Hydrogen-assisted fatigue in pipeline steel has become a major concern in the field of energy transition. In recent decades, growing awareness of environmental issues has significantly increased interest in hydrogen as an alternative energy source. Hydrogen is affordable and can be transported using the existing natural gas infrastructure. However, the structural integrity of pipeline steels, especially under hydrogen-assisted fatigue conditions, poses significant challenges to the durability and safety of modern energy infrastructure. Since fatigue is a common cause of pipeline failure, a deeper understanding of hydrogen’s impact on the fatigue behaviour of these pipeline materials is essential. This project investigates the fatigue crack growth (FCG) behaviour of API X65 pipeline steel and its welding material in hydrogen enriched environments. To achieve this, a modular hydrogen autoclave testing device was designed and implemented to conduct in-situ hydrogen gaseous fatigue tests on eccentrically single-edge notched tension (ESE(T)) specimens. The fatigue crack growth relationship (da/dN vs. ∆K) was measured, and hydrogen-assisted fatigue crack growth was observed and compared to benchmarks in nitrogen and air. The study employs two testing frequencies to examine the influence of loading frequency under constant load and hydrogen pressure. This approach evaluates the combined effects of hydrogen and the microstructures of both base and weld metal. The investigation focuses on material responses in terms of crack propagation rates, measured using direct current potential drop, and fractographic analysis. To determine the required hydrogen pre-charging time, a hydrogen diffusivity FEA (Finite Element Analysis) modelling approach was used to estimate the hydrogen equilibrium concentration in the specimens before testing. The experimental setup involved pre-charging the specimens in a hydrogen environment at 10 MPa, followed by fatigue testing at frequencies of 0.1 Hz and 10 Hz. The findings indicate that the hydrogen environment accelerates fatigue crack growth compared to inert gases and air. The base metal (BM) at 10 Hz showed approximately two times greater susceptibility to hydrogen-assisted cracking compared to the weld metal (WM). At higher ∆K values (∆K > 50 MPa√m), the crack growth rates of base metal (BM) and weld metal (WM) were similar. However, at lower ∆K values (20-30 MPa√m)), the BM exhibited about one order of magnitude higher crack growth rates. The study emphasizes the importance of microstructural characteristics, including grain boundaries, size, and micro-hardness, in affecting fatigue crack growth. The study confirms that loading frequency significantly influences H-AFCG rates, where at loading frequency f = 0.1 Hz, FCGR increases by 2.3 times for BM and 3.9 times for WM compared to results at 10 Hz frequency, with lower frequencies allowing more hydrogen diffusion and promoting the transition from ductile to brittle cracking. Further findings indicated that finer grains in WM enhance hydrogen fatigue resistance due to increased grain boundary densities. However, this advantage diminishes at lower frequencies where hydrogen diffusion is enhanced, increasing susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG). Analysis of fracture surfaces reveals a significant transition from ductile to brittle fracture modes when moving from inert environments (air or nitrogen) to hydrogen. In hydrogen environment, the presence of quasi-cleavage and intergranular facets was prevalent, compared to ductile transgranular features in air or nitrogen. Additionally, in hydrogen environment, secondary microcrack branching a characteristic of ductile fracture is notably absent, especially at lower stress intensity factors (∆K).

Additive manufacturing (AM) is quickly becoming one of the more popular methods to manufacture components made of Ti-6Al-4V in the aerospace and automobile industry due to its flexibility in producing complex geometries and reducing tooling costs. As the world of additive manufacturing is still relatively young, there is no globally accepted method for the certification of AM parts, allowing freedom to develop procedures to utilize the advantages of AM. To greatly reduce material wastage, the use of smaller specimens has been explored to characterize the mechanical properties of the material.

This study aims to analyze the influence of test specimen size on the near threshold fatigue properties of Ti-6Al-4V manufactured by conventional processes and Laser powder bed fusion (LPBF) . Two different specimen sizes were utilized, the standard size and the sub-sized specimens. The effect of two heat treatments, stress relief (SR) and annealing (AN) on the LPBF material on the fatigue properties was also studied. In addition, the effect of anisotropy on the fatigue threshold value was also touched upon in this study. A fatigue test setup for the sub-sized samples was designed and a constant Kmax test was employed to characterize the threshold properties. The direct current potential drop (DCPD) method was employed to measure the crack length during the test.

