Long Fiber Reinforced Thermoplastics Research Articles

Process- and material-induced heterogeneities in recycled thermoplastic composites

A novel recycling solution for thermoplastic composites (TPCs) was recently implemented. The processing steps comprise shredding of TPC offcuts to flakes of a few centimetres, melting and blending of the flakes in a low-shear mixer, extrusion of a molten mixed dough and subsequent compression moulding in a press. This material and process are similar to the compression moulding of long-fibre thermoplastics (LFTs) that have been in the market for decades, such as glass mat thermoplastics (GMT) or direct-LFT. However, the input material in this recycling route consists of multi-layered woven flakes, which is very different from the pellets or chopped rovings of other LFTs. Process- and material-induced heterogeneities such as fibre orientation, percolation, variation of fibre fraction, or fibre attrition may be different for this new material. The development of this recycling technology and future industrial applications require more confidence in the material and process. The objective of this study is to characterise these heterogeneities for this recycling solution, and compare them to those generated in regular LFTs. It was found that the process- and material-induced heterogeneities of the recycled TPCs are similar to other LFTs, for the aspects listed here: fibre orientation, percolation, variation of fibre fraction and fibre attrition. In comparison to GMT, the effect of the mixing step is particularly noticeable on the local variation of fibre fraction within the panels. Industrial applications of this recycling route will benefit from this similarity, as it improves the confidence in the material and process combination.

Tissue engineering stent model with long fiber-reinforced thermoplastic technique.

This study aims to construct tissue engineering stents by using the long fiber-reinforced thermoplastic (LFT) technique to develop artery stents. The experimental method combines fibers, the LFT technique, and electrospinning technique. First, the biodegradable polyvinyl alcohol yarns are twisted and coated in polycaprolactone/polyethylene glycol blends through the LFT technique. Next, the weft-knitting and heat treatment are used to establish the stent structure, after which poly(ethylene oxide) (PEO) is electrospun to coat the stents. The morphology, mechanical, and biological properties of tissue engineering stents are evaluated. The test results indicated that the use of the LFT technique retains the softness of filaments, which facilitates the subsequent weft-knitting process. The coating of blends and electrospinning of PEO have a positive influence on the tissue engineering stents, as demonstrated by the tensile strength of 59.93 N and compressive strength of 6.10 N. Moreover, the in vitro degradation of stents exhibits a stabilized process. The water contact angle is 20.33°, and the cell survival rate in 24 h is over 80%. The proposed tissue engineering stents are good candidates for artery stent structure.

Calibration of Fiber Orientation Simulations for LFT—A New Approach

Short fiber reinforced thermoplastics (SFT) are extensively used due to their excellent mechanical properties and low processing costs. Long fiber reinforced thermoplastics (LFT) show an even more interesting property profile and are increasingly used for structural parts. However, their processing by injection molding is not as simple as for SFT, and their anisotropic properties resulting from the fiber microstructure (fiber orientation, length, and concentration) pose a challenge with regard to the engineering design process. To reliably predict the structural mechanical properties of fiber reinforced thermoplastics by means of micromechanical models, it is also necessary to reliable predict the fiber microstructure. Therefore, it is crucial to calibrate the underlying prediction models, such as the fiber orientation model, within the process simulation. In general, these models may be adjusted manually, but this is usually ineffective and time-consuming. To overcome this challenge, a new calibration method was developed to automatically calibrate the fiber orientation model parameters of the injection molding simulation by means of optimization methods. This optimization routine is based on experimentally determined fiber orientation distributions and leads to optimized parameters for the fiber orientation prediction model within a few minutes. To better understand the influence of the model parameters, different versions of the fiber orientation model, as well as process and material influences on the resulting fiber orientation distribution, were investigated. Finally, the developed approach to calibrate the fiber orientation model was compared with a classical approach, a direct optimization of the whole process simulation. Thereby, the new optimization approach shows a calculation time reduced by the factor 15 with comparable error variance.

