Fiber Reorientation Research Articles

Comparison of fibre reorientation of short-and long-fibre reinforced polypropylene by injection molding with a rotating mold core

Abstract With a rotating mold core during the injection molding of fibre-reinforced plastics, the rotational shear caused by the rotation is superimposed on the injection-induced shear. This allows the fibre orientation in this area to be intentionally manipulated so that, for example, in the case of internal pressure loading, the fibres can be oriented in the tangential main loading direction. This paper deals with the impact of a rotating mold core on the fibre orientation and burst strength of short-and long-fibre-reinforced polypropylene. It is shown that the fibre orientation and strength can be significantly influenced for both short and long fibres, whereby increases in bursting strength of mostly over 80%, in some cases over 200%, could be achieved. The ultimate strength depends, among other things, on the wall thickness used and the fibre content. Major differences between the short-and long-fibre-reinforced polypropylene are less evident in the strength and more in the fibre orientation.

Non-isothermal direct bundle simulation of SMC compression molding with a non-Newtonian compressible matrix

Compression molding of Sheet Molding Compounds (SMC) is a manufacturing process in which a stack of discontinuous fiber-reinforced thermoset sheets is formed in a hot mold. The reorientation of fibers during this molding process can be either described by macroscale models based on Jeffery's equation or by direct mesoscale simulations of individual fiber bundles. In complex geometries and for long fibers, direct bundle simulations outperform the accuracy of state-of-the-art macroscale approaches in terms of fiber orientation and fiber volume fraction. However, it remains to be shown that they are able to predict the necessary compression forces considering non-isothermal, non-Newtonian and compaction behavior. In this contribution, both approaches are applied to the elongational flow in a press rheometer and compared to experiments with 23% glass fiber volume fraction. The results show that both models predict contributions to the total compression force and orientation reasonably well for short flow paths. For long flow paths and thick stacks, complex deformation mechanisms arise and potential origins for deviation between simulations models and experimental observations are discussed. Furthermore, Jeffery's basic model is able to predict orientations similar to the high-fidelity mesoscale model. For planar SMC flow, this basic model appears to be even better suited than the more advanced orientation models with diffusion terms developed for injection molding.

Influence of fiber breakage on flow behavior in fiber length- and orientation-dependent injection molding simulations

• Orientation dependent fiber breakage modeling approach • Fiber length and orientation dependent flow modeling • Coupling of transient fiber breakage modeling and fiber dependent viscosity and flow modeling Injection molding is one of the most important processes for manufacturing discontinuous fiber reinforced polymers (FRPs). The matrix of FRPs shows a transient chemo-thermomechanical behavior and the fibers create anisotropy influencing physical properties. Hence, FRPs are complex materials, but also likely used in volume production. In this work, the fiber-induced anisotropic behavior during mold filling is modelled with an anisotropic fourth order viscosity tensor. The viscosity tensor takes second and fourth order fiber orientation tensor, fiber length and non-Newtonian matrix viscosity into account. In this way, the macroscopic simulation captures the influence of the flow field on the fiber re-orientation and vice versa. The fiber orientation tensor is used to determine reference fibers in every element for calculation of hydrodynamic forces. This information is used in a novel fiber breakage model, based on buckling of fibers in Jeffery's orbit. The result is a macroscopic molding simulation with not only transient fiber orientation distribution, but also fiber length distribution. Due to the anisotropic viscosity tensor, the predicted fiber breakage influences the material's viscosity and flow behavior, which is also visible in the simulated cavity pressure. The results are validated with injection molding experiments, performed with a glass fiber reinforced phenolic compound, showing good agreement.

Non-affine Fiber Reorientation in Finite Inelasticity

This paper introduces a model for the mechanical response of anisotropic soft materials undergoing large inelastic deformations. The material is considered made by a isotropic matrix with embedded fibers, each component having its own relaxation dynamics. The constitutive equations are provided in terms of the free energy density and the dissipation density, which are both required to be thermodynamically consistent and structural frame-indifferent, i.e., independent of a rotation overimposed on the intermediate natural state of both matrix and fibers. This is in contrast to many of the currently used anisotropic inelastic models, which do not deal with the lack of uniqueness of the intermediate state. This issue is thoroughly discussed and in terms of two possible choices satisfying structural-frame indifference and leading to different flow rules of the inelastic processes. It is shown that different models from the literature can be incorporated in the proposed formulation including anisotropic viscoelasticity and growth.

