Fiber-matrix Debonding Research Articles

Progressive Micromechanical Modeling for Pullout Energy of Hooked-end Steel Fiber in Cement-based Composites

A progressive micromechanical damage-plasticity formulation is proposed to analyze the single hooked-end steel fiber pullout energy from the surrounding cement-based matrix within the context of hooked-end steel fiber-reinforced cementitious composites (HSFRCC). As the hooked-end steel fiber has a unique fiber geometry, its fiber pullout energy from the cement-based matrix consists of the interfacial fiber-matrix debonding, the frictional sliding and pullout energy, and the elastoplastic deformation energy of the steel fiber hooked end. The aforementioned energy components are analytically derived first, and the superposition principle is subsequently employed to obtain the total energy dissipation during the fiber pullout process. Good agreement is obtained for comparisons between the experimental results of single hooked-end steel fiber pullout tests and the proposed analytical predictions. This satisfactory verification supports the validity and applicability of the proposed damage-plasticity energy prediction formulation, and suggests the applicability of this methodology to further investigation on micromechanical fracture energy prediction of HSFRCC during flexural macro-cracking.

An experimental investigation on the low velocity impact response of composite plates repaired by VARIM and hand lay-up processes

This paper presents results of an experimental investigation on the impact response of repaired and unrepaired glass/epoxy composite plates. Repaired samples were prepared by two different manufacturing methods; vacuum assisted resin infusion process and hand lay-up technique. In order to compare impact response of the repaired and unrepaired samples a number of single impact tests were performed under various impact energies. Damage process of the samples is analyzed from cross-examining load–deflection curves and damaged specimens. From the visual inspection, for the impacted side of the samples, it is noted that the main damage modes for repaired samples are matrix and fiber cracks around point of impact and delaminations while severe matrix cracks expanded through fiber directions are the dominant damage mode for unrepaired samples. At the back surfaces, delaminations and fiber–matrix debonding oriented in the fiber directions are observed for unrepaired samples. However, for repaired samples the fiber fractures through repair line as well as the delaminations become dominant modes. For a reasoning justification in discussing impact test results, interlaminar fracture toughness (Mode I and Mode II) and flexural tests for repaired and non-repaired samples were also conducted.

The impact damage response of plain woven natural silk/epoxy laminated composite plates

This paper presents the impact response of woven natural silk (WNS)/Epoxy composite. The composite specimens were prepared in configurations of 10, 15, 20, 25 and 30 ply of WNS. Drop weight impact test was carried out under varied impact energies of 32J, 48J and 64J. Examinations of load bearing capability, energy absorption capability and damage process from initiation of damage to final perforation, with regards to increasing number of WNS Ply and increasing impact loads were investigated. It was observed that load bearing capability increases with increase in the number of WNS ply. Further investigation of each plate revealed that load bearing capability increases with increase in impact load. The energy absorption capability also increased with increase in the number of WNS ply. It is important to note that as the thickness of the specimen increases, delamination and surface area damage increased. The scanning electronics microscope (SEM) result showed mechanism of failure as a combination of delamination, fibre-matrix debonding, fibre breakage and interlaminar matrix cracking. Keywords: Impact damage, laminated composites, energy absorption, woven natural silk.

Durability of GFRP Rebars in Simulated Concrete Solutions under Accelerated Aging Conditions

This paper discusses the potential ingress and effects of simulated concrete pore solution species on glass fiber-reinforced polymer (GFRP) rebars. Knowing that diffusion of moisture into the fiber-matrix interphase in a composite could cause fiber-matrix debonding and that the presence of alkalis at the location of the glass surface would lead to fiber degradation, particular attention was devoted to investigating whether GFRP rebars allow both species to penetrate or allow only water, while blocking the alkalis. To investigate this scenario, GFRP rebars were immersed in five types of simulated concrete pore solutions at elevated temperatures. Penetration of alkalis was assessed using X-ray mapping of backscattered electron images (BEI) and crosschecked by line and point energy dispersive spectroscopy (EDS) techniques. The effects of diffusion at the fiber-matrix interphase were observed by investigating the degradation of constituents at the interphase. Degradation of polymer matrix was assessed by a Fourier transform infrared (FTIR) spectroscopy. Degradation of fiber was investigated by observing highly magnified scanning electron microscopy (SEM) images and point EDS analyses close to the interphase. Debonding of fibers from the matrix was investigated using a SEM technique. Fiber-matrix debonding took place in a few samples, despite the fact that the glass fibers and polymer matrix remained essentially intact and that no penetration of alkalis into the GFRP rebars was observed. This debonding, which occurred only for samples exposed to 75°C, is believed to be caused by hydrolysis of fiber sizing at high temperatures. Finally, this study shows that the vinyl ester (VE) polymer matrix used acts as an effective semipermeable membrane by allowing the penetration of water, while blocking alkali ions.