Applying TPU blends and composite carbon fibers to flexible electromagnetic-shielding fabrics: Long-fiber-reinforced thermoplastics technique

A novel manufacturing technique called long-fiber-reinforced thermoplastic (LFT) is proposed to produce continuous flexible composite carbon fibers and fabrics. During the process, a polyester hot-melt adhesive (MPTU) is added to thermoplastic polyurethane (TPU) to form TPU blends that can improve the interfacial-bonding level. The TPU blends are extruded to enwrap carbon fibers and form an outer layer. The effects of the TPU blends on the composite are examined to determine the optimal parameters. At a TPU/MPTU ratio of 85/15 wt%, the blend exhibits good miscibility and the fiber tows show the optimal cluster effect and knot properties. Consequently, the specific strength of the optimal composite carbon fibers reaches 349.27 MPa·g−1·cm3. The resulting of composite carbon fabrics shows flexibility, and an average electromagnetic-interference shielding efficiency of 28.35 dB. Overall, this study reveals that LFT technique is an effective and novel method of developing carbon fiber-reinforced thermoplastics.

An effective model for fiber breakage prediction of injection-molded long fiber reinforced thermoplastics

Fiber length is an important factor affecting the mechanical properties of long fiber reinforced thermoplastic (LFRT). When LFRT is processed by injection molding, the strong shear flow usually leads to severe fiber breakage. Therefore, it is a crucial issue to reduce the loss of fiber length as much as possible during composite molding. Current work focused on constructing an effective model for predicting fiber breaking caused by shear stress during melt filling. Based on the Oseen formula, the disturbance of liquid flow caused by a single external force was studied, and the force acceptance formula of fiber immersed in flow field was derived. A mechanical model for characterizing the degree of fibers buckling and breaking and the shear stress was constructed by the Euler buckling criterion. To verify the model, glass fiber reinforced polypropylene (GF/PP) composites with initial fiber length of 3 mm and 6 mm was subjected to shear at the specific shear rate by using a rotating rheometer. The length of GF after sheared was measured by fiber length distribution analyser. The breaking ratio of fibers was predicted using the new model, and the predicted results were in good agreement with experiment, although more comparisons with experiments are necessary.

Fabrication and Analysis of Long Fiber Reinforced Polypropylene Prepared via Injection Molding

Fiber reinforcement leads to an improvement in mechanical properties of composites. In particular, long fiber thermoplastics (LFT) show more remarkable enhancement in the properties than short fiber thermoplastics (SFT). In this study, different fiber lengths, fiber loadings, and processing methods were considered to understand their effects on the physical properties more in-depth. It was found that significant breakage of fibers occurred during the screw based injection molding, which yielded a significant reduction in the properties. The orientation of fiber was observed experimentally and compared with the result of numerical simulation.

A generalized distribution function of fiber orientation for injection molded composites

The ability to predict fiber orientation accurately is critical for determining the improvement of mechanical properties of injection molded composites. Based on the cumulative distribution function (CDF) monotonically increasing, a smoothing model of piecewise linear approximation function is proposed by assuming that the first derivative is continuous. Taking into account the ability of Weibull distribution reducing to an exponential decay function, a generalized distribution function (GDF) of fiber orientation is proposed by introducing the core thickness ratio (hc) to describe the sandwich layered morphology. Compared with the experimental results in the literatures, the cumulative generalized distribution function fits well (R2 > 0.98), and there is a good correlation (RE<6%) between the experimental results and the elastic moduli computed based on the GDF's FOD and the modified Weibull distribution for FLD. In comparison to RSC model, the GDF improves more than 5% in prediction precisions of elastic moduli for long fiber-reinforced thermoplastics (LFT).

Effect of Fiber Length on Mechanical Properties of Injection Molded Long-Fiber-Reinforced Thermoplastics

Fibers of long-fiber-reinforced thermoplastic (LFT) pellets are longer than those of short-fiber-reinforced thermoplastic (SFT) pellets produced by extrusion. To overcome the fiber length limitations of the SFT pellets, LFT pellets were prepared by a pultrusion process using continuous fibers, which afforded longer fibers. Currently, long glass-fiber-reinforced polypropylene (LGF-PP) materials, which are one of the growing markets of the plastics industry, are typically used, particularly for applications in the automotive industry where lightweight performance and cost savings are required. The LFT pellets are primarily converted into the final product by injection molding. During the injection molding process, the LFT pellets are broken and the fiber length decreases as a consequence. In this study, glass and carbon-fiber-reinforced polypropylene of various pellet lengths (3-16 mm) were prepared by impregnating pultrusion process to evaluate the effect of mechanical properties according to fiber length. The length of the residual fibers recorded after injection molding were observed to decrease in proportion to the pellet length. Even with LFTs having the same material composition, the mechanical properties was different after injection molding according to the residual fiber length. In this injection molded LFT, the flexural modulus was higher as the residual fiber length was shorter. The impact strength was larger in samples without adhesive resin and the observed differences were larger for longer fiber lengths. From these results, the mechanical properties of LFT can be improved by understanding the properties of long fibers.

