Laser Shock Peening is a surface treatment used to increase fatigue life, resistance to stress corrosion cracking, and fretting resistance. The compressive residual stress induced at the surface inhibits crack initiation and retards propagation in components undergoing cyclic stress. In this study, peening was applied to aluminium samples representative of aircraft fuselage fastener joints as a potential alternative to cold hole expansion. Since crack initiation is preferential at locations of stress concentration such as fastener holes, only the area of the joint around the highest loaded holes was peened. The peening was performed before the drilling of the holes for fixing. The effect of the drilling process for the fastener holes on the residual stress distribution was investigated with two different techniques: incremental hole drilling (IHD), and synchrotron X-ray diffraction (SXRD). Results show a good agreement with the results from the two methods, with a very high level of spatial accuracy from the SXRD data. Drilling of the holes leads to limited residual stress relaxation, and some intensification of the residual stress in proximity of the holes, leaving compressive residual stress at the surfaces.

Experimental analysis of residual stress in carbon-fibre-reinforced polymer (CFRP) composites is rare owing to the difficulties of determining residual stress accurately in these materials. This can lead to non-conservatism in design and poor understanding of potential failure mechanisms. In this study we have determined the residual stress in a cross-ply CFRP laminate. For the first time, the contour method of residual stress measurement was applied using wire electric-discharge machining (WEDM), the process that is accepted as the best way to ensure an accurate relaxed surface profile is obtained to back calculate the residual stress. A novel incremental linear smoothing approach was introduced for the measured displacement data which gave better approximation than the conventional spline smoothing method used in the standard contour method for metals. Measurements show tensile residual stress in the direction transverse to the fibres with a maximum value of 40 MPa, and compressive residual stress along the fibre direction with a maximum value of -130 MPa. The accuracy of the calculated residual stress was improved by considering the measured thickness of each ply cluster rather than the nominal thickness. The results of modified contour method are compared with an analytical solution based on the classical laminate theory.

In nuclear power plants, thermal fatigue can occur at pipe mixing points where hot and cold water combine, leading to failure at these critical locations. This study investigates the effect of temperature fluctuations on fatigue failure at critical locations and welded joints, which has received limited attention in previous research. A numerical approach was used, starting with highly-resolved unsteady conjugate heat transfer simulations to assess heat flux at the pipe wall. This was followed by structural analysis using the finite element method, and finally, a fatigue assessment to predict failure locations and estimate component lifespan. Temperature differences of 80 degrees C and 160 degrees C were investigated at the different weld locations. Results showed that at a temperature difference of 160 degrees C between the main pipe and the branch pipe, a full penetration butt weld would fail after 1462 to 19,119 h. A shorter failure time was observed at the stress concentration area upstream of the T-junction under the same conditions. These findings were applied to the well-documented 1998 failure at the Civaux1 plant in France, to help understand the potential causes of that failure.

Additive manufacturing (AM) processes are known to produce anisotropic microstructures and thereby, anisotropic mechanical properties. However, fundamental understanding on the anisotropic mechanical behaviour of AM-built Ti6Al4V is limited, particularly for high-deposition rate wire feed directed energy deposition AM processes. The present study provides insights into the role of anisotropic microstructure and associated texture on the tensile deformation and damage accumulation in wire feed directed energy deposition Ti6Al4V. Materials were deposited using oscillation-pass and parallel-pass build strategies. In situ neutron diffraction studies were performed on samples with tensile loading applied parallel and perpendicular to the built layers. Dissimilar thermal histories experienced in the parallel-pass strategy resulted in thinner columnar (3 grains and finer transformed microstructure, the latter leading to higher yield strength compared to the oscillation strategy. The presence of strong columnar (3 fibre textures in both build strategies led to anisotropic deformation. When loaded perpendicular to the columnar grains, elastic strain accumulation is more crystallographically homogeneous and includes strain accumulation between basal, prismatic, and pyramidal planes in both build strategies. Conversely, when loaded parallel to the columnar (3 fibre texture, the majority of the pyramidal orientations preferentially aligned along the loading axis and were subjected to significant elastic strains. Similar anisotropy was inferred under plastic deformation where tensile strain appeared to be accommodated primarily by prismatic slip but was not detected when loaded parallel to the columnar grains.

Metastable n-Ti alloys are important alloys for the aerospace industry. Understanding the often complex microstructure evolution in these alloys is essential to control their mechanical properties. Hence, we conducted heat treatment experiments combined with in situ electrical resistance measurements to investigate the phase transformation behaviour of a metastable n-Ti alloy, Ti-5Al-5Mo-5V-3Cr (Ti-5553). The results were condensed into a Time-Temperature-Transformation diagram, which revealed three distinct phase transformation regions: w-phase region, alpha-phase region, and the likely presence of an alpha 2-phase region. The phase field 'nose' for alpha-phase formation lies at-600 degrees C. Additionally, the results showed that the transition between pseudo-spinodal and isothermal w-phase-assisted mechanisms for alpha-phase formation occurs at a heating rate of-30 degrees C/min.

