Fiber-matrix Debonding Research Articles

The role of hydrophobic silane coating on Si stamps in nanoimprint lithography.

Hydrophobic silane coatings have been successfully applied to the surface of Si stamps to improve demolding in nanoimprint lithography (NIL). However, the role of the silane coating has only been studied either indirectly, by measuring adhesion or friction coefficients for Si and substrate surfaces without patterns, or collectively, by measuring the overall demolding force that does not differentiate contributions of friction dissipation, stored elastic energy, and adhesion. Here, for the first time, we present experimental evidence on the role of the silane coating in improving demolding in UV-NIL by using different silane coatings. The silane coatings were characterized by x-ray photoelectron spectroscopy, water contact angle, and friction force measurements. Then, the work of demolding was systematically measured for different silane coatings using stamps with the same micropattern but different pattern depths. Comparison of the results to the theoretical model developed for fiber-matrix debonding energy by Sutcu and Hillig [Acta Metall. Mater. 38(12), 2653-2662] indicated that with a hydrophobic silane coating, the main parameter contributing to overall demolding work shifts from adhesion to stored elastic energy and frictional dissipation as surface adhesion keeps decreasing. The results confirm that the main role of the silane coating in reducing the demolding is to reduce surface adhesion rather than friction at the stamp/substrate interface.

Effect of fatigue testing on the properties of Glass-Epoxy composites using the acoustic tool

This study presents the experimental results of the influence of mechanical fatigue on composite material. The plates of Glass fiber with SR 1500 epoxy resin with SD 2505 composite were realized by vacuum molding. Experimental tests were carried out on a standard hydraulic machine INSTRON 8516. The machine is interfaced with a dedicated computer for controlling and data acquisition. The fatigue tests were performed using sinusoidal type of waveform at a displacement control with frequency of 10 Hz. The evolution of Young’s modulus and strain based on fatigue gives us an idea about the resistance of the material. Degradation of mechanical properties was observed, and the results have showed that the Young’s modulus of plates undergo only minor changes. In fact, the residual stiffness and residual strength decrease when the cycle number of fatigue increase (100 to 50000 cycles), indicating that the studied composites have experienced some forms of mechanical damage.The mechanical tests were backed by Acoustic Emission Monitoring (AEM) during the load cycle, in order to understand the nature of the failure process in the composites such as fiber breakage, matrix crazing, matrix debonding and delamination etc.

Machinability characteristics evolution of CFRP in a continuum of fiber orientation angles

ABSTRACTCircular orthogonal milling tests on unidirectional carbon fiber reinforced polymer (UD-CFRP) laminate disks were conducted to investigate the machinability characteristics evolution in a continuum of fiber orientation angles (β). Experimental results show that surface cavities are the main machined surface damages, which elevate the surface roughness Ra. The surface cavities systematically appear between angles β = 25° and β = 75° for down-milling and β = 20° and β = 50° for up-milling and their occurrence may not be influenced by the cutting velocity and the feed rate. In the surface cavity region, low-level cutting forces are generated, which is highly correlated with the occurrence of fiber–matrix debonding for both down-milling and up-milling. SEM micrographs show that the formation of the surface cavities is mainly caused by the occurrence and propagation of fiber–matrix debonding, followed by bending-induced fiber fractures and compression-induced fiber fractures. Based on those observations, a novel milling strategy taking the fiber orientation angle into consideration is proposed to avoid the surface cavities; this unique approach provides a new way to milling typical aerospace CFRP laminates with high machined surface quality.

Formulation of off-axial interfacial debonding and sliding problem by constrained conditional finite element method

In ceramics matrix composites (CMCs), fiber-matrix interfacial debonding and sliding are the main toughening mechanisms. An interfacial debonding and sliding problem was formulated in this study, using the constrained conditional finite-element method (CC-FEM). In this formulation, the equivalence of nodal displacements at the interface and the equilibrium of contact forces are assumed as constrained conditions in which Coulomb’s law of friction is taken into account. As a distinguished advantage, numerical solutions are obtainable by a single calculation without an iterative algorism. We earlier treated a case in which fibers were oriented along the loading direction. In actual CMCs, however, fibers are not necessarily oriented along the loading direction. The fiber diameter also fluctuates along the axis. In this study, therefore, the off-axial interfacial debonding and sliding problem based on CC-FEM was formulated. Its validity was discussed by comparison with ANSYS. In both cases of on-axis and off-axis, the resultant fiber and matrix stress distributions agreed well with those of ANSYS. Comparison between on-axis and off-axis cases showed that the matrix stress in the latter recovered more steeply because of the higher equivalent friction coefficient.

