Brittle Fibers Research Articles

Fabrication and fracture behaviors of the continuous brittle fiber reinforced tungsten composites fabricated via field-assisted sintering technology

Fabrication and fracture behaviors of the continuous brittle fiber reinforced tungsten composites fabricated via field-assisted sintering technology

Heterogeneous components removal mechanism and grinding force model from energy aspect in ultrasonic grinding continuous fiber reinforced metal matrix composites

Continuous fiber reinforced metal matrix composites (CFMMCs) offer higher specific strength, specific modulus, and operating temperature than matrix metals due to the unique enforcement mechanism of the one-to-one scale arrangement of matrix and reinforced phases. Due to the heterogeneous characteristics between the plastic matrix and brittle fibers, the removal mechanism of CFMMCs during processing is exceptionally complex. Ultrasonic vibration-assisted grinding (UVAG) shows great advantages in machining difficult-to-cut materials (i.e., ceramics and composites) by changing the motion trajectory between grains and workpieces, effectively reducing grinding force and improving machining quality. However, little is known about the removal mechanism of UVAG for CFMMCs composed of the ductile (i.e., metal matrix) and brittle (i.e., SiC fiber) phases with highly anisotropic structure characteristics. This raises the question of how CFMMCs with heterogeneous components perform under abrasive processing and how to predict their processing forces. Hence, UVAG and conventional grinding (CG) experiments with single CBN grain were carried out on SiC fiber reinforced TC17 matrix composites (SiCf/TC17) in this work. A grinding force model considering both phases and materials structure from energy aspect was proposed. A theoretical model for suppressing SiC fiber damage has been proposed, which is expected to guide low-damage processing of brittle materials. According to the results, the removal models of CFMMCs are revealed including: i) macro fracture of SiC fiber, ii) neat fracture of SiC fiber, and iii) TC17 matrix massive adhesion on the SiC fiber. Besides, no cracks crossing fibers are observed on the subsurface of SiC fiber under both UVAG and CG due to the good support of the TC17 matrix on SiC fibers. The grinding force predicted model error decreases as ap increases. When ap is 50 μm, the errors between predicted and experimental values are 7.8 % and 9.1 % for normal forces (Fn) and tangential forces (Ft), respectively. Ultrasound suppresses the severe wear behavior of grains, thereby improving the tool life. This paper aims to comprehensively reveal the characteristics of abrasive processing of CFMMCs from various aspects (surface morphology, subsurface features, grinding force prediction, and tool wear), which will promote the industrial application of CFMMCs.

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.

Synergistic ductility deformation and helical design of carbon nanotube fiber composites

Twisting attracts more attention in adjusting the mechanical behavior and failure mechanism of materials, which could be implemented to understand and design the deformation behavior of helical structures. Herein, we developed a general helical model to theoretically investigate the mechanical response and geometric evolution of twisted carbon nanotube (CNT) fiber. The in-situ experimental results of CNT fibers in scanning electron microscopy were used to verify the correctness of the theory. For the structure with a large helical angle, the interfacial force and uncoordinated interlayer deformation during elongation are the key factors determining its unique tensile-torsional coupled motion. By helical design, the elongation and deformation of brittle fiber in composite structure can be delayed to achieve synergistic fracture with ductile fiber. The breaking order of these two fibers under stretching can be further regulated by changing the degree of twisting. Based on these theoretical results, the outer brittle fibers could also be used to predict the tensile necking and fracture locations of inner ductile materials. This approach can be achieved by changing the helical geometry and failure strain, and this work would throw light on the mechanical design of multi-layer cables and fiber composites.