Laser shock peening is a novel surface engineering method that improves component performance by introducing deep compressive residual stress. It has demonstrated significant benefits in safety-critical applications such as aerospace engines. Ongoing research focuses on lowering the cost and increasing equipment flexibility.

Crack initiation in AISI 316 stainless steel has been investigated. Persistent slip bands (PSBs) were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). PSBs on the surface of the material increase the surface roughness and result in crack initiation. EBSD data from near the crack initiation region were used to correlate the global and local misorientations of the grains, plastic deformation, and Schmid factor with the fatigue life of specimens. The crack initiation region was found to have the highest misorientations. The region near crack initiation was found to have more plastic deformation, which was severe in specimens loaded with higher stresses. The kernel average misorientation (KAM) and grain reference orientation deviation (GROD) maps from the EBSD data were investigated for specimens that failed at different fatigue cycles. It was found that the interaction of high dislocation density, substructuring, and misorientation of low-angle grain boundaries in the region of plastic deformation resulted in fatigue crack initiation.

Multi-material laser powder bed fusion (MM-LPBF) offers the possibility of components with material and compositional complexity, as well as the geometric complexity for which additive manufacturing is known. LPBF materials are susceptible to fatigue failures due to stress concentrating roughness and porosity defects. Understanding fatigue failure processes is therefore important to enable adoption of multi-material parts, and suitable combinations of materials may offer a strategy to enhance fatigue performance by resisting crack propagation. This study focused on fatigue crack propagation in 316L/15-5 precipitation hardened (PH) bi-material stainless steels (SS), and the effect of residual stress distribution and yield stress gradient on fatigue crack propagation through the interface. The expected yield stress gradient effect in bi-materials (soft to hard interface) was simulated using FE models, showing a slight shielding effect with a drop in J-integral value. Contour cutting measurements detected a residual stress distribution near the bi-material interface that was tensile in 316L layer and compressive in 15-5PH layer. Fatigue crack propagation rates in bi-materials deviated from those in the corresponding single-material specimens. A relatively small shielding effect due to the yield stress gradient was detected within a short distance of the crack tip from the interface. However, the effects of residual stress were more pronounced and inhibited the crack growth rate by up to 77.8 % in regions of 15-5PH SS under residual compression, which suggesting that MM-LPBF parts can be designed such that the compressive residual stress is positioned to intercept and suppress propagating cracks to improve damage tolerance.

This paper proposes a transfer learning approach to recalibrate our previously developed Wheel Odometry Neural Network (WhONet) for vehicle positioning in environments where Global Navigation Satellite Systems (GNSS) are unavailable. The WhONet has been shown to possess the capability to learn the uncertainties in the wheel speed measurements needed for correction and accurate positioning of vehicles. These uncertainties may be manifested as tyre pressure changes from driving on muddy and uneven terrains or wheel slips. However, a common cause for concern for data-driven approaches, such as the WhONet model, is usually the inability to generalise the models to a new vehicle. In scenarios where machine learning models are trained in a specific domain but deployed in another domain, the model's performance degrades. In real-life scenarios, several factors are influential to this degradation, from changes to the dynamics of the vehicle to new pattern distributions of the sensor's noise, and bias will make the test sensor data vary from training data. Therefore, the challenge is to explore techniques that allow the trained machine learning models to spontaneously adjust to new vehicle domains. As such, we propose the Recalibrated-Wheel Odometry neural Network (R-WhONet), that adapts the WhONet model from its source domain (a vehicle and environment on which the model is initially trained) to the target domain (a new vehicle on which the trained model is to be deployed). Through a performance evaluation on several GNSS outage scenarios - short-term complex driving scenarios, and on longer-term GNSS outage scenarios. We demonstrate that a model trained in the source domain does not generalise well to a new vehicle in the target domain. However, we show that our new proposed framework improves the generalisation of the WhONet model to new vehicles in the target domains by up to 32%.

Oxide-dispersion-strengthened (ODS) steels are candidate materials for components in current and future nuclear power plants. One of the issues with using ODS steels is the difficulty of joining them without loss of mechanical performance. In this study, austenitic ODS 316L stainless steel was diffusion-bonded to Inconel 718 superalloy. Having optimized the bonding conditions, a number of samples were made at 1200 °C with a bonding time of 1 hour and pressure of 10 MPa. Preliminary mechanical and microstructural analyses indicated the formation of a sound joint interface, despite slight grain growth in the ODS 316L. A post-bonding thermo-mechanical treatment (TMT) was conducted to refine and restore the microstructure of the ODS 316L. Comparative TEM investigations of the parent ODS alloy and the bonded samples (with and without TMT) along with statistical analyses showed that the Y–Ti–O oxide size distribution remains unaffected by the bonding and complementary TMT cycles, indicating stability of such particles even at very high temperatures and suitability of the devised route for joining the ODS 316L steel.

