Fiber Push-out Research Articles

Evaluating Residual Stress in Carbon Fiber-Reinforced Polymer (CFRP) at Microscale Using Fiber Push-Out Experiment and Finite Element Modeling.

Microscale residual stress may develop during the manufacturing of Carbon Fiber-Reinforced Polymer (CFRP) composites and negatively affect apparent macroscale mechanical properties. Accordingly, accurately capturing residual stress may be essential in computational methods used for composite material design. This work presents a new data-driven methodology for the evaluation of microscale residual stress in CFRPs using fiber push-out experiments with in situ scanning electron microscopy (SEM) imaging. SEM images reveal significant through-thickness matrix sink-in deformation in resin-rich areas after nearby fibers are pushed out, which is attributed to the release of microscale process-induced residual stress. The sink-in deformation is measured experimentally, and a Finite Element Model Updating (FEMU) method is used to retrieve the associated residual stress. The finite element (FE) analysis includes simulation of the curing process, test sample machining, and fiber push-out experiment. Significant out-of-plane matrix deformation larger than 1% of the specimen thickness is reported and associated with a high level of residual stress in resin-rich areas. This work emphasizes the importance of in situ data-driven characterization for integrated computational materials engineering (ICME) and material design.

A fracture mechanics analysis of the micromechanical events in finite thickness fibre push-out tests

Understanding the micromechanical events of interfacial failure in fibre reinforced composites is vital to accurately characterising micromechanical properties and, consequently, the macroscopic properties of the composite. A fracture mechanics model of the fibre push-out test is developed, with an emphasis on the effect of sample thickness and residual stresses on the mechanisms of interfacial crack advancement. The model is applied to both a SiCf-SiC ceramic matrix composite and a SiCf-Ti metal matrix composite. The model demonstrates that previous assumptions about the micromechanical events of interfacial cracking are consistent with the measured values of interfacial fracture energy for ceramic matrix composites. Moreover, the model can identify the range of geometries for which different micromechanical cracking mechanisms occur simultaneously in a given material system. Identifying this range is important in choosing the sample geometry for fibre push-out testing because the interaction of advancing cracks affects the measurement of interfacial fracture energy by classical models.

Effect of the interplay between fiber surface chemistry and sizing reactivity on fiber matrix interaction in carbon fiber reinforced epoxy resin

ABSTRACT A comprehensive picture of the interaction between fibers and matrix in carbon fiber–reinforced epoxy resin is given. We present new data on fiber wetting by a resin, interphase formation, and fiber matrix adhesion and develop a model for fiber matrix interaction, considering earlier studies on fiber surface chemistry and interaction with sizings. The effect of surface chemistry is investigated by oxygen functionalization of carbon fibers by anodic oxidation utilizing two electrolytes. To address the influence of sizing reactivity, two epoxy-based sizings are applied with different number of epoxide groups per molecule. The fiber matrix adhesion is investigated by single fiber push-out tests. The resulting interfacial fracture toughness of functionalized fibers in epoxy resins depends on the fiber surface chemistry. Based on quantitative measurements, an influence of surface topography on the fiber matrix adhesion can be ruled out. Application of a standard sizing does not change the composite’s interfacial fracture toughness compared to the unsized case. In contrast, use of a reactive sizing generally increases fiber matrix adhesion. The systematic variation of interfacial fracture toughness is discussed based on the number of bonds formed between fiber and resin. Bonding between hydroxyl surface functionalities and epoxy groups of resin or sizing is assumed to dominate.

Enhancement and Evaluation of Interfacial Adhesion between Active Screen Plasma Surface-Functionalised Carbon Fibres and the Epoxy Substrate.

