Fiber-matrix Debonding Research Articles

Degradation Kinetics of Pultruded E-Glass/Vinylester in Alkaline Media

The use of fiber-reinforced polymer (FRP) composites, in the form of externally bonded reinforcement for rehabilitation or in the form of externally bonded reinforcement for rehabilitation of in the form of reinforcing bars and tendons for new reinforcement, provides a potential means of slowing or even preventing, the deterioration seen in conventional materials such as steel and concrete. However, there is a concern regarding the long-term durability of E-glass reinforced FRP with concrete. This article presents the results from a 75-week investigation aimed at characterization of deterioration mechanisms and performance of E-glass/vinylester in an alkaline environment. The study used dynamic mechanical thermal analysis, Fourier-transform infrared spectroscopy, gravimetric moisture uptake, tensile and short-beam-shear tests, and microscopy. Results show a range of deterioration mechanisms initiating with reversible plasticization of the resin and transitioning to irreversible fiber-matrix debonding, hydrolysis, and chain scission of the resin, and pitting and material loss of the fiber. Reconditioning, following periods of immersion, results in some regain in tensile strength over at least half the exposure period, whereas after 15 weeks there is almost no regain in interlaminar shear strength. The authors predict that long-term response will match fairly closely with experimental results at the 75-week level. Under self-similar continuation of deterioration, 26.89% of the original tensile strength and 58.24% of the short-beam-shear strength can be expected to be retained after 50 years. The authors conclude with a brief discussion of real-life conditions versus the testing conditions.

Helicopter rotor blade frequency evolution with damage growth and signal processing

Structural damage in materials evolves over time due to growth of fatigue cracks in homogenous materials and a complicated process of matrix cracking, delamination, fiber breakage and fiber matrix debonding in composite materials. In this study, a finite element model of the helicopter rotor blade is used to analyze the effect of damage growth on the modal frequencies in a qualitative manner. Phenomenological models of material degradation for homogenous and composite materials are used. Results show that damage can be detected by monitoring changes in lower as well as higher mode flap (out-of-plane bending), lag (in-plane bending) and torsion rotating frequencies, especially for composite materials where the onset of the last stage of damage of fiber breakage is most critical. Curve fits are also proposed for mathematical modeling of the relationship between rotating frequencies and cycles. Finally, since operational data are noisy and also contaminated with outliers, denoising algorithms based on recursive median filters and radial basis function neural networks and wavelets are studied and compared with a moving average filter using simulated data for improved health-monitoring application. A novel recursive median filter is designed using integer programming through genetic algorithm and is found to have comparable performance to neural networks with much less complexity and is better than wavelet denoising for outlier removal. This filter is proposed as a tool for denoising time series of damage indicators.

Finite Element Simulation of the Fiber–Matrix Debonding in Polymer Composites Produced by a Sliding Indentor: Part I – Normally Oriented Fibers

To study the contact and debonding behaviors between a CF–PEEK fiber-reinforced polymer composite specimen and a sliding diamond indentor, finite element macro- and micro-models have been developed. Around each fiber, interface elements were introduced in order to detect the tension-type and also the shear-type debondings for different cases. If initial or final debonding has occurred, a “control algorithm” checked the limit strain conditions for the interface elements and changed the material properties according to the actual debonding condition. As a final result, it can be concluded, that the dominant debonding under compression is due to the shear loading conditions.

Radiation effects on the mechanical integrity of novel organic insulators for the ITER magnet coils

High quality glass fiber reinforced composites will be used as electrical insulation systems for the windings of the superconducting magnet coils of the ITER fusion device. Recently, considerable interest in coil technology has focused on improving the radiation hardness of the organic impregnation resins, in order to avoid serious material damage, such as fiber-matrix debonding (delamination). This paper reports on first screening tests of novel cyanate-ester based glass fiber reinforced composites, which were developed especially for ITER-relevant applications. The material behavior was investigated at 77 K prior to and after reactor irradiation to a neutron fluence of 1 × 10 22 m −2 ( E>0.1 MeV) using the tensile and the short-beam-shear test, respectively. Furthermore, tension–tension fatigue measurements were carried out at low temperatures, in order to assess the mechanical performance under the pulsed operating conditions of ITER.