The fatigue threshold of the SR microstructure did not show any effect due to sample size. This was attributed to the partly intergranular crack propagation observed playing a prominent role in the fatigue threshold. The conventional and annealed samples witnessed a drop in threshold values of about 0.5 MPa√m when sub-sized samples were used. It was observed that the sub-sized samples had a greater crack depth at threshold leading to increased constraint at the crack tip and hence smaller degree of crack closure explaining the drop in threshold values.

The current study has thus established successfully the feasibility of using sub-sized specimens to characterize the near threshold fatigue properties of conventionally and additively manufactured Ti-6Al-4V.

With fossil fuels being phased out and growing global interest in a hydrogen economy, there is demand for re-purposing existing pipelines for transportation of hydrogen gas. However, hydrogen is known to have adverse affects on the properties of steels. Hydrogen will dissolve in the steel matrix and contribute to a reduction of mechanical properties, causing Hydrogen Embrittlement (HE). There is currently a knowledge gap about the behaviour of pipeline steels and their welds in a gaseous hydrogen environment that prevents re-purposing of pipelines for hydrogen transport. In this work, a combined approach of modelling and in-situ mechanical testing was used to assess the HE susceptibility of X60 pipeline steel and its girth welds. To this end, a novel tensile setup featuring in-situ charging with high pressure H2 gas and a sample geometry representing miniature pipelines was developed and validated. To our best knowledge, this setup design has not been reported elsewhere, making this a breakthrough design. An FEA modelling approach was used to estimate the pre-charging duration as well as to gain an insight into the stress states inside the notched samples. It was found that both the base and weld metals lose ductility when subjected to gaseous H2. The base metal showed 27% loss of ductility when subjected to 100 bar H2, which further increased to 40% in notched samples. A trend of increasing loss of ductility was found with increasing pressure for the weld metal, which showed up to 14% loss of ductility at 100 bar H2. The weld metal also retained more of its reduction in cross-sectional area after fracture in H2 as compared to N2 than the base metal. Other characteristics like yield strength and UTS were not affected by the hydrogen gas. The fracture mechanism in the both metals was found to change from microvoid coalescence (MVC) fracture to quasi-cleavage (QC) fracture. The base metal fracture mechanism changed to QC completely, while the weld metal only showed partial QC fracture. In the base metal, ductile fracture mechanisms like HELP and possibly AIDE were found to be dominant even in the QC fracture mode. It was concluded that the weld metal is less susceptible to HE than the base metal in a gaseous hydrogen environment. For both metals, HE effects were only observed at high amounts of plastic strain, which is outside of the operating conditions of a pipeline. However, before pipelines can be repurposed for hydrogen transport, fatigue testing should be performed to assess the influence of existing defects and cyclic loading conditions on the HE performance of both steels.

