Wire arc additive manufacturing (WAAM) can create large parts, due to its high deposition rate. WAAM can be used to create functionally graded materials (FGMs) such as a strong core of high strength low alloy (HSLA) steel and corrosion-resistant cladding of austenitic stainless (AUS) steel. However, HSLA and AUS steel are dissimilar metals with different microstructure and mechanical properties. Joining dissimilar metals with WAAM creates a complex interface with a different set of properties compared to parent materials. The current state of the art covers the microstructure, tensile properties, and hardness of such FGM. However, there is a lack of understanding about the fatigue properties at the interface. This study investigates the fatigue crack growth behaviour at the interface of ER70S-6 (HSLA steel) and ER316L (AUS steel). Initially, a benchmark study of the HSLA and AUS steel was done to obtain Paris parameters, threshold stress intensity range (∆Kth), and hardness. HSLA steel showed 43 to 77% lower Paris slope parameters than AUS steel, which suggests a better fatigue resistance for HSLA steel. However, no significant difference in ∆Kth values was observed. The hardness of HSLA and AUS steel were 247.0±6.1 HV and 227.7±5.5 HV respectively, which suggests a correlation between hardness and fatigue resistance. Further analysis showed a power law relation between the Paris slope parameter and hardness for HSLA steel. This was attributed to the grain size. Smaller grain sizes improve fatigue resistance and increase hardness. Hence, hardness can be an indirect measure of fatigue resistance. Under constant ∆K experiments, AUS steel showed up to a 61.7% difference in fatigue crack growth rate (FCGR). The anisotropy in AUS steels is due to the large grain size in the range of 100 μm and the structure of the weld beads, resulting in heterogeneity. After the benchmark study, the graded materials were tested under similar conditions. The crack in the graded material grows from AUS steel to a 2 mm wide interface and then to HSLA steel. A variation of FCGR at the interface region was measured, notably 1) a decrease of 5.1E-05 to 6.1E-05 mm/cycle before the interface region, 2) a gradual increase of 5.5E-05 to 1.32E-04 mm/cycle within the interface region, and 3) a sudden increase of up to 3E-04 mm/cycle. To understand these behaviour microscopy, fractography, and electron backscatter diffraction (EBSD) analyses were utilised. The sudden increase of FCGR in HSLA steel was attributed to process-induced porous defects. The decrease of FCGR before the interface region was due to the increase in grain boundaries, secondary cracks, an increased crack closure effect, and martensite formation through transformation-induced plasticity (TRIP) in the interface region. The gradual acceleration of FCGR within the interface region was due to the martensite phases formed during solidification. The martensite formed during solidification had twice the hardness in comparison to HSLA steel. The martensite formed by the TRIP effect also resulted in 25-50% higher hardness near the crack flanks compared to the regions away from it. For the application of this FGM, the interface can act as a protective layer by reducing the FCGR. However, the martensite formed during solidification in the interface should be mitigated.

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.

This thesis addresses infection risks associated with orthopedic implants and focuses on challenges associated with tackling bacterial biofilms comprising antibiotic resistant organisms. Here, a proof-of-concept drug delivery system for prevention of implant-associated infections is presented. The aim of this study was to characterize two materials, PluronicF108andF127, by evaluating their physical and drug releasing properties to develop a controllable, thermosensitive on-demand drug delivery. The two hydrogels were characterized in terms of their rheological and micelle forming properties at various concentrations, using the inverted tube test and dynamic light scattering, respectively. Their stability was assessed by recording the weight loss ratio of the hydrogel and the drug release characteristics were assessed by monitoring the release of a hydrophobic dye. The cytotoxicity of the system was also tested in vitro.

The rheological assessment showed that the lower sol-gel temperature was in the range of 20-35◦ C and the upper gel-sol transition was in the range of 45-65◦ C, depending on the mass fraction concentration, hydrophilicity and the type of solvent. Furthermore, these characteristics also influence the critical micellar concentration, stability and therefore their release. The most stable hydrogel com position was 20 wt% Pluronic F127 in 1x phosphate-buffered saline (PBS). This study demonstrated a proof-of-concept of an on-demand drug releasing system com prising Pluronic F127 and polycaprolactone (PCL) as thermoresponsive components. The system showed that it can be used for single as well as intermittent release. Increasing the stability of the Pluronic, however, is further desirable. The system offers a non-invasive approach for an on-demand drug delivery. It requires, however, further work to enhance stability of the hydrogel to minimize passive release.

