Single-fiber Test Research Articles

High-temperature dynamic mechanical testing of ceramic fibers: Apparatus and preliminary results

A test apparatus is described for the dynamic testing of single ceramic fibers from room temperature to 1600°C using forced vibration sine wave displacements from 0.1 to 25 Hz. Both storage (elastic, real or in-phase) and loss (dissipative, imaginary or out-of-phase) components of the dynamic modulus are obtained. The deconvolution equations required to extract isothermal data from a non-uniform (parabolic) temperature distribution are presented. Preliminary data on sapphire filaments and CVD silicon carbide fibers are discussed. The results show the validity of the testing method and data, which are in good agreement with available literature data on similar materials.

Pairing-specific long-term depression of Purkinje cell excitatory postsynaptic potentials results from a classical conditioning procedure in the rabbit cerebellar slice.

1. Using a rabbit cerebellar slice preparation, we stimulated a classical conditioning procedure by stimulating parallel fiber inputs to Purkinje cells with the use of a brief, high-frequency train of eight constant-current pulses 80 ms before climbing fiber inputs to the same Purkinje cell were stimulated with the use of a brief, lower frequency train of three constant-current pulses. In all experiments, we assessed the effects of stimulation by measuring the peak amplitude of Purkinje cell excitatory postsynaptic potentials (EPSPs) to single parallel fiber test pulses. 2. Intradendritically recorded Purkinje cell EPSPs underwent a long-term (> 20 min) reduction in peak amplitude (30%) after paired stimulation of the parallel and climbing fibers but not after unpaired or parallel fiber alone stimulation. We call this phenomenon pairing-specific long-term depression (PSD). 3. Facilitation of the peak amplitude of a second EPSP elicited by a parallel fiber train occurred both before and after paired stimulation suggesting that the locus of depression was not presynaptic. Depression of the peak amplitude of a depolarizing response to focal application of glutamate following pairings of parallel and climbing fiber stimulation added support to a suggested postsynaptic locus of the PSD effect. 4. The application of aniracetam potentiated EPSP peak amplitude by 40%, but these values returned to baseline as a result of pairings. With the removal of aniracetam from the bath 20 min after pairings, normal levels of pairing-specific EPSP depression were observed, indicating that the effect did not result from direct desensitization of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA) receptors. 5. Incubation of slices in the protein kinase inhibitor H-7 potentiated EPSP peak amplitudes slightly (9%), but peak amplitudes returned to baseline levels after pairings. The net reduction in EPSP peak amplitude of < 10% after pairings suggested that H-7 partially blocked PSD and that, in turn, PSD involved protein kinases. 6. The means of induction and the specificity of those means suggest that the phenomenology of PSD is fundamentally different from that of long-term depression. PSD only occurs with pairings of trains of parallel fiber and climbing fiber stimulation; it occurs without the need for bicuculline; and it can overcome the blocking effects of aniracetam. 7. Nevertheless, the involvement of protein kinases and the potential role of calcium suggest that the mechanisms involved in the induction of PSD and long-term depression have a number of features in common. 8. Because of the pairing-specific nature of the long-term synaptic depression observed in these experiments, PSD provides a mechanism that may contribute to the role of the cerebellar cortex in classical conditioning.

Incorporating Fiber Damage in a Micromechanical Analysis of Metal Matrix Composite Laminates

A micromechanics based formulation involving the method of cells is developed to analyze metal matrix composite behavior in the presence of fiber fragmentation. The effects of fiber fracture are accounted for by determining an instantaneous effective fiber modulus from a modified chain-of-bundles approach. The analysis assumes a uniform density of fiber breakage throughout the composite. This crack density is determined from Weibull statistics whose parameters may be obtained from single-fiber tests or estimated. The ultimate strength of titanium-based metal matrix composite (MMC) laminates as well as their inelastic stress-strain response in the presence of fiber fragmentation are predicted from the present analysis which are in good agreement with the experimental counterparts. Also, the applicability of the present analysis to predict the low cycle/high stress fatigue lives of MMC laminates is demonstrated.

