Thermoplastic Structures Research Articles

On the random vibration-based progressive fatigue damage detection and classification for thermoplastic coupons under population and operational uncertainty

The problem of vibration-based progressive fatigue damage detection and Fatigue State (FS) determination for thermoplastic coupons is experimentally addressed under significant population and experimental uncertainty. The study involves 13 coupons subjected to tension–tension fatigue testing with interruptions at 10,000-cycle intervals. Utilizing non-parametric random vibration analysis and a robust, uncertainty-tolerant methodology, the research employs Multiple Model Representations of uncertain dynamics through parametric ARMA(AutoRegressive-Moving Average)-type random vibration response modeling. Results, validated by ultrasonic C-Scan testing, reveal (a) the significance of uncertainties, (b) a resonant structural frequency indicating fatigue accumulation, (c) remarkable detection performance, achieving 100 % accuracy even at 10,000 cycles, and (d) FS determination with over 80 % accuracy across all coupons. This study offers promising avenues for automated fatigue damage detection and FS determination in thermoplastic structures, eliminating the need to interrupt their operational cycles.

Application of machine learning methods in the analysis of interface bonding strength for overmolded hybrid thermoset‐thermoplastic composites

AbstractThe focus of this study is to apply finite element method (FEM) and machine learning methods to investigate the interfacial bonding strength of continuous fiber reinforced thermoset composite (TSC)‐thermoplastic structures manufactured through co‐curing and overmolding processes, with polyamide 6 (PA 6) as the thermoplastic material. A model for interfacial healing degree in TSC‐PA 6 structures was developed. Then, FEM was employed to study the influence of various injection overmolding process parameters on the interfacial bonding strength of TSC‐PA 6 structures. The results show that there is a strong correlation between the degree of interface healing and the bonding strength. Subsequently, six machine learning methods were employed to correlate interfacial healing degree with diverse injection molding process parameters. Simulation data were utilized for training, calibration, and validation of the six machine learning models. Based on the results of simulation and machine learning predictions, a quantitative analysis of the significance of injection molding process parameters on healing degree was conducted. These parameters are ranked in descending order of importance as follows: insert temperature, melt temperature, and injection rate.HighlightsThe interfacial healing degree model of PA 6 was established and validated through the integration FEM and experiment.Six machine learning models were built to predict the interfacial healing degree.Grad boosting performed best in predicting the interfacial healing degree.Insert temperature had the greatest impact on the interface healing degree.

Carbon fiber reinforced polyethylene composite adhesive at elevated temperatures

The development of multi-layer-multi-material thermoplastic structures necessitates the bonding of dissimilar materials with varying mechanical, chemical, barrier, and thermal properties. Thermoplastic adhesives, such as maleic anhydride grafted polyethylene (PE-MAH), are advantageous over thermoset adhesives given their similar manufacturability, and numerous benefits as sustainable materials in compliance with strict environmental regulation in construction, automotive, and energy industries. A key drawback is that PE-MAH demonstrates weak adhesive properties at elevated temperatures. In this study, the adhesion strength of carbon fiber (CF) reinforced PE-MAH at elevated temperatures were evaluated using a novel method of T-peel test under a controlled environment. Significant improvements of 71% and 295% increase in peel strength at 90 °C and 110 °C, respectively, were achieved with CF reinforcement over neat PE-MAH. The improving mechanisms were discussed on both macro-level, i.e., enhanced peel zone and eliminating crazing, and micro-level, i.e., transferred stress and dispersed energy into micro peel zones. This study demonstrates the efficacy of tailoring the adhesion strength of thermoplastic composite adhesive for significantly improved performance at elevated temperatures.

Identification of Spatial Modulus Distribution of Chopped Carbon Fiber Tape-reinforced Thermoplastic Structures Using Modal Test-based Inverse Analysis

Identification of Spatial Modulus Distribution of Chopped Carbon Fiber Tape-reinforced Thermoplastic Structures Using Modal Test-based Inverse Analysis

Influence of void contour on the elastic behavior of parts produced by material extrusion

The significant void content is the most noticeable flaw of 3D printed thermoplastic structures. The voids run parallel to the printed filaments, resulting in non-ellipsoidal geometries that are directly related to the printing variables. Until recently, only numerical solutions for accurate simulation of void geometry have been published, while analytical models have been limited to solutions of ideal geometries such as long cylinders. The impact of the geometry of void inclusions on the elastic behavior of acrylonitrile butadiene styrene (ABS) produced by material extrusion is examined in this study, and a unique semi-analytical method for real void geometries is proposed using, as basis, finite element (FE) solutions of Eshelby’s tensor. The FE solutions of Eshelby’s tensor are obtained by employing the FE approach to solve the elasticity problem of a large matrix containing one void inclusion. The numerical solutions are then used to evaluate the effective elastic properties using the effective field methods. The results of the semi-analytical method are in good agreement with numerical and experimental results.