Ex vivo experimental characterizations for understanding the interrelationship between tissue mechanics and collagen microstructure of porcine mitral valve leaflets

Unidirectional blood flow in the left side of the heart is regulated by the mitral valve. To better understand the mitral valve function, researchers have examined the structural and mechanical properties of the mitral valve leaflets; however, limitations of the previous studies include the use of mechanics- and structure-altering tissue modifications (e.g., optical clearing) that limit the ability to quantify the unique load-dependent reorientation and realignment of the collagen fibers as well as their interrelation with the valve tissue mechanics. Herein, we aimed to circumvent these limitations by utilizing an integrated polarized-light imaging and biaxial testing system for understanding the mechanics–microstructure interrelationship for porcine mitral valve leaflets. We further performed constitutive modeling and evaluated the accuracy of the affine fiber kinematics theory. From the tissue mechanics perspective, the posterior leaflet was more extensible in the radial direction than the anterior leaflet (14.2% difference in radial tissue stretch), while exhibiting smaller collagen and elastin moduli based on the determined constitutive model parameters. From the collagen microstructure’s standpoint, the posterior leaflet had smaller increases in optical anisotropy (closely related to the degree of fiber alignment) than the anterior leaflet (32.8±7.7% vs. 50.0±19.7%). Further, the leaflets were found to possess two distinct fiber families – one family oriented along the circumferential tissue direction, and another more disperse family with a 30°–40° offset from the first fiber family. Finally, affine fiber kinematics consistently underpredicted the collagen fiber reorientations Overall, this study improved our understanding of the mitral valve leaflets that is essential for facilitating tissue-emulated valve replacement and cardiac valve modeling frameworks.

Bending, buckling and linear vibration of bio-inspired composite plates

Biological composite structures with helicoidal schemes and designs have good damage tolerance and high impact energy absorption. However, the effect of redirection and the reorientation of fibers inside the matrix of helicoid structure on their mechanical performance and response have not been comprehensively examined. Therefore, this works aims to cover this topic and investigate the static, buckling and vibration behaviors of laminated composite plate structure with helicoidal orientation scheme for the first time. The formulation and analysis is based on the hypothesis of first order shear deformation theory (FOSDT) of plate. Various sets of laminates including Fibonacci sequence are considered in this study to get the optimum scheme of layer's orientation. Based on energy principle, the governing equations of motion and associated boundary conditions are derived in detail. The obtained partial differential equations of motion, in terms of displacement fields, are solved by two-dimensional differential quadrature method (2D-DQM). Verification with previous works is presented to validate the present mathematical model and solution technique. The influences of different parameters such as laminate sequence, boundary conditions, material and geometric properties on the static, buckling stability and vibration responses of composite plate are studied and discussed. It is found that the laminate sequence has a significant influence on bending, critical buckling and natural frequencies, as well as this effect varies with various aspect ratios and side to thickness ratios, and boundary conditions of the composite plate.

Passive and active fiber reorientation in anisotropic materials

We present a continuum model to describe the reorientation of an anisotropic material structure, characterized by two fiber families able to modify their orientations following different evolution dynamics. The evolution equations are derived in a thermodynamically consistent way, and passive and active contributions to the reorientation process are identified. It is shown that a weaker extension of a well-known coaxiality result holds. The transversely isotropic and orthotropic cases are then recovered by imposing the proper constraint on the fiber rotation. Applications to biological experiments on cell layers under stretch are discussed, showing a good agreement between the model and the experimental results. Even though we focus on cell layers, our framework remains general and may be employed to describe reorientation in engineering materials.