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

This master's thesis provides knowledge on non-destructive testing of cement and supplementary cementitious materials composition in concrete with a handheld X-ray Fluorescence analyser.

Today, concrete production is responsible for 8% of the world’s CO2 emissions. Recycling concrete material can directly contribute to the reduction of CO2 emissions. This process is costly and time-consuming. A possible solution would be to use a handheld X-Ray fluorescence spectrometer on-site to determine the concrete chemical composition.

No existing research indicates if concrete identification with a handheld X-Ray Fluorescence analyser is possible. This thesis intends to prove that this technique can differentiate concrete with various chemical composition.

Fourteen concrete cubes of fourteen different chemical compositions were analysed to fulfil this objective. The fourteen compositions reflect the concrete design used in the Netherlands. Experimental programs conducted on the concrete revealed the impact of different factors on the results obtained from the handheld-XRF. These factors include measurement time, moisture, surface carbonation, and matrix effect. Each factor impacts various oxides in different proportions, leading to distinct patterns. After investigating their impacts, a protocol was written to test all the mixes. Finally, the reproducibility of the protocol was assessed, and the mixes were tested using the protocol.

The primary outcome of this thesis is proof that twelve of the fourteen mixes were differentiated based on their alumina content. This oxide proved to be less impacted by moisture and surface carbonation than the other oxides. The influence of the different factors on measurements was identified and quantified. These studies also revealed that twenty measurements were sufficient to identify the mixes. The protocol improved the control of the factors but also appeared limited by the concrete matrix.

A possible approach to circumvent this problem would be considering oxides content as thresholds rather than numbers. Determining these thresholds requires testing many samples. Another further study is the possibility of reducing the impact of moisture and surface carbonation on-site.

Hall spars, a leading innovator in the composite mast-building industry with a long history of successful projects, provided a challenge which is the inspiration of this thesis. This thesis aims to contribute to the challenge: Joining techniques for the internal bonding of carbon fibre-reinforced polymer components into carbon fibre-reinforced polymer structures. ”Internal bonding” can also be called ”bonding.

Currently, the epoxy adhesive film is used for these types of joints. Adhesive co-bonding raises challenges due to the limited access to the mast. Challenges concerning surface preparation, low mechanical strength, reliability and labour intensity. Other techniques will be reviewed for the internally bonding limited access hollow tubes. The aim is that no new tooling is required, and the currently used materials and process parameters can be used. The thermoset resin used is an epoxy carbon fibre prepreg.

According to the literature study, potential joining techniques for this application are joining with partially cured thermosets and joining with fusion welding of thermoset composites. Differential scanning calorimetry measurements are performed to analyse the thermoset resin. Material characterisation is required to apply the joining methods with the currently used material and process parameters of Hall Spars. Process parameters for the curing model, and glass transition temperature model are derived. Using this material model allows creating the insight that partial cured joining with the thermoset resin of Hall Spars is not feasible. The structural integrity of the joined parts can not be guaranteed in the curing cycle.

Polyetherimide is used as thermoplastic material to apply the fusion welding technique on the thermoset. This thermoplastic film is co-cured with the thermoset and used as a coupling layer to join the two thermoset parts. Performing an interphase analysis with the scanning electron microscope indicates interphase formation at the PEI/epoxy interface. The higher the isothermal curing temperature, the thicker this interphase. Gelation causes the interphase formation to slow down. The interphase formation at the interface is not been observed as expected based on the literature review. This required a test if the interphase formation would have occurred at a smaller scale than the scanning electron microscopy could observe. This can be achieved by performing single-lap shear experiments. These joints are processed according to Hall Spars process parameters. Joints created with the fusion coupling technique with the PEI film show promising results. However, due to high void content, a lower ultimate lap shear strength is observed. Therefore it is advised to investigate a curing cycle with a second dwell phase. This changes the viscosity profile of the resin. Therefore allows the trapped air at the PEI/epoxy interface to leave the joint. This could potentially lead to more reliable joints.