ANN-predictive modeling and GA-optimization for minimizing dimensional tolerance in Polyjet Additive Manufacturing

Polyjet Additive Manufacturing (AM) is gaining attention owing to its ability to manufacture intricate parts with microscopic resolution. While extensive research and development have optimized the accuracy and strength of 3D printed structures, however, only a few prediction models exist for predicting the accuracy of the thermoplastic-based components using Polyjet technique. The study presents an accuracy-based predictive model for polyjet AM of thermoplastic structures which significantly raises the fabrication standards and reduces the number of unnecessary experimental attempts. Multivariate Regression Analysis (MVRA) statistical approach and the Generalized Regression Neural Networks (GRNN) approach is carried out to evaluate the effect of numerous significant polyjet printing parameters such as Support Material, Print Mode, Print Orientation, and Thermoplastic resin on the dimensional stability of the printed structures, examined under an ultra-compact 3D laser sensor. The outputs are optimized using the Genetic Algorithm (GA), and the findings are found consistent with experimental trials. This study sets new standards for additive manufacturing of thermoplastic components by examining the influence of polyjet printing factors on the accuracy of the manufactured fastener along the X, Y, and Z axes, respectively. The study may eventually substitute metal fasteners in certain industrial applications such as in fluidics, electronics, biomedical engineering, food packaging, and automobile industries.

Erratum to: “Anisotropic thermal behavior of extrusion‐based large scale additively manufactured carbon‐fiber reinforced thermoplastic structures” [Polymer Composites 2022; 43: 3678–3690

Erratum to: “Anisotropic thermal behavior of extrusion‐based large scale additively manufactured carbon‐fiber reinforced thermoplastic structures” [Polymer Composites 2022; 43: 3678–3690

Tunable soft–stiff hybridized fiber-reinforced thermoplastic composites using controllable multimaterial additive manufacturing technology

As a revolutionary manufacturing technology, material extrusion exhibits great potential for fabricating thermoplastic composites. However, the application of controllable multiple-thermoplastic materials for producing a tunable soft–stiff hybridized fiber-reinforced thermoplastic composite (CCFRTP-TSSH) based on material extrusion has yet to be extensively studied. Thus, this study proposes a controllable multimaterial additive manufacturing process for integrating continuous carbon fibers (CCF) as reinforcement for the thermoplastic structures. Structural reinforcement and tunable soft–stiff hybridization were achieved by adopting thermoplastic polyurethane (TPU) and polylactic acid (PLA) as the soft and stiff materials, respectively. Additionally, the mechanical properties of the CCFRTP-TSSH specimens were systematically investigated and discussed based on various process parameters, such as nozzle temperature, layer thickness, and supporting resin volume ratio (SRVR). The experimental results revealed that the layer thickness had a significant influence on the tensile properties, followed by the SRVR and nozzle temperature, wherein the highest tensile strength obtained was 90.89 MPa. Moreover, the bending strength depends on both the layer thickness and loading direction, with the largest bending strength of 65.96 MPa. Furthermore, the influence of SRVR on various fusion interfaces, e.g., PLA/ TPU, PLA/CCF, and PLA/PLA, was analyzed using scanning electron microscopy, including the influence of process parameters on the wall thickness of the stiff and soft resins, to realize an enhanced tunable soft–stiff hybridization. Ultimately, several distinct components, including a thin-walled hollow cylinder, round table, and a socket for a limb prosthetic, were fabricated to demonstrate the feasibility and accuracy of the proposed method. Therefore, this study presents a novel approach for the integrated manufacturing of soft–stiff hybridized composites, particularly in the field of wearable devices.

Experimental quality assessment of thermoplastic composite corner regions manufactured using laser-assisted tape placement

Over the past 25 years, interest in thermoplastic composites in aircraft has steadily increased. Combining winding and laser-assisted tape placement is a promising method to manufacture thermoplastic structures using in-situ consolidation, as shown recently by manufacturing a variable stiffness, unitized, integrated-stiffener thermoplastic wingbox at the University of Limerick. The corner regions are a critical point of the structure and require in-depth characterization studies, for example by unfolding L-shaped samples in a 4-point bend test. In this work, samples with radii varying from 2 to 10 mm were manufactured and tested. Two manufacturing parameters were varied: the rotational speed and acceleration of the tool. Test data show that decreasing the radius increases the corner strength, but an optimum radius exists to withstand a maximum unfolding force/moment. In addition, the slowest deposition rate with least acceleration of the head used during manufacturing lead to the highest corner strength for the same radius.

