Preferred Fiber Orientation Research Articles

Performance of long fiber reinforced thermoplastics subjected to transverse intermediate velocity blunt object impact

An understanding of long fiber reinforced thermoplastic (LFT) behavior while undergoing high energy, high strain rate impact is essential in promoting its use in automotive and other markets. In this work, damage tolerance of polypropylene (PP)/E-glass LFTs subjected to transverse blunt object impact (BOI) was investigated, treated from an experimental standpoint, in order to characterize energy dissipation and damage modes. LFT subjected to BOI exhibited high impact energy dissipation, which increased linearly with increasing areal density. The average impact energy dissipation at the critical velocity (ballistic limit) was 167 J and 121 J for a 4.61 g cm −2 specimen impacted by flat and conically shaped projectiles, respectively. The fiber orientation also played a large role in energy dissipation; failure appeared to occur along planes of preferential fiber orientation. The critical energy for specimen perforation did not vary with the mass of the impactor, i.e. strain rate, in the range investigated. This overall work advances the state-of-the-art in LFTs with an automotive focus.

Direct observation and measurement of fiber architecture in short fiber-polymer composite foam through micro-CT imaging

A non-destructive X-ray imaging technique was used to determine internal structure in a polymer foam reinforced with short fibers. The technique, known as micro-CT (for computerized tomography), was used to measure the fiber length distribution (FLD) and fiber orientation distribution (FOD), two parameters that are critical to the behavior of short-fiber-reinforced composites. Phenolic foam reinforced with short glass fibers was used as an exemplar to demonstrate the potential of this technique, exploiting the large difference in density between the two components. Direct 2D and 3D images were generated in which individual fibers were clearly resolved, along with portions of the foam structure. The images were analyzed using computer software to obtain quantitative FLD and FOD data. A distinct preferred orientation of fibers was revealed that was attributed to shear flow during foam expansion. For quantitative analysis of microstructure in short fiber composites, the micro-CT technique affords numerous advantages over the conventional approach of parallel dissection followed by image analysis of polished surfaces, and may be useful for determining FLD and FOD in polymer composites with dense matrices.

Modeling and characterization of damping in carbon nanofiber/polypropylene composites

The objective of this paper is to summarize recent analytical and experimental investigations of the dynamic mechanical properties of carbon nanofiber/polypropylene composites. In this case, the carbon nanofibers are vapor grown carbon fibers (VGCF) which are grown catalytically from gaseous hydrocarbons using metallic catalyst particles. The mechanical damping and storage modulus of these materials are measured by using a Dynamic Mechanical Thermal Analyzer (DMTA). For the analytical model, the elastic–viscoelastic correspondence principle is used to transform the elastic Halpin–Tsai equations to complex form. The model is then validated for the prediction of the damping and stiffness of carbon nanofiber/polypropylene composites having a preferential fiber orientation in the direction of injection. Good agreement between measured and predicted storage values is found over the range of fiber volume fractions investigated. Although predicted and measured damping values are in the same range, the predicted values do not show the measured decrease in damping with increasing fiber volume fraction. Predictions show that composites having very low fiber aspect ratios should have higher damping than those having high fiber aspect ratios, and that the carbon nanofiber aspect ratios of approximately l/ d=19 are in the range which yields the highest predicted damping.

Remodelling of continuously distributed collagen fibres in soft connective tissues

Extracellular matrix remodelling plays an essential role in tissue engineering of load-bearing structures. The goal of this study is to model changes in collagen fibre content and orientation in soft connective tissues due to mechanical stimuli. A theory is presented describing the mechanical condition within the tissue and accounting for the effects of collagen fibre alignment and changes in fibre content. A fibre orientation tensor is defined to represent the continuous distribution of collagen fibre directions. A constitutive model is introduced to relate the fibre configuration to the macroscopic stress within the material. The constitutive model is extended with a structural parameter, the fibre volume fraction, to account for the amount of fibres present within the material. It is hypothesised that collagen fibre reorientation is induced by macroscopic deformations and the amount of collagen fibres is assumed to increase with the mean fibre stretch. The capabilities of the model are demonstrated by considering remodelling within a biaxially stretched cube. The model is then applied to analyse remodelling within a closed stented aortic heart valve. The computed preferred fibre orientation runs from commissure to commissure and resembles the fibre directions in the native aortic valve.