This paper investigated the modification of the advanced active screen plasma (ASP) technology on PAN-derived carbon fibres (CFs) with gas mixtures of N2-H2 and N2-H2-Ar, separately. A more-than-30% improvement was found in the interfacial shear strength (IFSS) between the modified CFs and the epoxy substrate in the resulting composites, as disclosed by single fibre push-out tests. Based on the study of surface morphology, surface chemistry and water-sorption behaviour, the interfacial adhesion enhancement mechanisms were attributed to (1) the increased chemical bonding between the introduced functional groups on the fibre surface and the matrix; (2) the improved surface hydrophilicity of CFs; and (3) the enhanced van der Waals bonding due to the removal of surface contaminations.

Micromechanical properties of vapour-exposed SiCf/BN/SiC ceramic-matrix composites

SiCf/BN/SiC Ceramic-Matrix Composites are candidate materials for aero-engines, but their interphase stability after potential low-temperature water-vapour exposures during flight cycles is not well known. The examination of these composites exposed for 50, 250 and 500 h at low temperature (65 °C) and 95% relative humidity was therefore performed, in order to understand if resulting oxidation products affected the mechanical properties at the micro-scale. The composites were subject to fibre push-out tests to compare pristine from degraded composites. It was found that whilst the sample exposed for 50 h had no significant change from pristine, the samples exposed for 250 and 500 h had a clear decrease in interfacial shear strengths measured. Parallel studies also revealed that whilst damage was strongly localised, the diffusion of water within the composite was not fully complete at exposure times between 50 and 250 h. The permeability of the CMC was shown to be affected at longer exposure times where differences in mechanical performances even between tows and within tows were noted.

Design, Analysis and Experiment of the Fiber Push-Out Device Based on Piezoelectric Actuator.

In this study, a fiber push-out device based on a piezoelectric actuator was designed, analyzed and tested, and its experimental environment was designed. The piezoelectric actuator includes a flexible beam. By using response surface analysis (RSM), taking the large displacement as the objective function and on the premise of meeting the strength requirements, the structural parameters of the flexible beam were analyzed. In the process of fiber push-out, the interfacial shear stress was estimated by establishing the system integrating the fiber-matrix-composite three-phase model and the piezoelectric actuator model using the analytic method, and the theoretical analysis results of the fiber interface mechanical properties were given. A prototype of the system was made, and the performance tests of the piezoelectric actuator and the fiber push-out device were carried out. The test results showed that the designed piezoelectric actuator can achieve a stepping resolution of 6.67 μm and a maximum displacement of about 100 μm at the input voltage of 150 V, which is consistent with the design results. The extrusion test of a single fiber was carried out using a piezoelectric actuator. The mechanical properties of the interfacial layer during the push-out process were measured and the interfacial shear strength was calculated, which is consistent with the theoretical analysis results. Finally, based on the mechanical properties obtained from the test, the loading failure process of the fiber was simulated by finite element analysis, which well explained the failure process of the fiber, thus verifying the feasibility of the designed fiber push-out device.

Calibrating a fiber–matrix interface failure model to single fiber push-out tests and numerical simulations

To characterize the fiber–matrix interface of a glass-fiber reinforced sheet molding compound (SMC), single-fiber push-out tests are performed and simulated numerically. The parameters of a cohesive zone model for the interface are calibrated on the single-fiber push-out tests. The fracture-toughness/energy release rate therein is determined from cyclic (loading–unloading) experiments. The matrix model, consisting of the nonlinear-elastic Neo-Hooke law with a Prony series to model viscoelastic behavior, is calibrated with data from nanoindentation tests by adjusting simulation curves to their experimental counterparts. Using the calibrated model of the single-fiber push-out, the influence of neighboring fibers and thermally induced residual stresses is shown. The interface damage initiates in the single-fiber push-out test at the indented fiber at positions closest to other fibers under the surface. In addition this is the position where the radially largest fiber expansion due to the Poisson effect is found. The results reveal that although the push-out test is simple to perform, the interpretation of its results might be a complicated task.

