Fiber Waviness Research Articles

Improved fibre placement in filament-based 3D printing of continuous carbon fibre reinforced thermoplastic composites

This study investigates the mechanism of fibre deposition in conventional 3D printing and presents an aligned fibre deposition (AFD) method to improve the fibre placement. The deposition mechanism reveals that the cracking of the filament in the transition zone and the torsional deformation during the steering path are the main cause of the defects in the conventional printing process. The AFD method is shown to reduce and mitigate the fibre waviness and twisting, hence producing smooth filament deformation, introducing less air voids and fibre breakage during printing. It is found that the AFD method improves the fibre alignment angle from ±25° to ±12° and reduces the void content to 0.27 % during straight-line deposition. During curved-line printing at a large radius of curvature, tow shearing instead of fibre folding is produced by AFD method, resulting in smooth edges with less fibre breakage.

Thickness variation effect on compressive properties of ultra-thick CFRP laminates

As ultra-thick laminates are widely used in the field of aerospace and marine as the main load-bearing structure, extreme service conditions lead to increasing thickness of composite structures. However, the effect of the increase in thickness on the compressive properties of ultra-thick laminates is still unclear. Herein parameters including fiber volume fraction, fiber waviness, void content and free edge effect that affect the compressive strength with increasing thickness are studied. The results show that the fiber volume fraction of ultra-thick laminates does not change significantly with the increase of thickness due to the adoption of the zero-bleeding process, while the fiber waviness and void contribute to the thickness effect on compressive strength. On the other hand, the high interlaminar stress of the free edge contributes to the premature delamination of the specimen, which dominates the compressive strength of the ultra-thick laminate. Finally, compared with the thin laminate (2 mm), the compressive strength of ultra-thick laminate drops sharply (nearly 28%), but when the thickness increases from 30 to 70 mm, the drop of compressive strength is not obvious (only 10%). When the material and curing process are consistent, the weak interlaminar properties of ultra-thick laminates does not change significantly with the increase of thickness, which leads to the insignificant decrease in compression strength of ultra-thick laminates with the increase of thickness. The proposed empirical relationship between the compressive strength and thickness of quasi-isotropic laminates can provide important theoretical basis and experimental reference for the design of ultra-thick composite structures.

A novel method based on eddy‐current equilibrium for improving the sensitivity of fiber orientation detection

AbstractFiber orientation deviation is a typical defect type in carbon fiber reinforced polymer composites (CFRPs) manufacturing process. Unlike other defect types, fiber orientation deviation has minimal influence on the local conductivity of CFRPs, so it is difficult to be detected by eddy current (EC) detection methods. Based on the principle of eddy‐current equilibrium, a new method is proposed in this paper to improve the sensitivity of fiber orientation detection. In this method, an EC field with eddy‐current equilibrium distribution inside the specimen is excited by means of a special double D‐shaped probe. When there is a deviation from the fiber orientation, the eddy‐current equilibrium state is broken, resulting in a rapid increase in the intensity and distribution range of the induced EC inside the specimen, and finally lead to a drastic change in the probe signal. The experimental results show that the double D‐shaped probe with an eddy‐current equilibrium effect can detect the fiber orientation of CFRPs, and the changes in fiber orientation can cause up to a 423.1% variation in the probe signal. Compared with the traditional rectangular probe, the designed probe is more sensitive than the former to small changes in fiber orientation in the range of −20° to 20° under high frequency excitation above 3 MHz. In addition, the proposed method can also be used for rapidly identifying fiber waviness with a deviation angle of 7.4° inside unidirectional CFRPs, which can be achieved effectively and intuitively through defect imaging.

