Long Fiber Reinforced Thermoplastics Research Articles

Uncertainty quantification of fiber orientation distribution measurements for long-fiber-reinforced thermoplastic composites

We present a detailed methodology for experimental measurement of fiber orientation distribution in injection-molded discontinuous fiber composites using the method of ellipses on two-dimensional cross sections. Best practices to avoid biases occurring during surface preparation and optical imaging of carbon-fiber-reinforced thermoplastics are discussed. A marker-based watershed transform routine for efficient image segmentation and the separation of touching fiber ellipses is developed. The sensitivity of the averaged orientation tensor to the image sample size is studied for the case of long-fiber thermoplastics. A Mori–Tanaka implementation of the Eshelby model is then employed to quantify the sensitivity of elastic stiffness predictions to biases in the fiber orientation distribution measurements.

Reliability in the characterization of fiber length distributions of injection molded long carbon fiber composites

The mechanical properties of a discontinuous fiber reinforced composite material are dependent on the length of the reinforcing fibers. Long‐fiber thermoplastic (LFT) composites offer a compromise between ease of manufacture and mechanical properties for structural applications and are conventionally made using fibers that are initially 10–14 mm long. However, fiber attrition during the injection molding process significantly reduces the final fiber lengths in molded composite and produces a fiber length distribution (FLD), which depends on the processing parameters. Thus, accurate FLD measurements are important for reliable predictions of the composite properties. Here, we present a detailed methodology for experimentally measuring the FLD in LFT composites based on best practices. Possible biases occurring during the measurement process are discussed and their effect on the measured FLD and averaged fiber length values are evaluated. We then employ a Mori–Tanaka implementation of the Eshelby model to predict the elastic stiffness of the LFT composite and analyze the sensitivity of the stiffness values to the biases in the FLD measurements. POLYM. COMPOS., 39:4594–4604, 2018. © 2017 Society of Plastics Engineers

Compressive behavior of automotive side impact beam with continuous glass fiber reinforced thermoplastics incorporating long fiber thermoplastics ribs

Long fiber thermoplastics (LFT) ribs co-molded with woven glass fabric prepregs were considered to design an automotive side impact beam (SIB). The LFT ribs were designed to locate at the center of the continous fiber reinfoced SIB. The woven fabric prepregs of different 90°(Weft)/0°(Warp) ratios of 20/80 and 50/50 were considered to evaluate the composite SIB. A finite element simulation was conducted to predict the failure behavior of the SIB. The SIB made of woven fabric prepregs with 50/50 ratio which were laminated with 45° and additionally reinforced by placing the unidirectional tapes on the flange showed the best performance value under compression, comparable to steel-made SIB.

Numerical predictions of fiber orientation and mechanical properties for injection-molded long-glass-fiber thermoplastic composites

Achieving accurate numerical predictions of the orientation of fiber in a shell-core structure is a critical requirement for improving mechanical performance and carrying out structural analysis for long fiber-reinforced thermoplastics (LFRT) in injection molding. When using 3D-mesh computation, the orientation in the shell layer was predicted fairly well, but resulted in over-predictive deviation in relation to the core region. Recently, an objective orientation model, iARD-RPR (improved ARD model and Retarding Principal Rate model), was developed in the rheological field of fiber suspension. We therefore used the iARD-RPR model to enhance the prior fiber orientation predictions in 3D injection molding simulation. For various LFRT composites with the same concentration (40 wt%) of glass fiber immersed in two different polymer matrices (polypropylene and polyamide 6,6), the mechanical properties depending on fiber orientation are estimated to a significant degree, and their reinforced performance further compared.

