Due to environmental challenges and need for action with regard to CO2 emission, reducing the weight of vehicles has become one of the most important goals of car manufacturers in Europe. Materials like fibre-reinforced plastics and aluminium are the core of the research for lightweight design. However, efficiently joining these materials together is still a challenge. When thermoplastic composites are used, direct joining (i.e. without adhesives or fasteners) with the metal substrate can be obtained using welding technologies which melt the thermoplastic at the interface. In this study, ultrasonic plastic welding was investigated as a candidate technology for joining aluminium and carbon fibre-reinforced thermoplastics. The goal was to understand the main mechanisms involved in the welding process 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 aluminium were analysed to improve the performance in terms of lap shear strength; mechanical, chemical and physical treatments were also carried out. With laser structuring, strengths comparable to adhesive bonded joints were obtained, but in a much shorter process time. Other treatments 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.

In previous work, single-spot ultrasonically welded joints were found to feature similar load carrying capability in shear but significantly low capability in peel as joints with a representative single-mechanical fastener. This leads to questioning welding as an appropriate solution for the commonly-used single-lap joint configuration. The present paper investigates the mechanical performance of spot welded single-lap joints in thermoplastic composites in comparison to their mechanically fastened counterparts. Single-row joints, double-row joints with varying inter-row distance and multi-row joints with varying number of rows were investigated in this study. The results showed that, owing to higher joint stiffness and hence lower secondary bending and peel stresses, the load carrying capability of the spot welded joints was comparable to that of the mechanically fastened joints in all considered cases. Likewise, the effects of increasing the inter-row distance and of increasing the number of rows were similar for both types of the joints.

In ultrasonic welding of thermoplastic composites an energy director (ED) (i.e. neat thermoplastic film), is used between the two adherends to be welded, to promote frictional and viscoelastic heating. For welding of thermoset composites (TSC), a thermoplastic coupling layer is co-cured on the surface to be welded as typical procedure to make the TSC “weldable”. This study focuses on investigating whether a polyetherimide (PEI) coupling layer by itself has the potential to promote heat generation during ultrasonic welding of CF/epoxy and CF/PEI samples, without the need for a separate ED, and if so, what thickness should that coupling layer be. The main findings were that welding without a loose ED resulted in overheating of the CF/PEI adherend and/or coupling layer due to the inability of the latter to promote heat generation efficiently. However, welding of CF/ epoxy and CF/PEI samples with the use of a loose ED resulted in high-strength welds.

This article presents research about the propagation of ultrasonic guided waves in ultrasonically welded thermoplastic composite joints. The goal of the study was to understand the effect of weld manufacturing defects on guided wave transmission across the joint. Triangular energy-directors integrated into the lower composite adherends enabled the production of defective joints in a controlled manner. The produced defect types were unwelded areas and adherend fibre bundle distortion. The reference condition corresponded to the fully welded stage which showed the highest single-lap shear strength. It was possible to detect adherend fibre bundle distortion through the increase in the negative shift of the signal characteristic frequency. Evidence of the presence of unwelded areas was found in the increase of Time-of-Flight of the maximum amplitude Lamb wave group. The sensitivity of the diagnostic parameters was found to be dependent on the ultrasonic guided wave excitation frequency.

The influence of ultrasonic welding on the crystallinity degree at the welding interface of carbon fibre reinforced polyphenylene sulphide(CF/PPS) joints was investigated. Two sets of welding force and vibration amplitude were used, (1000N, 86.2microns) and (300N, 52.8microns), representing short and long welding times, respectively. The evolution of the temperature with time at the centre of the joint overlap was recorded using thermocouples while the crystallinity degree of PPS was measured using differential scanning calorimetry (DSC). The cooling rate dependency of crystallinity was determined through fast scanning calorimetry (FSC) measurements. It was found that high force and high amplitude resulted in faster cooling rates and predominantly amorphous PPS, while low force and low amplitude resulted in slower cooling rates and yielded PPS of moderate crystallinity. It is suggested that the capability of PPS to crystallize despite the very fast cooling rates could be attributed to strain-induced crystallization during the welding process.