Fiber distribution of long fiber reinforced polyamide and effect of fiber orientation on mechanical behavior

AbstractThe objective of this paper is to research fiber distribution and mechanical behavior (tensile, flexural, and Charpy impact properties) of long fiber reinforced thermoplastic (LFT) plates made by injection molding. The plates were made of long glass fiber reinforced polyamide 6,6 granules and injected using two types of molds, including center‐gated and end‐gated injecting modes, to obtain different fiber orientation state. In addition, end‐gated plate was cut along three directions (0°/45°/90°) to main flow path, in order to experimentally investigate the effects of fiber orientation on mechanical properties. It is found that there is skin‐shell‐transition‐core structure across thickness of plate, resulted from different fiber orientation. Furthermore, end‐gated plate has more obvious fiber orientation at shell and core regions than center‐gated plate. The experimental results demonstrate that fiber orientation has significant effects on modulus, strength, toughness, and full‐field strain evolution. Tensile and flexural properties decrease by over 50%, as the cutting angle with respect to flow direction changes from 0° to 90° in end‐gated plate. Moreover, unnotched and notched strength drops by 66% and 33%, respectively, from 0° to 90° off‐axis angle. These results show apparent mechanical anisotropy of LFT. Shell‐core structure also significantly influences strain distribution and fracture morphology.

An anisotropic creep model for continuously and discontinuously fiber reinforced materials

AbstractThe present study is concerned with the formulation, implementation and validation of anisotropic creep laws for neat thermoplastic matrix materials, discontinuously long fiber reinforced thermoplastic (LFT) composite materials with process dependent fiber preference directions as well as unidirectionally fiber reinforced thermoplastic tapes. Based on a simple three term Kelvin‐Voigt approach, an anisotropic model is formulated and implemented into a finite element program. The model is validated against an experimental data base of creep experiments on continuously and discontinuously fiber reinforced materials.

Fiber aspect ratio effect on mechanical and thermal properties of carbon fiber/ABS composites via extrusion and long fiber thermoplastic processes

The present study explores the effect of the aspect ratio of carbon fiber on the mechanical, thermal, and impact properties of carbon fiber-reinforced acrylonitrile-butadiene-styrene (ABS) matrix composites, which were produced using: (1) chopped carbon fibers with different lengths by means of twin-screw extrusion followed by injection molding; and (2) long fiber thermoplastic (LFT) pellets with different lengths by means of LFT followed by injection molding. The results revealed that the carbon fiber/ABS composites made by using carbon fiber/ABS LFT pellets exhibited the tensile, flexural, dynamic mechanical thermal, heat deflection temperature, and impact properties higher than those by using conventionally extruded carbon fiber/ABS pellets. It is noted that 12-mm-long LFT pellets contributed most to improving the mechanical and thermal properties of carbon fiber/ABS composites, compared to 6- and 24-mm-long LFT pellets. Meanwhile, the Izod impact strength of the composites was highest in the case of 24-mm-long LFT pellets. The improvements in the mechanical and thermal properties of carbon fiber/ABS composites were explained by different length distribution and aspect ratio of carbon fibers existing in the composites produced via conventional twin-screw extrusion and LFT processes, respectively.

Hybrid forming of sheet metals with long Fiber-reinforced thermoplastics (LFT) by a combined deep drawing and compression molding process

In this article, a newly developed method for forming metal sheets together with long fiber-reinforced thermoplastics (LFT) to hybrid parts is presented. The idea of combining two different materials in one part is nothing new, but most of the existing processes use at least separate production techniques for each material and an additional joining operation. The special characteristic of this new process is realized by a combined forming tool for both sheet metal and LFT to create a hybrid part with only one necessary and simultaneously executed process step. Besides contact to the punch, the metal sheet is also formed by the molten LFT material, which behaves like a hydraulic fluid in hydroforming processes. With this method, it is possible to reduce the thickness of sheet metal components by adding a LFT reinforcement structure. The interface connection between the metal sheet and LFT is realized by using a bonding agent, which is previously applied to the metal sheet via coil or powder coating. To achieve this hybrid forming process, new tool and sealing concepts have been developed and the corresponding process parameters were identified and optimized. As a result, the innovative process offers a cost- and time-efficient solution for multi-material lightweight design.