Local field effects in titanium matrix composites subject to fiber-matrix debonding

This paper addresses fiber-matrix debonding in titanium matrix composites (TMCs) using a recently developed micromechanics model known as the high-fidelity generalized method of cells (HFGMC). By employing a higher-order displacement field, this model supercedes its predecessor, the generalized method of cells (GMC), in terms of micro-scale field accuracy. The import of this micro-scale accuracy is amplified in the case of fiber-matrix debonding as the debonding phenomenon is dominated by local field effects. Via inclusion of appropriate constitutive relations for inelastic deformation and fiber-matrix debonding, both HFGMC and GMC have been applied herein to model the transverse deformation of titanium matrix composites, which exhibit obvious effects of interfacial debonding. Results indicate that HFGMC is considerably more quantitatively accurate than GMC for analysis of composites with debonding, enabling realistic predictions of the TMC transverse response. The improved accuracy of the HFGMC local fields also enables investigation of some qualitative aspects of the debonding phenomenon within TMCs.

The effect of fiber orientation on the toughening of short fiber‐reinforced polymers

AbstractThe effect of fiber orientation on the toughening of polymers by short glass fibers generally below their critical length was investigated using specimens with either well‐aligned or randomly oriented fibers. The fibers were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned fibers in three orientations with respect to the crack plane and propagation direction. Specimens with fibers aligned normal to the fracture plane were the most tough, those with randomly oriented fibers were less tough, and those with fibers aligned within the fracture plane were the least tough. The fracture behaviors compared favorably with predictions based on observed processes accounting for fiber orientation. The processes considered were fiber pull‐out (including snubbing), fiber breakage, fiber–matrix debonding, and localized matrix‐yielding adjacent to fibers bridging the fracture plane. Fibers not quite perpendicular to the fracture plane provided the greatest toughening; these fibers pulled out completely and gave a significant contribution from snubbing. Fibers at higher angles provided less toughening, involving nearly equal contributions from pull‐out, breakage, and debonding. Fibers within the fracture plane provided the least toughening, involving debonding alone. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2740–2751, 2003

GF/PE 복합재료의 충격파괴거동에 관한 연구

Many of researches regarding mechanical properties of composite materials are associated with humid environment and temperature. Especially the temperature is a very important factor influencing the design of thermoplastic composites. However, the effect of temperature on impact behavior of reinforced composites have not yet been fully explored. An approach which predicts critical fracture toughness G<TEX>$_{IC}$</TEX> was performed by the impact test in this work. The main goal of this work is to study the effect of temperature and span of specimen supports on the results of Charpy impact test for GF/PE composite. The critical fracture energy and failure mechanism of GF/PE composites were investigated in the temperature range of <TEX>$60^{\circ}C;to;-50^{\circ}C$</TEX> by the Charpy impact test. The critical fracture energy showed the maximum at the ambient temperature, and it tended to decrease as the temperature increased or decreased from the ambient temperature. The major failure mechanisms are the fiber matrix debonding, the fiber pull-out and/or delamination and the matrix deformation.n.

Debonding of short fibres among particulates in a metal matrix composite

A numerical analysis is carried out for the development of damage by fibre–matrix debonding in aluminium reinforced by aligned, short SiC fibres. A unit cell-model that has earlier been applied to study materials with arrays of transversely staggered fibres is here extended to contain a number of differently shaped fibres or particulates in each unit cell, thus representing debonding of a relatively long discontinuous fibre among particulates that do not debond. Interfacial failure is modelled in terms of a cohesive zone model that accounts for decohesion by normal separation as well as by tangential separation. It is found that the evolution of failure can depend rather strongly on the distribution of particulates around a fibre subject to debonding.

An experimental investigation into the orthogonal cutting of unidirectional fibre reinforced plastics

This paper aims to understand the machinability of epoxy composites reinforced by unidirectional carbon fibres when subjected to orthogonal cutting. It was found that the subsurface damage and its mechanisms of a machined component are greatly influenced by fibre orientation. The material’s bouncing back is a characteristic phenomenon associated with the cutting of a fibre-reinforced composite. Three distinct deformation zones appear, i.e., chipping, pressing and bouncing when the fibre orientation is <90°. Otherwise, fibre-bending during cutting will become more significant and subsurface damage caused by fibre-matrix debonding will be severer. As a result, surface roughness, subsurface damage and cutting forces all change dramatically with the fibre orientation. It was also found that the curing conditions of making the composites do not have an obvious effect on the machinability, though the mechanical properties of the materials vary.

Cohesive zone representations of failure between elastic or rigid solids and ductile solids

A cohesive zone model that describes tangential separation as well as normal separation along an interface is reviewed. The model is based on nonlinear traction–separation relations between the normal and tangential components of the interface tractions and relative displacements. To illustrate the application of the cohesive zone model in studies of material failure or crack growth, analyses of matrix–fibre debonding in metal matrix composites are presented, taking into account effects of residual stresses or of nonlocal plasticity for the matrix. Also studies of interface crack growth under mixed mode conditions are discussed.

