Single-fiber Push-out Tests Research Articles

Micromechanical modeling of longitudinal tensile behavior and failure mechanism of unidirectional carbon fiber/aluminum composites involving fiber strength dispersion

This paper examines the longitudinal tensile behavior and failure mechanism of a new unidirectional carbon fiber reinforced aluminum composite through experiments and simulations. A Weibull distribution model was established to describe the fiber strength dispersion based on single-fiber tensile tests for carbon fibers extracted from the composite. The constitutive models for the matrix and interface were established based on the uniaxial tensile and single-fiber push-out tests, respectively. Then, a 3D micromechanical numerical model, innovatively considering the fiber strength dispersion by use of the weakest link and Weibull distribution theories, was established to simulate the progressive failure behavior of the composite under longitudinal tension. Due to the dispersion of fiber strength, the weakest link of the fiber first fractures, and stress concentration occurs in the surrounding fibers, interfaces, and matrix. The maximum stress concentration factor for neighboring fibers varies nonlinearly with the distance from the fractured fiber. Both isolated and clustered fractured fibers are present during the progressive failure process of the composite. The expansion of fractured fiber clusters intensifies stress concentration and material degradation which in turn enlarges the fractured fiber clusters, and their mutual action leads to the final collapse of the composite.

Degradation in interfacial shear strength of carbon fiber/ vinyl ester composites due to long-term exposure to seawater using push-out tests

In this study, single fiber (∼7-μm diameter) push-out tests are conducted to evaluate hygrothermal effects on the interfacial shear strength (IFSS) of carbon fiber/vinyl ester (CF/VE) composites. Hygrothermal conditioning is achieved by saturating samples in simulated seawater at 40 °C for two years. An investigation has been conducted on the preparation, validity, and interpretation of the push-out test results. First, the authors present a polishing methodology that results in thin films of CF/VE composites in the thickness range of 15–120 μm and produces an average 41.2% drop in IFSS due to long-term hygrothermal exposure. Using scanning electron microscopy (SEM), we show that during the push-out tests, the failure initiates locally at the zone of minimum bond strength at the bottom (away from the indenter), then propagates along the length of the interface. The influence of radial tensile stresses originating due to bending is found to be negligible. Using the SEM imaging of the pushed-out fibers, we validate the failure of the interface to be the primary source of failure. The associated results are found to depend on the thickness of the interface. We then reevaluate the results using the Weibull distribution, knowing that the failure mechanism is analogous to the weakest link theory. The results show a 25.5% drop in the IFSS of the CFVE composite, measured at an infinitesimal scale due to long-term to hygrothermal conditioning at 40 °C. A significant drop in IFSS was observed after reheating above glass transition temperature (Tg) and cooling.

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.

3D printing as an automated manufacturing method for a carbon fiber-reinforced cementitious composite with outstanding flexural strength (105\xa0N/mm2)

As research interest in the additive manufacturing of cementitious materials for structural uses has been continuously increasing, the question of how to incorporate tensile reinforcement in an automated process has gained further importance. Our research describes a carbon fiber-reinforced cementitious composite produced by common extrusion techniques applied in 3D printing as a means to effectively control fiber alignment. Optimization of the mixture design and consistency allows for admixing up to 3 vol.-% chopped carbon fibers, leading to specimens that can reach a flexural strength exceeding 100 N/mm2 without the addition of further continuous reinforcement. Fiber integrity during the process was checked using optical microscopy. Analysis of the microstructure shows that approximately 70% of the fibers are aligned within ± 5° of a preferential direction. Micromechanical single-fiber push-out tests confirm an interfacial fracture toughness typical for strain-hardening systems. The first insights into a ‘lost formwork’ approach commonly employed in 3D printing show that the reinforcement remains effective even when combined with nonreinforced mortar.

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.

Investigation of interface debonding behaviour in a unidirectional SiC fibre reinforced Ti-17 composite by a single fibre push-out technique

The debonding behaviour at the fibre/matrix interface in a unidirectional continuous carbon-coated SiC fibre reinforced Ti-17 composite was investigated. Single fibre push-out testing on thin slice specimen by micro-indentation was employed to evaluate the interface debonding strength. In order to investigate interface debonding modes, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilised to observe the microstructure at the fibre/matrix interface region after the fibres were pushed out. It is found that the interface debonding strength is evaluated to be 108.2 MPa in the composite. The interface debonding occurs at the interface between the interface reaction zone (RZ) and the carbon coating on the fibre surface. The carbon coating/SiC fibre interface and the RZ/matrix interface remain intact after the fibre was pushed out.

