Composite are now widely used in the aircraft industry. The use of light-weight, high-temperature resistant composite materials will allow for aircraft to have high-performance and economical designs. The composites used in the aircraft industry can be classified into two types: Thermosets and thermoplastics. Thermosets are polymers that are obtained by curing, where there are chemical cross-links between the polymer chains. Wings, fuselages, and bulkheads are some applications in aerospace which are made of thermoset composites. The interest in and the application of thermoplastic composites is increasing in the aerospace industry due to their advantages over thermoset composites. They have the advantage that they can be re-molten and reshaped upon heating, that they are recyclable, and have a high material toughness. The first advantage can lead to major cost-benefits since it makes efficient forming and welding techniques possible. As a result, different welding techniques have been developed for joining thermoplastic composite structures such as resistance, induction, and ultrasonic welding. Ultrasonic welding is generally the fastest and most energy-efficient welding technique.

This thesis explores the continuous ultrasonic welding (CUW) of unidirectional carbon fiber-reinforced thermoplastic composites, with a focus on the impact of adherend thickness on the welding process and joint quality. As the demand for lightweight, high-performance materials in industries such as aerospace and automotive continues to rise, efficient and scalable welding techniques for thermoplastic composites are becoming essential. CUW presents a promising method for joining large composite structures, offering speed and energy efficiency advantages.

Through experimental investigations, this study evaluates the effects of increasing adherend thickness on defects such as overheating and fiber squeeze-out. Two different clamping setups, the bar clamp and picture frame jigs, were used to assess their impact on weld quality. It was concluded that the welds made on the bar-clamped jig overall had a much higher energy requirement compared to those made on the picture frame jigs. The picture frame jig, due to its lack of constraints on the thick adherends, resulted in much better parallelism throughout the process. The welds made using the picture frame jig also experienced less overheating at the interface compared to the bar-clamped jig. The resulting weld uniformity and strength were also higher when using this jig. However, one issue with the picture frame jig was the extent of top surface scraping, which was found to be prevalent regardless of the weld speed. The changes in process parameters were also studied. Contrary to findings from static welds, increasing the welding force resulted in through-the-thickness heating when the thickest adherend was welded and did not have much effect on the weld strength of the thickest adherend on the picture frame jig. The findings from this thesis result in the need for further research to mitigate the effects of the thicker adherends.

The effect of creating parallel welds in a single overlap using continuous ultrasonic welding on the weld quality has been investigated, with the goal of creating three high-quality parallel welds. To achieve this, multiple aspects were examined to understand the creation of high-quality parallel welds. The weld quality is highly influenced by a change in boundary conditions. The number of welds, the order of welding, and the energy director strategy all change the boundary conditions. Increasing the number of parallel welds will make it more difficult to create high-quality welds. The order of welding has a large effect on the quality of the welds. The order of welding will determine if a low- or high-quality weld is created with constant welding parameters. Two energy director strategies were compared: a ‘full overlap energy director’ and ‘energy director strips’. Both can create high-quality welds, but energy director strips create an overall better-quality weld.

The goal of the project was to produce slotted single-lap shear specimens using continuous ultrasonic welding, which had not been done before. These aim to provide more reliable lap shear strengths than currently used coupons due to increased assembly stiffness and eliminated squeeze-out. By increasing the weld width and stacking smaller rotated adherends, fully welded coupons could be extracted and subsequently slotted. However, high-quality welds were not yet obtained consistently due to pre-heating of the adherends, resulting in overheating during about the second half of these welds. The produced slotted coupons showed the viability of using this configuration, the best achieving 1% under-welding and 90% of the co-consolidated slotted coupon reference lap shear strength, despite not being fully optimized. However, slot precision is essential. In contrast, welded conventional coupons likely yield non-conservative strengths, reaching 109% of the reference, possibly due to the squeeze-out and adherend geometrical distortions resulting from welding.

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.

Components of novel propulsion systems in electric aviation contain materials defined as critical by the European Union: electric motors contain Rare Earth Elements neodymium and dysprosium, and state-of-the-art batteries materials like lithium and cobalt. Such critical raw materials have high supply risks but are crucial for the economy. The demand of all these materials is forecasted to increase drastically in the coming years, which means that the rate of supply might no longer suffice to fill the demand. Circular economy principles have been suggested as a solution. Materials recovered from end-of-life components can secure supplies and reduce the environmental impact of products.