Placement defects in thermoset‐impregnated rovings deposited along curved paths

AbstractWet Fiber Placement (WFP) is a novel process for manufacturing complex shaped parts from continuous fiber‐reinforced polymers without semi‐finished products. Here, dry fiber bundles (rovings) are in‐line impregnated with a thermoset resin, followed by almost tension‐free deposition. The freshly impregnated and limp rovings allow for placement in radii. Due to the lack of compression in deposition, subsequent consolidation, for example, via vacuum bagging, is required. However, this consolidation cannot compensate for defects already present in the deposited lay‐up as they will typically occur when depositing rovings along curved paths. In this study, we deposited two carbon fiber rovings in radii between 5 and 2000 mm by WFP. The occurring defects were visually evaluated. Three main defects were observed: Fiber waviness, up‐folding and twists. The extent of the defects was evaluated using image analysis techniques. Results show, that up‐folding and twists are dominant for radii <100 mm, while for greater radii fiber waviness is the main defect. Up‐folding and twists almost completely disappear for radii >500 mm, but a slight waviness is still observed. However, the effect of fiber waviness on the longitudinal stiffness is estimated to be small for radii >500 mm and perhaps acceptable for some applications.

Mimicking the Graded Wavy Structure of the Anterior Cruciate Ligament.

Anterior cruciate ligament (ACL) is the connective tissue providing mechanical stability to the knee joint. ACL reconstruction upon rupture remains a clinical challenge due to the high mechanical properties required for proper functioning. ACL owes its outstanding mechanical properties to the arrangement of the extracellular matrix (ECM) and to the cells with distinct phenotypes present along the length of the tissue. Tissue regeneration appears as an ideal alternative. In this study, a tri-phasic fibrous scaffold that mimics the structure of collagen in the native ECM is developed, presenting a wavy intermediate zone and two aligned uncurled extremes. The mechanical properties of the wavy scaffolds present a toe region, characteristic of the native ACL, and an extended yield and ultimate strain compared to aligned scaffolds. The presentation of a wavy fiber arrangement affects cell organization and the deposition of a specific ECM characteristic of fibrocartilage. Cells cultured in wavy scaffolds grow in aggregates, deposit an abundant ECM rich in fibronectin and collagen II, and express higher amounts of collagen II, X, and tenomodulin as compared to aligned scaffolds. In vivo implantation in rabbits shows a high cellular infiltration and the formation of an oriented ECM compared to aligned scaffolds.

Location specific multi-scale characterization and constitutive modeling of pig aorta

The mechanical and structural behavior of the aorta depend on physiological functions and vary from proximal to distal. Understanding the relation between regionally varying mechanical and multi-scale structural response of aorta can be helpful to assess the disease outcomes. Therefore, this study investigated the variation in mechanical and multi-scale structural properties among the major segments of aorta such as ascending aorta (AA), descending aorta (DA) and abdominal aorta (ABA), and established a relation between mechanical and multi-structural parameters. The obtained results showed significant increase in anisotropy and nonlinearity from proximal to distal aorta. The change in periphery length and radii between load and stress free configuration was also found increasing far from the heart. Opening angle was significantly large for ABA than AA and DA (AA/DA vs ABA; p = 0.001). Mean circumferential residual stretch (ratio of mean periphery length at load and stress free configurations) was found decreasing between AA and DA, and then increasing between DA to ABA and its value was significantly more for ABA (AA vs DA; p = 0.041, AA vs ABA; p = 0.001, DA vs ABA; p = 0.001). The waviness of collagen fibers, collagen fiber content, collagen fibril diameter and total protein content were found significantly increasing from proximal to distal. Pearson correlation test showed a significant linear correlation between variation in mechanical and multi-scale structural parameters over the aortic length. Residual stretch was found positively correlated with collagen fiber content (r = 0.82) whereas opening angel was found positively correlated with total protein content (TPC) (r = 0.76).

