Main Fiber Orientation Research Articles

Finite Element Simulation and Experimental Assessment of Laser Cutting Unidirectional CFRP at Cutting Angles of 45° and 90°.

Laser cutting of carbon fibre-reinforced plastics (CFRP) is a promising alternative to traditional manufacturing methods due to its non-contact nature and high automation potential. To establish the process for an industrial application, it is necessary to predict the temperature fields arising as a result of the laser energy input. Elevated temperatures during the cutting process can lead to damage in the composite's matrix material, resulting in local changes in the structural properties and reduced material strength. To address this, a three-dimensional finite element model is developed to predict the temporal and spatial temperature evolution during laser cutting. Experimental values are compared with simulated temperatures, and the cutting kerf geometry is examined. Experiments are conducted at 45° and 90° cutting angles relative to the main fibre orientation using a 1.1 mm thick epoxy-based laminate. The simulation accurately captures the overall temperature field expansion caused by multiple laser beam passes over the workpiece. The influence of fibre orientation is evident, with deviations in specific temperature data indicating differences between the estimated and real material properties. The model tends to overestimate the ablation rate in the kerf geometry, attributed to mesh resolution limitations. Within the parameters investigated, hardly any expansion of a heat affected zone (HAZ) is visible, which is confirmed by the simulation results.

Decoding the microstructural properties of white matter using realistic models

Multi-echo gradient echo (ME-GRE) magnetic resonance signal evolution in white matter has a strong dependence on the orientation of myelinated axons with respect to the main static field. Although analytical solutions have been able to predict some of the white matter (WM) signal behaviour of the hollow cylinder model, it has been shown that realistic models of WM offer a better description of the signal behaviour observed.In this work, we present a pipeline to (i) generate realistic 2D WM models with their microstructure based on real axon morphology with adjustable fiber volume fraction (FVF) and g-ratio. We (ii) simulate their interaction with the static magnetic field to be able to simulate their MR signal. For the first time, we (iii) demonstrate that realistic 2D WM models can be used to simulate a MR signal that provides a good approximation of the signal obtained from a real 3D WM model derived from electron microscopy. We then (iv) demonstrate in silico that 2D WM models can be used to predict microstructural parameters in a robust way if ME-GRE multi-orientation data is available and the main fiber orientation in each pixel is known using DTI. A deep learning network was trained and characterized in its ability to recover the desired microstructural parameters such as FVF, g-ratio, free and bound water transverse relaxation and magnetic susceptibility. Finally, the network was trained to recover these micro-structural parameters from an ex vivo dataset acquired in 9 orientations with respect to the magnetic field and 12 echo times. We demonstrate that this is an overdetermined problem and that as few as 3 orientations can already provide comparable results for some of the decoded metrics.

The intervertebral disc from embryonic development to disc degeneration: insights into spatial cellular organization

Background contextLow back pain is commonly attributed to intervertebral disc (IVD) degeneration. IVD resembles articular cartilage in its biochemical and cellular composition in many ways. For articular cartilage, degeneration stage-specific characteristic spatial chondrocyte patterns have recently been described. PurposeThis study addresses how spatial chondrocyte organization in the IVD changes from early embryonic development to end stage degeneration. Study designEx vivo immunohistochemical analysis. MethodsWe immunohistochemically investigated bovine IVD-tissue (n=72) from early embryonic development to early disc degeneration and human adult IVD-tissue (n=25) operated for trauma or degeneration for cellular density and chondrocyte spatial organization. IVD samples were sectioned along the main collagen fiber orientation. Nuclei were stained with DAPI and their number and spatial patterns were analyzed in an area of 250,000 µm² for each tissue category. ResultsThe initially very high cellular density in the early embryonic bovine disc (11,431 cells/mm²) steadily decreases during gestation, growth and maturation to about 71 cell/mm² in the fully grown cattle. Interestingly, in human degenerative discs, a new increase in this figure could be noted (184 cells/mm). The IVD chondrocytes appear to be predominantly present as single cells. Especially in the time after birth, string-formations represent up to 32% of all cells in the anulus fibrosus, although single cells are the predominant spatial pattern (>50%) over the entire time. With increasing degeneration, the relative proportion of single cells in human IVDs continuously decreases (12%). At the same time, the share of cells organized in clusters increases (70%). ConclusionSimilar to articular cartilage, spatial chondrocyte organization appears to be a strong indicator for local tissue degeneration in the IVD. Clinical SignificanceIn the future these findings may be important for the detection and therapy of IVD degeneration in early stages.