Micromechanical analysis of periodic composites by prescribing the average stress

A new method for the micromechanical finite element analysis of unidirectional composites is presented. The method is especially effective in time-dependent analyses, as those involving viscoplasticity and viscoelasticity. The microstructure of the composites under consideration is characterized by periodicity and central symmetry, which allow analysing half of a unit cell for solving the micromechanical problem. Half of the unit cell contains only half of the continuous fibre and this makes the numerical analyses inexpensive in terms of computer memory usage and processing time. New boundary conditions are presented in order to prescribe the average stress on half of the unit cell of hexagonal and rectangular distributions of heterogeneities in the case of static analysis. Then, the new boundary conditions are used to prescribe the precise rate of the average stress in time-dependent analyses. With respect to other existing procedures, the proposed method is easy to adopt in commercial software and it does not require the modification of parts of the source code that are not usually accessible to the user. The proposed method is applied to the interesting case of composites with aligned long fibres imperfectly bonded to a viscoelastic matrix. The numerical simulations carried out in this work provide the loci of the average stress and strain corresponding to the initiation of the fibre-matrix debonding, which determines a considerable decay of the composite stiffness and strength. The influence of the geometrical properties of the microstructure is evaluated by analysing both hexagonal and rectangular distributions of fibres. The numerical results show how the inelastic behaviour of the matrix affects the loci corresponding to the initiation of the debonding.

Finite-Element Model for Failure Study of Two-Dimensional Triaxially Braided Composite

A new three-dimensional finite element model of two-dimensional triaxially braided composites is presented in this paper. This meso-scale modeling technique is used to examine and predict the deformation and damage observed in tests of straight sided specimens. A unit cell based approach is used to take into account the braiding architecture as well as the mechanical properties of the fiber tows, the matrix and the fiber tow-matrix interface. A 0 deg / plus or minus 60 deg. braiding configuration has been investigated by conducting static finite element analyses. Failure initiation and progressive degradation has been simulated in the fiber tows by use of the Hashin failure criteria and a damage evolution law. The fiber tow-matrix interface was modeled by using a cohesive zone approach to capture any fiber-matrix debonding. By comparing the analytical results to those obtained experimentally, the applicability of the developed model was assessed and the failure process was investigated.

Metal Thickness, Fiber Volume Fraction Effect on the Tensile Properties, Debonding of Hybrid Laminates

Hybrid aluminum glass fiber laminates consist of glass fiber reinforced plastic (GFRP) bonded with thin aluminum sheets on either side. Laminates with different aluminum thickness and fiber volume fractions were fabricated by hand layup method and were tested under tensile loading. The variations in the elastic modulus, yield strength, and ultimate strength as well as the extent of delamination were studied. Rule of mixtures predictions of laminate properties under tensile loading compared well with the experimental results. Delamination was found to depend upon the thickness of aluminum alloy sheet, among other factors. Scanning electron microscopic pictures on the fractured hybrid specimen showed that the GFRP failure was due to fiber fracture, fiber pullout, and fiber matrix debonding. On an overall basis, the hybrid laminate showed a significant improvement in the mechanical properties when compared with the GFRP or monolithic aluminum. Though an increase in fiber volume fraction increases the elastic modulus, it does not have any notable effect on the failure mechanism or debonding of the hybrid aluminum glass fiber laminate.

Influence of annealing duration on the erosive wear behavior of polyphenylenesulphide composites

The influence of annealing duration on the erosive wear behavior of short glass fiber (40% w/w) and CaCO3 mineral particulate (25% w/w)–short glass fiber (40% w/w) (total: 65% w/w) reinforced PPS composites has been characterized under various experimental conditions by differential scanning calorimetry (DSC) and erosion measurements. The erosive wear of the composites have been evaluated at different impingement angles (30, 45, 60, and 90°) and at four different annealing periods (30, 60, 90, and 120 min). Increase in the total crystallization causes an improvement in the erosive wear properties of the samples. Annealing time controls the morphology by influencing the degree of crystallinity in the matrix and in the fiber–matrix interface. This formation restricts fiber–matrix debonding. There is no linear proportionality between annealing time and relative degree of crystallization. The results indicate that PPS composites show maximum in wear versus impact angle relation at 60° confirming their semi-ductile failure behavior. The morphologies of eroded surface are examined by the scanning electron microscope.

