Welding is a joining method for thermoplastic composites (TPCs) that offers multiple advantages over the more traditional methods of mechanical fastening and adhesive bonding. A particularly promising welding technique is ultrasonic welding, which features very short process times as a result of the high heating rates that can be achieved. This spot welding technique is hypothesized to have potential for improved damage tolerance compared to more commonly used continuous welding techniques: in a multi-spot welded joint, evolving damage will need to re-initiate in subsequent spots. The fact that damage initiation will need to occur multiple times might delay overall damage evolution through the joint compared to a continuous welded joint, where damage initiation needs to occur only once. This work is a first exploratory step into the domain of fatigue of multi-spot welded joints in TPCs. Existing research on the fatigue behavior of four-spot welded steel joints in various layouts served as the main reference throughout this research: its methodology was transferred to four-spot welded joints in TPCs. By comparing fatigue behavior across both materials, it was evaluated to what extent existing knowledge and design rules for steel could potentially be transferred to TPCs. Differences were observed in the results obtained for TPC and steel joints. Most notably, in steel joints the dominant failure mode was seen to change from spot fracture to sheet fracture at higher fatigue lives. In TPCs, joints consistently showed spot fracture across all load levels. A different interrelation between layout performances was seen in the steel and thermoplastic composite joints, assumed to be a result of localized material strengthening in the steel joints from interference of adjacent heat-affected zones. These results indicate that existing knowledge on multi-spot welded joints in steel cannot be readily transferred to TPCs, as failure modes and material mechanisms may differ. It was discovered that, when one spot failed prematurely as a result of existing damage in the joint, the remaining layout no longer seemed to have an effect on fatigue life performance. This was attributed to asymmetry in the remaining joint layout, meaning one spot would always become a preferred location for damage initiation and subsequent evolution. Therefore, subsequent damage evolution would only be restricted by a single spot up to the point where the shear strength of the joint was exceeded.

Emerging in the domain of composite manufacturing, thermoplastic polymers can enable the reduction of process times, costs, and waste. In this study, lifecycle assessment (LCA) is used to evaluate a design for a carbon fibre-reinforced thermoplastic (CFRTP) wing rib, made from carbon fibre and polyetherketoneketone (CF/PEKK). The CF/PEKK rib is compared to several hypothetical alternatives, considering autoclave and resin transfer moulding of CF/epoxy and milled aluminium alloy. The comparison uses novel and state-of-the-art techniques. Using scenario analysis, several perspectives are considered: recyclability, mass-induced energy demand, and alternative energy carriers. The analysis of energy carriers and end-of-life processes incorporates prospective methods to explore the effects of the energy transition. Across these scenarios, it was found that, when there is a mass difference among alternatives of 2% or more, the lighter alternative will be preferred, regardless of other factors. Through sensitivity analyses, potential was found for this margin to grow to 3% under extreme conditions, and to around 5-10% when shifting the whole lifecycle into the future. When dealing with smaller mass differences, material production and manufacturing waste become distinguishers of environmental performance. These insights are valuable when exploring novel materials and manufacturing methods for commercial aviation. The approach defined in this thesis can be extended to any other application which has a lightweighting imperative, such as automotive, shipping, rail, or wind turbines. Building on this thesis, guidance can be provided on how and where to apply novel materials across multiple product lifecycles.

Companies let their employees take short-haul flights for business trips while there are alternative, more sustainable, modes of transport. Understanding the motivation of the company and their employees for flying is essential to create a design solution which could change their behavior. To reach the goal of a maximum global temperature rise of 1.5 degrees by 2050, we have to change our polluting behavior as much as possible. The data reported in this report is drawn from both a literature study and a series of qualitative, semi-structured interviews. The results of the research show that for the company the motivation to go on a business trip is driven by social motivators. The motivation of employees is driven by emotional motivators. Both the company and the employees experience physical motivators when choose to fly (compared to other modes of transport). The design solution is a seminar that helps creating awareness about the impact of short-haul flights in combination with a website, where users are guided in their decision to go on a business trip and in their choice for the best suited (sustainable) mode of transport.