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.

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.

Nitinol shape memory alloys (SMAs) have a unique combination of shape memory capability, making it an attractive material for various engineering and biomedical applications. Additive manufacturing (AM) by laser powder bed fusion (L-PBF) allows to produce Nitinol net shape parts, which broadens its applications. Due to the high heating and cooling rate during L-PBF process, there always exists supersaturated solute elements and metastable structures in L-PBF Nitinol parts. Microstructure and precipitate characteristics have detrimental effects on phase transformation and shape memory behavior of Nitinol alloys. Proper heat treatment is an important method to mitigate these detrimental effects and improve the properties of Nitinol. In this project, the effect of heat treatment on shape memory behavior of equiatomic Nitinol fabricated by L-PBF is studied. The heat treatment process is optimized, which is annealing at 950 °C for 5.5h and subsequent aging at 350 °C for 18 hours. By applying the optimized heat treatment process, the cyclic stability of the Ti50Ni50 SMA are improved by 50% for recoverable strain and 70 MPa for applied stress. Additionally, the relationship among the microstructure and precipitates, functional and mechanical properties, and heat treatment parameters are investigated.

Many manufacturing industries have been impacted by the innovation of additive manufacturing (AM), and biomaterials manufacturing is no exception. One group of biomaterials impacted by the innovation of 3D printing are degradable biomaterials. Degradable biomaterials could eliminate the need for surgery to remove the implant. 3D printed porous degradable biomaterials provide both mechanical support and space for bone ingrowth. As of today, there is an absence of materials for this application that are biodegradable, biocompatible and can be 3D printed. Magnesium can be used as a degradable biomaterial but its corrosion resistance is not yet adequate for application in the human body. Applying zinc as an alloying element to magnesium increases its corrosion resistance compared to pure magnesium. As the addition of alloying elements changes the microstructure, which in turn changes its corrosion behaviour.

This thesis analysed the effect of microstructure on the corrosion behaviour of extrusion-based 3D printed porous Mg-4Zn (wt.%) scaffolds, using localised electrochemical techniques i.e. scanning Kelvin probe force microscopy (SKPFM) and scanning electrochemical microscopy (SECM). The microstructure of the Mg-4Zn scaffolds includes grains with secondary phases precipitated along the grain boundaries with the presence of micropores. The secondary phase particles showed increased Volta-potential compared to the magnesium-based matrix. Therefore, the addition of zinc caused micro-galvanic coupling between secondary phase particles and the matrix, but their contribution to corrosion is minimal due to postponed contact with the electrolyte and the protection by the corrosion products. Micropores in the Mg-4Zn scaffold increased the surface exposed to fluids and were pitting corrosion initiation sites. During corrosion however, the surface was covered with a more stable corrosion product compared to pure magnesium. As a result of this, the corrosion resistance of Mg-4Zn scaffolds is better than pure magnesium scaffolds.

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.

