Fiber Buckling Research Articles

Progressive failure analysis of composite pi joint with 3D FE modeling

Aimed at the complex failure modes of composite pi joint, the material elastic modulus degradation method was applied to simulate the progressive failure procedure. A three-dimensional finite element model of one type composite pi joint was established based on the commercial finite element software Abaqus. Two progressive failure analysis subroutines considering both the composite laminates and adhesive failures were developed within Abaqus/Standard. One of them is based on Hashin three-dimensional failure criteria and Ye-delamination criterion to distinguish five failure modes of composite laminates: fiber breakage, fiber buckling, matrix tensile cracking, matrix compression, and delamination failure. The other is based on Tsai-Wu criterion to discriminate the composite failure. Additionally, an elastic-perfectly plastic material model was applied in the subroutines to simulate the adhesive failure. The established pi joint model was analyzed under given increasing tensile loads. The numerical results s...

A Device to Measure the Shrinkage and Heat Transfers during the Curing Cycle of Thermoset Composites

Residual stresses development during manufacturing of composites depends mostly on the shrinkage behaviour of the polymer matrix from the point where stresses cannot be relaxed anymore. The matrix shrinkage may have a thermal and/or chemical origin and can leads to dimensional instability, ply cracking, delamination and fibre buckling. The approaches for measuring cure shrinkage can be classified as volume and non-volume dilatometry. Each technique has corresponding advantages and drawbacks but volume dilatometry is the one that is mostly used. In the present article, we report a home-built apparatus, named PVT-a mould, on which temperature, volume change and reaction conversion degree are measured simultaneously for an applied pressure. It can also be used to study the composite during curing and for the bulk samples having several millimetre thicknesses. The instrument is preferred over other techniques as it works in conditions close to the industrial ones. This device was used to measure cure shrinkage of resin and thermoset composite material with different fibre fractions as a function of temperature and reaction conversion degree. The heat of cure of the resin measured by PVT-a mould was compared to the results obtained by DSC.

Investigation on Failure Mode of Spruce under Different Loading Conditions

Compression experiments of spruce along axial, radial and tangential loading directions are implemented by INSTRON equipment. Mechanical properties of spruce along three directions are gained. Spruce microscopic cell failure modes under axial, radial and tangential compression condition are observed by scan electron microscope. Results show that failure modes of spruce are fiber buckling and wrinkle when loading direction is along the grain. When loading direction is along radial or tangential across to the grain, failure modes are wood fiber slippage and delamination. Theory analytic solution to single wood cell failure under different direction compression is done. The obtained expression shows that mean limit loading is relative to yield stress, cell structure dimension and wrinkle length for complete wrinkle cases.

Energy Absorption of Spruce Wood under Three Kinds of Quasi-Static Compression Conditions

The curves of stress versus strain along spruce wood axial, radial and tangential directions are gained by static compression experiments. Moisture content and density of the spruce wood are 12.72% and 413 kg/m3respectively. The results indicate that spruce compression process includes elastic, yield and compaction phases. Failure modes of spruce subjected to axial compression are fiber buckling and wrinkle. And failure modes under radial or tangential compression are wood fiber slippage and delamination. Axial compression yield strength is about nine times as that of radial and tangential compression. Radial and tangential compression yield strengths are almost equal. Energy absorption efficiency and ideality energy absorption efficiency of spruce along different loading directions are analyzed. And theory analytic solution to single wood cell buckling under axial compression is done. The obtained expression shows that the mean limit loading is relative to yield stress, cell structure dimension and wrinkle length for complete wrinkle cases.

The blast resistance of a woven carbon fiber-reinforced epoxy composite

The blast resistance of a carbon fiber-reinforced epoxy (CFRE) resin has been investigated through experiments on a range of panels. The panels were subjected to blast loading by detonating small quantities of plastic explosive at a fixed stand-off distance. A ballistic pendulum was used to provide a measure of the impulse imparted to the specimen. Tests were undertaken at impulses up to those required to completely destroy the laminates. An examination of the damaged panels highlighted a number of different fracture mechanisms including delamination, top surface fiber buckling, fiber fracture, and shear failure along the edges of the plates. The impulses associated with the onset of fiber fracture and complete target failure were found to increase in a linear fashion for the range of panels considered here. The experimental data are compared with previously published results from similar tests on a woven carbon fiber-reinforced poly-ether-imide. Here, it was demonstrated that the CFRE offered a similar blast resistance to that of the thermoplastic—matrix composite.