This paper investigates welding of carbon/epoxy and carbon/PEEK composites using the following procedure. Firstly, the carbon/epoxy composite was made “weldable” through a very thin PEI thermoplastic film co-cured on its surface. During the curing cycle, the PEI resin and the components of the epoxy resin system partially diffused into each other generating a gradient interphase between the original epoxy and PEI resins. Subsequently, the carbon/PEEK composite adherend was welded onto the PEI-rich surface of the weldable carbon/epoxy adherend, exploiting the total miscibility between PEI and PEEK. Thermal degradation of the carbon/epoxy adherend during the welding process was avoided via the ultra-short heating times enabled by the ultrasonic welding technology. In this research, mechanical testing was used to evaluate the weld strength relative to reference joints. Additionally, cross-section scanning electron microscopy was used to assess the morphology of the PEI/epoxy interphase before and after the welding process.

The research presented in this paper is an essential part of a bigger effort on developing robust sequential ultrasonic welding of multi-spot welded joints in thermoplastic composites. It mainly focused on assessing the impact of the changes in boundary conditions on the welding process and whether it could be circumvented by using an appropriate process control strategy. A two-step approach was followed by investigating: (1) the effect of boundary conditions on displacement- and energy-controlled single-spot welded joints and (2) displacement- and energy-controlled sequential ultrasonic welding of double-spot welded joints. The results showed that previous spots indeed affect the welding energy required to obtain an optimum new welded spot, which challenges the use of energy-controlled welding for this application. Contrarily, displacement-controlled welding was shown to provide consistent-quality welds with a constant set of welding parameters and it was hence identified as the most promising welding strategy for sequential ultrasonic welding of thermoplastic composite structures.

The present work focuses on the applicability of the mandrel peel test to quantify the interlaminar fracture toughness of 5 harness satin woven fabric carbon/PEEK composites. For this purpose, the Mandrel Peel (MP) test was compared to the Double Cantilever Beam (DCB) and End-Loaded Split (ELS) test in terms of experimental procedure and obtained results. The interlaminar toughness of the 5 harness carbon/PEEK was measured both parallel and perpendicular to the predominant fibre direction at the interface. While stable crack propagation was observed in the ELS test, unstable crack propagation (stick-slip) was observed during both the DCB and the mandrel peel tests. In the case of the mandrel peel test, however, the unstable propagation was immediately arrested by the mandrel, limiting the instability and providing numerous crack re-initiation values per unit of crack length. This effect is expected to increase the statistical relevance of a single test and thereby to increase the reliability of the measured values as compared to DCB tests. A fractographic analysis was performed to study the nature of the crack propagation for the different testing techniques. The mandrel peel test was found to be a potentially plausible alternative to the DCB test for woven fabric reinforced composites.

In this work, the effect of temperature exposure on the strength of resistance welded joints is analysed. Glass fibre reinforced polyphenylene sulphide (GF/PPS) adherends were joined using the resistance welding technique, using a stainless steel mesh as the heating element. Single lap shear tests were performed at temperatures ranging between -50 °C and 150 °C to evaluate the strength of the welded joints. The results showed that the lap shear strength decreased with increasing temperature, except for the region between 50 °C and 90 °C where it remained constant. Fractography analysis revealed that the main failure mechanism was glass fibre/matrix de-bonding and the connection between the mesh and the matrix was not the weakest link at the interface of the joint at any temperatures under study. The fibre/matrix interfacial strength and the stress distribution at the joint overlap were identified as the main factors influencing the behaviour of lap shear strength with temperature.

approach for joining thermosetting matrix composites (TSCs) proposed in this study is based on the use of a low melting co-cured thermoplastic film, added as a last ply in the stacking sequence of the composite laminate. During curing, the thermoplastic film partially penetrates in the first layer of the thermosetting composite, leading to macro-mechanical interlocking as the main connection mechanism between the thermoplastic film and the underlying composite. After curing, the thermosetting composite joints with the thermoplastic modified surface can be assembled by welding. Welding of the TSC-TSC joints is performed by ultrasonic and induction welding. The weld strength is investigated by morphological characterization of cross sections and failure surfaces and by mechanical testing. The effect of the thermoplastic film thickness on the welding process and on its outcome is also analyzed. Both induction and ultrasonic welding mostly result in good-quality welded joints. The welding process used as well as the initial thickness of the thermoplastic film are found to have a significant effect on the final thickness of the weld line and on the location of failure. Thicker thermoplastic films are found to ease the welding processes.