Additive Manufacturing (AM), commonly known as 3D printing, exemplifies the recently emerging processing methodologies that aim to substitute the conventional routes, such as to produce parts with complex geometries and eliminate expensive tooling. AM also allows high degree of freedom and rapid prototyping for functional part optimization. This has led to a renewed interest in the Functionally Graded Materials (FGMs). FGMs are a class of novel materials designed to have graded compositions or microstructures with tailored properties. Selective Laser Melting (SLM) is one of the most widely used AM method showing great potential to produce parts made from Inconel 718, a Nickel-based superalloy.This study aims to investigate the microstructural gradients in Inconel 718 produced with SLM by manipulating the thermal fields acting during the production and their subsequent effect on fatigue behavior. Two different laser powers, 950 W and 250 W were used to develop coarse grained and fine grained microstructures respectively. Ungraded and graded specimens were produced to study the fatigue crack growth in individual as well as graded microstructures under cycling loading. The effect of standard post-process heat treatments on the microstructure and fatigue properties of as-printed (AP) AM Inconel 718 was also studied. The two heat treatments under discussion here are homogenisation + solution + aging (HT) and hot isostatic pressing + HT (HIPHT). Direct Current Potential Drop (DCPD) method was used to measure the fatigue crack growth rates in standard tests to identify the fatigue properties of ungraded microstructures. A new approach of using a constant ΔK procedure was employed for graded specimens to investigate the crack growth rate as a function of the crack interaction with local microstructure.The coarse columnar grains with preferred <001> texture were found elongated along the building direction (BD) and their axis of elongation in the specimens changed as a function of BD. Fine grains were found equiaxed and randomly oriented. HT had no significant effect on this observed trend, while HIPHT entirely altered the printed microstructure. The grain sizes, orientation as well as heat treatments were found to be affecting the fatigue behavior of individual microstructures. Fine grained microstructures showed a slower fatigue crack propagation (by ≈70% in AP, ≈40% in HT and ≈45% in HIPHT) than coarse grained. Fatigue cracks propagated slower in coarse grains oriented perpendicular to the crack path in AP (≈80%) while they were slower when oriented parallel to the crack path in HT (≈75%) and HIPHT (≈9%). The interfaces produced in AP and HT graded specimens were seen to introduce barriers for crack propagation and reduce the local crack growth rate. The same was not observed in HIPHT due to diminished gradients, resulting from grain coarsening.Thus, this study has successfully demonstrated the feasibility of using AM to fabricate future FGMs featuring altered fatigue response of the local microstructures.

Functionally graded materials (FGMs) represent a new class of materials that consist of graded compositional and microstructural patterns allowing to tailor properties. Additive manufacturing (AM) or more commonly known as 3D-printing offers a paradigm shift in FGMs and engineering design due to its ability to produce components with complex geometries and functional part optimization. This study aims in investigate the concept of microstructural grading enabled via 3D-printing process and its feasibility to tailor site-specific fatigue behaviour. Non-graded and graded specimens of Inconel 718 featuring a range of microstructural gradients were produced by selective laser melting (SLM). In the current work the grain size, grain orientation and precipitates, resulting in distinctly different properties, were used to develop microstructural gradients. Post-process heat treatment in terms of hot isostatic pressing (HIP) and aging was also investigated. Microstructural and elastic properties as well as in-depth fatigue crack growth behaviour were characterised and compared with conventionally manufactured wrought Inconel 718. Fatigue crack growth in FGMs under cyclic loading was investigated via novel experiments and FE approach. Results from homogeneous specimens were used for estimating spatial property distribution and crack-extension effect in the graded specimens. Hence, for the graded material a constant ΔK procedure was designed to investigate the change in crack growth rate as a function of different gradients. Additionally, Digital Image Correlation (DIC) and Potential Drop (PD) were conducted to validate the constant ΔK tests. It was found that fatigue behaviour of the individual microstructures was primarily affected by the grain size, grain orientation and heat treatment. As opposed to the general trend, fine grained material (range of 10-70 µm) had a better fatigue crack propagation resistance than coarse grained (100 – 500 µm), which was attributed to the high compressive residual stresses developed during the 3D-printing. With respect to the wrought material, the 3D-printed material with fine grains showed higher ΔKth and lower da/dn values. The crack growth rate in the graded material at a constant ΔK was found to change from one microstructure to the other following the same trend as the individual microstructures. The effect of grain orientation was found to significantly affect the crack growth rate, with grains elongated parallel to crack growth showing the larger and more continuous crack growth gradient. Additionally, in the heat treated (HIP+H/T) graded material, the difference in properties between the two gradients was almost entirely diminished, indicating the necessity to optimize heat treatment regimens more suitable for developed herein FGMs. The current study not only delivered fatigue parameters and FE model for various graded and non-graded 3D-printed Inconel 718, but also successfully demonstrated the feasibility of using 3D-printing process to design microstructures and alter the fatigue behaviour in FGMs components.