Dynamic Debonding Strength of Fiber Glass Composite

A laminated composite may fail by various mechanisms: debonding, delamination, fiber fracture, matrix cracking, and fiber buckling. The fiber fracture possess the greatest threat the structure integrity. However, the progressive debonding often increases the load sustained by the fiber; therefore, it may promote the fiber fracture. It is clear that data of the debonding strength are crucial. Unfortunately, the experiment for the purpose is often difficult particularly in a high-strain rate case. In this paper, we present an experimental technique to obtain the mechanical properties of the debonding in the high-strain rate case.

Measurement of contractile forces generated by individual fibroblasts on self-standing fiber scaffolds

Contractility of cells in wound site is important to understand pathological wound healing and develop therapeutic strategies. In particular, contractile force generated by cells is a basic element for designing artificial three-dimensional cell culture scaffolds. Direct assessment of deformation of three-dimensional structured materials has been used to calculate contractile forces by averaging total forces with respect to the cell population number. However, macroscopic methods have offered only lower bounds of contractility due to experimental assumptions and the large variance of the spatial and temporal cell response. In the present study, cell contractility was examined microscopically in order to measure contractile forces generated by individual cells on self-standing fiber scaffolds that were fabricated via femtosecond laser-induced two-photon polymerization. Experimental assumptions and calculation errors that arose in previous studies of macroscopic and microscopic contractile force measurements could be reduced by adopting a columnar buckling model on individual, standing fiber scaffolds. Via quantifying eccentric critical loads for the buckling of fibers with various diameters, contractile forces of single cells were calculated in the range between 30–116 nN. In the present study, a force magnitude of approximately 200 nN is suggested as upper bound of the contractile force exerted by single cells. In addition, contractile forces by multiple cells on a single fiber were calculated in the range between 241–709 nN.

Enhancing the Deformation of Shape Memory Sandwich Panels

Abstract: Composite sandwich panels fabricated using a thermosetting shape memory polymer matrix material and a corresponding thermoset shape memory polymer foam core offer the potential to demonstrate large, recoverable, deformations in otherwise stiff structures, under flexural loading. However, as with flexure of thin, fibre-reinforced shape memory matrix laminates, deflection is limited by fibre compression buckling because of the reduced shape memory matrix stiffness at elevated temperature. A hybrid matrix concept has been developed for sandwich panels loaded in flexure in a single direction. This concept uses a non-shape memory resin as the matrix for a fraction of the plies on the surface of the facesheet loaded in compression. It is predicted that, at the elevated temperatures required for the generation of deformation in the shape memory structural sandwich panel, the shape memory matrix and foam moduli will be substantially reduced, while the modulus of the non-shape memory resin will not. Thus, at elevated temperature this effectively leads to a shift of the neutral axis towards the non-shape memory surface, keeping the low stiffness shape memory matrix material in tension and extending the range of deformation prior to onset of fibre buckling. The experiments performed demonstrate that this hybrid matrix approach enables a three-fold increase in mid-span deformation prior to buckling of fibres in the compression surface plies. Furthermore, the force measured to attain the deformed geometry, at elevated temperature, only increases approximately 10–15%, while the magnitude of the force required remains very low. Thus, the hybrid matrix approach functions as predicted and enables the development of sandwich panel structural elements which can undergo large, recoverable deformations.

