Fiber Buckling Research Articles

Influence of stacking sequences of woven jute-carbon hybrid composites: Diffusion mechanism and mechanical characterization

In this study, different stacking orders of carbon-jute fiber mats were used to fabricate the composites and hybrids. The impact of different stacking orientations on mechanical properties was investigated experimentally. The experimental results were verified by finite element analysis (FEA). In addition, the impact of the fiber orientation on the loading direction was also investigated. Four layers of fabrics were piled to fabricate the composites by hand-layup with the cold press process. A NaCl solution was used to create a corrosive medium (pH 8), and the composites and hybrids were immersed in that medium, and their mechanical properties were compared. In this research, it has been found that the laminates with the stacking sequence of type-C (C/J/J/C) show the best result among other composites. Jute composites where all the fibers are in the loading direction possess a high strain compared with the other composites and hybrids. On the scanning electron microscope (SEM) image, delamination, buckling of fiber, fiber pullout, and voids were observed. In addition, the kinetics of the diffusion mechanism were also investigated, and it was found from the experimental results that the composites and hybrids immersed in pH 8 absorbed water via Fickian diffusion.

Investigation of the static and low‐velocity impact performance of 3D braided spacer composites

AbstractTo meet the lightweight requirements of advanced composite materials, the ultralightweight three‐dimensional (3D) braided spacer composites were designed and manufactured by ultra‐high molecular weight polyethylene (UHMWPE). The key structural effects of core column distances and core column heights on the static bending, compression, and low‐velocity impact properties of the composites were further investigated. Results indicated that the mechanical properties of 3D braided spacer composites was closely depended on their structural parameters. The flexure and compression of 3D braided spacer composites decreased with increasing core column distance and core column height. W1/H1 specimen showed the optimal bending and compressive performance. The maximum flexural strength and flexural modulus were approximately 54.77 MPa and 4.95 GPa, respectively. The low‐velocity impact properties were further investigated where the stiffness, energy absorption, and impact resistance of the composite decreased with increasing core column distance. The peak load and energy absorption of the W1 specimen were 2.39 kN and 7.99 J, respectively. While the low‐velocity impact properties increased with increasing core column distance. The H3 specimen exhibited the greatest stiffness, impact resistance and energy absorption with peak loads and energy absorption of 2.81 kN and 8.49 J, respectively. In addition, the macro morphology and scanning electron microscopy (SEM)micrographs were examined to investigate the damage morphology and failure mechanism of the composites. The main failure modes were matrix cracking, interface debonding, fiber buckling, and dislocation, as well as tilting, and failure of the core columns. This work provides guidance for structural design and engineering application of 3D lightweight braided spacer composite.

A numerical study on tensile strength of low-density Kagome networks made of brittle fibers

In this paper, we present a numerical study on the tensile strength of low-density Kagome networks made of brittle fibers. First, an elastic beam model is employed to analytically predict the effective elastic properties and tensile strength, as well as the critical condition for buckling of the fibers in Kagome networks. A series of finite element analyses are then conducted to simulate the elastic deformation and failure of Kagome networks under tension. The numerical results based on unit-cell models reveal four possible failure modes of the Kagome networks subject to uniaxial tension, summarized in a phase diagram in terms of the relative density and the fiber strength. The pre-buckling failure mode is restricted to cases with relatively high density and low fiber strength. A low-density Kagome network is likely to fail by one of the post-buckling modes, with an effective tensile strength much lower than the prediction by the elastic beam model. For Kagome networks consisting of a large number of unit cells, the effect of boundary conditions on the tensile strength is examined. Under periodic boundary conditions, the effective tensile strength is nearly identical to that predicted by the unit-cell model, independent of the model size. Under a roller boundary condition, with damage initiation near the free edges followed by a diffusive damage progression, the effective tensile strength is lower than that under periodic boundary conditions for the cases of relatively low fiber strengths. Under a clamped boundary condition, the effective tensile strength is higher than that under periodic boundary conditions for the cases of relatively high fiber strengths, where fiber buckling is largely suppressed by the clamped boundaries. Finally, the effect of a crack-like defect on the effective tensile strength is studied for Kagome networks under the clamped boundary condition. With a small defect, the effective strength is nearly independent of the defect size. In contrast, with a relatively long defect, the effective strength decreases almost linearly with the length of the crack-like defect. The effective toughness for damage initiation and steady-state damage progression in the Kagome networks is discussed from an energetic perspective.

