Long Fiber Reinforced Thermoplastics Research Articles

A model for fiber length attrition in injection-molded long-fiber composites

Long-fiber thermoplastic (LFT) composites consist of an engineering thermoplastic matrix with glass or carbon reinforcing fibers that are initially 10–13mm long. When an LFT is injection molded, flow during mold filling degrades the fiber length. Here we present a detailed quantitative model for fiber length attrition in a flowing fiber suspension. The model tracks a discrete fiber length distribution at each spatial node. A conservation equation for total fiber length is combined with a breakage rate that is based on buckling of fibers due to hydrodynamic forces. The model is combined with a mold filling simulation to predict spatial and temporal variations in fiber length distribution in a mold cavity during filling. The predictions compare well to experiments on a glass–fiber/PP LFT molding. Fiber length distributions predicted by the model are easily incorporated into micromechanics models to predict the stress–strain behavior of molded LFT materials.

Influence of natural long fiber in mechanical, thermal and recycling properties of thermoplastic composites in automotive components

Natural fiber thermoplastic components in the automotive industry can afford the advantages of weight, cost reduction and recyclability, compared to conventional materials. Handlings of natural fibers in automotive exterior and interior components are essential to recover eco-efficiency and renewability. Natural fibers have recently become affordable to automotive industry as an alternative reinforcement for glass fiber reinforced thermoplastics. The best way to boost the fuel efficiency without sacrificing safety is to employ fiber reinforced composite materials in the body of the cars so that weight reduction can be achieved. The goals are to amplify the performance of the long fiber thermoplastics (LFRT) components by replacing the glass fibers with natural kenaf fiber and also to trim down the production cost. This research is focused on development of partially eco-friendly injection molded hybrid long fiber reinforced thermo plastics with natural kenaf fiber by impregnation process to improve the desired mechanical, thermal properties and recycling of the automotive components. Key words: Long fiber reinforced thermoplastics, kenaf fiber, hybrid, impregnation process.

Advanced Reinforced Thermoplastic Composites for Mass Transit and Heavy Truck Applications

The objective of the research effort is to develop cost-effective high performance thermoplastic composite materials and product forms and manufacturing of structural components for mass transit, heavy truck and military transport applications. Thermoplastic composites technologies possess the potential to provide structures with lower cost, lighter weight, and improved performance. This paper discusses an integrated processing and product development approach for select components such as seats, floor segments, and roof cover doors generic to mass transit, heavy transit and military transport. Long fiber thermoplastic (LFT) and continuous thermoplastic fiber forms have been used in processes ranging from extrusion-compression to vacuum thermoforming. About 40-60% weight savings with equivalent or enhanced performance have been achieved in these applications. Process modeling such as fiber/polymer melt flow in the LFT process has also been discussed in this paper.

A multiscale modeling approach to analyze filament-wound composite pressure vessels

A multiscale modeling approach to analyze filament-wound composite pressure vessels is developed in this article. The approach, which extends the Nguyen et al. [Prediction of the elastic-plastic stress/strain response for injection-molded long-fiber thermoplastics. J Compos Mater 2009; 43: 217–246.] model developed for discontinuous fiber composites to continuous fiber ones, spans three modeling scales. The microscale considers the unidirectional elastic fibers embedded in an elastic–plastic matrix obeying the Ramberg–Osgood relation and J2 deformation theory of plasticity. The mesoscale behavior representing the composite lamina is obtained through an incremental Mori–Tanaka [Average stress in matrix and average elastic energy of materials with misfitting inclusions. Acta Metall 1973; 21: 571–574.] type model and the Eshelby [The determination of the elastic field of an ellipsoidal inclusion and related problems. Proc R Soc Lond, Ser A 1957; 241: 376–396.] equivalent inclusion method. The implementation of the micro–meso constitutive relations in the ABAQUS® finite element package (via user subroutines) allows the analysis of a filament-wound composite pressure vessel (macroscale) to be performed. Failure of the composite lamina is predicted by a criterion that accounts for the strengths of the fibers and of the matrix as well as of their interface. The developed approach is validated in the analysis of an aluminum liner – T300 carbon/epoxy pressure vessel to predict the burst pressure. The predictions compare favorably with the numerical and experimental results by Lifshitz and Dayan [Filament-wound pressure vessel with thick metal liner. Compos Struct 1995; 32: 313–323]. The approach will be further demonstrated in the study of the effects of the lamina thickness, helical angle, and fiber–matrix material combination on the burst pressure.