In this research, the implementation of circular economy approaches to address critical raw material demand in electric aviation is studied. According to the developed models, the material demand is negligible in comparison to other industries until 2050. However, as the electric aircraft technologies are still in development, there is a lot of uncertainty around the demand.

Beyond 2050, components will start reaching their end-of-life stage. In this case, a circular strategy considered feasible for electric motors is remanufacturing. After the Rare Earth Element magnets in the motors become obsolete, they can be recycled to recover the critical raw materials. Both hydrogen decrepitation and a combination of hydro- and pyrometallurgical processes can be used to regain materials for magnets in aviation or other applications.

Although circular economy strategies will not be able to significantly reduce the primary material demand in electric aviation by 2050, these can still lower the environmental impacts from production. Additionally, well-established circular practices could address the material demand more substantially in the future, after 2050, if electric technologies are more widely adopted then.

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.

Research and development of thermoplastic composites has been ever increasing owing to the advantages brought by it over conventional materials such as metals and thermoset composites. Higher processing rates, automation, the possibility to recycle, weldability and better mechanical and chemical properties are some of them. Fusion bonding of TPCs that involves application of heat and pressure at the localised bonding area promises faster assembly times and weight reduction and is thus attractive to the aerospace industry. When comparing different welding techniques, ultrasonic welding has the lowest welding times and energy consumption and also overcomes certain challenges that come with the other techniques. The ultrasonic welding process has been a part of the plastic industry for decades, and with time, the research has also shown promising results in welding advanced thermoplastic composites. Heat generation in this mechanical/frictional welding technique occurs due to surface friction and viscoelastic heating caused due to high frequency and low amplitude vibrations. The process creates quality welds in seconds and can be controlled in-situ, ensuring robustness and repeatability. Scaling up the static process to a continuous process enables continuous sealed welds having a more uniform distribution of load and higher load-carrying capability. However, the continuous process brings challenges. The state of the art continuous ultrasonic welding (CUW) equipment at TU Delft has demonstrated quality welds for CF/PPS fabric laminates. However, problems like excessive through the thickness heating (TTH) and deconsolidation of welds need to be understood and overcome.

As the industry moves toward welding larger and more complex composites structures such as stringers, frames and skin joints, the CUW process becomes more attractive. The primary thermoplastic structures would involve the use of unidirectional materials. The material that has been proposed for the future structures is the Carbon fiber/ Low Melt - PolyArylEtherKetone composite. LM-PAEK polymer exhibits comparable properties to the other advanced thermoplastic materials like PEEK and PEKK but has a lower processing temperature enabling faster processing. To the author's knowledge, no literature is currently available related to CUW of this material. It was believed that when welding UD / LM-PAEK, the problem of excessive through the thickness heating might be present, leading to fibre and polymer squeeze-out and porosity in the adherends and the need for longer consolidation time. Therefore, this research was focused on investigating the extent of TTH in the ultrasonic welding of this UD material and the solutions to mitigate the effects of TTH on the welds. Static and continuous welding experiments were conducted, and temperature readings were obtained from the adherends and the interface. Along with this, different characterisation techniques such as lap shear tests, fractography and cross-sectional microscopy were used to observe the effects of TTH.

CUW showed overall higher TTH. It was observed that changing the fibre architecture from fabric to UD exacerbates the effects of excessive TTH in the adherends. Also, the change in the boundary conditions from that of a static weld to that of a spot or a continuous weld brings in the differences in the viscoelastic bulk heat generation in the top adherend. The cause of this difference is yet to be understood. Changing the process parameters and shape of the sonotrode helped in mitigating the overall TTH in the CUW. The use of a round sonotrode helped in reducing the preheating during welding and the lower welding speeds used, brought the advantage of consolidating the welds for longer times. In conclusion, it was found that changes in the welding and consolidation parameters, and the equipment, can help mitigate the effects of excessive TTH on the adherends. Overcoming the problem of excessive TTH is one of the first steps toward enabling CUW of UD/LM-PAEK composites for future applications.

Fiber-reinforced polymer composites have gained relevance in the aerospace industry due to their great potential for weight reduction. This, thanks to their outstanding specific properties and ability to be tailored to different applications. Developing efficient composite joining techniques is a challenge that requires great attention as it is key to a more sustainable industry. Thermoplastic composites present the great advantage that they can be joined via fusion bonding, a joining technique in which the interface is virtually erased and is generally faster than other available techniques such as mechanical fastening and adhesion bonding. A fusion bonding technology that stands out for its short processing times and cost-efficiency is ultrasonic welding, a process in which the joint is developed via low-amplitude and high-frequency mechanical vibrations that heat up the interface. Although the effect of different parameters is well known, the particular effect of the part -commonly referred to as adherend- thickness is yet not fully understood.