Mechanical and Thermal Behavior of Fibrous Carbon Materials.

The ability of various commercial fibrous carbon materials to withstand stress and conduct heat has been evaluated through experimental and analytical studies. The combined effects of different micro/macro-structural characteristics were discussed and compared. Large differences in mechanical behavior were observed between the different groups or subgroups of fibrous materials, due to the different types of fibers and the mechanical and/or chemical bonds between them. The application of the Mooney–Rivlin model made it possible to determine the elastic modulus of soft felts, with a few exceptions, which were studied in-depth. The possible use of two different mechanical test methods allowed a comparison of the results in terms of elastic modulus obtained under different deformation regimes. The effective thermal conductivity of the same fibrous materials was also studied and found to be much lower than that of a single carbon fiber due to the high porosity, and varied with the bulk density and the fiber organization involving more or less thermal contact resistances. The thermal conductivity of most materials is highly anisotropic, with higher values in the direction of preferential fiber orientation. Finally, the combination of compression and transient thermal conductivity measurement techniques allowed the heat conduction properties of the commercial fibrous carbons to be investigated experimentally when compressed. It was observed that thermal conductivity is strongly affected under compression, especially perpendicular to the main fiber orientation.

Effects from natural weathering on long-term structural performance of wood-polymer composite cladding in the building envelope

The structural use of wood-polymer composites (WPC) in façades has not been fully investigated, which can be attributed to a lack of practical experience. In this study, three different extruded compounds were tested in Central Europe after one year of natural weathering. Their strength loss was quantified using the flexural strength and pull-though resistance of screws positioned differently in the test sheets. It was found that pull-through is affected by fiber orientation in the same way as is bending strength. Screws at the panel edges parallel to the main fiber orientation showed 26% higher values, whereas the difference between 10 mm and 15 mm edge distance was not significant. Also, round pre-drilled holes instead of slotted ones did not translate into higher resistance values. As far as aging is concerned, the flexural strength became far more degraded by global radiation than did fastener pull-through strength. It can therefore be argued that flexural characteristics alone are not a realistic indicator of the structural durability of a WPC façade.

Temperature distribution during laser based heat conduction welding of CFRP

For the implementation of carbon fiber reinforced (CFRP) parts, these need to be assembled to more complex structures. Therefore, laser transmission welding was transferred to heat conduction welding for joining thermoplastic CFRP to itself. The goal of these investigations was to determine the influence of the focal point geometry and the main fiber orientation within the CFRP on the temperature distribution at the upper joining member. A set-up was chosen consisting of two thermo cameras in order to measure the process temperatures on top and underneath the upper joining member. Furthermore, the heat affected width was determined and correlated to the process temperatures.

Non-destructive characterization of fiber orientation in reinforced SMC as input for simulation based design

The macroscopic properties of materials are strongly influenced by their microstructure. This holds in particular for fiber reinforced composites where fiber distribution and orientation are crucial for the reinforcement to serve its purpose. This essential microstructural information can be obtained from high-resolution images using appropriate methods for quantitative image analysis. Sheet molding compounds feature a very dense layered system of reinforcing fibers and a particularly strong X-ray absorption. Therefore, in this case, state-of-the-art fiber orientation analysis based on 3D images obtained by X-ray microtomography faces problems. In this paper, we determine the local fiber orientation in each pixel by the orientation of the anisotropic Gaussian filter yielding the strongest filter response. Hence, the local fiber orientation can be computed without identifying individual fibers. From the thus determined area weighted orientation distribution, the degree of anisotropy and the main fiber orientation are derived. This extremely robust analysis method is applied to 2D slice images from scanning acoustic microscopy and high-resolution 3D microtomography. We show that the Gaussian filter based fiber orientation analysis method yields comparable results for both imaging techniques. Moreover, comparison with fatigue tests performed on the same specimens proves the image analytically determined fiber orientation and the failure behaviour to be strongly correlated. In particular, a critical degree of anisotropy could be identified. For degrees of anisotropy higher than this limit, the samples behave mechanically like a uniaxial material. The paper thus provides experimentally validated evidence for calibrating micro-mechanical models for subsequent simulation of macroscopic material properties using the combination of high-resolution imaging techniques and quantitative image analysis.

