The linear friction welding process is currently commercially used solely to produce titanium alloy aeroengine bladed disks (blisks). The process can, however, be potentially used to produce welds in non-aeroengine materials for non-aeroengine applications. The aim of this thesis was therefore to demonstrate the capabilities of the process to join materials not commonly used in the aeroengine industry and to develop understanding of the process. The focus of this thesis has been on the linear friction welding of 316L stainless steel and the linear friction welding of dissimilar materials: aluminium to copper, aluminium to magnesium and aluminium to steel. For all of these studies it was seen that weld line and near weld line microstructure could be altered by the use of different welding parameters. This property of linear friction welding was used to advantage to optimise microstructures in dissimilar welds. Intermetallic formation is usually a major challenge to the achievement of welds with good mechanical properties, and for work in this thesis welding parameters were optimised to allow welds with limited intermetallic formation. The welding of 316L and the dissimilar welding of aluminium to copper proved viable. For the welding of 316L and aluminium to copper, fracture during tensile testing occurred in the parent material (aluminium parent material for aluminium to copper). The welding of aluminium to magnesium and aluminium to steel showed promising results, with weld strength comparable to the aluminium parent material for aluminium to steel and comparable to the parent materials in O temper for aluminium to magnesium. However, repeatability of mechanical properties was poor for these dissimilar welds, which would be a significant barrier to commercial exploitation. Further work needs to be conducted to improve repeatability. Weld microstructures were characterised using optical and scanning electron microscopy as well as electron backscatter diffraction and X-ray diffraction techniques.

In the past few decades, brake pad-rotor interface friction studies have gained high importance in the automotive industry. The goal of these studies has been to improve the design to maximize the contact area and performance in brakes. In these studies, friction coefficient has always assumed to be the same for linear and rotational motion. In our study, we show that the rotational and linear friction process have different friction coefficients. We use semi-metallic and ceramic brake material pads reduced into brake samples using scaling laws of physics. The samples were mounted on the Universal Mechanical Tester and experimented for linear and rotational friction process against Pearlitic Gray cast iron rotor. From results, it proved friction coefficients of linear movement is always higher than the rotational movement. The linear friction coefficient was found to be 43% higher on an average than the rotational friction coefficient in both the materials tested at 1MPa and 10 mm/s. These results will help industry in gaining better fundamental understanding about the friction coefficients of rotor- brake contact interfaces.

No description available.

No description available.

Viscoelastic materials have been used widely in different applications, such as constructing tires, artificial joints, shoe heels, and soles. A study on the different characteristics of viscoelastic materials has always been a matter of interest in order to improve their properties for various applications. In the automotive industry, rubber, as a viscoelastic material, has been used in several subsystems, such as vehicle interior, suspension, steering joints, and tires. The tire and terrain's contact characteristics are among the essential factors for assessing the performance of the tire and the vehicle in general. Friction and tread wear are two of these contact characteristics. Considering the tire's functionality, for most applications, it is desired to have higher friction to have better traction and a lower wear rate to minimize the material loss of the tread. The friction coefficient and the rubber's wear rate depend on various parameters such as rubber material properties, terrain characteristics, temperature (tread and the environment), and the load. To obtain the wear rate and friction of a viscoelastic material, three approaches have been used for this study: Experimental, Analytical, and Numerical. The results obtained using these approaches have been compared and validated. Several test setups have been designed and implemented to study the wear and friction of the rubber experimentally. Also, a new linear friction tester has been designed and manufactured by the author to achieve this project's objectives. The new test setup has several advantages over existing test setups in this field, such as covering a higher range of velocities while maintaining high precision. The designed Linear Friction tester and the modified dynamic friction tester at the CenTiRe laboratory at Virginia Tech were used to measure the rubber's friction and wear for different testing conditions such as different normal loads, different velocities, and various surfaces such asphalt and sandpaper. The data collected by the experiment will later be used for the validation of the developed models. In order to obtain the wear rate of the rubber using the analytical approach, the real contact area and friction of the rubber were calculated using Persson's model. The simulation has created the surface to obtain the friction coefficient and the real contact area. After obtaining the friction coefficient and the real contact area, the rubber's wear rate was calculated using a novel approach by combining the Persson Powdery Rubber Wear model with the Crack Propagation model. The results from the improved model compare well with the results from the original model. For the last step of this project, a Finite Element approach was used for modeling a tread block and round rubber sample. A new semi-empirical model for wear was developed by improving the Archard wear model. The novel approach was implemented to Abaqus by using the Umeshmotion subroutine and adaptive mesh motion (ALE) and subroutine UFric and UFric_Coef in two categories: The Node base method and the Ribbon base method. For finite element modeling, the visco-hyper elastic material model has been used to define the rubber's material properties. / Doctor of Philosophy / Viscoelastic materials have been used widely in different applications, such as constructing tires, artificial joints, shoe heels, and soles. Therefore, studying the different characteristics of viscoelastic materials has always been a matter of interest in improving their properties for various applications. In the automotive industry, rubber, as a viscoelastic material, has been used in several subsystems, such as vehicle interior, suspension, steering joints, and tires. The tire and terrain's contact characteristics are among the essential factors for assessing the performance of the tire and the vehicle in general. Friction and tread wear are two of these contact characteristics. Considering the tire's functionality, for most applications, it is desired to have higher friction to have better traction and a lower wear rate to minimize the material loss of the tread. This study used different approaches such as experimental, analytical, and numerical methods to predict the friction and wear of the rubber sample in contact with different surfaces. For the experimental parts, the author designed and manufactured a linear friction test setup. For the numerical parts, a new semi-empirical model was created to predict rubber samples' wear accurately.

