Erratum to: 36th International Symposium on Intensive Care and Emergency Medicine

[This corrects the article DOI: 10.1186/s13054-016-1208-6.]

Multi-Terminal DC Networks: System Integration, Dynamics and Control

When large amounts of electricity need to be transported for long distances, or when underground or submarine cables are involved, using direct current high-voltage transmission systems is more efficient and cost effective than using traditional high-voltage alternating current transmission. Therefore, the main thesis objective is to study to what extent can multi-terminal dc networks provide an optimal platform to foster the integration of remotely located renewable resources, with particular focus on the integration of offshore wind farms in the North Sea. In this thesis, five main challenges were identified before high-voltage multi-terminal dc networks – which can promote the inclusion of remotely located renewable sources while strengthening the existing ac power system networks – can finally become widespread: system integration, power flow control, dynamic behaviour, stability and fault behaviour. These challenges are investigated through a comprehensive literature review, a series of detailed simulation models, and an experimental laboratory setup of a three-node multi-terminal dc network. A novel strategy to control the power flow in multi-terminal dc networks – called Distributed Voltage Control – was developed in this thesis. In the experimental setup, three voltage-source converters were successfully operated in a parallel-radial multi-terminal dc network with a symmetric monopolar configuration. Lastly, a real-time digital simulator was used to emulate the behaviour of an offshore wind farm. Real measurements from the Dutch offshore wind farm Egmond aan Zee were used to validate the Distributed Voltage Control strategy, which successfully controlled the power flow inside the three-terminal low-voltage dc network with high overall precision, while providing the complete system with a fast dynamic response.Electrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc

BGP Route Propagation between Neighboring Domains

Electrical Engineering, Mathematics and Computer Scienc

Application of value of information theory in adaptive metamodeling for reliability assessment

The present paper discusses the application principles of value of information theory in adaptive metamodeling for reliability analysis. Metamodeling for reliability purposes has become particularly relevant in recent years. The usage of metamodels allows surrogating the, costly to evaluate, performance functions of engineering structures. Adaptive Kriging procedures are examples of the successful application of metamodel- ing in reliability analysis. Efficient adaptive Kriging involves the usage of some notion of improvement in what ultimately is an unsupervised decision making scheme that selects points to enrich the model. Therefore, the decision to select a point to enrich the experimental design should consider the utility of each candidate in the expectation of improvement of the metamodeling accuracy. Within this context, a comprehensive discussion on the application of value of information for reliability metamodeling is presented. Since the candidate points and surrogate are jointly built in a virtually costless model, it is possible to know the virtual outcome of the enrich- ment decisions. In many circumstances, points in the experimental design may provide redundant information. Furthermore, a priori knowledge on the performance function may be applied to weight the expected outcome of exploration and exploitation. Value of information considerations adds value to reliability metamodeling that uses adaptive methods, and is of interest for efficient design and optimization of complex structures, such as bridge structures.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Integral Design and Managemen

Promoting extrinsic bridging of adhesively-bonded CFRP joints through the adhesive layer architecture

Carbon fiber-reinforced polymers (CFRPs) have widely attracted the aerospace and automotive industries due to high stiffness and lightweight. Secondary adhesive bonding of CFRPs is a promising research field to fully explore their potential. However, multiple challenges have limited the further application of adhesively-bonded composite joints since it is difficult to inspect the premature debonding, which leads to catastrophic failure once initiated. Thus, it is crucial to introduce crack arrest features, to slow down (or even stop) the crack growth and achieve progressive failure. Various methods have been reported to introduce crack arrest features, including z-pins and corrugated substrates. Our previous work directly utilized the adhesive layer to bridge the separating CFRP parts, through the extrinsic bridging of adhesive ligaments. The bridging adhesive ligaments are triggered by the patterning of distinct surface treatments. These extrinsic bridging ligaments largely enhance the energy release rate (ERR) and successfully arrest the crack propagation. However, a large portion of the required energy for the further crack propagation is stored elastically in the stretching ligaments, which would cause catastrophic fast joint debonding after the failure of ligaments. In this work, the adhesive layer was architected in order to improve its plasticity. By promoting the plastic energy dissipation, the bridging, stretching, and failure of generated adhesive ligaments could result in tougher and safer joints. CFRP substrates were alternatively patterned by two distinct surface treatments to achieve different interfacial strength and toughness values. Then, double-cantilever beams (DCB) were manufactured by bonding treated substrates with the architected adhesive material, such as integrating 3D-printed nylon wires or newly synthesized adhesive material. Results showed that the proposed joint toughening strategy could improve ERR compared to conventional uniform treatments and increasd adhesive plasticity could also stabilize the crack propagation, leading to a safer joint.Structural Integrity & Composite

Multi-material adhesive joints with thick bond-lines: Crack onset and crack deflection

This study investigates the fracture onset and crack deflection in multi-material adhesive joints with thick bond-lines (≈10 mm) under global mode I loading. The role of adherend-adhesive modulus-mismatch and pre-crack length are scrutinized. The parameters controlling the crack path directional stability are also discussed. Single-material (i.e. steel-steel and GFRP-GFRP) and bi-material (i.e. steel-GFRP) double-cantilever beam joints bonded with a structural epoxy adhesive are tested. The joints are modelled analytically, considering a beam on elastic-plastic foundation, to include characteristic length scales of the problem (e.g. adhesive thickness, plastic zone) and numerically using Finite Element Model. An empirical relation, in terms of geometrical and material properties of the joints, that defines the transition between non-cohesive and cohesive fracture onset is found. Above a specific pre-crack length the stress singularity at pre-crack tip rules over the stress singularity near bi-material corners, resulting in mid-adhesive thickness cohesive fracture onset. However, the cracking direction rapidly deflects out from the adhesive layer centre-line. Positive T-stress along the crack tip is found to be one of the factors for the unstable crack path.Structural Integrity & Composite