Mechanisms of energy absorption in the creep fracture of woven ceramic composites

Abstract A micromechanical model is developed which accounts for the energy absorbed in the creep deformation and fracture of a 2.5D SiC/C/SiC composite, representative of the new generation of non-oxide CMCs. The model quantifies the influence of the geometrical, mechanical and material parameters and, in particular, is very sensitive to the interfacial sliding stress. The effect of the sliding stress on the contribution of the different energy absorbing mechanisms in the creep fracture of CMCs is described. It is concluded that, for all testing conditions, most of the energy absorbed in the creep fracture is controlled by fibre–matrix debonding and fibre pull-out.

Time-dependent Microcracking Detected in a Rubber-toughened Carbon-epoxy Composite by the Modal Acoustic Emission Method

A method of modal acoustic emission monitoring and waveform analysis is developed as a means for tracking two of the primary damage mechanisms in unidirectional composite lamina, matrix-cracking and fiber–matrix debonding. Unidirectional 30, 45 and 90° coupons of a rubber-toughened carbon-epoxy are monitored in this way for various loading histories. A method of comparing waveforms from different samples is proposed in order to establish the similarity of microcracking regardless of fiber direction. Histograms of waveform energy are used as a measure of average arrested crack length versus stress level. Larger faster cracks will emit more energy. Finally, an interpretation of the AE data is given based on an initial population of existing flaws where a cumulative distribution function (CDF) of microcracking is defined and used to study effects of stress history.

Cohesive-Shear-Lag Model for Cycling Stress-Strain Behavior of Unidirectional Ceramic Matrix Composites

During fatigue tests of fiber-reinforced ceramic matrix composites (CMCs), the stress-strain hysteresis loops undergo typically through various changes in shape and size before they stabilize after many cycles. These hysteresis loops are a good representation of intrinsic deformation and damage modes occurring in the materials. The present study proposes a cohesive-shear-lag model to analyze the cyclic stress-strain behavior of unidirectional fiber-reinforced CMCs. The model, as a modification to classical shear-lag model, takes into account the effects of spatially distributed fracture of a partial number of fibers as well as matrix cracking and partial interfacial debonding. The effect of the distributed fibers fracture is modeled in an average sense by using a cohesive damage law. The proposed model has been utilized to investigate the stress-strain hysteresis loops of a unidirectional fiber-reinforced ceramic-glass matrix composite, SiC/1723. The results demonstrate its capability not only in characterizing the cycling stress-strain behavior but also in tracking the progressive damage process that occurred during the tension-tension fatigue of the composite. The dominant progressive damage mechanisms in this case were found to be accumulation of fibers breakage, growth of fiber-matrix interfacial debond, and smoothening of frictional debonded interface, following matrix cracking, which presumably occurred in the first few cycles.

On a damage mesomodel for laminates: micromechanics basis and improvement

The so-called “damage mesomodel for laminates” developed in the past fifteen years, particularly at Cachan, is being reconsidered in the light of the recent works, theoretical as well as experimental, done on the microscale. Previous works have already proved that this mesomodel can be interpreted as the homogenized result of micromodels involving common microdamage mechanisms: transverse microcracking, delamination at the tips of transverse microcracks and fiber–matrix debonding. Here, we are going one step further by considering microcracking in skin plies of various thicknesses. To fit experimental results better, we introduced a modification of the coupling of the three microdamage scenarios in all plies. Contrary to the classical approach, the critical values of the energy release rate are not associated with a healthy material but with a damaged material; the first damage mechanism to occur is associated with non-transverse microcracks due to nearly uniformly distributed fiber–matrix debonding in the damaged ply. These mechanisms and this coupling are homogenized on the mesoscale, which leads to an improved version of the “damage mesomodel for laminates” involving only a small number of material constants which can be interpreted on the microscale. Thickness effects are taken into account: the improved version, unlike the previous one, is valid for arbitrary values of the thickness. We discuss experimental data and identification using carbon–epoxy laminates at room temperature.

