This paper investigates the use of multi-walled carbon nanotube/polypropylene (MWCNT/PP) films to enable ultrasonic welding and damage detection of glass fiber/polypropylene (GF/PP) composite joints ...in a single lap configuration. Three MWCNT concentrations above electrical percolation (15–25 wt%) were chosen to weld GF/PP adherends. The effect of MWCNT content on the welding process was evaluated by comparing weld quality through lap shear strength (LSS) tests and fractography analysis. Films containing 15 and 20 wt% MWCNT had no significant effect on the welding process, while 25 wt% reduced LSS by up to 39% compared to pure PP films due to their brittleness. Welded joints with MWCNT/PP films showed potential for damage detection through real-time electrical resistance changes at the interface. Thus, the multifunctional films proposed in this study represent a novel and promising approach for ultrasonic welding and damage detection of thermoplastic composite joints, but future work should focus on improving 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.
This manuscript explores the disassembly potential of ultrasonically welded thermoplastic composite joints for reuse or recycling through resistance heating via a nanocomposite film located at the ...welded interface. Nanocomposite films containing multi-walled carbon nanotubes (MWCNTs) were characterized for thermo-electrical behavior to assess self-heating. It was generally observed that maximum temperature increased with MWCNT and film thickness. To demonstrate potential for disassembly, glass fiber/polypropylene adherends were welded with nanocomposite films. Shear stress during disassembly was measured for three initial adherend’s surface temperatures. It was found that the required tensile load decreased by over 90% at the highest temperatures, effectively demonstrating the potential for disassembly via electrically conductive films. Fracture surfaces suggested that disassembly was facilitated through a combination of nanocomposite and matrix melting and weakened fiber–matrix interface. Limitations, such as slow heating rates and the loss of contact at the interface, imply that the method could be more suited for recycling, instead of repair and reuse, as the heat-affected zone extended through the adherends’ thickness at the overlap during heating.
The research 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 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 composites.
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.
As the application of composite materials in structural parts continues to increase, so does the importance to monitor the structure’s health condition during their entire service life. This paper ...presents a comprehensive review on damage monitoring methods focusing on fiber-reinforced polymer joints: adhesively bonded, mechanically fastened, and welded. In this review, structural health monitoring (SHM) methods are classified into two major groups: intrinsic sensing (embedded at the bond line) and extrinsic sensing (placed outside the interface). Main intrinsic techniques include fiber optic sensors, piezoelectric sensors (e.g., electromechanical impedance), and nanocomposite-based monitoring. The latter has attracted significant interest in the past years and is emphasized from its applications for different joining methods. For extrinsic sensing, several non-destructive testing techniques are considered to evaluate the integrity of composite joints, including acoustic emission, acousto-ultrasonic wave (e.g., guided wave), structural vibrations and acoustics, laser shock adhesion test, electromechanical impedance (e.g., piezoelectric sensors), and ultrasonic non-destructive testing (NDT). For each sensing method, damage monitoring is discussed from three aspects: i) damage type, ii) damage location, and iii) damage severity. Novel methods, significant results, current trends, and challenges are summarized. Finally, further research efforts needed in this field are recommended.
Repair is a common process to extend the service life of composite structures, and there is a need for techniques compatible with thermoplastic composites (TPCs). As the assembly of TPC joints ...through welding is gaining importance, evaluating their potential for repairability is essential. This work aims to develop novel techniques to repair welded TPC joints: 1) Successive repair (three cycles) of broken joints by ultrasonic-assisted method with neat and nanocomposite films, and 2) one-step repair using ultrasonic welding (USW) and resistance heating. It was found from tensile testing that multi-walled carbon nanotube/polypropylene (MWCNT/PP) films partially restored the lap shear strength (LSS), but pure PP films enabled significant recovery of the LSS as 94.5%, 89.4%, and 86.7% after the first, second, and third repair cycle, respectively. Nanocomposite films were however promising for monitoring the initiation and propagation of the damage occurring within the repaired joints, even after three repair cycles. Moreover, results from one-step repair using USW revealed that a stronger and more uniform repair interface was obtained, compared to resistance heating. Overall, the repair methods developed in this study exhibited promise for TPC joint repair, with strength recovery between 63.8% and 94.5%.
Continuous ultrasonic welding of plastic films, fabrics, and even thermoplastic composite prepreg tape is a 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.
Cable-driven continuum robots, which are robots with a continuously flexible backbone and no identifiable joints that are actuated by cables, have shown great potential for many applications in ...unstructured, uncertain environments. However, the standard design for a cable-driven continuum robot segment, which bends a continuous backbone along a circular arc, has many compliant modes of deformation which are uncontrolled, and which may result in buckling or other undesirable behaviors if not ameliorated. In this paper, we detail an approach for using additional cables to selectively stiffen planar cable-driven robots without substantial coupling to the actuating cables. A mechanics-based model based on the planar Cosserat equations is used to find the design conditions under which additional cables can be routed without coupling of the cable lengths for small deformations. Simulations show that even for relatively large deformations, coupling remains small. A prototype is evaluated, and it is demonstrated that the compliance of the robot is substantially modified relative to the same robot without stiffening cables. Additional stiffening cables are shown to increase the end-effector output stiffness by a factor of approximately 10 over a typical design with actuating cables.
Ultrasonic welding is a common fusion bonding technique to join unreinforced and reinforced thermoplastics. It is expected that applying ultrasonic vibrations to thermoset prepregs can produce heat ...generation to promote resin flow and consolidation. This paper discusses the feasibility of using ultrasonic vibrations as a high-speed repair technique for carbon fiber/epoxy prepregs to replace the traditional vacuum-bagging scarf setup. Three material types were investigated: out-of-autoclave unidirectional and plain weave prepregs (Cycom® 5320) and a general purpose twill weave prepreg (AS4/Newport 301). Two welding modes were considered: time and travel (vibrations stop once the desired vertical displacement is reached). For each mode, vibration time, travel, force, and amplitude were investigated. Cross-sectional analysis showed that void content equal to or below the vacuum-bagged samples could be achieved with ultrasonic consolidation to meet aerospace standards (≤2%). The following ultrasonic parameters were recommended to preserve prepreg tows integrity and minimize void content: vibration time below 1.0 s, travel between 12.5% and 50% of sample's initial thickness, force equal to or below 100 N, and amplitude below 41.3 μm. Temperature values recorded during the ultrasonic process reached the manufacturer's cure temperature range (120℃ to 180℃), with a predicted maximum degree of cure of 0.24. Interlaminar shear strength values were comparable for ultrasonically consolidated and vacuum-bagged samples. Soft and hard repair patches were applied to open-hole tensile coupons, with up to 50% strength recovery for both repair methods. Overall, ultrasonic consolidation has potential as a time- and cost-efficient repair method for thermoset prepregs.
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