Effect Of Fiber Waviness Research Articles

The Effect of Fiber Waviness on the Stiffness of a Polymer Composite Material

The Effect of Fiber Waviness on the Stiffness of a Polymer Composite Material

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.

The Effect of Fiber Waviness on the Residual Stress State and Its Prediction by the Hole Drilling Method in Fiber Metal Laminates: A Global-Local Finite Element Analysis

In this paper, fiber waviness, as one of the most frequently occurring defects in fiber reinforced composites, is numerically investigated with regard to the formation of residual stresses in fiber metal laminates. Furthermore, the prediction of the residual stress state in the thickness direction by means of the simulated hole drilling method is studied. To this regard, a global-local finite element analysis based on the submodel technique is presented. The submodel technique essentially consists of two governing steps: In the first step, a global model is first utilized to calculate and analyze the residual stress distribution and deformation in the intrinsically joined hybrid structure. Effective cure-dependent thermo–elastic properties predicted by a numerical homogenization procedure were used to simulate the curing-process and analyze the residual stresses state. However, the dimension of the intrinsically manufactured hybrid plate is large compared to the diameter of the drilled hole (2 mm), so that a local model is necessary, which provides only a geometric partial portion of the global model. The local model takes the global stress state into account and is subsequently used to simulate the incremental hole drilling method with a refined mesh discretization. The production-related fiber waviness is modeled by an element-wise orientation approximating a sinus function. In order to validate the global-local modeling approach, a comparison between numerical results and experimental data from literature is presented. The comparison between global residual stress state (global model) and the simulated hole drilling method (local model) is used to assess the applicability and reliability of the hole drilling method in case of fiber waviness. It is found that an in-plane fiber waviness leads to a rather low variance of residual stresses over thickness. In case of an out-of-plane fiber waviness, oscillating residual stress fields occur over the entire thickness along the fiber direction. Moreover, the current limits of the incremental hole drilling method could be pointed out by the presented investigations. It is seen that the simulated results of the incremental hole drilling method are sensitive to waviness, even if the amplitude-wavelength-ratio is small. Without further adjustment of the calibration coefficients the oscillating stress and strain fields lead, in particular fiber waviness in thickness direction, to unreliable predictions. For the experimental application it can be concluded that the specimens have to be carefully examined with regard to fiber waviness.

Effect of fibre waviness on the compressive fatigue behavior of woven carbon/epoxy laminates

In the present work, two woven carbon/epoxy laminates with different areal weight were tested under tension-compression (R = −1) and compression-compression (R = -∞) cyclic loadings. Their performance was evaluated in terms life to final separation and stiffness degradation during the fatigue life. In addition, microscopic observations were carried out at regular intervals during the tests to identify the damage evolution at the micro-scale. The bundle waviness was found to have a large influence on the damage mechanisms and the fatigue life of the laminates both for tension-compression and compression-compression loadings, while the stiffness loss remained small until final failure in all cases.

Effects of Fiber Waviness and Misalignment of Reinforcing Fiber on Deformation Strength in Rubber-Composite Material

Effects of Fiber Waviness and Misalignment of Reinforcing Fiber on Deformation Strength in Rubber-Composite Material

Failure Behavior of Unidirectional Composites under Compression Loading: Effect of Fiber Waviness.

The key objective of this work is to highlight the effect of manufacturing-induced fiber waviness defects on the compressive failure of glass fiber-reinforced unidirectional specimens. For this purpose, in-plane, through-thickness waviness defects (with different waviness severities) are induced during the manufacturing of the laminate. Numerical and experimental results show that the compressive strength of the composites decreases as the severity of the waviness defects increases. A reduction of up to 75% is noted with a wave severity of 0.075. Optical and scanning electron microscopy observations of the failed specimens reveal that kink-bands are created in the wavy regions and lead to failure.