Characterization of Metal Matrix Composites by Synchrotron Refraction Computed Topography

X-ray computed tomography is an important tool for evaluating the three dimensional microstructure of modern materials non-destructively. To resolve material structures in the micrometre range and below high brilliance synchrotron radiation has to be taken. But materials of low absorption or mixed phases show a weak absorption contrast at there interfaces. A Contrast enhancement can be achieved by exploiting the refraction of X-rays at interfaces. This technique was developed and applied at the NDT department of the Federal Institute for Materials Research and Testing (BAM) during the last decade. It meets the actual demand for improved non-destructive characterisation of high performance composites, ceramics and other low density materials and components. The technique is based on Ultra Small Angle Scattering (USAXS) by micro structural elements causing phase related effects like refraction and total reflection at a few minutes of arc as the refractive index of X-rays is nearly unity. The extraordinary refraction contrast of inner surfaces is far beyond absorption effects and hence especially useful for materials of low absorption or mixed phases, showing similar X-ray absorption properties. Crack orientation and fibre-matrix debonding in plastics, polymers, ceramics and metal-matrix-composites after cyclic loading and hydro thermal aging can be visualized. By combining the refraction technique with the computed tomography technique the three dimensional imaging of the micro structure of the materials is obtained. In most cases the investigated inner surface and interface structures correlate to mechanical properties. Recent results with a sub-micrometer resolution will be presented.

Acoustic Emission Characteristics of Mode I Delamination in Glass/Polyester Composites

This article proposes an experimental study on the mode I interlaminar fracture of glass/polyester composites by using acoustic emission (AE) to analyze the damage evolution and evaluate the interlaminar performance of polymeric composites. A delamination process simulated with a double cantilever beam in opening mode (Mode I) coupled with an AE technique has been employed. The microscopic observation (scanning electron microscopy) is used to determine the correlation between different fracture mechanisms and their corresponding AE signal frequency content. Selected emissions are classified as matrix cracking, fiber breakage, or interface processes (fiber—matrix debonding) based on their total power in defined frequency intervals of the spectral power density. A correlation was established between the mechanical energy release rate and the AE energy rate. Analysis in the frequency domain shows AE parameters are powerful indicators of the intensity of the damage.

Cyclic Damage Evolution in Polymer Matrix Composites

Damage mechanics has been applied to describe the cyclic behaviour of glass fibre-polyester and carbon fibre-epoxy composites with different lay-ups under various loading conditions. Damage evolution was determined by continually monitoring fatigue modulus degradation and measuring the crack density. These methods complemented each other. They showed that the damage could be separated into two stages. Damage evolved rapidly for the first 10% of life, followed by a more gradual and linear accumulation for the remainder of life. In general, the transition from the first to the second stage indicated a change from transverse matrix cracking to fibre-matrix debonding and coalescence. Damage mechanics was applied to the fatigue modulus changes that occurred in the stress-strain hysteresis loops, monitored throughout life. A two-stage model was applied to express damage evolution using the modulus- and crack-based damage parameters. This model successfully described cyclic damage evolution for different lay ups of the PMCs. The significance of which was that the amount of fatigue damage for any stress level at the end of the initial stage could be used to accurately predict fatigue life and construct a stress-life diagram for the given composite

Modelling of chip separation in machining unidirectional FRP composites by stiffness degradation concept

Progressive failure of unidirectional glass fiber-reinforced polymer composites (FRP) was studied using finite element analysis in orthogonal machining. Chip formation process and damage modes such as matrix cracking, fiber–matrix debonding and fiber breaking were modelled by degrading the material properties. Damage analysis was carried out using Hashin, Maximum stress and Hoffman failure criteria. After damage was detected, selective stiffness degradation was applied to the workpiece material. The objective of this study is to better understand the chip formation process and to analyse the cutting-induced damage from initiation stage until complete chip formation. The effect of the fiber orientation on cutting forces and sub-surface damage was investigated with different failure criteria. The results were addressed in terms of cutting forces evolution and damage progression in the composite structure during machining. It was demonstrated that the use of the stiffness degradation concept with the appropriate failure criterion responds potentially in a predictable fashion to changes in chip formation process for machining of FRPs.

