Uniaxial tensile and creep behaviour of an alumina fibre-reinforced ceramic matrix composite: II. Modelling of tertiary creep

The tertiary creep of an alumina fibre-reinforced silicon carbide composite is modelled on the basis of the damage mechanisms activated during tensile creep tests carried out under vacuum at 1100°C. Progressive fibre-matrix debonding induced by the difference between the radial creep strain of the fibres and that of the matrix mantle is used to explain the axial creep behaviour. Fibre failure and the subsequent stress redistribution are also taken into account. The modelling approach successfully describes: (i) the time evolution of the creep rate, (ii) the decrease of the elastic modulus, (iii) the failure mode after tertiary creep and (iv) the stress dependence of the creep rate in the secondary stage and of the time to rupture of the composite. It is shown that a conventional creep stress exponent cannot be determined in this and similar composite materials because of the stress dependence of the damage accumulated in the composite before the secondary stage is reached.     

Creep behaviour of a SiC/Si-B-C composite with a self-healing multilayered matrix

The creep behaviour of a SiC/Si-B-C composite at 1200 °C in argon is investigated under static and cyclic loading conditions. The SiC/Si-B-C composite consists of a multilayered self healing matrix reinforced with Nicalon fibers. It was produced via chemical vapor infiltration (CVI). The creep behaviour is examined with respect to the extent of damage created during an initial step of monotonic loading and controlled through the applied strain. The creep rate is shown to be dictated mainly by creep of fibers and interfacial debonding, whereas no significant creep induced matrix cracking was detected.     

Compressive behaviour of rigid rod polymer fibres and their adhesion to composite matrixes

Abstract

The deformation behaviour of the new high performance polymer fibres, poly(p-phenylene benzobisoxazole) (PBO) and polypyridobisimidazole (PIPD) and their adhesion to an epoxy composite matrix have been investigated. Both fibres give well defined Raman spectra, and the deformation micromechanics of PBO and PIPD single fibres and composites were studied from stress induced Raman band shifts. Single fibre stress-strain curves were determined in both tension and compression, thus providing an estimate of the compressive strength of these fibres. It was found that the PIPD fibre has a higher compressive strength (~1 GPa) than PBO (~0·3 GPa) and other high performance polymer fibres, because hydrogen bond formation is possible between PIPD molecules. It has been shown that when PBO and PIPD fibres are incorporated into an epoxy resin matrix, the resulting composites show very different interfacial failure mechanisms. The fibre strain distribution in the PBO-epoxy composites follows that predicted by the full bonding, shear lag model at low matrix strains, but deviations occur at higher matrix strains due to debonding at the fibre/matrix interface. For PIPD-epoxy composites, however, no debonding was observed before fibre fragmentation, indicating better adhesion than for PBO as a result of reactive groups on the PIPD fibre surface.     

Development and characterisation of high-density oxide fibre-reinforced oxide ceramic matrix composites with improved mechanical properties

A low cost and reliable ceramic matrix composite fabrication route has been developed. It involves the coating of 2D woven ceramic fibres (Nextel? 720) with oxide nano-size ceramic particles by electrophoretic deposition (EPD) followed by impregnation of the coated fibres with ceramic matrix and warm pressing at 180 °C to produce the “green” component ready for pressureless sintering. The effects of two different weak interface materials, NdPO₄ and ZrO₂, on the thermomechanical properties of the composites are also examined. Damage mechanisms, such as debonding, fibre fracture, delamination and matrix cracking within the composite plates subjected to tensile loading are analysed using acoustic emission technique and correlated with microstructure. It is shown that the composites with NdPO₄ interface, 10% porosity and 40 vol.% fibre loading have superior themomechanical properties in terms of strength and damage-tolerant behaviour in multilayer plate form. The improved sinterability and microstructure stability at moderate temperatures ensure both the fibre integrity and load transfer efficiency resulting in high strength damage-tolerant composites. The final components produced are considered to be suitable for use as shroud seals and insulating plates for combustor chambers in aircraft engines.     

