Fiber Axial Stress Research Articles

Study of the effect of interfacial damage and friction on stress transfer in short fiber-reinforced composites

The purpose of this study is to investigate the influence of different stages of different interfaces evolution under external forces on stress transfer within composite materials, which is crucial for analyzing reinforcement mechanisms in composite materials. Analytical solutions are derived to explore the impact of these distinct phases, both at the interfaces along the fiber length direction and at the fiber ends, on the complex stress distribution profiles within composite materials. Furthermore, the frictional effect at the interface serves to impede the debonding process in the composite. Under the same load, the debonding length of the interface decreases as the frictional effect increases. The increase in fiber aspect ratio (AR) effectively reduces the length of the damage and debonding interface and increases the axial fiber stress. Additionally, the theoretical results agree well with numerical simulation and experimental results. In essence, this model provides analytical solutions that are instrumental for analyzing stress transfer in fiber-reinforced composites during different stages of interface evolution.

Tensile strength prediction of unidirectional polyacrylonitrile (PAN)-based carbon fiber reinforced plastic composites considering stress distribution around fiber break points

Recent advancements in enhancing the mechanical characteristics of carbon fibers open up new application possibilities for carbon fibre-reinforced plastic (CFRP) composites. Particularly in unidirectional CFRPs, which form the basal structure of CFRP laminates, developing a micromechanics model capable of predicting the tensile strength of unidirectional CFRPs based on carbon fiber mechanical characteristics is a current aspiration. This study conducted a stress distribution analysis around the fiber fracture point to predict the tensile strengths of unidirectional CFRPs prepared with five types of polyacrylonitrile (PAN)-based carbon fibers, each with unique mechanical characteristics. Numerical simulation results obtained using a unidirectional CFRP model that considered the stress concentration, fiber axial stress, and bimodal Weibull distribution were reasonably consistent with the experimental results for the tensile strengths of unidirectional CFRP composites, regardless of the differences in the mechanical characteristics of the fiber. Our findings can provide guidance for designing further enhanced high-performance CFRP materials.

Temperature-dependent strength modeling of fiber-reinforced composites considering critical properties evolution

As the aerospace field rapidly develops, the application of fiber reinforced polymer (FRP) composites in extreme environments, particularly at elevated temperatures, has become increasingly widespread. The theoretical study on the temperature-dependent tensile strength (TDTS) of FRP composites has always been a significant research issue, which closely concerns the service safety of FRP composites in different temperature environment. Herein, the distribution expressions of temperature-dependent fiber axial stress as a function of axial position was first derived in the bonding and debonding zone of FRP composites. Then, based on the Force-Heat Equivalence Energy Density (FHEED) principle and introducing the effect of a wide range of fiber content, we proposed a novel TDTS model of FRP composites with physically-based theoretical method. The presented model can quantify the combined influences of temperature, fiber/matrix properties, residual thermal stress, and especially the critical properties evolution including the interfacial properties and fiber agglomeration. The TDTS of FRP composites with different interfacial properties and fiber contents can be readily predicted by the model, and attain a reasonable agreement with the available experiments. Additionally, the critical parameters affecting the mechanical properties and the evolution with temperature were analyzed, which can effectively provide the beneficial insights for composites strengthening. The work provides a novel tensile strength theoretical model of FRP composites at different temperatures and fiber contents, and further deepens the understanding of the mechanical properties degradation and composites improvement.

Effect of fiber roughness on interfacial friction of fiber reinforced ceramic matrix composites

Interfacial friction plays a crucial role on mechanical behaviors of fiber reinforced ceramic matrix composites (FRCMCs), such as matrix cracking, damage evolution, and fracture, and the fiber surface roughness has been shown to enhance the interface friction. However, the influence of interphase thickness and residual stress on the friction enhancement of fiber roughness was neglected in previous works. Therefore, in this paper a thoroughly theoretical model is established and the additional radial stress is derived to intuitively reflect the contribution of fiber roughness to the interfacial friction. The results indicate that the interfacial friction can usually be enhanced by increasing the fiber roughness and reducing the interphase thickness, which however is severely overestimated by more than 5 times by the previous models. Moreover, the residual stress and fiber axial stress have a nonnegligible effect on the contribution of fiber roughness. For a given fiber axial stress, the residual stress that is too large or too small will weaken or even eliminate the contribution of fiber roughness, and only when the residual stress is within a proper range, can the fiber roughness play an effective role. On the other hand, for a given target load and ambient temperature, a critical fiber roughness that can enhance the interfacial friction is analytically derived. This work can provide valuable insights into the design of fiber roughness and interphase thickness for FRCMCs.