Theoretical Study of a Spinning Triangle with Its Application in a Modified Ring Spinning System

The spinning triangle is a critical area in the spinning process of staple yarns. Its geometry influences the distribution of fiber tension in the spinning triangle and the properties of spun yarns. In earlier theoretical models, the spinning tension of the yarn acting on the convergence point is assumed to be perpendicular to the nip line of front rollers. This assumption is reasonable in the study of the conventional ring spinning triangle, but may not be appropriate for modified ring spinning systems in which the yarn spinning tension has an obvious angle with the vertical axis perpendicular to the nip line. Thus in this paper a new theoretical model is proposed by considering the inclination angle of the spinning tension. Based on the principle of minimum potential energy, the theoretical model quantitatively describes the fiber tension distribution and its relationships with the spinning parameters, shape of the spinning triangle and the inclination angle of yarn spinning tension. As an application of the proposed method, spinning triangles of a modified ring spinning system were investigated under various spinning parameters. The fiber tension distributions in the spinning triangle both with and without fiber buckling were numerically simulated and the results were compared with those from our earlier model. In addition, the properties of spun yarns produced by the modified system were evaluated and analyzed.

How age influences unravelling morphology of annular lamellae – a study of interfibre cohesivity in the lumbar disc

Although age- and degeneration-related changes in the morphology and biochemistry of the annulus fibrosus have been extensively reported, studies of tensile strength changes show only a weak correlation with maturity. Given that the disc is a tissue system in which significant levels of deformation occur with normal physiological loading, there may be structure-related properties that provide a better indicator of the influence of ageing on its function. This study is a morphological investigation of lamellar interfibre cohesivity with respect to maturity. Anterior segments of ovine lumbar discs in two age groups were cut at one of two section angles to generate intralamellar and interlamellar slices. These slices of hydrated annular tissue were subjected separately to microtensile and swelling forces, and examined using differential interference contrast microscopy. There were distinct differences in microstructural responses to transverse extension between the immature and mature intralamellar slices. The immature tissue exhibited a diffuse expansion of the array to form a fine fibrous net. In contrast, the mature tissue displayed a discontinuous expansion with the development of clefts and localized fibre buckling. A difference was also observed in the free-swelling response; the immature slices remained planar, whereas the cropped lamellar fibres in the mature slices exhibited a folded, buckled morphology. Morphological evidence from these experiments infers differences in fibre cohesivity between the immature and mature tissues, consistent with biochemical and histological studies. More extreme levels of deformation in the mature tissue could result in discontinuous opening of the fibrous arrays, which might have the potential to lead to cleft formation. These clefts may, in turn, provide micropaths through which nuclear material could extrude. Importantly, with many animal studies carried out on immature discs, the results here suggest that some caution is required with respect to extrapolating annular behaviour beyond this age group.

Effects of Geometry of Ring Spinning Triangle on Yarn Torque: Part II: Distribution of Fiber Tension within a Yarn and Its Effects on Yarn Residual Torque

This paper presents a theoretical analysis of fiber tension distribution within a ring spun yarn without relaxation. Yarn residual torque is determined on the basis of the translation of fiber tension at the spinning triangle, as presented in the first part of this series of papers, into the fiber tension within the yarn. The results from numerical simulations indicate that, with fiber buckling, the yarn exhibits a lower average fiber tension and, thus, a much reduced yarn residual torque than that without fiber buckling. Comparison with experiments confirmed that fiber buckling exists in ring yarns while the assumption of no fiber buckling is not realistic. Generally, a low yarn twist results in low average fiber tension in the yarn and, thus, a reduced yarn residual torque. A symmetrical spinning triangle leads to a slightly higher yarn residual torque than a right-angle spinning triangle when the yarn counts and twists are identical and fiber buckling occurs.