Improvement of interfacial compatibility of SiCf/Ti-6Al-4V composites by applying fiber coating and heat treatment

A new approach to improving the interfacial compatibility of SiCf/Ti-6Al-4V compositesby using fiber coating coupled with subsequent heat treatment was presented. The SiCf/Ti-6Al-4V composites with C-coated and C/Mo-coated fibers were prepared using the foil-fiber-foilmethod and subsequently heat treated in vacuum at 750 °C to obtain several kinds of the SiCf/Ti-6Al-4V composites with different interfacial characteristics. Interfacial microstructures of these composites were systematically characterized by means ofscanning electron microscopy and X-ray energy dispersive spectroscopy to investigate the interfacial microstructure evolution and reaction dynamics. The effects of interfacial microstructures oninterfacial mechanical propertieswere also investigated using thin-slice fiber push-out tests. The results show that the brittle interfacial reaction layer of TiC in theC-coated SiCf/Ti-6Al-4V composites was obviously thickened and some microvoids even formed near the matrix with increasing heat treatment durations, whereas the matrix adjacent to Mo coating in the C/Mo-coated SiCf/Ti-6Al-4V composites gradually transformed into a ductile β-Ti layer. Interfacial shear strengths of the C/Mo-coated composites were slightly increased with increasing the heat treatment time, while those of theC-coated composites were remarkably improved. Critical issues onmodifying interfacial compatibility of the SiCf/Ti composites for further improving the mechanical behavior of the composites were discussed.

Reducing the raw material usage for room temperature infusible and polymerisable thermoplastic CFRPs through reuse of recycled waste matrix material

In this work, a closed loop recycling process is investigated, which allows polymerised bulk thermoplastic matrix (Elium 150) from production waste (also referred to as recyclate) to be reused as additive in composite manufacturing by vacuum assisted resin infusion (VARI) of virgin Elium 150 monomer. It is shown that this process can save up to 7.5 wt% of virgin material usage in each processing cycle. At the same time, the thermal stability and stiffness of the composite increases with the proportion of recyclate introduced. Contemporarily, the shear and bending properties have also been observed to improve. Gel permeation chromatography (GPC) showed that the changes observed are due to an increase in molecular weight with the recyclate content. In particular, a correlation between the molecular weight and the shear properties of the composite was discovered using single fibre push-out tests.

A novel trench fibre push-out method to evaluate interfacial failure in long fibre composites

Traditional fibre push-outs for the evaluation of interfacial properties in long fibre ceramic matrix composites present their limitations—solutions for which are addressed in this work by introducing the novel trench push-out test. The trench push-out makes use of a FIB milling system and an SEM in-situ nanoindenter to probe a fibre pushed into a trench underneath, allowing in-situ observations to be directly correlated with micromechanical events. SiCf/BN/SiC composites—candidate material for turbine engines—were used as model materials in this work. Different fibre types (Hi-Nicalon and Tyranno type SA3) were coated with BN interphases, presenting mean interfacial shear stresses of 14 ± 7 MPa and 20 ± 2 MPa, respectively, during fibre sliding. The micromechanical technique enabled visualisation of how defects in the interphase (voids, inclusions & milled notches) or in the fibre (surface asperities, non-uniform coatings) affected the variability of interfacial property measurement.Graphic abstract

On stability and damage behavior of asymmetric sandwich panels under uniaxial compression

In practical applications, asymmetric sandwich panels with tapered and junction regions are frequently used. Under uniaxial compressions, asymmetric sandwich panels are collapsed with complex failure mechanism, and its mechanical behaviour is affected by tapered regions. For structure safety consideration, it is necessary to systematically investigate the stability and damage behavior of full-scaled asymmetric sandwich panels under uniaxial compression experimentally and numerically. For comparison, symmetric sandwich panels were also studied. Quasi-static compression tests and numerical modeling were conducted. Experimental data were sufficiently recorded and carefully analyzed. Meanwhile a nonlinear finite element model was advanced for evaluating stability and predicting failure behaviours and load-bearing capacity of test specimen. Comparisons between numerical predictions and experimental measurements showed that a good agreement was achieved. The tapered region had significant influence on stability behavior and load-bearing capacity. It intensified the asymmetry and introduced additional bending moments into asymmetric sandwich panels and thus caused early local buckling. Thicker core thickness enhanced the structural stiffness and load-bearing capacity. Large amounts of matrix cracking, fibre push-out and core-skin delamination were captured in asymmetric sandwich panels. Overall, buckling occurred in symmetric sandwich panels instead of local buckling in asymmetric sandwich panels, and brittle fibre breakage dominated the failure.

