Fiber Push-out Tests Research Articles

Elastic FEM Analysis of Fiber Push-Out Test for C/C Composites

To study mechanical properties of fiber, matrix and their interface in C/C composites, fiber push-out test has been carried out. Where, to decompose the deformation, measured from indenter displacement, into local deformation is essential for mechanistic understandings. But up to now, there has been no systematic study on this effect. In this study, as the preliminary analysis of the push-out tests, those with a variety of groove width on specimen holder were performed and the elastic deformations of specimen were analyzed by a finite-element method (FEM). Materials used were unidirectionally reinforced C/C composites. Fiber push-out tests were carried out by utilizing dynamic ultra micro indentation test machine. For the cases of 20 and 100 μm groove, plateaus which indicate the occurrence of fiber push-out could be seen in Load-displacement curves ( indentation curves) and there was a difference in the duration of plateau. On the other hand, for all the cases, any difference could not be seen before plateaus. Now by using axis symmetrical FEM model, elastic deformation of C/C composites before push-out was calculated. From the results of calculation, the effect of groove width on indentation curves within elastic regime was considered to be little. But there was a large difference in the distribution of the stress transverse to carbon fiber direction on fiber-matrix interface between 20 and 100 μm, and this was considered to be the origin of the difference of the duration of plateau.

Influence of Residual Stresses, Interface Roughness, and Fiber Coatings on Interfacial Properties in Ceramic Composites

Fiber pushout tests are performed on zircon‐matrix composites especially fabricated with a variety of silicon carbide reinforcing fibers and fiber coatings in order to create samples with different interfacial properties, surface roughness, and possibly in different states of residual stress to demonstrate their role on the interfacial and mechanical properties of fiber‐reinforced composites. The data obtained from fiber pushout tests are analyzed using linear, shear‐lag, and progressive debonding models to extract important interfacial properties, residual stresses, and surface roughness. The nature and magnitude of residual stresses in composites are independently characterized by measuring the coefficient of thermal expansion of the fiber, the matrix, and the composite for comparison with similar values measured using the fiber pushout tests. These results are then compared for self‐consistency among different ways of analyzing data and with independently measured and calculated values. The results have shown that independent and complementary methods of data acquisition and analysis are required to fully understand interfacial properties in ceramic composites. In particular, independent measures of the coefficient of thermal expansion, residual stresses, and surface roughness are required to confidently interpret interfacial properties obtained by different analytical approaches and then relate them to the overall mechanical response of composites. It is also shown that composites with optimum mechanical response can be created by suitably engineering the interface using multiple fiber coatings.

Interfaces and fatigue Damage in a Ti-15–3/SCS-6 [0/90]2s Composite

AbstractThe key role of interfaces in the initiation and propagation of fatigue damage is elucidated for a model eight ply [0/90]2s Ti-15V-3Cr-3Al-3Sn (Ti-15–3) composite reinforced with SiC (SCS-6) fibers. Interfacial strengths obtained from fiber push-out tests are used in the quantification of shielding due to crack bridging. Composite fatigue tests are predicted using an idealized fracture mechanics model. The possible effects of interfacial strength variation on the predicted fatigue lives are also elucidated.

EVALUATION OF INTERFACIAL SHEAR PROPERTIES OF METAL MATRIX COMPOSITES FROM FIBRE PUSH-OUT TESTS

SUMMARY Fibre push-out test is commonly used to characterize the fibre-matrix interfacial behaviour. In the case of metallic and intermetallic matrix composites (MMCs and IMCs), the presence of high levels of thermal residual stresses, very small thickness of the specimens and ductility of the matrix material make the interpretation of the test results difficult. In this paper, single fibre push-out test is studied using finite element methods, with the objective of extracting interfacial properties from the experimental test results. The fibre-matrix interface is modelled using a contact-friction formulation, and debonding of the interface is predicted using a failure criterion based on the local stress state at the interface. Load versus displacement behaviour of the push-out tests is numerically simulated as a function of different interfacial strengths. The data is then used to generate a calibration curve to predict the actual interfacial properties for a given experimentally measured peak push-out l...