Finite element modelling of the single fibre composite fragmentation test with comparison to experiments

This paper develops a finite element (FE) model of the single fibre fragmentation test designed for direct comparison with experimental results on an E-glass/epoxy system by McCarthy et al. (2015). Interface behaviour is modelled via a cohesive surface, and stochastic Weibull fibre strengths (determined by independent experiments) assigned at random to the elements along the fibre. Predictions from the model agree with experiment for a range of outputs: The evolution of the number of fibre breaks with strain is similar and breaks occur at random locations as required. The model also captures a transition to a Uniform (rather than Weibull) statistical distribution of break locations at later stages of the test consistent with recent experiments. The evolution of the cumulative distribution of fragment lengths is also similar to that of the experiment. In addition, fibre axial stress and interfacial shear stress distributions conform with experimental observation. Correct model predictions of break locations confirm the approach taken on assigning stochastic (Weibull) strengths along the fibre. The effectiveness of the FE model in capturing a number of key aspects of the fragmentation phenomenon suggest its usefulness as a tool in analysing and interpreting fibre fragmentation tests, including back-calculation of interfacial shear strength.

Experimental investigation on fatigue property at room temperature of C/SiC composites machined by rotary ultrasonic milling

Rotary ultrasonic milling technology (RUM), as a surface strengthening machining method, was proposed to utilize in processing of C/SiC composites for enhancing anti-fatigue performance innovatively. Static tensile, intermittent fatigue and residual strength test were carried out. Due to constant impingement of high-frequency and low-amplitude vibration, surface residual compressive stress was formed near 2 GPa maximally. Axial thermal residual stress in fiber achieved -662.4 MPa proved by loading-unloading test. The peak value of fatigue damage parameter was reduced significantly. RUM surface restrains most of interface cracks because of residual compressive stress, and hinder the growth of fiber cracks for better machined surface quality. The damage accumulation, the first stress redistribution and fiber reinforcement stage were delayed. Average damage rate was decreased by 80.5 %. Residual tensile strength of RUM C/SiC was improved after fatigue, up to 95.8 % of tensile strength. The strengthening effect from RUM on fatigue property of C/SiC is significant and valuable.

High-temperature axial stress evolution mechanism of polyacrylonitrile-based carbon fiber

Temperature and stretching are important factors in the high-temperature treatment of carbon fiber. The axial stress during carbon-fiber high-temperature treatment affects its ability to stretch. The high-temperature axial stress evolution mechanism of polyacrylonitrile-based carbon fiber was studied through in situ tension tests, Raman spectroscopy, X-ray diffractometry, elemental analysis, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, thermal expansion coefficient tests, and density methods. The high-temperature axial stress evolution of polyacrylonitrile-based carbon fiber involved three stages: rapid increase, rapid decrease, and relaxation. The highest stress and relaxation temperatures of the polyacrylonitrile-based carbon fiber were 1600°C and 1950°C, respectively. The main factors that affected the fiber axial stress included carbon-structure rearrangement and the effect of thermal expansion and cold shrinkage on fiber length. During the first stage ( T < 1600°C), carbon-structure rearrangement after nitrogen atom removal increased the fiber axial stress. In the second stage (1600 ⩽ T ⩽ 1950°C), the difference in the thermal expansion of fibers that entered the graphite furnace and the cold shrinkage of fibers that exited the graphite furnace increased gradually, which resulted in a decrease in fiber axial stress by up to 1950°C, where the fiber relaxed and the third stage ( T > 1950°C) began. The difference between expansion and shrinkage increased significantly, which increased fiber relaxation. Carbon fibers with fewer nitrogen atoms and more regular structures had a lower axial stress during high-temperature treatment, but the trend and characteristic temperature remained unchanged. The corresponding fiber high-temperature maximum stretching ratio and axial stress showed opposite trends below 1950°C. The ability to stretch the carbon fiber increased above 1950°C, which differed from the axial stress relaxation.

Modeling the effects of interfacial properties on the temperature dependent tensile strength of fiber reinforced polymer composites

In this study, the stress distribution of fiber axial stress and interfacial shear stress in fiber reinforced polymer (FRP) composites at different temperatures was derived as a function of fiber axial position. The maximum shear stress criterion was extended to be temperature dependent. Based on the extended criterion, the temperature dependent fiber debonding length of FRP composites was predicted, which is indirectly verified by other scholar's experimental study. Then, by introducing the maximum fiber axial stress transferred from the matrix to fiber by their interface including the effect of partial interfacial debonding, we modified the law of mixtures, and finally established the temperature dependent tensile strength model for FRP composites. The model considers the combined effects of temperature, constituents' properties, interfacial properties and residual thermal stress. Reasonable agreement is obtained between the model predictions and available temperature dependent tensile strength of FRP composites. Additionally, the influencing factors analysis for FRP composites is performed in detail. This work provides some important insights on the strength control mechanisms of FRP composites at different temperatures, which are beneficial to the material evaluation, strengthening, and optimization.