Anisotropic thermal behavior of extrusion‐based large scale additively manufactured carbon‐fiber reinforced thermoplastic structures

AbstractLarge format additive manufacturing (AM) enables rapid manufacturing of large parts and structures with minimum waste in material and energy. Extrusion‐based AM deposition processes provide parts with highly anisotropic thermal properties, which are not typically reflected in textbook values for these materials. In order to develop accurate models that describe the directionally dependent thermal behavior of these materials in processing and service, accurate measurements of specific heat capacity and thermal conductivity are required. This work characterizes, documents, and analyzes the effect of the anisotropic nature of the extrusion‐based deposition process on the specific heat capacity and thermal conductivity of the resulting AM products. All measurements were made over a temperature range of 20–180°C using the transient plane source technique, also referred to as the hot disk technique. Three of the most commonly used large format AM feedstock materials that utilize carbon fiber reinforcement were examined in this work: acrylonitrile butadiene styrene, polyphenylene sulfide and polyphenylsulfone. These findings can serve as a thermal design/process guideline for future large format AM applications.

Assessment of controllable shape transformation, potential applications, and tensile shape memory properties of 3D printed PETG

In this research, for the first time, highly controllable self-coiling and tensile shape memory behaviors of the 3D printed (Poly-Ethylene Terephthalate Glycol) PETG thermoplastic structures, as a novel shape memory polymer (SMP), are introduced. The results showed that the maximum printing-induced pre-strain of the PETG is stored in the first printed layer. Manipulating the infill printing direction of the 3D printed PETG strip provides different controllable shape transformation modes, including bending and spiraling. The wall effect experiments indicated that the shape transformation related to the infill pattern weakens by increasing the wall size. Employing a cross printing pattern resulted in the strip's more intense self-spiraling behavior, making the final shape transformation mode less influenced by the wall size. The shape transformation of the planar rectangular surfaces printed with single and cross printing patterns caused a self-tubing shape transformation in half and full tubes, respectively. Mechanical and biomedical potential applications of the PETG 3D printed parts' shape transformation are defined as a self-deployable spiral stent, self-conforming supporting splint, mechanical fastening, and self-locking planarly printed gripper to grab spherical, slippery, and soft objects that are hard to grab. The tensile shape memory test was done on a dog-bone tensile sample printed with a cross pattern to evaluate the effect of the first layer's printing direction, programming temperature, and strain rate on the tensile shape memory performance. Hot programmed samples (Tg+10 °C) exhibited a higher shape fixity and weaker strain recovery than warm programmed samples (Tg). Warm programmed samples showed full strain recovery with a considerable downward self-bending in their recovered shape, but hot programmed samples did not show any curvature. This behavior intensifies by increasing the stretching percentage. Also, changing the first printed layer's direction from the longitudinal to the transverse condition prevented the from self-bending during the recovery process. A twentyfold increase in the deformation speed for the warm programming condition resulted in a doubled gentler undesired self-bending and a 14.3% higher shape recovery.

Considerations on the Plastic Structure of a UAV Payload Made by 3D Printing Technology

With the development of unmanned aerial vehicle (UAV) systems for a multitude of real-time applications, 3D printing technologies have been developed to make thermoplastic structures by fusing filament Fused Deposition Modeling (FDM) or Fused Filament Fabrication (FFF). However, we consider that the realization of new technologies of experimental models / technological demonstrators / prototypes becomes profitable by using 3D printing technologies. The main aim of the paper is to highlight how the use of three types of materials, which are processed differently, influences the Von Mises stresses of the payload used for a UAV, with the mission of photographing and filming from high altitude.

On the bead design in LFT structures: the influence of manufacturing-induced residual stresses

Abstract In the design of long fibre reinforced thermoplastic (LFT) structures, there is a direct dependency on the manufacturing. Therefore, it is indispensable to integrate the manufacturing influences into the design process. This not only offers new opportunities for material- and load-adapted designs, but also reduces cost-intensive modifications in later stages. The goal of this contribution is to make the complexity manageable by presenting a method which couples LFT manufacturing and structural simulations in an automated optimization loop. Herein, the influence of linear-elastic, local anisotropic material properties as well as residual stresses resulting from the compression molding of LFT on the stiffness-optimized design of beaded plates is investigated. Based on the simulation studies in this contribution, it can be summarized that the resulting bead height and flank angle, considering anisotropies and residual stresses, are smaller compared to isotropic modelling. As a conclusion, the strength constraint limits the maximum bead height and the flank angle needs to be additionally chosen as a consequence of the local fibre orientations and residual stresses resulting from manufacturing. Optimized bead cross sections are only valid for a specific system under investigation, as they depend on the defined boundary conditions (load case, initial charge geometry and position, fibre orientations, etc.).