Efficient Characterization and Modelling of Material Behaviour of LFT for Component Simulations

Modeling failure and progressive damage of long fibre reinforced thermoplastics (LFT) presents a challenging task since local inhomogeneities, anisotropic fibre orientations, and strain-rate dependence must be taken into account also on the microscale. We show that for simple geometries, the material behaviour of the composite can be modelled using layered geometrical models. But for more complex geometries, this approach fails since the fibre orientation distribution is inhomogeneous. In this case, multiscale methods allow the accurate and efficient prediction of the material behaviour with the local fibre orientation taken from an injection molding simulation. This material model can be extended to viscoplasticity and integrated into the NTFA-TSO method from Michel &amp; Suquet (2016). In this way, we can obtain an accurate and efficient multiscale method for the realistic modelling of LFT.

Tensile fatigue characterization of polyamide 66/carbon fiber direct/in-line compounded long fiber thermoplastic composites

Fiber reinforced polymer (FRP) composite materials can be advantageous alternatives to structural metals due to their high specific stiffness and strength. One potentially useful class of FRPs are Long-Fiber Thermoplastic composites (LFT) as these materials have significantly higher aspect ratios for the fibers. A possible lower cost variant of LFTs, Direct Long Fiber Thermoplastics (DLFT), where the long discontinuous fibers are created directly during the extrusion process from fiber rovings. The objective of this study was to determine the fatigue performance of DLFT composites. Therefore, tensile fatigue characterization was carried out for compression molded direct/in-line compounded (DLFT) 40% (by weight) carbon fiber/polyamide 66 composites. This characterization yielded fatigue life prediction curves (stress ratio of 0.1) for 0°, 45°, and 90° orientations with respect to flow. The evaluations showed a reasonable match to a power law approximation for the relationship of peak stress to number of cycles to failure for these materials under these test conditions (23 °C, dry as molded, R = 0.1, 3 Hz). Peak stresses at which the samples achieved 106 cycles were 105 MPa for samples oriented in the flow direction, 72 MPa for samples oriented 45° to the flow direction, and 53 MPa for samples oriented 90° to flow.

Experimental characterisation of the mechanical properties of a carbon fibre/PA66 LFT automotive seatback under quasi-static and impact loading

Few studies/bodies of data are available in the open literature on the topic of mechanical material characterisation and structural performance for direct/inline compounded long fibre thermoplastic (LFT) automotive parts, particularly incorporating carbon fibre. Therefore, this study aimed to comprehensively document the mechanical properties of a seatback for a lightweight automotive seating design, compression moulded from carbon fibre/polyamide 66 on a Dieffenbacher LFT-D manufacturing line. The methodology consisted of extracting and characterising mechanical behaviour of tensile specimens from full seatback components and completing quasi-static and low velocity impact loading on these seatbacks. Both longitudinal and transverse charge placements within the compression mould were considered. Main findings from the tensile tests noted a dependency of the mechanical properties based upon the location from which specimens were extracted; from the charge region, the elastic moduli in the 0° and 90° directions were similar (0°: 14.0 GPa & 90° 15.9 GPa with coefficients of variation (CoV) of 56% and 36%, respectively) or, for the case of specimens extracted from the flow region, exhibited significant anisotropy (elastic moduli in the 0° direction 9.79 GPa and 90° direction 21.6 GPa, with CoV’s of 16% and 23%). In terms of anisotropy of the full seatback component, under low velocity impact loading a local force maxima was observed for seatbacks produced with a longitudinal charge orientation at deflections of approximately 15 to 20 mm. No local maxima were consistently observed for transverse charge seatbacks. In terms of rate effects, stiffness was 550% higher for low velocity impact with respect to quasi-static loading. Digital image correlation identified a greater level of deformation, particularly away from the indenter, for quasi-static loading. Catastrophic failure occurred at larger deflections for low velocity impact (36% increase for longitudinal charge placement, 24% for transverse). The results of this investigation have practical applications for engineers in industry considering direct/inline compounded carbon fibre LFT materials in cases where quasi-static and dynamic loading will be present, for example, automotive chassis/body structural components.