One of the advantages of thermoplastic composites relative to their thermoset counterparts is the possibility of assembling components through welding. Ultrasonic welding in particular is very promising for industrialization. However, uncertainty in the fatigue and fracture behaviour of composites is still an obstacle to the full utilisation of these materials. Health monitoring is then of vital importance, and Lamb wave techniques have been widely recognised as some of the most promising approaches for that end. This paper presents the first experimental study about the influence of welding travel on the transmission of Lamb waves across ultrasonically welded thermoplastic composite joints in single-lap configuration. The main aim of this research is to start to understand how guided waves interact with the internal structure of ultrasonic welds, so that benign, manufacture-related structural features can be distinguished from damaging ones in signal interpretation. The power transmission coefficient and the correlation coefficient proved to be suitable for analysing the wave propagation phenomena, allowing quantitative identification of small variations of weld-line thickness and intermolecular diffusion at the weld interface. The conclusions are used to develop a tentative damage detection criterion which can later on assist the design of a Lamb-wave-based structural health monitoring system for thermoplastic composite structures. The Lamb wave test results are backed-up by phased-array inspections, which also provide some extra insight on the internal structure of ultrasonic welds.

It is a common practice in fusion bonding of thermoplastic composites to add a matrix layer between the two substrates to be joined. The aim is to ensure proper wetting of the two parts. The effect of this additional matrix layer on the mechanical performance was studied by mode I fracture toughness measurements. The additional matrix was inserted at the interface in the form of films of various thicknesses. Three different manufacturing techniques, namely autoclave consolidation, press consolidation and stamp forming, were used to prepare different sets of specimens with varying resin-rich bond line thickness. The occurrence of fibre migration towards the matrix rich interface was induced by the manufacturing techniques used due to their different processing times. The interlaminar fracture toughness was observed to increase with increasing amount of extra-matrix at the interface, while no effects of the fibre migration on the fracture toughness were observed.

The effect of cooling rate on the interlaminar fracture toughness of Carbon reinforced PPS laminates was investigated experimentally. A typical stamp forming process was utilised in a novel manner to achieve high average cooling rates, of up to 3500 °C/minute, while ensuring a good consolidation quality. Differential scanning calorimetry measurements were used to characterise the degree of crystallinity of the samples, while the interlaminar fracture toughness of the laminates was characterised under mode I using the Double Cantilever Beam test. Finally, micrographic analysis of the fracture surfaces was carried out to correlate the degree of crystallinity to the failure modes. A strong correlation between fracture toughness and degree of crystallinity was found. The samples with a low degree of crystallinity showed a high interlaminar fracture toughness and large plastic deformation of the matrix during fracture.

Ultrasonic welding of thermoplastic composite materials is a promising joining technique that is now moving towards up-scaling, i.e. the assembling of large industrial parts. Despite its growing technological maturation, the assumed physical mechanisms underlying ultrasonic heating (viscoelastic heating, friction) are still insufficiently understood and modelled. In particular, the hammering phenomenon, resulting from the periodic loss of contact between the sonotrode and adherends due to the high frequency vibration caused to the former, directly impacts the heating efficiency. We propose in this work an original experimental and modelling approach towards a better understanding of the hammering effect. This approach makes combined use of: (i) an experimental static welding setup provided with a high-frequency laser sensor to analyse the vibration amplitude transmitted to the adherends and (ii) an improvement of the multiphysical finite element model already presented in previous works. Results show it is possible to obtain a good estimation of the vibration transmitted to the upper adherend from laser measurements close to the sonotrode. The hammering effect is shown to decrease during the welding process, due to the heating of the interface which directly affects further heat generation. Quantitative introduction of this hammering effect in the existing numerical model results in improved predictions in terms of dissipated power in time.

This paper presents a fully experimental study on melting, flow and weld strength development during ultrasonic welding of CF/PPS composites with integrated triangular energy directors. The main goal of this research was assessing whether the heating time to achieve maximum weld strength could be significantly reduced as compared to ultrasonic welding with flat energy directors. The main conclusion is that, in the specific case under study, the triangular energy directors did heat up, melt and collapse approximately two times faster than the time it took for the flat energy directors to melt and significantly flow. However the heating time needed to achieve maximum weld strength for the integrated triangular energy directors did not differ drastically from that for flat energy directors. This was caused by the fact that a fully welded overlap was not directly achieved right after the collapsing of the triangular energy directors. Instead a solidified resin-rich interface was created which needed to be re-melted as a whole in order to achieve a fully welded overlap and hence maximum weld strength.