The noteworthy tensile plasticity and strength of CoFeSiB metallic glass fiber reinforced epoxy composites

AbstractMetallic glass fibers have high strength, poor plasticity and low Young's modulus. In this study, it was used as a reinforcement to prepare metallic glass fiber reinforced epoxy composites with different fiber volume contents (20%, 30%, 40%, and 50%). The effect of fiber volume contents on the tensile properties of the metallic glass fiber reinforced epoxy composites was studied by experiment and the finite element analysis method. The experiment and simulation results are in good agreement. The tensile strength and plasticity of the metallic glass fiber reinforced epoxy composites increase with the increase of fiber volume contents. When the fiber volume content is 40% or 50%, a sharply necking feature appeared on the fibers and exhibited improved plasticity, due to the formation of interacting and arresting events of shear bands occurring at the interface between the metallic glass fibers and the epoxy. In addition, the strain hardening due to plastic deformation of the metallic glass fibers further enhanced the tensile strength of the metallic glass fiber reinforced epoxy composites. This is a first step toward toughening the inherently brittle metallic glass fibers under tensile conditions.Highlights A novel CoFeSiB metallic glass fiber reinforced epoxy resin composite The increase of fiber volume fraction improves the tensile strength and plasticity of fiber reinforced composites. The strain hardening due to plastic deformation of the metallic glass fibers further enhanced the tensile strength of the metallic glass fiber reinforced epoxy composites.

Distributed strain sensing using Bi-metallic coated fiber optic sensors embedded in stainless steel

Silica fiber optic sensors are resistant to corrosive environments and high temperatures, making them attractive candidates for harsh conditions found in nuclear and aerospace industries. Moreover, fibers can be deployed remotely for continuous measuring of spatially distributed temperatures and strains. This study investigated embedding a Ni/Cu bi-metallic coated fiber in a stainless-steel 316 (SS316) matrix using laser powder bed fusion towards functionalizing metal components for site-specific health monitoring. The embedded fiber was continuously interrogated during controlled heating to 1000°C. The measured fiber strains were similar to the expected differential thermal strains between the fiber and the SS316 matrix, until divergent behavior was observed at temperatures >500°C. No debonding at the matrix–coating–fiber interfaces was observed during microscopy, but significant interactions between the coatings and matrix resulted in diffusion-driven chemistry variations and Kirkendall void formation. Applying the strain-lag theory revealed plastic behavior in the Ni coating at temperatures >500°C, limiting the strain transfer to the fiber at higher temperatures. It was estimated that the elastic modulus in the Ni coating had decreased from ∼200 GPa at room temperature to below 40 GPa, starting at 600°C. The low elastic modulus above 600°C is within the margin of what the tangent modulus would be in the case of bilinear isotropic hardening. Regardless of the divergent strain transfer at higher temperatures, the fiber was exposed to the equivalent of 1.9 % engineering strain at 1000°C, but measured only a 0.7 % engineering strain due to the poor strain transfer. Although compensating for the plastic behavior of Ni proved challenging, the bonding of a brittle silica fiber to a metal matrix surviving to 1000°C invites potential iterations on coating material for future application. For example, the embedded fiber is sufficient for acoustic energy transfer, realizing high temperature distributed acoustic sensing.

Chemo-mechanical ageing of paper: effect of acidity, moisture and micro-structural features

A multi-scale modeling framework is proposed for the prediction of the chemo-mechanical degradation of paper, with the particular aim of uncovering the key factors affecting the degradation process. Paper is represented as a two-dimensional, periodic repetition of a fibrous network unit cell, where the fibers are characterized by a moisture-dependent chemo-hygro-mechanical constitutive behavior. The degradation of paper occurs primarily as a result of the hydrolysis of cellulose, which causes a reduction of the degree of polymerization and a consequent decrease of the effective mechanical properties, ultimately leading to fiber embrittlement and a loss of material integrity. The interplay between the acidity of the paper, the ambient environmental conditions, and its chemo-mechanical degradation behaviour is a complex process. In the model, these interactions are accounted for by determining the coupled temporal evolution of the degree of polymerization, the acidity of the paper, and the moisture content, from which the time-dependent tensile strength of the paper is calculated. The internal stresses developing in the fibrous network under a change in moisture content lead to brittle fiber fracture once they reach the fiber tensile strength. The successive breakage of individual fibers results in damage development in the fibrous network, altering its effective constitutive properties. The temporal evolution of the effective hygro-mechanical properties of the fibrous network is calculated by employing asymptotic homogenization. For obtaining accurate model input, the strength and stiffness properties of individual fibers and the degree of polymerization of paper samples are measured at different ageing times by carrying out dedicated experiments. Subsequently, a series of numerical simulations is performed to analyze the chemo-mechanical degradation process of paper, highlighting the influence of the time-evolving acidity and moisture content. The numerical study further considers the effects of micro-structural features (i.e., the anisotropy of the fibrous network orientation and the fiber longitudinal elastic modulus) on the macroscopic degradation response of paper. The results of this work may help conservators of cultural heritage institutions determining optimal environmental conditions to limit or delay the time-dependent degradation of valuable historical paper artefacts.

