A multi-material structure combines diverse materials, allowing for precise tailoring of properties in specific regions. The integration of dissimilar materials leads to superior performance compared to single-material components. Bimetallic structures, a subset of multi-material systems, offer site-specific performance owing to the contrasting properties of the materials involved. This thesis focused on fabricating a thin, functionally graded bi-metallic wall using Wire Arc Additive Manufacturing (WAAM) with Inconel 625 and HSLA steel. Functional grading of metallic materials, like Inconel 625 and HSLA steel, enables precise customisation of component properties to fulfil distinct functions within a structure. Despite its promise, this combination presents challenges, notably the formation of intermetallic compounds that act as a catalyst for forming solidification cracks in combination with varying material composition and high heat input. Optimal process parameters, notably reduced heat input, are critical in mitigating these issues. The current study involved process parameters optimisation for Inconel 625 single bead-on-plate welds to control dilution levels at dissimilar interfaces. Extensive characterisation using Light Optical Microscopy (LOM) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS) was conducted to study the microstructural evolution of the as-fabricated sandwich structure. Results indicated that interface 1 (Inconel 625/HSLA steel) exhibited minimal dilution and zero defects, while interface 2 (HSLA Steel/Inconel 625) exhibited substantial dilution and was prone to solidification cracks. The EDS results affirmed that the variations of elemental composition and heavy dilution at interface 2 lead to inhomogeneous microstructural features, elemental segregations and the formation of brittle intermetallic phases, thereby leading to a solidification crack at this particular interface. X-ray diffraction (XRD) measurements were conducted to identify phases at the dissimilar interfaces, corroborating the microstructural features observed using SEM and LOM. The identified phases confirm the presence of brittle intermetallics, such as the laves phase, extensively present at interface 2. Vickers hardness testing was performed to assess the mechanical behaviour at both interfaces, revealing a consistent trend of decreasing values with a sudden increase in hardness values at both interfaces due to microstructural transition. Based on the EDS results, an optimal range of elemental compositions was speculated, focusing primarily on the significant elements Fe, Ni, and Cr. These compositions, with Ni content of approximately 20-25 wt%, Fe content of around 70-75 wt%, and Cr content of approximately 5-10 wt%, are crucial in reducing cracking susceptibility at interface 2. This study elucidates the initiation and propagation of solidification cracks at interface 2, establishing a definitive link between microstructural evolution, mechanical behaviour, and crack formation. Ultimately, this thesis lays the foundation for future research to delve deeper into these insights, fine-tune process parameters, and fabricate compositionally graded multi-material structures with enhanced susceptibility to solidification cracking at dissimilar interfaces.

The current research work investigates the possibility of using machine learning models to deduce the relationship between WAAM (wire arc additive manufacturing) sensor responses and defect presence in the printed part. The work specifically focuses on three materials from the nickel alloy family – Inconel 718, Invar 36 and Inconel 625, and uses three sensor responses (welding voltage, welding current and welding audio) for predictions. A variety of types of prints, including ramp tests, single bead depositions, and walls were explored. Three different machine learning models are used – artificial neural networks (ANNs), K-Means clustering and random forests (RF), and the performances are compared. In addition to separate material analysis, cross-material predictions are conducted using two supervised models to investigate the prediction capabilities of such an approach. The results indicate that models are indeed capable of finding connections between welding parameters and defect formation, and the accuracies range from 60% to 90% and the correlation coefficient is less than 0.5 (indicating weak positive correlation) depending on the model and material. The cross-material predictions are significantly worse, with accuracies ranging from 20% to 27% and very weak correlation coefficients (less than 0.1). Analysis of the results indicates that the importance of audio sensor response depends on the nature of defect, and that additional sensors like spectrometers could give a wider range of information to cover more types of defects, potentially raising the performance of cross-material predictions. Between the models, random forest is found to perform the best overall, with ANNs coming in a close second. The versatility of ANNs indicates that increasing the dataset size and resolving the class imbalance could potentially tip the scales in the favor of ANNs.

