As a natural composite material, wood exhibits complex structural char-acteristics and diverse behavior under compression and tensile forces. Itsanisotropic nature results in mechanical properties that vary depending onthe load direction along its longitudinal, tangential, and radial grains. An im-portant property of anisotropic materials is the modulus of elasticity, whichrelates stress to strain and demonstrates directional variations.This study focuses on investigating the mechanical properties of pinewood,particularly stiffness, and deformation, in the longitudinal direction duringcompression, taking into account the effect of the sample size. The digitalimage correlation (DIC) method is utilized to measure deformation, an op-tical technique that involves tracking motion in DIC patterns. Wood, beinga renewable and natural composite resource, has been widely used as a con-struction material and for various other purposes for centuries. Its physicaland mechanical properties encompass a broad spectrum influenced by factorssuch as species, moisture content, density, and temperature.This research aims to analyze the size-dependent effects on deformation andstiffness in pine wood samples using the DIC method. Initially, three sam-ple sizes were compared, namely 10 × 10 × 10mm3, 20 × 20 × 20mm3, and25 × 25 × 100mm3 these were selected based on Afshar (2022) and Walley& Rogers (2022). However, the sample size of 20 × 20 × 20mm3 producedinsufficient results due to equipment limitations to test them under sufficientload. Among the remaining sample groups, nine specimens from each groupwere tested and compared in terms of stiffness and deformation. The exper-imental results did not provide statistically significant data supporting thepresence of a noticeable size effect between the dimensions of the samples10 × 10 × 10mm3 and 25 × 25 × 100mm3. / Trä är ett förnybart och naturligt kompositmaterial och har i århundradenanvänts som byggmaterial och för olika ändamål. Dess fysikaliska och mekaniskaegenskaper omfattar ett brett spektrum som påverkas av faktorer som fuk-thalt, densitet och temperatur. Trä som ett naturligt kompositmaterial ochdess komplexa strukturella egenskaper visar varierande beteende vid tryckoch dragkrafter. Träets anisotropa natur gör att dess mekaniska egenskapervarierar beroende på belastningsriktning, längs dess längd, tangential ellerradial riktning. För anisotropa material är elasticitetsmodulen en viktig egen-skap som varierar längs de olika riktningarna. Vilket är ett samband mellanspänning och deformationen och visar riktningsvariationer.Den här studien syftar till att undersöka de mekaniska egenskaper hos furu.Den här studien undersöker storleks-effekt i materialet furus styvhet och de-formationen längs fiberriktning (längd) vid kompression med hjälp av DigitalImage Correlation (DIC). Metoden DIC är en optisk teknik som mäter de-formationen hos material med hjälp av spårningsmönster på materialet.För att se storleks effekten hos trä jämfördes initiellt tre provstorlekar, näm-ligen 10 × 10 × 10mm3, 20 × 20 × 20mm3, och 25 × 25 × 100mm3 dessa provs-torlekar valdes baserat på Afshar (2022) och Walley & Rogers (2022). Dockproducerade provstorleken 20 × 20 × 20mm3 otillräckliga resultat på grundav begränsningar i utrustningen för att testa dem under tillräcklig belast-ning. Bland de återstående provgrupperna testades och jämfördes nio provfrån varje grupp när det gäller styvhet och deformation. De experimentellaresultaten gav inte statistiskt signifikanta data som stödjer förekomsten aven märkbar storlekseffekt mellan dimensionerna hos proven 10 × 10 × 10mm3och 25 × 25 × 100mm3.

<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>