Bainitic steels are the type of steels which are widely in use, especially in automotive industries, for its good mechanical behavior, i.e., with coexistence of strength, ductility and fatigue resistance. A better understanding on the mechanism and transformation kinetics of bainite are necessary in order to improve the performance of the bainitic steel further. This research mainly studies the kinetics of bainitic transformation during isothermal treatments. The chemical composition, in particular silicon (Si), affects the transformation kinetics of bainite, and therefore the fractions of retained austenite and bainitic ferrite. The current research focuses on the bainite transformation kinetics during isothermal heat treatment. Steel specimens are austempered at 250°C, 300°C and 350°C and held for 30, 60 and120 minutes, respectively. The steel in the current research has the alloy composition of Fe-0.61C-1.62Si-0.85Mn-0.32Cr (wt.%). Microstructures observed using optical micrographs consist of bainite and retained austenite after the austempering process. Quantitative measurements of the retained austenite (RA) fraction are performed by magnetization technique. The results show that, at austempering temperatures of 250°C and 300°C, the fraction of retained austenite decreases gradually with increasing holding times and increases with increasing austempering temperatures. However, a different affect is observed in the steel austempered at 350°C for 30minutes. The fraction of retained austenite increases from 30 to 60 minutes and subsequently decreases from 60 to 120 minutes. In order to study the effect of retained austenite on hardness resulting in decrease in overall hardness with increasing austempering temperatures and increases with increasing hold duration. The JMAK model has been fitted to the observed fraction of austenite as function of time and temperature, which results in the parameters such as the rate constant and Avrami exponent. The fitted results suggest a one-dimensional grain growth of the bainite in this steel. The predictions from a thermodynamic analysis using the para-equilibrium model and the T0-temperature are compared to the experimentally observed fractions of RA, which results in obtaining higher fractions than obtained experimentally. This thermodynamic analysis predicts an increase in the fraction of remaining austenite with increasing austempering temperature as observed experimentally in most cases. The thermodynamic study on the effect of Si concentration (1.62 wt.%)results shows that there is insufficient Si present in the steel to hinder the formation of cementite.

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.

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 most commonly used material in the automotive industry is steel. Steel used in making Body-In-White (BIW) for cars are broadly classified as Advanced High Strength Steels (AHSS). These AHSS are produced by a well-defined alloying and specific annealing procedure. However, external oxidation of the constituent alloying elements from the steel is observed the annealing step . This selective oxidation of alloying elements on the steel surface affects the adhesion of the Zinc to the steel which is essential for corrosion protection. The main goals of this research work were to define the optimal parametric windows for the Electrolytic Plasma Cleaning technique to form a plasma capable of cleaning a steel surface by removing external oxides formed during the annealing process. Electrolytic Plasma Cleaning uses the traditional electrochemical cell that consists of an anode, a cathode, an external potential and an electrolyte but at higher input potentials. An experimental setup was developed with the aim of finding the ideal working parameters, establishing the groundwork for future large-scale experimentation.

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.

Although many studies have been performed on porous absorbable iron scaffolds created by the space holder method, no studies have considered the influence of variations in space holders on absorbable scaffolds. The present study aimed to clarify the effects of space holder particle size and volume fraction, on the pore structure, resultant mechanical properties and absorbability of iron scaffolds.
Spherical iron particles were used as the matrix powder and mixed with coarse and fine rectangular urea space holding particles in volume fractions of 20, 40 and 60%, to create six different scaffolds with the space holder method. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to observe the scaffolds surface. Thereafter x-ray microtomography and Archimedes’ tests were used to characterize the scaffolds topology. The absorbability of scaffolds was characterized by potentiodynamic polarization measurements and weight loss measurements after 3 and 11 of days immersion in modified simulated body fluid (m-SBF). Mechanical properties were determined by performing diametral compression of the scaffolds, both before and after immersion in m-SBF. SEM images showed that morphology of the final pores was similar to that of space holder particles. Pores at lower porosities are isolated and become more connective or fully connected at higher porosities. Archimedes’ data showed that space holder fraction was directly related to porosity. Pore connectivity, measured by 휇CT, is higher for fine space holding particles. Until at 60% space holder volume fraction, the high amount of space holder particles agglomerated. Furthermore, it was observed that space holding particles had a large preference to move in a plane perpendicular to the applied compaction pressure. Diametral compression showed that increasing porosity resulted in lower yield strength and elastic modulus of scaffolds. The effects of space holder particle size and immersion time on mechanical properties were less pronounced. PDP measurements showed that the corrosion mechanism of scaffolds did not change due to change in space holder particle size or volume fraction. Weight loss measurements after 3 and 11 day immersion in m-SBF showed that increasing porosity resulted in higher weight loss of scaffolds. Weight loss was marginally affected by space holder particle size. Space holder particle size affected the created pore size and surface area yet also changed the pore connectivity. Similarly, the space holder volume fraction affected the created porosity and pore connectivity yet also changed the pore size and shape. This means that the pore structure not independently changes due to space holder particle size or fraction, but rather as a combination of both. Furthermore, in this study it was mainly the created porosity that influenced the mechanical properties and absorption rate of the scaffold. Space holder particle size did not show the same differentiating results, although this might be caused by agglomerating space holder particles and its dependency on volume fraction. Differentiating the created pore structure more and using a longer immersion time should be the next objectives in the determination of the effect of space holder particles on the performance of absorbable iron scaffolds for orthopedic applications.