Interfaces and fatigue damage in a metastable beta titanium matrix composite

The results of a systematic study of the effects of interfacial microstructure on fatigue damage in a metastable β Ti15V3Cr3Al3Sn/SiC (SCS-9) composite are presented. Interfacial microstructure is controlled by heat treatment in the β phase field of the matrix, which promotes coarsening of the fiber-matrix interface without significant changes in the metastable β matrix microstructure (grain size). The effects of interfacial microstructure on debone and friction strengths are also discussed using results from fiber push-out tests. Fatigue damage initiation and propagation mechanisms are elucidated via optical/scanning electron microscopy and acoustic emission analysis of specimens that were deformed to failure in incremental cyclic loading steps. The effects of cyclic deformation on matrix hardness and composite modulus are also examined prior to the presentation of a fracture mechanics (micromechanics) approach for the prediction of fatigue life, and the effects of crack-tip shielding via bridging mechanisms. The paper highlights the potential for the development of accurate life prediction methodologies that are based on experimental observations of damage in titanium matrix composites. It also illustrates the need for multidisciplinary mechanics and materials approaches in the study of composite behavior.

Characterization of modified ceramic fiber surfaces using AFM

Several models have been developed to describe the interfacial behavior of fiber-reinforced ceramic matrix composites. However, these models are limited in that they do not take into account the effect of fiber surface roughness on the fiber sliding resistance. Clearly, the fiber surface roughness will play an important role in extensive fiber/matrix debonding and fiber pull-out behavior which are characteristic of a toughened composite. In this study, a Topometrix TMX2010 Atomic Force Microscope (AFM) was used to quantitatively evaluate the surface topography of a SiC fiber subjected to a series of surface modification treatments. The relationship between interfacial properties determined from fiber push-out tests and the surface roughness will be used to modify existing mathematical models.In order to systematically study the effect of surface roughness on the interfacial properties of ceramic matrix composites, a “model” single fiber composite having varying degrees of fiber surface roughness was developed. This was accomplished by using a single manufacturer's ceramic fiber which was subjected to a series of treatments in order to alter the surface.

Influence of Cr and W alloying on the fiber-matrix interfacial shear strength in cast and directionally solidified sapphire nial composites

Sapphire-reinforced NiAl matrix composites with chromium or tungsten as alloying additions were synthesized using casting and zone directional solidification (DS) techniques and characterized by a fiber pushout test as well as by microhardness measurements. The sapphire-NiAl(Cr) specimens exhibited an interlayer of Cr rich eutectic at the fiber-matrix interface and a higher interfacial shear strength compared to unalloyed sapphire-NiAl specimens processed under identical conditions. In contrast, the sapphire-NiAl(W) specimens did not show interfacial excess of tungsten rich phases, although the interfacial shear strength was high and comparable to that of sapphire-NiAl(Cr). The postdebond sliding stress was higher in sapphire-NiAl(Cr) than in sapphire-NiAl(W) due to interface enrichment with chromium particles. The matrix microhardness progressively decreased with increasing distance from the interface in both DS NiAl and NiAl(Cr) specimens. The study highlights the potential of casting and DS techniques to improve the toughness and strength of NiAl by designing dual-phase microstructures in NiAl alloys reinforced with sapphire fibers.