Synergistic effects of fiber/matrix interface wear and fibers fracture on matrix multiple cracking in fiber-reinforced ceramic-matrix composites

ABSTRACTIn this paper, the synergistic effects of fiber/matrix interface wear and fibers fracture on matrix multiple cracking in fiber-reinforced ceramic-matrix composites (CMCs) are investigated using the critical matrix strain energy (CMSE) criterion. The shear-lag model combined with the fiber/matrix interface wear model, fibers fracture model and the fiber/matrix interface fracture mechanics debonding criterion is adopted to analyze the fiber and matrix axial stress distribution inside of the damaged composite. The relationships between the matrix multiple cracking, fatigue peak stress, applied cyclic number, fiber/matrix interface wear and debonding and fibers failure are established. The effects of fiber volume fraction, fiber/matrix interface shear stress, fiber/matrix interface debonded energy, cycle number, fatigue peak stress and fibers strength on matrix multiple cracking evolution are discussed. Comparisons of matrix multiple cracking with/without cyclic fatigue loading are analyzed. The experimental multiple matrix cracking of unidirectional SiC/CAS, SiC/CAS-II, SiC/Borosilicate and mini-SiC/SiC composites with/without cyclic fatigue loading are predicted.

Predicting tensile behaviors of short flax fiber-reinforced polymer–matrix composites using a modified shear-lag model

Natural fiber-reinforced composites are increasingly being used in the industry. The fiber–matrix interfacial properties of the composites are influenced by many factors, including chemical treatment of the natural fiber, type of polymer matrix, composites fabrication method, and process and the service environment of the composites. In this paper, a modified shear-lag model based on a cohesive fiber/matrix interface is proposed and applied to the analysis of the stress–transfer characteristics and the tensile properties of unidirectional short flax fiber-reinforced composites. The model takes into account of the interfacial shear stiffness, bonding strength between fiber end face and matrix, fiber aspect ratio and fiber volume fraction. 3D finite element models of the composites using a cohesive zone method are used to verify the accuracy of the modified shear-lag model. The fiber tensile strength and the composite tensile elastic modulus are significantly influenced by the interfacial shear stiffness, fiber aspect ratio, and fiber volume fraction. The bonding strength between the fiber end face and the matrix only has an effect when the interfacial shear stiffness is low. The predicted results from the modified shear-lag model show good agreement with the finite element analysis and experimental results in the literature. The modified cohesive shear-lag model provides a simple and effective method for analyzing fiber axial stress, shear stress in the fiber/matrix interface, and tensile elastic modulus of the final composite.

A Large-scale Finite Element Model on Micromechanical Damage and Failure of Carbon Fiber/Epoxy Composites Including Thermal Residual Stress

The purpose of this paper is to study micromechanical progressive failure properties of carbon fiber/epoxy composites with thermal residual stress by finite element analysis (FEA). Composite microstructures with hexagonal fiber distribution are used for the representative volume element (RVE), where an initial fiber breakage is assumed. Fiber breakage with random fiber strength is predicted using Monte Carlo simulation, progressive matrix damage is predicted by proposing a continuum damage mechanics model and interface failure is simulated using Xu and Needleman’s cohesive model. Temperature dependent thermal expansion coefficients for epoxy matrix are used. FEA by developing numerical codes using ANSYS finite element software is divided into two steps: 1. Thermal residual stresses due to mismatch between fiber and matrix are calculated; 2. Longitudinal tensile load is further exerted on the RVE to perform progressive failure analysis of carbon fiber/epoxy composites. Numerical convergence is solved by introducing the viscous damping effect properly. The extended Mori-Tanaka method that considers interface debonding is used to get homogenized mechanical responses of composites. Three main results by FEA are obtained: 1. the real-time matrix cracking, fiber breakage and interface debonding with increasing tensile strain is simulated. 2. the stress concentration coefficients on neighbouring fibers near the initial broken fiber and the axial fiber stress distribution along the broken fiber are predicted, compared with the results using the global and local load-sharing models based on the shear-lag theory. 3. the tensile strength of composite by FEA is compared with those by the shear-lag theory and experiments. Finally, the tensile stress-strain curve of composites by FEA is applied to the progressive failure analysis of composite pressure vessel.