This paper presents a detailed experimental assessment of the effect of the thickness of flat energy directors (ED) on heat generation at the interface during ultrasonic welding. Power and displacement data showed clear differences caused by the change of thickness, related to heat concentration at the weld line during the process. The extent of the heat-affected zone was assessed by welding specimens without consolidation at different stages of the process. It was confirmed through optical microscopy that heat is generated at the interface and transferred to the bulk adherends earlier in the process for thinner ED. The analysis of their fracture surface under optimum welding conditions revealed signs of matrix degradation, leading to less consistent quality, likely due to faster heat generation rate in both the ED and the substrates, and incidentally, higher temperatures surrounding the energy director.

The in-plane and out-of-plane mechanical behaviour of both ultrasonically spot-welded and mechanically fastened joints was investigated by double-lap shear and pull-through tests, respectively. Spotwelded specimens showed comparable onset failure load and significantly higher joint stiffness compared to mechanical fasteners when carrying shear load. The failure modes and the damage within specimens were analysed after mechanical tests. Intralaminar failure and very limited damage on the out-most ply were found for welded specimens, whereas catastrophic through-the-thickness failure was observed for mechanically fastened joints. Based on the experimental outcomes, the mechanical performance and failure mechanisms of spot-welded joints were critically assessed in comparison to the mechanical fasteners.

Carbon fibre reinforced thermoplastic composite laminates have been press-consolidated using various release media. The potential contamination of the laminate’s surface by the release media and its effect on the performance of the joint after fusion bonding was studied. Before bonding, the physical and chemical state of the bonding surfaces were analysed by measurement the surface energy and roughness. The laminate surfaces chemical composition was investigated by X-ray photoelectron spectroscopy (XPS). Subsequently, the laminates were fusion bonded through an autoclave cycle. The mechanical performance of the joints was characterized by mode-I fracture toughness and short beam strength tests. The surface of some of the composite laminates were found to contain high concentration of the release medium residues after consolidation. This contamination showed a significant effect on the mechanical performance of fusion bonded joints. It was demonstrated that the performance of the joint can be regained by employing a simple cleaning method

The process-induced voids during resistance welding of glass fabric-reinforced polyetherimide was investigated. The mechanisms of void formation in adherends, in particular, the residual volatile-induced voids and the fibre de-compaction-induced voids, were analysed. Due to the non-uniform temperature and stress distributions in the joints during welding, a non-uniform void distribution was observed in the joints with more voids generated in the middle of the joints than at the edges. Welding temperature and pressure were shown to have a large influence on void formation. Increasing of welding pressure was shown to effectively reduce the voids, while the residual moisture-induced voids were found more difficult to be eliminated than the fibre de-compaction-induced void

Continuous ultrasonic welding of plastic films, fabrics and even thermoplastic composite prepreg tape is common industrial practice. However, continuous ultrasonic welding of stiff thermoplastic composite plates is challenging due to squeeze flow of resin at the welding interface, and significant local deformation of the welding stack, that are generally needed to achieve strong welds. This paper presents a novel approach to continuous ultrasonic welding of thermoplastic composite plates based on zero-flow welding. The proposed technique can create strong welds before any squeeze flow takes place at the interface. It is enabled by the use of very thin flat energy directors, owing to simultaneous melting of both energy director and adherends’ matrix. The results prove the feasibility and indicate the potential for high-strength welds between thermoplastic composite plates at very high speed.

Abstract

This paper presents a novel straightforward method for ultrasonic welding of thermoplastic-composite coupons in optimum processing conditions. The ultrasonic welding process described in this paper is based on three main pillars. Firstly, flat energy directors are used for preferential heat generation at the joining interface during the welding process. A flat energy director is a neat thermoplastic resin film tha is placed between the parts to be joined prior to the welding process and heats up preferentially owing to its lower compressive stiffness relative to the composite substrates. Consequently, flat energy directors provide a simple solution that does not require molding of resin protrusions on the surfaces of the composite substrates, as opposed to ultrasonic welding of unreinforced plastics. Secondly, the process data provided by the ultrasonic welder is used to rapidly define the optimum welding parameters for any thermoplastic composite material combination. Thirdly, displacement control is used in the welding process to ensure consistent quality of the welded joints. According to this method, thermoplastic composite flat coupons are individually welded in a single lap configuration. Mechanical testing of the welded coupons allows determining the apparent lap shear strength of the joints, which is one of the properties most commonly used to quantify the strength of thermoplastic composite welded joints.