Environmental effects on fiber debonding and sliding in an SCS-6 SiC fiber reinforced reaction-bonded Si 3N 4 composite

Because fiber/matrix interfacial mechanical properties are critical to the overall strength and toughness of fiber-reinforced composites, several test methods have been developed to evaluate fiber debonding and sliding behavior. Most interface testing of metal or ceramic matrix composites has been performed in room (laboratory) air at room temperature without considering the effects of moisture or testing environment on the test results. Even though chemical reactivity at the fiber/matrix interface may be insignificant at room temperature for these composites, it is well known that friction and wear between sliding surfaces can be strongly dependent on environment, even at room temperature. Therefore, awareness of the environmental sensitivity of interface testing is necessary for the reliable interpretation and comparison of interfacial mechanical property tests. In addition, such awareness is needed to prevent the potentially inaccurate prediction of the performance of composites intended for service in high altitude or space environments based on interface tests completed in laboratory ambient. This paper will demonstrate the effect of environment on fiber debonding and sliding as evaluated by fiber push-out testing at room temperature of an SCS-6 SiC fiber reinforced reaction-bonded Si{sub 4}N{sub 4} matrix composite (SCS-76/RBSN). SCS-6/RBSN is a promising candidate material for component applications inmore » advanced heat engines, and its low fiber/matrix interfacial shear strength is a key determinant in its demonstrated toughness (as evidenced by graceful failure beyond matrix cracking).« less

Measurement of interfacial shear properties of composites of Ti-1100 alloy reinforced with SCS-6 SiC monofilament fiber

The commonly used thin-slice fiber push-out or pull-out tests to measure interface properties of fiber-reinforced metal-matrix composites can introduce biaxial bending of metallic matrix around the fiber and generate complex interfacial stress states. These factors complicate the interpretation of results from these tests. Also, the results from a single fiber push-out (or pull-out) test might not directly relate to the shear properties associated with an assemblage of fibers as in an actual composite body. In this paper, a technique has been presented in which a model, single-ply multi-fiber composite specimen is tested in a state of nearly pure shear. The chief advantages of this technique are that specimen preparation and test methodology are simplified, and the influence of non-shear stresses, such as those associated with the fiber push-out or pull-out test, are minimized. Composite shear strength has been measured as a function of fiber packing density. A method of interpreting the shear strength data, which includes contributions from fiber-matrix debonding strength as well as shear fracture strength of the matrix, has been developed. The method permits measurement of the interfacial shear strength and an assessment of matrix residual stress generated due to CTE difference between the matrix and the fiber. The method has been used to study the effect of surface coatings on SiC fiber, e.g. C-rich coating (SCS-6) and TiN coating, on the interfacial strength in Ti-1100/SiC fiber composites. Microstructures and interface chemistries of as-fabricated composites and sheared composite surfaces are also presented.

Thin-slice fiber pushout and specimen bending in metallic matrix composite tests

For researchers interested in fiber pushout testing of MMC's (including intermetallic matrix composites), we have shown that • • Composites containing optically transparent fibers can be metal backplated using photolithographic techniques to provide a support with a hole size matching the fiber diameter. • • Computational results indicate that backplated specimens exhibit negligible specimen flexure during thin-slice fiber pushout testing. • • Results from thin-slice fiber pushout tests of three different sapphire-reinforced MMC's indicate that the interfacial shear strength values are the same regardless of whether the specimen was supported by the Ni backplate or by a hole 1.6 times the fiber diameter or by a groove 2 times the fiber diameter. Thus at least for fiber aspect ratios of ≥2 and groove width/specimen thickness ratios ≤2, specimen bending does not significantly affect the measured value of the average interfacial shear strength at maximum fiber push-out load in the MMC systems studied.

Analyses of fibre push-out test based on the fracture mechanics approach

Theoretical analyses based on the concept of fracture mechanics have been developed to evaluate the interfacial properties of ceramic matrix composites in single fibre push-out tests. Two different fibre push-out models, i.e. single fibre-matrix model and three-cylinder model, were presented for different specimen geometries. The interfacial debonding and fibre push-out stresses were analyzed similar to the fibre pull-out test ( Zhou et al., J. Mater. Sci. 27, 3155–3166 and 29, 5541–5550). In the three-cylinder model, the push-out of a single fibre in a specimen with multiple fibres was treated as a three-cylinder composite in which a fibre is located at the centre of a coaxial cylindrical shell of the matrix matrerial which in turn is surrounded by a transversely isotropic elastic material with elastic properties of the bulk composite. The radius of the matrix cylinder in this case is related to the volume fraction of the fibre which significantly affects the behaviour of interfacial debonding and fibre push-out. A simple methodology is presented on the basis of these analyses to determine the interfacial properties, including interfacial fracture toughness G ic, residual clamping stress q 0 and coefficient of interfacial friction μ, from the experimental results of maximum debond stress σ d ∗ (or initial debond stress σ d 0) and initial frictional push-out stress σ fr measured in a fibre push-out test. The effects of the fibre volume fraction, the transversely isotropic property and the Poisson expansion of the fibre are all considered in the analyses.