3D printing of a continuous fiber-reinforced composite based on a coaxial Kevlar/PLA filament

Additive manufacturing has allowed for the production of complex and mass customized geometries, but often at the expense of mechanical performance, a penalty which can be in part mitigated with the fabrication of composite parts. Thermoplastic structures fabricated with material extrusion additive manufacturing stand to be improved in terms of fracture toughness with the integration of continuous fibers. The present research program has investigated the production of a continuously reinforced filament to be used in open-source fused filament fabrication systems. Three different volume fractions of Kevlar fibers were incorporated into a polylactic acid (PLA) thermoplastic filament. It was observed that a 20% fiber volume fraction resulted in a doubling of the tensile strength relative to the unreinforced PLA parts. High-velocity impact tests were also performed on the reinforced printed thermoplastic material, and it was observed that the composite with the highest fiber volume fraction provided an impact energy resistance improved by a factor of four, relative to the plain PLA. The reinforced fibers have shown to restrain the penetration of the projectile at velocities similar to those that perforated the unreinforced PLA. The present work has demonstrated the production of printed composites without the need of modifying the extruding systems of a commercial 3D printer. This approach could represent an alternate and feasible process for producing continuously reinforced 3D-printed thermoplastic parts with utility for high-velocity impact applications.

Hybrid fused filament fabrication for manufacturing of Al microfilm reinforced PLA structures

The quality of 3D printed thermoplastic structures mainly depends upon the various aspects of deposition pattern, processing conditions, and layer bonding. The incomplete layer-to-layer adhesion during the additive manufacturing process is the most critical issue since thermoplastics are bad heat conductors. In this study, aluminum (Al) microfilms have been deposited to promote the adhesion between the additive layers. The composite structures (as per ASTM D 695) of polylactic acid thermoplastic were manufactured by fused filament fabrication (FFF) process and consecutively reinforced with Al spray. The composite structures were subjected to compressive loading to investigate the influence of input process variables like; in-between microfilm layers (1–5 layers), bed temperature (60–100 °C), and infill percentage (40–100%). The results of the study suggested that using microfilm in-between additive layers is a promising technique for improving the compressive properties. The compressive strength has been observed maximum by performing FFF with 3 layers of Al microfilm, 70% of infill percentage, and 100 °C bed temperature. The results are supported by scanning electron microscopy, energy-dispersive spectroscopy, and differential scanning calorimeter analysis. An optimization study was successfully conducted using the analytic hierarchy process, which predicted the optimum parameter settings based on the relative importance of each response variable.

Additive batch electrospinning patterning of tethered gelatin hydrogel fibres with swelling-induced fibre curling

Limited methods exist to integrate patterning of free-spanning hydrogel microfibres as part of an additive manufacturing strategy. We demonstrate 3D additive batch electrospinning (3D-abES) workflow to efficiently produce layered hydrogel gelatin microfibres, tethered to 3D printed thermoplastic structures of various shapes. Enabled by the digital-design approach, multiple 3D printed fibre devices can be fabricated in a batch efficiently with minimised sample-to-sample variance. Customised fibre patterns and device geometries can be rapidly altered to fit with potential applications in 6- to 24- well plate formats. The diameter of as-produced dry fibres is in a range of ∼2−4 μm. With the intended device applications in aqueous environments, we investigate the effect of combinations of processing parameters on the gelatin fibre integrity, and its swelling behaviour when immersed in water. Aided by the parametric study, patterns of swelling-induced fibre curling, from straight to wavy, can be tuned. Our findings could serve as a guide to optimise the 3D fabrication and patterning of electrospun hydrogel-like fibres made of crosslinked hydrophilic polymers and oligomers, for cell culture or bio-sensing applications.