Influence of the Manufacturing Process (Comparison MuCell and Direct-LFT) of Foamed and Fiber Reinforced Polymer Sandwich Structures on the Fiber Length

The automotive and aerospace industries ask for lightweight and cost effective materials, material combinations and structures for their products. To achieve these goals certain composite material groups have to be optimized with respect to their lightweight potential and production methods.Thermoplastic sandwich composites - which consist of a core structure (transferring the load) and two face-layers (absorbing tensile and compression loads occurring at bending) - suite the need of minimizing weight per area under bending loads. Reduction of process steps can be achieved by connecting the face layers and core in-situ via in mold assembly process using foam injection molding (FIM). FIM uses a single material system and – as within this work – physical blowing agents (PBA) for foaming. To increase the strength and stiffness of FIM parts, (long glass) fibers are in cooperated to create long fiber reinforced thermoplastic (LFT) materials.A commercially available version of the LFT-FIM process is the MuCell® process (Trexel, Inc.). LFT granulate (~ 11 mm length) is fed into the injection molding machine, melted and combined with nitrogen as PBA. To skip the needed compounding process step of the rod granules Fraunhofer ICT developed a Direct LFT-FIM process where polymer and continuous fibers are fed into a twin-screw extruder, melted and mixed with nitrogen. This single phase solution then is transferred to an injection unit.Within this work, these two foam injection molding processes will be compared concerning their achievable fiber length. For that purpose, (foamed) long fiber reinforced polypropylene (PP) blanks were manufactured using identical raw materials such as polymer, additives and glass fibers. The semi-finished product, starting material for the MuCell® process, were manufactured by EASICOMP GmbH using the same raw materials as with the D-LFT process. The different blanks – foamed and fiber reinforced PP (30 wt% and 40 wt %) – were manufactured using one injection unit for the MuCell® process and one for the D-LFT-FIM process. To compare the fiber length the same mold optimized to reduce fiber breakage was used in both processes.

Simulation prediction of the fiber breakage history in regular and barrier structure screws in injection molding

In recent years, the use of long fiber‐reinforced thermoplastics (LFRT) has become one of the most popular materials in lightweight applications. However, the microstructure of the fiber inside the plastic matrix is quite complex, making it hard to fully control the fiber effect in the injection molding process. On the other hand, the screw structure and operation will affect the residual fiber length significantly. However, the mechanism of how the screw influences the fiber breakage is still not yet clear. Thus, in this study, we apply the screw plastication in the injection molding process to investigate the fiber microstructure mechanism. The results show that more severe fiber breakage occurs when the compression force increases abruptly. Generally speaking, the breakage increases from the compression to the metering section. One effective method to reduce the fiber breakage is to increase the channel depths in both the feeding and the metering sections while taking the plastic melt quality into consideration. Moreover, the fiber breakage phenomena for two types of fibers are also investigated. Evidently, carbon fiber is easier to be broken. Its length distribution has a higher peak at the exit of the screw. This discovery is consistent with the experimental results of the current literature studies. POLYM. ENG. SCI., 58:452–459, 2018. © 2017 Society of Plastics Engineers

Preparation and characterization of compression‐molded green composite sheets made of poly(3‐hydroxybutyrate) reinforced with long pita fibers

AbstractNovel green composites were successfully prepared from bacterial poly(3‐hydroxybutyrate) (PHB) and pita fibers derived from the agave plant (Agave americana). Various weight contents (10, 20, 30, and 40 wt.‐%) of pita fibers at different lengths (5, 15, and 20 mm) were successfully incorporated into PHB by compression molding. The newly prepared PHB/pita fibers composite sheets were characterized in terms of their mechanical and thermomechanical properties and then related to their morphology after fracture. Attained results indicated that the mechanical stiffness of PHB significantly improved with both the content and length of pita fibers, although ductile properties were reduced. In particular, the elastic modulus of the 40 wt.‐% PHB composite sheets containing 20‐mm‐long pita fibers was approximately 55% higher than the unfilled PHB sheet. Shore D hardness also improved, achieving the shortest pita fibers the highest improvement. Pita fibers with lengths of 15 and 20 mm also increased the Vicat softening point and heat deflection temperature (HDT) by 38 and 21°C, respectively. Due to their optimal shape, it is concluded that pita fibers with lengths above 15 mm can potentially reinforce and improve the performance of PHB biopolymer. In addition, the compression‐molding methodology described in this research work represents a cost‐effective pathway to feasibly prepare long‐fiber‐reinforced thermoplastics (LFRTs) fully based on renewable raw materials. Resultant green composite sheets can be of interest for the development of sustainable parts in the automotive industry and other advanced applications in polymer technology.