Influence of matrix cracking on mechanical properties of a ceramic matrix composite

The first apparent evidence of damage in ceramic matrix composites occurs via matrix cracking when these composites are subjected to stresses beyond the elastic limit. The presence of these matrix cracks may have several consequences to the mechanical response of composites both at room and elevated temperatures. For example, composites containing matrix cracks may show reduced modulus and enhanced susceptibility to damage by fatigue and oxidation at elevated temperatures. In addition, many composites have inherrent matrix cracks/ porosity produced either because of processing limitations (chemical vapour infiltration (CVI), solgel) or as a result of a mismatch in fibre/matrix thermal expansion. Therefore, an understanding of the influence of matrix cracks on the mechanical response of composites is important for designing composites and for assessing their performance under service conditions. In this study, a fully dense and crack-free zircon composite reinforced with SiC whiskers and SiC continuous fibres was loaded in three-point flexure up to the matrix cracking stress, then unloaded. Subsequently, these samples were repeatedly loaded and unloaded below the original matrix cracking stress to study the influence of matrix cracking on the elastic modulus of the composite. The objective of this letter is to report some of the initial observations of such mechanical testing on the mechanical response of a zircon composite reinforced with SiC fibres and whiskers. Some of these observations on matrix cracking behaviour are related to predictions of the steady state and nonsteady state micromechanical models for matrix cracking. This study was performed on a zircon matrix composite reinforced with SiC whiskers and uniaxially aligned SiC continuous filaments. Silicon carbide whiskers were Tateho SCW-1 and SiC monofilaments were SCS-6 type obtained from Textron Corporation. All of the composites tested in this study were processed in a manner described in a previous study [1]. As-fabricated composites were fully dense with an insignificant (< 1%) porosity, and contained about 20 vol % SiC whiskers in the zircon matrix phase and about 25% SiC monofilaments in the entire composite volume (including the zircon matrix, SiC whiskers and SiC filaments). The matrix phase containing zircon and SiC whiskers had an average grain size of about 3/~m, and X-ray diffraction (XRD) analysis of the composite revealed zircon and SiC as major matrix phases with a small but detectable amount of the free-zirconia phase. The zirconia phase was not detected in the starting zircon powder. Therefore, it must have been produced because of the decomposition of zircon [2] at the high processing temperature of 1610 °C. Average mechanical and fibre-matrix interracial properties of as-fabricated composites were measured by three-point flexure and fibre pushout tests, respectively. The details of these tests were also given elsewhere [1]. Typically, composite samples with dimensions of 3.2 cm length, 0.8 cm width and 0.15 cm thickness were individually ground to a finish of 60/~m and then tested in three-point flexure with lower support pins 2.54 cm apart (span-tothickness ratio: 17). All of the tests were performed at a crosshead rate of 0.0127 cmmin -1 to generate load-deflection data. A number of average mechanical properties were determined from these loaddeflection data. These included elastic modulus (E), first matrix cracking strength (Ocr), ultimate strength (~ru) and work-of-fracture (WOF), as described previously [1]. At least four samples of each type were tested to measure the average mechanical properties, which are summarized in Table I. Fibrematrix interracial shear strength was measured by a fibre pushout test [3, 4], and at least 10 fibres in each sample were pushed to calculate an average value of the interfacial shear strength. Average interfacial shear strength was measured during the first push as well as upon re-push (second push) of the already pushed fibres. These results are also summarized in Table I. In this study, a zircon composite reinforced with SiC whiskers and uniaxially aligned SCS-6 monofila-