Strength and fracture properties of industrially prepared steel fibre reinforced concrete

The paper reports on a study of steel fibre reinforced concrete (SFRC) which was prepared using normal industrial mixing, compaction and curing conditions. Both strength (compressive and tensile) and fracture (toughness measurements) characteristics have been investigated with test specimens prepared from 5 m long SFRC piles. The piles contained only steel fibre reinforcement and were manufactured in exactly the same way as ordinary piles. Slight differences in the tensile strengths (determined via torsion tests) were observed due to the existence of preferential fibre orientation. Flexural tests on notched beams (to evaluate fracture characteristics) produced a much more stable, reproducible, test than that observed for un-notched beams. Hence, it is concluded that the notched beam is a better geometry in terms of test stability and reliability. The results showed that tests specimens taken from industrially prepared SFRC displayed similar characteristics compared to that observed with test specimens prepared under laboratory conditions, with regards to the strength, fracture characteristics and, in particular, the variation observed.

Uniaxial tension test for steel fibre reinforced concrete––a parametric study

Abstract A RILEM Draft Recommendation was proposed in 2001 for obtaining the stress versus crack opening (σ–w) response of steel fibre reinforced concrete through a uniaxial tension test. The present study analyses the robustness of the recommended test through a parametric study. Furthermore, the methodology is extrapolated to cores extracted from cast elements. Also, the effect of the coring direction with respect to the preferential fibre orientation caused by the compaction procedure is examined. The study demonstrates that the test is robust and representative of the material response, and could be used for determining the σ–w relation of the material that may be needed for comparing the performance of different fibres or for providing input for finite element analysis. No significant influence of the characteristics of the specimen or problems of instability due to the loss of control were encountered. There is some relative rotation between the crack faces but its influence on the σ–w response is expected to be negligible. The parameters obtained from the tests exhibit coefficients of variation of up to 30%, which is mainly attributed to the randomness of the number of fibres bridging the crack, considering the relatively small cross-section of the specimen.

Extrusion blow molding of long fiber reinforced polyolefins

AbstractA 58% (by weight) long glass fiber reinforced (LGF)‐HDPE master batch was blended with a typical blow molding HDPE grade. HDPE composites having between 5% and 20% (by weight) long fiber content were extruded at different processing conditions (extrusion speed, die gap, hang time). The parison swell (diameter and thickness) decreased with increasing fiber content. Although the HDPE exhibited significant shear rate dependence, the LGF/HDPE composites were shear rate insensitive. Both the diameter and weight swell results also indicated very different sagging behavior. The LGF/HDPE parisons did sag as a solid‐body (equal speed at different axial locations) governed by the orientation caused by the flow in the die. Samples taken from blown bottles showed that fiber lengths decreased to 1‐3 mm, from the original 11 mm fiber length fed to the extruder. No significant difference in fiber length distribution was found when samples for different regions of the bottle were analyzed. SEM micrographs corroborate the absence of fiber segregation and clustering or the occurrence of fiber bundles (homogeneous spatial fiber distribution) as well as a preferential fiber orientation with the direction of flow. The blowing step did not change the orientation of the fibers. Five‐percent (5%) and 10% LGF/HDPE composites could be blown with very slight variations to the neat HDPE inflation conditions. However, 20% LGF/HDPE composites could not be consistently inflated. Problems related to blowouts and incomplete weldlines were the major source of problems.

Draw-ratio-dependent morphology of biaxially oriented polypropylene films as determined by atomic force microscopy

Using atomic force microscopy we have examined the surface morphology of sequentially biaxially oriented polypropylene (BOPP) films. The surface was shown to be dominated by a nanometer-scale fiber-like network structure, the configuration of which was found to be determined by the relationship between the draw ratios used in the bi-directional stretching processes [machine draw (MD) and transverse draw (TD)]. For the film fabricated with MD and TD ratios of 5.2:1 and 9:1, respectively, preferential orientation of fine fibers to the TD direction and larger veins to the MD direction were observed. When MD and TD ratios became similar, no predominant TD direction fiber alignment and no larger veins were observed. We have shown that the residual effects of the first stretching of the film surface can provide information on the way in which morphological development of the BOPP occurs.