Wire and Arc based Additive Manufacturing (WAAM) processes are novel technologies that are seen as promising candidates for fabricating complex 3-Dimensional large scale components. These processes offer avenues to locally tailor metallurgy of the melt pool and thus produce functionally and compositionally graded components for improved performance in extreme conditions. This thesis explores in-situ alloying of commercial Manganese Aluminium Bronze (MAB) with copper rich Cu3Si filler wire during deposition, as a microstructure control strategy for designing components that are resistant against selective phase corrosion. Evolution of microstructure during WAAM processing of MAB and during in-situ alloying with Cu3Si wire is investigated using optical and electron microscopy, x-ray diffraction and fluorescence. The microstructure of the as-deposited MAB bead is observed to change from continuous connected dual phase to cellular single phase upon controlled addition of Cu3Si in a MAB melt pool. Corrosion tests showed reduced severity of surface attack and an improvement in the rate of corrosion by 3 times for the new alloy. Thus this technique can be used in industries to improve corrosion resistance as an energy efficient alternative of conventional high temperature aging treatments. Additionally, the possibilities of improving mechanical behaviour of the new alloy (cellular single phase) are explored using the principles of grain refinement in the context of WAAM.

Functionally graded materials (FGM) are a class of materials in which the chemical composition or microstructure varies as a function of position, offering unique material properties. In this study, a functionally graded structure was manufactured using Gas Tungsten Arc Welding and a double wire feed device. The wire feed rate was changed step by step of the austenitic AISI 316L and ferritic AISI 430L stainless steel wire, creating a chemically graded duplex stainless steel structure. The structure, approximately 30 mm in height, was investigated using Optical Microscopy, X-Ray Diffraction, X-Ray Fluorescence and Energy Dispersive X-Ray Spectroscopy. The transverse section is composed of large elongated grains. The chemical analysis revealed a relatively smooth change in Nickel and Molybdenum composition over the section, due to remelting of previously deposited layers. The graded material showed a gradual transition in phase fractions from mainly austenite and some ferrite to mainly ferrite and some austenite, to fully ferric structure. There were no brittle phases detected in the structure.

For in-­situ resource utilisation start-up Maana Electric, an investigation was undertaken to determine whether a cover glass for solar panels can be produced using only desert sand as the raw material. During this investigation, the composition of desert sand, melt formation, processing temperatures, and mechanical and optical properties were considered. The composition of 18 desert sands was analysed by means of X­ray fluorescence and estimations of mineralogical composition were made, after which an attempt was made to melt the unmodified sand samples in a microwave furnace built for the purpose. Melt formation was further observed by melting binary combinations of store bought minerals that were found in the desert sands. The composition data and modelling of temperature-viscosity curves were employed to explore lowering the practical melting of the sand point by modification of the composition through benificiation. `Synthetic benificiated desert sand' was produced and melted based on the results. Glass samples produced were characterized using X­ray fluorescence, visual inspection, optical spectrometry, and fracture mirror analysis. It was found that about half of the desert sand samples assessed contain over 90 wt% silica, making it less feasible for use as raw material for glass due to high melting temperatures and/or large waste streams from benificiation, while sands containing larger fractions of carbonates and/or feldspars will form a melt at less than 1650 degrees Celsius if the SiO2 content is less than 55 wt%. Transmission of 85 % of ~550 nm wavelength light was shown to be possible for desert sand glass of 3 mm thickness if Fe2O3 content is lower than 0.1 wt%, while for the same transmission in the complete effective spectrum of silicon based solar cells the iron content needs to be lowered further. Known absorbing species such as Cr2O3, NiO and CuO were detected in desert sand in trace amounts, but were not present in the synthetic mixtures, the influence of these contaminants on transmission requires further research. Mechanical analysis was inconclusive due to a limited number and low quality of the samples produced, but a review of the literature implies that a Young's modulus of >70 GPa and flexural strength of >45 MPa are attainable in a glass produced from desert sand components.

