Thermoplastic Composite Structures Research Articles

Multi-scale finite element analysis of laser-assisted forming of CF/PEEK composite corner structures

Laser-assisted forming can significantly increase the forming efficiency of thermoplastic composites. However, when forming CF/PEEK curved corner structures using laser-assisted forming, bending spring-back generated by the prepreg tape constrains the shape accuracy of the component. The bending deformation of unidirectional resin matrix composite materials is highly nonlinear. In this study, a multi-scale finite element method is used to obtain the bending properties of CF/PEEK composites for bending spring-back phenomenon of thermoplastic composite samples and predict the spring-back angle of CF/PEEK corner structures fabricated via laser-assisted forming. The average material properties were first calculated using RVE. Following this, a mesoscale model was developed to analyze kink deformation of the composite. Finally, the macroscopic prepreg bending spring-back was calculated. It was determined that the main mechanism of corner forming corresponds to the occurrence of a fiber kinking phenomenon on the interior of the corner. Furthermore, spring-back deformation is caused by residual stresses resulting from insufficient fiber kinking. The accuracy of the simulation results is verified via corner forming experiments. This study contributes to a deeper understanding of the spring-back mechanisms of laser-assisted forming of corner structures and provides a theoretical guidance for precision forming of multilayer thermoplastic composite bent structures.

Exploring ultrasonic and resistance welding for thermoplastic composite structures: Process development and application potential

Thermoplastic composites outperform in aerospace applications compared to thermoset composites due to their weldability, a key advantage. This feature streamlines manufacturing, allowing for precise assembly of complex shapes, reducing costs, and accelerating production. Additionally, it simplifies maintenance, enabling easy repair or replacement of damaged parts, thereby extending vehicle lifespan and minimizing downtime. With exceptional strength-to-weight ratio and resistance to corrosion and fatigue, weldable thermoplastic composites ensure reliable performance in the demanding aerospace environment. Overall, their weldability enhances efficiency, versatility, and safety in aerospace applications. This paper first presents an overview of various welding processes and discusses the special challenges of assembly processes in the aviation industry. This work then focuses from the authors’ perspective with regard to industrializability on the two most promising processes, resistance and ultrasonic welding.

The technology and current applications of continuous fiber‐reinforced thermoplastic composites

AbstractBecause of its inherent properties, the use of thermoplastic composites is expanding in a number of industries, including aerospace, rail transportation, and wind power generation. However, the development of 2D and 3D reinforced thermoplastic composite structures is hampered by the high viscosity and poor wetting of thermoplastic matrices, resulting in their application being mainly limited to nonload‐bearing components. This article provides a comprehensive overview of the technology and current applications of thermoplastic composites, covering materials, preform structures, manufacturing methods, process parameters, performance, and applications. It also provides an outlook on their future development.Highlights Thermoplastic composite has economic and environmental benefits. Summarized the current technology and application of thermoplastic composites. The manufacturing difficulty is the thermoplastic polymers wet fibers. One promising approach to the issue is prepreg technology. Prepreg technology make 3D thermoplastic composites become possible.

Effect of prefabricating process on remelting interfacial properties for thermoplastic composite stiffened structures: Molecular dynamics simulation and experimental verification

The forming interface is the weak link of complex thermoplastic composite structures. Aiming at the bottleneck problem of “weak bonding” at the remelting interface for the “Prefabricating + Remelting bonding” forming process of stiffened structures, through a combination of molecular dynamics (MD) simulation and experimental verification, a study was conducted on the influence of different prefabricating systems on the remelting bonding performance in the subsequent process. The results showed that prolonged high-temperature melting would lead to resin crosslinking reactions in the prefabricating stage and affect the remelting interface's molecular diffusion and penetration. Establishing a time-temperature condition function for the resin not to undergo crosslinking reaction during the prefabricating stage, the negative impact of the prefabricating process on the remelting bonding quality was reduced effectively, which provides theoretical and technical support for the high-quality manufacturing of complex thermoplastic composite structures.