Analyses of fiber push-out tests performed on Nicalon/SiC composites with tailored interfaces

The interfacial properties of Nicalon fiber-reinforced SiC composites with tailored (i.e. weak or strong) interfaces are characterized using single-fiber push-out tests. A simple analysis is proposed to include the surface roughness of the fiber in the push-out model. Specifically, the roughness effect is characterized by the difference between the fiber push-out stress and the fiber reseating stress, which is obtained from fiber push-back tests. A method is developed to analyze the loading stress vs fiber-end displacement relation (i.e. the push-out curve) during the push-out process to evaluate the interfacial properties. The push-out curve for composites with weak fiber bonding can readily be analyzed by the existing push-out model incorporated with the roughness effect. However, premature catastrophic debonding at the interface is required to interpret the push-out curves for composites with strong fiber bonding.

Residual Stresses in Silicon Carbide–Zircon Composites from Thermal Expansion Measurements and Fiber Pushout Tests

Coefficients of thermal expansion (CTE) in the axial direction of two types of SiC fibers, monolithic zircon, monolithic SiC, and several SiCf‐zircon composites were measured in the temperature range of 50 to 1380C. The measured CTE values of composites were compared with values predicted by the rule‐of‐mixtures approach, and a small difference in measured and calculated values was ascribed to the nature of interfacial bonding and assumptions implicit in the rule‐of‐mixtures approach. Fiber pushout tests were performed on these composites and the residual stresses were extracted from the analysis of the load–displacement plots in terms of the shear‐lag and progressive debonding models. The radial and axial residual stresses arising from the mismatch in CTE were calculated and compared with values obtained from the fiber pushout tests. The fiber pushout tests in general produced lower values of the residual stresses, but the residual stresses obtained using shear‐lag analysis were in good agreement with the calculated values based on the CTE mismatch in composites with lower values of the interfacial shear stress. The influence of anisotropic fiber expansion in the radial and axial directions on the radial and axial residual stresses in composites were also examined.

Effect of Interfacial Roughness Parameters on the Fiber Pushout Behavior of a Model Composite

The effect of interfacial roughness on the frictional sliding in composites has been studied using fiber pushout and pushback tests on a model composite of Plexiglas rods in an epoxy matrix. Different extents of roughness were introduced on the Plexiglas rods and the resulting roughness profiles measured. The roughness profiles were characterized using six different roughness parameters. An attempt was made to find a correlation between the sliding resistance and the selected roughness parameters. A parameter defined as the maximum coefficient in the Fourier transform of the roughness profile was found to yield the best correlation. If the roughness introduced is periodic, then the pushout traces exhibit periodic dips, but the magnitude of this periodic dip is significantly smaller than the seating drop obtained from pushback tests.

Mechanics of the push-out process from in situ measurement of the stress distribution along embedded sapphire fibers

The basic mechanics of the fiber push-out process are studied by measuring the stress distributions along c-axis sapphire fibers in γ-TiAl matrices. The stress measurements were made in situ during fiber push-out tests using a piezo-spectroscopy method. By focussing an optical microprobe into a sapphire fiber, the frequency shift distribution of the characteristics R lines of Cr 3+, and hence the stress distribution along the fiber were determined at each increment of applied load. By comparing the measured stress distributions with ones calculated using finite element method, key mechanical parameters including residual stress before composite slicing, debond length at each load, debond energy and the frictional stress along the interface in both debonding and sliding phases were determined. Two coating systems (Mo/Al 2O 3 and CVD-C/Al 2O 3) were studied. The results also show that while the debond energies differ by an order of magnitude, the frictional stresses are very similar in magnitude, suggesting that the interface morphology is of greater importance than the nature of the coating.