The effect of fiber waviness on the fatigue life of CFRP materials

For carbon fiber reinforced polymer parts featuring manufacturing defects the balance between safety and cost has to be maintained. In this work, a non-crimp fabric with induced out-of-plane fiber waviness, which is a common defect, is investigated. Thereby its effect on the fatigue life under axial loading is assessed experimentally and numerically. For a profound understanding of damage initiation and progression, different damage modes are investigated up to final failure.A geometry based defect metric is established and the influence of this metric on the compression fatigue life of the considered material is quantitatively assessed, such that the proposed metric can be used for defining acceptance/rejection criteria for composite parts containing this specific defect.

Investigation into the effects of fiber waviness in standard notched composite specimens

This study presents a numerical and experimental evaluation of the standardized material testing specimens (tension and compression) used for determining the strength of composite materials in the presence of defects. The composite specimens contain through the thickness fiber waviness and a circular cutout. The intentional waviness levels have been applied in the out-of-plane direction of the multi and unidirectional laminate during the curing process. The standardized face-stabilized open-hole compression test based on ASTM D6484 and the open-hole tension test according to ASTM D5766 are used for evaluating the interaction of the fiber waviness and the circular cutouts. Temporal evaluations of the load-deformation response in the specimens are coupled with optical microscopy to understand the failure modes and damage progression. Laminates with multidirectional layups show different failure modes and a different damage trajectory when compared with the response in the unidirectional laminates. The damage trajectory is dominated by the notch region and also influenced by notch size and free edge effects.

Effect of Fiber Waviness on Tensile Strength of a Flax-Sliver-Reinforced Composite Material

Recently, a composite material made from natural fibers and biodegradable resin, “green composite,” is attracting attention as an alternative composite material for the replacement of glass fiber-reinforced plastics. Plant-based natural fibers such as kenaf and flax have already been used as composite reinforcement materials because they are more environmentally friendly and costless fibers than artificial fibers. A problem of using natural fibers is the fiber waviness, which affects the tensile properties. Fiber waviness is fluctuation in the fiber orientation that is inherent in the sliver morphology of plant-based natural fibers. This study was conducted to clarify the relation between quantified parameters of fiber waviness and a composite’s tensile strength. First, the fiber orientation angles on a flax-sliver-reinforced composite were measured. Then the angle distribution was quantified through spatial autocorrelation analysis methods: Local Moran’sIand Local Geary’sc. Finally, the relation between the resultant tensile strength and quantified parameters was discussed.

The Effect of Fiber Geometry and Interfacial Properties on the Elastic Properties of Cementitious Nanocomposite Material

This paper investigates the elastic (Young’s) modulus of carbon Nanotube‐ (CNT‐) reinforced cement paste using 3D and axisymmetric models using Abaqus software. The behavior of the CNT and the cement matrix was assumed to be fully elastic while the cohesive surface framework was used to model the interface. To investigate the effect of fiber waviness on the value of the elastic modulus, 3D models were developed assuming different distributions of fibers. The results obtained using the 3D model were compared to those obtained using the simplified three‐phase axisymmetric model which consists of one single CNT aligned in the center of composite unit cell, an interface, and cement matrix. A parametric study was then carried out using the axisymmetric model to study the role of the interface in the composite elastic modulus without accounting for the presence of the interfacial transition zone (ITZ or interphase). The results showed that the CNTs waviness significantly reduced their reinforcing capability in the cement paste. On the other hand, the results obtained using the axisymmetric model were found to be in good agreement with those obtained using the 3D model. Moreover, the results of the parametric study showed that the interface properties significantly affect the composite elastic modulus and alter its behavior.

Effects of CNT waviness on the effective elastic responses of CNT-reinforced polymer composites

In this study, the effects of fiber waviness on the effective elastic responses of CNT–polymer composites are investigated based on the framework of micromechanics and homogenization. By taking advantage of an ad hoc Eshelby tensor, the load-transfer capability of wavy carbon nanotube (CNT) embedded in the polymer matrix is accounted for. Further, the effective elastic responses of composites are simulated by using the multi-phase Mori–Tanaka method to study the influence of randomly oriented wavy CNT. It is demonstrated that the proposed micromechanics-based closed form solution is effective to tackle the underlying problem. The present predictions and the comparisons with the available experimental data indicate that the CNT waviness leads to the degradation of effective responses of composites. Finally, in addition to the effect of CNT waviness, the significance of CNT interface is briefly discussed based on the experimental observations.