This thesis investigated the effect of the adherends’ thickness on the process response and weld evolution on static ultrasonic welding of CF/LMPAEK thermoplastic composites. Different characterization techniques were used to assess the latter, such as the output from the welding equipment (power and displacement), temperature readings, microscopy analysis, high-speed camera recordings, and numerical models. The results showed that increasing only the top adherend’s thickness gradually increases the heating in this adherend up to a point where significant fiber squeeze-out is observed at early stages of the process. This overheating was associated with hammering and the differences in compliance when increasing the thickness. It is hypothesized that this hammering may contribute to overheating. Welding with a higher force significantly decreased the overheating in the top adherend as hammering is reduced, whereas changing the amplitude did not show to be as influential. In contrast, increasing only the bottom adherend’s thickness did not have as much effect on the process. The dissipated power and cooling rate were the two variables that showed to be most affected, and both were associated with more bulk viscoelastic dissipation as the thickness increases. Finally, changing from fabric to UD reinforcement and welding only different top adherend’s thicknesses resulted in less hammering and fewer differences in the process response and weld evolution between thin and thick adherends, which was associated with the variation in compliance.

The findings of this study indeed contribute to the understanding of the effect of the adherends' thickness. However, more research is required to fully grasp this effect on ultrasonic welding of thermoplastic composites. For example, developing a process envelope that quantifies the thickness limitations for different process parameters and investigating the effect of the thickness in continuous ultrasonic welding are some of the next steps towards a more robust and well-understood joining technology for thermoplastic composites.

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.

With the emerging demand for energy-efficient products, the composite material has become a point of attraction for many product manufacturers. Furthermore, with its high strength to weight ratio, long life span, low maintenance and design flexibility, composite materials offer tailored properties by choosing an appropriate combination of reinforcement and matrix material. Being one of the leading users of composite material, the Netherlands is a global player in the field of design, automation, material development and sustainability in the field of high-grade-reinforced plastic.

Recycling of the decommissioned composite material remains underdeveloped research. The current shredding and inefficient recycling result in the loss of material property, struggling to find its scalable application in the market. However, the composite material has a longer life than its entire product, resulting in premature decommissioning of the material due to constant development in the product design. One way around this problem is repurposing. Therefore, there is a need to look for an alternative approach.

Repurposing aims to keep the material alive by utilising its shape and remaining potential in serving an additional product life with a different application. However, the concept of repurposing is undiscovered in the field of composite material.

This graduation thesis aims to create a set of guidelines for industrial designers and companies, guiding them to efficiently utilise the composite material’s value with the practice of scalable repurposing.

Further to industrialise and streamline the repurposing practice, there is a need for product lifecycle management, assisting repurposing industrial designers and companies with the set of prerequisite information.

By case study of repurposing an aircraft galley made from the glass fibre reinforced plastic into a bicycle cart for kids, various operations, including disassembly, dismantling and re-manufacturing, were executed to gain a closer look into the repurposing. In addition, different stakeholders’ roles in repurposing were identified by conducting interviews with various actors involved in the composite industry. With the help of the theoretical and practical insights gathered during the exploratory phase, the PLM framework and guidelines were formulated focusing on scalable repurposing of the composite material.

A revised set of guidelines was proposed by evaluating the draft guidelines through an interview with the repurposing company and a co-creation session involving various stakeholders, education experts and industrial designers. Based on the evaluation, it can be concluded that the guidelines can serve as a base for implementing the repurposing practice, where the industrial designer can connect the detached stakeholder and guide the implementation of composite repurposing.

However, to make these guidelines well qualified for all the composite products from different sectors, a wide range of case studies should be conducted. Although these guidelines are formulated to support industrial designers to impact repurposing positively, further work is needed to conduct a business model, real case scenario, and policy management to look into the guidelines’ pertinency.

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.

Thermoplastic composites are gaining prominence in the aerospace industry owing to their higher damage tolerance, cost-efficient means of manufacturing, and the possibility to be recycled. One of the major benefits of thermoplastics compared to thermosets is their ability to be welded, and one of the most promising welding techniques for thermoplastic composites is ultrasonic welding. Ultrasonic welding is the fastest welding technique currently known, with typical weld times of a few hundred milliseconds.