Long Fiber Thermoplastic(LFT) 사출성형 공정에서 캐비티 내 압력 측정 및 CAE해석을 활용한 점도 추정

본 연구에서는 미지시료 또는 LFT와 같은 고점도 수지의 점도를 실제 사출성형 공정과 CAE 해석을 통하여 신뢰할 수 있는 새로운 점도의 추정 방법을 제안하였다. 우선 금형 내에 캐비티 압력을 측정할 수 있는 시스템을 구성하였고, 이 시스템을 이용해서 실제 사출과정에서 나타나는 압력 변화를 측정하는 것이다. 상용화 된 CAE 프로그램(Moldflow)은 사출공정에서 캐비티 내부를 흐르는 수지의 압력변화를 모사할 수 있다. 만약, CAE D/B에 있는 수지의 점도 데이터가 정확하다고 가정하면, 실험에서 측정한 압력 프로파일과 CAE로부터 계산 된 압력 프로파일이 일치해야 한다. 이것이 실험값과 일치하지 않으면 가정한 값을 CAE D/B에 입력해서 일치할 때까지 반복함으로써 신뢰성 있는 점도를 추정 할 수 있다. 이러한 알고리즘의 타당성을 검증하기 위해서 PP수지에 대하여 적용한 결과, 일반적인 점도계로 측정한 값과 일치하는 결과를 얻었다. 한편, PP-LFT에 대하여 적용하여 최적화 된 점도 데이터도 추정할 수 있었다. 【In this study, we proposed a new method that can estimate viscosity curves of unknown samples or high viscous resins like LFT(Long Fiber Thermoplastics). First, we built the system that could detect the pressure of melt during filling the cavity in a mold. It consists of both pressure sensors which are installed in a mold and the Kit which can convert analog signal to digital signal. The kit measures the melt pressure in mold cavity. We could also simulate the cavity pressure during filling process with commercialized CAE softwares(ex, Moldflow). If the viscosity data in CAE Database were correct, the simulated pressure profile coincided with the measured one. According to our proposed algorithm, we obtained correct viscosity data by iterating the process of comparing the simulated profile with the measured one until both coincided each other. In order to verify this algorithm, we selected well-defined PP resin and concluded that the experimental profile comply with the CAE profile. We could also estimate the optimized viscosity curves for PP-LFT by applying our method.】

QT/RR Coherence Is Associated with Testosterone Levels in Men with Chronic Spinal Cord Injury

Aim: To evaluate the effect of hypogonadism on temporal characteristics of ventricular repolarization (VR) and QT/RR coherence in men with spinal cord injury (SCI). Methods: Thirty-four men with SCI (>1 year postinjury) were studied. After clinical evaluation, 20 subjects were diagnosed as hypogonadal and 14 as eugonadal. QT and RT time, heart rate (HR), and Bazett QTc were determined from a digital electrocardiogram during quiet rest and a cold pressor test (CPT). QT/RR coherence was calculated across three spectral frequencies including mean (Mean Coh; 0–0.2 Hz), low (LF Coh; 0.04–0.15 Hz) and high (HF Coh; 0.15–0.40 Hz). Respiration was collected via impedance pneumography. Results: No statistical differences were observed among demographics. Pairwise comparisons demonstrated a significantly reduced Mean Coh at rest in the hypogonadal group compared to the eugonadal; during the CPT, the hypogonadal group had significantly elevated Mean and HF Coh. No group main effects were observed with analysis of variance. Condition main effects were observed in QT, Mean, LF and HF Coh. Significant group-condition interactions were present in Mean and HF Coh, while trends toward significance were observed on LF Coh, HR, QT and RT. Post-hoc pairwise comparisons revealed that the change from rest to CPT in Mean, LF and HF Coh was significant in the eugonadal group, but not the hypogonadal. Conclusion: Hypogonadism in SCI appears to adversely affect temporal VR characteristics independently of QTc. QT/RR coherence suggests an autonomic synergism with testosterone. These findings may represent a pathologic sequelae that precedes the well-documented hypogonadic effects on QTc.