Surface roughness controls the mechanical performance and durability (e.g., wear and corrosion resistance) of laser powder bed fusion (LPBF) components. The evolution mechanisms of surface roughness during LPBF are not well understood due to a lack of in situ characterisation methods. Here, we quantified key processes and defect dynamics using synchrotron X-ray imaging and ex situ optical imaging and explained the evolution mechanisms of side-skin and top-skin roughness during multi-layer LPBF of Ti-6Al-4V (where down-skin roughness was out of the project scope). We found that the average surface roughness alone is not an accurate representation of surface topology of an LPBF component and that the surface topology is multimodal (e.g., containing both roughness and waviness) and multiscale (e.g., from 25 mu m sintered powder features to 250 mu m molten pool wavelength). Both roughness and topology are significantly affected by the formation of pre-layer humping, spatter, and rippling defects. We developed a surface topology matrix that accurately describes surface features by combining 8 different metrics: average roughness, root mean square roughness, maximum profile peak height, maximum profile valley height, mean height, mean width, skewness, and melt pool size ratio. This matrix provides a guide to determine the appropriate linear energy density to achieve the optimum surface finish of Ti-6Al-4V thin-wall builds. This work lays a foundation for surface texture control which is critical for build design, metrology, and performance in LPBF.

The detection of illicit radiological materials is critical to establishing a robust second line of defence in nuclear security. Neutron-capture prompt-gamma activation analysis (PGAA) can be used to detect multiple radioactive materials across the entire Periodic Table. However, long detection times and a high rate of false positives pose a significant hindrance in the deployment of PGAA-based systems to identify the presence of illicit substances in nuclear forensics. In the present work, six different machine-learning algorithms were developed to classify radioactive elements based on the PGAA energy spectra. The model performance was evaluated using standard classification metrics and trend curves with an emphasis on comparing the effectiveness of algorithms that are best suited for classifying imbalanced datasets. We analyse the classification performance based on Precision, Recall, F1-score, Specificity, Confusion matrix, ROC-AUC curves, and Geometric Mean Score (GMS) measures. The tree-based algorithms (Decision Trees, Random Forest and AdaBoost) have consistently outperformed Support Vector Machine and K-Nearest Neighbours. Based on the results presented, AdaBoost is the preferred classifier to analyse data containing PGAA spectral information due to the high recall and minimal false negatives reported in the minority class.

Making laser powder bed fusion (L-PBF) additive manufacturing process sustainable requires effective powder recycling. Recycling of Ti6Al4V powder in L-PBF can lead to powder oxidation, however, such impact on laser-matter interactions, process, and defect dynamics during L-PBF are not well understood. This study reveals and quantifies the effects of processing Ti6Al4V powders with low (0.12 wt%) and high (0.40 wt%) oxygen content during multilayer thin-wall L-PBF using in situ high speed synchrotron X-ray imaging. Our results reveal that high oxygen content Ti6Al4V powder can reduce melt ejections, surface roughness, and defect population in the built parts. With increasing oxygen content in the part, there is an increase in microhardness due to solid solution strengthening and no significant change in the microstructure is evident.

Plain bend bars made from FV566 martensitic stainless steel were extracted from the root of ex-service power plant turbine blades and several industry-relevant notch geometries were introduced. Some of the samples were shot peened. The notched bend bars were loaded plastically in the low-cycle fatigue regime and finite element (FE) modeling carried out to investigate the effects of changing notch geometry, combined with shot peening, on fatigue behaviors such as crack initiation, short crack growth, and coalescence. Shot peening damaged the notch surface, accelerating initiation behaviors, but had a lifetime-extending effect by retarding short crack growth in all tested notch geometries. At a total strain range higher than 1.2%, the lifetime extension benefit from shot peening was diminished due to compressive residual stress relaxation in the notch stress field. Notch geometry (and the associated varying constraint levels and stress/strain gradients) was found to have no notable difference on fatigue life when tested at identical notch-root strain ranges.