Studies have been conducted in plenty regarding static ultrasonic welding of thermoplastic composites. But the industrialisation of the process involves the development of a robust continuous ultrasonic welding process which can weld the entire span of the joints, thus enabling higher load transfer and reduced stress concentrations. However, the state-of-the-art continuous ultrasonic welded joints contain voids at various locations within the weld which are assumed to appear due to a lack of consolidation during the welding process. Unlike the static ultrasonic welding, where the sonotrode can both transfer the vibrational energy to the adherends being welded and provide consolidation force, the continuous ultrasonic welding requires a separate consolidation device to provide consolidation pressure application. This makes it necessary to expand the understanding of the consolidation process to improve the weld quality and increase the Technology Readiness Level (TRL) of the ultrasonic welding process before it can be industrially used. While a lot of research to date focused on the vibration phase of the process, not much information is available regarding the consolidation phase. This research project thus explores the effect of consolidation pressure and time on (de)consolidation in ultrasonic welding of thermoplastic composites.

An experimental study was carried out on the consolidation in static welding of CF/PPS test coupons, and the knowledge obtained was extended to the continuous ultrasonic welding process. The consolidation in the continuous ultrasonic welding process was provided by a separate consolidation device or ”consolidator” placed behind the sonotrode. Various characterisation techniques including lap shear strength, void content assessment and fracture surface analysis were used to analyse the results obtained. The experiments revealed that for semi-crystalline polymer PPS, consolidation should start when the polymer is in its melt state and extend until the interface temperature of the weld drops below the crystallisation temperature of the polymer. The results obtained indicated that the voids in ultrasonic welding were formed due to a combination of shrinkage due to crystallisation, fibre decompaction, the choice of the clamps used and excessive squeeze out of the resin. In continuous ultrasonic welding, the location of the consolidator behind the sonotrode and the consolidation pressure was found to influence the weld quality. The research conclusions serve as a first step towards developing a robust consolidation process in continuous ultrasonic welding of thermoplastic composites.

Ultrasonic Welding is a fusion bonding method that can create high-strength thermoplastic composite joints at very fast speeds. This makes it a very promising method for use in the aerospace industry, though further research is required to mature it. One topic of interest is how the crystallinity of the polymer matrix in the joint is affected by the ultrasonic welding process. This thesis specifically investigated the degree of crystallinity of carbon fibre reinforced lower melting polyaryl ether ketone (LM PAEK) composite. An experimental methodology was developed to measure the temperature at different locations through the thickness of the joint, and extract samples for Differential Scanning Calorimetry analysis. The calculated degree of crystallinity was related to the temperature evolution for different locations in the joint and variation in the welding force and amplitude.

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.

Ultrasonic Welding (USW) is a rapidly upcoming technology for joining high performance Thermoplastic Composite (TPC) structures in the automotive industry. With the focus shifting from metals to composites for primary structures, USW process is seen as a fast, clean and efficient alternative for the conventional joining methods. Continuous Ultrasonic Welding (CUW) is an innovative, upscaled version of the USW process, where the weld is made over a moving part to create a weld line instead of a single spot. Currently, there has been very limited research done in this field and the technology is in very early stages. The key to success of this technology lies in creating a uniform weld of an acceptable lap shear strength (LSS). Recent research has suggested that using a woven mesh as an energy director (ED) at the interface provides a good uniformity along the weld line, along with an acceptable LSS value. The research shows how the mesh deforms prior to the melting process, creating an intimate contact between the adherends and how this has a positive effect on the weld uniformity. This study uses this data to design and manufacture a new ED that has these beneficial features incorporated in it. The study also uses an expanded mesh as an ED to study the effect of open areas in the weld uniformity. The EDs are used in the CUW process and compared based on their different features. Through this study, a deeper understanding of the behaviour of EDs during a CUW process is established which will be useful for developing new means of optimizing this process.