Modeling of evolving damage in high temperature polymer matrix composites subjected to thermal oxidation

This paper describes mechanism-based modeling of damage evolution in high temperature polymer matrix composites (HTPMC) under thermo-oxidative aging conditions. Specifically, a multi-scale model based on micro-mechanics analysis in conjunction with continuum damage mechanics (CDM) is developed to simulate the accelerated fiber–matrix debond growth in the longitudinal direction of a unidirectional HTPMC. Using this approach, one can relate the behavior of composites at the micro-level (representative volume element) to the macro-level (structural element) in a computationally tractable manner. Thermo-oxidative aging is simulated with diffusion-reaction model in which temperature, oxygen concentration, and weight loss effects are considered. For debond growth simulation, a model based on Darcy’s laws for oxygen permeation in the fiber–matrix interface is employed, that, when coupled with polymer shrinkage, provides a mechanism for permeation-controlled debond growth in HTPMC. Benchmark of model prediction with experimental observations of oxidation layer growth is presented, together with a laminate thermo-oxidative life prediction model based on CDM to demonstrate proof-of-concept.

Homogenization based continuum damage mechanics model for monotonic and cyclic damage evolution in 3D composites

This paper develops a 3D homogenization based continuum damage mechanics (HCDM) model for fiber reinforced composites undergoing micromechanical damage under monotonic and cyclic loading. Micromechanical damage in a representative volume element (RVE) of the material occurs by fiber-matrix interfacial debonding, which is incorporated in the model through a hysteretic bilinear cohesive zone model. The proposed model expresses a damage evolution surface in the strain space in the principal damage coordinate system or PDCS. PDCS enables the model to account for the effect of non-proportional load history. The loading/unloading criterion during cyclic loading is based on the scalar product of the strain increment and the normal to the damage surface in strain space. The material constitutive law involves a fourth order orthotropic tensor with stiffness characterized as a macroscopic internal variable. Three dimensional damage in composites is accounted for through functional forms of the fourth order damage tensor in terms of components of macroscopic strain and elastic stiffness tensors. The HCDM model parameters are calibrated from homogenization of micromechanical solutions of the RVE for a few representative strain histories. The proposed model is validated by comparing results of the HCDM model with pure micromechanical analysis results followed by homogenization. Finally, the potential of HCDM model as a design tool is demonstrated through macro-micro analysis of monotonic and cyclic damage progression in composite structures.

Thermo-oxidative behavior of high-temperature PMR-15 resin and composites

The present study examines the thermo-oxidative behavior of high-temperature polymer matrix composite materials. Thermo-oxidative aging in neat resin is simulated with a diffusion-reaction model in which temperature, oxygen concentration and weight loss effects are considered. The thermo-oxidative behavior of the composite, on the other hand, is significantly different from that of the constituents as the composite microstructure, including the fiber/matrix interphase/interface, introduces anisotropy in the diffusion and oxidation behavior. Unit cell analyses are carried out using three-dimensional finite element analysis of repeated volume elements representing the fiber, matrix and interphase regions, and the resin oxidation model. Parametric studies illustrating the anisotropy in the oxidative region growth and the effect of fiber and interphase diffusivity on the oxidation layer growth are discussed. It is suggested that fiber–matrix debonding could provide additional diffusion paths to explain the extent of observed anisotropic oxidation growth.

Effects of Water Absorption on Creep Behavior of Wood—Plastic Composites

The effects of filler loading and immersion in water on the creep/ recovery behavior of composites made from MDF (medium density fiberboard) flour (as natural fiber) and recycled HDPE (high density polyethylene) (as resin) were studied (at 60, 70, and 80% by weight fiber loadings). Nominal density and dimensions of the manufactured panels were 1 g/cm3 and 35 × 35 × 1 cm3, respectively. Maximum values of flexural modulus and strength of panels were obtained at 70 and 60% fiber content, respectively. The creep strain decreased as the lignocellulosic flour level increased. Water absorption has negative effect on creep behavior of MDF flour/HDPE composites. For all filler contents, it can be seen that the creep strain increases when the immersion time increases. This is believed to occur as a result of the cumulative effect of absorbed water on fiber matrix debonding and easier relaxation of the molecules at higher moisture contents leading to larger deformations at longer times.