Induction welding is an effective technique for joining unidirectional carbon fiber reinforced thermoplastic composites and L-joints can be produced through quick and cost-effective processing steps. However, due to high localized stresses in the skin-stiffener interface, these L-joints are often avoided in primary aircraft structures. Also, no international testing standards have been developed for testing of such joints. A method was developed for the implementation of a neat thermoplastic resin fillet between the L-joint skin and stiffener web using the induction welding process in an attempt to remove the high stress concentration at this location. A 35.4% increase in quasi-static pull-off strength was measured with a weight penalty of less than 0.5%. This result was compared with a similar autoclave co-consolidated joint, which showed an 80.9% improvement. An ANSYS Parametric Design Language finite element model was developed based on the virtual crack closure technique and it showed that the joint pull-off performance is strongly dependent on geometric parameters such as the skin and stiffener thickness. Also, a new test setup was developed, which reduced internal stresses created by the setup compared to those commonly used in literature. By further improving the method through which the fillet is joined to the induction welded L-joint, a performance increase similar to that of the co-consolidated joint should be achievable. Test results have shown that the use of this type of fillet can lead to the skin-stiffener interface no longer being the critical failure point for realistic joints in primary aircraft structures.

Due to EU regulations on fuel consumption, reducing the weight of vehicles has become one of the most important goals of car manufacturers in Europe. Among them, Toyota Motor Europe is one of the worldwide leaders in the research for a sustainable future. Materials like fiber-reinforced plastics and aluminum play a significant role in the research for lightweight design, thanks to their very good strength-to-weight ratio. However, joining these materials efficiently together is still a challenge. When thermoplastic composites are used, direct joining with the metal substrate can be obtained using welding technologies which melts the thermoplastic at the interface. Ultrasonic welding is well-known for being a fast, reliable and effective technology for metal/metal or plastic/plastic joining. In this study, a collaboration between Toyota and TU Delft, ultrasonic plastic welding was investigated as candidate joining technology for aluminum/thermoplastic joints in automotive applications. The goal was to understand the main mechanisms involved in the adhesion and how they affect the performance of the joint. Initially, the technique proved to be successful, but moderate strengths were obtained. Therefore, several surface pre-treatments of the aluminum were analyzed to improve performance in terms of strength and durability of the joint; mechanical, chemical and physical treatments were carried out. With laser structuring, strengths comparable to adhesive bonded joints were obtained, but in a much shorter process time. Other treatments such as conversion coating, sandblasting and plasma led to considerable improvements as well. The encouraging results achieved represent an important step in the development of ultrasonic plastic welding for multi-material joining in the automotive industry. Additional research could help Toyota and other car manufacturers realizing a better design to further decrease weight and CO2 emissions of vehicles.

Disassembly of fusion bonded joints aided by induction heating was investigated. Single-lap shear experiments were performed while heating the joint with an induction coil to research the effect on force required and damage inflicted during disassembly. Co-consolidated CF/PEEK samples with and without a metal mesh susceptor and ultrasonically welded samples were tested and compared. An induction heating model was built to facilitate experimental design and to help analyse the effect of the susceptor on the heating process. Induction heating appeared successful in lowering the disassembly force. Large reductions were achieved, but this came at the cost of thermal damage in the disassembled adherends. For a reduction to 37% of the original strength, no thermal damage was inflicted during disassembly, except for one outlier. A susceptor facilitated disassembly for co-consolidated joints, while PEEK energy director remnants hindered heat development in ultrasonically welded joints. Further research is required to develop the method.

Having seen exponential growth in demand for air travel, the aviation industry has found itself trying to find a balance between economic growth, technological development, and environmental sustainability. This saw a shift in attention towards materials such as fiber reinforced composites, predominantly thermoset in the past with higher strength-to-weight fractions. Relatively recent was the introduction of high-performance fiber reinforced thermoplastic polymer composite materials possessing more promising prospects of circularity in addition to the lightweighting capabilities. But as is, these only form for qualitative claims with no indication on how the ecological effects would pan out over the life cycle phases objectively, as well as on a relative scale. Extending beyond the orthodox considerations and measures of aircraft performance, life cycle assessment studies encompass a comprehensive analysis of the environmental impact associated with aerospace products through the various phases of their life cycle including material extraction/production, manufacturing, operation, and the respective end-of-life treatment. The primary objective is to quantify the environmental impact of the system, offering a holistic view of the emissions, energy demand, and resource consumption. To this end, this study constructed a comparative environmental profile, modelling for five material/manufacturing systems, namely numerically machined aluminium alloy, autoclave cured and resin transfer molded carbon fiber reinforced epoxy, autoclave consolidated, and press consolidated carbon fiber reinforced Polyetherketoneketone (PEKK) over the cradle-to-gate and the cradle-to-end of service phases in an attempt to find the best variant from an environmental perspective, while also adding a novel, semi-quantitative, robust framework of data quality assessment to the state-of-the-art. The characterization results, under the assumption of each scenario yielding a product of the same mass and equal importance being given to each impact category (equal weighting), indicated the press consolidated carbon fiber reinforced PEKK product to be the scenario with the lowest impact over the cradle-to-gate (including only material production/extraction and product manufacturing). Over the cradle-to-end of service phases (including material production/extraction, product manufacturing, and the operational phase of the aircraft), the operational phase was observed to have an exponentially larger impact compared to the other life cycle phases causing the comparative profile to homogenize. This was reiterated by outcomes of the performed contribution analyses. Sensitivity analyses were conducted to explore the environmental benefits of lightweighting and processing waste optimization (buy-to-fly ratios quantifying the relative, quantitative benefits of lighter products and leaner manufacturing systems.

