscale and microscale structure of the fault zone, specifically to characterize the shape, preferred orientation, and size distribution of the survivor clasts. Using various image processing techniques, survivor clast shape and size are characterized in 3D by best-fit ellipsoids. Renderings of survivor clasts illustrate that survivor clasts have fine tips reminiscent of sigma type tails of porphyroclasts observed in myolonites. The resolution of the XCT imaging permits characterization of survivor clasts with equivalent spherical diameters greater than 0.63 mm. The survivor clast population in both the SDZ and CDZ gouge layers have similar particle size distributions (PSD) which fit a power law with a slope of approximately -3; aspect ratio (major to minor axis ratios) distributions also are similar throughout ranging between 1.5 and 4, with the majority occurring between 2-2.5. The volume- and shape- distributions vary little with position across the gouge zones. A strong shape preferred orientation (SPO) exists in both creeping zones. In both the SDZ and CDZ the minor axes form a SPO approximately normal to the plane of the San Andreas Fault (SAF), and the major axes define a lineation in the plane of the SAF. The observation that the size-, shape- and orientation-distributions of mesoscale, matrix-supported clasts are similar in the SDZ and CDZ gouge layers, and vary little with position in each gouge layer, is consistent with the hypothesis that aseismic creep in the SDZ and CDZ is achieved by distributed, shearing. The consistency between the SPO and simple-shear, strike-slip kinematics, and the marked difference of PSD, fabric, cohesion and clast lithology of the gouge with that of the adjacent rock, is consistent with the hypothesis that the vast majority of the shear displacement on the SAF at SAFOD is accommodated within the gouge layers and the gouge displays a mature, nearly steady-state structure.

The variations of residual stress distributions on solder balls under the cyclic thermal load and aging processes are investigated in this thesis. The solidification phenomena for different shapes and materials of the solder balls during the reflow process are predicted by using the Surface Evolver program. The distribution of residual stress in the solder ball is calculated by employing the MARC finite element package. The temperature dependent material properties of the solders, i.e. 63Sn/37Pb and 96.5Sn/3.5Ag, are used in the residual stresses calculations. The variation of the residual stresses distributions of different solder balls under the temperature cycling test ( between ¡V40¢J and 85¢J ) and the aging test ( at 85¢J ) are simulated and studied. The effects of the solder parameters, i.e. the solder height and the pad geometry shapes on the residual stresses distribution are also studied. Besides, the same simulation and analysis has also applied on the solder ball with an unleaded solder 96.5Sn/3.5Ag. A better understanding about the variation of the residual stress in a solder ball is expected from this analysis.