The emerging field of 3D printing has expanded to the fabrication of metallic components during the last decade. Among the most prominent applications is the production of aluminium bronze marine propellers by the Wire+Arc Additive Manufacturing (WAAM) method. This method incorporates a welding system attached to a robotic arm. The final product results after sequential bead depositions. The main assets of the method are the efficient material usage and the minimization of lead time, which both have a positive environmental and economic impact.  The aim of this project is to evaluate the feasibility to produce 3D printed aluminium bronze (CMA and NAB alloys) blocks with the WAAM method and compare the mechanical and corrosion properties of the blocks with the market requirements. In order to achieve that, rectangular blocks were manufactured at the facilities of Delft University of Technology and at Rotterdam Additive Manufacturing Fieldlab (RAMLAB). Cross sectional areas were extracted and used for microstructural investigation and for hardness measurement. Subsequently, the blocks were machined to produce tensile and Charpy specimens along the built height. Finally, corrosion tests were performed, including open circuit potential measurements, polarization experiments and Scanning Kelvin Probe (SKP) tests. The microstructural investigation revealed that the 3D printed CMA block consisted of a banded structure. The deposited layers consisted of two dominant phases, α and β, and a variety of precipitates. The Widmanstätten α phase nucleates at the grain boundaries mainly. The tempering promotes the growth of the α phase, making the grain boundaries more indistinguishable, while the β phase decomposes. The mechanical testing results depicted that the hardness, the tensile strength and the absorption energy of the 3D printed blocks exceeded the specifications of the cast products, according to the ASTM standards. The built height direction is weaker than the welding direction; however, the deposition height plays no significant role in the mechanical properties. Samples were also tested after a heat treatment of 675 °C for 6 hours, as recommended in the literature. The result was a 25% reduction of the tensile yield strength and a 10% reduction of the ultimate tensile strength. However, the scatter in the measured values was reduced too.  Regarding the corrosion results, the built height has little effect on the corrosion susceptibility, according to the polarization curves. The material exhibits a remarkable low corrosion rate, which justifies its use in marine applications. The Scanning Kelvin Probe (SKP) tests illustrated the beneficial aspect of the tempering heat treatment, which alleviates the large potential differences of adjacent deposited areas. It can be concluded that the CMA alloys are tolerable to the oscillations of the production parameters, making them appealing to the additive manufacturing industry. The mechanical properties achieved, outmatch not only the specifications for the cast CMA products, but also the performance of similar 3D printed aluminium bronze structures, found in the literature.

Conversion coatings are generally required to enhance organic coating adhesion and corrosion resistance on galvanized steel. Until a few decades ago, chromate conversion coatings were the most common conversion coatings in the industry owing to their exceptional performance in this regard. However, the adverse effects associated with hexavalent chromium as found in chromate conversion coatings and certain corrosion inhibitor pigments are now widely known. As a result, various initiatives have been deployed around the globe to restrict and regulate the use of hexavalent chromium. Finding suitable alternatives for chromate conversion coatings has therefore become one of the most pertinent research topics of the moment in the field of corrosion protection. A number of chromium-free conversion coatings have been found to show comparable corrosion resistance relative to chromate conversion coatings. For galvanized steel, conversion coatings based on zirconium and titanium have been found to be suitable alternatives to chromium. Organic additives may be incorporated in these conversion treatment solutions for galvanized steel to enhance the adhesion of organic coatings to the substrate. The effect of a given organic additive is highly dependent on the surface composition of the substrate as has been observed with the polymers polyacrylic acid (PAA), polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) on hot-dip galvanized steel and a Zn-Mg-Al alloy coated steel studied in this project. The durability of corrosion protection may also be enhanced by embedding corrosion inhibitors in the organic coating. An increased concern for sustainability and environmental-friendliness, has resulted in a global effort toward developing and using corrosion inhibitors which are safe for the environment and for human health. The electrochemical behaviour of galvanized steel in electrolytes with green inhibitors based on silicates, phosphates, zinc oxide and calcium at various concentrations are investigated. This project considers a multi-layer corrosion protection coating system comprising a conversion layer based on Zr/Ti cations and adhesion-enhancing polymer additives, and an organic coating embedded with 'green' corrosion inhibitors. The overarching aim is to investigate and establish new knowledge regarding the effect of polymer additives in conversion coatings on the adhesion of organic coatings as well as the identification of suitable green corrosion inhibitors. The outcome is an indication of which types of polymer additives and corrosion inhibitors work best for the two substrates tested – MagiZinc® and hot-dip galvanized steel, both supplied by Tata Steel BV.

In floors without dilation joints drying shrinkage cracking can occur. Shrinkage reducing agents (SRAs) can reduce the amount of drying shrinkage and improve the durability of the concrete structure. The mechanism of shrinkage reduction and the influences on the hydration process and development of mechanical properties of the SRA Sika Control 40 were researched. Shrinkage reducing agents lower the surface tension of water which reduces the capillary tensile stresses and ultimately reducing the drying shrinkage. Experiments were performed on cement paste samples to investigate influences on the hydration process of cement (surface tension, rate of hydration, porosity, shrinkage, flexural and compressive strength). In addition experiments were performed on concrete samples to investigate the influences of applying shrinkage reducing agents on the mechanical properties of concrete (compressive strength, shrinkage, elastic modulus). Overall, the experimental results showed that applying Sika Control 40 is an effective method for reducing drying shrinkage when concentrations of 1.56 w% and 2.61 w% are used. The shrinkage reducing agent has positive effects on the surface tension, weight loss and drying shrinkage for concentrations of 1.56 w% and 2.61 w% of cement. It has negative effects on rate and degree of hydration, porosity, pore size distribution, compressive and flexural strength for concentrations of 1.56 w% and 2.61 w%. The elastic modulus lacks adequate data to observe a trend, however the results are within the range of expected values for concrete for mixtures 156 and 261. , An SRA concentration of 0.52 w% of cement resulted in unpredictable and contradicting results.

