Fiber Buckling Research Articles

Field Assembly Splice Technique With Solid Refractive Index Matching Material

Solid refractive index matching material enables us to splice fiber with air holes as well as conventional single-mode fiber. In this paper, we report our study of the applicability of solid refractive index matching material to a field assembly splice technique. We investigate pressing force and bending loss characteristics as a function of fiber buckling experimentally. We also investigate the impact of the hardness of the solid material on the splice loss characteristics of the field assembly splice technique. Then we derive appropriate splice conditions for both the hardness of the solid material and fiber buckling. Finally, we confirm the validity of the derived splice condition by using a single-mode hole assisted fiber.

Compressive damage mechanism of GFRP composites under off-axis loading: Experimental and numerical investigations

Experimental and computational studies of the microscale mechanisms of damage formation and evolution in unidirectional glass fiber reinforced polymer composites (GFRP) under axial and off-axis compressive loading are carried out. A series of compressive testing of the composites with different angles between the loading vector and fiber direction were carried out under scanning electron microscopy (SEM) in situ observation. The damage mechanisms as well as stress strain curves were obtained in the experiments. It was shown that the compressive strength of composites drastically reduces when the angle between the fiber direction and the loading vector goes from 0° to 45° (by 2.3–2.6 times), and then slightly increases (when the angle approaches 80–90°). At the low angles between the fiber and the loading vector, fiber buckling and kinking are the main mechanisms of fiber failure. With increasing the angle between the fiber and applied loading, failure of glass fibers is mainly controlled by shear cracking. For the computational analysis of the damage mechanisms, 3D multifiber unit cell models of GFRP composites and X-FEM approach to the fracture modeling were used. The computational results correspond well to the experimental observations.

Post microbuckling mechanics of fibre-reinforced shape-memory polymers undergoing flexure deformation

The buckling mechanics of fibre-reinforced shape-memory polymer composites (SMPCs) under finite flexure deformation is investigated. The analytical expressions of the key parameters during the buckling deformation of the materials were determined, and the local post-buckling mechanics of the unidirectional fibre-reinforced SMPC were further discussed. The cross section of SMPC under flexural deformation can be divided into three areas: the non-buckling stretching area, non-buckling compression area and buckling compression area. These areas were described by three variables: the critical buckling position, the neutral plane position and the fibre buckling half-wavelength. A strain energy expression of the SMPC thermodynamic system is developed. According to the principle of minimum energy, the analytical expressions of key parameters in the flexural deformation process is determined, including the critical buckling curvature, critical buckling position, position of the neutral plane, wavelength of the buckling fibre, amplitude of the buckling fibre and macroscopic structural strain of the composite material. The results showed that fibre buckling occurred in the material when the curvature increasing from infinitesimal to the critical value. If the curvature is greater than the critical curvature, the neutral plane of the material will move towards the outboard tensile area, and the critical buckling position will move towards the neutral plane. Consequently, the half-wavelength of the buckling fibre was relatively stabilised, with the amplitude increasing dramatically. Along with the increasing of the shear modulus, the critical curvature and buckling amplitude increase, while the critical half-wavelength of the fibre buckling decrease and the critical strain of the composite material increase. Finally, we conducted experiments to verify the correction of the key parameters describing SMPC materials under flexural deformation. The values determined by the experiments proved that the theoretical prediction is correct. Additionally, the buckling deformation of the carbon fibre generated a large macroscopic structural strain of the composite material and obtained a resulting large flexural curvature of the structure with minimal material strain of the carbon fibre.

Strain history dependence of the nonlinear stress response of fibrin and collagen networks

We show that the nonlinear mechanical response of networks formed from un-cross-linked fibrin or collagen type I continually changes in response to repeated large-strain loading. We demonstrate that this dynamic evolution of the mechanical response arises from a shift of a characteristic nonlinear stress-strain relationship to higher strains. Therefore, the imposed loading does not weaken the underlying matrices but instead delays the occurrence of the strain stiffening. Using confocal microscopy, we present direct evidence that this behavior results from persistent lengthening of individual fibers caused by an interplay between fiber stretching and fiber buckling when the networks are repeatedly strained. Moreover, we show that covalent cross-linking of fibrin or collagen inhibits the shift of the nonlinear material response, suggesting that the molecular origin of individual fiber lengthening may be slip of monomers within the fibers. Thus, a fibrous architecture in combination with constituents that exhibit internal plasticity creates a material whose mechanical response adapts to external loading conditions. This design principle may be useful to engineer novel materials with this capability.