The creep behavior of long fiber reinforced thermoplastics examined by microstructural simulations

Long fiber reinforced thermoplastics (LFT) appear to be promising for the cost efficient manufacture of lightweight structures by injection or compression molding. One major concern persists in their inherent tendency to creep due to continuous sliding within the thermoplastic matrix. To enable the application of LFT components under significant static loads, a profound knowledge of the interactions between the viscoelastic matrix and the nonwoven, discontinuous fiber reinforcement is necessary. In the present work, these interactions are investigated by micromechanical finite element simulations of computer generated LFT structures. The viscoelastic properties of the neat matrix are experimentally characterized and implemented into the microstructural models by an appropriate constitutive law in the form of a four parameter Burgers model. Since a distinct degree of nonlinearity is observed, the applied model is extended to the nonlinear viscoelastic regime and found to be suitable for an accurate reproduction of the experimental data. Micromechanical creep simulations which incorporate the viscoelastic matrix behavior are then validated against creep experiments on LFT specimens of three material variants with a different fiber fraction (PPGF10, PPGF20 and PPGF30), which are loaded under two different orientations at multiple stress levels. The model predictions show a good agreement to the experimental results in particular for the lower and medium stress levels, whereas a slightly increased error can be observed for the highest stress levels. By the application of different variants of the viscoelastic matrix model it is shown that the effects of nonlinearity on the effective creep behavior of the composite are quite considerable. Finally, the evolution of stress and strain within the microstructure during the creep period is visualized by contour plots at different times. The redistribution of stress from the viscoelastic matrix to the elastic fibers can clearly be observed.

Mechanical Behavior of Long Carbon Fiber Reinforced Polyarylamide at Elevated Temperature

Long fiber reinforced thermoplastic (LFT) composites have recently found increasing use in transportation, military and aerospace applications and become well established as high volume and low cost materials with high specific modulus and strength, superior damage tolerance, and excellent fracture toughness. This study is conducted to evaluate the performance of long fiber reinforced thermoplastic composite at elevated high temperature. Long carbon fiber reinforced polyarylamide (CF/PAA) composites containing 20 wt% and 30 wt% carbon fibers are used and processed using extrusion compression molding. Flexural and tensile samples are tested at three temperatures, room temperature, medium temperature (MD 65°C) and glass transition temperature (TG 80°C). Samples in both longitudinal and transverse directions are prepared to show the effect of the orientation on mechanical properties at different temperatures. The testing results show that as temperature increases, both of the flexural and tensile properties of the CF/PAA decrease as expected. Both of the flexural and tensile modulus reduce more dramatically than the flexural and tensile strength, indicating that the temperature has more pronounced effect on modulus than strength. The transversely oriented samples generally show larger reduction in properties than the longitudinally oriented samples. Temperature significantly affects flexural strength at the elevated temperature section between MD and TG temperature.

Effect of long fiber thermoplastic extrusion process on fiber dispersion and mechanical properties of viscose fiber/polypropylene composites