Bifurcation buckling and the effect of imperfections on the microbuckling of soft materials with periodic microstructure by the finite strain HFGMC micromechanics

Two approaches are presented for the prediction of the microbuckling of composites subjected to compressive loading and undergoing large deformations. In the first one, the finite strain high-fidelity generalized method of cells (HFGMC) micromechanical analysis is successively implemented for the prediction of the ideal bifurcation buckling stress of the composite. In the second approach which considers composites which possesses imperfections (e.g. wavy fibers), a perturbation expansion is adopted. The resulting zero and first-order problems are solved by employing the finite strain HFGMC micromechanics. Here, the ideal critical stress can be readily predicted, and from the form of the waviness growth with applied loading it is possible to estimate the actual bifurcation stress. In addition, in the particular case of a multilayered periodic composites, an exact solution is derived for the prediction of the bifurcation buckling stress. This exact solution is employed to verify the micromechanically predicted critical stresses of bi-layered composites. For fiber-reinforced composites, a comparison with available finite element solution is utilized to verify the micromechanical predictions. Applications of the offered approaches are given for periodically bi-layered composites that consist of five types of hyperelastic materials, and two types for fiber-reinforced composites. The present methods are also implemented to predict the microbuckling stresses of a porous hyperelastic material and lattice blocks whose elements consist of a hyperelastic material.

Microstructural and damage parameter effect analysis on the failure mechanism of fibrous soft tissues with a structure-based unit cell model

The presence of hierarchical microstructures in natural materials, such as connective tissues, makes them possess exceptional mechanical performance and undergo complex damage behaviors. Accordingly, in this study, a structure-based unit cell model (UCM) in which controllable irregular crimped fiber is embedded into soft matrix is constructed for the analysis of microstructural effects. An anisotropic hyperelastic constitutive model enhanced with different damage parameters is applied in finite element program, and the effects of damage parameters on the mechanical responses of fibrous soft tissues are investigated. The numerical results indicate that the fiber waviness plays a significant role in determining the stiffness of the tissues and that the microstructure controlled by the fiber crimp amplitude H, waviness χ and number of inflection points ω, mainly determines the fluctuation of the nominal stress–strain curves of these tissues with damaged fibers. In addition, a damage parameter analysis indicates that the attenuation of the strength and amplitude of the stress strongly depend on the strain energy limiter, and the parameter m mainly determines the rate of damage accumulation by the fibers with smaller strain energy limiters. The results of this study deepen the understanding of the mechanical behaviors of such fibrous soft tissues and provides a template for the optimization of structural and material designs for bioinspired composites.

An analytical formulation of unidirectional composite curved beam with out-of-plane fiber waviness under bending

A closed-form analytical solution is developed for analyzing laminated composite curved beam with out-of-plane fiber waviness under bending. Timoshenko beam theory was adopted in derivation where the effects of shear deformation was considered for moderately thick beam. Explicit expressions for evaluating equivalent axial and bending stiffness are formulated based upon the modified lamination theory and taking the structural deformation characteristics of narrow section beam into consideration. Stress distribution at any given location calculated by the proposed method is investigated. Significant increase of radial normal stress and shear stresses are observed when out-of-plane fiber waviness is introduced. The comparisons between present analytical results and numerical data indicate that the present methodology have ability to capture radial normal, circumferential normal, and shear stress distributions of composite curved beam with out-of-plane fiber waviness and the level of stress predicted is satisfactory with numerical results.

Fibre waviness characterisation and modelling by Filtered Canny Misalignment Analysis (FCMA)

Fibre waviness characterisation and modelling by Filtered Canny Misalignment Analysis (FCMA)

Tow placement effects on fiber waviness and tensile performance of 3D-tow reinforced hybrid-molded composites

Three-dimensional (3D) tow reinforced hybrid molding combines continuous fiber composite tow with injection overmolding to improve mechanical performance of molded components, but tensile performance is not accurately captured by conventional modeling processes. When wrapped around load introduction points, the fibers of a thick tow traverse a shorter distance at the inner radius than the outer radius leading to waviness on the inner region of each wrap. The Hsaio and Daniel model was used to predict local elastic properties of the wavy fiber composite and spatially varying material properties were applied to 3D finite element models of a suspension link. Neglecting fiber waviness overpredicted experimental tensile stiffness and strength by 36% and 33% respectively while modeling waviness overpredicted stiffness and strength by only 9% and 14% respectively. Tow wrap configuration, waviness propagation, and material parameters have significant effect on tensile performance while the tow has little effect on compressive performance.