Parametric Optimization Erosion Wear of Polyester-GF-Alumina Hybrid Composites using the Taguchi Method

The improved performance of polymer-based hybrid composites in tribological applications has recently been a subject of considerable interest. A hybrid composite consists of the matrix reinforced with both fibers and particulate fillers. Alumina has the potential to be used as filler in such a multi-component system. This article investigates the effect of alumina filling on the erosion wear performance of glass fiber-reinforced polyester composites. For this purpose, an air jet type erosion test configuration and the design of experiment approach utilizing Taguchi's orthogonal arrays are used. Taguchi's design eliminates the need for repeated experiments; thus saving time, materials, and cost. The systematic experimentation leads to identifying significant factors and their interactions that predominantly influence the erosion wear. Pure glass-polyester composite without filler shows greater erosion rate whereas a significant improvement in the erosion resistance is observed with alumina fillers. This may be due to restriction of fiber-matrix debonding. The morphologies of the eroded surface are examined by a scanning electron microscope. Finally, optimal factor settings for minimum wear rate have been determined using genetic algorithm.

Damage characterization of polymer-based composite materials: Multivariable analysis and wavelet transform for clustering acoustic emission data

In the present work, a procedure for the investigation of local damage in composite materials based on the analysis of the signals of acoustic emission (AE) is presented. One of the remaining problems is the analysis of the AE signals in order to identify the most critical damage mechanisms. In this work, unsupervised pattern recognition analyses (fuzzy C-means clustering) associated with a principal component analysis are the tools that are used for the classification of the monitored AE events. A cluster analysis of AE data is achieved and the resulting clusters are correlated to the damage mechanisms of the material under investigation. After being validated on model samples composed of unidirectional fiber-matrix composites, this method is applied to actual composites such as glass fiber/polyester cross-ply composites and sheet molding compound (SMC). Furthermore, AE signals generated by heterogeneous materials are not stationary. Thus, time-scale methods are used to determine new relevant descriptors to be introduced in the classification process in order to improve the characterization and the discrimination of the damage mechanisms. Continuous and discrete wavelet transforms are applied on typical damage mechanisms AE signals of glass fiber/polyester composites such as matrix cracking, fiber-matrix debonding. Time-scale descriptors are defined from these wavelet analyses. They provide a better discrimination of damage mechanisms than some time-based descriptors.

Finite element analysis of load transfer at a fibre–matrix interface during pull-out loading

Load transfer ability of the fibre–matrix interface is well known to mainly control the mechanical behaviour of fibre-reinforced materials. This load transfer phenomenon is of great importance in dentistry when a post is used for fixing a ceramic crown on the tooth. The pull-out test has been well accepted as the most important micromechanical test for evaluating the interaction properties between the fibre and matrix. In this study, a finite element model is developed to analyse the pull-out process of a steel fibre from an epoxy matrix. Based on the pull-out force–displacement curves, developed in our previous experimental work, specific load transfer laws at the fibre–matrix interface have been proposed for each stage of the pull-out process, i.e., before and after fibre–matrix debonding. Predicted initial extraction forces for different implantation lengths were fitted to experimental values and an initial interference fit of 4 μm was determined. An interfacial shear strength of 21 MPa was then determined by fitting the predicted debonding forces for different implantation lengths to the experimental values. According to the load transfer laws considered, analysis of the interfacial shear stress indicates that fibre–matrix debonding initiates simultaneously at both the lower and upper extremities of the interface.

Cycling tests of a fibre-reinforced polymer honeycomb sandwich deck panel at very cold temperatures

The FRP deck panel with an overall geometry of 2.134×0.340×0.178 m was cyclically tested under a three-point loading setup in a cold room with the temperature as low as −55°C. The Classic Beam Theory (CBT) method and the Finite Element (FE) method were also used to predict the panel behaviour at room temperatures. Based on this research, the following conclusions were made: (1) The low-temperature stiffness was about 3.8% higher than at the high temperature as long as the low temperature was above a certain degree; (2) When the temperature was lowered from −20 to −55°C, the low-temperature stiffness decreased 11.5% comparing with the stiffness at room temperatures. This degradation in stiffness could not be recovered by raising the panel temperature; (3) During the ultimate load test at −55°C, the panel failed at 171.1 KN with a very loud noise. Visual inspections indicated the debonding of the face sheet finally led to the local buckling of the exterior plate on both sides of the panel; (4) Very cold temperature exposure resulted in matrix hardening, matrix micro-cracking and fibre-matrix debonding as demonstrated by the AE testing data at different temperatures; and (5) At the room temperatures, the FRP panel showed a little stiffer performance than both the CBT method and the FE method would predict.