Fiber Push-out Tests Research Articles

A simplified methodology for assessing the interface of CFRPs with fibre push-out testing

This work proposes a simplified methodology for examining the interface of fibre reinforced composites with fibre push-out testing. Demonstrated in a carbon fibre (CF) reinforced polymer (CFRP) case study, it examines the interface of a commercial epoxy with the 31E, 50C, and F0E T700 CFs. Atomic force microscopy, attenuated total reflection – Fourier transform infrared spectroscopy, and scanning electron microscopy are used to characterise the fibres, and the CFRP samples are manufactured with wet winding. To create trenches of suitable thickness within the bulk composite, femtosecond-laser processing is proposed as a localised material subtractive method, facilitating the fibre push-out testing with the flat nanoindenter tip. The results highlight the influence of the CF sizing agent on the CFRP interface, and overall, a methodology is defined that simplifies the specimen preparation for assessing the interface of FRPs. This advancement holds promise for a broader adoption of such tests from both academia and industry.

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.

Interfacial properties of SiCf/SiC minicomposites with a scheelite coating

Unidirectional SiCf/SiC minicomposite with a scheelite (CaWO4) interphase coating was fabricated through the precursor infiltration and pyrolysis method. Fractography of the SiCf/SiC minicomposites indicated that weak fiber/matrix bonding can be provided by the CaWO4 interphase. Furthermore, interfacial debonding stress of SiCf/CaWO4/SiC minicomposite was evaluated through the fiber push-out test, and estimated to be 80.7 ± 4.6 MPa. In-situ tensile SEM observation of SiCf/CaWO4/SiC minicomposite after oxidation at 1000–1100 °C was carried out, and thermal compatibility between CaWO4 interphase coating and SiC fiber or matrix after heat treatment at 1300 °C was investigated.

The effect of multi-material architecture on the ex vivo osteochondral integration of bioprinted constructs

Extrusion bioprinted constructs for osteochondral tissue engineering were fabricated to study the effect of multi-material architecture on encapsulated human mesenchymal stem cells’ tissue-specific matrix deposition and integration into an ex vivo porcine osteochondral explant model. Two extrusion fiber architecture groups with differing transition regions and degrees of bone- and cartilage-like bioink mixing were employed. The gradient fiber (G-Fib) architecture group showed an increase in chondral integration over time, 18.5 ± 0.7 kPa on Day 21 compared to 9.6 ± 1.6 kPa on Day 1 for the required peak push-out force, and the segmented fiber (S-Fib) architecture group did not, which corresponded to the increase in sulfated glycosaminoglycan deposition noted only in the G-Fib group and the staining for cellularity and tissue-specific matrix deposition at the fiber-defect boundary. Conversely, the S-Fib architecture was associated with significant mineralization over time, but the G-Fib architecture was not. Notably, both fiber groups also had similar chondral integration as a re-inserted osteochondral tissue control. While architecture did dictate differences in the cells’ responses to their environment, architecture was not shown to distinguish a statistically significant difference in tissue integration via fiber push-out testing within a given time point or explant region. Use of this three-week osteochondral model demonstrates that these bioink formulations support the fabrication of cell-laden constructs that integrate into explanted tissue as capably as natural tissue and encapsulate osteochondral matrix-producing cells, and it also highlights the important role that spatial architecture plays in the engineering of multi-phasic tissue environments. Statement of significanceHere, an ex vivo model was used to interrogate fundamental questions about the effect of multi-material scaffold architectural choices on osteochondral tissue integration. Cell-encapsulating constructs resembling stratified osteochondral tissue were 3D printed with architecture consisting of either gradient transitions or segmented transitions between the bone-like and cartilage-like bioink regions. The printed constructs were assessed alongside re-inserted natural tissue plugs via mechanical tissue integration push-out testing, biochemical assays, and histology. Differences in osteochondral matrix deposition were observed based on architecture, and both printed groups demonstrated cartilage integration similar to the native tissue plug group. As 3D printing becomes commonplace within biomaterials and tissue engineering, this work illustrates critical 3D co-culture interactions and demonstrates the importance of considering architecture when interpreting the results of studies utilizing spatially complex, multi-material scaffolds.

Relationship between both thickness and degree of crystallisation of BN interphases and the mechanical behaviour of SiC/SiC composites

SiC/BN/SiC-Si single-ply composites with Hi-Nicalon S™ fibres were synthesised by chemical vapour infiltration followed by the liquid route. These are model composites representing the behaviour of industrial composites intended to be used as structural parts in the hot sections of aeronautical engines. Four interphases with two different degrees of crystallisation and, in each case, two different thicknesses were tested. The interfacial shear stress measured by fibre push-out test is significantly higher when the crystallisation degree of the BN interphase is low. Yet, in this case, it decreases with the thickness of the interphase. This higher interfacial bonding is also associated, albeit less significantly, with better macro-mechanical properties as measured by tensile and shear tests. The cracks observed in the composite with different microscopy techniques are also less long and open. Depending on the interphases, the debonds are more or less pronounced and occur preferentially at different locations in the interfacial system.

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.

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.

Interface Controlled Micro- and Macro-Mechanical Properties of Vibration Processed Carbon Fiber/Epoxy Composites.

The fiber-resin interface is an important component that significantly affects mechanical properties of composites. Random vibration-assisted vacuum processing (RVAVP), a new method to improve the adhesion of the fiber-resin interface, was presented. The effects of different curing processes on mechanical properties were comprehensively assessed by combining the fiber push-out test, finite element model simulation, cure monitoring approach, and short-beam three-point bending test, and the correlation between fiber volume fraction and mechanical properties was quantified by a facile thermogravimetric analysis-based methodology. The results revealed that application of random vibration during the curing process can promote the impregnation of resin into fibers and impede the growth of interface defects while improving mechanical properties at the same time. For this reason, the laminates produced by RVAVP exhibited the average interfacial shear strength of 78.02 MPa and the average interface fracture toughness of 51.7 J/m2, which is obtained a 48.26% and 90.77% improvement compared with the 0 MPa autoclave process. With the large observed increase in micro-mechanical properties, the average interlaminar shear strength of 93.91 MPa showed a slight reduction of 5.07% compared with the 0.6 MPa autoclave process. Meanwhile, the mechanical properties tended to be stable at the fiber volume fraction of 65.5%.

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.

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.

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.

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.

Flexural modulus of SiC/SiC composites sintered by microwave and conventional heating

To deeply study the variation mechanisms of mechanical properties, flexural modulus of SiC fibers reinforced SiC matrix (SiC/SiC) composites prepared by conventional and microwave heating at 800 °C–1100 °C was discussed in detail. The elastic modulus of fibers and matrix, interface bonding strength and porosity of SiC/SiC composites were considered together to analyze the changing tendencies and differences in their flexural modulus. The elastic modulus of fiber and matrix was determined by nanoindentation technique and interface characteristics applying fiber push-out test. The porosity and microstructure examinations were characterized by mercury intrusion method, X-ray Diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, two conflicts between the changing trends of elastic modulus and chemical compositions of composite components were focused and explained. Results indicate that three factors played different roles in the flexural modulus of SiC/SiC composites and residual tensile stress in composite components led to the conflicts between their elastic modulus and chemical compositions.