Effect of polymer crosslinking to the failure behavior of thermoplastic composite remelting interface: Molecular dynamics simulation and experimental verification

AbstractThe forming interface is the weak link of thermoplastic composite complex structures. To explore the effect of polymer crosslinking on the failure behavior of thermoplastic composite remelting interface using “Preforming + Remelting bonding” process, the all‐atom models of different types of interfaces were established, and a molecular dynamics (MD) simulation was conducted on the uniaxial tensile failure process of the remelting interface. The failure behavior of the remelting interface with different preforming processes was tested to verify the correctness of MD simulation, and the influencing mechanism of polymer crosslinking reactions in the preforming stage to the remelting interface properties was clarified. The results showed that the yield stress of the uncrosslinked interface reaches 283.2 MPa and 16.0% and 50.6% higher than that of the unilateral and bilateral crosslinked interfaces respectively. The polymer crosslinking reaction during the preforming stage will reduce the interlaminar bonding performance. The interlaminar shear strength (ILSS) of the uncrosslinked interface is 87.46 MPa and 7.21% and 27.36% higher than that of the unilateral and bilateral crosslinked interface respectively, and only 3.95% lower than that of the non‐remelting sample formed by standard process. This research provides a new approach for predicting and analyzing the effect of polymer crosslinking on remelting interface properties of thermoplastic composite using “Preforming + Remelting bonding” process.Highlights The failure behavior of different thermoplastic composite remelting interfaces was studied. MD simulation was conducted on the uniaxial tensile failure process of the remelting interface. The influencing mechanism of polymer crosslinking reactions in the preforming stage to the remelting interface properties was clarified. It is effective to explore the failure mechanism of thermoplastic remelting interface by MD simulation.

Fabrication and mechanical properties of thermoplastic corrugated sandwich panel with enhanced face‐core interface

AbstractThermoplastic sandwich structures are widely utilized in engineering fields due to their high specific strength and stiffness, recyclability, and impact toughness. The face‐core interface strength is crucial to maximize the performance benefits of sandwich structures. Two sizes of corrugated sandwich panels with hot‐melt bonding and resistance welding connection methods were fabricated. The mechanical properties of sandwich panels were compared using a combination of experimental, numerical, and analytical investigation. Edgewise compression (EC) and 3‐point bending (3 PB) tests were conducted to obtain load–displacement curves and investigate different failure modes of sandwich panels. The peak edge compression and bending loads of the resistance welded structures were 42% and 92% higher, respectively, and the face‐core interface was always well‐connected compared to those of the hot‐melt bonding. The results indicate that the resistance welding method can be utilized as a core bonding technique for thermoplastic composite sandwich structures and has great potential for aerospace application.Highlights A method that includes hot‐melt bonding and resistance welding are develop to produce thermoplastic composite corrugated sandwich panels (TPC‐CSPs). The TPC‐CSPs fabricated by resistance welding exhibit superior interface connection performance compared to hot‐melt bonding and this method enhance the structural load bearing capacity. The numerical and analytical method were presented to investigate the failure model, which was consistent with the experimental results.

Innovative welding integration of acousto-ultrasonic composite transducers onto thermoplastic composite structures

Acousto-ultrasonic composite transducers (AUCTs), comprising piezoceramic materials in a reinforced polymeric matrix, show promise for structural health monitoring in composite structures. Challenges arise when integrating AUCTs onto highly loaded thermoplastic composites, especially low-surface-energy materials like polyaryletherketone composites. To address this, the study explores the viability of attaching AUCTs to low-melting polyaryletherketone carbon fiber-reinforced thermoplastic composite structures using ultrasonic welding. This welding technique forms a joint where the interface material fuses with the AUCT embedment and the structure matrix, providing a reliable and automatable process. The investigation includes a comparative analysis of an ultrasonic welded joint with an external energy director and a reference AUCT system integrated using a vacuum bagging oven procedure. Results highlight the potential of AUCT configurations integrated by ultrasonic welding as an alternative solution, acknowledging challenges that persist for further development and increased reliability in structural health monitoring applications.

Press Conduction Welding for Secondary Bonding of Aircraft Skin/Stiffener Assemblies Using Carbon Fiber/PEKK Thermoplastic Composites and PEI Adhesive.