Experimental and Numerical Evaluation of the Mechanical Behavior of Strongly Anisotropic Light-Weight Metallic Fiber Structures under Static and Dynamic Compressive Loading.

Rigid metallic fiber structures made from a variety of different metals and alloys have been investigated mainly with regard to their functional properties such as heat transfer, pressure drop, or filtration characteristics. With the recent advent of aluminum and magnesium-based fiber structures, the application of such structures in light-weight crash absorbers has become conceivable. The present paper therefore elucidates the mechanical behavior of rigid sintered fiber structures under quasi-static and dynamic loading. Special attention is paid to the strongly anisotropic properties observed for different directions of loading in relation to the main fiber orientation. Basically, the structures show an orthotropic behavior; however, a finite thickness of the fiber slabs results in moderate deviations from a purely orthotropic behavior. The morphology of the tested specimens is examined by computed tomography, and experimental results for different directions of loading as well as different relative densities are presented. Numerical calculations were carried out using real structural data derived from the computed tomography data. Depending on the direction of loading, the fiber structures show a distinctively different deformation behavior both experimentally and numerically. Based on these results, the prevalent modes of deformation are discussed and a first comparison with an established polymer foam and an assessment of the applicability of aluminum fiber structures in crash protection devices is attempted.

Influence of the Initial Fiber Orientation on the Weld Strength in Welding of Glass Fiber Reinforced Thermoplastics

The welding factors are significantly lower in welding of fiber reinforced thermoplastics than in welding of unreinforced thermoplastics due to the fiber orientation in the weld. This paper presents results from investigations on the influence of the initial fiber orientation on the weld strength in hot plate and vibration welding for glass fiber reinforced polypropylene and polyamide 6. Injection molded specimens are compared to specimens with main initial fiber orientation being longitudinal and transverse to the joining direction. The results of CT analysis of the fiber orientation in the weld show the opportunity to achieve a higher weld strength by using specimens with fibers being initially oriented longitudinally to the joining direction. The influence of the initial fiber orientation in the parts to be welded on the weld strength in hot plate welding is more distinct than in vibration welding.

Validation of In utero Tractography of Human Fetal Commissural and Internal Capsule Fibers with Histological Structure Tensor Analysis.

Diffusion tensor imaging (DTI) and tractography offer the unique possibility to visualize the developing white matter macroanatomy of the human fetal brain in vivo and in utero and are currently under investigation for their potential use in the diagnosis of developmental pathologies of the human central nervous system. However, in order to establish in utero DTI as a clinical imaging tool, an independent comparison between macroscopic imaging and microscopic histology data in the same subject is needed. The present study aimed to cross-validate normal as well as abnormal in utero tractography results of commissural and internal capsule fibers in human fetal brains using postmortem histological structure tensor (ST) analysis. In utero tractography findings from two structurally unremarkable and five abnormal fetal brains were compared to the results of postmortem ST analysis applied to digitalized whole hemisphere sections of the same subjects. An approach to perform ST-based deterministic tractography in histological sections was implemented to overcome limitations in correlating in utero tractography to postmortem histology data. ST analysis and histology-based tractography of fetal brain sections enabled the direct assessment of the anisotropic organization and main fiber orientation of fetal telencephalic layers on a micro- and macroscopic scale, and validated in utero tractography results of corpus callosum and internal capsule fiber tracts. Cross-validation of abnormal in utero tractography results could be achieved in four subjects with agenesis of the corpus callosum (ACC) and in two cases with malformations of internal capsule fibers. In addition, potential limitations of current DTI-based in utero tractography could be demonstrated in several brain regions. Combining the three-dimensional nature of DTI-based in utero tractography with the microscopic resolution provided by histological ST analysis may ultimately facilitate a more complete morphologic characterization of axon guidance disorders at prenatal stages of human brain development.

