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 research focused on the question whether it is possible to predict the density of tropical hardwoods with acceptable accuracy based on drill resistance measurements. To assess the feasibility, the influence of the characteristics of the drill resistance measurement tool; the Resistograph® and the wood species have been investigated. With respect to the measurement tool, drill sharpness, feed speed and rotation rate were examined. For the wood samples, the influence of extractives, moisture content, drill direction and density were investigated. It was found that the influence of drill sharpness cannot be ignored and a calibration procedure was developed. No significant difference in drill resistance was observed between drilling in radial and tangential directions; however, drilling in the longitudinal direction resulted in a significant higher drill resistance. Moisture content influences the drill resistance; however, the effect is small and not statistically significant, it can usually be ignored. The influence of extractives on the drill resistance most likely depends not only on the mass fraction of extractives but also on the sort and the composition of the cell wall. Meranti has a typical density within the range 350 to 860 kg/m3. With an appropriate calibration procedure, it is possible to predict the density of meranti with 95% confidence to an accuracy of ± 70 kg/m3

Fatigue cracks could occur in the material of offshore and marine structures that are cyclically loaded. These cracks occur due to the cyclic stresses induced by the waves causing premature failure. Monitoring these fatigue cracks using a wireless system gives the opportunity to guarantee the integrity of the structure without manual inspection, making this a safer way of inspection. In addition, inspection is very costly due to the man hours and because of the fact that the locations of fatigue cracks are mostly not easily accessible. Also, visual evaluation of the crack length can be difficult to assess, which could result in early retirement of structures or unnecessary and costly maintenance.

A wireless crack monitoring system based on the Self Magnetic Flux Leakage (SMFL) method could be a solution for this problem. This method suggests that the ferromagnetic material is passively magnetized by the Earth’s magnetic field and the magnetic signal changes when a crack appears in the material. The measured data is sent wirelessly to the control room of the marine structure. Here, the data can be assessed to determine the size of the fatigue crack. With this information, it can be decided what course of action should be taken to ensure the integrity of the marine structure as long as possible. This will result in a safer way of working and being economically more efficient.

For accurate crack sizing, the SMFL must be interpreted correctly. During cyclic loading the stresses in the material change, affecting the magnetic signal. This is called the stress-induced magnetization. The aim of this research is to investigate the effect of stress-induced magnetization on the SMFL in the stress concentration zone of a structural steel plate, and its implications for crack monitoring by the SMFL method.

The measured stress magnetization curves are obtained by means of an experiment. In this experiment, a steel plate with an elliptical hole is cyclically loaded up to the yield stress. The magnetic signal is measured in a grid around the hole. The results show a maximum variation of 25 µT. Depending on the application, the stress-induced magnetization may need to be considered for the interpretation of the measured signals for crack monitoring using the SMFL method.

In the past decades the understanding of fracture of ductile material has increased substantially. Research has shown that the onset of fracture highly depends on the full state of stress inside the material. Better understanding of fracture resulted in more advanced and complex fracture models being conceived, allowing researchers to predict fracture in ductile solids with improved accuracy.


Nonlinear finite element analysis is a powerful tool at the disposal of researchers to predict the response of ship and offshore structures. When it comes to simulating accidental loads, such as collisions, often basic criteria are applied to include fracture in a finite element model. However, accurate fracture prediction is of great importance to determine the ice resilience of vessels, or to obtain reliable estimates of the sustained damage due to maritime collision. This research focuses on the latter.


This thesis is concerned with bridging the gap between the recent developments in fracture prediction and the application of failure criteria in finite element analysis of ship collision. A selection of recently published fracture models has been made and experiments have been conducted on S235 structural steel for calibration and validation of these models. Four small scale experiments have been conducted. These experiments serve a dual purpose: first, to gather information on the material behaviour during deformation. Second, to obtain information on the effect of different stress conditions on fracture. An iterative method has been employed to accurately model the material behaviour. For the calibration of the fracture models a method has been conceived and applied that takes into account the full histories of stress and strain during the deformation process up to fracture.

Before application as failure criteria for finite elements, the calibrated fracture models require a correction based on the size of the elements: a modification to an already existing theoretical framework has been proposed and applied to obtain element-size dependent failure criteria.


A large scale drop tower experiment has been designed to simulate a so called raking damage scenario. This experiment has been conducted on the same material as the small scale experiments and serves as a validation for a finite element model that has been created using the information on the material behaviour obtained from the small scale experiments. The different failure criteria have been implemented into the commercial finite element package LS-DYNA and have been applied to the finite element model. The results have been compared to the results of the raking damage experiment.


It was concluded that the application of complex multi-parameter failure models in analysis of maritime collision does not necessarily provide an improvement over conventional fracture prediction methods. The inability of shell elements to accurately describe strain concentrations and the effect of the element size introduce uncertainties that overrule the benefits of stress-state dependent prediction of element failure.