This study investigates the secondary bonding of aircraft skin/stiffener assemblies using press conduction welding with carbon fiber/polyetherketoneketone thermoplastic composites and polyetherimide adhesive. Recognizing the challenges posed by conventional welding methods in maintaining material integrity and uniformity, this research explores an alternative methodology that mitigates these issues while ensuring high-strength bonds. The press conduction welding parameters were selected based on single-lap shear tests and applied in the bonding of skin and omega stiffener components. The temperature range was determined using differential scanning calorimetry. The pressure was held at 1 MPa for 180 s. The welding temperature that produced a high-bonding strength was identified experimentally; these key variables were then used in the welding process of the skin and omega stiffener. By analyzing how the fibers tear and the effectiveness of interdiffusion between the plies, we were able to gain insights into the bonding strength and fractured surface. The findings suggest that press conduction welding provides a viable route for secondary bonding in thermoplastic composite structures, highlighting its advantages in terms of processing efficiency and integrity. This study contributes to the understanding of the mechanical behaviors of bonded joints and underscores the significance of temperature control in the welding process.

Arkema and Hexcel complete the first aeronautical structure made of thermoplastic composites

On March 7, 2024, Arkema announced it had completed the first aeronautical structure made of thermoplastic composites. This was a result from the strategic partnership between Hexcel and Arkema. The high-performance thermoplastic composite structure was designed and manufactured using HexPly® thermoplastic tapes. These aerospace-grade materials are developed from Arkema's Kepstan® PEKK resin and Hexcel HexTow® AS7 and IM7 carbon fibers. The demonstrator project was carried out as part of the collaborative project known as HAICoPAS.

DOD awards $3.7 million to enhance advanced thermoplastics production

On November 27, 2023, the Department of Defense announced the award of $3.7 million via the Industrial Base Analysis and Sustainment (IBAS) program to Qarbon Aerospace to design and manufacture advanced lightweight continuous fiber thermoplastic composite structures for national defense aviation applications.

Joint Performance of a Continuous Glass Fiber/Polypropylene Composite.

Thermoplastic composite structures possess superior properties compared with thermosetting composites, including recyclability and high damage tolerance. However, the poor adhesion properties of thermoplastic composites make their joining process challenging. In this research, three bonding techniques, namely adhesive, mechanical joining, and hybrid bonding, are investigated using lap shear specimens to evaluate their mechanical properties and failure modes. The stress distributions at the joints of the three bonding techniques are analyzed by numerical simulation. The findings demonstrate that hybrid bonding enhances the strength of composite joints, albeit at the expense of some stiffness due to the presence of an open hole. This method is particularly suitable for applications that necessitate robust connections requiring high strength.

Dynamic response optimization of a thermoplastic composite sandwich beam under random vibration

The dynamic response of a thermoplastic composite sandwich structure is optimized under random vibration. First, the experimental modal analysis data of a set of test samples are processed by a sequential set of statistical analysis such as descriptive statistics, factor analysis, and paired sample t-test. Then, the sample with the highest ability to represent the group is taken as the reference data. Three different computational models, which are defined according to whether the solid to be meshed is considered an area or a volume, are constructed. Modal analysis results of the computational models are compared to the reference experimental data to evaluate the performance of the models. To predict the dynamic response of the sandwich beam, it is excited through a random signal in the transverse direction. The nodal acceleration responses are computed in 17 evenly spaced points located on the upper finishing layer of the sandwich beam. Finally, a geometry optimization study is conducted to predict the optimum thicknesses of the 7 layers bonded together to form the sandwich beam. The optimum layer thicknesses that minimize the nodal accelerations at 17 evenly spaced points on the sandwich beam are computed. The current study shows that the shell model has the closest values to the experimental data compared to other models. As far as the dynamic response of a TPC sandwich structure is concerned, it is concluded that the shell model better represents the structure during the modeling phase and leads to concurrently reduced weight and nodal acceleration, when optimized.

Vacuum-bag-only consolidation of C/PEKK fiber placed preforms with engineered gas evacuation channels

Vacuum-bag-only (VBO) consolidation of automated fiber placed (AFP) thermoplastic composites can be a cost-effective processing route for large and complex thermoplastic composite structures. This paper explores the possibility of two-step AFP and VBO consolidation of advanced thermoplastic composites. Two commercially available carbon fiber reinforced Poly-Ether-Ketone-Ketone (C/PEKK) thermoplastic tapes were evaluated. Different process parameters were needed for the two material systems. Further, for the first time, an attempt was made to create in-plane air evacuation channels by deliberately introducing gaps between the tapes in the preforms during the placement. In the case of one material system, the VBO consolidation of preforms with and without gaps yielded good consolidation quality with less than 1% voids. In the other case, VBO consolidation of preforms with gaps showed a improved consolidation quality with less than 1% voids than the preform without gaps, indicating that the gaps possibly help accelerate the gas evacuation process.