A three-dimensional finite element model of the induction welding of carbon fiber/polyphenylene sulfide thermoplastic composites is developed. The model takes into account a stainless steel mesh heating element located at the interface of the two composite adherends to be welded. This heating element serves to localize the heating where it is needed most, i.e. at the weld interface. The magnetic, electrical, and thermal properties of the carbon fiber/polyphenylene sulfide composite and other materials are identified experimentally or estimated and implemented in the model. The model predicts the temperature–time curves during the heating of the composite and is used to define processing parameters leading to high-quality welded joints. The effect of the heating element size and input current on the thermal behavior is investigated, both experimentally and using the developed model. The welds quality is assessed through microscopic observations of the weld interfaces, mechanical testing, and observations of the fracture surfaces. A comparison with two other welding processes, namely resistance welding and ultrasonic welding is finally conducted.

Abstract

One of the major constraints in welding thermoplastic and thermoset composites is thermal degradation of the thermoset resin under the high temperatures required to achieve fusion bonding of the thermoplastic resin. This paper presents a procedure to successfully prevent thermal degradation of the thermoset resin during high-temperature welding of thermoplastic to thermoplastic composites. The procedure is based on reducing the heating time to fractions of a second during the welding process. In order to achieve such short heating times, which are much too short for commercial welding techniques such as resistance or induction welding, ultrasonic welding is used in this work. A particularly challenging scenario is analysed by considering welding of carbon-fibre reinforced poly-ether-ether-ketone, with a melting temperature of 340 deg. C, to carbon-fibre reinforced epoxy with a glass transition temperature of 157 deg. C.

Abstract

Ultrasonic welding is a very fast joining technique well suited for thermoplastic composites, which does not require the use of foreign materials at the welding interface for either carbon or glass fibre-reinforced substrates. Despite very interesting investigations carried out by several researchers on different aspects of the process, ultrasonic welding of thermoplastic composite parts is not well understood yet. This article presents a deep experimental analysis of the transformations and heating mechanisms at the welding interface and their relationship with the dissipated power and the displacement of the sonotrode as provided by a microprocessor-controlled ultrasonic welder. The main aim of this research is to build up the knowledge to enable straightforward monitoring of the process and ultimately of the weld quality through the feedback provided by the ultrasonic welder.

Abstract

A process model composed of electrical and heat transfer models was developed to simulate continuous resistance welding of thermoplastic composites. Glass fabric reinforced polyphenylenesulfide welded in a lap-shear configuration with a stainless steel mesh as the heating element was considered for modelling and experimental validation of the numerical results. The welding temperatures predicted by the model showed good agreement with the experimental results. Welding input power and welding speed were found to be the two most important parameters influencing the welding temperature. The contact quality between the electrical connectors and the heating element was found to influence the distribution of the welding temperature transverse to the welding direction. Moreover, the size of the electrical connectors was found to influence the achievable welding speed and required power input for a certain welding temperature.

Abstract

Resistance welding is one of the most suitable and mature welding techniques for thermoplastic composites. It uses the heat generated at the welding interface when electric current flows through in a resistive element, frequently a metal mesh. Closed-loop resistance welding relies on indirect temperature feedback from the weld line for process control. Its implementation is more complex than the most common open-loop welding, but on the contrary it does not in principle require the definition of processing windows for each welding configuration and it allows for constant-temperature welding.

The temperature at the welding interface can be indirectly monitored through the resistance of the heating element. The relationship between resistance and temperature, expected to be approximately linear for a metal mesh heating element, can then be used to translate the welding temperature into a target resistance value for the process control routine. Despite the apparent straightforwardness of this procedure, the research results presented in this paper prove that different types of characterization tests yield different resistance versus temperature relations for a metal mesh heating element, which can lead to significant temperature deviations when used in closed-loop processes.

Abstract

The evolution of weld displacement, or the thickness of welding stack, with welding time during resistance welding of thermoplastic composites was characterised, and based on this the possibility of using displacement data for process monitoring and processing window definition was investigated. Resistance welding of glass fabric reinforced polyetherimide using a metal mesh as the heating element was studied, and weld displacement was detected using a laser sensor. The effect of welding parameters on the displacement curve was studied. Welding defects, such as voids and squeeze flow, could be detected by monitoring the weld displacement. Fast definition of the welding processing window was found to be possible using displacement curves, and the predicted processing window showed good agreement with the processing window determined from mechanical tests.