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.

The Adhesion of Plasma Nanocoatings Controls the Shear Properties of GF/Polyester Composite.

High-performance fibre-reinforced polymer composites are important construction materials based not only on the specific properties of the reinforcing fibres and the flexible polymer matrix but also on the compatible properties of the composite interphase. First, oxygen-free (a-CSi:H) and oxygen-binding (a-CSiO:H) plasma nanocoatings of different mechanical and tribological properties were deposited on planar silicon dioxide substrates that closely mimic E-glass. The nanoscratch test was used to characterize the nanocoating adhesion expressed in terms of critical normal load and work of adhesion. Next, the same nanocoatings were deposited on E-glass fibres, which were used as reinforcements in the polyester composite to affect its interphase properties. The shear properties of the polymer composite were characterized by macro- and micromechanical tests, namely a short beam shear test to determine the short-beam strength and a single fibre push-out test to determine the interfacial shear strength. The results of the polymer composites showed a strong correlation between the short-beam strength and the interfacial shear strength, proving that both tests are sensitive to changes in fibre-matrix adhesion due to different surface modifications of glass fibres (GF). Finally, a strong correlation between the shear properties of the GF/polyester composite and the adhesion of the plasma nanocoating expressed through the work of adhesion was demonstrated. Thus, increasing the work of adhesion of plasma nanocoatings from 0.8 to 1.5 mJ·m−2 increased the short-beam strength from 23.1 to 45.2 MPa. The results confirmed that the work of adhesion is a more suitable parameter in characterising the level of nanocoating adhesion in comparison with the critical normal load.

Mechanical properties and strengthening mechanism of SiCf/SiC mini-composites modified by SiC nanowires

The effects of the SiC nanowires (SiCNWs) and PyC interface layers on the mechanical and anti-oxidation properties of SiC fiber (SiCf)/SiC composites were investigated. To achieve this, the PyC layer was coated on the SiCf using a chemical vapour infiltration (CVI) method. Then, SiCNWs were successfully coated on the surface of SiCf/PyC using the electrophoretic deposition method. Finally, a thin PyC layer was coated on the surface of SiCf/PyC/SiCNWs. Three mini-composites, SiCf/PyC/SiC, SiCf/PyC/SiCNWs/SiC, and SiCf/PyC/SiCNWs/PyC/SiC, were fabricated using the typical precursor infiltration and pyrolysis method. The morphologies of the samples were examined using scanning electron microscopy and energy dispersive X-ray spectrometry. Tensile and single-fibre push-out tests were carried out to investigate the mechanical performance and interfacial shear strength of the composites before and after oxidization at 1200 °C. The results revealed that the SiCf/PyC/SiCNWs/SiC composites showed the best mechanical and anti-oxidation performance among all the composites investigated. The strengthening and toughening is mainly achieved by SiCNWs optimization of the interfacial bonding strength of the composite and its own nano-toughening. On the basis of the results, the effects of SiCNWs on the oxidation process and retardation mechanism of the SiCf/SiC mini-composites were investigated.

Fiber reinforced metal matrix composites - a review

Modern technology has placed increasing demands on materials. The need of hour is to develop the new materials so as to overcome the limitations of the materials existed presently regarding applications in different fields. The materials with properties such as better wear resistance, high specific strength and stability at high temperatures have been in great demand in the past few decades. The properties of conventional materials can be improved by the method of reinforcing whiskers, fibers and particles in to polymers, metals and ceramics. This has resulted in the manufacturing of composite materials. The mechanical properties of fiber reinforced metal matrix composites (MMCs) make them the suitable choice for applications in the fields of automotive and aerospace etc. The paper outlines the overview of fiber reinforced MMCs. The applications, manufacturing methods and mechanical properties of these composites are discussed. Furthermore, this review describes the single fiber push-out test which is developed in the past years so as to measure quantitatively the properties of the interface between the fiber and the matrix in these fiber reinforced MMCs.