Evaluation of the interfacial strength of carbon-fiber-reinforced temperature-resistant polymer composites by the micro-droplet test

This paper presents an evaluation method for the fiber/matrix interfacial strength. The interfacial strength is determined by comparing experimental data with numerical simulations. The micro-droplet test is conducted, and the fiber axial stress at the point of interface debonding is obtained. A numerical simulation is performed with ABAQUS using an axisymmetric finite-element model. In the numerical simulation, an accurate value of the thermal residual stress based on the thermo-viscoelasticity and the damage to the resin around the blade-contacting point is considered to simulate the experimental phenomena ideally. In the thermal residual stress analysis, the actual thermal residual stress is calculated by considering the relaxation modulus and the time–temperature superposition principle for the resin. Damage initiation criteria for both dilatational and shear cases, based on continuum damage mechanics, are considered for the resin. Interfacial debonding is simulated using a cohesive zone model, and the interfacial strength is taken as the strength of the cohesive zone element at the simulated fiber maximum stress corresponding to the experimental value.

Semi-analytically presenting the creep strain rate and quasi shear-lag model as well as finite element method prediction of creep debonding in short fiber composites

In this research work, semi-analytical method (SAM) is presented to predict composite creep strain rate and quasi shear-lag (QSL) formulation directly, as well as, finite element method (FEM) is employed for predicting partial creep debonding at the interface in steady state creep of short fiber composites under tensile axial stress. Also, new formulation QSL is introduced to obtain the average axial stress in fiber which its results are similar to the results of shear lag (SL) model. Then, it is shown that FEM can approximately predict the partial debonding in some regions of the interface. As a result, interfacial debonding can be caused by high tensile axial and circumferential stresses, high shear and equivalent stresses, and low compressive radial stresses with considering stress concentration. The results obtained from SAM are in good agreement with the available experimental results. Finally, it is concluded that FEM simulation can be useful for predicting some defects such as interfacial debonding and also better designing the fibrous composites.

Stress Transfer of a Kevlar 49 Fiber Pullout Test Studied by Micro-Raman Spectroscopy

The interfacial stress transfer behavior of a Kevlar 49 aramid fiber-epoxy matrix was studied with fiber pullout tests, the fibers of which were stretched by a homemade microloading device. Raman spectra on the embedded fiber were recorded by micro-Raman spectroscopy, under different strain levels. Then, the fiber axial stress was obtained by the relationship between the stress and Raman shift of the aramid fiber. Experimental results revealed that the fiber axial stress increased significantly with the load. The shear stress concentration occurred at the fiber entry to the epoxy resin. Thus, interfacial friction stages exist in the debonded fiber segment, and the interfacial friction shear stress is constant within one stage. The experimental results are consistent with the theoretical model predictions.

The effect of a graded interphase on the mechanism of stress transfer in a fiber-reinforced composite

Based on an improved shear-lag model, the effect of an inhomogeneous interphase on the mechanism of stress transfer in fiber-reinforced composites is investigated. The inhomogeneity of the interphase is represented by the graded feature of the Young’s modulus varying according to a power law or a linear one in the radius direction, while the Poisson’s ratio and thermal expansion coefficient are assumed to be constants. Considering the effects of the inhomogeneous interphase as well as the Poisson’s contraction and thermal residual stress, closed-form solutions to the axial fiber stress and interfacial shear stress are obtained analytically. Comparing the case with a power law to that with a linear one, we find that the fiber stress increases significantly in the former case, while it decreases slightly in the latter one with an increasing interphase thickness. With the same external tensile load and interphase thickness, it is found that the fiber in the power law case is subjected to a larger tensile stress than that in the linear variation one. However, the interfacial shear stress is not sensitive to the interphase thickness in both cases, except that near the two ends of fiber. Under the same external load, the maximum shear stress in the interphase is much smaller in the latter than that in the former. All the phenomena can be characterized by one parameter, i.e., the average Young’s modulus of interphase, and denote that an interphase with a power variation law is more effective for stress transfer while the linearly graded one is more advantageous to avoid shear failure. The results should be helpful for engineers to properly design the interphase in novel composites, e.g. a carbon-fiber reinforced epoxy one.