Oriented nanofibrous membranes for tissue engineering applications: Electrospinning with secondary field control

Electrospinning is an electrical field driven method to produce polymer fibre membranes by deposition of a charged polymer jet onto a grounded collector. Fibre alignment within these mats is usually achieved by a fast collector movement, which is not feasible for all collector geometries, such as small diameter tubes or free-form moulds. The aim of this study was to evaluate the use of charged deflector plates to apply a dynamic, alternating electrical field perpendicular to the spinning direction, in order to directly control the fibre trajectory. Different field signal types, deflector plate voltages and deflection frequency ranges have been investigated. 210 poly(ɛ)caprolactone (PCL) membranes were electrospun. SEM images of each membrane were analysed using ImageJ. Main fibre diameter and orientation, as well as the degree of fibre alignment, were calculated, while a subset of the spun scaffolds were tested for their tensile properties. Higher deflector plate voltage amplitude resulted in a better fibre alignment. The best alignment was observed in a low deflection frequency range from 2 to 10 Hz. Mean main fibre direction was 87±18°, relative to the deflection axis, while fibre alignment had only a minor effect on the average fibre diameter. Young′s modulus and yield stress increased with the ratio of the parallel fibre component. The feasibility of the described method to achieve fibre alignment was demonstrated. However, the main fibre direction is not aligned with the deflection axis, but consistently perpendicular to it, which is also reflected in the tensile properties of spun samples.

Angle-sensitive MRI for quantitative analysis of fiber-network deformations in compressed cartilage

To demonstrate the feasibility of a novel experimental method to quantitatively analyze fiber-network deformation in compressed cartilage by angle-sensitive magnetic resonance imaging (MRI) of cartilage. Three knee cartilage samples of an adult sheep were imaged in a high-resolution MRI scanner at 7 T. Main fiber orientation and its "offset" from the direction perpendicular to the bone-cartilage boundary were derived from MR images taken at different orientations with respect to B0. Bending of the collagen fibers was determined from weight-bearing MRI with the load (up to 1.0 MPa) applied over the whole sample surface. A "fascicle" model of the cartilage ultrastructure was assumed to analyze characteristic intensity variations in T2-weighted images under load. T2-weighted MR images showed a strong variation of the signal intensities with sample orientation. In the T2-weighted weight-bearing series, regions of high signal intensity underwent shifts from the lateral to the central parts in all three cartilage samples. The bending of the collagen fibers was determined to be 27.2°, 35.4°, and 40.0° per MPa, respectively. Assuming a "fascicle" model, the presented MRI method provides quantitative measures of structural adjustments in compressed cartilage. Our preliminary analysis suggests that cartilage fiber deformation includes both bending and crimping.

Imaging and 3D morphological analysis of collagen fibrils

The recent booming of multiphoton imaging of collagen fibrils by means of second harmonic generation microscopy generates the need for the development and automation of quantitative methods for image analysis. Standard approaches sequentially analyse two-dimensional (2D) slices to gain knowledge on the spatial arrangement and dimension of the fibrils, whereas the reconstructed three-dimensional (3D) image yields better information about these characteristics. In this work, a 3D analysis method is proposed for second harmonic generation images of collagen fibrils, based on a recently developed 3D fibre quantification method. This analysis uses operators from mathematical morphology. The fibril structure is scanned with a directional distance transform. Inertia moments of the directional distances yield the main fibre orientation, corresponding to the main inertia axis. The collaboration of directional distances and fibre orientation delivers a geometrical estimate of the fibre radius. The results include local maps as well as global distribution of orientation and radius of the fibrils over the 3D image. They also bring a segmentation of the image into foreground and background, as well as a classification of the foreground pixels into the preferred orientations. This accurate determination of the spatial arrangement of the fibrils within a 3D data set will be most relevant in biomedical applications. It brings the possibility to monitor remodelling of collagen tissues upon a variety of injuries and to guide tissues engineering because biomimetic 3D organizations and density are requested for better integration of implants.

Idiopathic REM Sleep Behavior Disorder and Parkinson's Disease—Two Sides of One Coin?

Idiopathic REM Sleep Behavior Disorder and Parkinson's Disease—Two Sides of One Coin?