Microscale deformation mechanisms in paperboard during continuous tensile loading and 4D synchrotron X‐ray tomography

AbstractA better physical understanding of mesoscale and microscale mechanisms behind deformation and failure of paperboard material is important to optimize industrial packaging converting processes and decrease waste. In this study, these mechanisms were investigated using synchrotron X‐ray tomography during in situ continuous uniaxial tensile loading. High spatial and temporal data resolution enabled quantification of rapid changes in the material occurring before, during and after material failure. The evolution of 3D strain fields, fibre orientations and sample thickness showed that deformation and failure mechanisms differ significantly between samples tested in machine direction (MD), cross direction (CD) and 45° from the loading direction. In 45° and CD, gradual failure processes could be followed across several load steps. Immediately after failure, the in‐plane fracture region was significantly larger in both 45° and CD compared to MD. Both fracture characteristics and strain field distributions differed between the three material directions. Significant fibre reorientation was an active deformation mechanism in 45° already from the beginning of the loading, also present in CD after peak load but absent in MD. The MD‐dependent mechanisms interpreted and quantified at the scale of the fibre network in this study can help guide model development and likely have wider applicability to other paper‐based materials.

Highly stretchable three-dimensional thermoelectric fabrics exploiting woven structure deformability and passivation-induced fiber elasticity

Accommodating localized strains and deformations imposed by human body movements is indispensable to reliable operation of wearable thermoelectric (TE) fabrics. Despite their inherent flexibility and conformability, TE fabrics can suffer from significantly compromised TE performance and mechanical reliability at large applied strains, owing to the limited stretchability of the constituting fibers/components. Here, we propose three-dimensional woven fabrics of high TE performance and exceptional mechanical reliability, encouraged simultaneously by passivation of TE fibers. Immersion-coating in carbon nanotube (CNT) ink effortlessly deposits TE sheath layers on polyurethane (PU) core fibers. PU passivation layers are encapsulated on the coated fibers not only to enhance the intrinsic fiber stretchability by enabling fiber reorientation of individual CNTs, but also to augment the TE properties by regulating heat transfer along PN legs. The passivated fibers are woven into three-dimensional fabrics that harvest heat in the out-of-plane direction with the highest normalized open-circuit voltage of 8.0 mV K−1 and normalized power per leg of 1.1 × 10−4 μW K−2 among the reported woven-type TE fabrics. The woven structure deformability and the passivation-induced fiber elasticity cooperatively achieve reliable fabric mechanical stability against bending and stretching up to 100% strain. Conforming seamlessly to the curved human forearm, the TE woven fabrics hold promises for efficient body heat harvesting to realize self-powered wearable electronics.

The accuracy of restoration of femoral head centre of rotation in the anteroposterior plane after uncemented total hip arthroplasty

AimsHip arthroplasty aims to accurately recreate joint biomechanics. Considerable attention has been paid to vertical and horizontal offset, but femoral head centre in the anteroposterior (AP) plane has received little attention. This study investigates the accuracy of restoration of joint centre of rotation in the AP plane.MethodsPostoperative CT scans of 40 patients who underwent unilateral uncemented total hip arthroplasty were analyzed. Anteroposterior offset (APO) and femoral anteversion were measured on both the operated and non-operated sides. Sagittal tilt of the femoral stem was also measured. APO measured on axial slices was defined as the perpendicular distance between a line drawn from the anterior most point of the proximal femur (anterior reference line) to the centre of the femoral head. The anterior reference line was made parallel to the posterior condylar axis of the knee to correct for rotation.ResultsOverall, 26/40 hips had a centre of rotation displaced posteriorly compared to the contralateral hip, increasing to 33/40 once corrected for sagittal tilt, with a mean posterior displacement of 7 mm. Linear regression analysis indicated that stem anteversion needed to be increased by 10.8° to recreate the head centre in the AP plane. Merely matching the native version would result in a 12 mm posterior displacement.ConclusionThis study demonstrates the significant incidence of posterior displacement of the head centre in uncemented hip arthroplasty. Effects of such displacement include a reduction in impingement free range of motion, potential alterations in muscle force vectors and lever arms, and impaired proprioception due to muscle fibre reorientation.Cite this article: Bone Joint Res 2022;11(3):180–188.