Microstructural Evolution and Failure in Fibrous Network Materials: Failure Mode Transition from the Competition between Bond and Fiber.

For the complex structure of fibrous network materials, it is a challenge to analyze the network strength and deformation mechanism. Here, we identify a failure mode transition within the network material comprising brittle fibers and bonds, which is related to the strength ratio of the bond to the fiber. A failure criterion for this type of fibrous network is proposed to quantitatively characterize this transition between bond damage and fiber damage. Additionally, tensile experiments on carbon and ceramic fibrous network materials were conducted, and the experimental results show that the failure modes of these network materials satisfy the theoretical prediction. The relationship between the failure mode, the relative density of network and strength of the components is established based on finite element analysis of the 3D network model. The failure mode transforms from bond damage to fiber damage as increasing of bond strength. According to the transition of the failure modes in the brittle fibrous network, it is possible to tailor the mechanical properties of fibrous network material by balancing the competition between bond and fiber properties, which is significant for optimizing material design and engineering applications.

Revealing low temperature-mechanical coupling failure mechanisms in CFRP laminates with in-situ observations

The number of layers and ply orientations determined the service performance of carbon fiber-reinforced polymer (CFRP) materials. Under low-temperature service conditions, the unclear weakening mechanism of mechanical properties of CFRP aggravates the failure risks of aerospace structures. In present work, the unclear correlation between layers with alternating orientations and the tensile-failure mechanisms of CFRP, the low temperature-mechanical coupling micromechanical behavior, and the failure mechanism of CFRP was experimentally revealed by using a horizontal testing instrument integrated with a low-temperature loading module and real-time optical imaging function. Based on various temperatures in a range from −40 °C to room temperature (RT) and two strain rates, the gradually decreasing trends of tensile strength, Young's modulus, and elongation after fracture with decreasing temperature are obtained. Three failure modes including fiber brittle fracture, fiber pull-out, and interface debonding are proposed to reveal the low temperature-mechanical coupling failure mechanism. Low-temperature asynchronous shrinkage of carbon fiber and epoxy resin promoted the transition from fiber brittle fracture to delamination at the interface. The proposed instrument could establish the approximatively actual service conditions involving low temperature and mechanical load.

Effect of fiber layout on low-velocity impact response of intralaminar hybrid carbon/glass fiber braided composite pipes under internal pressure

This study investigated the low-velocity impact behaviors of intralaminar hybrid carbon/glass braided composite pipes under internal pressure. The effects of intralaminar fiber layout and internal pressure on the mechanical responses, damage extension, and material cost-effectiveness were analyzed. The internal damage distribution of composite pipes with different hybrid fiber alternating numbers but identical mix ratios was evaluated through Micro-CT three-dimensional damage reconstructions. The yarn splitting of brittle carbon fiber materials was mutually prevented by the induction of intra-yarn debonding and delamination damages by hybridization with ductile glass fiber materials, which were prone to produce out-of-plane displacement and shear stresses under impact loading. A smaller hybrid fiber alternating unit was conducive to exploiting the hybrid impact toughening effect by avoiding pure reinforced fiber materials aggregated under the impactor. Internal pressure had a supporting effect on the composite pipe and was effectively synergistic with the hybridization effect. The change in the dominant damage mode resulted in significant reductions in impact deformation and damage severity compared to the unpressurized case.