Contact compliance effects in the frictional response of bioinspired fibrillar adhesives.

The shear failure and friction mechanisms of bioinspired adhesives consisting of elastomer arrays of microfibres terminated by mushroom-shaped tips are investigated in contact with a rigid lens. In order to reveal the interplay between the vertical and lateral loading directions, experiments are carried out using a custom friction set-up in which normal stiffness can be made either high or low when compared with the stiffness of the contact between the fibrillar adhesive and the lens. Using in situ contact imaging, the shear failure of the adhesive is found to involve two successive mechanisms: (i) cavitation and peeling at the contact interface between the mushroom-shaped fibre tip endings and the lens; and (ii) side re-adhesion of the fibre's stem to the lens. The extent of these mechanisms and their implications regarding static friction forces is found to depend on the crosstalk between the normal and lateral loading directions that can result in contact instabilities associated with fibre buckling. In addition, the effects of the viscoelastic behaviour of the polyurethane material on the rate dependence of the shear response of the adhesive are accounted for.

Photoelastic study of stresses in the vicinity of a unique void in a fibre-reinforced model composite under compression

Abstract The subject of this work is the investigation of the stress field around a unique void in a fibre-reinforced model composite under compression. Voids were introduced into the epoxy material by injection. The idea was to imitate a real void in fibre reinforced plastics (FRP) where voids appear between fibres and in resin rich areas. To investigate the mechanical response of the composite material under compression, a test jig was designed, which fits into a light microscope with an integrated photoelasticity device. That way in situ measurements were conducted. It could be demonstrated, that voids have a significant influence on the stress distribution in the matrix which leads to stress concentrations particularly around the end caps and the longitudinal side of the voids. Furthermore, different failure mechanisms like fibre matrix debonding and fibre buckling could be observed.

A model for fiber length attrition in injection-molded long-fiber composites

Long-fiber thermoplastic (LFT) composites consist of an engineering thermoplastic matrix with glass or carbon reinforcing fibers that are initially 10–13mm long. When an LFT is injection molded, flow during mold filling degrades the fiber length. Here we present a detailed quantitative model for fiber length attrition in a flowing fiber suspension. The model tracks a discrete fiber length distribution at each spatial node. A conservation equation for total fiber length is combined with a breakage rate that is based on buckling of fibers due to hydrodynamic forces. The model is combined with a mold filling simulation to predict spatial and temporal variations in fiber length distribution in a mold cavity during filling. The predictions compare well to experiments on a glass–fiber/PP LFT molding. Fiber length distributions predicted by the model are easily incorporated into micromechanics models to predict the stress–strain behavior of molded LFT materials.

Development and application of an experimental system for the study of thin composites undergoing large deformations in combined bending–compression loading

To increase understanding of damage evolution in advanced composite material systems, stereo digital image correlation has been integrated with a compression–bending mechanical loading system to obtain full-field deformations on both compression and tension surfaces throughout the loading process. The integrated system is employed to simultaneously quantify full-field deformations along the length of the specimen. Specifically, the integrated system is employed to experimentally study the progressive failure behavior of thin, woven glass–epoxy composite specimens undergoing both cyclic and monotonic compression–bending loading resulting in large out-of-plane bending deformations with end conditions that allow free out-of-plane rotation. Experimental results obtained using the measurement system for specimens undergoing both linear and highly non-linear deformations during monotonic loading are presented. Results clearly show (a) the presence and magnitude of anticlastic (double) specimen curvature near mid-length for all fiber angles, (b) the distinct differences in the strain fields between the tension and compression surfaces at the critical location, (c) the corresponding disparity in local material failure mechanisms between the tension (e.g. matrix cracking) and compression (e.g. fiber buckling) surfaces in the critical regions and (d) the highly localized character of the strain fields, focused in regions of increased damage.