Flexural creep of long fiber-reinforced thermoplastic composites: Effect of processing-dependent fiber variables on creep response

Flexural creep properties were studied as a function of fiber weight fraction and processing-induced fiber alignment in extrusion/compression-molded, long fiber-reinforced thermoplastic (LFT) nylon 6/6, polypropylene, and high-density polyethylene and their 10 wt.% and 40 wt.% E-glass fiber reinforced LFT composites. The residual fiber lengths and probability distribution parameters were near-equal, regardless of the initial fiber length and processing. Creep compliances decreased with increasing fiber weight fraction, and clear influence of fiber alignment was found in model parameters. Processing-induced fiber alignment imaged using X-ray radiography, was correlated with the creep compliances of strategically sectioned specimens, and tested as per ASTM D-2990. Longitudinal fibers aided in lowering the creep compliance, and the range in compliance decreased with lower preferential fiber alignment. Creep compliances from flexural creep tests and dynamic mechanical analysis/static creep tests were combined using time–temperature–stress superposition (TTSSP) to construct long-term master curves that correlated closely with long-term tests.

Direct Molding of Long Fiber Reinforced Thermoplastics

Direct Molding of Long Fiber Reinforced Thermoplastics

An Elastic-plastic Damage Model for Long-fiber Thermoplastics

This article proposes an elastic-plastic damage model that combines micromechanical modeling with continuum damage mechanics to predict the stress— strain response of injection-molded long-fiber thermoplastics. The model accounts for distributions of orientation and length of elastic fibers embedded in a thermoplastic matrix whose behavior is elastic-plastic and damageable. The elastic-plastic damage behavior of the matrix is described by the modified Ramberg—Osgood relation and the 3D damage model in deformation assuming isotropic hardening. Fiber/matrix debonding is accounted for using a parameter that governs the fiber/matrix interface compliance. A linear relationship between this parameter and the matrix damage variable is assumed. First, the elastic-plastic damage behavior of the reference aligned fiber composite containing the same fiber volume fraction and length distribution as the actual composite is computed using an incremental Eshelby—Mori—Tanaka mean field approach. The incremental response of the latter is then obtained from the solution for the aligned-fiber composite by averaging over all fiber orientations. The model is validated against the experimental stress—strain results obtained for long-glass-fiber/polypropylene specimens.

Rheological characterization of long fiber thermoplastics – Effect of temperature, fiber length and weight fraction

Isothermal squeeze flow tests were conducted on E-Glass/polypropylene long fiber thermoplastics (LFT) to obtain the rheological characteristics of the material over a range of squeeze rates (0.5–60 mm/min). A transversely isotropic power-law model has been incorporated to capture the combined effect of shear and extensional flow behavior. Scott’s approach [Bird RB. Useful non-Newtonian models; 1976. p. 13–34] was used to obtain the shear power-law parameters, which were then used to calculate the radial velocity in the r-direction. The continuity equation was used to calculate transverse velocity in the z-direction. Radial and through the thickness velocity profiles were determined to obtain the extensional and the shear strain rates. Finally the extensional and shear viscosities were determined at strain rates calculated. Good agreement between the experimental applied stress and the predicted curves from the model was achieved. Effects of mold separation, mold temperature, and fiber length on viscosity at constant fiber weight fraction were examined. Effect of fiber weight fraction on viscosity at constant fiber length, mold separation and temperature was examined. Results indicate that viscosities decrease with either increase in mold temperature or decrease in fiber length at constant mold separation and fiber weight fraction. Viscosities also decreased with decrease in fiber weight fraction.

Effect of Compounding and Injection Molding on the Mechanical Properties of Flax Fiber Polypropylene Composites

The present study focuses on short flax fiber, as well as long flax fiber-reinforced polypropylene (flax/PP) composites, manufactured by the injection molding method. Compounding of flax with two different grades of PP (with and without maleic anhydride (MA-PP) grafting) is carried out by four methods: kneading process, Henschel kinetic mixer, extrusion compounding, and production of long fiber thermoplastic (LFT) granules through pultrusion. The effect of the compounding method and injection molding on the fiber length and mechanical properties of the composites is being investigated. Furthermore, the effect of fiber—matrix adhesion on the mechanical response is being discussed. It can be concluded that the reduction in fiber length, associated with injection molding, did not affect the tensile properties significantly for the studied systems due to improvements in fiber orientation along the polymer flow direction and increased fiber efficiency through dimensional changes due to fiber opening. The addition of MA-PP led to improvements in the tensile strength of injection-molded composites. Kneader compounded composites showed maximum tensile strength as well as stiffness when compared with other compounding methods.