Finite Element Analysis of Creep Behavior of AZ91D Magnesium Matrix Composites Reinforced with Aluminum Silicate Short Fibers

By Computer finite element analysis, the impact of the interface thickness, the interface module and the short fiber orientation of Al2O3-SiO2(sf)/AZ91D composite on the maximum fiber axial stress and the steady creep rate is studied. Maximum axial stress of the short fiber is in the fiber center, and the axial stress gradually decreases along the direction of the fiber length. When the external stress is constant, the maximum fiber axial stress increases with decreasing of the thickness of the interface, and the steady creep rate increases with the increasing of thickness of the interface. The maximum fiber axial stress increases with the increasing of the interface modulus, the increasing of the interface module improves the load transfer and the creep resistance. Finite element simulation results and experimental results can be well matched to better explain the creep behavior.

Steady state creep behavior of short fiber composites by mapping, logarithmic functions (MF) and dimensionless parameter (DP) techniques

A new mathematical insight based on logarithmic, polynomial mapping functions (MF), and dimensionless parameter (DP) models is presented for determination of some unknowns in the steady state creep stage of short fiber composites subjected to axial loading. These unknowns are displacement rate in outer surface of the unit cell, shear and equivalent stresses at interface and outer surface of the unit cell, and average axial stress in fiber. Dimensionless parameter technique is presented for determination of displacement rate and equivalent stress in outer surface of the unit cell. However, the polynomial mapping function is presented for determination of average axial stress in fiber. Most important novelty of the present research work is determination of the mentioned unknowns in steady state creep by DP and MF techniques without using the shear-lag theory unlike the previous researches. Good agreements are found among the new approaches and previous analytical results based on the shear-lag theory and also numerical solutions (FEM) for predicting the steady state creep behavior in short fiber composites.

Micromechanical Analysis of Single-Fiber Pull-Out Test of Fiber-Reinforced Viscoelastic Matrix Composites

A micromechanical model for single-fiber pull-out test of fiber-reinforced viscoelastic matrix composites is established. It includes fiber, interphase and viscoelastic matrix. The formulas to calculate the fiber axial stress, the interphase shear stress, and the matrix axial and shear stress are obtained. Moreover, for Kevlar aramid fiber reinforced viscoelastic matrix composites, the influences of the interphase thickness, the fiber embedded length and volume fraction on the stress distributions of fiber and interphase is studied. Some analysis results show that, with the increase of normalized fiber axial distance, the fiber axial stress increases monotonically, but the interphase shear stress decreases. The stress distributions of fiber and interphase change with the variation of the interphase thickness, the fiber embedded length and volume fraction.

Effect of Matrix Hardening on Tensile Strength of Alumina-Fiber Reinforced Aluminum Matrix Composites

This paper examines the stress distribution around a fiber break in alumina-fiber reinforced aluminum matrix (Al2O3/Al) composites using finite element analysis and predicts the tensile strength using tensile failure simulations. In particular, we discuss the effect of the matrix hardening on the tensile failure of the Al2O3/Al composites. First, we clarify the differences in the stress distribution around a fiber break between an elastic-perfect plastic matrix and an elastic-plastic hardening matrix using finite element analysis. Second, the procedure for simulating fiber damage evolution in the Al2O3/Al composites is presented. The simulation incorporates the analytical solution for the axial fiber stress distribution of a broken fiber in the spring element model for the stress analysis of the whole composite. Finally, we conduct Monte Carlo simulations of fiber damage evolution to predict the tensile strength of the Al2O3/Al composites, and discuss the effect of matrix hardening on the tensile strength of the Al2O3/Al composites. Coupled with size-scaling analysis, the simulated results express the size effect on the strength of the composites, which is seen in experimental results.

Modelling of polypropylene fibre-matrix composites using finite element analysis

Polypropylene (PP) fibre-matrix composites previously prepared and studied experimentally were modelled using finite element analysis (FEA) in this work. FEA confirmed that fibre content and composition controlled stress distri- bution in all-PP composites. The stress concentration at the fibre-matrix interface became greater with less fibre content. Variations in fibre composition were more significant in higher stress regions of the composites. When fibre modulus increased, the stress concentration at the fibres decreased and the shear stress at the fibre-matrix interface became more intense. The ratio between matrix modulus and fibre modulus was important, as was the interfacial stress in reducing pre- mature interfacial failure and increasing mechanical properties. The model demonstrated that with low fibre concentration, there were insufficient fibres to distribute the applied stress. Under these conditions the matrix yielded when the applied stress reached the matrix yield stress, resulting in increased fibre axial stress. When the fibre content was high, there was matrix depletion and stress transfer was inefficient. The predictions of the FEA model were consistent with experimental and published data.