Residual Stresses in Microcomposites and Macrocomposites

Abstract Existing models for built-in residual stresses in composite materials are reviewed and discussed. In particular, the thermal longitudinal stress present in the fiber prior to a single-fiber fragmentation experiment is studied using various model composite data. It is found that this stress is typically compressive in nature and that, quantitatively, it depends on the fiber content, the degree of undercooling, and the thermoelastic constants of the fiber and the matrix. In the case of single-fiber composites (or microcomposites), the thermal longitudinal stress present in the fiber is high enough to either induce fiber sinewave buckling (such as in E-glass/epoxy), or extensive fiber fragmentation (such as in graphite HM/polypropylene) that may then be used to measure the dependence of compressive fiber strength upon length. This has to be accounted for in quantitative models that calculate interfacial adhesion parameters using single-fiber tests, such as the fragmentation test or the microbond tes...

The use of a single fibre test technique to investigate interfacial phenomena in SiC/Ti3Al‐based composites

SummaryA study of the chemical compatibility of a Ti3 Al‐based alloy (Ti–24Al–10Nb, at.%) with two silicon carbide continuous fibres (SCS‐6, SM 1240) has been conducted. Owing to the difficulty in processing intermetallic matrix composites, this type of material has been simulated in the present work by sputtering a thin titanium aluminide layer onto fibres and heat treating at temperatures representative of fabrication conditions. The degradation of the fibre strength due to its interaction with the matrix was correlated with analytical studies of the fibre/matrix interface using a combination of SEM, TEM, EELS and a submicrometre ion probe.

Fiber interactions in the multi-fiber composite fragmentation test

The single-fiber fragmentation test has been extended to a multi-fiber fragmentation test that demonstrates the fiber/fiber interactions that are present in real composites. The model composites are of Nicalon fibers embedded in an epoxy resin and have a controlled inter-fiber separation. An important qualitative result is that the interfacial failure in each fiber in an array is correlated with that of its neighbors, even when the individual fiber breaks are not at the same location. The fiber bundle as a whole thus has well defined regions of interfacial failure. Quantitatively, fibers in an array have a larger mean fragment length than a single fiber, and the fragment length increases with smaller inter-fiber separation or more embedded fibers. Shear-lag theory predicts the opposite effect. The results suggest that the presence of neighboring fibers prevent failure at some flaws. The flaw density function changes as a result of fiber/fiber interactions and this affects the fiber fragmentation process. Multi-fiber fragmentation tests show that the embedded single fiber test is insufficient to model fiber behavior within real composites.

An advanced equipment for single-fibre pull-out test designed to monitor the fracture process

A newly designed single-fibre pull-out machine with optimized stiffness in all components was developed, in order to obtain stable crack propagation during the fibre pull-out. On the basis of a mathematical model, which simulates the debonding process during a single-fibre pull-out experiment, calculations of the stress distribution, the force-displacement traces and the fracture propagation were made. Some of these results are compared with pull-out measurements using glass fibres embedded in thermoplastic matrices. The agreement between simulation and test results is good, and shows that stable crack propagation is achievable. Because friction has an important influence on pull-out forces, the interpretation of single-fibre test results has to be revised.

Plasma Modifications to the Interface in Carbon Fiber Reinforced Thermoplastic Matrices

Abstract Radio frequency glow discharge oxygen plasma was used to modify the surfaces of PAN-based and mesophase pitch-based carbon fibers. Surface chemical changes to the fibers were monitored by X-ray photoelectron spectroscopy and by fiber wetting studies evaluated in terms of dispersive-polar components of surface energy and acid-base contribution to the work of adhesion. Physical changes to these fibers were monitored by scanning electron microscopy. Stress transferability of these fibers was evaluated by the embedded single fiber test in poly(methyl methacrylate), poly(ethyl methacrylate), poly(methacrylonitrile) and poly(vinyl chloride) as these matrices offered varying degrees of dispersive-polar and acid-base character. Experimentally determined critical aspect ratios were compared to the theoretical work of adhesion determined by dispersive-polar interactions and with the Lewis acid-base nature of the matrices.