Numerical Simulation of Matrix Reinforced Composite Materials Subjected to Compression Loads

This paper reviews the most common formulations to obtain the compression strength of long fiber composites due to fiber buckling. This failure mode was first studied by Rosen (Fibre Composite Materials, pp. 37–45, 1965) who defined two different fiber buckling modes, extensional and transverse. Further studies improved the first model proposed by Rosen by defining with more accuracy the mechanics of the problem. Although each formulation use a different approach to solve the problem, all of them agree in the dependence of fiber buckling on three main parameters: matrix shear strength, fiber initial misalignments and volumetric participation of the fibers in the composite. Once having described the different approaches used, and the parameters on which they depend, this paper describes a new formulation capable of obtaining the compression strength of composites taking into account the fiber buckling phenomenon. This formulation uses the serial/parallel mixing theory developed by Rastellini et al. (Comput. Struct. 86(9):879–896, 2008) to simulate the composite, and takes advantage of knowing the mechanical performance of the composite constituents to simulate the fiber buckling phenomenon. This is done with an homogenization procedure. It consists in introducing the interaction between fibers and matrix into their respective constitutive equations. The interaction between fiber and matrix takes into account fiber initial misalignments, its volumetric participation and the mechanical properties of both constituents. The new formulation proposed is implemented in a finite element code, taking into account that fibers can have different misalignment levels, and that the composite behaves differently if it is under tensile or compression forces. The mechanical performance of the formulation proposed is studied with several finite element simulations of compressed composites. Finally, the correctness of the formulation is proved by comparing the numerical results with the experimental tests provided by Barbero and Tomblin (Int. J. Solids Struct. 33(29):4379–4393, 1996), Tomblin et al. (Int. J. Solids Struct. 34(13):1667–1679, 1997).

Buckling of composite FRP structural plates using the complex finite strip method

Abstract A finite strip method is presented to predict local, distortional, and lateral buckling of composite fiber reinforced plastic (FRP) structural plates. Each plate may be subjected to a combination of longitudinal compression, longitudinal in-plane bending, or shear stress. A sinusoidal function is assumed in the longitudinal direction for a buckling mode of whatever type and a polynomial function is used in the transversal direction. The critical stress and critical moment of I-shape and box and channel sections under bending and uniform loading are obtained by solving an eigenvalue problem. Using this solution technique, a simple expression is developed for prediction of I-shape section beams’ buckling stresses in three design curves. The critical stresses for different E x / E y values are calculated using the design curves.

Correlation of Microstructure and Mechanical Properties of Tantalum-Continuous-Fiber-Reinforced Amorphous Matrix Composite Processed by Liquid Pressing

A Zr-based amorphous alloy matrix composite reinforced with tantalum continuous fibers was processed without pores or misinfiltration by liquid pressing, and their microstructures and mechanical properties were investigated. About 60 vol pct of tantalum fibers were homogeneously distributed inside the amorphous matrix, which contained a few crystalline particles. The ductility of the composite under tensile or compressive loading was dramatically improved over that of the monolithic amorphous alloy, while maintaining high strength. The composite showed a high tensile elongation of 7.2 pct because of the formation of multiple shear bands, crack deflection in the matrix, multiple necking, and obstruction of crack propagation by ductile fibers. Under compressive loading, fibers prevented the shear fracture of the amorphous matrix and caused buckling of fibers, thereby leading to a high plastic strain of 45 pct or higher. This suggested that the liquid pressing process was useful for the development of continuous-fiber-reinforced amorphous matrix composites with improved ductility.

Buckling behaviour of electrospun microtubes: a simple theoretical model and experimental observations

The final form of tubular nanofibres produced by the co-electrospinning of two solutions (core/shell) is largely determined by the kinetics governing the buckling phenomenon. The buckling mechanism involves the evaporation of the core solution through a solidified shell resulting in a pressure difference across the fibre shell. Buckling can take place when the pressure drop across the fibre shell exceeds a critical value. In this work the physical conditions leading to fibre buckling are analysed from a kinetic point of view. A time interval, Δt, during which buckling may occur, is introduced as a single criterion determining the buckling probability. Different core/shell systems were spun by varying the surface tension, viscosity, flow rate, electric field and the diffusion coefficient of the core solvent through the fibre shell. The imaged as-spun nanofibres were analysed statistically to determine the buckling probability, and the corresponding Δt values were calculated using the values of the spinning parameters. The obtained data were fitted with an exponential distribution function which afforded determination of the characteristic time to buckling, tb. The results provide a means of predicting the buckling of tubular nanofibres. In particular, one can conclude that the dominant parameter determining the final form of the as-spun tubular nanofibres is the diffusion coefficient of the core solvent through the fibre shell.