This paper reports on the adoption of TiB2 as an inoculant to fabricate IN718 via the direct energy deposition (DED) process. Effective grain refinement and low texture were achieved in IN718/TiB2 using a TiB2 powder size of d(90) = 10 mu m and mass fraction of 1.5 wt%. The use of low linear energy density (33.08 J/mm) produced IN718/TiB2 deposits free from large grains (>300 mu m), however at the cost of the formation of interlayer defects. By comparison, the large grains were present in deposits made with a linear energy density of 78.74 J/mm and also in deposits manufactured without an included inoculant. Production of deposits at lower energies without interlayer defects was possible by reducing the powder flow rate to 7 g/min. However, this caused a moderate increase in grain size. The TiB2 inoculant reduced the Laves phase network by replacing it with homogeneously distributed Cr-, Mo-, Nb-, and B-enriched needle-shaped precipitates. Tensile strength increased by 300-500 MPa with TiB2 addition, but at the cost of significant ductility drop, regardless of the deposition conditions. The IN718 deposit displayed many micro-cracks at the network of Laves phase during tensile loading, whereas micro-cracks in the IN718/TiB2 occurred at the interface between the needle-shaped precipitates and the matrix. The strength enhancement in IN718/TiB2 was by a combination of strengthening mechanisms: grain boundary, dislocation structure formation, Orowan-type and load transfer related to the needle-shaped precipitates.

We performed high-throughput in situ electrical resistance measurement experimentation to understand the effect of cooling rate on the phase transformation behaviour of Ti-5Al-5Mo-5V-3Cr beta-Ti alloy. The results suggest the presence of two alpha-phase variants and that the formation of alpha-phase starts with a cooling rate of 0.5 degrees C/s. A slow cooling rate (0.1 degrees C/s) can be used to obtain similar microstructure and hardness (similar to 430 Hv) than fast cooling followed by an ageing treatment.

Ti-6Al-4V was fabricated by powder-bed fusion using different laser scanning strategies. The microstructure and deformation properties were investigated in the as-built condition, and also after the material had been subjected to a laser-shock-peening (LSP) treatment. The microstructure in each condition was surveyed using 3D optical microscopy, EBSD, and TEM. The post-manufacture residual stresses were determined. The results indicate a correlation between the residual stresses and the substructures observed in TEM: tensile residual stresses from the surface down to 1 mm depth were observed in the as-built material, corresponding to extensive deformation through twinning of the {1012} type and wavy slip structures; while after LSP the alloy showed a variety of dislocation arrangements, especially planar and in significantly higher density, along with {1122} twins and with the presence of compressive residual stresses. The findings indicate that the deformation capability is mecha-nistically aided by the peening process, which effectively promotes the replacement of tensile residual stresses by compressive ones, offering routes for potentially improving the mechanical properties of the additively manu-factured Ti-6Al-4V, as well as its usability.

Room-temperature high-cycle fatigue (HCF) of IN718 repaired joint via laser direct energy deposition (DED) were studied with the fatigue axis perpendicular to the joint interface. Solution treated and aged (STA) were compared with directly aged (DA) conditions. The wrought IN718 substrate showed equiaxed grains with a size of-90 mu m and a high fraction of annealing twins, whereas the DED deposit revealed a mixture of equiaxed and columnar grains with an average size of-20 mu m. There was little difference between the STA and DA conditions in the grain length-scale. Micro-hardness results highlighted the need for the heat treatment as it can remove the heat-affected zone and hardness dip, creating a uniform hardness profile across the joint. Although the mono-lithic DED deposit had a similar tensile strength to the wrought substrate, the DED joint exhibited an overall decreased HCF performance, regardless of the heat treatment conditions. When the fatigue stress was low, the STA condition had a better HCF performance than the DA, however, the opposite trend appeared for the high stress, resulting in a cross-over point on the stress-life S -N plot. Interrupted fatigue tests, combined with mi-croscopy and fractography, revealed that the fatigue failure occurred in the substrate for the DED joint in the DA condition, whilst in the deposit zone for the STA condition due to the distribution and fracture of the Laves and delta phases. Grain boundary cracking in the substrate near the substrate-to-deposit interface can occur in both cases, probably due to the Nb-rich liquid films.

Wire + arc additive manufacturing (WAAM) is a modern manufacturing process that has opened new possibilities for rapid builds and reductions in material wastage. This paper explores residual stress in WAAM Ti-6Al-4V walls built using three different deposition strategies: single bead, parallel path, and oscillation path. The effect of interlayer hammer peening and interlayer temperature was investigated for the single bead walls. We also examined the residual stress in compact-tension (C(T)) coupons extracted from large builds (walls) with crack orientation either parallel with or perpendicular to the build direction. This type of sample is often used for the measurement of the fatigue crack growth rate. The contour method was used for experimental determinations of residual stress. In addtion, residual stress in the C(T) coupons was estimated by finite element (FE) analysis. A good agreement was achieved between the contour method and FE analysis. The oscillation-path wall had the lowest residual stress values. For the single bead walls built with various process conditions, residual stress was significantly reduced after removing the substrate. A interlayer temperature of 110 °C resulted in much higher residual stress values in the wall (both tensile and compressive) compared to the continuous build, with much higher interlayer temperature.