A review of Long fibre thermoplastic (LFT) composites

ABSTRACTLong fibre-reinforced thermoplastic or long fibre thermoplastic (LFT) composites possess superior specific modulus and strength, excellent impact resistance, and other advantages such as ease of processability, recyclability, and excellent corrosion resistance. These advantages make LFT composites one of the most advanced lightweight engineering materials and enable their increasing use in various applications. This review paper summarises the research and development work that has been conducted on LFT composites since their initial development. Different aspects of LFTs, such as process development, fibre orientation distribution (FOD), fibre length distribution (FLD), and their effects on the mechanical properties of LFT composites are described. The characterisation of the FOD and FLD in the LFT composites using advanced imaging technology such as high-resolution 3D micro-CT scanning technique is summarised. Research and development of LFT hybridisation and LFT additives are also discussed. Finally, conclusions are made and the future outlook of LFT composites is given.

In Situ Joining of Unidirectional Tapes on Long Fiber Reinforced Thermoplastic Structures by Thermoplastic Automated Fiber Placement for Scientific Sounding Rocket Applications

Automated Fiber Placement allows the automated lay-up of tailored laminates in aerospace quality. For thermoplastic matrix materials, the use of closed-loop temperature control enables an in situ consolidation. This makes autoclave curing superfluous and increases the overall process efficiency. Scientific sounding rockets typically consist of several aluminum modules carrying the scientific payload. The design of a module includes a cylindrical outer shell and two load input rings with a defined assembly interface. Reducing the structural weight would allow higher payloads, higher apogees or reduced fuel consumption. A new manufacturing concept using the Thermoplastic Automated Fiber Placement (TP-AFP) process was developed to manufacture a lightweight composite module.This paper presents the developed concept and focuses on the characterization of an in situ joint of TP-AFP tapes on a long fiber reinforced thermoplastic (LFT) structure. This represents the joint of the cylindrical shell on the load input rings of the rocket module. The joint was characterized with single lap shear tests for two different sample extraction areas at room temperature and at elevated temperature on pre-treated and untreated LFT surface.The variations did not show significant effects on the resulting shear strength. The results were considered for the design of the composite module allowing a weight reduction of more than 40 % compared to the aluminum modules. The new module proved its airworthiness as part of the sounding rocket mission REXUS-23 in March 2019. Beyond sounding rockets, the concept of in situ bonded TP-AFP tapes on complex LFT structures has many potential applications within aerospace but also automotive structures.

Long fiber thermoplastics are a key technology in expanding existing markets for composites

Long fiber reinforced thermoplastics (LFT) are like the middle child of reinforced plastics technology. They are sometimes overlooked but hold the composites family together by bridging the gap between low-cost short fiber filled thermoplastics and heavy-duty fabric reinforced engineering polymers. We talk to the specialists of PlastiComp, a company dedicated to expanding the application of LFT.

Impact and energy absorption of long fiber-reinforced thermoplastic based on two-phase modeling and experiments

Impact behavior and energy absorption of long fiber-reinforced thermoplastic (LFT) plates under low velocity impact load is investigated based on two-phase modeling and experiments. In this study, strain rate dependent properties and fiber distribution are characterized by mechanical tests and X-ray CT scans. A two-phase homogenized material model is established and validated by impact test data. On this basis, this paper presents a method for the assessment of the energy absorption capability of LFT plates, which defined as a power law equation normalized by penetration energy. The accuracy and applicability of the proposed approach is validated by finite element simulations of three impactor diameters. The results show a good consistency and indicate that the exponent of the power law is probably independent of the fiber content distributions and impact conditions. The expression of minimum penetration energy is derived and can help in a quick design of fiber volume fraction and plate thickness.

Strain rate dependent multiaxial characterization of long fiber reinforced plastic

Abstract Long-fiber reinforced thermoplastics (LFT) were characterized for automotive applications with high rate experiments in a range of strain rate from 10 − 3 s − 1 up to 10 2 s − 1 . High-speed video imaging with digital image correlation (DIC) and a high-speed infra-red camera are applied to investigate the strain and temperature development during four different testing types. Different states of stress were investigated to provide adequate input data for simulation. A new fracture initiation criterion is introduced. Nearly pure shear load could be reached with a specifically designed specimen. It was found that global deformations rise with the strain rate, while local strains decrease. The adiabatic temperature-rise enlarges the deformation zone, impedes strain localization and leads to higher energy absorption at higher strain rates.