Laser surface treatments offer interesting prospects for the creation of architectured microstructures formed by distinct phases in metastable austenitic steels. In the present study, a laser-based localised heat treatment was developed to locally create an austenitic region in a quenched Fe25Ni0.2C martensitic microstructure. The highly localised laser heat flux gives rise to high spatial gradients in peak temperature and heating rate. This results in strong variations in the microstructures observed over short distances, which are related to local changes in the martensite to austenite phase transformation temperatures and formation mechanisms, the occurrence of grain growth and recrystallization in the newly formed austenite, and the tempering of the initial martensite. Moreover, thermal stresses and surface effects influence the final microstructure. In this work, effects of heating rates and peak temperatures are studied by dilatometry whereas Electron Backscatter Diffraction (EBSD) and optical microscopy are used to assess austenite grain size and morphology. This information is linked to a Finite Element Model of the local thermal history to investigate the evolution of the local microstructure throughout the zone affected by the laser heat source. This research provides insight into localised microstructural control of steels with laser surface treatment and provides a thermal model for detailed understanding of the mechanisms controlling the microstructural changes taking place during these treatments.

Additive manufacturing (AM), or 3D printing, is emerging as a technology for different applications made of steel. It is expected that in the coming years the construction industry will benefit from the free and lightweight forms that can be fabricated and the possible material savings. A few experimental results of 3D printed stainless steel are published, while no data is available on the material properties of wire and arc additively manufactured (WAAM) low carbon steel nor on specific connections. Carbon steel is widely applied in the construction industry and is less expensive, compared to other types of steel, as stainless, which was investigated for WAAM in other studies, at the TU Delft and abroad. Therefore, WAAM low carbon steel plates with thicknesses of 3 and 6 mm, produced by the company MX3D, were investigated thoroughly in this research. Tensile coupon tests were performed to determine the strength, stiffness and ultimate strain. The surface roughness, effective thickness and the influence of the thickness and the printing direction on the material properties were investigated by experiments and an evaluation of 3D scanning and Digital Image Correlation (DIC). The Archimedes’ principle was used to establish an effective thickness. The experiments were conceived to predict the tear-out failure behaviour of 3D printed plates. This is one of the basic failure modes in bolted connections. The conducted experiments provide the necessary evidence for the behaviour of single bolts and pins that interact with the WAAM material. A design formula for bolted and pinned connections is proposed based on experimental results. The results of the tensile and tear-out tests were compared with conventionally produced carbon steel and existing findings on WAAM stainless steel. The influence of the end distance of a double lap bolted connection is evaluated by comparing the experimental results with the existing standards and studies on rolled carbon steel. This research was performed to assess the applicability of current design standards for conventionally produced carbon steel to WAAM low carbon steel connections. The reliability of this method was checked by a statistical analysis. A new design factor for the tear-out strength of single bolts and pins in a WAAM low carbon steel plate is recommended. This factor reduces the design resistance of a tear-out connection and includes the ultimate tensile strength of the WAAM low carbon steel, the end distance, effective thickness of the plate and printing direction, compared to the applied force. The reduction factors that originate from the effective thickness determination and that reduce the ultimate tensile strength due to the surface roughness effect have to be incorporated for WAAM material as well. The starting hypothesis was confirmed by the investigation and following is concluded: WAAM carbon steel is a suitable material for structural applications, due to its lower costs and higher stiffness compared to the stainless steel alternatives. The experimental results of this research are meant to be used for design purposes and as input for Finite Element Modelling, so the resistance of more complex geometries can be studied accordingly. It is expected that with topology optimisation (TO) of connections, considering the printing direction, and the consequential material efficiency, full potential of 3D printing can be yielded for the manufacturing of structural parts.