Flexural creep behavior of discontinuous thermoplastic composites: Non-linear viscoelastic modeling and time–temperature–stress superposition

Flexural creep behavior of nylon 6/6, polypropylene and high-density polyethylene long fiber thermoplastic (LFT) composites was studied according to ASTM D-2990. Neat polymers were tested for baseline data and compared with the 40 wt.% E-glass reinforced LFTs, all processed by compression molding. All materials exhibited non-linear viscoelasticity and showed a succession in creep resistance consistent with static flexural yield strength. A four parameter empirical model used for short fiber thermoplastics (SFT), proposed by Hadid et al., was found to provide an excellent fit to the experimental data. Time-compliance data from flexural creep and dynamic mechanical analysis (DMA) were combined to utilize short-term flexural creep tests to predict lifetime of the composites. A time–temperature–stress superposition (TTSSP) procedure was used, where stress-based vertical shifts were applied in addition to horizontal shifts used in a traditional time–temperature superposition (TTSP). Master curves obtained by this method projected the long-term creep properties, the order of creep resistance being consistent with the flexural creep data.

Injection Molding of Long Fiber Reinforced Thermoplastic Composites

Abstract Long fiber reinforced thermoplastic composites are one of the fastest growing segments experiencing 30% growth per year, over the last decade. This study describes the development of long fiber reinforced thermoplastic (LFRT) composites and the effect of fiber length on mold and process parameters of injection molding process. LFRT pellets of different sizes were prepared by an extrusion process using a specially designed impregnation die and palletized into different lengths. These pellets were injection molded to develop LFRT composites. Fiber length distributions were analyzed using a profile projector and an image analyzer software system. Maleic anhydride grafted polypropylene (MA-g-PP) was added to improve the adhesion between glass fibers and polypropylene. Improvement in mechanical properties were analyzed for different pellet sizes with different fiber content for long fiber reinforced polypropylene (LFPP) and long fiber reinforced polyamide (LFPA) composite.

Characterization of fatigue behavior of long fiber reinforced thermoplastic (LFT) composites

Fatigue behavior of long fiber reinforced thermoplastic composites (polypropylene/20 vol.% E-glass fiber) is presented in terms of stress – number of cycles to failure curves. Samples tested along longitudinal direction showed a higher fatigue life than the transverse samples which can be explained by the preferential orientation of the fibers along the longitudinal direction developed during the processing. Fatigue life decreased with increase in frequency. Hysteretic loss and temperature rise were measured; they depended on the stress amplitude as well as the cyclic frequency. Long fiber reinforced thermoplastic composite showed a lower temperature rise compared to unreinforced PP because long fiber reinforced thermoplastic has higher thermal conductivity than unreinforced PP and thus faster heat dissipation to the surroundings occur. The hysteretic heating also led to decrease in the modulus of long fiber reinforced thermoplastic as a function of number of cycles due to the softening of the matrix during fatigue cycling and depended on stress amplitude and frequency of the test.

Prediction of the Elastic—Plastic Stress/Strain Response for Injection-Molded Long-Fiber Thermoplastics

This article proposes a model to predict the elastic—plastic response of injection-molded long-fiber thermoplastics (LFTs). The model accounts for elastic fibers embedded in a thermoplastic resin that exhibits the elastic—plastic behavior obeying the Ramberg—Osgood relation and J-2 deformation theory of plasticity. It also accounts for fiber length and orientation distributions in the composite formed by the injection-molding process. Fiber orientation was predicted using an anisotropic rotary diffusion model recently developed for LFTs. An incremental procedure using Eshelby's equivalent inclusion method and the Mori—Tanaka assumption is applied to compute the overall stress increment resulting from an overall strain increment for an aligned-fiber composite that contains the same fiber volume fraction and length distribution as the actual composite. The incremental response of the latter is then obtained from the solution for the aligned-fiber composite by averaging over all fiber orientations. Failure during incremental loading is predicted using the Van Hattum—Bernado model that is adapted to the composite elastic—plastic behavior. The model is validated against the experimental stress—strain results obtained for long-glass-fiber/polypropylene specimens.

An anisotropic rotary diffusion model for fiber orientation in short- and long-fiber thermoplastics

The Folgar–Tucker model, which is widely-used to predict fiber orientation in injection-molded composites, accounts for fiber–fiber interactions using isotropic rotary diffusion. However, this model does not match all aspects of experimental fiber orientation data, especially for composites with long discontinuous fibers. This paper develops a fiber orientation model that incorporates anisotropic rotary diffusion. From kinetic theory we derive the evolution equation for the second-order orientation tensor, correcting some errors in earlier treatments. The diffusivity is assumed to depend on a second-order space tensor, which is taken to be a function of the orientation state and the rate of deformation. Model parameters are selected by matching the experimental steady-state orientation in simple shear flow, and by requiring stable steady states and physically realizable solutions. Also, concentrated fiber suspensions align more slowly with respect to strain than models based on Jeffery's equation, and we incorporate this behavior in an objective way. The final model is suitable for use in mold filling and other flow simulations, and it gives improved predictions of fiber orientation for injection molded long-fiber composites.