Impact performance of two bamboo-based laminated composites

The present work aims to determine the impact performance of two bamboo-based laminated composites [bamboo/poplar laminated composite (BPLC) and bamboo/glass fiber laminated composite (BGFLC)] using low-velocity impact tests by a drop tower. In addition, fracture characteristics were evaluated using computed tomography (CT). Results showed that BPLC presented better impact properties in both directions than BGFLC. Three stages are noted in impact load–deflection curves. The load–deflection curve characteristics of two composites are different in different stages. Matrix cracking, fiber-matrix interface debonding and delamination, and fiber breakage are the three main fracture mechanisms of two composites. Structural characteristics of the components and bonding strength are the important factors for impact properties and fracture mechanism of both bamboo-based laminated composites.

Long‐term durability of a water‐contaminated quartz‐reinforced bismaleimide laminate

The long‐term structural viability of a six‐ply water‐contaminated quartz fiber‐reinforced bismaleimide laminate is investigated via dynamic mechanical analysis and flexural strength assessment over a four‐year experimental time frame. Water contamination is achieved via immersion in water at 25°C for short‐term (2 weeks and 1 month), long‐term (6 months), and very long‐term (4 years) duration. Long‐term moisture uptake exhibits distinctly non‐Fickian behavior. Maximum moisture content exceeds 1.5% by weight after four years of immersion. Laminates exhibit a remarkable resistance to degradation for all exposure durations. Flexural strength decreased by roughly 5% in the worst case. Further, no significant change in glass transition temperature was observed. Scanning electron microscopy revealed no micro‐crack formation and a relatively low incidence of fiber–matrix debond. Experimental evidence suggests that quartz fiber‐reinforced bismaleimide is a viable option for long‐term moisture‐prone structural applications at moderate temperatures. POLYM. COMPOS., 39:2643–2649, 2018. © 2016 Society of Plastics Engineers

An Investigation of burrs, chip formation, hole size, circularity and delamination during drilling operation of GLARE using ANOVA

While there has been a significant amount of research on drilling composite-metal stacks, limited work has been carried out on the machinability of fibre metal laminates (FMLs) used in aerospace structures. Challenges faced in drilling FMLs include those which exist in metals and composites such as delamination, chip and burr formations. The aim of this work is to extend the knowledge of machining fibre metal laminates through the assessment of twist drilling operations in order to improve workpiece quality. The current work presents an experimental study to analyse the effects of drilling parameters (spindle speed and feed rate) on hole quality in two grades of GLARE (2B & 3). The evaluation includes inspecting the hole size, circularity error, entry and exit burrs, chip formations and damage described at the macro level (delamination area) using computerised tomography CT scan, and at the micro level (fibre matrix debonding, chipping, adhesions, cracks) using scanning electron microscopy (SEM). In addition, the results are statistically analysed using analysis of variance (ANOVA) to determine the contribution of cutting parameters on investigated hole quality parameters.

Occurrence and formation mechanism of surface cavity defects during orthogonal milling of CFRP laminates

Carbon fiber reinforced polymer (CFRP) has been commonly used in the aviation industry and related machining demands are increasing. During machining CFRP, severe machining defects, such as delamination, burrs and subsurface damages etc., usually occur due to the prominent anisotropy and heterogeneity of CFRP. Surface cavity defects are a major manifestation of subsurface damages leading to the decline of the machined surface quality. This paper aims to investigate the occurrence and formation mechanism of the surface cavity defects in a continuum of fiber cutting angles (θ) ranging from 0°–180° by circular orthogonal milling of unidirectional CFRP laminate disks. The experimental results showed that the surface cavities only occur on 50–60% of the against fiber cutting region (0° < θ < 90°). It was found that the surface cavities were mainly caused by the occurrence and propagation of fiber-matrix debonding followed by bending-induced fiber fractures and shear-induced fiber fractures. Results also showed that the surface cavities could be avoided by employing “inclination milling” strategy during edge trimming of typical multidirectional CFRP laminates.

A numerical micro-mechanical study of the influence of fiber–matrix interphase failure on carbon/epoxy material properties

A finite element micromechanical study of unidirectional carbon–epoxy system is performed in order to investigate the role of fiber–matrix debonding in the degradation of mechanical properties and in the onset of failure for this class of composite materials.The presence of interphase flaws, that can be induced during the manufacturing processes, into micro-scale FE models is obtained by means of an original damage injection technique developed by the authors. The fibers are considered as transversally isotropic solids and the matrix is modeled as an isotropic, elasto-plastic, material with damage.The effect of fiber–matrix debonding is analyzed by means of a quasi 3-D unitary cell with a single fiber, with periodic boundary conditions, for different loading cases. Subsequently, multi-fiber representative volume elements are investigated with the same boundary and loading conditions. Finally, the effect of a 3-D debonding propagation is studied via single fiber model with an increased fiber-wise depth.

