Thermoplastic Composite Structures Research Articles

Fabrication and Flatwise Compression Property of Glass Fiber-Reinforced Polypropylene Corrugated Sandwich Panel

Composite sandwich structures with cellular cores have wide application in many fields such as aerospace due to their excellent properties. Thermoplastic composite structure has superior impact resistance and recycling ability compared with conventional thermosetting. The glass fiber-reinforced polypropylene corrugated sandwich panels were fabricated by hot-pressing and hot-melting methods, and the flatwise compression property was experimentally investigated. Numerical simulations by use of ABAQUS VUMAT were subsequently carried out, which captured the main experimental features. The classic buckling theory was used to establish the analytical prediction. Experimental results were used to fit the boundary condition factor between face sheet and corrugated core. The fabricated thermoplastic corrugation has competing compression strength with some metal lattice cores and outperforms the commercial aluminum foams with the same density.

Definition of process parameters for manufacturing of thermoplastic composite sandwiches – Part A

Thermoplastic composite sandwich structures offer great potential to meet the demands of lightweight structures for aeronautical applications. In this study, compression moulding of sandwich components, consisting of carbon fibre reinforced polyether ether ketone (CF/PEEK) skins and polyetherimide (PEI) core structures, is studied by modelling the effect of processing conditions on the properties of the sandwich structure, particularly the skin to core bond. In order to predict the skin-to-core tensile bond strength, a theoretical model is deduced, which is based on intimate contact and autohesion, the two mechanisms governing the fusion bonding process. The bond model allows the prediction of the tensile bond strength depending on the skin and core pre-heat temperatures and allows a prognostication about the expected failure mechanisms. According to the model, sandwiches manufactured with skin pre-heat temperatures above 290°C and a core kept at room temperature will feature a sufficient bond strength to fail cohesively within the core. In addition, the model predicts that pre-heating the core improves the bond strength. A verification of the model will be published in a follow-up paper.

Hybrid bicomponent fibres for thermoplastic composite preforms

Hybrid bicomponent fibres – materials in which reinforcement fibres are individually sheathed in thermoplastic polymer – are proposed as a novel class of preform materials for thermoplastic composites. We assert that by reducing the scale of hybridization between the reinforcement fibres and the matrix polymer to the level of the fibre, a thermoplastic intermediate material with both high drapeability and short consolidation times can be developed. The manufacture of hybrid bicomponent fibres is demonstrated in a scalable, coating process in which glass fibres are combined with several thermoplastic polymer matrix systems in dimensions and proportions suitable for use in thermoplastic composite structures. This novel class of thermoplastic composite preforms is expected to expedite the high volume production of geometrically complex thermoplastic composite parts.

Definition of process parameters for manufacturing of thermoplastic composite sandwiches – Part B

Thermoplastic composite (TPC) sandwich structures offer great potential to meet the demands of lightweight structures for aeronautical applications. In this study, compression moulding of sandwich components, consisting of carbon fibre-reinforced polyetheretherketone (CF/PEEK) skins and polyetherimide (PEI) cores, is studied with the aim of verifying a previously proposed model. This model predicts the influence of processing conditions on the properties of the sandwich structure, particularly the skin to core bond. Sandwich specimens are manufactured according to the parameters resulting from the model, in particular to verify skin and core pre-heat temperatures. The resulting bond quality is characterized by flatwise plane tensile testing and compared to the predicted values. This comparison shows a reasonable good agreement between predicted and experimentally obtained tensile strengths, which mainly depend on the skin pre-heat temperature. In addition, the specimens mostly fail as predicted by the model. While the model predicts that pre-heating the core leads to an improvement of the bond strength, the experiments show that core pre-heating leads often to core collapsing, which was not considered in the model. Therefore, it is proposed to complement the modelling approach with an analysis of the heat flow into core.

Mechanical performance of CF/PEEK–PEI foam core sandwich structures

Previous work showed that thermoplastic composite sandwich structures offer great potential to meet the demands of lightweight structures for aviation applications. In this study, the influence of several processing parameters on the mechanical properties of thermoplastic sandwich components, consisting of carbon fibre reinforced polyetheretherketone skins and polyetherimide foam cores, is characterised. Sandwich specimens are manufactured with varying skin temperatures, core compaction distances and different polyetherimide concentrations at the skin–core interface. Following, sandwich samples are mechanically tested to characterise the bond strength, the core performance as well as the performance of the whole sandwich. The results show that in most cases the processing parameters significantly affect the cell structure of the sandwich core, provided that a proper fusion bond between skins and core exists. Thereby, the core performance seems to be weakened and failure predominantly occurs in the transition between affected and original cell structures.