Bionic artificial penile Tunica albuginea

Bionic artificial penile Tunica albuginea

A Review of Self-Sensing in Carbon Fiber Structural Composite Materials

Sensing is a basic ability of smart structures. Self-sensing involves the structural material sensing itself. No device incorporation is needed, thus resulting in cost reduction, durability enhancement, sensing volume increase and absence of mechanical property diminution. Carbon fiber renders electrical conductivity to a composite material. The effect of strain/damage on the electrical conductivity enables self-sensing. This review addresses self-sensing in structural composite materials that contain carbon fiber reinforcement. The composites include polymer-matrix composites with continuous carbon fiber reinforcement (relevant to aircraft and other lightweight structures) and cement–matrix composites with short carbon fiber reinforcement (relevant to the civil infrastructure). The sensing mechanisms differ for these two types of composite materials, due to the difference in structures, which affects the electrical and electromechanical behaviors. For the polymer–matrix composites with continuous carbon fiber reinforcement, the longitudinal resistivity in the fiber direction decreases upon uniaxial tension, due to the fiber residual compressive stress reduction, while the through-thickness resistivity increases, due to the fiber waviness reduction; upon flexure, the tension surface resistance increases, because of the reduction in the current penetration from the surface, while the compression surface resistance decreases. These strain effects are reversible. The through-thickness resistance, oblique resistance and interlaminar interfacial resistivity increase irreversibly upon fiber fracture, delamination or subtle irreversible change in the microstructure. For the cement–matrix composites with short carbon fiber reinforcement, the resistivity increases upon tension, due to the fiber–matrix interface weakening, and decreases upon compression; upon flexure, the tension surface resistance increases, while the compression surface resistance decreases. Strain and damage cause reversible and irreversible resistance changes, respectively. The incorporation of carbon nanofiber or nanotube to these composites adds to the costs, while the sensing performance is improved marginally, if any. The self-sensing involves resistance or capacitance measurement. Strain and damage cause reversible and irreversible capacitance changes, respectively. The fringing electric field that bows out of the coplanar electrodes serves as a probe, with the capacitance decreased when the fringing field encounters an imperfection. For the cement-based materials, a conductive admixture is not required for capacitance-based self-sensing.

C-X-C Chemokine Receptor Type 4 (CXCR-4) Functionally-Selective Allosteric Agonist ATI2341 Promotes the Recovery of Uterosacral Ligament

This study intends to assess whether CXCR4 functionally-selective allosteric agonist ATI2341 recovers uterosacral ligament. The 50 female rats were assigned into five groups including A group (normal healthy rats), B group (rats with uterine ligament injury), C group (injury rats treated with UC-MSCs cells), D group (treated with ATI2341); E group (treated with UC-MSCs cells and ATI2341) followed by analysis of uterus pathological changes by H&E staining and the expression of CD44, CD90, CXCR4, and SDF-1 by Western Blot or PT-PCR. There was regular and pyknotic fibrillar connective tissue and few small vessels in A group without infiltration of inflammatory cells. However, B group showed infiltration of inflammatory cells with few fibroblasts of fibrous tissue. The quantity of infiltration of inflammatory cells in C group and D group was less than that in B group with few visible new-born vessels. The improvement of pathological condition in uterus tissue in E group was the most among treatment groups. The number of wavy fiber was increased gradually and fibrillar connective tissue was changed into dense with elevated new-born vessels in ligament. The expression CD44, CD90, CXCR4 and SDF-1 was upregulated effectively by ATI2341. In conclusion, ATI2341 can upregulate the expression of CD44, CD90, CXCR4 and SDF-1 and promote the recovery of uterine ligament in rats, indicating that it might be a new approach for the treatment of uterine ligament.