Effect of fiber waviness on the bending behavior of S-glass/epoxy composites

A study was conducted on the influence of localized ply waviness on the strength of uniaxial fiberglass composites subjected to three-point bending. A series of experimental three-point bending tests were performed to discover how wave geometry will affect the strength of these materials. It is shown that any waviness will decrease the strength, but this reduction is maximized by increased amplitude, deceased wavelength, and concave up waves as well as increased total specimen length. As much as 37% reduction in strength compared to a flat specimen is seen in the case of large concave up waves. The dominant failure mode for unidirectional wavy composites under bending was shown to be interlaminar shear, which in turn leads to microbuckling. Finally, a failure criterion is derived which can predict the load and location where failure will begin.

New Fatigue Data for Wind Turbine Blade Materials

This paper reports on recent fatigue data of interest to the wind turbine industry in several areas: (a) very high-cycle fatigue data; (b) refined Goodman Diagram; (c) effects of fiber waviness; and (d) large-tow carbon fibers. Tensile fatigue results from a specialized high-frequency small strand testing facility have been carried out to 1010 cycles in some cases, beyond the expected cycle range for turbines. While the data cannot be used directly in design due to the specialized test specimen, the data trends help to clarify the proper models for extrapolating from standard coupons to higher cycles. The results for various fiber and matrix systems also provide insight into basic failure mechanisms. For spectrum loading predictions, a more detailed Goodman Diagram has been developed with additional R-values (R is the ratio of minimum to maximum stress in a cycle). The data of greatest interest were obtained for tensile fatigue with low cyclic amplitudes, close to R=1.0, to clarify the shape of the diagram as the cyclic amplitude approaches zero. These data may significantly shorten lifetime predictions compared with traditional Goodman Diagram constructions based on more limited data. The effects of material/process induced flaws on properties continues to be a major concern, particularly with large-tow carbon fabrics. The results of a study of fiber waviness effects on compressive strength show significant strength reductions for severe waviness which can be introduced in resin infusion processes. The final section presents new fatigue results for large-tow carbon/fiberglass hybrid composites. Epoxy resin laminates show marginally higher compressive strength and fatigue resistance with carbon fibers. Improved compressive static and fatigue performance is found with stitched fabrics as compared with woven fabrics.

Effects of material and geometric nonlinearities on the tensile and compressive behavior of composite materials with fiber waviness

The effects of fiber waviness on the nonlinear behavior of unidirectional composites under tensile and compressive loadings have been investigated theoretically and experimentally. Unidirectional composites examined were composed of continuous fibers with sinusoidal waviness in a matrix. As a consequence of material and geometric factors, both the tensile and compressive behavior of these composites was generally nonlinear under finite deformation. Analytical models were proposed for predicting the nonlinear tensile and compressive behavior as a function of fiber waviness for three types of fiber waviness pattern: uniform, graded and localized fiber waviness. The material and geometric nonlinearities due to fiber waviness were incorporated into the models based on complementary energy density and an incremental method. Specimens with various degrees of fiber waviness were fabricated. Tensile and compressive tests were conducted on the specimens to obtain the elastic properties and behaviors of the composite materials with fiber waviness. The experimental results were in good agreement with the predictions.

Effect of Fibre Waviness on Compression Failure of Cross-Ply Carbon/Epoxy Laminates

Composites were made as cross-ply laminates with artificially introduced waviness near the middle, with different severity of waviness and different fractions of wavy layers. The laminates had configuration (0/(90 0 2 ) n /core/ (0 2 90) n / 0) with n = 2, 4, and 6 in samples with 24, 36, and 48 layers respectively. The fraction of wavy layers varied from 6 to 37%, and the severity of waviness, measured as the ratio of amplitude over half wavelength - 2a/λ, varied from 0.03 to 0.08, which corresponded to the angular deviations from 7 to 14°. The composites were tested in compression to determine the effect of wave severity and fraction of wavy layers on compression strength. Failure of tested coupons was examined to establish the mechanism of fracture initiation and propagation. Two compression failure mechanisms were observed.