This thesis is a comparative study of the Thermo-Mechanical Fatigue (TMF) performance of different grades of SiMo nodular cast iron for heavy-duty diesel engine exhaust gas manifold applications. The TMF performance of the current SiMo variant used to manufacture exhaust manifolds - SiMo 5.10 (C-3.25Si-4.45Mo-0.76), is compared with that of the variants SiMo 4.05 (C-3.22Si-4.66Mo-0.56) and SiMoNi (C-3.3Si-4.5Mo-1Ni-1.3) by performing three out-of-phase (OP) TMF test series under partial constraint conditions. A benchmark TMF test series in the temperature range: 50 ˚C to 550 ˚C with a hold time of 30 s at 550 ˚C showed that SiMo 5.10 had relatively better performance due to development of lower mechanical crack driving forces compared to other variants. However, a long holding time of 600 s at 550 ˚C saw a larger decrease of average TMF lifetimes for SiMo 5.10 than that of SiMo 4.05 despite similar crack driving forces. An investigation of the stress relaxation during TMF of the two variants showed that the SiMo 4.05 performs better during long hold time due to better stress relaxation properties. The SiMoNi variant which is very brittle at low temperatures was found to fail by a fracture by overloading mechanism taking over quite early in the fatigue cycle; which is confirmed by examination of the fracture surfaces and numerical estimations. This also explained the low lifetimes and scatter in previously performed TMF tests under total constraint conditions. The TMF test series performed in the temperature range: 150 ˚C to 550 ˚C with a hold time of 30 s at 550 ˚C found that a heat-treatment seemed to reduce the TMF performance of the SiMo 5.10 variant. Metallographic investigations and hardness measurements of as-cast and heat-treated materials revealed that the distribution of the Mo-rich phase from the grain boundary regions into the matrix due to an annealing heat-treatment seemed to affect the TMF performance.

Research has shown the Circumferentially Notched Tensile (CNT) Specimen to potentially be a size-independent fracture toughness specimen. This research focusses on expanding the practical applicability of the CNT specimen as well as using it to perform fracture toughness tests on S690QT steel and simulated CGHAZ material in S690.
Based on literature research the research goals of the research were to investigate different pre-cracking methods with the focus of creating cracks with little or no eccentricity, improve the accuracy of CTOD measurements, determine the equivalency between CNT and conventional fracture toughness, determine the size of the plastic zone at the crack tip specimen and find alternative methods of microstructure simulation.
In order to improve pre-cracking rotational bending (RB) and standalone compression – compression (C-C) fatigue were used, both lead to low eccentricity cracks, where the length of the crack could be influenced by changing the process parameters or specimen geometry. Room temperature CTOD measurements were performed on both, where RB specimen showed an average CTOD of 0.12 mm and C-C specimen of 0.18 mm. This difference is believed to be caused by the unique stress free situation at the crack tip of the C-C specimen.
By using digital image correlation (DIC) and optical microscopy, the most accurate approximation of the CTOD was determined by comparing extensometer data to optical measurements. Influence of the eccentricity of the ligament was on the fracture toughness was found to increase for brittle material. Equivalency between CNT and conventional specimen was determined by comparing both RB and C-C specimen’s CTOD at low temperature (-100 °C). C-C specimen were found to show comparable fracture toughness, while RB showed an increased value.
The size of the plastic zone was measured using EBSD, measurements on annealed S690 and low carbon steel failed to show distinctive plasticity, possibly caused by insufficient plastic strain.
Inductive heating was tested as a replacement for Gleeble heat treatment with promising results, however the Gleeble was used for producing all S690 CGHAZ samples. C-C was found to be the only viable pre-fatiguing method, creating average crack length extensions of 0.49mm and an eccentricity of 0.18 mm. CNT results showed a slightly lower room temperature CTOD compared to conventional specimen (0.12 mm vs 0.08 mm), low temperature results showed comparable CTOD values.
More research is recommended in further development of the new standalone C-C pre-fatiguing method, improving the inductive heating and cooling setup, expanding the DIC test setup, determining the plastic zone size by performing more EBSD measurements or by performing nanoindentation measurements and using CNT specimen on very brittle materials.