Low-velocity impact behaviors of a fully thermoplastic composite laminate fabricated with an innovative acrylic resin

The current study investigates the low-velocity impact behavior of newly developed ultra-high molecular weight polyethylene (UHMWPE) fiber-reinforced polymer composites (FRPCs) manufactured at room temperature with an innovative liquid methylmethacrylate (MMA) thermoplastic resin, Elium®. Because of the extremely high molecular mass, UHMWPE fibers exhibit outstanding impact strength and energy dissipation properties. Thus, to evaluate the out-of-plane mechanical properties of the newly developed laminates and compare the results with those of traditional thermosetting and thermoplastic laminates, impact tests at various energy levels from 15 J to 40 J are conducted. Different impact characteristics, namely maximum load, displacement, absorbed energy, structural integrity, and damage failure modes are analyzed for Elium®-based laminates and compared with those of thermosetting laminates fabricated with UHMWPE fibers. During the tests, the thermoplastic composite laminate showed a ductile behavior and developed extended plasticity. The results revealed that the newly developed thermoplastic structures at room temperature can obtain higher impact load and lower absorbed energy up to 40% compared to those thermosetting counterparts. Furthermore, it was found that replacing the thermosetting resin with the thermoplastic resin significantly improved the structural integrity of the laminate by 240%.

Integrated self-monitoring and self-healing continuous carbon fiber reinforced thermoplastic structures using dual-material three-dimensional printing technology

Polymer-matrix composite structures with both self-monitoring and self-healing properties are proposed using a dual-material additive manufacturing technique. A double-nozzle three-dimensional (3D) printer system was adopted to fabricate these composite structures. A thermoplastic matrix with healing-agent containers was printed using one of the nozzles. The continuous carbon fibers, serving as both a sensory element and reinforcement, were embedded into the thermoplastic matrix using the other nozzle. The feasibility and effectiveness of the structures were verified through compression and three-point bending tests. Real-time self-monitoring of structural damage was performed using electrical resistance measurements. The results demonstrated that addition of 2120 Epoxy Cure agent and 2000 Epoxy Resin agent into the polymer matrix could help to repair the structural damage, with average healing efficiencies of 27.84% for non-reinforced specimens and 30.15% for fiber-reinforced specimens. In addition, digital image correlation (DIC) was utilized to analyze the reinforcing mechanism and failure behavior of the 3D printed thermoplastic composite structures reinforced with continuous carbon fibers. Integration of continuous carbon fibers and healing agents within the polymer matrix can create smart structures that can not only monitor their own structural health state, but also repair structural damage.

Methodology of Accelerated Characterization for long-term creep prediction of polymer structures to ensure their service life

Abstract Predicting long-term creep behaviour (10 or 20 years) is important in the development and design of thermoplastic structures to ensure their service life. But providing robust material data to engineers is not easy, as testing is expensive and time-consuming. The purpose of this paper is to propose a new Methodology of Accelerated Characterization for long-term creep Prediction (MACcreeP) which gives more material information much faster and at a lower cost, than Classical Methodology (CM) used today by engineers. In order to prove the effectiveness of this new method, the material used for the study is a well-known high density polyethylene grade (HDPE). More precisely MACcreeP is a characterization protocol based on creep tests performed in true tensile stress conditions over short time periods (24 h) and at different temperatures combined with the Time-Temperature Superposition Principle to construct master curves. In addition to the material data obtained over a short period of time, the data generated by the behaviour laws and the use of the CREEP law in a FEA software allows a more predictive numerical response than in CM. Finally, this methodology, which implies very few tests, allows to compare and choose the appropriate material properties to design a thermoplastic structure which will comply with end-use product requirements.

Porous graphitic biocarbon and reclaimed carbon fiber derived environmentally benign lightweight composites

Bamboo-derived biocarbon (BA900) and wood-derived biocarbon (THOC700) have exhibited graphite-like characteristics through transmission electron microscopy, X-ray diffraction (XRD), and Attenuated Total Reflectance (ATR) spectroscopy analysis. Lightweight composites of biocarbons were manufactured by a mechanism of shear controlled melt-phase mixing, ensuring the preservation of biocarbon pore structures and simultaneously taking full advantage of low density polyolefin substrates. Effective tensile strength was improved by approximately 10% in the polypropylene-based bamboo carbon composite, whereas no appreciable improvement was observed in the tensile and impact strength of bamboo-derived biocarbon formulations compared to neat polymer. However, the tensile and flexural moduli and flexural strength of the THOC700-PP composites were significantly enhanced, by 56%, 67%, and 19%, respectively, compared to neat polymer. The most significant finding of the investigation was the retention of density in polyolefin polymer (ρPP = 0.91; ρTHOC = 0.95; ρBA900 = 0.99), with enhanced mechanical performance useful for lightweighting applications. Bamboo biocarbon provides a viable alternative to another abundantly available industrial carbon feedstock, reclaimed carbon fiber (RCF), in manufacturing thermoplastic composites. The origin of the carbon plays an important role in defining ultimate composite performance. A mechanism for retaining lightweight structural performance has been proposed in this original work, paving the way to develop next-generation lightweight thermoplastic structures for transportation and other industrial and consumer products.