<div>In this thesis, the role of location-specific microstructural features in the fatigue performance of the safety-critical aerospace components made of Nickel (Ni)-base superalloys and linear friction welded (LFW) Titanium (Ti) alloys has been studied using crystal plasticity finite element (CPFE) simulations, energy dispersive X-ray diffraction (EDD), backscatter electron (BSE) images and digital image correlation (DIC).</div><div><br></div><div>In order to develop a microstructure-sensitive fatigue life prediction framework, first, it is essential to build trust in the quantitative prediction from CPFE analysis by quantifying uncertainties in the mechanical response from CPFE simulations. Second, it is necessary to construct a unified fatigue life prediction metric, applicable to multiple material systems; and a calibration strategy of the unified fatigue life model parameter accounting for uncertainties originating from CPFE simulations and inherent in the experimental calibration dataset. To achieve the first task, a genetic algorithm framework is used to obtain the statistical distributions of the crystal plasticity (CP) parameters. Subsequently, these distributions are used in a first-order, second-moment method to compute the mean and the standard deviation for the stress along the loading direction (σ_load), plastic strain accumulation (PSA), and stored plastic strain energy density (SPSED). The results suggest that an ~10% variability in σ_load and 20%-25% variability in the PSA and SPSED values may exist due to the uncertainty in the CP parameter estimation. Further, the contribution of a specific CP parameter to the overall uncertainty is path-dependent and varies based on the load step under consideration. To accomplish the second goal, in this thesis, it is postulated that a critical value of the SPSED is associated with fatigue failure in metals and independent of the applied load. Unlike the classical approach of estimating the (homogenized) SPSED as the cumulative area enclosed within the macroscopic stress-strain hysteresis loops, CPFE simulations are used to compute the (local) SPSED at each material point within polycrystalline aggregates of 718Plus, an additively manufactured Ni-base superalloy. A Bayesian inference method is utilized to calibrate the critical SPSED, which is subsequently used to predict fatigue lives at nine different strain ranges, including strain ratios of 0.05 and -1, using nine statistically equivalent microstructures. For each strain range, the predicted lives from all simulated microstructures follow a log-normal distribution; for a given strain ratio, the predicted scatter is seen to be increasing with decreasing strain amplitude and are indicative of the scatter observed in the fatigue experiments. Further, the log-normal mean lives at each strain range are in good agreement with the experimental evidence. Since the critical SPSED captures the experimental data with reasonable accuracy across various loading regimes, it is hypothesized to be a material property and sufficient to predict the fatigue life.</div><div><br></div><div>Inclusions are unavoidable in Ni-base superalloys, which lead to two competing failure modes, namely inclusion- and matrix-driven failures. Each factor related to the inclusion, which may contribute to crack initiation, is isolated and systematically investigated within RR1000, a powder metallurgy produced Ni-base superalloy, using CPFE simulations. Specifically, the role of the inclusion stiffness, loading regime, loading direction, a debonded region in the inclusion-matrix interface, microstructural variability around the inclusion, inclusion size, dissimilar coefficient of thermal expansion (CTE), temperature, residual stress, and distance of the inclusion from the free surface are studied in the emergence of two failure modes. The CPFE analysis indicates that the emergence of a failure mode is an outcome of the complex interaction between the aforementioned factors. However, the possibility of a higher probability of failure due to inclusions is observed with increasing temperature, if the CTE of the inclusion is higher than the matrix, and vice versa. Any overall correlation between the inclusion size and its propensity for damage is not found, based on inclusion that is of the order of the mean grain size. Further, the CPFE simulations indicate that the surface inclusions are more damaging than the interior inclusions for similar surrounding microstructures. These observations are utilized to instantiate twenty realistic statistically equivalent microstructures of RR1000 – ten containing inclusions and remaining ten without inclusions. Using CPFE simulations with these microstructures at four different temperatures and three strain ranges for each temperature, the critical SPSED is calibrated as a function of temperature for RR1000. The results suggest that critical SPSED decreases almost linearly with increasing temperature and is appropriate to predict the realistic emergence of the competing failure modes as a function of applied strain range and temperature.