New evaluation technique of interfacial properties in GFRP

A new method was proposed to evaluate the mechanical properties at the interface between the fibres and the matrix in composites using an embedded single fibre coupon test. A mechanical parameter at the interface (called the interfacial transmissibility, κ) was derived from the fibre strength and the apparent stress of the fibre immediately before the first fracture of embedded fibre, σfa. This parameter indicated the degree of the mechanical transmission from the matrix to the fibre through the interface. This avoided some complicated problems such as the stress distribution along fibre fragments and the critical fragment state in a typical single-fibre test. This new method was tried to determine the κ-values for a fibre glass/epoxy resin with different amounts of a coupling agent at the interface. In order to measure the stress at the first fracture, the fracture process was monitored with a video camera during the single fibre test. The stress values at the first fracture for many coupons were analysed as a function of the three-parameter Weibull distribution. The resulting average stress and its coefficient of variation indicated that the reliability of the measurement for the stress at the first fracture was not less than that obtained by the usual single-fibre test. The change of interfacial transmissibility with amount of the coupling agent revealed the existence of an optimal interface.

The embedded single-fibre dynamic load test — a new non-destructive interphase investigation tool

Embedded single-fibre tests are commonly used to investigate fibre/matrix adhesion. However, unstable crack propagation renders these destructive tests hard to interpret. Most of the relevant theories apply an energy or a strength criterion to indirectly deduce interphase properties from the obtained results. A new method, the embedded single-fibre dynamic load (SFD) test, was developed to measure interphasial viscoelastic properties directly in a non-destructive way. Using different fibres in combination with poly(phenylene sulfide) in a comparative study, it is shown that results from the SFD test correlate well with those from single-fibre pull-out (SFP) tests. The morphology is established as a key link for an understanding of the fracture behaviour as well as the dynamic-mechanical behaviour of the interphase in fibre-reinforced composites.

Electric Resistive Heat Curing of the Fiber-Matrix Interphase in Graphite/Epoxy Composites

A novel method for tailoring the interphase of carbon fiber-polymer composites by resistive electric heating is presented. The single fiber-epoxy resin tensile test is used to investigate the adhesion and fracture properties of the interphase. Electric resistive heating is shown to increase adhesion and toughness at the interphase region. In analyzing the results, the strength and fracture energy of the interphase are related to the thermal postcure conditions created by resistive electric heating. For this purpose, difference analysis method is used to obtain numerical solution for heat conduction problem in the single fiber test specimen and the temperature distributions are plotted. Improvements obtained using resistive electric heating via carbon fiber are compared with those obtained by postcuring of the whole sample via convective thermal postcuring. The results obtained using these two different postcure methods seem to be similar.

The Use of a Single-Fiber Fragmentation Test to Study Environmental Durability of Interfaces/Interphases Between Epoxy and a Glass Fiber

AbstractThis work examines the usefulness of the single-fiber fragmentation test in studying the durability of fiber/matrix interfaces/interphases. This test measures the critical length/diameter ratio (L/D) of the fiber fragments formed in the test and relates this length to the interface's strength, or ability to transfer load. In the work reported here, we immersed samples of epoxy containing a single-glass fiber - that was previously sized with an epoxy-compatible coating - in either 65 or 75 °C water and tested after different times of exposure. In general, this ratio increased as a function of time of exposure to water. During exposure at 75 °C, the fibers' L/D in the samples did not increase significantly until after the sample reached its “apparent” equilibrium content of water ∼ (3.0 wt%). Because there was no significant measurable change in the tensile modulus between wet and dry samples, we cannot attribute these differences in L/D to changes in the resin's properties due to plasticizing by water. A small percentage of samples exposed at 65 °C did not show a significant increase in L/D, and in these cases the moisture produced a marked roughening of the fiber surface along the fiber/matrix interface. One possible explanation is that the attack by moisture degrades the interface, thus reducing its strength with a corresponding increase in the L/D. To varying degrees, however, the attack by moisture also degrades the E-glass fiber. This attack by moisture roughened the surfaces of the fibers and increased the distribution and/or size of the critical flaws, thus reducing both the strength of the fiber and the L/D. Based on our preliminary results, it appears that the singlefiber test has the potential to be useful for studying the durability of the resin/matrix interface providing that the influence of the environmental agent on all of the components of the model composite: resin, fiber, and interface/phase, is considered.