Effect of Fiber Waviness on Buckling Strength of Composite Plates

This paper explores the idea of tailoring the profile of reinforcing fibers to improve the buckling strength of composite plates. This paper analyzes the uniaxial buckling behavior of composite laminates in which fibers are placed along parallel sinusoidal curves, instead of the conventional straight line pattern. The sinusoidal fiber orientation results in variable elastic stiffness and nonuniform prebuckling stress fields, which are shown to have a pronounced influence on the buckling strength. For example, changing the fiber orientation from straight line to a sinusoidal pattern can increase the buckling load of a simply supported laminate by more than 80%. For the analysis, a computerized Rayleigh-Ritz procedure is developed that exploits an analogy between bending and stretching formulations to model a variety of boundary conditions.

Effect of Fiber Waviness on the High-Strain-Rate Behavior of Composites

The effect of fiber waviness on the high-strain-rate behavior of carbon/epoxy composite materials was investigated. Falling weight impact and Split Hopkinson Pressure Bar systems were developed for basic characterization of unidirectional aligned composites in compression at strain rates up to 1800 s-1. Seventy-two and forty-eight-ply unidirectional carbon/epoxy (IM6G/3501-6) laminates loaded in the longitudinal and transverse directions were characterized. Compression tests on 45° off-axis laminates of the same unidirectional material were also conducted to obtain the in-plane shear stress-strain behavior. Strain rates over a wide range, from quasi-static up to 1800 s-1, were recorded. An incremental analysis was then used to predict the high-strain-rate behavior of composites with fiber waviness using the results obtained from the strain rate characterization. It is shown that, under longitudinal compressive loading, the nonlinear stress-strain behavior shows significant stiffening as the strain rate increases due to the shear component involved.

NONLINEAR BEHAVIORS OF THICK COMPOSITES WITH FIBER WAVINESS UNDER PURE BENDING

The effects of fiber waviness on the nonlinear behavior of unidirectional composites under pure bending were studied theoretically and experimentally. Constitutive models were proposed for the predictions of the flexural properties and nonlinear behavior of composites with fiber waviness. Three types of wavy patterns (uniform, graded and localized fiber waviness) were considered. The material nonlinearity and geometrical nonlinearity due to fiber waviness were incorporated into the analyses. Specimens with various degrees of fiber waviness were fabricated. Bending tests were conducted to compare the experimental results with the predictions for the uniform fiber waviness model. It was found that the experimental results were in good agreement with the predictions.

A Study of the Effects of Fiber Waviness on the Buckling Characteristics of Laminated Plates

The equations of equilibrium of a composite plate within-plane fiber waviness are derived and the effects of waviness on the buckling loads are studied. The plate is assumed to be in cylindrical bending and the resulting eigenvalue problem is solved using a combination of a shooting algorithm and the secant method. A numerical example of a plate with sinusoidal fiber waviness shows that the buckling load decreases as the amplitude of the waviness increases. However, results indicate that it is possible to enhance the stability characteristics of the plate by introducing artificial fiber waviness to increase its buckling load.

The Effect of Fibre Waviness on the Relationship between Compressive and Flexural Strengths of Unidirectional Composites

A non-linear finite element analysis has been carried out which shows that the same mechanism of shear instability due to fibre waviness can occur under pure bending as under pure compressive loading. Results for carbon fibre-epoxy indicate a significantly higher maximum compressive stress in bending than in compression due to the presence of the stress gradient, and resulting through thickness stresses. For thick specimens. where the stress gradient is low, the maximum compressive stress in bending tends towards the value in pure compression. As the thickness decreases. the bending stress increases due to the constraint of the adjacent less highly loaded fibres. The predicted maximum stress in bending was up to 73% higher than in compression. The results show that where failure is controlled by shear instability due to fibre waviness. compressive strength can be expected to be a function of stress gradient and therefore cannot be regarded as a true material property.