Ultrasonic welding of epoxy- to polyetheretherketone- based composites: Investigation on the material of the energy director and the thickness of the coupling layer

Ultrasonic welding is a highly promising technique for joining thermoplastic to thermoset composites. A neat thermoplastic coupling layer is co-cured on the surface to be welded to make the thermoset composite ‘weldable’. A reliable bond is attained when miscible thermoplastic and thermoset materials are chosen. For welding carbon fibre/polyetheretherketone (PEEK) to thermoset composite samples, a PEEK film is not preferable due to its immiscibility with epoxy resins. On the other hand, polyetherimide is an excellent candidate, since it is known to be miscible to most epoxy systems at high temperatures and PEEK polymers. This study focusses on two main subjects; firstly, the nature of the material of the energy director, i.e. a flat thermoplastic film used to promote heat generation at the interface. In this case, the energy director can be either polyetherimide, as in the coupling layer or PEEK material, as in the matrix of the thermoplastic composite adherend. It was found that both materials can produce welds with similar mechanical performance. This study focusses secondly on the thickness of the coupling layer. Due to the high melting temperature of the PEEK matrix, a 60-µm-thick coupling layer was seemingly too thin to act as a thermal barrier for the epoxy resin for heating times long enough to produce fully welded joints. Such an issue was found to be overcome by increasing the thickness of the coupling layer to 250 µm, which resulted in high-strength welds.Aerospace Manufacturing TechnologiesStructural Integrity & CompositesAerospace Structures & Computational Mechanic

Acoustic emission approach for identifying fracture mechanisms in composite bonded Joints: A study on varying Substrate's stacking sequence

This study uses the acoustic emission structural health monitoring method to identify fracture mechanisms in composite bonded joints when varying the substrate stacking sequence. Quasi-static mode I loading tests were performed on secondary adhesively bonded multidirectional composite substrates (0, 90, 45, −45, 60 and −60° fibre orientations). An unsupervised artificial neural network combined with the visual fracture evaluation of the specimens and the Morlet continuous wavelet transform was used to cluster and give the acoustic emission signals a physical meaning. Different fracture mechanisms could be identified within the adhesive layer (i.e., cohesive failure) and in the composite substrates, including non-visible damage mechanisms (matrix micro-cracking, fibre/matrix debonding, fibre pull-out and fibre breakage). Using the Morlet continuous wavelet transform, it was possible to recognise that the highest peak frequency does not always represent the most relevant signature of the fracture mechanism. Moreover, multiple peak frequencies can be associated with multiple fracture mechanisms, such as the fibre pull-out that occurs in the combination of matrix cracking and fibre breakage. Furthermore, no differences were observed in mode I loading conditions between the acoustic emission signatures from the cohesive failure in the adhesive layer and the matrix cracking within the composite substrate. The findings of this study present a great opportunity to gain more insight into the fracture behaviour of polymer materials and fibre-reinforced polymer materials and to improve the quality of adhesively bonded joints.Group DransfeldQN/AfdelingsbureauGroup Teixeira De Freita

Ultrasonic welding of CF/epoxy to CF/PEEK composites: Effect of the energy director material on the welding process

With its short heating times, ultrasonic welding is a highly promising technique for joining thermoplastic (TPC) to thermoset (TSC) composites, to prevent thermal degradation of the thermoset adherend. A neat thermoplastic coupling layer is co-cured on the surface to be welded to make the TSC “weldable”. For welding CF/PEEK to a TSC adherend, it would be logical to use PEEK as the coupling layer. However PEEK and epoxy are not miscible with each other, therefore a bond created after co-curing of these two materials is not reliable. PEI on the other hand is known to be miscible to most epoxy systems at high temperatures and PEEK polymers, hence it is an excellent candidate for the coupling layer material. The other necessary element for ultrasonic welding is the energy director (ED), a neat TP film placed at the interface to help promote heat generation through preferential frictional and viscoelastic heating. Usually EDs are made from the same material as the TP matrix, but in this case ED can be either PEI or PEEK. Mechanical testing and fractographic analysis showed that the usage of a PEEK ED is the most successful approach. This research is part of the European project EFFICOMP.Aerospace Manufacturing TechnologiesStructural Integrity & Composite

On the influence of overlap topology on tensile strength of composite bonded joints: A multistacking design

The goal of this study is to investigate new designs of composite bonded joints in order to improve their strength under tensile loading. Multiple stacked overlaps are compared with single overlap designs. The concept of multiple stacking is well known as ply-interleaving technique for co-curing dissimilar materials. For a secondary bonding process, a similar concept is used in tongue-and-groove joints. However, it is so far limited to one stacking level due to the complexity of the design. By means of thin unidirectional layers, the tongue-and-groove design is expanded further to two stacking sequences and applied to secondary bonding of CFRP adherends. Single lap joints of 12.7 and 25.4 mm overlap length were compared to finger joints with 1 and 2 overlaps of 12.7 mm overlap length, stacked through the thickness. Specimens were tested according to ASTM D-5868-01. The initial and final failure load were recorded. The study shows that for the same overlap length in a multi-stacked configuration, there is a potential for higher average lap strength, in comparison with an increase in overlap length of a single overlap. This effect might be mainly due to the reduction of secondary bending moment and by load distribution over multiple interfaces.Structural Integrity & Composite