Assessing pseudo-ductile behavior of woven thermoplastic composites under tension and bending

Most fiber-reinforced composites are inherently brittle and fail suddenly at low strains without yielding and energy-absorbing capability. Still, under some conditions, they can demonstrate ductile like response known as pseudo-ductility. To investigate such a response, experimental analysis of carbon- and glass-fabric reinforced thermoplastic polymer (C/GFRP) composites was performed in on- and off-axis orientations under service loading conditions of tension and bending. Tensile tests of off-axis specimens were conducted with a full-field strain-measurement digital image correlation (DIC) technique. Cyclic bending tests of on- and off-axis C/GFRP specimens were performed to assess their ductility and damage behavior. The tests revealed that on-axis CFRP laminates failed due to fracture of brittle carbon fibers under tension, monotonic and cyclic bending. The on-axis GFRP samples demonstrated a linear-elastic brittle response under tension but a visco-elasto-plastic nonlinear behavior under monotonic and cyclic bending with hysteresis and energy absorption. This nonlinearity could be due to stress stiffening of thin laminates under large-deflection bending and mesoscopic effects in the woven fabrics. The off-axis C/GFRP specimens exhibited ductile behavior akin to metals, enduring high strains with permanent deformation before ultimate failure, and absorbing substantial amounts of energy. The pseudo-ductile response of the off-axis specimens is attributed to plasticity due to matrix cracking, fiber-matrix debonding as well as fiber trellising, whereas in the on-axis GFRP specimens, it is primarily due to visco-elasto-plastic behavior of glass fibers and the TPU matrix. It is concluded that material's response can be tailored for stiffness, strength and ductility for specific applications.

Machining SiC fibre reinforced metal matrix composites – How do different matrix materials affect the cutting performance?

SiC fibre reinforced metal matrix composites find widespread application but show major machining difficulties due to significant variations in constituents' properties. In this sense, while the SiC fibre plays significant strengthening effects, the properties mismatch between the brittle fibre and ductile matrix materials becomes important in their machining performance. Through machining tests, SiC fibre reinforced MMCs with Al (soft) and Ti (hard) matrix alloys are evaluated, showing the variation of interaction between inserts and fibres in cutting due to the different matrix properties. This leads to less tool wear but compromised surface integrity in Al-based composite than in Ti-based one.

Splitting Mode-I fracture toughness of carbon fibers using nanoindentation

In this study, we conducted a thorough experimental and computational investigation to extend the pillar-splitting technique, developed initially for ceramics, to brittle fibers in reinforced composites. In pillar-splitting methodology, micropillars with circular cross-sections are tested to evaluate the load required to cause instability. The instability load is then correlated with the fracture toughness using the parameter γ which is evaluated using the cohesive zone finite element method (CZ-FEM). In reinforced composites, a unique opportunity arises due to the existence of already present micropillars (reinforcing fibers) for fracture toughness evaluation. In this work, the authors successfully applied the pillar-splitting technique to carbon fibers by investigating the effects of fiber/matrix assembly and the shape of the fiber upon the error in instability load. The minimum height required to avoid matrix effects is found to be equal to the diameter of the fiber reinforcements for a fiber–matrix system with a perfect interface. By contrast, the required protruding height is reduced to zero for a weak fiber–matrix interface. A new parameter, β, is introduced to quantify the deviation of the cross-sectional shape from circularity. An effective γ is then evaluated and used in the pillar-splitting experiments to correlate the instability load to Mode I fracture toughness. Finally, the results are validated experimentally, and the Mode I fracture toughness is evaluated for a range of Toray PAN carbon fibers. Fracture toughness is found to decrease as the modulus of the carbon fibers increases.