Third-generation advanced high strength steels (AHSS) are a new class of steels that offer superior functional properties and significant weight savings in the body-in-white (BIW) structure of a car. Weight savings directly translate to reduced CO2 emissions from cars which aids automotive manufactures to meet the vehicle emission guidelines put forth by regulatory bodies around the world. Further increase in weight savings and productivity can be realised when BIW components are fabricated using laser beam welding (LBW). However, the phenomenon of solidification cracking of AHSS during LBW poses challenges to not only its application in the automotive industry, and also in the production lines of steel manufacturers. From the body of literature pertaining to solidification cracking two fundamental conditions can be identified that result in solidification cracking in alloys. First is the development of thermo-mechanical stresses/strains during liquid melt solidification and second, is the formation of a crack susceptible microstructure. In addition to this, the welding conditions can influence the susceptibility of alloys to solidification cracking. The objective of the present study is to understand the influence of variable processing conditions during LBW on solidification cracking tendency of AHSS and how to control these conditions to minimise it. In particular, an attempt is made to understand the effect of keyhole configuration, welding speed and laser beam spot size on solidification cracking. Furthermore, finite element analysis is used to predict the size and shape of the weld pool during LBW, and to determine the net process efficiency which in turn is compared with the calculated process efficiency from the existing analytical model. Bead-on-plate LBW following the testing procedure of the VDEh (German Steel Institute) standard hot cracking test was performed on three third-generation AHSS at two LBW facilities with different beam quality. Due to this, the keyhole during the tests at the two facilities was identified to exist in the closed keyhole configuration and more towards the open keyhole configuration. The susceptibility to solidification cracking was found to increase when the keyhole prevailed in the closed keyhole configuration during LBW. The keyhole configuration was varied by altering the process parameters (welding speed and spot size of the laser beam) in comparison to the parameters corresponding to the closed keyhole configuration. Reducing the welding speed, while keeping the laser power and spot size constant, resulted in the open keyhole configuration and subsequent reduction in the solidification cracking tendency but, until a limit. Similarly, reducing the spot size, while keeping the welding speed and laser power constant, also reduced the solidification cracking tendency as the open keyhole configuration was enforced. The macroscopic area of the fusion zone (weld size) was found to corroborate with the solidification cracking tendency of the alloys. Consequently, the spot size of the laser beam was varied to determine the critical spot size which resulted in a critical weld size at which solidification cracking did not occur. Corresponding to this, the critical process efficiency was determined which is representative of the critical heat input above which solidification cracking occurs. However, the magnitude of the critical spot size, weld size and process efficiency is dependent on the solidification cracking susceptibility of the alloy in question.

In recent years, Wire and Arc Additive Manufacturing (WAAM), is gaining attractions both from academia and industry, because it has the potential to substitute or complement the traditional manufacturing methods. Traditional materials and methods are reaching their limitation when demanding and special applications are considered. The current growth in WAAM applications will boost the WAAM consumable material development in the coming future. WAAM applications mostly refer to small batch production or prototypes, which often requires special wire compositions or can benefit from tailoring the consumable composition for the desired components. On one hand, solid wire production is only economically viable when large volumes are involved. On the other hand, metal-cored wires are particularly suitable to produce tailored or small-batch consumable compositions, which is very attractive for WAAM. However, only limited research has been carried out in the use of metal-cored wires in additive manufacturing. In this research, the focus is to explore the use of metal-cored wire in WAAM applications. Two chemical compositions, one ferrous, medium carbon low steel alloy (AM-XC-45), and one non-ferrous, Stellite 6 (cobalt-based superalloy), metal-cored wires were investigated based on industrial interests at RAMLAB. The microstructure of the WAAM AM-XC-45 thin wall was characterized using optical microscopy and scanning electron microscopy. Pearlite, ferrite, bainite, and martensite are present in the deposited wall. Columnar grains are found near the fusion line. The repeated thermal cycles cause the grains to become finer from the top to bottom layers. The mechanical properties including microhardness and tensile strength were tested and compared with the traditional processing methods like casting, milling, and forging. It showed a comparable or superior microhardness and tensile strength to the traditional process whereas the relative lower elongation of the deposited AM-XC-45 thin wall indicates that further post heat treatment is needed to improve the ductility of the part. Stellite 6 is a cobalt-based superalloy, which has good wear and corrosion resistance and retains these properties at high temperatures. In this study, Stellite 6 metal-cored wire (WEARTECH® WT-6 GMAW-C, AWS A5.21 ERCCoCr-A) is selected and optimized to obtain results comparable to the commonly employed and more costly laser deposition, in terms of microstructure and mechanical properties. The optimal deposition parameters were developed using the S355 steel substrate, and then adopted on depositing on the AISI 420 stainless steel substrate. Characterization showed that Stellite 6 layer mainly contains Co-Cr-Fe solid solution with FCC crystal structure as the matrix and the Cr7C3 and Cr3C2 were identified in the microstructure through SEM, EDS, and XRD. These carbides can contribute to the strength. In addition, the identified Co4W2C carbides can contribute to the wear properties of the Stellite 6 WAAM deposits. The dilution and hardness of the WAAM deposit can reach the same level as laser deposition. Additionally, a 3D temperature distribution model is developed by adopting the moving mesh technique to numerically study the WAAM process. The model helps to gain a better understanding of the physical phenomena (heat and mass transfer) during the WAAM deposition. The AM-XC-45 was used to validate the developed model. The simulated and experimental results were compared. The dilution and HAZ of WAAM deposited single bead was used as validation criteria. The A1 line (1100K) simulation is in a good agreement with the experimental result. The fusion line (1800K) shows some deviation. Compared with laser deposition, the main differences between the two processes lie on the heat source, the boundary conditions, and the material responses to the process.