Identification of defects in composite laminates by comparison of mode shapes from electronic speckle pattern interferometry

A novel technique for identifying defects in carbon fibre reinforced plates has been developed. Firstly, modal analysis by impact excitation was performed to obtain the first five resonant frequencies for three defect-free and three defective specimens with in-plane fibre waviness. Then amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) was used to obtain the mode shapes at these frequencies. The contours of the nodal regions visible in the mode shapes were extracted using a specially-developed algorithm employing density-based spatial clustering of applications with noise (DBSCAN). Fourier descriptors, that are invariant to rotations, translations, and scaling, were used to decompose the contours to reduce data dimensionality and make comparisons. The differences in contours between the two sets of specimens showed that the 4th and 5th mode shapes can be used for identifying the presence of the waviness defects. This technique for nodal region comparison was found to greatly simplify the comparison of fringe patterns for the purpose of damage assessment and could potentially be used as part of validation procedures.

Predicting and Improving Interlaminar Bonding Uniformity during the Robotic Fiber Steering Process.

With their high specific stiffness, corrosion resistance and other characteristics, especially their outstanding performance in product weight loss, fiber-reinforced resin matrix composites are widely used in the aviation, shipbuilding and automotive fields. The difficulties in minimizing defects are an important factor in the high cost of composite material component fabrication. Fiber steering is one of the typical means of producing composite parts with increased strength or stiffness. However, fiber waviness is an important defect induced by fiber steering during the fiber placement process. Meanwhile, the laying speeds of the inner and outer tows along the path width direction are different during the fiber steering process, resulting in different interlaminar bond strengths. Therefore, the fiber waviness and uneven interlaminar bonding strength during fiber steering not only affect the dimensions of a composite product, but also influence the mechanical properties of the part. This study aims to reduce fiber waviness and improve interlaminar bonding uniformity along the path width direction using a multi-piece compaction roller. By analyzing the mechanism of the generation of fiber waviness, the interlaminar bonding strength for each tow during fiber steering is investigated. Through analyzing and optimizing the compaction force, laying temperature and laying velocity during fiber steering experiments, the optimization approach is verified.

Tailoring elastomeric meshes with desired 1D tensile behavior using an inverse design algorithm and material extrusion printing

Additive Manufacturing (AM) is a highly attractive technology in the production of medical devices due to its capability to tailor product functionalities in line with a person's complex body structure. AM can resolve the chronic discomfort of body-supporting devices felt by people, especially those that suffer from imprecise fits, inconvenience with adjustments required in these devices to fit the body, or limitation of body movements to normal operating ranges. To enhance the user experience of those who wear medical devices, elastomeric meshes with programmed “waves” can be additively produced to enable an adjustment of the tensile stiffness in small-strain and large-strain sections. In this study, we demonstrate a method by which to adjust the stiffness transition point from low small-strain stiffness to high large-strain stiffness of additively manufactured sinusoidal and semicircular elastic fibers and determine the key design factors for transition points. Mechanics model predicts the 1D tensile behavior and experiments with controlled mechanical testing verifies the model. We reveal the major design factors that can induce the difference in the stiffness akin to on-off before and after transition. In addition, we produce meshes by the superposition of different wavy fibers, leading to multi-transition tensile behavior and thus expanding the area where mechanical properties can be controlled. An inverse design strategy is developed to improve the utility of an elastomeric mesh which enables the facile customization of the mesh with desired properties. Finally, we demonstrate that users can implement desired functions - not only the size but also the degree of body restriction - through proper settings by producing locally tailored wearable mesh-aid devices. • We clarify the major design factors for tailoring the nonlinear tensile behavior of a material extruded wavy elastomeric fiber. • Wavy fibers can transit their tensile stiffness to 1,400 fold in maximum at transition points. • Overall 1D tensile behavior of a mesh is the superposition of composed wavy fibers. • Inverse design accompanied by additive manufacturing well duplicates the targeted non-linear tensile behavior.