Abstract

Energy directors, responsible for local heat generation in ultrasonic welding, are neat resin protrusions traditionally moulded on the surfaces to be welded. This study evaluates an alternative energy directing solution for ultrasonic welding of thermoplastic composites based on the usage of a loose flat layer of neat resin at the welding interface, referred to as ‘flat energy director’. Analysis of dissipated power, displacement of the sonotrode, welding energy and time as well as weld strength compared to more traditional energy directing solutions showed that flat energy directors, which significantly simplify ultrasonic welding of thermoplastic composites, do not have any substantial negative impact in the welding process or the quality of the welded joints.

Abstract

Ultrasonic welding of thermoplastic composites is a very interesting joining technique as a result of good quality joints, very short welding times and the fact that no foreign material, e.g. a metal mesh, is required at the welding interface in any case. This paper describes one further advantage, the ability to relate weld strength to the welding process data, namely dissipated power and displacement of the sonotrode, in ultrasonic welding of thermoplastic composite parts with flat energy directors. This relationship, combined with displacement-controlled welding, allows for fast definition of optimum welding parameters which consistently result in high-strength welded joints.

Abstract

A model for the mechanics (oscillating deformation), heat transfer including viscoelastic heat generation and friction dissipation, and degree of adhesion (intimate contact and healing) is proposed for the initial transient heating phase.
Numerical resolution was performed using a multi-physical finite element code. The predicted dissipated power evolution exhibits a good correlation with previous experimental measurement of delivered power, and shows that the apparatus has a global efficiency of 13%. The predicted degree of adhesion also confirms the experimental observation that adhesion starts at the edge of the contact area, and progressively extends to the whole contact area.
The numerical model was further used to investigate the physical mechanisms occurring during the welding process. As suggested in the literature, the first heating mechanism is confirmed to be due to interfacial friction. Bulk viscoelastic dissipation becomes predominant when the interface reaches higher temperatures. The dissipated power is suddenly increased when the whole interface reaches the glass transition temperature.

Abstract

The strength and failure modes of resistancewelded thermoplastic composites were investigated. Special attention was paid to the effect of basic characteristics of the adherends such as fibre–matrix adhesion and fibre orientation. 8HS woven GF/PEI composites were resistance welded. Intralaminar failure was found to be the major failure mechanism for the well welded joints, consisting of either fibre–matrix debonding or laminate tearing. An improved fibre–matrix adhesion was found to result in significantly higher lap shear strength. Besides, the main apparent orientation of the fibres on the welding surfaces was found to have an effect on the strength of the joints.

Abstract

The possibility of assembling through welding is one of the major features of thermoplastic composites and it positively contributes to their cost-effectiveness in manufacturing. This paper presents a comparative evaluation of ultrasonic, induction and resistance welding of individual CF/PPS thermoplastic composite samples that comprises an analysis of the static and dynamic mechanical behaviour of the joints as well as of the main process variables. The induction welding process as used in this research benefited from the conductive nature of the reinforcing fibres. Hence no susceptor was placed at the welding interface. Resistance welding used a fine woven stainless-steel mesh as the heating element and low welding pressures and times were applied to prevent current leakage. Triangular energy directors moulded on a separate tape of PPS resin were used to concentrate ultrasonic heat at the welding interface. The static single lap shear strength of the joints was found similar for induction and ultrasonic welding. A 15% drop in the static mechanical properties of the resistance welded joints was attributed to incomplete welded overlaps following current leakage prevention. However, the fatigue performance relative to the static one was similar for the three sorts of joints. As expected, ultrasonic welding showed the shortest welding times but the highest power requirements as well. The lowest power requirements were for resistance welding, although they are directly related with the size of the area to be joined. Intermediate power, time and energy values characterised the induction welding process.

Abstract

Ultrasonic welding is considered as one of the most promising welding techniques for continuous fiber-reinforced thermoplastic composites. Intermolecular friction within the bulk, resulting fromthe application of ultrasonic waves applied on the surfaces, generates the heat required for welding to take place at the interface of the joining members via the so-called “energy directors” (EDs). Energy directors consist of resin protrusions or artificially produced asperities on the composite surfaces and play an important role both in the welding process and in the quality of the resulting welds. This paper presents the results of a study on the effects of configuration of different EDs on the ultrasonic welding of carbon fiber/polyetherimide advanced thermoplastic composites in a near-field setup. Triangular EDs were molded on the surface of consolidated composite laminates with a hot platen press. Single lap-shear-welded samples were produced to investigate the influence of the orientation of the EDs with respect to the load direction, as well as the configuration of multiple EDs. The results indicate that the configuration of multiple transverse EDs was more effective in covering the overlap area, once the resin has melted, causing only a minimum fiber disruption at the welding interface.