Fiber push-in failure in carbon fiber epoxy composites

Micromechanical quasi-static and cyclic fiber push-in tests performed with an indenter in-situ in a scanning electron microscope on polished slices (thickness around 300 μm) of carbon-fiber epoxy composites provide information on the effect of fiber density on the fiber-matrix debonding and the fiber failure. Specifically, "close-packed", i.e., fibers surrounded by six nearest neighbor fibers (called "hexagonal" hereafter) versus "isolated" fibers, defined as fibers at least one fiber diameter distant from the next nearest fiber, were compared. In spite of the more compliant behavior of isolated fibers under quasi-static and cyclic indentation loads, initiation of fiber-matrix debonding and push-in failure (fiber splitting into two or three parts) occurred at roughly comparable loads for both, "hexagonal" and "isolated" fibers. Earlier tests on thinner slices of such composites (thickness around 30 μm) had resulted in full debonding and fiber push-out without fiber failure. The fibers in the thicker slices showed first indenter imprints on the surface and an increasing hysteretic behavior in the load-displacement curves before debonding. The analysis of the cyclic load-displacement curves yields the elastic-plastic and the hysteretic (debonding) energy contributions.

Interfacial Reactions and Mechanical Properties Studies of C-Coated and C/B4C Duplex-Coated SiC Fiber-Reinforced Ti2AlNb Composites.

Continuous SiC fiber-reinforced Ti2AlNb matrix composites have a great potential for high-temperature aviation structure applications, and their properties strongly depend on the microstructure of the interfacial reaction layer. Notably, introducing diffusion barrier coatings has still been a popular strategy for optimizing the interfacial structure and interfacial properties of SiCf/Ti. In this work, C coating and C/B4C duplex coating were successfully fabricated onto SiC fibers via chemical vapor deposition (CVD), then consolidated into the SiCf/C/Ti2AlNb and the SiCf/C/B4C/Ti2AlNb composites, respectively, via hot isostatic pressing (HIP) under the condition of 970 °C, 150 MPa, 120 min, and finally furnace cooled to room temperature. The C- and C/B4C-dominated interfacial reactions in the SiCf/C/Ti2AlNb and the SiCf/C/B4C/Ti2AlNb were explored, revealing two different reaction products sequences: The different-sized TiC and the coarse-grained (Ti,Nb)C + AlNb3 for the SiCf/C/Ti2AlNb; and the fine-grained TiB2 + TiC, the needle-shaped (Ti,Nb)B2/NbB + (Ti,Nb)C, the coarse-grained (Ti,Nb)C + AlNb2 for the SiCf/C/B4C/Ti2AlNb. Annealing experiments were further carried out to verify the different reaction kinetics caused by C coating and C/B4C duplex coating. The reaction layer (RL)-dominated interfacial strength and tensile strength estimations showed that higher interface strength and tensile strength occurred in the SiCf/C/Ti2AlNb instead of the SiCf/C/B4C/Ti2AlNb, when the same failure mode of fiber push-out took place.

Investigation of the fiber-matrix interaction in carbon fiber-reinforced polyether ether ketone by cyclic single fiber push-out and push-back tests

ABSTRACTTo characterize the fiber-matrix interaction of carbon fiber-reinforced polyether ether ketone, a high-performance thermoplastic composite material, micromechanical cyclic single fiber push-out and push-back tests were performed. A new type of successive push-out behavior was observed in addition to the well-known abrupt fiber push-out. To determine the interfacial fracture toughness from the successive push-out load-displacement curves, an advanced energy-based method was developed. It was found that the two different types of push-out correspond to two different values of interfacial fracture toughness. Thermal expansion measurements indicate that residual thermal stress within the investigated sample induces the two different push-out behaviors. The findings suggest that the fiber-matrix interaction might be one reason for the considerable impact of thermal stress on interface dominated macroscopic parameters.