Particle-resin systems for additive manufacturing of rigid and elastic magnetic polymeric composites

Fiber-reinforced polymeric composites (FRPCs) with selective fiber orientations are finding applications in building biological constructs, smart materials and energy devices. Field-assisted additive manufacturing (AM) has emerged as one of the promising methods for creating FRPC structures. Compared to other AM processes and due to the liquid form of the raw materials, vat photopolymerization has shown higher potential in building FRPCs with selective fiber orientation. Compared to other methods of controlling the fiber orientation during AM processes, magnetic field-assisted fiber reorientation, is a relatively easy-to-use contactless method that has been effectively paired with vat photopolymerization. One of the major issues in magnetic field-assisted vat photopolymerization is the dispersion instability of the particles in resin system, which leads to separation between the resin components and dispersed particles and results in non-uniform particle distribution during the printing process. In this study, ferromagnetic particle-loaded photocurable resins are developed through surface modification of the particles to achieve more uniform particle dispersion, more dispersion-stable resins systems and enhanced mechanical properties of FRPCs. Magnetite particles were functionalized under silanization reactions and mixed with two photocurable resins. Fourier-transform infrared spectroscopy (FTIR) analysis was used to select the matching composition of silane compounds and verify the functionality of the treated magnetite particles for each resin mixture. Dispersion stability of the developed resin system was verified by visual observation of the resin mixture and image analysis of microscope images. To provide a better guideline for 3D printing the developed resin system, cure depth of each resin system containing different particle fractions is studied. Tensile tests are devised to understand the effects of particle content, orientation, and surface treatment on mechanical properties of the 3D printed FRPCs. Results of this study suggest that an optimized particle-resin system can be developed through surface treatment of magnetite particles with functional groups.

Experimental and Numerical Analysis of SMC Compression Molding in Confined Regions—A Comparison of Simulation Approaches

The compression molding process of sheet molding compound (SMC) is an economical manufacturing process for lightweight parts. However, molding defects, such as fiber matrix separation, and fiber re-orientation, may develop during the molding process in confined regions, such as ribs and bosses. Hence, the mechanical properties of the composite depend on the local fiber architecture. Consequently, this work compares the predictive capabilities of tensor-based and directly modeled process simulation approaches regarding compression force, fiber volume content and fiber orientation on the example of honeycomb structures molded from SMC. The results are validated by micro-computed tomography and thermal gravimetric analysis. The fiber orientation in the honeycomb varies between individual samples because a sheet molding compound is macroscopically heterogeneous and thus the fiber architecture is strongly influenced by random events. Tensor-based fiber orientation models can not reliably predict fiber volume content and fiber orientation in the part’s thickness direction if there is a lack of scale separation. Therefore, directly modeled process simulations should be preferred in cases in which fiber length and mold dimensions prohibit scale separation. The prediction of fiber volume content is a difficult task and no simulation can predict the severity of fiber matrix separation precisely in all cases.

A mathematical model for viscoelastic properties of biological soft tissue.

A quaternary viscoelastic structure model with two characteristic times is presented to describe the viscoelastic properties of parallel-fibered collagen tissue. The comparison results of model prediction and experimental data of rabbit medial collateral ligaments show that the model could accurately describe viscoelastic behavior such as stress-relaxation, strain-strengthening and creep of bio-soft-tissue within a small scope of errors. To study the biomechanical mechanism of viscoelasticity that biological soft tissue shows, the influence of model parameters on viscoelastic behavior of bio-soft-tissue is analyzed and researched, which indicated that the major influential elements of stress-relaxation in bio-soft-tissue are elastic modulus, relaxation time and strain rate of proteoglycan-rich matrix. The influence of elastic modulus of collagen fibers on stress-relaxation is not significant. However, the nonlinearity of stress-strain curve and viscoelastic behavior of bio-soft-tissue mostly depends on recruitment and reorientation of collagen fibers under external loading.

Stress fiber growth and remodeling determines cellular morphomechanics under uniaxial cyclic stretch.

Stress fibers in the cytoskeleton are essential in maintaining cellular shape and influence cellular adhesion and migration. Cyclic uniaxial stretching results in cellular reorientation orthogonal to the applied stretch direction. The mechanistic cues underlying changes to cellular form and function to stretch stimuli are currently underexplored. We show stretch-induced stress fiber lengthening, their realignment, and increased cortical actin inNIH 3T3 fibroblasts stretched over varied amplitudes and durations. Higher amounts of actin and stress fiber alignment were accompanied with an increase in the effective elastic modulus of cells. Microtubules did not contribute to the measured stiffness or reorientation response but were essential to the nuclear reorientation. We used a phenomenological growth and remodeling law, based on the experimental data, to model stress fiber elongation and reorientation dynamics based on a nonlinear, orthotropic, fiber-reinforced continuum representation of the cell. The model predicts the changes observed fibroblast morphology and increased cellular stiffness under uniaxial cyclic stretch which agrees with experimental results. Such studies are important in exploring the differences underlying mechanotransduction and cellular contractility under stretch.