In-Line Integration of Sensors in Thermoplastic Composite Structures Using Novel Continuous Orbital Winding Technology

Currently there is a great demand for energy and resource efficient and also function integrating manufacturing processes. Therefore, suitable technologies and corresponding foundational researches are being pursued in the federal cluster of excellence “MERGE Technologies for Multifunctional Lightweight Structures” at the Technische Universität Chemnitz. A part of this project is the development of the continuous orbital winding (COW) technology including the goal of a large-scale process used for special fiber-reinforced thermoplastic semi-finished products. This method is an inverted winding process. The winding core needs to perform only the feed motion. Furthermore, this allows synchronization to upstream and downstream process chains.Due to the modular structure of the machine concept, it is possible to integrate a sensor system during production without interrupting the process. For this purpose, a textile carrier tape with integrated electrically conductive fibers and applied sensors is embedded. Various silicon sensors, e.g. acceleration, pressure or stress sensors are applied by micro-injection molding. A so-called “interposer” is used as an electrically adapter between the microstructures of the sensor system and the mesostructures of the textile.In this article, basic investigations for the continuous processing of semi-finished thermoplastic structures and the integration of sensors are presented. It is intended to determine the bonding properties, possible structural thickening by the sensors and the resistance of the sensor systems and its electronic components to the process conditions.In summary, investigations are carried out to determine the parameters of the machine system as well as to determine the optimum processing conditions for the application of additional elements.

Experimental Investigations on Integrated Conductor Paths in Fiber-Reinforced Thermoplastic Composite Structures

By the benefit of functional integration the advantages of fiber reinforced plastics (FRP) as construction material can be increased due to the possibilities of integrating sensors and actuators. In Regard to the layer-by-layer definition of the wall thickness, this class of material offers a high potential for the integration of additional smart elements within the stacking and forming process. In addition to the actual integration methods of sensors or actuators, the electrical signal transmission and contacting is of great importance for smart structures. Various approaches can be followed. On the one hand, the conductor path can be defined by means of a wire and, on the other hand, the definition of conductor paths can be accomplished by functionalized films (by means of printing technology). Within this paper, experimental investigations are intended to demonstrate the suitability of screen-printed conductor paths for the press-technical transformation of FRP structures. In addition to the variation of the screen printing material and the film material, for a material-homogeneous integration, an evaluation of a corresponding selection of materials takes place with respect to the stresses derived from the deformation-technical boundary conditions.

Modified foam cores for full thermoplastic composite sandwich structures

Full thermoplastic composite sandwich structures with a foam core offer the possibility to be manufactured by fusion bonding in significant shorter cycle times than thermoset-based sandwiches. However, the application of foam cores results in lower mechanical properties such as compression and shear strength compared to honeycomb cores, therefore foam-based sandwiches cannot compete with sandwich structures based on Aramid/phenolic honeycomb cores, the current state of the art. In order to improve the mechanical performance of foam core-based sandwiches while maintaining their advantages, concepts to reinforce the foams were developed in this study. By introducing rods either orthogonally or diagonally to the skin plane, which are fusion bonded to the skins during processing, the compression and shear properties can be improved by up to 1000% and 72%, respectively. Even when correcting for the weight increase, an improved specific compression strength could be achieved. And therefore, the pinning looks especially promising when only applied locally in highly loaded areas for example.

Experimental assessment of the influence of welding process parameters on Lamb wave transmission across ultrasonically welded thermoplastic composite joints

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, manufacturing-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.

Long-fiber reinforced thermoplastic composite lattice structures: Fabrication and compressive properties

Recyclable lightweight materials with advanced processing techniques are essential for the sustainable development of future transportation. Thermoplastic composites lattice structures were developed to meet this demand. An additive manufacturing method is presented here to fabricate such lattice structures by reversible assembly of several long-fiber reinforced thermoplastic composite parts (LFTCPs) which were economically processed by injection molding. The resulting thermoplastic lattice structures (density of 30kg·m−3) assembled with different sequences and connections are structurally evaluated and compared. Out-of-plane compression tests revealed that their mechanical properties were more sensitive to the presence of the connections rather than their assembly sequence, although their structural failure mode was always brake of inclined struts followed by fracture of the horizontal struts. Potential solutions to the problem of internal stresses, induced during assembly, are also explored by designing novel LFTCPs. The novel fabrication route for thermoplastic lattice structures will improve the prospects for their industrial application.