Behaviour of structural fibre composite sandwich panels under point load and uniformly distributed load

An innovative fibre composite sandwich panel made of glass fibre reinforced polymer skins and a modified phenolic core material was developed for building and other structural applications. The behaviour of this new generation sandwich panel was studied with reference to the main fibre orientation in floor applications, so that the effect due to erroneous installation could be evaluated. The two- and four-edge supported sandwich panels with different fibre orientations and fixity systems between panel and joist were tested under point load and uniformly distributed load (UDL) to determine their strength and failure mechanisms. The results of this experimental investigation show that the panels behave similarly under both loading conditions. Moreover, the fixity does not have a major effect on its failure mode and deflection.

Tough Lessons From Bone: Extreme Mechanical Anisotropy at the Mesoscale

AbstractBone is mechanically and structurally anisotropic with oriented collagen fibrils and nanometer‐sized mineral particles aggregating into lamellar or woven bone.[1] Direct measurements of anisotropic mechanical properties of sublamellar tissue constituents are complicated by the existence of an intrinsic hierarchical architecture. Methods such as nanoindentation provide insight into effective modulus values; however, bulk material properties cannot sufficiently be characterized since such measurements represent properties of near‐surface volumes and are partially averaged over fibril orientations.[2–5] In this study, we focus on the material properties of bone at one single level of hierarchy. By measuring properties of individual parallel‐fibered units of fibrollamellar bone under tension under controlled humidity conditions, an unusually high anisotropy is found. Here, we clearly demonstrate ratios as large as 1:20 in elastic modulus and 1:15 in tensile strength between orientations perpendicular and parallel to the main collagen fiber orientation in native wet bone; these ratios reduce to 1:8 and 1:7, respectively, under dry conditions. This extreme anisotropy appears to be caused by the existence of periodic, weak interfaces at the mesoscopic length scale. These interfaces are thought to be relevant to the proper mechanical and physiological performance of bone.

Determination of the Anisotropic Thermal Conductivity of a Carbon Aerogel–Fiber Composite by a Non-contact Thermographic Technique

The carbon-composite under investigation consists of a felt of carbon fibers infiltrated by a nanoporous sol-gel derived carbon aerogel; in addition, cracks caused by the shrinkage of the gel upon drying are present within the composite. Due to the anisotropic structure of the felt, consisting of a pinned stack of fiber mats, the thermal conductivity of the compound is anisotropic. The components of the thermal conductivity in the different directions were measured at room temperature using a non-contact technique, which was formerly established for fibers and foils. Here, a high-power laser diode is used for non-contact heating and a thermographic system for non-contact detection of the surface temperature of the samples. The stationary profile of the surface temperature next to the laser focus was evaluated with respect to the lateral thermal conductivity. Using a line-shaped laser focus, a one-dimensional heat flow within the samples was established and the two relevant components of the thermal conductivity in the different directions could be separated by investigating slices of the composite with different orientation to the main fiber orientation of the felt. The anisotropy of the thermal conductivity, i.e., the ratio of the components perpendicular and parallel to the felt surface, was determined to be about two under vacuum conditions. This relatively small thermal anisotropy was in qualitative agreement with preliminary tests of the electrical conductivity. In addition, local inhomogeneities due to macroscopic voids within the samples influenced the observed temperature profile. As alternative measurement variants, point-shaped laser-heating of sample discs was used and evaluated in terms of a two-dimensional heat flow and periodic, i.e., dynamical heating of the sample slices with a line-shaped laser focus was used. The reliability of the results was also tested by comparing them to data derived with a guarded-hot-plate measurement of a (1.1 × 15 × 15) cm3 large tile.

Organization and distribution of intramuscular connective tissue in normal and immobilized skeletal muscles. An immunohistochemical, polarization and scanning electron microscopic study.