Reduced kinematic multiscale model for tissue engineering electrospun scaffolds

To this day, there is still a need for a direct relationship between the microscopic material properties and network microstructure configuration with the macroscopic mechanical response in order to optimize the design loops of biomimetic electrospun grafts. Multiscale mechanical modeling arises as a useful alternative, which allows to represent the individual nanofibers mechanical response and how the interaction between fibers results in the final macroscopic behavior. In this work, a micromechanical model that accounts for fiber interaction, progressive straightening (i.e. progressive recruitment) and reorientation is presented. An RVE is generated by means of a virtual deposition algorithm that mimics the electrospinning process itself, thus obtaining geometries that resemble the observed electrospun microstructure. These geometries were then validated by comparison with analysis of SEM images, taking special interest in the diameter, orientation and tortuosity distributions. Then, an elastic–plastic constitutive law for the nanofibers is implemented along with a simplified kinematic description that results in a significant reduction of the number of Degrees of Freedom of the discretized mechanical equilibrium problem. Finally, the micromechanical model is validated against uniaxial tensile tests of electrospun PLLA bone-shaped samples, reproducing the experimentally observed behavior while employing realistic geometric and constitutive parameters to characterize the fibers.

J integral and local strain energy density approach to characterize the cracks in anisotropic hyperelastic skin type composite materials

It has been proposed that the response of soft tissues and skin type materials critically depends on the orientation and re-orientation of reinforcing fiber. Here, we quantify the effect of fiber orientation on the mechanical and fracture properties of anisotropic hyperelastic glass fiber (GF)/polydimethylsiloxane (PDMS) composites through a unified experimental and computational analysis. Experimental results show that the tensile strength and fracture toughness of skin type materials depends on the initial orientation of fiber families and is maximum for 0–90 fiber orientation. The re-orientation of fibers under tensile loading is experimentally demonstrated. The experimental findings are well complimented by the finite element (FE) simulations performed using bilinear strain stiffening fiber and matrix (BLFM) material model for soft tissues. The material model parameters are obtained by fitting the experimental data for a meaningful comparison. It is observed that fiber rotation along the loading direction indeed enhances the resistance to crack growth in skin type materials. The size of process zone (RC) ahead of the crack tip and relative contribution of anisotropic energy is characterized using J integral and average strain energy density (ASED) criteria. The RC and anisotropic energy contribution (ϕaniso-avg/ ϕavg) depends on the fiber orientation with higher values for 0–90 fiber orientation. The larger process zone enhances the fracture toughness for 0–90 orientation in comparison to other orientations. Moreover, RC and ϕaniso-avg/ ϕavg are found to be independent of initial crack length and therefore, they can be considered as material parameters for a given fiber orientation. The findings of the present investigations may be used to explore GF/PDMS composite as a possible candidate for soft actuator and bio-medical applications with enhanced toughness.

The Effects of Healthy Aging on Right Ventricular Structure and Biomechanical Properties: A Pilot Study.

Healthy aging has been associated with alterations in pulmonary vascular and right ventricular (RV) hemodynamics, potentially leading to RV remodeling. Despite the current evidence suggesting an association between aging and alterations in RV function and higher prevalence of pulmonary hypertension in the elderly, limited data exist on age-related differences in RV structure and biomechanics. In this work, we report our preliminary findings on the effects of healthy aging on RV structure, function, and biomechanical properties. Hemodynamic measurements, biaxial mechanical testing, constitutive modeling, and quantitative transmural histological analysis were employed to study two groups of male Sprague-Dawley rats: control (11 weeks) and aging (80 weeks). Aging was associated with increases in RV peak pressures (+17%, p = 0.017), RV contractility (+52%, p = 0.004), and RV wall thickness (+38%, p = 0.001). Longitudinal realignment of RV collagen (16.4°, p = 0.013) and myofibers (14.6°, p = 0.017) were observed with aging, accompanied by transmural cardiomyocyte loss and fibrosis. Aging led to increased RV myofiber stiffness (+141%, p = 0.003), in addition to a bimodal alteration in the biaxial biomechanical properties of the RV free wall, resulting in increased tissue-level stiffness in the low-strain region, while progressing into decreased stiffness at higher strains. Our results demonstrate that healthy aging may modulate RV remodeling via increased peak pressures, cardiomyocyte loss, fibrosis, fiber reorientation, and altered mechanical properties in male Sprague-Dawley rats. Similarities were observed between aging-induced remodeling patterns and those of RV remodeling in pressure overload. These findings may help our understanding of age-related changes in the cardiovascular fitness and response to disease.