Digital image correlation (DIC) is a surface measurement technique that can be applied whilst performing experimental tests. At Delft University of Technology, a fatigue test is to be conducted and DIC will be used to measure surface displacements around the developing crack. These surface displacements are directly related to the crack geometry parameters. These crack geometry parameters can be found numerically via a minimisation algorithm where the difference between the surface displacements measured via DIC and surface displacement found via finite element analysis is minimised. Aim of present study is to perform a preliminary analysis before the experiment is performed in order to assess what is to be expected. It is looked into whether the out-of-plane surface displacement information that has to be taken into account since this influences the DIC hardware to be used during the experiment. Secondly, it has been looked into the noise as consequence of using DIC, and the influence of the noise on the minimisation to be performed. Finally it has been looked at the influence of the DIC noise on the accuracy of the minimisation. Since present study describes a preliminary analysis, simulated DIC surface measurements have been used. A finite element (FE) model of the specimen to be tested has been used in order to perform the study for uniaxial tension only. An algorithm using the FE model to perform the minimisation has been build and tested for simple 2D cases and a simpler 3D first before performing the 3D analysis with geometry considered. The simulated DIC noise taken is 0%, 1%, 2%, 5% and 10% of the mean measured displacement field in order investigate the influence of the noise on the minimisation. Present study showed that neglecting the out-of-plane displacement information introduces an error of around 2x10^-2 millimetres for 1000 MPa uniaxial tension applied. It is assumed that the algorithm used finds the same crack geometry parameters as have been used to construct the simulated DIC surface displacement field in case no noise is applied. It is found that this is not the case in present study. Reason is the algorithm converging to local minima located very close to the global minimum. Present study showed the location of the global minimum becoming less defined when noise applied is increased, i.e. the area in which the global minimum is located becomes larger. Present study shows that the algorithm can convergence towards the target crack geometry parameters. Accuracy of the algorithm as function of the noise has been checked for the full displacement field and the displacement field where out-of-plane information is neglected. Confidence intervals increase for increasing DIC noise, as can be expected. Due to the local minima the algorithm might converge to, it is not possible to draw conclusions on whether the full displacement field should be taken into account in order to have the accuracy required. Therefore, it is strongly recommended to change the algorithm to take extra information into account to make the algorithm converge to the global minimum. Furthermore, it is recommended to improve the numerical model, in order to improve the speed of the algorithm.

In order to save weight and reduce the impact on the environment, the aerospace industry keeps searching for strong and lightweight materials to reduce fuel consumption. A relatively new manufacturing technique made it possible to print nylon, reinforced with continuous unidirectional carbon fibres on a layer-by-layer basis and praised itself for making materials as strong as Aluminium 6061-T6. In this study, several coupons were 3D printed and tested in tensile, compression and three point bending mode to determine the material properties. When loaded in the direction of the fibres, the material did not live up to the upper limit of the rule of mixtures for fibre reinforced composites, which is mainly due to the amount of voids in the material. These voids were discovered when studying the micro-structure of the material and were both in between the layers, as well as in the layers themselves.
The mechanical tests were performed to validate constructed finite element models, simulated in Abaqus/CAE. The simulation results showed some mismatches with the mechanical test results, which were caused by the assumptions made to simplify the models. Finally, an air plane chair bracket, which has as task to transfer loads from the back seat panel to the chair, was modelled, simulated and designed. Major issues in the designing process were the limitations of the printer, such as not being able to choose which exact areas to reinforce in a specific layer. The final design of the chair bracket was manufactured and was tested to see if it could withstand a load of 375 N. The part did not fracture under the load, but a high displacement of the back seat was observed, as well as damage occurring at other locations than the simulations indicated.

The study showed that this 3D printing technique is able to print structural aerospace parts, given that the part is not too complex and that it is loaded in a static nature. More freedom in the design software of the 3D printer could allow the user to successfully design more complex structural aerospace parts. Since the constructed parts were only tested under static loading, further research has to be conduction into dynamic loading of these parts before it is possible to actually implement these in air planes.

Research into multi-axial fatigue requires knowledge on the through time and thickness development of the propagating crack front. In order to provide this knowledge methods must be developed to monitor crack initiation and growth during a fatigue test. The acoustic emission method has been recognised as a possible means to this end.
Fatigue is a failure mode that is characterised by the accumulation of damage during cyclic loading. Fatigue initiates on the microscale as dislocation movement, and subsequently develops into crack growth and eventual failure. In the assessment of fatigue damage it is critical to estimate the remaining lifetime. Prediction models have been developed for uni-axial loaded joints. However in the field of multi-axial fatigue these models are inadequate. To improve upon these models knowledge is required on the development of the fracture during cyclic multi-axial loading.
The acoustic emission method is a procedure for non-destructive testing that is based on detecting elastic stress waves originating from changes in the microstructure of an object. In the industry two common applications of acoustic emission exist. One of these aims to extract insights into the severity of damage from waveform features, the other aims to localise and correlate acoustic events. Novel applications focus mainly on the improvement of these two fields. A state of the art method is proposed which aims to localise acoustic sources in small scale volumes as a means of imaging the crack front.
Localisation of the sources is performed using a time difference of arrive scheme, which is also known as multilateration. The accuracy of this scheme is highly dependent on the accuracy of the input parameters. Therefore the times of arrival, receiver positions and speed of sound must be established meticulously.
Extensive processing is required to transform the recorded acoustic emission data into a representation of the crack propagation. For this purpose a post-processing system has been developed which automates time of arrival picking, event building and event localisation. The developed system is an extension to the AMSY-6 acoustic emission measurement system developed by Vallen Systeme.
In order to validate the accuracy of the extended measurement system an experiment has been designed. In a volumetric geometry sources are simulated in order to assess the bias and the variance, additionally noise is added to the simulation to evaluate the sensitivity of the procedures.
The study has shown that, under ideal noise conditions, the precision can amount to a cluster of about a millimetre in size. Additionally the bias of this cluster can be reduced to a minimum by means of careful calibration of the receiver positions and the speed of sound. Both of these observations indicate that an accurate representation of the propagating crack front may be obtained through a regression model, if the conditions are known and within acceptable limits. Regarding non-ideal conditions the analysis of the noise sensitivity has shown that the accuracy is relatively stable as long as the signal to noise ratio is kept above SNR ≥ 30 dB.
In conclusion this has shown that monitoring the propagation of the crack front using volumetric acoustic emission source localisation may be possible if the conditions are right. This means that the signal to noise ratio must be kept above SNR ≥ 30 dB. If this is the case, and if enough events are recorded to perform regression both the bias and the variance should pose a problem to monitoring crack growth.