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.

Orthotropic steel decks are widely used in bridge construction nowadays, especially in case of long span bridges. One of the main problems of bridges that utilize this technology is the susceptibility to fatigue failure. Cyclic loading due to wheel loads, in combination with the complex network of welded details, make such types of decks prone to fatigue cracking, which poses a threat to the structure's integrity.
One of the factors that contribute to crack initiation and propagation under cyclic loading are welding-induced residual stresses. The localized heat input, due to welding, and the subsequent cooling, force the adjacent area to undergo expansion and shrinkage, which are, however, restricted by the surrounding material. As a result, residual stresses are formed, which can reach the yield strength of the material in the region near the weld. When these are tensile, they can accelerate the crack propagation rate, thus severely reducing the fatigue life and endangering the structure's safety. Knowing the residual stress distribution in welded joints of orthotropic steel decks can prove to be useful in the evaluation of resistance against fatigue.
In this research project, a finite element model which is able to predict residual stresses in normal strength steel weldments, has been developed. In the first stage of the research, a small-scale welding experiment has been conducted, during which temperature and distortion measurements were derived. The welding procedure was then modelled in the finite element software ABAQUS. Experimental data have been used to validate the accuracy of the adopted heat source model and parametric studies have been carried out, in order to investigate the influence of the heat source parameters and the heat losses on the temperature distribution.
The validated methodology was then applied on a trough-to-deck plate welded connection of an orthotropic steel deck. Longitudinal and transverse residual stresses have been calculated at various positions of the deck and stiffeners. The influence of the welding sequence on the residual stresses has been investigated, by considering two welding scenarios; parallel and sequential welding. The effect of the adopted stress-strain relationship under elevated temperatures on the residual stress field has been examined, by considering an ideal plastic material model and one that includes strain hardening. In addition to these, the impact of the boundary conditions has also been studied. Finally, the effect of welding on the appearance of imperfections is discussed, by considering the deflection of the deck plate, after the process. As a result, valuable information is obtained about the shape of residual stresses, the associated deformations, and the factors that affect them, in a welded connection of an orthotropic steel deck.

Research to analyse and/or develop novel materials and techniques is necessary to address competition and safeguard craftmanship in shipbuilding. In this master thesis, two new approaches to reduce the production and maintenance costs are studied: {metal foam core, metal face sheet} sandwich materials and corner adstir fillet stationary shoulder friction stir welding (FSW). The current project is a preparatory analysis for the research in which the two components will be combined. It is anticipated that their combination may generate more added value due to mutual reinforcement.
The study of metal foam based sandwich materials showed that looking at the material level, the {aluminium foam core, steel face sheet} sandwich material is theoretically promising with respect to the conventional steel solid plate in specific situations and applications. When the sandwich material is implemented as part of the stiffened panel, the expected benefit could not be realised. This confirms the search for a more selective use, such in naval vessels when one needs a high resistance to impacts and blasting, and when one wants to conserve the limited internal space.
Compared with the currently existing arc welding technique of double sided T-joints, the corner adstir fillet stationary shoulder FSW scores better on hardness and fatigue resistance of the weld. Since the properties of the weld material correspond to those of the parent material, the weld can no longer be seen as the weakest link of the structure. The fatigue experiment occasionally revealed a new failure type at the undercut, being a sharp corner created by the modified FSW shoulder. This second failure type did not impact on the overall expected failure resistance; however, the limited occurrence does not allow a final statistical interpretation.
Based upon our analyses, there is not yet a place for widespread use of {aluminium foam core, steel face sheet} sandwich materials, but there is an added value for the corner adstir fillet stationary shoulder FSW.