Consolidation of aerospace-grade high-temperature thermoplastic carbon fiber composites via nano-engineered electrothermal heating

A carbon nanotube (CNT) electrothermal heater is shown to enable consolidation of high-temperature thermoplastic composite laminates for the first time, leveraging the CNT film's stable operation in air above 500 °C. This out-of-oven (OoO) conductive heating method without applied pressure (i.e., vacuum bag only) is used to process aerospace-grade carbon fiber/polyether ether ketone (PEEK) composite prepreg, which requires heating to 380–400 °C and is normally processed with significant applied pressure (∼2 ± 1 MPa) via a hot-press or autoclave. Micro-computed tomography verifies that the processed laminates exhibit low porosity of ≤0.1 vol%, which more than meets the desired target of <1 vol% voids. A range of thermophysical (including degree of crystallinity) and mechanical (interfacial shear strength and bending) tests show equal or improved properties vs. both hot press and autoclave processing. The CNT heaters in the OoO approach are observed to most closely match the desired processing cycle, with the observed benefit of being the only process studied to hit the target range of % crystallinity. The energy consumption comparison indicates a 98% energy saving of the CNT heater compared to the hot press. This study demonstrates autoclave-quality processing of high-temperature thermoplastic composite structures via conformal CNT heaters that can be fabricated efficiently and effectively in an out-of-autoclave (OoA) process.

An Efficient Procedure for Bonding Piezoelectric Transducers to Thermoplastic Composite Structures for SHM Application and Its Durability in Aeronautical Environmental Conditions

Piezoceramic transducers (PCTs) bonded to carbon fiber-reinforced plastic (CFRP) composite structures must be durable as well as remain properly bonded to the structure in order to provide reliable data for accurate guided-wave-based structural health monitoring (SHM) of aeronautical components. The current method of bonding transducers to composite structures through epoxy adhesives faces some shortcomings, such as difficult reparability, lack of weldability, longer curing cycles, and shorter shelf life. To overcome these shortcomings, a new efficient procedure for bonding the transducers to thermoplastic (TP) composite structures was developed by utilizing TP adhesive films. Application-suitable TP films (TPFs) were identified and characterized through standard differential scanning calorimetry (DSC) and single lap shear (SLS) tests to study their melting behavior and bonding strength, respectively. Special PCTs called acousto-ultrasonic composite transducers (AUCTs) were bonded to high-performance TP composites (carbon fiber Poly-Ether-Ether-Ketone) coupons with a reference adhesive (Loctite EA 9695) and the selected TPFs. The integrity and durability of the bonded AUCTs in aeronautical operational environmental conditions (AOEC) were assessed in accordance to the standard Radio Technical Commission for Aeronautics DO-160. The AOEC tests performed were operating low and high temperatures, thermal cycling, hot-wet, and fluid susceptibility tests. The health and bonding quality of the AUCTs were evaluated by the electro-mechanical impedance (EMI) spectroscopy method and ultrasonic inspections. The AUCT defects were created artificially and their influence on the susceptance spectra (SS) was measured to compare them with the AOEC-tested AUCTs. The results show that a small change occurred in the SS characteristics of the bonded AUCTs in all of the adhesive cases after the AOEC tests. After comparing the changes in SS characteristics of simulated defects with that of the AOEC-tested AUCTs, the change is relatively smaller and therefore it can be concluded that no serious degradation of the AUCT or the adhesive layer has occurred. It was observed that the most critical tests among the AOEC tests are the fluid susceptibility tests, which can cause the biggest change in the SS characteristics. Comparing the performance of the AUCTs bonded with the reference adhesive and the selected TPFs in the AOEC tests, it was seen that some of the TPFs, e.g., Pontacol 22.100 outperforms the reference adhesive, while the other TPFs have similar performance to that of the reference adhesive. Therefore, in conclusion, the AUCTs bonded with the selected TPFs can withstand the operational and environmental conditions of an aircraft structure, and hence, the proposed procedure is easily installed, reparable, and a more reliable method of bonding sensors to aircraft structures.