Mechanical characterisation of mullite-based ceramic matrix composites at test temperatures up to 1200°C

Nextel 610 fibre-reinforced mullite-based matrix fabricated by Dornier Forschung was characterised at DLR Institute of Materials Research. The material was produced by the polymer route after coating the fibres with a 0.1 μm thick carbon layer. The composite was manufactured by infiltrating the fibres with a slurry containing a diluted polymer and mullite powder, curing in an autoclave and subsequently heat treating and pyrolysis of the polymer. A final heat treatment in air is performed to remove the carbon coating and to reduce the residual stresses. A (0/90/0/90/0/90)_s-laminate was produced with an average fibre volume fraction of 45.6% and a porosity of 15.9%. Dog-bone-type tensile specimens with a width of 10 mm were cut from the plate by water jet and tested at temperatures up to 1200°C in air. The tensile strength at room temperature measured 177.4 MPa and linearly decreased to 145.2 MPa at a temperature of 800°C. A stronger decrease occurred at 1000 and 1200°C. In contradiction to ceramic matrix composites manufactured by the CVI-route the stress–strain behaviour is nearly linear up to failure. The modulus of the composite (at room temperature 108.8 GPa) is analysed on the basis of the expected moduli of the fibres and the mullite matrix. It can be concluded that the contribution of the matrix to the modulus of the composite is low, caused by porosity and components other than mullite. The intralaminar shear strength at room temperature measured 36 MPa. This value reflecting shear transfer capability of fibre to matrix limits the amount of fibre pull-out.     

A study of the large strain deformation and failure behaviour of mixed biopolymer gels via in situ ESEM

A mixed biopolymer gel, consisting of a protein (gelatin) and polysaccharide (maltodextrin) mixture has been investigated. By controlling the composition it was possible to construct an ‘emulsion-like’ structure, with included spherical particles of one phase (maltodextrin) within a continuous matrix of the second (gelatin). Large strain deformation and failure behaviour of this system has been examined via in situ environmental scanning electron microscopy (ESEM). ESEM has been employed to explore the changes in the structure of the material, whilst allowing the sample to stay hydrated as it was subjected to tensile strain, thereby allowing simultaneous imaging and determination of stress-strain data of the native sample. Ductile behaviour was observed, which has been attributed to the stretching, tearing and fracture of gelatin ligaments and debonding at the interface between the maltodextrin particles and continuous gelatin matrix. Deformation and fracture of the maltodextrin particles during tensile testing was also observed. The interfacial fracture energy of the composite has been calculated following an elastomer composite-debonding model, although there are several limitations to this approach for the mixed gel. It was found in samples tested after different ageing times that the debonding stress and strain was decreasing with ageing leading to a lower interfacial fracture energy. Samples were also tested after successive loading cycles, which resulted in a mechanical strength decrease after each cycle.     

Creep of a Nextel™720/alumina ceramic composite containing an array of small holes at 1200°C in air and in steam

Tensile stress-strain and tensile creep behaviors of an oxide-oxide composite containing an array of small circular holes were evaluated at 1200°C. The composite consists of Nextel™720 alumina-mullite fibers in a porous alumina matrix. Test specimens contained an array of 17 holes with 0.5-mm diameter drilled using a CO₂ laser. The presence of holes caused reduction in tensile strength and modulus. Tensile creep tests were conducted at 1200°C in air and in steam at creep stresses ranging from 38 to 140 MPa. Primary, secondary, and tertiary creep regimes were noted in air and in steam. The presence of the laser-drilled holes accelerates the steady-state creep rates. Creep run-out, defined as 100 hours at creep stress, was attained for stress levels <60 MPa in air and for stresses <40 MPa in steam. The presence of the laser-drilled holes significantly degrades creep resistance of the composite. The retained tensile properties of all specimens that attained run-out were determined. Composite microstructure was examined; the damage and failure mechanisms were considered. The degradation of tensile properties and creep resistance are attributed to damage caused to composite microstructure by laser drilling.     

Tensile properties and creep response of polypropylene fibre composites with variation of fibre diameter

The mechanical properties and morphology of polypropylene (PP) long‐fibre reinforced random poly(propylene‐co‐ethylene) (PPE) composites (50/50 % vol/vol) have been investigated with reference to the fibre diameter with constant length. There is an improvement in the mechanical properties of PPE matrix by incorporation of long PP fibres into the matrix. The elastic modulus of the composite increased with decrease in the fibre diameter to 50 µm, to 0.91 GPa, which was 5 times higher than for pure PPE. However, composite stiffness decreased with decreasing fibre diameter of less than 50 µm and this is discussed in term of the fibre stiffness, packing, stress concentration and aspect ratio. Creep resistance of the composites showed the same behaviour. Morphology of the composites was investigated using scanning electron microscopy. This showed that there was a thin layer of matrix on the reinforcement, which was attributed to good impregnation and wetting of the fibres. Moreover, prediction of tensile modulus using the Cox model correlated well with experimental data. Copyright © 2004 Society of Chemical Industry     

Resonant frequency study of tensile and shear elasticity moduli of carbon fibre reinforced composites (CFRC)