Viscose fiber reinforced polypropylene (PP/VF) composites were manufactured using long fiber thermoplastic (LFT) extrusion techniques with two different methods namely LFT-l and LFT-2. The compatibilizer [maleated polypropylene (MAPP)] and dispersing agent [stearic acid (SA)] were added to the PP/VF in order to improve the fiber dispersion and interfacial adhesion. The PP/VF composites manufactured using LFT-2 showed better fiber dispersion with higher tensile and flexural properties compared to the composites manufactured using LFT-1 method. Similarly, the impact strength and toughness of the LET-2 composites showed an improvement of 36 and 20% than LFT-1 whereas the average fiber length of composites was decreased from 6.9 mm to 4.4 mm because of the increase in shear energy as a result of residence time. Further, the addition of SA and MAPP to LFT-2 process has significantly improved the fiber dispersion and mechanical performance. The fiber dispersion and fracture behavior of the LFT-1 and LFT-2 composites were studied using scanning electron microscopy analysis. The Fourier transformation infrared spectra were also studied to ascertain the existence of type of interfacial bonds. Copyright © 2015 John Wiley & Sons, Ltd.

Parameter Identification by Inverse Modelling of Biaxial Tensile Tests for Discontinous Fiber Reinforced Polymers

AbstractIn the present article we discuss the characterization of the mechanical properties of long fiber reinforced thermoplastics (LFT) and sheet molding compounds (SMC) with biaxial tensile tests and the inverse parameter identification. The full 3D strain field is measured via digital image correlation (DIC). The anisotropic viscoelastic material properties are identified through inverse modelling by comparison of the heterogeneous experimental and simulated strain fields. A Gauss‐Newton type algorithm is used to identify the optimal parameter set [1]. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-scale structural mechanical modeling of long fiber reinforced thermoplastics

Abstract The mechanical properties of long fiber reinforced thermoplastics (LFT), which highly depend on the fiber orientation induced through manufacturing on a direct LFT line, are predicted for compression molded rectangular plates. Therefore, three two-scale structural mechanical simulation schemes are applied and discussed: a two-step approach, the Mori-Tanaka scheme and the self-consistent method. Fiber orientation tensors based on measured micro computed tomography data of selected samples as well as on filling simulations are used for the determination of mechanical properties, as e.g. the storage modulus. The results have been compared with dynamic mechanical analysis measurements of tensile specimens. The influence of the initial strand position on the effective mechanical properties of the plate and the variation of those are examined.

Pullout Performance of Common Construction Fasteners from Long Fiber Thermoplastic Composites

The recent increase in cyclone activity in the last 20 years has pressured the construction industry to look for alternative materials for roof construction. Although plywood is a cheap material, its mechanical properties are highly dependent on moisture content. Long fiber thermoplastics are an emerging type of plastic materials that feature enhanced mechanical properties due to high aspect ratio discontinuous fibers in a tough polymer matrix. The present study focuses on comparing the pullout strength of commonplace fasteners from plywood and long fiber thermoplastic composite panels. Specimens were tested according to ASTM standards D1761-88 and D6117-97 for mechanical fasteners in wood and long fiber thermoplastic (LFT) composites. Common construction nails like 6D, 10D and 16D and a #8 screw were used to compare the performance in terms of withdrawal. The data showed that LFT composites had withdrawal strength on average two to three times that of plywood for all tested fasteners types. Experimental data was analytically verified by APA TT-109 guidelines.

Highly enhanced mechanical properties of polypropylene-long carbon fiber composites by a combined method of coupling agent and surface modification of long carbon fiber

The aim of this study was to enhance the mechanical strengths of polypropylene/long carbon fiber thermoplastic (PP/LCFT) composite by increasing the adhesion between the PP matrix and the long carbon fiber (LCF). Bi-functional group grafted polypropylene (BFPP) was used as a coupling agent with surface modified LCF (SMLCF) in a long fiber thermoplastic (LFT) melt impregnation system to increase the interaction between the PP matrix and the LCF. The BFPP was produced by melt compounding of maleic anhydride grafted polypropylene (MAPP) and polyether amine (PEA). The surface modified LCF (SMLCF) was produced by dipping LCF into a sizing bath with 3-methacryloxypropyltrimethoxysilane (MPTS) to obtain oxygen functional groups. The composites were produced with a LFT melt impregnation system with PP, BFPP, and SMLCF. Tensile and flexural tests and scanning electron microscopy (SEM) results of the PP/BFPP/SMLCFT composite showed significantly enhanced mechanical strengths, compared with those of the common PP/LCF composite with a conventional maleic anhydride grafted polypropylene (MAPP) coupling agent. These improvements in mechanical properties are attributed to better fiber/matrix interfacial adhesion, as confirmed by micro droplet specimen tests and SEM micrographs of the fracture surface after inter laminar shear testing. The PP/SMLCFT composite with BFPP 5 wt% as coupling agent showed the highest tensile strength and flexural strength, which increased by 1.5 times and 1.7 times respectively, compared with PP/LCFT with a conventional MAPP coupling agent. The composite produced by this effective combination method of a coupling agent and surface modification of long carbon fiber can potentially be applied to automobile materials, leading to the replacement of metal parts and car weight reduction.