Microstructure and mechanical performance of SiCf/BN/SiC mini-composites oxidized at elevated temperature from ambient temperature to 1500 °C in air

SiCf/BN/SiC mini-composites comprising single tow SiC fibre-reinforced SiC with chemical vapour deposited (CVD) BN interface layers were fabricated. The mechanical performance and binding strength of the composites and the fibre/interface for the oxidized SiCf/BN/SiC mini-composite samples (oxidation at 1000, 1200, 1300, 1400 and 1500 °C in air for two hours) were investigated by tensile tests and fibre push-out tests, respectively. The value of oxidation mass change was also measured. Some unusual phenomena for the SiCf/BN/SiC mini-composites oxidized at 1000 °C were discovered in this work. The tensile strength reached a maximum value, and the mass gain rate showed as a singular negative value, while the shear strength between the fibre and the matrix was moderate. Scanning electron microscopy, energy dispersive spectrometry, infrared spectroscopy and X-ray diffraction characterization methods were used to reveal the microstructural evolution and investigate the unusual phenomenon during oxidation procedures. This work will provide guidance for predicting the service life of SiCf/BN/SiC composite materials and may enable these materials to become a backbone for thermal structure systems in aerospace applications.

Metamaterials with Poisson’s ratio sign toggling by means of microstructural duality

Neither positive nor negative Poisson’s ratio materials are fully advantageous across a wide range of load bearing applications. In some cases, it is advantageous for structures to possess duality in material properties so as to take advantage of auxetic behavior under one loading condition as well as conventional behavior when the loading condition changes. This paper proposes two metamaterials—the hybrid rhombic–re-entrant metamaterial and the hybrid kite-arrowhead metamaterial—in which the prescription of either compressive or tensile load always leads to lateral expansion. Specifically, the Poisson’s ratio of these metamaterials range from negative infinity to zero upon tensile load, and from zero to positive infinity upon compressive load. This duality of auxetic and conventional behavior for each of these metamaterial is achieved by employing a simultaneous lock and slide mechanism that is interchangeable upon load reversal. When loaded in one direction, the locked part takes effect while the sliding part becomes redundant. Upon load direction reversal, the previously locked and sliding parts convert to sliding and locked parts, respectively, thereby activating the redundant part while deactivating the functional part. The change in effective microstructural components, therefore, facilitates Poisson’s ratio sign toggling with load reversal. Due to the indiscriminate lateral expansion regardless of the axial loading direction, such behavior in a fiber is useful to resist both fiber pull-out as well as fiber push-out from the matrix material.

Use of interphase in geopolymer matrix composites for improved toughness

Geopolymers are a lightweight material with excellent fire and thermal resistance. When combined with continuous fiber reinforcement, geopolymers demonstrate potential to replace heavier, structural components, especially those that are exposed to elevated temperature regimes (> 400 °C) in air. Oxidation resistance is essential to continuous operations in these environments. While oxide fibers can drastically improve the mechanical properties of geopolymers, their incorporation has been met with challenges. These geopolymer matrix composites (GMC) lack the necessary toughness to find much use as a structural component. In this study, concepts and materials used to reduce interfacial strength in ceramic matrix composites for the purpose of improving toughness are employed in GMCs. To investigate the potential to improve toughness, composites containing an alumina-based fiber and geopolymer were prepared with three different interfacial conditions: heat cleaned, carbon coated, and monazite coated. From the fabricated composites, specimens for four-point flexural testing and single fiber pushout were prepared. Additionally, composite samples were heat treated to 650 °C in oxidizing and non-oxidizing environments. To varying degrees, composites that utilized a fiber coating demonstrated improvements in toughness and relative damage tolerance as compared to those without any modification to the interface. In some cases, the improvement was over 160% as compared to those without a coating. Pushout testing showed specimens with coated fiber to have weaker interfacial conditions. The combined results support the use of tailored interface concepts in GMCs for improving toughness.