For the use of screening potential glassmaking recipes the seeding method has been applied to a Molecular Dynamics simulated CaO-SiO₂ system in order to attain the parameters for the Classical Nucleation Theory and construct a Time-Temperature-Transformation (TTT) diagram. Using this TTT diagram, the non-crystallisation temperature and the critical cooling rate of the material was determined, two quantities important for the prevention of crystallisation during the glassmaking process. The implementation of the seeding method on the CaO-SiO₂ system involved creating a novel local bond order based detection method for distinguishing the crystalline structure of Wollastonite-1A from the glass melt. Regrettably this method had less than desirable precision which resulted in results for the nucleation rate that can only be used qualitatively. In contrast the results for the crystalline growth rate can be used quantitatively, while the resulting TTT diagram again can only be used qualitatively.

Preventing solidification cracking is an essential prerequisite for the safety of a welded structure. An undetected solidification crack has the potential to cause premature failure during service. Two conditions generated by a weld thermal cycle are responsible for the initiation of solidification cracks. The first is the presence of excessive stresses/strains imposed on the solidifying weld metal and the second is the existence of a weak solidifying microstructure. For more than five decades, weld solidification cracking has been a subject of considerable interest. Cracking has been observed in various alloys, used in a wide range of engineering applications. Despite achieving a better understanding over this period, an accurate prediction of the occurrence of solidification cracking under a specific set of conditions remains difficult. An alloy with a high susceptibility to solidification cracking can still exhibit good weldability upon selection of appropriate welding conditions. Conversely, an alloy with supposedly high resistance to cracking, can still fail when subjected to inappropriate welding conditions. The objective of the research work reported in this dissertation is to study and elucidate the solidification cracking phenomenon in two popular and commercially available automotive sheet steels, namely transformation-induced plasticity (TRIP) and dual phase (DP) steels. In particular, the effect of restraint (strain imposed), shape of the weld pool, solidification morphology, segregation, solidification temperature range, dendrite coherency and interdendritic liquid feeding on susceptibility to solidification cracking is considered.@en

Hydrogen as an alternative energy source has risen in popularity due to increased environmental awareness. Existing natural gas infrastructure is considered as a means to transport hydrogen, due to practicality and financial aspects. However, hydrogen can deteriorate the fatigue behaviour and thus induce premature failure. Since fatigue is a common failure mode in pipelines, a more thorough understanding of the effects of hydrogen on fatigue behaviour is required. In this work, the hydrogen fatigue of X60 pipeline steel and its girth welds was investigated through a combined approach of modelling and in-situ fatigue testing. A novel in-situ gaseous hydrogen charging fatigue set-up was developed, which involves a sample geometry that mimics a small-scale pipeline with high internal hydrogen gas pressure. The specimen geometry involved an internal circumferential notch that induces a stress concentration factor (Kt = 3.0) related to the worst case scenario for pipelines. The effect of hydrogen was investigated by measuring the onset of crack initiation and growth using a newly designed direct current potential drop setup which probes the outer surface of the specimen. A FEA modelling approach was used to estimate the hydrogen equilibrium concentration in the specimens, as well as to determine the stress states in the material. Results showed that both materials experienced a reduction in fatigue life in the presence of hydrogen. For the base metal, the reduction in fatigue life (37%) manifested solely in the crack growth phase; hydrogen accelerated the crack growth (factor 4). In contrast, the reduction in fatigue life (68%) of the weld metal was due to accelerated crack growth (factor 8) and a decrease in resistance to crack initiation (57%). Varying the hydrogen gas pressure from 70 barg to 150 barg did not cause any differences in the fatigue behaviour. The presence of hydrogen influenced the fracture mechanisms of both materials. The fracture path of the base metal transitioned from transgranular and ductile in nature, to a mixed-mode transgranular and intergranular quasi-cleavage fracture. The weld metal exhibited a similar transition, however in the inert environment some intergranular features were observed at the prior austenite grain boundaries. The presence of hydrogen reduced the crack tortuosity. This is associated with a decrease in roughness- and plasticity-induced crack closure, thereby accelerating the crack growth. It is inferred that hydrogen-enhanced localised plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) were the dominant types of hydrogen embrittlement mechanisms during fatigue of this pipeline material. It was concluded that the weld metal is more susceptible to hydrogen fatigue than the base metal in a gaseous hydrogen environment. The worst-case scenario for pipelines is in the case of weld defects. The weld defects involved in this work were macropores (0.5-1.0 mm) with a spheroid morphology. When these defects were located at the notch surface, the resistance to crack initiation decreased by 92% compared to non-porous specimens in nitrogen. The existing natural gas infrastructure could have accumulated similar flaws during service life, which would make them unreliable for safe hydrogen transport. The costs associated with the repurposing of these pipe segments could raise unexpected economic hurdles, hindering the transition to a hydrogen economy.