Analysis of the damage mechanisms in mixed-mode delamination of laminated composites using acoustic emission data clustering

In this study, acoustic emission (AE) technique is used to investigate different time-to-failure mechanisms of delamination in glass/epoxy composite laminates. Woven and unidirectional layups were subjected to the double cantilever beam, end notch flexure, and mixed-mode bending tests and the generated AE signals were captured. Discrimination of the AE events, caused by different types of the damage mechanisms, was performed using wavelet packet transform (WPT) and fuzzy clustering method (FCM) associated with a principal component analysis (PCA). The FCM and WPT analyses identified three dominant damage mechanisms. Furthermore, different interface layups and different GII/GT modal ratio values (ratio of mode II strain energy release rate per total strain energy release rate) indicated different time-to-failure mechanisms incidence. Additionally, the damaged mechanisms were observed using scanning electron microscopic (SEM) analysis. The results showed that the dominant damage mechanisms in all the specimens are matrix cracking and fiber–matrix debonding. Besides, some fiber breakage appeared during the tests, and the percentage of this damage mechanism in the unidirectional specimens and mode I condition was higher than those in the woven specimens and mode II. SEM observations were also in good agreement with the obtained results. It was found that the presented methods can be utilized to improve the characterization and discrimination of damage mechanisms in the actual occurring modes of delamination in composite structures.

Micromechanical damage analysis in laminated composites with randomly distributed fibers

Damage mechanisms in uni-directional fiber-reinforced plastics have been studied at the micro-scale by developing three-dimensional-repeating unit cells with randomly distributed fibers. Three damage mechanisms have been considered, viz., matrix damage, fiber failure and fiber-matrix interface debonding. The development of these damage modes and their effect on the overall stress–strain response of the micro-structure due to varying fiber volume fractions, loading conditions (longitudinal, transverse and combined transverse tension and shear), spatial distribution of fibers and fiber strength distributions (deterministic and Weibull) are of interest. A numerical framework has been developed that allows for conducting such studies for the chosen parameters. Microscopic images of real micro-structure of uni-directional fiber-reinforced plastics have been used to develop the random micro-structure for the three-dimensional-repeating unit cell. In the longitudinal loading scenario, fibers with Weibull strength distribution are closer to reality. While the three-dimensional-repeating unit cell under longitudinal loading fails in predominantly fiber failure mode, debonding and matrix damage play a major role in determining the average response of the micro-structure under transverse and combined transverse and shear loading.

Hierarchical analysis of the degradation of fibre-reinforced polymers under the presence of void imperfections.

The subject of this work is the investigation of the influence of voids on the mechanical properties of fibre-reinforced polymers (FRPs) under compression loading. To specify the damage accumulation of FRPs in the presence of voids, the complex three-dimensional structure of the composite including voids was analysed and a reduced mechanical model composite was derived. The hierarchical analysis of the model composite on a micro-scale level implies the description of the stress and strain behaviour of the matrix using the photoelasticity technique and digital image correlation technology. These studies are presented along with an analytical examination of the stability of a single fibre. As a result of the experimental and analytical studies, the stiffness of the matrix and fibre as well as their bonding, the initial fibre orientation and the fibre diameter have the highest impact on the failure initiation. All these facts lead to a premature fibre-matrix debonding with ongoing loss of stability of the fibre and followed by kink-band formation. Additional studies on the meso-scale of transparent glass FRPs including a unique void showed that the experiments carried out on the model composites could be transferred to real composites. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

Comparative assessment of the fire structural performance of carbon-epoxy composite and aluminium alloy used in aerospace structures

An experimental investigation is presented comparing the fire structural survivability of two important types of aerospace materials: carbon fibre-epoxy composite and aluminium alloy. The thermal response, softening rate, deformation behaviour and structural survivability for the two materials are compared when exposed to a thermal flux. Simulated fire structural tests are performed on quasi-isotropic carbon-epoxy composite and aluminium alloy (AA2024-T3) involving one-sided unsteady-state heating and constant tension loading. When exposed to the same radiant thermal flux, the surface and internal temperatures of the composite and aluminium alloy were different due to differences in their thermal conductivity as well as fire-induced damage (e.g. delamination cracks, fibre-matrix debonding) and decomposition to the composite. Under tensile loading, the aluminium deformed when exposed to fire via elastic, plastic and creep-induced softening processes whereas the composite deformed at a much slower rate due to matrix softening and decomposition. For the experimental test conditions, the tensile load-bearing capacity of the composite was superior to the aluminium alloy when exposed to the thermal flux, and this was due to the capacity of the load-bearing carbon fibres to retain high tensile stiffness and strength to much higher temperatures.