Adhesion of carbon fibers to amine hardened epoxy resin: Influence of ammonia plasma functionalization of carbon fibers

The modification of carbon fiber surface properties represents a powerful tool to improve the adhesion between fibers and polymeric matrix in carbon fiber reinforced polymers. In the presented work the surface of untreated carbon fibers, taken from the production process directly after carbonization, is functionalized by a low pressure ammonia plasma treatment. Compared to the untreated fibers an enhanced concentration of surface nitrogen functionalities and an increased surface energy are observed. This leads to an improved wetting behavior, similar to that of carbon fibers activated by anodic oxidation. No changes of the fiber surface topography on micro- and on nanoscale are induced by the plasma treatment. Compared to the untreated fiber no increase of interfacial fracture toughness between the plasma treated fibers and an amine hardened epoxy resin is detected by single fiber push-out tests. Anodically oxidized fibers, in contrast, show a significant increase of interfacial fracture toughness. The results suggest, that adhesion of carbon fibers to an amine hardened epoxy resin is dominantly enhanced by interactions between surface oxygen functionalities and the components of the resin. In contrast, nitrogen surface functionalities appear to be of minor importance for fiber matrix adhesion in carbon fiber reinforced amine hardened epoxy resin.

Scalable manufacturing of carbon nanotubes on continuous carbon fibers surface from chemical vapor deposition

A novel process has been successfully developed to grow carbon nanotubes (CNTs) on continuously moving carbon fibers (CF) surface by a unique open-ended chemical vapor deposition (CVD) furnace. Systematic researches were carried out under various deposition temperatures, velocities of continuous carbon fibers and catalyst concentrations. The morphologies and structures of CNTs were investigated by field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM), which indicated clearly that carbon fibers with uniformly distributed CNTs were achieved, with the optimum parameters of 650 °C deposition temperature and 6 cm min−1 velocity of continuous carbon fibers and 0.05 M catalyst concentration, respectively. By means of the single fiber push-out tests, the interfacial shear strength (IFSS) of carbon fibers growing CNTs was increased significantly by 84.4% compared to the desized carbon fibers. Furthermore, the results of single fiber tensile test verified that there is scarcely any degradation in the mechanical properties of carbon fibers after the growth of CNTs with the optimum parameters. This study provides a new and original vision for scalable manufacturing of CNTs on continuous moving substrate, when compared to the traditional batch process.

Carbon nanotube-grafted carbon fiber polymer composites: Damage characterization on the micro-scale

Abstract Multiwall carbon nanotubes (CNTs) – carbon fibers (CFs) hybrid materials were produced by directly growing CNTs on CFs by means of chemical vapor deposition. For the latter, the oxidative dehydrogenation reaction of C2H2 and CO2 was applied, which allows growing CNTs without damaging the CF surface. Uni-directional nano-engineered carbon fiber reinforced composites (nFRCs) were fabricated by impregnating these hybrid materials with epoxy. The nFRCs subjected to single fiber push-out tests revealed a decrease of the interfacial shear strength (IFSS) of about 36% compared to the carbon fiber composites without CNTs. By means of transverse three-point bending tests performed on pre-notched composite beams inside a scanning electron microscope, the fracturing behavior parallel to the fibers was studied in-situ. The nFRCs showed significantly reduced fiber/matrix debonding while CNTs pull-out, CNTs bridging as well as matrix failure occurred. These results demonstrate that the presence of CNTs in nFRCs affects the stress distribution and consequently the damage initiation as well as the damage propagation. The presence of CNTs suppresses the stress concentration at the fiber/matrix interface and reduces the debonding of CFs from the matrix. However, our results indicate that the stress concentration shifts towards the CNTs' ends/matrix interface and causes promoted matrix failure leading to lower IFSS.