Collagen fiber network is a major contributor to the coherence and tensile strength of normal skeletal muscle. Despite the well-recognized importance of the intramuscular connective tissue to the normal integrity and function of the skeletal muscle, the specific architecture including the location and three-dimensional orientation of the intramuscular connective tissue within the muscle tissue is poorly described. The structure of the intramuscular connective tissue was studied by immunohistochemistry, polarization microscopy (the crimp length and angle of the collagen fibers) and scanning electron microscope (SEM) in rat skeletal muscles (gastrocnemius, soleus and tibialis anterior) in normal situation and after 3 weeks of disuse (immobilization). Three separate networks of collagen fibers were distinguished by SEM in the normal endomysium; fibers running longitudinally on the surface of the muscle fibers (the main collagen orientation), fibers running perpendicularly to the long axis of the muscle fibers and having contacts with adjacent muscle fibers, and fibers attached to the intramuscular nerves and arteries. Similarly, the SEM analysis also disclosed three distinct collagen fiber networks running in different directions in the perimysium, but, contrary to the endomysium, the main fiber orientation could not be established. Immobilization resulted in a marked increase in the endo- and perimysial connective tissue, the majority of the increased endomysial collagen being deposited directly on the sarcolemma of the muscle cells. Immobilization also resulted in substantial increase in the number of perpendicularly oriented collagen fibers with contacts to two adjacent muscle fibers in the endomysium. Further, immobilization clearly disturbed the normal structure of the endomysium making it impossible to distinguish the various networks of fibers from each other. In the perimysium, immobilization-induced changes were similar, the number of longitudinally oriented collagen fibers was increased, the connective tissue was very dense, the number of irregularly oriented collagen fibers was markedly increased, and consequently, the different networks of collagen fibers could not be distinguished from each other. Of the three studied intact muscles, the crimp angle of the collagen fibers was lowest in the soleus and highest in the gastrocnemius muscle, and the crimp angle decreased over 10% in all muscles after the immobilization-period. Altogether, the above described quantitative and qualitative changes in the intramuscular connective tissue are likely to contribute to the deteriorated function and biomechanical properties of the immobilized skeletal muscle.

Mechanical Properties of Thermoplastic Butadiene-Styrene (SBS) and Oxidized Short Carbon Fibre Composites

This work reports on some results of research conducted on composite materials consisting of a butadiene-styrene (SBS) thermoplastic elastomer matrix filled with short carbon fibres previously subjected to oxidative treatment to increase the surface functionality. Scanning electron microscopy confirms the existence of interactions between the matrix and the fibre, which are not observed for commercial fibre fillers and which translate into mechanical strength increments, in terms of the Young's modulus, tensile and tear strengths for the oxidized fibre composites. The stress-strain curves of the composites show yield point phenomena as strain is applied longitudinally to the main fibre orientation. In oxidized fibre composites the stress and strain coordinates are a function of the degree of oxidation (greater strain for more strongly oxidized fibre) and fibre strength (lower stress for longer treatment times).

The heart’s fibre alignment assessed by comparing two digitizing systems. Methodological investigation into the inclination angle towards wall thickness

Myocardial contractile pathways which are not aligned strictly parallel to the heart's epicardial surface, give rise to forces which also act in the ventricular dilating direction. We developed a method which allows us to assess any fibre orientation in the three-dimensional myocardial weave. Decollagenized hearts were prepared by peeling-off fibre strands, following their main fibre orientation down to near the endocardium. In the subepicardium the strands followed a course more or less parallel to the epicardium, whereas from the mid-wall on they tended to dive progressively deeper into the wall. The preparation displays more or less rugged surfaces rather than smooth layers. The grooves and crests on the exposed surfaces were sequentially digitized by two methods: (1) Using a magnet tablet (3 Draw Digitizer System, Polhemus, Cochester VTO 5446, USA) on a dilated pig heart we manually followed the crests using a stylus, handling each groove and crest as an individual contractile pathway. (2) A constricted cow heart was digitized using a contact-free optical system (opto TOP, Dr. Breuckmann, Meersburg, Germany), which is based on the principle of imaging triangulation. Using specially developed software the inclination angles of selected crests and grooves with respect to the epicardial surface were calculated. The two digitizing methods yield comparable results. We found a depth- and side-specific weave component inclined to the epi-endocardial direction. This oblique netting component was more pronounced in the inner 1/3 of the wall than in the subepicardium. The inclination angle probably increases with increasing wall thickness during the ejection period. Manual digitizing is an easy and fast method which delivers consistent results comparable with those obtained by the cumbersome high resolution optical method. The rationales for the assessment of transmural fibre inclination are (1) the putative existence of dilating forces inherent in the myocardial weave and (2) the possible overproportional increase in the oblique transmural weave component during myocardial hypertrophy, which would entail a reduction in efficiency of ventricular performance in terms of haemodynamic work.