This master thesis research investigates the possibility to repair carbon fiber thermoplastic aircraft
structures using induction welding. Carbon fiber is conductive and heats up when placed inside an
alternating magnetic field. A generator, coil, and pressure frame are needed to perform induction
welding. The support plates needed to pressurize the carbon fiber parts need to be non-conductive,
non-magnetic, temperature resistant, stiff at high temperatures, and thermally insulating.
The material used is five harness satin weave carbon fiber PPS supplied in pre-consolidated plates
and unconsolidated semi-preg. Three different joint geometries are used in this investigation: a conventional
scarf, a continuous scarf, and a stepped lap joint. The stepped lap joint and the conventional
scarf are milled using a CNC machine. The continuous scarf is produced in a press using specialized
tooling and a press program prescribed by TenCate.
Of each type two specimens are produced: an induction welded specimen and a press joined specimen.
The specimens are tested in tension to determine the tensile strength and stiffness. This gives
the performance of the induction welded joints with respect to the press joined specimens. Additionally,
both of the welded specimens are compared to the pristine specimens.
By measuring the temperature along the weld-line of multiple test specimens, an induction welding
program is obtained for each joint type. Achieving a consistent temperature along the weld-line is
challenging due to the thermal conductivity of carbon fibers and other effects inside the laminate. Similar
to the continuous scarfed specimens, the press joined specimens are created using specialized tooling.
Before testing the specimens in tension, they are scanned using a C-scan. The press bonded
specimens show no flaws, whereas the induction welded specimens do not return the signal to the
transducer. Micrography of the specimens shows similar results as the C-scan. The press joined
specimens show little to no flaws and the induction welded specimens show a significant amount of
voids and small unjoined sections near the tips for the continuous scarf and stepped lap specimen.
The tensile tests show that all induction welded joints perform less than the press joined specimens
with a percentage of about 73-78%. The conventional scarf and the continuous scarf result in a similar
failure strength recovery of about 44% of the pristine failure strength for the press joined specimens.
Both the conventional scarf and the continuous scarf show similar behavior in welding and the tensile
testing, but the continuous scarf is more difficult to produce. Therefore the conventional scarf joint is
preferred over the continuous scarf joint. The stepped lap joint has the best performance, recovering
about 59% of the pristine stiffness. The stiffness of all tested specimens is between 92%-99% of the
pristine stiffness.
Inspection using digital image correlation and finite element modeling indicates the failure initiates
inside the PPS matrix in between the parts at the location of the first or last step. Finite element modeling
also suggests that a smaller angle for the scarf joint or a longer overlap for the stepped lap joint would
increase failure strength.

This project investigates the combination of PPS thermoplastic UD and fabric composite materials with novel mass-produced vertically aligned carbon nanotube (VACNT) materials at the interlaminar region. There are two goals for this project, the first is to understand how the VACNT materials can be embedded and consolidated with the use of elevated temperatures and pressures and how that impacts the resulting morphology of the VACNTs. The second is to understand if VACNTs can increase the interlaminar related mechanical properties of the PPS composite materials similar to that seen with thermoset materials. Processes for embedding of VACNTs to a single ply was found to be possible and repeatable, though secondary multi-ply consolidation lead to matting of the VACNTs at the interlaminar layer. Subsequent mechanical testing showed that this matting of VACNTs led to a decrease in shear and compression strengths by 10.6% and 8.5% respectively against control specimens.

The joining of high performance Thermoplastic Composite (TPC) structures by Ultrasonic Welding (USW) is considered a promising alternative for mechanical joining or adhesive bonding methods. The process is fast, clean, and highly automated. In the USW process, an intermediate, unreinforced polymer layer is required at the weld interface to concentrate the heat generation at this weld interface and is called the Energy Director (ED). This research investigates a new method to manufacture and apply this ED at the weld interface, being Fused Deposition Modeling (FDM).

With FDM, complex products can be manufactured directly from a Computer Aided Design (CAD) model, without additional mould or special tooling required. The goal is to simultaneously manufacture and adhere a dedicated ED geometry on a consolidated TPC substrate with FDM, such that no additional fixation step of the ED is required before welding. The new technique is successfully applied with a PEEK (Polyether ether Ketone) ED and a consolidated CF/PPS (Carbon fibre/Polyphenylene Sulfide) substrate. The deposited ED has sufficient bonding with the substrate that it remained fixated during handling prior to and during the weld process. This resulted in a successful weld, having similar strength and quality as found with welds having a loose, flat film as ED.

A challenge encountered in using FDM as manufacturing method of the EDs, is a non-uniform thickness distribution occurring in the ED. Additional research on the influence of a variation in thickness is done, where it is found to reduce the overall single Lap Shear Strength (LSS), and divide the fracture surface of a single weld in two areas welded to different weld stages.