Friction spot joining (FSpJ) of aluminum-CFRP hybrid structures

The employment of various materials (such as lightweight metal alloys and composites) with distinct physicochemical properties in the automotive and aerospace industries has opened a new field of research into the joining of dissimilar materials. Several alternative methods have recently been developed for joining metal-composite multi-material structures. Friction spot joining (FSpJ) is an innovative technique within welding-based joining technologies suitable for metal-composite structures. This work aims to address and overview different aspects of FSpJ. Case-study overlap joints using aluminum alloy AA2024-T3 and carbon-fiber-re-inforced poly(phenylene sulfide) (CF-PPS) were produced. Peak temperatures of up to 474 °C were recorded during the process. Such temperatures are well below thermal decomposition of PPS, and extensive thermal degradation of PPS was not detected by thermal analysis in this work. Microstructure analysis was performed showing usual metallurgical phenomena (recovery and dynamic re-crystallization) taking place with friction-based aluminum joining. Microstructural changes caused an alteration to the local mechanical properties as confirmed by microhardness and nanohardness measurements. Moreover, microstructural analysis of the composite part revealed the formation of a small number of volumetric defects such as pores and fiber-matrix debonding. Bonding mechanisms at the interface were studied into details by microscopy analysis and X-ray photoelectron spectroscopy. The influence of various aluminum surface pre-treatments on the bonding mechanisms and mechanical performance of single-lap shear joints was studied. In addition, fatigue life of the joints was investigated using an exponential model to obtain S-N curves. Finally, the quasi-static strength of the friction spot joints was compared with the state-of-the-art adhesive bonding. Friction spot joints showed 50 % stronger joints than adhesively bonded joints, indicating the potential of the technique to be used for joining lightweight metals to composite materials.

Fiber-Matrix Interfacial Behavior of Hooked-End Steel Fiber-Reinforced Concrete

AbstractConcrete has inherent low tensile strength and fails at low tensile strains. These drawbacks have been bypassed by the introduction of discontinuous randomly distributed fibers to the concrete. The fiber-matrix interfacial behavior is of critical importance to the properties of fiber-reinforced concrete (FRC), which is usually assessed by fiber pullout tests. In this paper, a rigorous pullout test setup was developed with 50-mm-diameter × 100-mm-height cylindrical concrete specimens and utilized to investigate fiber-matrix interfacial behavior of hooked-end steel FRC with two fiber sizes and various embedment depths. The relation of fiber pullout load-end slip displacement and its average curves established the efficiency of the utilized fiber pullout test setup concerning the repeatability and reliability. Four components of the fiber matrix interactions were observed for hooked-end steel fibers. These include the elastic fiber-matrix bond, partial fiber-matrix debonding, full fiber-matrix debond...

High strain-rate failure in carbon/Kevlar hybrid woven composites via a novel SHPB-AE coupled test

Abstract The high strain-rate induced failure characteristics in a carbon/Kevlar hybrid composite subjected to high strain-rate compressive loading were studied using a novel SHPB-AE coupled test. The tests were performed based on a split-Hopkinson pressure bar (SHPB) apparatus and an acoustic emission (AE) technique, respectively. Within the strain-rate range of 1002–1941 s −1 , a high strain-rate compressive test was conducted on the cylindrical carbon/Kevlar hybrid composite specimens located between the incident and transmitted bars. A wide bandwidth type AE sensor was connected to the specimen with a fine copper waveguide to monitor the AE signals in real time during the test. Specific types of failure mechanisms were observed with optical microscopy and scanning electron microscopy. First, AE characteristics originating from the specimens were investigated profoundly to distinguish the AE signals from diverse damage or failure sources. AE signals were then analyzed in terms of the AE amplitude, the AE count, the slope of cumulative AE count and the peak frequency. Finally, signals were classified into four types based on the waveform and corresponding peak frequency by a short time Fourier transforms (STFT). As a result, under high strain-rate compressive loading, the strain level at the damage initiation was shortened with increasing strain-rate. The failure process of the carbon/Kevlar hybrid woven composite showed initial matrix fracture and then brittle carbon fiber breakage. Subsequently, multiple failure mechanisms appeared, such as fiber-matrix debonding, fiber pull-out, excessive deformation and breakage in the Kevlar fiber tips including splitting and fibrillation. The application of the novel SHPB-AE coupled test to the carbon/Kevlar hybrid composite was discussed in depth for characterizing the failure process, and it was an effective and relevant methodology to grasp in situ information on the failures under high strain-rate compressive loading.