Thermoplastic composite beam structures from mixtures of recycled HDPE and rubber crumb for acoustic energy absorption

The use of recycled rubber crumb in the design and production of thermoplastic-rubber composites as sound absorbers can provide solutions to noise pollution and for the recovery of post-consumer materials from both packaging and waste tyres. The work of this study is concerned with the effect of rubber crumb incorporation in high-density polyethylene (HDPE) and also in HDPE glass fibre composites on acoustic, mechanical and physical properties. Recycled HDPE compounds containing variable concentrations of cured rubber crumb particles were prepared by twin screw extrusion. Thermal analysis has revealed a significant increase in the level of crystallinity of the HDPE component by increasing the rubber content in the mixes. Standard three-point bending and notched impact test specimens were manufactured by injection moulding, and large-scale beam samples were produced by compression moulding using an ad hoc method that allows variation of the through-thickness elastomer content as a means of obtaining composition gradients. The flexural modulus and impact strength varied monotonically with rubber crumb concentration. A fast Fourier transform technique was used to determine the acoustic performance of the beams over a wide frequency range. The graded structures produced large improvements in acoustic absorption properties in the frequency range of 2–6 kHz, notably from composite beams containing 20% rubber and also in some multilayer beams with rubber concentration gradients.

Manufacturing of thermoplastic composite sandwich structures

Composite sandwich structures show promising lightweight properties for the aviation industry. Nowadays time-consuming manufacturing methods still prevent an extensive application of composite sandwiches, which can be overcome by the use of thermoplastic polymers in skins and core. During manufacturing of thermoplastic composite (TPC) sandwich structures, the joining of skins and core is a critical step. Therefore, several skin–core joining methods have been under investigation and development in the published literature, which can be categorized into adhesive bonding or fusion bonding. Fusion bonding by means of vacuum moulding, compression moulding or in situ foaming shows great potential for joining sandwich skins and core. Although various phenomena such as core collapsing or skin deconsolidation challenge the processes. This article aims to present an overview of research that has been done in the area of manufacturing TPC sandwich structures and will serve as a baseline and aid for further research and development efforts.

The role of material characterisation in the crush modelling of thermoplastic composite structures

The predictive capability of high fidelity finite element modelling, to accurately capture damage and crush behaviour of composite structures, relies on the acquisition of accurate material properties, some of which have necessitated the development of novel approaches. This paper details the measurement of interlaminar and intralaminar fracture toughness, the non-linear shear behaviour of carbon fibre (AS4)/thermoplastic Polyetherketoneketone (PEKK) composite laminates and the utilisation of these properties for the accurate computational modelling of crush. Double-cantilever-beam (DCB), four-point end-notched flexure (4ENF) and Mixed-mode bending (MMB) test configurations were used to determine the initiation and propagation fracture toughness in mode I, mode II and mixed-mode loading, respectively. Compact Tension (CT) and Compact Compression (CC) test samples were employed to determine the intralaminar longitudinal tensile and compressive fracture toughness. V-notched rail shear tests were used to measure the highly non-linear shear behaviour, associated with thermoplastic composites, and fracture toughness. Corresponding numerical models of these tests were developed for verification and yielded good correlation with the experimental response. This also confirmed the accuracy of the measured values which were then employed as input material parameters for modelling the crush behaviour of a corrugated test specimen.

Advanced laser welding of high-performance thermoplastic composites

Thermoplastic composite structures based on continuous carbon fiber reinforcements are gaining importance in many industrial applications. These comprise basic technical functions as well as high-performance applications in the aerospace sector. Welding techniques are applicable for composites based on thermoplastic matrix materials in contrast to thermoset systems. In this context, welding steps are not limited to the joining of carbon fiber reinforced plastic (CFRP) parts among themselves, but extend to the connection of various components, consisting of unreinforced and glass fiber reinforced thermoplastics to CFRP components. In this work, a laser transmission welding process is evaluated with respect to the influence of the carbon fiber reinforcement within the laser absorbing part as well as the glass fiber reinforcement within the laser transparent part on the weld seam formation. Thermoplastic base material nylon (PA 6.6) and polyphenylene sulfide are used. By applying two different strategies, contour and quasisimultaneous welding, the influence of continuous fiber reinforced composites on the welding process is studied. Significant differences to the process characteristics known from the joining of unreinforced thermoplastics emerge from the fiber reinforcement inducing high thermal conductivity and fluctuating absorption properties for the laser wavelength, resulting in an essentially altered plastification performance which is directly mirrored in the inhomogeneous formation of the weld seam structure.

Characteristics of carbon fiber based strain sensors for structural-health monitoring of textile-reinforced thermoplastic composites depending on the textile technological integration process