Experimental investigation on buckling of GFRP cylindrical shells subjected to axial compression

Composite cylindrical shells are being used in submarine, underground mines, aerospace applications and other civil engineering applications. Thin cylindrical shells are more prone to fail in buckling rather than material failure. An experimental study on buckling of glass fiber reinforced plastics layered composite cylindrical shells under displacement and load controlled static axial compression are reported The experimental results are compared with general purpose finite element program (ANSYS). Limit point loads evaluated for geometric imperfection magnitudes shows an excellent agreement with experimental results which clearly indicates the confidence gained on the numerical results presented. Present study finds direct application to qualitatively investigate the influence of geometric imperfection on other advanced grid-stiffened structures.

Characterization of the cure shrinkage, reaction kinetics, bulk modulus and thermal conductivity of thermoset resin from a single experiment

The use of thermoset composites has increased remarkably during the recent past in naval, automobile and aeronautical applications. Despite superior mechanical behaviour, certain problems, e.g. shape distortion, fibre buckling and matrix cracking, are induced in composite part, especially during fabrication due to the heterogeneous nature of such materials. Excellent control of the curing process is required for production of a composite part with required shape and properties. For an accurate simulation of the curing process, exact knowledge of cure-dependent polymer properties and heat transfer is needed. Several instruments are required to identify these parameters, which is time consuming, and costly. In the present study, results on the simultaneous characterization of bulk modulus, chemical shrinkage and degree of cure of vinylester resin using PVT-α device are presented. Determination of cure and temperature-dependent thermal conductivity of the matrix using the same device is also discussed. The obtained results are compared with the available literature results.

Onset of failure in a fiber reinforced elastomer under constrained bending

Fiber reinforced elastomers subjected to compressive loads are prone to failure initiated by fiber buckling, a phenomenon of great technological importance. A 2D bifurcation analysis for an infinite ply-reinforced elastomer, subjected to constrained bending, is hereby presented for determining the composite’s critical (i.e. lowest) curvature and associated eigenmode. The study is complemented by a full 3D analysis of the same composite.More specifically, the onset of bifurcation analysis is based on the Bloch wave representation of the 3D eigenmode and the periodic, over a 2D unit cell, principal solution of the infinite, perfect composite subjected to constrained (i.e. its top layer is bonded to an inextensible metallic plate) bending. The critical curvature and corresponding eigenmode are found by minimizing the lowest bifurcation curvature as a function of the eigenmode’s wavelengths. These semi-analytical results, based on a 2D Finite Element Method (F.E.M) representation of the unit cell, are found to be in remarkable agreement with the full 3D calculations of the corresponding imperfect composite, thus establishing the usefulness of the proposed analysis. A comparison of the numerical simulation results to some limited experimental data is also discussed.

The mechanical behavior of porous metal fiber sintered sheets

Porous metal fiber sintered sheets (MFSSs) are a type of low density cellular materials promising for functional and structural applications. A micromechanics random beam model is proposed to investigate the elasto-plastic behavior of MFSSs. The relative density dependence of the elastic constants and yield strength of MFSSs is predicted and found to agree well with available experimental results. Fiber stretching is identified as the dominant deformation mechanism under uniaxial and multiaxial loading. When compared with two-dimensional Voronoi foams and honeycombs, the stretching deformation dominated MFSSs exhibit higher stiffness and tensile strength, but lower compressive strength due to long fiber buckling. With the developed micromechanics model, the multiaxial elasto-plastic responses of MFSSs are simulated. A macroscopic phenomenological constitutive model with a segmented yield function is proposed to describe the predicted multiaxial responses. The yield function and its evolution can be fully calibrated in terms of the uniaxial tension and compression responses rather than complex multiaxial loading responses, which can greatly facilitate practical applications of the model. This constitutive model is also expected to be applicable to other fiber sintered materials with hydrostatic pressure sensitive and asymmetric tension–compression yielding behaviors.