Overcoming the brittleness of shear failure: A new FRP-RSC strengthening philosophy

Due to the linear elastic stress–strain behavior of carbon fiber reinforced polymer (CFRP), the ductility of CFRP shear-strengthened reinforced concrete (RC) beams will be significantly reduced. Compared with shear capacity enhancement, increasing the ductility of CFRP shear-strengthened RC members is a much more challenging task, and a satisfactory solution has yet to be found. This paper introduces a new methodology that combines the brittle fiber reinforced polymer (FRP) with ductile rubber support composite (RSC) to form the FRP-RSC strengthening system. The variable stiffness RSC can adjust the stiffness of the strengthening system, which is achieved by the changed shear transfer actions in FRP, transverse reinforcement and concrete. Eight specimens were tested to evaluate the effectiveness of the FRP-RSC strengthening method. Experimental tests demonstrated that the displacement ductility with this FRP-RSC technique was 2.15 times that of the conventional externally bonded (EB) FRP strengthening, reaching remarkably 5.24. The mechanisms for enhancements in ductility, deformability and shear capacity were revealed. Poor matches were observed between the predicted FRP shear strength contribution (Vf) per existing models and test data. A new model that can predict Vf of FRP-RSC strengthened RC beams was developed, and better agreements between predictions and experimental results were obtained.

TITANIUM BASED FIBRELESS COMPOSITES

Novel composites made of titanium and Ti-Ti5Si3 eutectic were obtained and their mechanical properties were studied. It was shown that the eutectic inclusions play a reinforcing role like brittle fibres do it with respect to metal matrix. It was also shown that the composite microstructure produced at some technological regimes loses its pronounced original layered microstructure and forms a new composite type. The mechanical properties of such type of the composite look attractive. The bending strength of composites based on pure titanium reaches 1300 MPa, this characteristic can be about 1500 MPa in case of using low strength OT4-0 alloy (Ti-(0.4-1.4)Al - (05- 1.3)Mn) as the matrix. The composites reveal sufficiently high plasticity.

A numerical model for calculating the impact-induced depression

Materials usually have a quasi–brittle behavior under impact loading, even the ductile materials when loading rate is high enough. In this paper, a new numerical model for calculating the transverse impact on a quasi–brittle target is developed and the theoretical deductions with a strict mathematic logicality are illustrated. Herein, the example of a stiff ball impacting a brittle fiber bundle is investigated and the issues of stress wave propagation, inertia force, energy transformation and stress equilibrium are considered. The numerical model that can describe the impacting procedure is derived. With timing, the deformation of brittle fiber bundle, the kinetic energy of stiff ball as well as their displacements are given. Thus, the overall perspective of the impacting procedure is calculated and analyzed. This work offers a physical illustration of impact events by developing a numerical model and displays the insights into the transient procedure, that will contribute to the development of impact mechanics for engineering.

Role of crack offset and loading condition on fracture behavior of wire specimen

Accurate stress intensity factor (SIF) solutions for cylindrical specimens with varied wire aspect ratios, crack aspect ratio, relative crack depth and crack offset are required to evaluate the fracture toughness of high strength yet brittle rods, cables, fibres and wire specimens. The mode I geometric factor (GF) solutions of various crack configurations in a cylindrical fracture specimen in tension have been determined using linear elastic fracture mechanics in earlier studies. In the present study, finite element analysis (FEA) was used to compute these as a function crack offset and wire aspect ratios for two different boundary conditions-constrained and unconstrained. In constrained loading condition, the GF was seen to decrease significantly with crack offset for wires at lower aspect ratio in all the 3 crack curvatures – concave, straight and convex. GF stabilized as the wire aspect ratio was increased beyond 40. In unconstrained loading the GF values were independent of the wire aspect ratio and crack offset for the 3 crack curvatures.