Hyperelasticity of blood clots: Bridging the gap between microscopic and continuum scales

The biomechanical properties of blood clots, which are dictated by their compositions and micro-structures, play a critical role in determining their fates, i.e., occlusion, persistency, or embolization in the human circulatory system. While numerous constitutive models have emerged to describe the biomechanics of blood clots, most of these models have primarily focused on the macroscopic deformation of the clots and the resultant strain-stress correlations without depicting the microscopic contributions from their structural components, such as fibrin fibers, fibrin network and red blood cells. This work addresses the gap in current scientific understanding by quantifying how changes in the microstructure of blood clots affect its mechanical responses under different external stresses. We leverage our previous published work to develop a hyperelastic potential model for blood clots, which incorporates six distinct strain-energy components to describe the alignment of fibers, the entropic and enthalpic stretching of fibrin fibers, the buckling of these fibers, clot densification, and clot jamming. These strain-energy components are represented by a combination of simple harmonic oscillators, one-sided harmonic potentials, and a Gaussian potential. The proposed model, which is C0, C1, and C2 continuous with a total of 13 parameters, has been validated against three datasets: 1) fibrin clot in tension, 2) blood clot in compression, and 3) blood clots in shear, demonstrating its robustness. Subsequent simulations of a microscopic blood clot model are performed to uncover mechanistic correlations for a majority of the hyperelastic potential's stiffness/strain parameters. Our results show that only one proposed term concerning fiber buckling needs further refinement, while the remaining five strain-energy terms appear to describe precisely what they were intended to. In summary, the proposed model provides insight into the behavior of thromboembolisms and assistance in computer-aided design of surgical tools and interventions such as thrombectomy.

Flexural fatigue behavior of ultra-high performance concrete under low temperatures

Ultra-high performance concrete (UHPC) shows superior mechanical performance, which leads to increasing applications in infrastructure constructions that are subjected to different loading (i.e., flexure, tension, compression, etc.) and environmental conditions (ambient, freeze, etc.). Among them, the flexural performance of UHPC under low temperatures (i.e., sub-zero temperature) is still little understood especially under fatigue loading. To investigate the flexural fatigue properties of UHPC under low temperatures, eleven UHPC prisms are subjected to cyclic bending at different stress levels from 0.40 to 0.80 (the ratio between applied maximum fatigue stress and static flexural strength) and temperatures of 20 °C, -10 °C, and -20 °C. Results indicate that the fracture interfaces for specimens under static and fatigue loading exhibit distinct differences. No fiber buckling appears for the monotonically-loaded specimens, while for specimens under fatigue loading, fibers buckled and fractured. Low temperature improves the mechanical properties of UHPC under monotonic loading due to enhanced matrix strength. Conversely, low temperature adversely affects the flexural fatigue performance of UHPC due to the cold brittleness nature of matrix and steel fibers, leading to the accumulation of matrix deterioration and the fracture of steel fiber. When the temperature drops from 20 °C to -10 °C and -20 °C, there is a 15.0 % and 12.7 % decline in the flexural fatigue strength of UHPC, respectively. In addition, low temperature accelerates the degradation of UHPC, resulting in a larger and faster accumulation of fatigue deformation, including tensile strain, mid-span deflection, and fatigue deformation modulus.

Tool wear influence on surface roughness, burrs and cracks in milling unidirectional carbon fiber reinforced plastics (UD-CFRP)

This study delves into the machining defects encountered during the milling of Carbon Fiber Reinforced Plastics (CFRP), with a special focus on fishbone-like cracks on the machined surface, which have never been focused before. Through a series of experiments, models for post-machining fiber burrs and matrix cracks were developed and analyzed, uncovering the causes of fishbone crack formation. The research reveals that the formation mechanisms of burrs vary with fiber orientation, burrs are not only from fiber bending under tool pressure but also from fiber buckling. Fishbone cracks are believed to be induced by growing burrs as the tool wear, and propagate along the in-plane and subsurface. Propagation downwards is along the fiber direction, while propagation inward is at an angle of approximately 40° ± 3°. This research is vital for enhancing the machining quality of composite materials and their engineering applications.