Bioresorbable screws reinforced with phosphate glass fibre: Manufacturing and mechanical property characterisation

Use of bioresorbable screws could eliminate disadvantages associated with metals such as removal operations, corrosion, MRI interference and stress shielding. Mechanical properties of bioresorbable polymers alone are insufficient for load bearing applications application as screws. Thus, reinforcement is necessary to try and match or surpass the mechanical properties of cortical bone. Phosphate based glass fibres were used to reinforce polylactic acid (PLA) in order to produce unidirectionally aligned (UD) and unidirectionally plus randomly distributed (UD/RM) composite screws (P40 UD and P40 UD/RM). The maximum flexural and push-out properties for the composite screws (P40 UD and P40 UD/RM) increased by almost 100% in comparison with the PLA screws. While the pull-out strength and stiffness of the headless composite screws were ∼80% (strength) and ∼130% (stiffness) higher than for PLA, those with heads exhibited properties lower than those for PLA alone as a result of failure at the heads. An increase in the maximum shear load and stiffness for the composite screws (∼30% and ∼40%) in comparison to the PLA screws was also seen. Maximum torque for the PLA screws was ∼1000mNm, while that for the composite screws were slightly lower. The SEM micrographs for P40 UD and P40 UD/RM screws revealed small gaps around the fibres, which were suggested to be due to buckling of the UD fibres during the manufacturing process.

Measuring the Optimum Fiber Content Ratio of E-Glass/Epoxy Composite through Microbuckling

When fiber-reinforced composites (FRC) are subjected to compressive load parallel to the fiber direction, they fail as a result of fiber buckling and/or transverse failure of the resin. Compressive loading brings about two buckling modes to fibers. The first mode is shear buckling, and the other is transverse buckling. Recent studies support the hypothesis that fiber buckling causes compressive rupture. In this study, finite element modeling software was employed to examine the behavior of a resin-embedded single fiber in terms of fiber content ratio. The performed modeling procedures illustrated that the single fiber experiences three discrete failure modes depending on fiber content ratio; and then a corrected equation was proposed for each mode. Fiber content ratio of the composite is one of effective parameters to determine the compressive strength value. Optimum fiber content ratio has been measured using finite element method. Numerical results are compared to experimental ones to analyze the obtained results. The optimum fiber content ratio calculated by the finite element modeling was measured 40% in this paper.

NUMERICAL AND EXPERIMENTAL STUDY OF BUCKLING OF ADVANCED FIBER COMPOSITE CYLINDERS UNDER AXIAL COMPRESSION

Advanced lightweight laminated composite shells are increasingly being used in modern aerospace structures, for enhancing their structural efficiency and performance. Such thin-walled structures are susceptible to buckling when subjected to static and dynamic compressive stresses. In this paper, details of a numerical (FEM) and an experimental study on buckling of carbon fiber reinforced plastics (CFRP) layered composite cylinders under displacement and load controlled static and dynamic axial compression are reported. The effects of different types of loadings, geometric properties, lamina lay-up and amplitudes of imperfection on the strength of the cylinders under compression are studied. Accurate measurement of imperfections in the cylindrical surface is carried out in the specimens tested. It is shown that the buckling behavior of thin composite cylindrical shells can be evaluated accurately by modeling measured imperfections and material properties in FEM.

Dynamic Deformation Behavior of Zr-Based Amorphous Alloy Matrix Composites Reinforced with STS304 or Tantalum Fibers

The Zr-based amorphous alloy matrix composites reinforced with stainless steel (STS) or tantalum (Ta) continuous fibers were fabricated without pores or defects by liquid pressing process, and their dynamic deformation behaviors were investigated. Dynamic compressive tests were conducted by a split Hopkinson pressure bar, and then the test data were analyzed in relation to the microstructures and the deformation modes. In the STS-fiber–reinforced composite, the STS fibers could interrupt the propagation of cracks initiated in the matrix and promoted the continuous deformation without fracture according to the strain-hardening effect of the fibers themselves. The Ta-fiber–reinforced composite showed the higher yield strength than the STS-fiber–reinforced composite, but the cracks were not interrupted properly by the Ta fibers according to the lower ductility and strain hardening of the Ta fibers. Both the Ta and STS fibers favorably affected the strength and ductility of the composites by interrupting the propagation of cracks formed in the amorphous matrix, by dispersing the stress applied to the matrix, and by promoting deformation mechanisms such as fiber buckling. The STS-fiber–reinforced composite showed the higher compressive strength and ductility than the Ta-fiber–reinforced composite because the STS fibers were higher in the resistance to deformation and fracture than the tantalum fibers.