Finite element study of compressively loaded fibres fractured during impact

This paper examines the compressive behaviour of plies with fibres previously fractured during impact. The analysis is conducted using finite element (FE) modelling in ABAQUS 6.7. Two- and three- dimensional models are used to consider the possibility of fibre penetration and “brooming” of fractured fibres, or fibre buckling. A parametric study of the influence of the input parameters of the Drucker Prager plasticity model was also conducted, to enable a better understanding of the model. This was used to capture the triaxial stress state in the matrix surrounding the fibres. The results suggest that fibre buckling is more likely to occur due to the geometry of fibres and fibre spacing in carbon fibre composites, but fibre penetration could still occur in regions of low fibre volume fraction.

The Blast Behavior of Fiber Reinforced Thermoplastic Laminates

This article presents the results of a series of blast tests on a carbon fiber-reinforced poly-ether-imide (PEI) and a glass fiber-reinforced PEI composite. Initially, the fracture properties of the two composite systems were characterized through a series of flexural and interlaminar fracture tests. Blast testing was then undertaken on a ballistic pendulum facility, capable of measuring the impulse imparted by the plastic explosive. Delamination, localized fiber buckling, fiber fracture, and shear failure at the boundary of the clamped plates were identified as the primary failure mechanisms in the laminates, with their severity depending on the panel thickness and the applied impulse. Delamination was very localized along the center plane of the laminate, a reflection of the very high interlaminar fracture toughness of these composites. The critical impulse for rear surface fiber fracture has been found to increase rapidly with laminate thickness for the range of panels considered here. The impulses associated with the onset of rear-surface fiber fracture and complete failure of the target were similar, suggesting that rear surface fiber fracture is a pre-cursor to complete failure in these laminates. Limited tests on the glass fiber-reinforced PEI system showed that it offers a superior blast resistance to its carbon fiber counterpart.

Mapping the depth dependence of shear properties in articular cartilage

Determining the depth dependence of the shear properties of articular cartilage is essential for understanding the structure–function relation in this tissue. Here, we measured spatial variations in the shear modulus G of bovine articular cartilage using a novel technique that combines shear testing, confocal imaging and force measurement. We found that G varied by up to two orders of magnitude across a single sample, exhibited a global minimum 50–250 μm below the articular surface in a region just below the superficial zone and was roughly constant at depths >1000 μm (the “plateau region”). For plateau strains γ plateau≈0.75% and overall compressive strains ε≈5%, G min and G plateau were ≈70 and ≈650 kPa, respectively. In addition, we found that the shear modulus profile depended strongly on the applied shear and axial strains. The greatest change in G occurred at the global minimum where the tissue was highly nonlinear, stiffening under increased shear strain, and weakening under increased compressive strain. Our results can be explained through a simple thought model describing the observed nonlinear behavior in terms of localized buckling of collagen fibers and suggest that compression may decrease the vulnerability of articular cartilage to shear-induced damage by lowering the effective strain on individual collagen fibrils.

Fracture morphology and deformation characteristics of repeatedly impacted thermoplastic matrix composites

Deformations of thermoplastic matrix composites during impact fatigue loading were investigated by instrumented impacts tester and scanning electron and optic microscopy (SEM). Curves of damage evolution against the number of repeated impacts to fracture the composites revealed three distinct zones: fibre micro buckling and shear fracture of fibres (1st region), initiation and propagation of delaminations and matrix deformations (2nd region), propagation of delaminations and fibre cracking and pull out especially in tensile area (3rd region). Intensive deformations observed primarily in compression region during impact-fatigue loading can be explained by lower compressive strength of composites compared to their tensile strength.

Buckling of fibers under distributed axial load

The analysis of buckling of fibers with continuously varying cross section under distributed axial load has been investigated by using numerical method. The eigenvalues and critical length of the vertical fiber column which the moment of inertia and the intensity of distributed axial load vary according to a power of the distance are given in a tabulated form.