The paper presents an effective online measurement technology for non-destructive and continuous monitoring of endless glass fiber reinforced thermoplastic (FRTP) composite structures using the numerous possibilities of textile process technologies for the design and one-step integration of one- and two-dimensional sensor structures based on piezo-resistive exPAN carbon filament yarn (CFY) in textile reinforced structures.The focus of this paper is directed at the analysis of such textile technological integrated sensors as an integral part of the textile reinforcement and their usability for in situ monitoring of local and/or global mechanical loading conditions in difficult or inaccessible areas within short distance of the structural load-bearing layers of FRTP's. If the adhesion of the carbon filament yarn sensor (CFYS) to the surrounding FRTP is sufficient enough, a change in geometry and thereby in resistivity of the carbon filaments can be measured by any mechanical load of the sensor-carrying FRTP component.The advantages and disadvantages of the tailored fiber placement (TFP), flat knitting and weaving textile processes for creating sensor structures will be shown, wherein the mentioned variety of interlacing techniques are used.The effects of the used textile manufacturing processes on the sensory properties are being compared. Furthermore, the effects of the integrated CFY on the mechanical properties of the basic structure are quantified. Sensor structures with minimal influence on the composite structure can be derived from it as well as sensors that enable a selective reinforcement of the composite structure. The textile integration provides a routing of one CFY through several layers of reinforcement. Thus, a damage-free and reproducible production of more complex, 3-dimensional, textile-based sensor networks is feasible in one step.

Research on Forming Technology of Thermoplastic Composite Blade for Wind Turbine

Thermoplastic composite has become preferred material for wind turbine blade with high performance, low cost and greenization. The fused mass of the thermoplastic resin has high viscosity and the forming of the thermoplastic composite materials is laborious, quality is not readily guaranteed, thus, the widespread use of thermoplastic composite blades for wind turbine is restricted. Based on the analysis of all kinds of the characteristics of thermoplastic forming technology, the paper has points out that the diaphragm forming is especially suitable for making a hyperboloid, variable thickness, large size wind turbine thermoplastic composite blade structure. The key to improving the forming quality and the efficiency of the thermoplastic blade forming is to establish finite element deformation model of a diaphragm forming process, to effectively control the process parameters such as temperature, pressure, forming rate. Conducting research on thermoplastic blade diaphragm forming technology lay the foundation for the industrialization of thermoplastic wind turbine blade.

Process Chain Modelling and Analysis for the High-Volume Production of Thermoplastic Composites with Embedded Piezoceramic Modules

Active composite structures based on thermoplastic matrix systems are highly suited to applications in lightweight structures ready for series production. The integration of additional functional components such as material-embedded piezoceramic actuators and sensors and an electronic network facilitates the targeted control and manipulation of structural behaviour. The current delay in the widespread application of such adaptive structures is primarily attributable to a lack of appropriate manufacturing technologies. It is against this backdrop that this paper contributes to the development of a novel manufacturing process chain characterized by robustness and efficiency and based on hot-pressing techniques tailored to specific materials and actuators. Special consideration is given to detailed process chain modelling and analysis focusing on interactions between technical and technological aspects. The development of a continuous process chain by means of the analysis of parameter influences is described. In conclusion, the use of parameter manipulation to successfully realize a unique manufacturing line designed for the high-volume production of adaptive thermoplastic composite structures is demonstrated.

Optimization of thermoplastic composites resistance welding parameters based on transient heat transfer finite element modeling

The use of resistance welding technology to join thermoplastic composite aerospace structures is still contingent upon a better understanding of the heat transfer mechanisms occurring during welding, which govern the joint quality and mechanical performance. In this study, two-dimensional (2D) and three-dimensional (3D) transient heat transfer finite element models were developed to simulate resistance welding of thermoplastic composites. The 2D model was used to investigate the effect of the length of the exposed areas of the heating element to air (clamping distance) on the local overheating at the edges and the effects of the input power level on the thermal behavior of the welds. It is shown that controlling the clamping distance improves the thermal uniformity of the weld. The 3D model shows that heat conduction along the length of the laminates influences the thermal uniformity of the weld interface. An optimization chart is developed in order to minimize the undesirable edge effect and to define the conditions required to obtain a complete weld. The results of the 3D model are compared with experimental data.

Consolidation behaviour during manufacture of adapted piezoceramic modules for functional integrated thermoplastic composite structures

Abstract For the series production of adaptive fibre-reinforced thermoplastic structures, the development of process-adapted piezoceramic modules is gaining central importance. Thermoplastic-compatible piezoceramic modules are being developed which are suitable for a matrix-homogeneous adhesive-free integration of the modules in fibre-reinforced thermoplastic structures during a simultaneous welding process. Aiming at the continuous improvement of the novel module design and its manufacturing technology based on a hot-pressing technology, an adapted physical nonlinear simulation model was developed in order to analyze the material and process dependent function and consolidation behaviour. Special regard is given to residual stresses caused by different coefficients of thermal expansion of the component materials and their specific use to enhance the module performance.

Numerical Simulation of Thermoplastic Composite Material by ANSYS

This paper proposes a material model of AS4/PEEK, a typical thermoplastic composite material, for the general purpose finite element code—ANSYS, which can be used to predict the mechanical behavior of AS4/PEEK composite structures. The computational result using this model has a good agreement with the test result. This investigation can lay the foundation for the numerical simulation of thermoplastic composite structures.