Axisymmetric finite element analysis of single fiber push-out test for stainless steel wire reinforced aluminum matrix composites

An axisymmetric cylindrical finite element (FE) model with wire-matrix interface is presented with the objective of evaluating the interfacial properties of stainless steel wire reinforced aluminum matrix composites. It consists of a wire of stainless spring steel 1.4310 (X10CrNi18-8), a matrix of aluminum alloy AA6060 and the interface. The FE model used to model the push-out test which has been developed over a number of years as a mean of quantifying the interfacial properties. The interface with a specified thickness is modeled using the cohesive layer concept where, in addition to the elastic parameters, a damage criterion based on the maximum nominal stress is also defined for the interface. Fracture energy is used to define the damage evolution which describes the rate at which material degrades, once the damage initiation criterion is met. The interface parameters are varied to adopt the model according to the experimental results reported in the past. The simulation results are in qualitative agreement with experimental results regarding the force–displacement graph. The simulation results obtained show the presence of compressive stresses at the top and tensile stresses at the bottom of the interface caused by the bending of the specimen. It indicates that, during fiber push-out test of metal matrix composites where thin-slice geometries are used, interfacial failure initiation favoured by tensile stress field, probably occurs at the bottom of the specimen even in the absence of thermally induced shear stresses. However, modeling the stepwise debonding of interface requires the more sophisticated implementation of interface behaviour concerning failure evolution and the friction between wire and the matrix.

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.

Carbon fiber surface modification for tailored fiber-matrix adhesion in the manufacture of C/C-SiC composites

The surface of a commercial PAN-based carbon fiber (CF) was modified to optimize fiber–matrix adhesion (FMA) in a CF-reinforced polymer (CFRP) preform used for the manufacture of CF-reinforced silicon carbide (C/C-SiC) composites. For these purposes, CFs were thermally desized and then electrochemically oxidized. The surface of the fibers was investigated by atomic-force microscopy. The CFs’ chemical composition, specific surface acidity and wettability were investigated by different analytical methods. CFRPs based on electrochemically oxidized and untreated CFs were subsequently carbonized and transformed into C/C-SiC via liquid silicon infiltration (LSI). The microstructure of the composites was investigated by scanning electron microscopy (SEM). Single-fiber push-out tests were used to characterize FMA and revealed the highest interfacial fracture toughness for the CFRP based on electrochemically oxidized CFs. The surface treatment also showed positive effects after siliconization, e.g., a lower conversion rate of the CFs and improved mechanical properties.

Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.

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.

Evaluation of interfacial properties in SiC composites using an improved cohesive element method

A two-dimensional axisymmetric finite element model based on an improved cohesive element method was developed to simulate interfacial debonding, sliding friction, and residual thermal stresses in SiC composites during single-fiber push-out tests to extract the interfacial bond strength and frictional stress. The numerical load–displacement curves agree well with experimental curves, indicating that this cohesive element method can be used for calculating the interfacial properties of SiC composites. The simulation results show that cracks are most likely to occur at the ends of the experimental sample, where the maximum shear stress is observed and that the interfacial shear strength and constant sliding friction stress decrease with an increase in temperature. Moreover, the load required to cause complete interfacial failure increases with the increase in critical shear strength, and the composite materials with higher fiber volume fractions have higher bearing capacities. In addition, the initial failure load increases with an increase in interphase thickness.

Interfacial Fracture Toughness of Fiber Reinforced Titanium Matrix Composites by Push out Test

Abstract The model for single fiber push-out test was developed to evaluate the fracture toughness GIIc of the fiber/matrix interface in titanium alloys reinforced by SiC monofilaments. Theoretical solution to GIIc was obtained from fracture mechanics, and the effects of several key factors such as the applied stress needed for crack advance, crack length, and interfacial frictional shear stress were discussed. The predictions by the model were compared with the previous finite element analysis results for the interfacial toughness of the composites including Sigma1240/Ti-6-4, SCS/Ti-6-4, SCS/Timetal 834 and SCS/Timetal 21s. The results show that the model can reliably predict the interfacial toughness of the titanium matrix composites, in which interfacial debonding usually occurs at the bottom of the samples.

Mechanical properties of SiCf/SiC composites with alternating PyC/BN multilayer interfaces

ABSTRACTAlternating pyrolytic carbon/boron nitride (PyC/BN)n multilayer coatings were applied to the KD–II silicon carbide (SiC) fibres by chemical vapour deposition technique to fabricate continuous SiC fibre-reinforced SiC matrix (SiCf/SiC) composites with improved flexural strength and fracture toughness. Three-dimensional SiCf/SiC composites with different interfaces were fabricated by polymer infiltration and pyrolysis process. The microstructure of the coating was characterised by scanning electron microscopy, X–photoelectron spectroscopy and transmission electron microscopy. The interfacial shear strength was determined by the single-fibre push-out test. Single-edge notched beam (SENB) test and three-point bending test were used to evaluate the influence of multilayer interfaces on the mechanical properties of SiCf/SiC composites. The results indicated that the (PyC/BN)n multilayer interface led to optimum flexural strength and fracture toughness of 566.0 MPa and 21.5 MPa m1/2, respectively, thus the fracture toughness of the composites was significantly improved.