Fiber Bundle Pushout: A Technique for the Measurement of Interfacial Sliding Properties

An experimental technique for the measurement of interfacial sliding properties in fiber‐reinforced materials is presented. The technique involves pushing a bundle of fibers simultaneously through a thin section of the composite. Using this technique, sample averages are immediately available, eliminating the need for repeated single‐fiber tests. The technique is particularly useful for composites that contain small‐diameter fibers. Salient features of the technique are demonstrated through tests on a CAS/SiC composite.

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part III. The Effects of Postcure

Abstract A novel method for tailoring the interphase of carbon fiber-polymer composites by resistive electric heating is presented. The single-fiber fragmentation test is used to investigate the adhesion and fracture properties of the interphase. Electric resistive heating is shown to increase adhesion and toughness at the interphase region. In analyzing the results, the strength and fracture energy of the interphase are related to the thermal postcure conditions created by resistive electric heating. For this purpose, a difference analysis method is used to obtain a numerical solution for the heat conduction problem in the single-fiber test specimen and the temperature distributions are determined. Improvements obtained by using resistive electric heating of the carbon fiber are compared with those obtained by postcuring of the whole sample via convective thermal postcuring. The results obtained using these two different postcure methods seem to be similar.

A variational mechanics analysis of the stresses around breaks in embedded fibers

Embedded single-fiber tests are often used to characterize the fiber/matrix interface, but their interpretation is usually limited by reliance on the qualitative view of the stresses provided by shear-lag analyses. This paper describes a new, three-dimensional, axisymmetric solution for the stresses around breaks in embedded fibers. The new solution is obtained using variational mechanics. It obeys equilibrium and traction boundary conditions exactly, obeys compatibility approximately, includes all components of the stresses, accounts for interacting fiber breaks, and includes residual thermal stresses. We apply the stress analysis to the single-fiber fragmentation test. In some sample calculations, we plot all components of stress at the fiber/matrix interface and give predictions for an “ideal” single-fiber fragmentation test. The stress analysis technique is readily adaptable to new problems such as the single-fiber pull-out test, the microdrop debond test, the description of interfacial fracture or yielding, and the effect of interfacial friction.

Zirconia and organotitanate film formation on graphite fiber reinforcement for metal matrix composites

The formation and characterization of zirconia coatings on graphite/carbon filaments were investigated. The objective was to eliminate or minimize degradative chemical reactions and improve bonding at the metal/carbon-fiber interface when the coated fiber is used as reinforcement in metal matrix composites. Thin, homogeneous films of zirconium oxide, ZrO2, less than 1μm thick, were formed on carbon monofilaments (35 μm diameter) from a zirconium oxychloride solution in water (less than 1.0 wt. % ZrO2) by dip coating and heating. Chemical changes during thermal decomposition and polycondensation were examined by FTIR spectroscopy. Through dynamic x-ray diffraction tests, the zirconia coating was found to transform first to a metastable tetragonal phase on heating to 330 °C, and then upon cooling, to a stable monoclinic structure. Organotitanate coatings were formed by electrodeposition of the ionizable organometallic complex, titanium di(dioctylpyrophosphate) oxyacetate (TDPA). Single fiber tests revealed a slight reduction in the strength of fibers with thicker coatings, probably due to crack initiation by brittle fracture of the ZrO2 coating. Thin coatings applied from 0.25% ZrOCl2 did not cause such strength reduction. Effective bonding of ZrO2 coatings to the filaments was revealed in single filament composite tests which showed the interfacial shear strength (IFSS) of the coated filaments in an epoxy matrix to be higher than that of the uncoated filaments. The IFSS of monofilaments electrocoated by the titanate was also higher. Examination of the fracture surfaces showed fiber pull-out associated with poor bonding in the case of specimens prepared from uncoated monofilaments. The coated monofilaments showed no fiber pull-out, suggesting that maximum fiber strength was achieved and transferred through the interface to the matrix. Finally, the chemical and thermal stability of the interfacial region of coated and uncoated graphite rods embedded in an aluminum matrix were evaluated. The uncoated rod showed little or no interfacial bonding to the metal matrix, suggesting poor wetting, while the dip-coated and electrocoated rods showed good wetting and compatibility with the metal matrix.