Practical use of the statistically modified laminate model for injection moldings. Part 1: Method and verification

AbstractThe fibers in injection molded FRP provide the material's strength and stiffness; however, they also supply many of the problems. Preferential orientation of fibers during molding can reduce strength and stiffness below expected values in critical directions, or induce warpage in thin walled sections. Makers of short fiber reinforced injection molded products typically use computer aided engineering packages to optimize product performance and manufacturing variables. However, the reliability of the fiber orientation simulation can be limited, and the method is not easily understood, making an assessment of accuracy for a given situation difficult. In addition, the structural module of flow analysis packages is often a basic package with many features missing. This paper presents a structural analysis system for injection molded parts made of short fiber reinforced plastics. A full‐featured commercial structural analysis code is interfaced with a flow analysis program using a practical material model that takes into account the effects of local fiber orientation. The system is completely open to the user, and can be modified as required.

Interface formation in carbon fibre reinforced magnesium alloys (AZ91)

In metal matrix composites the adhesion between a ceramic fibre and the metal matrix should be submitted on the atomic scale to physical or chemical forces, without forming a reaction zone [1, 2]. Only in this case will fibres remain undamaged and a load transformation from the fibre to the matrix take place during composite application. For different metal matrix composites different processing methods have been developed to achieve this interface structure. In SiC-fibre reinforced titanium alloys the fibres are protected by a carbon coating [3, 4] and the mechanical properties of the composites remain unchanged as long as the protective coating prevents the reaction of titanium with the fibre surface [4]. In alumina fibre reinforced aluminium alloys fibre wetting and adhesion is submitted to atomic forces and can be supported by magnesium segregation in the interface [5, 6]. Magnesium atoms take up vacancy positions in the alumina surface, reducing the wetting angle during melt infiltration [6]. Thermal loading of these composites results in a fibre-matrix reaction that leads to a fibre and composite strength degradation [5, 6]. In carbon fibre reinforced magnesium alloys processed by hot pressing of fibre layers and matrix foils only unsufficient wetting and low adhesion between fibre and matrix result. At positions with moderate adhesion a thick magnesia layer was observed in these composites [7-9]. The magnesia formation was interpreted to arise from the oxidation of the matrix during processing, which was performed in an oxygen rich environment. This letter reports the results of transmission electron microscopy (TEM) studies on the interfaces of carbon fibre reinforced (Toray 300) magnesium alloys (AZ91:9 at% Zn, 1 at% A1, Mn, Si and Cu impurities) processed by squeeze casting of chopped carbon fibre preform bonded by an SiO2 binder (4 wt %). The fibre preform was heated to 600 °C before melt infiltration, which took place at 812 °C. Composites with a fibre volume fraction of 20% were obtained [10]. In the preform the fibres have a preferential orientation in a plane perpendicular to the infiltration direction. For TEM investigations sample preparation was performed parallel and perpendicular to the preferential fibre orientation plane. Preparation was performed in the usual way by grinding,

Dynamic properties of short fiber–EPDM matrix composites as a function of strain amplitude

AbstractThe dynamic properties of composite materials consisting of an ethylene–propylene rubber matrix (EPDM) and short polyester polyethylene‐terephthalate (PET) fiber vary with dynamic stress amplitude applied to the material. These variations support the statement that fiber treatment with 1,4‐carboxy‐sulphonyl‐diazide, which acts as a bridge between the fiber and the matrix and hence enhances the strength of the interface enabling it to resist greater strain applied to the composite and, as a consequence, yielding greater retention values of the storage modulus, measured longitudinally to preferential fiber orientation, E′L. By means of transversal measurements of the storage modulus, E′T, of these materials it is possible to determine a parameter b, which eventually indicates the degree of matrix–fiber bonding and which is consistently higher for materials filled with surface‐treated fiber. This enhanced phase adhesion is further confirmed by higher equivalent interfacial thickness values, ΔR, which, in addition, vary less with increasing dynamic strain amplitude. Finally dissipated energy variation or mechanical energy loss, Eloss, is studied as a function of fiber content and strain amplitude. Experimental findings show Eloss to increase with fiber content and strain amplitude, when measured at constant strain amplitude ϵ0, and to yield higher values for treated fiber samples. © 1994 John Wiley & Sons, Inc.