Characterisation of off-axis tensile behaviour and mesoscale deformation of woven carbon-fibre/PEEK using digital image correlation and X-ray computed tomography

The architectural intricacy of multi-ply woven composites, caused by differences in layer alignment, introduces complexity into their mesoscale response to on- and off-axis tensile loading. To better understand the relationship between architecture and deformation response, this work combines digital image correlation (DIC) and 3D micro-computed tomography ( μ CT) to examine deformation for 0°, 15°, 30° and 45° specimens. Tensile tests were conducted with a four-layer plain woven carbon/polyetheretherketone (C/PEEK) laminate. In addition to the evident variance of mechanical properties between orientations, significant mesoscale differences in topography and strain were observed resulting from the layer alignment of individual specimens. Alignment of the top two surface layers induced distinct topographical peaks under extension, whereas nested architectures formed continuous topographical ridges. Matching observed surface strains around microstructural cracks identified in μ CT images revealed both inter-tow and intra-tow shearing during off-axis extension and corresponding fibre reorientation, which effectively illustrates the potential benefits from combining DIC and μ CT. • X-ray computed tomography and digital image correlation investigation of woven C/PEEK. • Influence of architectural layer alignment on surface topography/strain revealed. • Mesoscale off-axis deformation mechanisms and damage characteristics ascertained. • Findings provide means to link future surface DIC results to probable internal structure.

Collagen Structure Orientation at the Bone–Cartilage Interface

Abstract The articular cartilage overlying subchondral bone consists of variously oriented collagen fibres, the detailed organisation of which is essential for cartilage integrity that has suffered damage in degenerative joint disease such as osteoarthritis (OA). A technique attracting particular interest is that of coherent small-angle X-ray scattering mapping; the present investigations are supported by PILATUS, a highly pixelated two-dimensional detector system developed at the Paul Scherrer Institute (PSI). The system has yielded information on the anatomical features that correspond to the large-scale organisation of collagen and the mineralised phase contained within the collagen fibres in the deep cartilage zone. Results obtained from a decalcified cartilage–bone sample have been plotted in terms of the orientation of cartilage and bone components for particular intervals of k-space, showing the organisation of collagen-II within superficial layers and collagen-X in the chondrocyte-rich deeper layers of cartilage. It is apparent in undamaged cartilage that there is a gradual reorientation of the collagen-II fibres of the cartilage, from parallel to the surface of the joint, to normal to the cartilage–bone interface. A similar pattern of orientation is seen below the cement line (the surface at which cartilage anchors to the bone), for the collagen type-I prevalent in bone. Changing the interval of k-space allows subchondral features such as the microvasculature to be seen.

Enthesis strength, toughness and stiffness: an image-based model comparing tendon insertions with varying bony attachment geometries.

Tendons of the body differ dramatically in their function, mechanics and range of motion, but all connect to bone via an enthesis. Effective force transfer at the enthesis enables joint stability and mobility, with strength and stiffness arising from a fibrous architecture. However, how enthesis toughness arises across tendons with diverse loading orientations remains unclear. To study this, we performed simultaneous imaging of the bone and tendon in entheses that represent the range of tendon-to-bone insertions and extended a mathematical model to account for variations in insertion and bone geometry. We tested the hypothesis that toughness, across a range of tendon entheses, could be explained by differences observed in interactions between fibre architecture and bone architecture. In the model, toughness arose from fibre reorientation, recruitment and rupture, mediated by interactions between fibres at the enthesis and the bony ridge abutting it. When applied to tendons sometimes characterized as either energy-storing or positional, the model predicted that entheses of the former prioritize toughness over strength, while those of the latter prioritize consistent stiffness across loading directions. Results provide insight into techniques for surgical repair of tendon-to-bone attachments, and more broadly into mechanisms for the attachment of highly dissimilar materials.