Micromechanical Modeling of Damage Development in Polymer Composites

Damage initiation and progression in long fibre unidirectional continuous polymer composites has been studied at the micro-scale considering a three dimensional repeating unit cell (3D-RUC) with square packing consisting of a single fibre in a polymer matrix. Three damage modes under static loading have been looked at, viz., matrix damage, fibre failure and fibre-matrix debonding. A progressive damage model for the matrix, fibre breakage model using maximum stress failure criterion and interface debonding using a traction-separation criterion via cohesive zone modelling (CZM) approach has been implemented. Homogenization of the said 3D-RUC has been conducted for various load cases that describes the averaged response of the microstructure under combined progressive damage modes.

An experimental investigation into multi-scale damage progression in laminated composites in bending

In laminated composite materials fibre–matrix debonding, as an initial damage mechanism, can initiate a damage sequence that can result in catastrophic failure of its structure by promoting intermediate damage mechanisms. This paper presents an in-depth experimental investigation into each of these damage mechanisms and how they transition from one state to the next, beginning at the micro-scale with fibre–matrix debonding and crack coalescence, to transverse ply fracture at the meso-scale through to formation of macroscopic delamination.In-situ SEM micro-mechanical testing is used to determine the onset of the aforementioned damage mechanisms and to follow their progression in laminates of both [0/90]s and [90/0]s stacking sequences. The damage progression of [0/90]s specimens is presented first, followed by the more progressive failure of [90/0]s specimens, yielding an in-depth analysis of both rapid and more progressive damage growth, respectively. The intralaminar cracking and delamination of [0/90]s laminates was found to be instantaneous and provided limited opportunity to characterise damage progression. For [90/0]s laminates, damage progression was much more progressive and various factors such as fibre positioning were shown to influence debonding initiation and crack path development before catastrophic failure and so these laminates form the bulk of the analysis presented for this paper.

Mechanics of fibre deformation and fracture in vibration-assisted cutting of unidirectional fibre-reinforced polymer composites

Fibre orientation significantly affects the surface integrity of the machined fibre-reinforced polymer (FRP) composites. Crushing or bending-dominated fibre fracture in the machining of FRPs is governed by fibre orientation relative to the cutting direction. This paper revealed the mechanics behind such phenomena and developed a reliable predictive model for cutting FRPs. The analysis integrated the effect of the whole fibre orientation range from 0° to 180°. Both traditional cutting and elliptic vibration-assisted (EVA) cutting of unidirectional FRPs were analysed by integrating microstructure-based and equivalent homogeneous deformation zones. A systematic experiment was also carried out. A comprehensive comparison of the mechanics predictions with the experiments shows that the model has captured the major material removal mechanisms and can be used to predict fibre deformation, fibre–matrix debonding, fibre fracture, subsurface damage and cutting forces in both traditional and vibration-assisted cutting. It was identified that fibre orientation affects subsurface damage and cutting forces significantly, and that EVA cutting can minimise the orientation effect. There is a critical fibre orientation at which subsurface damage and cutting force in feed direction would reach their worst situations.

Effects of accelerated aging on mechanical, thermal and morphological behavior of polyurethane/epoxy/fiberglass composites

Wind blades, an important application of polymeric composite materials, are subject to natural weathering. This study aims to evaluate mechanical, thermal and morphological behavior during accelerated aging in three thicknesses of epoxy and fiberglass polyurethane-coated composite plates used in wind turbines, in addition to testing with two acoustic emission techniques. An accelerated aging chamber simulated natural weathering mechanisms for 45, 90, 135 and 180 days. This degradation primarily reduced the mechanical properties of the thinner composites, with some damaged specimens exhibiting fiber-matrix debonding. Thermal properties deteriorated. There were no morphological changes on the polyurethane–epoxy interface; however, degradation occurred in the fiber-matrix interface on the surface exposed to radiation. The degree of chalking indicated coating deterioration on the external surface of the polyurethane. The acoustic wave propagation speed and attenuation coefficient measured prior to mechanical testing indicated the presence of damage areas.