The influence of thermal residual stresses on the transverse strength of CFRP using FEM

The failure of transversely loaded unidirectional CFRP has been investigated using mechanical and thermo-mechanical test methods as well as finite element analysis (FEA). The FEA analysis consists of two cases: a high interfacial strength between fiber and matrix, so that matrix failure governs the fracture process of the composite, as well as a weak interface, so that fiber matrix debonding is the dominating failure process of the composite. The failure dependence of the resin on the actual stress-state could be described. Furthermore, the influence of the thermal residual stresses on the initial matrix failure has been investigated, and the actual stiffness as well as the thermal expansion change of the epoxy resins and the composites as a function of temperature have been determined experimentally. The results of the mechanical and thermo-mechanical tests performed on the neat resin and on the composites were incorporated into FEA and compared with the transverse tensile properties of the composite laminates. In the FE-analysis, the local fiber volume fraction was varied over a wide range in order to investigate its influence on the thermal residual stresses and transverse composite strength. The results can explain the low strain to failure of transverse laminates under tensile loading. The calculated interfacial shear strength (ISS) and the interfacial normal strength (INS) are in good agreement with values found in the literature.

Low‐temperature hygrothermal degradation of ambient cured E‐glass/vinylester composites

AbstractFreeze and freeze‐thaw durability characteristics of fiber reinforced polymer (FRP) composites, especially in the presence of moisture, need to be investigated prior to the widespread implementation of these materials in civil, polar, and offshore structural components and systems. The hygrothermal degradation characteristics of an ambient cure E‐glass/vinylester system due to exposure to −10°C conditions and conditions of freeze‐thaw, including in the presence of water and seawater, was investigated. Changes in mechanical characteristics such as strength and modulus, and thermo‐mechanical dynamic characteristics such as storage and loss moduli, and glass‐transition temperature were measured, and short‐term effects of environmental exposure were assessed. It is seen that the presence of moisture/solution has a significant effect; both in terms of physical and chemical aging, and in terms of microcracking and fiber–matrix debond initiation. Results indicate the critical importance of cure characteristics and diffusion related phenomena. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2255–2260, 2002

Fully Coupled Micro/Macro Deformation, Damage, and Failure Prediction for SiC/Ti-15-3 Laminates

The deformation, failure, and low-cycle fatigue life of SCS-6/Ti-15-3 composites are predicted using a coupled deformation and damage approach in the context of the analytical generalized method of cells (GMC) micromechanics model. The local effects of inelastic deformation, fiber breakage, fiber-matrix interfacial debonding, and fatigue damage are included as submodels that operate on the microscale for the individual composite phases. For the laminate analysis, lamination theory is employed as the global or structural scale model, while GMC is embedded to operate on the mesoscale to simulate the behavior of the composite material within each laminate layer. While the analysis approach is quite complex and multifaceted, it is shown through comparison with experimental data to be quite accurate and realistic, while remaining extremely efficient.

Mechanical damage and strain in carbon fiber thermoplastic‐matrix composite, sensed by electrical resistivity measurement

AbstractThe volume electrical resistivity of a unidirectional continuous carbon fiber thermoplastic (nylon‐6) matrix composite was found to be an indicator of strain and damage during repeated loading in the fiber direction. The through‐thickness resistivity irreversibly and gradually decreased upon damage (probably fiber‐matrix debonding) during repeated compression or tension. Moreover, it reversibly and abruptly increased upon matrix damage, which occurred reversibly near the peak stress of a stress cycle. In addition, the resistivity increased reversibly upon tension in every stress cycle, and decreased reversibly upon compression in every stress cycle. On the other hand, the longitudinal resistivity irreversibly and gradually increased upon damage. Moreover, it decreased reversibly upon tension in every stress cycle and increased reversibly upon compression in every stress cycle. The through‐thickness resistivity was a better indicator of damage and strain than the longitudinal resistivity.

Response of Fiber Reinforced Polymer Confined Concrete Exposed to Freeze and Freeze-Thaw Regimes

Unidirectional carbon/vinylester composites and concrete cylinders wrapped with three layers of the same composite were exposed to -18°C (0°F) conditions both with and without prior saturation by moisture and to freeze-thaw cycling after saturation. All specimens show degradation in strength, with the maximum degradation being due to the saturated freeze-thaw condition caused by cyclic effects of absorption, subsequent crack-opening and fiber-matrix debonding. Analytical predictions, based on approximation of hygrothermomechanical response models for composites combined with a simple confinement model, are shown to correlate well with experimental data for most of the exposure conditions.

Étude de la décohésion fibre-matrice dans les arcs composites circulaires

In the framework of linear elasticity, we consider a composite arch constituted by a matrix and a fiber reinforcing the structure. This arch is clamped at one extremity, while at the other the fibre is subject to either prescribed displacements or forces. By making an asymptotic analysis based on the slenderness and loading parameters, we study by means of an energy criterion the fiber-matrix debonding, resulting from the inextensional displacements of the medium line. We show in particular that for these specific loading types, the debonding occurs brutally and the critical length of initiation is of order 1. To cite this article: K. Madani, C. R. Mecanique 330 (2002) 535–541.