The usage of carbon-fibre reinforced composite materials in the aerospace industry has been steadily increasing due to their light weight solution with respect to their metal counterparts. Using thermoplastic resins gives rise to induction heating as a joining technique. Induction heating can be applied by utilising the inherent material properties of the adherends themselves. This avoids having to input a foreign material in between adherends that has to heat up the joint. It is a form of non-contact heating, it is relatively cost-effective and it is a rapid form of heating. One of the major challenges in using induction heating is controlling the temperature distribution within adherends. The temperature distribution directly influences the quality of the bond and is controlled by numerous process parameters. Having computational models able to predict the influence of these process parameters is of key importance into optimising the bond quality in a cost-effective and time-efficient manner. This research focuses on the electromagnetic response of UD CF-PEKK laminates on an alternating electromagnetic flux. It shows the relation between the micro-structure and electric material properties with the thermal response to an electromagnetic wave. This research fills the gap in literature by modelling this response for unidirectional laminates heated without the use of susceptor materials. These models can ultimately be used as optimisation tools within the process of continuous induction welding.

The popularity of thermoplastic composites (TPCs) has been growing steadily in the last decades in the aircraft industry. This is not only because of their excellent material properties, but also owing to their fast and cost-effective manufacturing process. Fusion bonding, or welding, is a typical joining method for TPCs due to the intrinsic properties of thermoplastic polymers. Among different welding technologies, ultrasonic welding has been regarded as one of the most promising techniques for the assembly of TPC components. Ultrasonic welding is by nature a spot welding technique. As it is known that a series of problems result from using mechanical fasteners for joining composite structures, e.g. breaking fibres during drilling and extensive labour work, ultrasonic spot welding can be considered as a promising alternative from the perspective of fast manufacturing cycle. However, fundamental understanding is still lacking to achieve application of ultrasonic spot welding in composite structures to be achieved: @en

The use of high performance thermoplastic composites (TPCs) in the aviation industry has increased over the last years. TPCs have significant advantages such as superior damage tolerance, excellent chemical resistance and the ability to be welded. To date, mechanical fastening, primarily, and adhesive bonding are the two traditional joining methods used in aviation industry. However, welding of TPCs has attracted significant attention due to its advantageous qualities such as the very short cycle times and the minimised stress concentrations. Most of the research published on welding of TPCs concerns process optimization and the evaluation of the mechanical performance at room temperature (RT) conditions. However, aircraft operate in a wide range of temperatures, typically between -50 °C to 70 °C and depending on the conditions, even up to 93 °C. Considering the temperature dependency of polymer composites properties, the weld performance of TPCs at low and high temperatures needs to be addressed. To the author’s knowledge, prior to the year 2014 there was no available literature assessing the influence of temperature on the mechanical performance of TPCs welded joints and since then, only a few publications have addressed this topic. The primary objective of this thesis was to not only fill the literature gap but also to obtain deeper knowledge and clear understanding of the behaviour of thermoplastic composites welded joints at low and high temperatures. In other words, to understand the phenomena dictating the weld performance which, in turn, would pave the way for further improvement of material properties and weld design.@en

One of the most promising welding techniques for thermoplastic composites is ultrasonic welding. It is mainly known as a spot welding technique. The relatively new technology continuous ultrasonic welding of thermoplastic composites makes it possible to obtain large fully welded seams with higher load carrying capabilities. However, at the start of the research conducted for this dissertation very little knowledge on the process was available. The weld quality in the state-of-the-art research was significantly lower than the weld quality of the statically welded counterpart in terms weld uniformity as unwelded and overwelded areas were present simultaneously, the single-lap strength, and the presence of voids. Hence, before continuous ultrasonic welding can be industrially applied the weld quality needs to be improved. Consequently, the main objective of this dissertation was to acquire a deeper understanding of the continuous ultrasonic welding process of thermoplastic composites to promote its development for future industrial applications.@en