Micro-indentation testing has shown great promise in extracting local mechanical properties of ductile metal materials. Although the relevant contact physics has been well revealed since the 19th century, interpreting the indentation data still poses many challenges at the application level. Regarding one of the mainstream methodologies for extracting metal's representative stress-stain curve, the semi-analytical method has demonstrated remarkable performance towards the well-defined contact system involved. However, applying such a model to other indentation scenarios tends to cause some practical measuring problems. The validity of the results depends heavily on the practical experimental setup and the hardware testing calibration, which is inherently related to its accomplishment level in capturing the entire mechanical response. This thesis investigates such practical issues through a provided semi-analytical model validation. In addition, to capture a material's elastic modulus with less reliance on initial data, a novel analytical model has been proposed, with a special focus on the extensive unloading/reloading data. However, both the analytical and semi-analytical methods are not fully applicable. For the analytical model, the first unloading segment contributes the most matched estimates of effective modulus to the tensile reference value, with an average deviation error of 6%. But there still remain relatively large discrepancies between two similar samples. Besides, the validity of its results highly depends on accurate profile radius determination, which demands a more precise profilometry system. For the semi-analytical model validation, the resulting indentation strain-stress curves appear to exhibit post-yield behavior and fail to capture the effective modulus as well as the yield strength. It reveals the model's performance that heavily relies on the initial elastic data.

In recent decades, high strength steels in thick sections have been increasingly used in offshore structures where they are subjected to harsh service conditions such as freezing temperatures and high static/dynamic loading. At these conditions they are susceptible to a transition from ductile failure to a dangerous brittle (cleavage) predominant type of failure, which occurs well before yielding. One of the main challenges in employing thick sections is the through-thickness heterogeneous variance of microstructures as a result of the processing route owing to a gradient of cooling rates from the surface to the bulk. As the material’s mechanical and fracture behaviour strongly depend on the microstructure, the through-thickness microstructural heterogeneity leads to a significant scatter in mechanical and fracture properties, which makes it difficult to predict and control cleavage fracture. From the body of literature establishing microstructural dependence on cleavage failure, several features contributing to this type failure can be identified. Phases, grain size, grain boundary misorientations and the presence of secondary phase constituents can play a major role in failure through cleavage. Additionally, cleavage failure is also sensitive to the crack depth to width ratio (a/W). This study investigates the microstructural features contributing to cleavage failure in a 100 mm thick S690QT high strength steel plate by performing mechanical and fracture toughness tests. In order to improve mechanical properties and cleavage fracture toughness, an isoparametric study employing rapid cyclic heating with the objective of grain refinement was performed. The steel plate has coarser prior austenite grain (PAG) sizes, and a larger area and number fraction of inclusions in the middle section. Additionally, segregation bands as a result of solute segregation during the solidification process were observed to be dispersed throughout the middle section. The middle section was also characterized by lower hardness compared to the top section. The detrimental effects of the middle section were evidenced by inferior low temperature tensile properties and cleavage fracture toughness. This was attributed to the larger PAG sizes, larger area and number fractions of inclusions, and segregation bands in the middle section. The specimen orientation with respect to the rolling direction was found to have no effects on the tensile properties. Additionally, different a/W geometries and notch orientations with respect to the rolling direction were used to investigate the role of constraint effect and rolling orientation, respectively, in the fracture behaviour. Shallow-notched specimens representative of the defects found in offshore structures demonstrated a higher fracture toughness than the deep-notched specimens. This was attributed to lower hydrostatic stresses at the crack tip, which reduces stress triaxiality. The isoparametric study resulted in average grain size reduction by 41% and proved to improve micro-hardness, low temperature tensile properties and cleavage fracture toughness by 5%, 13% and 41% respectively. Fractographic analysis on the fracture toughness specimens revealed the presence of O, C-rich regions which are known to promote brittle behaviour.