Fabrication of long glass fiber reinforced polyacetal composites: Mechanical performance, microstructures, and isothermal crystallization kinetics

A thermoplastic pultrusion was carried out to prepare the long fiber reinforced thermoplastic (LFT) composites based on polyacetal (POM) matrix on the custom‐designed pultrusion equipment. The investigation on mechanical performance revealed that the POM‐based LFT composites achieved much higher tensile, flexural, and impact strength than the short glass fiber reinforced ones at the same fiber loadings. Such a promising reinforcement effect is attributed to the feature that the residual fiber length in the injection‐molded LFT products is greatly superior to that in short fiber reinforced ones. This takes full advantage of the strength of the reinforcing fiber itself. The scanning electronic microscopy demonstrated that the fiber fracture and fiber pull‐out concurred on the tensile and impact fracture surfaces, and the former preceded the latter. The isothermal crystallization kinetics of the POM‐based LFT composites was also intensively studied, and the results indicated that the crystallinity of POM domain was enhanced by the heterogeneous nucleation of glass fiber, but the crystallization rate was postponed due to the interspace restriction toward crystalline growth caused by long glass fiber. These kinetic parameters provided information on the processing conditions of POM‐based LFT composites for the injection and compression molding. POLYM. COMPOS., 36:1826–1839, 2015. © 2014 Society of Plastics Engineers

Colored inorganic-pigmented long-fiber thermoplastics

Long-fiber thermoplastic (LFT) composite materials are rapidly expanding in automotive, transportation, and recreational industry. Most of these materials are natural or black in color with a need for secondary painting of the manufactured products. Standard organic pigments and dyes are not stable above 250°C and degrade during processing. Alternatively, inorganic pigments are thermally stable to at least 800°C. High-performance inorganic pigments offer resistance to outdoor weathering, chemicals, and acids. However, in fiber-reinforced composites, the pigment causes fiber attrition and thereby shows reduction in strength. This work explores colored inorganic-pigmented LFT composites. The ability to integrate the color in the manufacturing steps eliminates the need for secondary painting. Pigment variables such as particle size, distribution, chemistry, and coatings have been investigated. The article presents the processing and performance envelopes of colored inorganic-pigmented LFTs in comparison with unpigmented standard LFTs.

Processing and Characterization of Continuous Fibre Tapes Co-Moulded with Long Fibre Reinforced Thermoplastics

An important advantage when designing with plastics is the ability to incorporate features such as ribs, grids and bosses in the part. Rib stiffened polymer matrix composites have been widely used in aerospace, automobile, and civil infrastructure applications due to their high impact and fatigue resistance, high strength and stiffness to weight ratio, and damage tolerance. However, for long fibre-reinforced polymer composites, the processing complexity increases for features including ribs, grids and bosses. An innovative method of replacing ribs is the use of pre-consolidated continuous fibre reinforced thermoplastic (CFRT) tapes that can be co-moulded with long fibre thermoplastics (LFT). This work focuses on processing and performance evaluation in terms of the static and dynamic properties of LFTs co-moulded with pre-consolidated CFRTs, referred to as endless long fibre thermoplastic (E-LFT). The E-LFT approach is an alternative to rib stiffened composites. The effect of the thickness and fibre type in tape reinforced LFT is compared to LFT (with and without ribs) of equivalent flexural rigidity for static flexure and low velocity impact (LVI) response. In all these conditions, E-LFT samples performed better that the LFTs with and without ribs. LFT samples with and without ribs exhibited a brittle failure, as opposed to the progressive failure exhibited by E-LFT.