Buckling of fibers in fiber-reinforced composites

Elastic stability of fibers in fiber-reinforced composite materials subject to compressive loading is studied. The transversal buckling mode is considered, and two limiting cases, the dilute and non-dilute composites are analyzed. In the case of a non-dilute composite, the cylindrical model and the lubrication approximation are applied. The original problem is reduced to a problem of stability of a rod on elastic foundation. Through the solution of this problem a simple formula for the buckling load is obtained. In the case of a dilute composite, the solution of a problem of stability of a compressed rod in elastic plane is used. On the basis of the obtained solutions in two limiting cases the interpolation formulae are derived. These formulae describe buckling of fiber in the fiber-reinforced composite for any value of the fiber volume fraction. Comparison with known numerical and experimental results is carried out, and the sufficient accuracy of the derived formulae is demonstrated.

Simultaneous contraction and buckling of stress fibers in individual cells

It has been proposed that buckling of actin stress fibers (SFs) may be associated with their disassembly. However, much of the detail remains unknown partly because the use of an elastic membrane sheet, conventionally necessary for inducing SF buckling with a mechanical compression to adherent cells, may limit high quality and quick imaging of the dynamic cellular events. Here, we present an alternate approach to induce buckling behavior of SFs on a readily observable glass plate. Actin SFs were extracted from cells, and constituent myosin II (MII) molecules were partially photo-inactivated in contractility. An addition of Mg-ATP allowed actin-myosin cross-bridge cycling and resultant contraction of only thick SFs that still contained active MII in the large volume. Meanwhile, thin SFs with virtually no active motor protein in the small volume had no choice but to buckle with the shortening movement of nearby thick SFs functioning as a compression-inducing element. This novel technique, thus allowing for selective inductions of contraction and buckling of SFs and measurements of the cellular prestress, may be applicable to not only investigations on their disassembly mechanisms but also to measurements of the relative thickness of individual SFs in each cell.

木材の縦圧縮クリープ限度

Static loading test, rapid loading test, plastic flow test, and 10-hour creep test were performed in order to find the creep limit of wood subjected to compressive stress parallel to the grain. Japanese cedar and SPF were employed to the four series tests. Major findings are as follows. The stable creep limit strain (SCL strain), i.e., the strain transient from stable to unstable stage of creep, is clearly identified at the inflection point of the creep curve. The SCL strain is lower-bounded by the fiber buckling strain (FB strain) of the wood. The FB strain is obtained from the rapid loading test, e.g., 0.35 to 0.37% for Japanese cedar and 0.25 to 0.29% for SPF in the absence of viscous strain. The unrecoverable strain in the plastic flow test appears gradually beyond the stress level of around 85% to the static strength, and the wood showing a larger plastic flow gives a larger SCL strain.

Postbuckling of functionally graded fiber reinforced composite laminated cylindrical shells, Part II: Numerical results

In this Part, the extensive parametric studies performed are reported and numerical results are presented for the buckling and postbuckling of fiber reinforced polymer matrix and metal matrix composite laminated shells subjected to axial compression or external pressure under different sets of environmental conditions. Two kinds of fiber reinforced composite laminated shells, namely, uniformly distributed (UD) and functionally graded (FG) reinforcements, are considered. The numerical results show that the buckling loads as well as postbuckling strength of the shell can be increased as a result of functionally graded fiber reinforcements. The results reveal that the effect of functionally graded fiber reinforcements on the buckling loads and postbuckling strength of shell with polymer matrix is more pronounced compared to the shell with metal matrix in the case of axial compression. In contrast, in the case of external pressure, the functionally graded fiber reinforcements may have a significant effect on the buckling pressure and postbuckling strength of the shell with metal matrix.