Experimental study and prediction on tensile behavior of steel fiber-reinforced-carbon/glass hybrid composite bars in concrete environment

The tensile behavior of steel fiber-reinforced-carbon/glass hybrid composite bars (S-CG HCBs) in concrete environment was experimentally investigated. The influences of the steel fiber content (VS) and the carbon-glass ratio (VC/VG) on the tensile behavior of corroded S-CG HCBs were analyzed. The optical microscope (OM) was used to reveal the morphology and corrosion mechanism of S-CG HCBs. The results show that under tensile load, S-CG HCBs exhibited a pseudo-yield platform due to the separate fracture of the three types of fibers and the buckling of steel fibers. The retentions of yield and ultimate strengths of the S-CG HCBs in alkaline solution increased with the VS or VC/VG increased due to the reduction of glass fiber content and its more inferior alkali resistance than carbon and steel fibers. After 63 days of corrosion at a 40 ℃, the specimen (S30C1G6), whose VS and VC/VG are 30% and 1/6, respectively, exhibited the highest tensile strength retention rate of 87.88% compared to other specimens. In addition, the tensile stress retention of S-CG HCBs serviced in concrete environment under various ambient temperatures was predicted based on the Arrhenius equation.

Hierarchical buckling of elastic fiber under transverse confinement

Hierarchical buckling is a novel phenomenon observed in elastic fibers subjected to transverse confinement; however, the deformation mechanisms and modal transitions of this unique phenomenon remain to be elucidated. This paper investigates the hierarchical buckling of elastic fibers with elliptical (circular) cross-sections under transverse confinement through analytical derivations and numerical simulations. Various magnitudes of hierarchical buckling of fibers are observed with the variation of the controlled elastic matrix stiffness. An analytical solution is first derived for the fiber’s buckling phenomenon, and the hierarchical buckling is accomplished through the superposition of buckling at various modes. The theoretical results are validated against the finite element simulations with good agreement. It is demonstrated from the parametric results that the hierarchical buckling phenomenon is primarily influenced by the stiffness of the external transverse confinement (matrix), which is defined as a dimensionless parameter. It is thus illustrated from the computational results that the buckling of elastic fibers within a solid or fluid matrix can be controlled and customized. The present work provides theoretical guidance for the application of elastic fibers in stretchable conductor fibers and flexible electronic devices.

Clots reveal anomalous elastic behavior of fiber networks.

The adaptive mechanical properties of soft and fibrous biological materials are relevant to their functionality. The emergence of the macroscopic response of these materials to external stress and intrinsic cell traction from local deformations of their structural components is not well understood. Here, we investigate the nonlinear elastic behavior of blood clots by combining microscopy, rheology, and an elastic network model that incorporates the stretching, bending, and buckling of constituent fibrin fibers. By inhibiting fibrin cross-linking in blood clots, we observe an anomalous softening regime in the macroscopic shear response as well as a reduction in platelet-induced clot contractility. Our model explains these observations from two independent macroscopic measurements in a unified manner, through a single mechanical parameter, the bending stiffness of individual fibers. Supported by experimental evidence, our mechanics-based model provides a framework for predicting and comprehending the nonlinear elastic behavior of blood clots and other active biopolymer networks in general.

Improved structural efficiency in composite manufacturing via hammered printing for large-curvature fiber paths

Maximizing the structural efficiency of composite materials is often limited by the minimum turning radius of fibers in composite manufacturing. Neglected defects, such as local fiber buckling, become more pronounced in regions with a large curvature, leading to reduced load-carrying capabilities and a deviation from the target design specifications. This study introduces “hammered printing” as a novel approach for achieving a large curvature of the fiber path in composite manufacturing. By utilizing the transition of the modulus with temperature of a thermoset epoxy pre-polymer, the proposed hammered printing technique generates a unique pressure effect that improves adhesion between the fiber tow and the substrate. This enables the production of accurate and uniform composite structures with a minimum curvature radius of 2 mm, while reducing the occurrence of wrinkling and bending deformations. A continuous fiber path with a small radius (2 mm) for open-hole plates was accurately printed using this printing pattern. Compared with conventional laminates and drilled composites, the obtained printed samples exhibit 75.3% and 28.8% increments in the tensile and bearing strengths, respectively, indicating that this continuous small-radius fiber path can effectively retain the strength and safety of fastened composite parts. Finally, comparisons indicate the great potential of the proposed technique as a large-curvature-path and high-performance manufacturing alternative for the cutout of composite assemblies in the aerospace and related engineering industries.