</div><div><br></div><div>LFW process leads to the development of significant residual stress in the components, and the role of residual stress in the fatigue performance of materials cannot be overstated. Hence, to ensure fatigue performance of the LFW Ti alloys, residual strains in LFW of similar (Ti-6Al-4V welded to Ti-6Al-4V or Ti64-Ti64) and dissimilar (Ti-6Al-4V welded to Ti-5Al-5V-5Mo-3Cr or Ti64-Ti5553) Ti alloys have been characterized using EDD. For each type of LFW, one sample is chosen in the as-welded (AW) condition and another sample is selected after a post-weld heat treatment (HT). Residual strains have been separately studied in the alpha and beta phases of the material, and five components (three axial and two shear) have been reported in each case. In-plane axial components of the residual strains show a smooth and symmetric behavior about the weld center for the Ti64-Ti64 LFW samples in the AW condition, whereas these components in the Ti64-Ti5553 LFW sample show a symmetric trend with jump discontinuities. Such jump discontinuities, observed in both the AW and HT conditions of the Ti64-Ti5553 samples, suggest different strain-free lattice parameters in the weld region and the parent material. In contrast, the results from the Ti64-Ti64 LFW samples in both AW and HT conditions suggest nearly uniform strain-free lattice parameters throughout the weld region. The observed trends in the in-plane axial residual strain components have been rationalized by the corresponding microstructural changes and variations across the weld region via BSE images. </div><div><br></div><div>In the literature, fatigue crack initiation in the LFW Ti-6Al-4V specimens does not usually take place in the seemingly weakest location, i.e., the weld region. From the BSE images, Ti-6Al-4V microstructure, at a distance from the weld-center, which is typically associated with crack initiation in the literature, are identified in both AW and HT samples and found to be identical, specifically, equiaxed alpha grains with beta phases present at the alpha grain boundaries and triple points. Hence, subsequent fatigue performance in LFW Ti-6Al-4V is analyzed considering the equiaxed alpha microstructure.</div><div><br></div><div>The LFW components made of Ti-6Al-4V are often designed for high cycle fatigue performance under high mean stress or high R ratios. In engineering practice, mean stress corrections are employed to assess the fatigue performance of a material or structure; albeit this is problematic for Ti-6Al-4V, which experiences anomalous behavior at high R ratios. To address this problem, high cycle fatigue analyses are performed on two Ti-6Al-4V specimens with equiaxed alpha microstructures at a high R ratio. In one specimen, two micro-textured regions (MTRs) having their c-axes near-parallel and perpendicular to the loading direction are identified. High-resolution DIC is performed in the MTRs to study grain-level strain localization. In the other specimen, DIC is performed on a larger area, and crack initiation is observed in a random-textured region. To accompany the experiments, CPFE simulations are performed to investigate the mechanistic aspects of crack initiation, and the relative activity of different families of slip systems as a function of R ratio. A critical soft-hard-soft grain combination is associated with crack initiation indicating possible dwell effect at high R ratios, which could be attributed to the high-applied mean stress and high creep sensitivity of Ti-6Al-4V at room temperature. Further, simulations indicated more heterogeneous deformation, specifically the activation of multiple families of slip systems with fewer grains being plasticized, at higher R ratios. Such behavior is exacerbated within MTRs, especially the MTR composed of grains with their c-axes near parallel to the loading direction. These features of micro-plasticity make the high R ratio regime more vulnerable to fatigue damage accumulation and justify the anomalous mean stress behavior experienced by Ti-6Al-4V at high R ratios.</div><div><br></div>