The dynamic elastic properties are important characteristics of composite materials. They control the vibrational behaviour of composite structures and are also an ideal tool for monitoring of the development of CFRCs’ mechanical properties during their processing (heat treatment, densification). The present studies have been performed to explore relations between the dynamic tensile and shear moduli and some structural features (viz., fibre fraction, fibre type, porosity, weave pattern of woven reinforcement) of various unidirectional or bi-directional fibre reinforced carbon/carbon composites, made out of PAN- or pitch-based fibres as reinforcements and phenolic resin or coal tar pitch as matrix precursors. The dynamic tensile and in-plane shear moduli were determined from resonant frequencies of a beam with free ends. The longitudinal dynamic Young’s modulus of unidirectional CFRC composites – besides its dependence on the original fibre modulus and fibre volume contents – also reflects changes induced in matrix and fibres by heat treatment. The in-plane shear modulus does not depend on the fibre type but there exists its distinct tendency to increase with increasing fibre fraction. For bi-directionally reinforced composites, the longitudinal tensile modulus is more sensitive to the fabric weave pattern than to the fibre type. Tensile modulus of diagonally cut specimens and in-plane shear modulus of longitudinally cut ones are mutually correlated and, therefore, simultaneously controlled by densification steps and graphitisation heat treatment.     

Fine diameter ceramic fibres

Two families of small diameter ceramic fibres exist. The oxide fibres, based on alumina and silica, which were initially produced as refractory insulation have also found use as reinforcements for light metal alloys. The production of SiC based fibres made possible the development of ceramic matrix composites. Improved understanding of the mechanisms which control the high temperature behaviour of these latter fibres has led to their evolution towards a near stoichiometric composition which results in strength retention at higher temperatures and lower creep rates. The SiC fibres will however be ultimately limited by oxidation so that there is an increasing interest in complex two phase oxide fibres composed of α-alumina and mullite as candidates for the reinforcement of ceramic matrices for use at very high temperatures. These fibres show low creep rates, comparable to the SiC based fibres but are revealed to be sensitive to alkaline contamination.     

Creep and Stress–Strain Behavior after Creep for SiC Fiber Reinforced, Melt-Infiltrated SiC Matrix Composites

Silicon carbide fiber (Hi-Nicalon Type S, Nippon Carbon) reinforced silicon carbide matrix composites containing melt-infiltrated silicon were subjected to creep at 1315°C at three different stress conditions. For the specimens that did not rupture after 100 h of tensile creep, fast-fracture experiments were performed immediately following the creep test at the creep temperature (1315°C) or after cooling to room temperature. All specimens demonstrated excellent creep resistance and compared well to the creep behavior published in the literature on similar composite systems. Tensile results on the after-creep specimens showed that the matrix cracking stress actually increased, which is attributed to stress redistribution between composite constituents during tensile creep.     

Kenaf‐filled biodegradable composites: rheological and mechanical behaviour

Biodegradable polymer composites, typically based on biodegradable polymer matrices and natural‐organic fillers, are gaining rising interest and importance over the last few years. Several natural‐organic fillers can be used but the most widespread so far is wood, in the form of fibres or flour. Alternative cellulosic fillers can ensure advantages in terms of resource utilization and properties of the final composite. In this work, Mater‐Bi^® based biodegradable composites were prepared with two kinds of wood flour, and directly compared with alternative composites containing kenaf fibres. The use of kenaf fibres allowed improved elastic modulus, tensile strength and interaction with the polymer matrix to be obtained, although the filler dispersion was worse. Rheological measurements evidenced higher viscosity and an increasingly elastic behaviour of the melt. Copyright © 2012 Society of Chemical Industry     

Strain rate effect in the single‐fiber‐fragmentation test

The single fiber fragmentation test (SFVU) has been widely used to characterize the interface it fiber‐reinforced polymers. The purpose of the work reported here was to determine the effect of strain rate on the fiber fragment lengths obtained in the SFFT. Three materials systems were used to make single‐fiber‐composite specimens: E‐glass fiber/polycarbonate matrix, AS4‐carbon fiber/polycarbonate matrix, and AS4‐carbon fiber/polycarbonate matrix. The fiber‐matrix adhesion in all three systems is based on physisorption rather than chemisorption. Each system was tested at strain rates ranging over four orders of magnitude. Results are reported in terms of fragment length, the dependent variable in this study, which is inversely related to the quality of the Interface. It was expected that the fragment length would show a systematic decrease with Increasing strain rate, but the expected trend was not found. Although the polycarbonate matrix exhibited rate‐dependent viscoelastic behavior typical of amorphous polymers below T_g, the fragment length at saturation did not show a statistically significant variation with strain rate for any of the three materials systems. A major contributor to the lack of observed effect was the inherent random variability associated with the SFFT; random variability in average fragment length was equal or greater than the 19% effect of rate predicted for ideal elastic systems with no debonding at the interface. In addition, considerable interfacial debonding occurred during the SFFT, not surprising for Interfaces based on physisorption alone. Debonding Interferes with transfer of applied load from matrix to fiber, and would thus interfere with transfer of the effect of rate from matrix to fiber. A tensile Impact test developed previously was also performed on single‐fiber composite specimens made from the same three materials systems. The results of the Impact tests differed from those obtained at controlled strain‐rates for only two of the materials systems.     