The effect of an interphase on the stress and energy distribution in the embedded single fibre test

The finite element method is used to study the behaviour of an E-glass/epoxy system containing an interphase tested by the embedded single fibre test. The effect of mechanical properties and thickness of an interphase on the stress distribution after fibre breakage is examined. Energy distribution within the composite and elastic energy released during propagation of an interfacial crack are evaluated. These results are used to develop a strain history for this system.

Continuous monitoring of the fragmentation phenomenon in single fiber composite materials

AbstractA continuously monitored single‐filament composite (CM‐SFC) test was conducted to measure the stress at which successive fiber breaks occur in the single fiber fragmentation process. This exercise was performed with a limited number of samples of various types. The purpose was to explore the possibility of using this test as a simple alternative means of (i) measuring the size effect in single fibers, (ii) calculating the Weibull shape and scale parameters for fiber strength, (iii) calculating the fiber/matrix interfacial shear strength from the extrapolated value of fiber strength using the loading history of a single fragmentation test, rather than from the value of fiber strength extrapolated from extensive testing of single fibers at various gage lengths, as is usually done. These are aspects of the SFC test that have largely been ignored so far. The results presented here confirm the possibility of using the CM‐SFC test for such purposes, with a certain degree of approximation, as discussed. Additional information supplied by this test as well as a possible effect of fiber pre‐tensioning on fragmentation results (including the value of the interfacial shear strength) are also briefly discussed.

Loss of Curing Agent During Thin Film (Droplet) Curing of Thermoset Material

Abstract Recently, it has been reported by our group and others1.2 that loss of curing agent is encountered during the curing of small droplets or thin films of amine cured epoxies. In our earlier study3 results were reported on loss of curing agent in small droplets used in conducting the rnicrobond, single fiber test for determination of interfacial shear strength (ISS). It was reported that use of a volatile curing agent (meta-phenylene diamine (m-PDA) with DGEBA resin) resulted in increasing amounts of curing agent being lost (as measured by T8 of the cured droplets) with decreasing droplet size during the processing procedure. Droplets smaller than 150 micrometers were seen to lose up to 40% of the curing agent leading to alteration of the mechanical properties of the droplet and, therefore, causing measured values of ISS to be exceedingly low. Use of a less volatile curing agent (Jeffamine 700, a polyether diamine, Texaco Specialty Chemicals) in combination with DGEBA resin produced results which in...

Measurement of the thermomechanical stability of interphases by the embedded single fiber test

In this review of results of the embedded single fiber composite test it is shown that stress transferability in E-glass and carbon fiber reinforced polymer composites can be characterized by a fragment length distribution, and that the mode of failure in most instances can be characterized by a fracture energy criterion. E-glass fiber systems appear to be bounded by fracture energies of the order of 230 J/m 2 for well-bonded interfaces to 60 J/m 2 for poorly-bonded interfaces, with values being only slightly influenced by temperature. The upper limit of fracture energy in carbon fiber systems may be somewhat lower than in glass fiber systems and the use of higher modulus carbon fiber seems to result in lower fracture energies. Fragment length distribution are sensitive to fiber surface treatment and temperature, and are useful for predicting the strength and stiffness properties of composites.