Mechanical properties of glass fiber/talc/polybutylene-terephthalate composites as processed by the radlite technique

In this investigation, the effect of glass fiber and talc loading on the mechanical and physical properties of chopped glass fiber reinforced polybutylene terephthalate composites has been studied. Further areas of investigation include the effect of fiber length, talc particle size and talc surface treatment. The so-called RADLITE process is employed on a laboratory scale to prepare composite sheets. Since the sheets show a preferred fiber orientation, testing was carried out in two directions, parallel and transverse to the direction of preferred fiber orientation (MFD and TFD). The fiber loading clearly dominates the composite performance and has a pronounced positive effect on it. Increasing the talc loading reduces strength. Under certain conditions, talc can be as efficient as glass fibers in increasing the composite modulus. Higher talc loadings do not impair the composite impact properties. A reduction in fiber length from 12·7 to 6·4mm causes improved properties in the MFD, which may be explained by a higher degree of fiber orientation. No significant effect of the talc particle size can be established as a result of measuring the variable over too narrow a range. A talc surface treatment does not result in improved composite performance.

Preferred orientation of pitch precursor fibers and carbon fibers prepared from isotropic pitch

The preferred orientation of pitch precursor fibers and carbon fibers from an isotropic pitch was studied using x-ray diffraction techniques. The φ scan profile of a pitch precursor fiber was divided into two parts: φ scan-dependent (anisotropic) and φ scan-independent (isotropic) profiles. The half width at half maximum (HWHM) intensity of the anisotropic part became smaller as the pitch viscosity increased during spinning and as the filament diameter decreased. The area ratio of the anisotropic part to the total area increased when spinning at higher pitch viscosities and when making the diameter thinner; no preferred orientation existed for pitch precursor fibers with very large diameters. A thermodynamic model is proposed for the preferred orientation formation when spinning an isotropic pitch. The carbon fiber exhibited no skin-core difference in preferred orientation. A crystallite with a larger crystallite size Lc(002) in the carbon fiber was shown to be better aligned along the fiber axis than that with a smaller Lc(002). Furthermore, a well-aligned crystallite possessed a larger Lc(002) than one that was misaligned. The pitch precursor fiber also exhibited such a correlation between Lc(002) and the preferred orientation, but the correlation was weak.

An in oculo model for quantitation of vascular growth in the rat heart

The aim was to describe, both qualitatively and quantitatively, vascularisation in an in oculo model that may prove useful for studying neovascularisation in the myocardium. Whole hearts from 13-14 d rat fetuses were implanted into the anterior chamber of host Sprague Dawley rats. Implant growth was monitored and at 3 months postimplantation grafts were recovered. Fixed tissue sections were histologically characterised with light and electron microscopy and morphometrically analysed using image analysis. At two weeks, 92% of implanted hearts were beating. At 1 month, well vascularised implants that were beating were slightly larger than those not beating, at 16.0(SD 1.0) v. 12.3(2.0)mm2. By 2 months, beating implants were significantly (p less than 0.05) larger than non-beating implants, at 23.5(3.0) v. 12.4(1.7)mm2. In non-beating implants, the larger the extent of vascularisation, the larger the implant size. Histologically, well vascularised beating implants showed normal adult cardiac myocyte structure, with normal appearing sarcomeres and intercalated discs, but no preferential fibre orientation. The amount of area occupied by muscle and connective tissue varied between implants. Implant vasculature, including arterioles and capillaries, appeared normal. The percentage of vascular area was consistent between grafts. A small percentage of total area was occupied by lymphocytes. This investigation indicates that sufficient vascularisation is important for growth of grafts, provides for the first time quantitative morphometric data characterising the in oculo cardiac implant model, and reports baseline data that will be useful for comparison to myocardium treated with putative angiogenic factors in the future.

Polyester moulding compounds with novel initial fibre orientation for use in injection moulding

This paper describes a systematic study of the effect of initial fibre orientation on the flexural and impact strength of injection moulded thermosetting polyester moulding compounds. A range of special compounds was prepared, with three different types of initial fibre orientation: fully random (3-D), random-in-plane (2-D) and unidirectional (1-D). It was found possible to produce injection mouldings with low levels of fibre breakage and significantly improved properties by using starting compounds with either 2-D or 1-D fibre orientation. This does not appear to be possible with 3-D compounds, such as dough moulding compounds. In addition to using a starting material with preferred fibre orientation, it is also desirable that the matrix viscosity under processing conditions be as low as possible. In the case of sheet moulding compounds, ‘physically thickened’ compounds give better performance than MgO-thickened materials for this reason. Comparisons between the behaviour of dough moulding compounds and that of compounds made by the continuous impregnation method confirm the findings of other workers, that the latter technique is capable of producing compounds with a much lower level of initial fibre degradation.