Effect of UV exposure on the microstructure and mechanical properties of long fiber thermoplastic (LFT) composites

In this work, the effect of ultraviolet (UV) exposure on the microstructure and dynamic Young’s modulus of polypropylene (PP)/21 vol.% E-glass LFT and neat PP was investigated. Microscopic observations revealed that the damage due to UV was confined to the surface region only in the form of surface cracking and exposure of fibers to the surface in the case of long fiber thermoplastic (LFT) and surface cracking in the case of neat PP. Fourier transform infrared spectroscopy showed that the crystallinity of PP in the damaged layer increased, both in neat PP as well as in LFT, with exposure time. This is due to chemicrystallization, which involves rearrangement of amorphous broken polymer chains into crystalline form. Crystallinity of PP in the damaged layer in LFT increased at a higher rate as compared to that in neat PP. Results of nanoindentation showed that the Young’s modulus of the PP in the damaged layer increased, with UV exposure time; the rate of modulus increase being higher in the case of LFT than in neat PP. Although the local Young’s modulus of the degraded layer increased, the dynamic Young’s modulus of the overall composite showed a decrease with UV exposure time.

Two-dimensional microstructure based modelling of Young's modulus of long fibre thermoplastic composite

The dependence of physical and mechanical properties on microstructure is well known. Various numerical and analytical methods are routinely employed to predict the properties of multiphase materials but these models make certain simplifying assumptions about the microstructure of the material, such as homogeneity, that are not accurate. In the present work, a microstructure based finite element code called object oriented finite element method (OOF) has been used to investigate the longitudinal elastic modulus of a long fibre reinforced thermoplastic (LFT) composite (glass fibre/polypropylene). The modulus value predicted by OOF was then compared with the experimental value and values predicted by various models. It is shown that by taking into account the important microstructural parameters in the composite, i.e. the actual fibre orientation and distribution, accurate prediction of modulus can be obtained.

Characterization of long fiber thermoplastic/metal laminates

Long fiber thermoplastic (LFT) composite/metal laminate (LML) is a hybrid composite consisting of alternate layers of metals such as aluminum and an LFT composite, which combines advantages from both the constituents. The LFT/Al laminates (LMLs) were processed by compression molding and were characterized for their Young’s modulus, mechanical strength, and low-velocity impact (LVI) properties. The average values of specific elastic modulus and specific tensile strength were approximately 20 GPa/(gcm−3) and 108.5 MPa/(gcm−3), respectively. Failure mechanisms included delamination between LFT composite and Al, fiber fracture and pullout in LFT composite, and shear fracture of aluminum and LFT composite layers. Rule-of-mixtures (ROM) predictions of laminate properties in tension compared well with the experimental values. Specific perforation energy of the laminates determined by LVI tests was 7.58 J/(kgm−2), which is significantly greater than that of the LFT composite alone, 1.72 J/(kgm−2). Overall, the LML showed significant improvement in the properties as compared to the LFT composite.

Process simulation, design and manufacturing of a long fiber thermoplastic composite for mass transit application

Abstract Long fiber thermoplastics (LFTs) have witnessed rapid growth in thermoplastics matrix composites, mainly due to developments in the automotive and transportation sector. In LFTs, pelletized thermoplastic polymer matrix is reinforced with long glass or carbon fibers (3–25 mm) are processed by extrusion-compression molding. The current work focuses on the applied science and manufacturing of E-glass/polypropylene (E-glass/PP) LFT composite material. Process simulation was conducted to evaluate the flow of fiber filled viscous charge during the compression molding of the LFT composite. Studies on optimum charge size and placement in the tool, press force, temperature of mold, shrinkage and warpage were also conducted. The flow pattern of the molten charge in the mold and the resulting fiber orientation predicted by process simulation are verified experimentally. The studies have been applied for a mass transit/transportation component namely, a LFT battery box access door for form-fit-function to replace a heavy metal door. Weight reduction of 60% was achieved using 40% weight percent E-glass/PP LFT over the metal design.