Mechanics of random discontinuous long-fiber thermoplastics. Part II: Direct simulation of uniaxial compression

The mechanical interactions between fibers in a dense random-fiber network transmit stress, cause fiber curvature, and influence fiber orientation in the processing of many types of composites. A few theories describe the mechanics of fiber networks, but almost no simulation results are available. Here, we report a direct numerical simulation of the mechanical behavior of random-fiber networks. The finite element method is used, and each fiber is represented by a small number of 3D beam elements. The calculations assume a periodic structure to avoid boundary effects, but within the unit cell, the fibers are placed randomly. A special algorithm that uses the random sequential adsorption process creates an initial structure of straight, random, nonintersecting fibers from which a unit cell with periodic boundary conditions is built automatically [A. I. Abd El-Rahman and C. L. Tucker III, “Mechanics of random discontinuous long-fiber thermoplastics. Part I: Generation and characterization of initial geometry...

Mechanics of Random Discontinuous Long-Fiber Thermoplastics—Part I: Generation and Characterization of Initial Geometry

The deformation mechanics of dry networks of large-aspect-ratio fibers with random orientation controls the processing of long-fiber thermoplastics (LFTs) and greatly affects the mechanical properties of the final composites. Here, we generate initial geometries of fiber networks in a cubic unit cell with a fiber aspect ratio of l/d = 100 and fully periodic boundary conditions for later numerical simulation. The irreversible random sequential adsorption (RSA) process is first used to generate a quasi-random structure due to the excluded-volume requirements. In order to investigate the nonequilibrium character of the RSA, a second method, which is similar to the mechanical contraction method (MCM) (Williams and Philipse, 2003, “Random Packings of Spheres and Spherocylinders Simulated by Mechanical Contraction,” Phys. Rev. E, 67, pp. 1–9) and based on a simplified Metropolis Monte Carlo (MC) simulation is then developed to produce quasi-equilibrium fiber geometries. The RSA packing results (ϕ ≈ 4.423% when using a fiber aspect ratio of 100) are in good agreement with the maximum unforced random packing limits (Evans and Gibson, 1986, “Prediction of the Maximum Packing Fraction Achievable in Randomly Oriented Short-Fibre Composites,” Compos. Sci. Technol., 25, pp. 149–162). The fiber structures were characterized by several distribution functions, including pair-spatial and pair-orientation distributions, based on either the center-to-center distance or the shortest distance between the particles. The results show that the structures generated by the RSA have an easily-detectable long-range spatial correlation but very little orientational correlation. In contrast, the quasi-equilibrium structures have reduced spatial correlation but increased short-range orientational correlation.

Young’s modulus prediction of long fiber reinforced thermoplastics

Abstract The aim of this paper is to provide analytical models able to predict the elastic properties of long fiber reinforced thermoplastics (LFT) in dependence of microstructural parameters such as the fiber volume content ( v f ), the fiber orientation distribution (FOD) and the fiber length distribution (FLD). The analytical predictions are compared to the experimental stiffness values from tensile tests on the composite material showing a good agreement. The FLD in terms of the probability density distribution as function of the fiber aspect ratio has been computed by an automated fiber separation and image analysis tool. On the other hand, the FOD in terms of the probability density distribution as function of fiber in-plane orientation was identified by using an image correlation procedure based on computer tomography scans of characteristic LFT specimens. Our analysis shows a good performance to predict Young’s modulus of LFT due to incorporation of the FOD as well as the FLD into the calculations.