Management of Katigraha Through Ayurveda – A Case Report

The description of Katigraha is not available as a separate entity in the classics. It is described amongst the eighty by Nanatmaja Vatavyadhi in the Charak Samhita. Ayurveda provides a horizon for such patients with its miraculous treatments of Panchakarma, Shamana, Rasayana, Agnikarma, etc. Lumbar canal stenosis is an abnormal narrowing of the spinal canal in the lumbar region, resulting in a neurological deficit that produces symptoms like pain, numbness, paraesthesia, and loss of motor function. Abnormal weight distribution combined with soft tissue laxity and instability over a prolonged period allows for excessive joint play and buckling of the posterior annular fibres of the intervertebral disc (IVD). The main treatment protocol, according to Ayurveda, is the use of both purification and palliative treatment. This case report represents a 56-year-old male patient who came to the OPD complaining of pain in his lumbar, hip, and legs; tingling sensation; stiffness; and walking difficulty. At Kati Pradesh, the trauma caused by the fall led to the vitiation of Vata and kha- vaigunya. Additionally, Agnimandya was already there, which caused ama to develop. He was given both Shodhana and Shamana Chikitsa. There was a significant improvement in the symptoms, and the patient could do his routine work without any support. This case and its results show that this therapy is effective in lumbar canal stenosis and should be done in a large population. Patients with lumbar canal stenosis can be effectively treated with Panchkarma procedures and oral medications without surgical intervention.

Investigating the transient angle in a unidirectional composite in tension and compression

In unidirectional composite laminates, when there is a slight deviation between fiber and load-applying direction, the failure mode in tensile and compression will be changed from fiber breakage and buckling, respectively, and will be turned to matrix cracking. The transient angle is the angle of misalignment between fiber and load-applying direction, in which the strength properties of the composite will sharply and considerably decrease due to the change of failure mode. In this paper, the transient angle has been investigated experimentally in fiber-reinforced glass epoxy unidirectional composites. To determine the transient angle, certain unidirectional composite specimens with specific digression angles from the load-applying direction were manufactured due to standards, and tensile and compression tests were performed on them. Results demonstrate that the tensile transient angle is between 4° and 8°, and the compressive transient angle is between 8° and 12°. Moreover, experimental results were compared with analytical calculations using Hashin’s failure criteria. Investigations showed that Hashin failure criteria can adequately predict the amount of load causing failure in unidirectional composite laminates before the transient angle. Still, by approaching this angle, Hashin’s failure criteria can’t correctly predict the load causing failure, and error will increase gradually as it passes the transient angle. This error will occur after the transient angle because Hashin’s Failure Criteria can’t predict changes in failure modes in unidirectional composite laminates.

Characteristics of compressive failure behavior of polyacrylonitrile‐based carbon fiber multifilament

AbstractThe deformation characteristics exhibited by the hierarchical structure in composites and their influence on compressive properties are investigated by controlling the deformation of carbon fiber multifilament under axial compressive loading. Our findings reveal that the deformation curve of carbon fiber composite materials under axial compression loads is nonlinear. Specifically, for 12 K high‐strength and medium‐modulus carbon fiber multifilament, when the compression load exceeds approximately 600 N, the buckling of individual fibers leads to shear deformation that damages the interface between the fiber and resin. Consequently, the compressive deformation of the multifilament composites transitions from elastic to plastic deformation. By enhancing the support force exerted by the resin on the multifilament composite and reducing the load span within the range of 2–4 mm, it is possible to effectively minimize or eliminate the overall buckling that may occur during the compression process of the multifilament composite. This results in a higher secant modulus under the same level of deformation. Additionally, it reduces the step difference between the compression and tension regions in the cross‐section of carbon fiber formed by the buckling, while the compression strength of the carbon fiber multifilament increases from 2.32 to 4.78 GPa.Highlights Characteristics of compression deformation of carbon fiber multifilament. Regulation of the degree of buckling of carbon fiber multifilament. Cross‐section morphology under different compression failure mechanisms. Influence of the degree of buckling on compressive properties of carbon fiber.