Form and Function in Organisms

SYNOPSIS. While form and function are always together in organisms, the studies of morphology and physiology often take place in different places by different people. In the 20th century some zoologists have re-united the two into a point of view called “functional morpholgy.” The concepts a student should know in order to set about a study in functional morphology include: (1) the scales of size and time, (2) structural hierarchy, (3) permission and constraint of function, (4) properties as descriptors of structure and function, (5) analogy and homology. The structural phenomena that a student will see in any organism include: (1) pattern, especially polarity and symmetry, (2) dimensions that signify the size scale, (3) heterogeneity, which includes the concepts: (4) composite materials and (5) anisotropy resulting from the preferred orientation of fibers and crystals. Rather than being a discipline, functional morphology is the combination of structural and physiological thinking that invites other points of view, especially those of development and evolution, to be added.

The Tangent Compliance of Staple-fibre Bundles in Tension

The mechanical properties of an untwisted oriented assembly or “bundle” of fibres differ from those of the constituent fibres and are most conveniently described by a tangent-compliance matrix whose non-zero entries can be readily related to various uniaxial tests. The most interesting and distinguishing characteristic of a staple-fibre bundle is its compliance when stretched in the direction of preferential fibre orientation. The resistance to deformation is highly dependent on the lateral pressure, which determines the amount of relative fibre slippage. A theoretical connexion between the properties of the individual fibres in the bundle and the behaviour of the bundle is established in this paper. The key fibre properties are shown to be the length distribution, the average tensile properties, and the single-fibre withdrawal (i.e., frictional) properties. A reasonable agreement with experimental data is found, although evaluation of the model is difficult. It is suggested that the theory will find appl...

Flow induced fiber orientation in an expanding channel tubing dies

AbstractIn the application of plastic pipes for fluid transport and for the protection of underground electrical cables, it is desirable to improve mechanical properties, particularly in the hoop direction. The use of orientable reinforcing particles such as chopped glass fibers could make possible such an improvement if the orientation of the fibers could be controlled. While conventional pipe extrusion dies tend to promote axial fiber orientation, the use of an expanding channel die has been proposed to produce a preferential hoop orientation of fibers. In this paper, a theoretical model of the flow of a fiber suspension through an expanding channel die that predicts the fiber orientation distribution at the die exit is described. The effects of Theological properties and die geometry on the final fiber orientation distribution are predicted. The results of an experimental study of fiber orientation in pipe extruded using an expanding channel die are shown to be in agreement with the theoretical predictions.

Electro-biorheology.

Studies on piezoelectricity in biopolymers are reviewed with some examples. By measuring anisotropy of elastic and piezoelectric constants in fish skin, the direction of preferred orientation of collagen fibers is determined. Temperature and moisture dependence of real and imaginary parts of the piezoelectric constant observed for oriented film of fibrin gel indicates that temperature-time equivalence holds for the piezoelectric relaxation. The piezoelectric constant for oriented film of polyhydroxybutyrate decreases above glass transition temperature, which is related to the elastic and dielectric relaxations. When alpha-helical molecules of poly-gamma-methyl-L-glutamate are electrically oriented in ethylene dichloride solution and shear stress is given by ultrasound, the piezoelectric polarization is observed. When the film of poly-gamma-methyl-glutamate is wrapped around the femur of rat, the formation of new bone is produced.

Collagen fibre reorientation around a crack in biaxially stretched aortic media

The distribution and orientation of collagen fibres in unstretched and biaxially stretched pig aortic media have been determined by an X-ray diffraction technique in order to analyse the reorientation of collagen fibres in response to the presence of a notch in biaxially stretched samples. The results show that collagen fibres align themselves following the force trajectories. This results in preferential fibre orientation perpendicular to the advancing crack tip, in agreement with the theoretical predictions of stress concentration effect due to the presence of an elliptical notch in an elastic plate.