Advanced Ultrasonic Testing and post-processing for detection of sustainable composites

Carbon-fibre reinforced composites (CFRP) and glass-fibre reinforced composites (GFRP) has seen widespread usage in various industries, from aerospace to utilities. With the 'The green shift' and sustainability movement gaining momentum, there is an increased pressure for sustainable / natural materials to be used. Natural materials such as flax fibre can be used as a replacement for glass fibres. With the expectation that these composites will see increased usage, there is a need to develop inspection technologies. In this paper, single element immersion ultrasonic testing is used to inspect the flax fibre composites. The initial step involved inspection optimisation to obtain ideal input parameters. The raw results had no clear detection of the seeded delamination (using Teflon sheets). An advanced post-processing technique used for fibre waviness detection in CFRP was applied to the data. This post-processing technique included frequency analysis, phase analysis and amplitude analysis. The three types of data were separated from the raw data and visualised using a C-scan. Using this post-processing method, the seeded delamination could be detected. As a follow-up, more samples of various thicknesses and varying values of fibre waviness were fabricated to understand the limitation of inspection of flax fibres composites. This was achieved by having different thicknesses of Teflon sheets. The aim is to eventually have an inspection technique for a natural composite. This inspection technique and post-processing techniques has the potential to be applied onto the composite fabrication process, to aid in the manufacturing, optimisation, and validation process.

Structural and biomechanical characterizations of acellular porcine mitral valve scaffolds: anterior leaflets, posterior leaflets, and chordae tendineae.

Mitral valve (MV) tissue engineering is still in its early stage, and one major challenge in MV tissue engineering is to identify appropriate scaffold materials. With the potential of acellular MV scaffolds being demonstrated recently, it is important to have a full understanding of the biomechanics of the native MV components and their acellular scaffolds. In this study, we have successfully characterized the structural and mechanical properties of porcine MV components, including anterior leaflet (AL), posterior leaflet (PL), strut chordae, and basal chordae, before and after decellularization. Quantitative DNA assay showed more than 90% reduction in DNA content, and Griffonia simplicifolia (GS) lectin immunohistochemistry confirmed the complete lack of porcine α-Gal antigen in the acellular MV components. In the acellular AL and PL, the atrialis, spongiosa, and fibrosa trilayered structure, along with its ECM constitutes, i.e., collagen fibers, elastin fibers, and portion of GAGs, were preserved. Nevertheless, the ECM of both AL and PL experienced a certain degree of disruption, exhibiting a less dense, porous ECM morphology. The overall anatomical morphology of the strut and basal chordae were also maintained after decellularization, with longitudinal morphology experiencing minimum disruption, but the cross-sectional morphology exhibiting evenly-distributed porous structure. In the acellular AL and PL, the nonlinear anisotropic biaxial mechanical behavior was overall preserved; however, uniaxial tensile tests showed that the removal of cellular content and the disruption of structural ECM did result in small decreases in maximum tensile modulus, tissue extensibility, failure stress, and failure strain for both MV leaflets and chordae.

Prediction of Residual Strains Due to In-Plane Fibre Waviness in Defective Carbon-Fibre Reinforced Polymers Using Ultrasound Data

Residual strains affect the properties and performance of composite components, therefore measuring and predicting them are important. The prediction of residual strains from a model can be achieved by two steps: the generation of a geometric ply map and the modelling based on that to predict 3D residual strains. A novel method for identifying the most effective algorithm for characterising fibre orientation for the geometric ply map using ultrasound C-scan data has been developed. Finite element models were generated based on the fibre-orientation data from three different algorithms: the Radon transform, 2D fast Fourier transform, and Sobel filter. The models were used to predict residual strains due to three different severities of in-plane fibre waviness induced in a set of 18 specimens. Stratified leave-one-out cross validation was applied to obtain optimum parameters for the three characterisation algorithms and to update the values of the coefficient of thermal expansion for the material. Residual strains on the surface of the specimens were obtained from calculations based on the out-of-plane displacements measured using a digital image correlation system. The predicted and measured residual strain maps were decomposed into feature vectors using orthogonal polynomials to reduce data dimensionality and make quantitative comparisons. The measured residual strains and the predictions based on models using optimised parameters showed good agreement. The differences in performance were quantified based on the accuracy of the predicted residual strains, which showed that the Radon transform performed best.