Failure prediction in a non-crimp basalt fibre reinforced epoxy composite

Failure analysis in non-crimp fabric (NCF) based fibre reinforced composite materials is challenging due to the complexity associated with multiaxial fabric architectures and the interaction of different failure modes. In this study an in-situ 3-point bend test under scanning electron microscopy (SEM) has been performed to analyse the failure behaviour of a NCF basalt epoxy composite manufactured by vacuum assisted resin transfer moulding (VaRTM) technique. The failure mechanisms observed during the in-situ 3-point bend test are compared to those from standard (ex-situ) flexure and interlaminar shear strength (ILSS) tests on the same material. Finite element analysis of each test configuration is also conducted to analyse the in-ply stress distributions and to predict the damage initiating stresses. The study reveals that fibre/matrix debonding leading to matrix cracking in the 90° sub-ply is the damage initiating failure mode, with final failure occurring due to fibre buckling in neighbouring 0° sub-plies. The finite element analysis reveals that matrix cracking in the 90° sub-ply is controlled by shear stresses ranging between 35 and 45 MPa.

Shape distortion in composites: sources, characterization, and remedies

This paper reviews the shape distortions in a polymer composite, its sources, characterization techniques, and remedies. The shape distortion may include warpage, spring-in, or change in enclosed diameter for flat, angled, and circular parts. Residual stresses are considered to be the major source of shape distortion (dimensional instability) in laminated composites. These stresses are the result of the difference in thermal expansion behavior of the different plies or between matrix and reinforcement. Additionally, fiber buckling, transversal cracking, and delamination are also produced in the composite, affecting its mechanical properties like tensile, flexural, and compression. The produced residual stresses are determined through different techniques like layer removal of symmetrical laminates, first ply failure, and x-ray diffraction. The common remedies for shape distortion reported in the literature include nanoparticle addition and variable thickness at the base or flanges.

Thermal oxidation and compressive failure behavior of ZrSi2–B4C modified phenolic resin-based composite

Phenolic resin-based ablative materials have gained significant attention in the aerospace sector in recent years. In this study, TGA-DSC, FT-IR, XRD and XPS were employed to the thermal stability and phase transformation of the ZrSi2–B4C modified quartz fiber/boron containing phenolic resin (QF/BPR) composite. The compressive failure behavior of the composite at 600–1200 °C was studied through SEM and high-speed camera techniques. The results indicated that the compression strength of the composite decreased to a minimum at 600 °C and increased at 800–1200 °C. The composite exhibited different failure modes at different temperature: shear fracture at room temperature (RT), debonding of the fiber bundles at 600 °C, fiber buckling and fiber shear fracture at 800 °C, fiber buckling and delamination at 1000–1200 °C. The improvement in thermal stability of composite was primarily attributed to the formation of ZrO2, SiO2 and B2O3, which covered the fiber and effectively transmitted the load, leading to an enhanced strength of the interface between the fiber and resin.

X-ray microtomography analysis of the damage mechanisms in 3D circular braided carbon fiber/epoxy resin composite tubes under axial impact compression

The dynamic behavior of three-dimensional (3D) circular braided composite tubes with different braided parameters was experimentally investigated subjected to axial impact compression using a Split Hopkinson Pressure Bar (SHPB). The damage characterization of the post-impacted tubes was examined using scanning electron microscope (SEM) and X-ray micro-computed tomography (micro-CT). The influence of braided parameters and strain rate, including braided angle, the number of braided layer and axial yarn, on the dynamic mechanical properties, progressive crush characteristics and failure mechanism of 3D braided composite tubes was analyzed in detail. It was found that the smaller braided angle and the larger numbers of braided layers contributed to better axial dynamic properties. The compressive strength, stiffness and specific energy absorption (SEA) of 3D five direction (3D5D) tubes with axial yarn were larger than those of 3D four direction (3D4D) tubes without axial yarn. Meanwhile, the dynamic mechanical response and damage patterns of braided tubes exhibited significant strain rate effects. The impact compression failure mechanisms of braided tubes were mainly fiber tows crushing, fiber breakage, fiber buckling, shear fracture, resin cracking, interfacial cracks, fiber-matrix debonding, etc.