On the use of a rounded sonotrode for the welding of thermoplastic composites

Continuous ultrasonic welding is an attractive welding technique for thermoplastic composite structures. In this process, a metallic sonotrode connected to a piezoelectric transducer and to a press moves along the parts to be welded applying ultrasonic vibrations and a static welding force on the welding overlap. Thus far, the research carried out on this topic makes use of sonotrodes featuring a flat contact surface with the parts to be welded, which limits the use of the process to the welding of overlaps with no curvature in the welding direction. With the final aim of assessing whether this process can also be applied to curved structures, this paper explores the feasibility of using a rounded sonotrode for continuous ultrasonic welding of thermoplastic composites. The main conclusions drawn from the results obtained in this research is that it is indeed possible to continuously weld thermoplastic composite panels with a rounded sonotrode and that high-quality welds can be obtained from such a process. Furthermore, the use of a rounded sonotrode has the positive effect of lowering the temperatures at the welding interface as well as the temperatures within the adherends. On the other hand, the use of such sonotrode leads to a decreased, although still competitive, welding speed and, potentially, an increased welding force, thereby setting boundary conditions that need to be considered for each specific application.

On-line monitoring of multi-parameters in thermoplastic composite structure forming based on fiber grating sensor

On-line monitoring of multi-parameters in thermoplastic composite structure forming based on fiber grating sensor

Evaluation of energy absorption enhancement of additively manufactured polymer composite lattice structures

The unique compressive behaviour of lattice cubic structures as well as their high specific strength and significant energy absorbing characteristics makes them an attractive solution for crashworthiness applications. Hence in this research study, the crashworthiness behaviour and energy absorbing characteristics of the thermoplastic polymer composite lattice cubic structures were experimentally investigated under quasi-static compression. Four design patterns such as Cuboctahedron, Kelvin cell, Truncated cube in square and dividend square geometrics were considered and fabricated through fused deposition modelling technique. The proposed structures were additively manufactured with four different thermoplastic polymer based filament materials and their influence on the crashworthiness characteristics were investigated experimentally. The obtained results revealed that the PLA-CF based KC configuration exhibited SEA of 2.50 kJ g−1 and the maximum value of CFE is 84.91% for PETG-CF based KC configuration. Furthermore, the experimental results indicated that the proposed thermoplastic polymer composite based lattice cubic structures are potentially a suitable component for crashworthiness applications owing to their significant energy absorption ability.

Resistance welding analysis of thermoplastic composite structures in aeronautical applications

Resistance welding analysis of thermoplastic composite structures in aeronautical applications

Drop-weight impact responses and energy absorption of lightweight glass fiber reinforced polypropylene composite hierarchical cylindrical structures

A high spike force at the beginning of impact and the load fluctuations caused by unstable plateau deformation are disadvantages to avoid for impact resistance structures. Lightweight glass fiber-reinforced polypropylene composite hierarchical cylindrical structures (namely GFRP-CHCS, with a relative density of less than 0.1) have the potential to achieve low initial force and high crushing force efficiency under dynamic conditions. This work explores the drop-weight impact behaviors and energy absorption capacity of GFRP-CHCS. Deformation modes, failure conditions, and impact load history responses are discussed. Results showed that GFRP-CHCS has no initial load peak and achieves gentle response curves under the low-velocity impact when the load speed is less than 10 m/s. Among the experiments where the impact energy was beyond the range of static energy absorption, more than 94% of the impact energy was absorbed or dissipated when the impact energy ranged from 200 J to 300 J. After unloaded, GFRP-CHCS maintained good integrality and recovered a significant deformation which was over 93% initial structural height. With ideal isotropic material properties, numerical and theoretical predictions were conducted regarding the loading velocity effects on structural deformation mode and mean impact response force. This work utilized a thermoplastic composite consisting of GFRP face sheets and polypropylene core. The results have reference significance for the anti-impact design of thermoplastic composite hierarchical structures.