Delamination is a significant failure mode that has been the subject of extensive research in laminated structures. The interface in assembly structures often represents the susceptibly weak link and necessitates careful consideration to guarantee structural stability. It is of utmost significance to understand the behaviour of the delamination phenomenon and precisely evaluate the fracture toughness (resistance to delamination) in composites. Numerous test techniques have been established over time, some of which have been recognized and adopted as standards. The complexity associated with the standardized tests (crack tip monitoring, design configuration) and the limitations in the scope of testing various interface configurations have led to favoring of alternative testing methods in recent years. This thesis primarily aims at exploiting the new potentials of the Mandrel Peel Test method by incorporating the novel “Multi-Mandrel Concept” in determining the fracture toughness of the Thermoplastic Composites. Mandrel Peel test, a modified adaptation of the standard 90-degree peel test, presents as a promising technique for the delamination fracture testing of composites by determining the energy necessary for peeling off a flexible adherend (generally referred to as the peel-arm) from a rigid substrate. In this thesis, the objective was to further the understanding of the Mandrel Peel test, investigate the sensitivity of specimen configuration and testing parameters on different thermoplastic materials systems, and evaluate the suitability of the Mandrel Peel test method for establishing a relationship between mandrel roller size and mixed mode fracture behavior. Comparative assessments were conducted between the multi-mandrel radii peel test and standardized tests, including DCB, ENF, and MMB tests. Experimental tests, fractography studies, and numerical validation were performed on UD GF/PP and UD CF/PPS composites. The results show that the average fracture toughness values are slightly higher and more consistent for 2-ply peel arm specimens compared to 1-ply peel arm specimens. This is attributed to the microstructure (matrix and fiber distribution) at the interface and its influence on damage mechanism and fracture propagation behavior. The thickness of the ply also affects delamination propagation behavior and depends on the microstructure, with thicker plies exhibiting non-uniform fiber and matrix distribution. The experimental investigation reveals that the fracture toughness values increase as the mandrel radius increases for UD GF/PP and UD CF/PPS composites with a 0|0 interface. The fracture toughness of CF/PPS peel specimens with a 0|90 interface shows limited variation with respect to the mandrel radius. The relationship between mandrel radius and fracture toughness may not follow a linear trend, and it can depend on other factors such as material thickness, interface orientation, and fracture energy at pure loading modes. Comparative assessments between standardized tests and mandrel peel tests show similar trends in fracture toughness values with increasing mode mixture and mandrel roller radii. Fractography analysis provides valuable insights into the fracture behavior of tested materials, and a correlation is identified between the failure modes observed in mixed-mode bending (MMB) tests and mandrel peel tests. The mandrel peel test enables establishing a relatively straight crack front during the delamination propagation, controlled by the mandrel’s kinematics, resulting in a more uniform distribution of strain energy across the specimen’s width, unlike standard tests. The findings suggest that utilizing the multi-mandrel concept in assessing mixed-mode fracture toughness offers the possibility of extrapolating pure mode I and II fracture toughness values. This approach presents a viable tool for characterizing the fracture energy of composites with non-zero fiber-oriented interfaces that cannot be effectively assessed using classical tests. The results support the claim that mandrel roller size influences the degree of mode-mixity and suggests considering mandrel size when assessing mode-mixity in fracture phenomena. Overall, this study provides valuable insights into the sensitivity of specimen configuration and testing parameters, the influence of mandrel roller size on mixed-mode fracture behavior, and the potential benefits of the mandrel peel test for delamination studies. By considering these findings, further advancements can be made in understanding and characterizing fracture properties in hybrid material systems. Keywords: Delamination, fracture toughness, mixed-mode fracture, DCB, ENF, MMB, Mandrel Peel test, thermoplastic composite, GF/PP, CF/PPS, Multi- Mandrel Concept, peel arm, mandrel roller, mode mixity