A comparative study of polylactic acid (PLA)-Based unidirectional green hybrid composites reinforced with natural fibers such as kenaf, bamboo and coir

Plant-based composites are considered as new generation materials that offer sustainability, eco-friendly and green chemistry, utilized by numerous sectors. Composites of green hybrid made from polylactic acid (PLA) and three different plant fibers were prepared using two symmetrical fibers stacking sequences; namely high modulus and brittle (HMB) fibers (kenaf-bamboo-coir/PLA) and low modulus and ductile (LMD) fibers (coir-bamboo-kenaf/PLA) in the outmost layers. The stacking sequence was prepared using a simple slot tool to properly align the fibers and it was dried at 50 °C before being compressed using hot press. It was found that the tensile strength and tensile modulus of both hybrid composites increased linearly up to 158 MPa and 7 GPa respectively. It shows that the stacking sequences had no significant effect on tensile strength, but fracture strain increased by 58% produced by LMD fibres. However, the use of HMB fibres in the outmost layers significantly improved flexural strength up to 49% higher than that of LMD fibres. In contrast, the impact strength of the green hybrid composites using LMD fibres in the outmost layers was 21% higher than that of its counterpart. Like other plant fiber composites, both composites had an increased water absorption capacity of up to 39% due to increased fibre content. These stacking sequence properties are essential to fabricate a specific application to ensure properties suitable for the job's features. High modulus and brittle composites require a high load to deform permanently, as well as low modulus and ductile composites. Both exhibit versatile mechanical characteristic, providing a balance of sealant toughness and the ability to retain overall shape, such as in sound absorption and vibration in transportation.

Fracture spacings of fiber inclusions in a ductile geological matrix and development of microboudins: 3D numerical modeling

Tensile fractures and resultant microboudinage structures of brittle fiber inclusions (e.g., tourmaline, piedmontite and amphibole) in the soft matrix of deforming minerals are of great significance for determining the finite strains and paleostresses of naturally deformed rocks. Using statistical strength theory, damage mechanics, and continuum mechanics, we have reproduced in a series of numerical models the sequential fractures of either homogeneous or heterogeneous fiber inclusions under axial tension in an elastoplastic matrix. The results clarify that: (1) The spacing between fractures in fibers is inversely proportional to the applied strain. As the applied strain increases, the fracture spacing systematically decreases as sequential fractures fill in until fracture saturation is reached. (2) As fiber length increases, the critical tensile strain for fracture saturation rises. For the same fiber diameter, saturation fracture spacing increases slightly with rising fiber length. For the same fiber length, however, saturation fracture spacing decreases significantly with lessening fiber diameter. Hence, fracture spacing at the saturation state depends on the volume fraction of fiber. (3) The rupture mode of fibers strongly depends on the non-uniform distribution of mechanical properties, which provides an effective approach for estimating the inhomogeneity of fibers by analyzing the formation of fractures. Furthermore, due to material heterogeneity, new fractures are unlikely to occur in the middle of existing adjacent fractures.

Bioinspired design optimization for pseudo-ductility in platelet fibre laminates

Platelet designs inspired by nacre shell microstructure are a novel approach to confer pseudo-ductile deformation properties to brittle fibre reinforced polymer laminates while retaining high stiffness and strength. Here we present an experimental and finite element modelling study to investigate the effects of tiling pattern, overlapping length and interlaminar toughness of the ply platelets on the tensile properties, pseudo-ductility, and failure modes of platelet laminates inspired by nacre. The findings are then used to optimize the design for maximal pseudo-ductility for a given tensile strength. Experimental results show that unidirectional platelet laminates can possess tensile strengths in excess of 1 GPa while pseudo-ductile deformation accounts for more than ∼ 50% of the ultimate failure strain. Finite element modelling of platelet laminates reveals that the amount of pseudo-ductility can be tailored via the design optimization of the tiling pattern, overlap length, composite thickness and mode II interlaminar toughness of the platelets. The mechanisms controlling the pseudo-ductility effect in the bioinspired platelet laminates is described.