Interfacial Microstructure and Chemistry of SiC/BN Dual‐Coated Nicalon‐Fiber‐Reinforced Glass‐Ceramic Matrix Composites

Glass‐ceramic composites with improved high‐temperature mechanical properties have been produced by incorporating continuous SiC fibers into a barium magnesium aluminosilicate matrix. Control of the fiber/matrix interface was achieved by a dual‐layer coating of SiC/BN(C) applied to the fibers by CVD. The weakly bonded interface resulted in composites with high fracture toughness and strength up to 1100°C, and the composite system was oxidatively stable during long‐term exposure to air at high temperatures. Composites with different thermal and mechanical histories were studied, and interfaces were characterized using transmission electron microscopy (TEM), Auger electron spectroscopy, and fiber pushout tests. Observations of interfacial microstructure were correlated with the mechanical properties of the composite and with interface properties determined from fiber push‐out tests.

Fiber push-out testing apparatus for elevated temperatures

A newly developed apparatus has been designed for performing fiber push-out testing on continuous fiber-reinforced composites at elevated temperatures. This test measures the force at which a fiber resists being pushed by a flat-bottomed indenter moving at a constant speed. The applied load versus time curve characterizes the fiber debonding and sliding behavior. Extending measurements to elevated temperatures required incorporating sample/indenter heating in a nonoxidizing environment. With this new apparatus, fiber push-out tests have been performed up to 1100 δC in a vacuum of 10-6 Torr. A line-of-sight to the sample is maintained during the test which allows video monitoring of the push-out process. Results are shown for SCS-6 SiC fiber-reinforced Ti-24Al-llNb (at. %) and Ti-15V-3Cr-3Sn-3Al (at. %) matrix composites. The results are discussed in terms of residual stresses, interfacial wear, matrix ductility, and changing modes of interfacial failure. The effect of temperature-dependent interfacial wear on the interfacial roughness contribution to frictional shear stresses during fiber sliding is examined.

Evaluation of in situ mechanical properties of composites by using nanoindentation techniques

The in situ mechanical properties of the fibers, matrices and interfaces in an Al 2O 3 fiber-reinforced β-21S Ti alloy have been evaluated by using two nanoscale indentation tests. The Al 2O 3 fibers were coated with a refractory metal and Y 2O 3 duplex coating which served as a diffusion barrier during the HIPing used to produce the metal matrix composites. The hardness of the fibers, interfaces and matrix were obtained by performing a series of indentations across the fiber/matrix interface. The hardness decreases from the Al 2O 3 fiber to the Ti matrix. Additionally, by doing fiber pushout tests, the interfacial shear strength, interfacial frictional stress and mode II interfacial fracture energy were obtained. Scanning electron microscopy and X-ray mapping were used for microstructural and chemical analysis. The mechanical properties of the interfaces were related to their chemical composition. The interfacial fracture was found to occur at the interface between the refractory metal and the Y 2O 3. The mode II interfacial fracture energy in this system is more than two orders of magnitude lower than the interfacial fracture energy of Ti/Al 2O 3 without the diffusion barrier.

Interfacial stability and mechanical properties of Al2O3 fiber reinforced Ti matrix composites

The mechanical properties of the interfaces in an Al2O3 fiber reinforced β-21S Ti alloy have been evaluated by using fiber pushout tests. The Al2O3 fibers were coated with a refractory metal and Y2O3 which served as a diffusion barrier during the HIPing used to produce the metal matrix composites. By doing fiber pushout tests, the interfacial fracture was found to occur at the interface between the refractory metal and the Y2O3. The interfacial shear strength and interfacial frictional stress were measured to be 323 and 312 ± 2 MPa, respectively. The interfacial frictional stress, which is due to asperity interlocking during the fiber sliding, was correlated to the surface roughness of the coated Al2O3 fiber obtained with the aid of an atomic force microscope. The measured surface roughness of 18.8 ± 2.2 nm was related to the frictional stress through Hutchinson's model.9 The frictional coefficient between the Al2O3 fiber and the Ti matrix is calculated to be 0.32 ± 0.02.