Temperature dependence analysis of mechanical properties and bending behaviors of shape memory programmable composites

Temperature significantly affects the mechanical properties of shape memory polymers (SMPs). The temperature dependence of mechanical properties and bending behaviors of shape memory polymer composites (SMPCs) was investigated in this study. Dynamic mechanical analysis and static mechanical tests were performed to investigate the temperature dependence on the mechanical properties of SMP and SMPC. The tensile strength and shear strength of SMP declined as the temperature rose, while the elongation at break increased first and subsequently reduced. SMPC exhibited brittle fracture at low temperatures and fiber buckling at high temperatures, respectively. Subsequently, a temperature-dependent model of the bending behavior of SMPC was developed. Theoretical model revealed the effect of temperature and fiber volume content on fiber buckling. The higher the temperature and fiber volume content is, the easier the fiber buckles. The effects of temperature, thickness and fiber volume content on critical buckling curvature, strain energy, half-wavelength and amplitude were investigated. Temperature and fiber volume content were positively related to critical buckling curvature, strain energy and amplitude, but negatively related to half-wavelength. Thickness was positively related to strain energy, half-wavelength and amplitude, but negatively correlated to critical buckling curvature, in which thickness does not determine whether SMPC buckling occurs. Eventually, the theoretical model was verified using the four-point bending test to ensure its credibility. The research results are expected to provide theoretical guidance for the design and application of SMPC.

Material database construction for data-driven computing via a continuous path-following method

Data-driven computational homogenization has been proposed recently for the analyses of composite structures. Its basic idea is to construct an equivalent stress–strain database of composites via offline homogenization on the representative volume element and conduct online macroscopic simulation through distance-minimizing data-driven computing. Thanks to the scale separation of concurrent multiscale systems, this framework allows for improving online computational efficiency. However, high-density database construction in the offline stage remains a burdensome and time-consuming task. To this end, this work proposed an efficient approach that associates computational homogenization with the Asymptotic Numerical Method (ANM) to construct a high-density database. Being a reliable and efficient perturbation technique, the ANM allows for accurate tracking of the displacement–load paths and easily generates abundant equivalent stress–strain data on the paths. A fiber reinforced composite material with fiber buckling has been considered to demonstrate the accuracy and efficiency of the proposed method for the database construction of composites.

A scientific view on composite inlays in running shoes: The influence of epoxy crosslink density on bending and fatigue properties

This study highlights the need for a deeper understanding of the structure-property relationships of carbon fiber reinforced thermosets applications, whenever considered as a functional part for running footwear. This is done, investigating the static and dynamic flexural properties of three different epoxy resins in prepreg-based carbon fiber composite laminates, varying the glass transition temperature (Tg). In this way, the inherent morphology of the resin was correlated with the fatigue properties of the laminate, which is crucial for the design of composite stiffening inlays for running shoes. First, it was shown that the crosslink density and aromatic content of the resin, which correlates with Tg, drastically affect the compressive strength of the matrix and are therefore crucial for the static and dynamic flexural properties of CFRP. Increasing the Tg from 147 °C to 269 °C leads to stiffening of the resin and thus increased resistance to fiber buckling, resulting in improved bending stiffness, flexural strength, bending angle, overall fatigue strength and thus durability of CFRP plates for running shoes. Moreover, the percentage of fibers in the loading direction can be considered as the main factor affecting the flexural properties of the laminate. Here, a quasi-isotropic stacking order leads to an increased bending angle but to a decrease in bending stiffness, flexural strength and fatigue strength compared to a unidirectionally reinforced composite due to a high load bearing capacity of the 0° fibers. This knowledge creates a basis to bridging the gap between composite material and biomechanical sports science for future work.