Tensile creep behaviour of polymethylpentene–silica nanocomposites

For the first time, poly(4‐methyl‐1‐pentene) (PMP) nanocomposites were prepared by melt compounding 2 vol% of fumed silica nanoparticles, in order to study the role of the nanofiller surface area and functionalization on the tensile mechanical response of the material, with particular focus on its creep behaviour. The high optical transparency of the polymer matrix was substantially preserved in the nanocomposites, while the mechanical properties (in particular the creep stability) were improved. Dynamic mechanical thermal analysis showed an improvement of the storage modulus, more evident above the glass transition temperature of the polymer matrix. Uniaxial tensile tests evidenced that the elastic modulus of the material was positively affected by the presence of silica nanoparticles, even if a slight reduction of the strain at break was detected. The reduction of the tensile creep compliance was proportional to the surface area of the nanofiller, being more evident at high stresses and elevated temperatures. Findley's law furnished a satisfactory fitting of the creep behaviour of the composites, even at high temperatures. It clearly emerges that the incorporation of fumed silica nanoparticles in PMP can be an effective way to overcome the problem of the poor creep stability of polyolefins, especially at high temperatures and high stresses. Moreover the possibility of retaining the original transparency of the material is fundamental for the production of completely transparent PMP components. Copyright © 2010 Society of Chemical Industry     

Tensile Creep and Creep-Recovery Behavior of a SiC-Fiber─Si3N4-Matrix Composite

The tensile creep and creep-recovery behavior of a unidirectional SiC-fiber/Si₃N₄-matrix composite was investigated at 1200°C in air. A primary objective of the study was to determine how various sustained and cyclic creep loading histories would influence the creep rate, accumulated creep strain, and the amount of strain recovered upon specimen unloading. The key results obtained from the investigation can be summarized as follows: (1) A threshold stress of 60 MPa was identified, below which the creep rate of the composite was exceedingly low (∼10⁻¹² s⁻¹). (2) Periodic fiber fracture was identified as a fundamental damage mode for sustained tensile creep at stresses of 200 and 250 MPa. (3) Because of transient stress redistribution between the fibers and matrix, the creep life and failure mode at 250 MPa. were strongly influenced by the rate at which the initial creep stress was applied. (4) Very dramatic creep-strain recovery occurred during cyclic creep; for cyclic loading between stress limits of 200 and 2 MPa, 80% of the prior creep strain was recovered during 50-h-creep/ 50-h-unloading cycles and over 90% during 300-s-creep/ 300-s-unloading cycles. (5) Cyclic loading significantly lowered the duration of primary creep and overall creep-strain accumulation. The implications of the results for microstructural and component design are discussed.     

A microdebonding test for in situ assessment of fibre/matrix bond strength in composite materials

A test technique which gives a quantitative measure of the in situ fibre/matrix bond strength in fibre-reinforced composite materials is described. The test involves the compressive loading of a fibre or region of fibres on a polished specimen surface to produce debonding. Results are presented for the debonding load for glass/epoxy, aramid/epoxy and graphite/epoxy composites. The change in debonding load is also followed as the interface degrades during moisture conditioning at different points through the thickness of the material. It is concluded that refinements to the technique are needed to simplify interpretation of the debonding force in terms of interface shear strength, and to make the test more reproducible.     

The viscoelastic extension of polymer fibres: complex loadings

The response of oriented polymer fibres to complex loading patterns is investigated. It is shown that the creep and stress relaxation is non-linear with the applied stress. The ratio of the creep rate and the stress-relaxation rate is given by the local slope of the tensile curve and not by the elastic modulus as predicted by linear viscoelastic theory. A consequence of this observation is that viscoelastic and yield deformations are coupled. By analysing the results of the step-creep and the strain-relaxation-strain experiments performed on poly(p-phenylene terephthalamide) fibres, it is shown that the linear superposition principle does not apply to the tensile deformation of polymer fibres above the yield point. Finally the various components of the tensile deformation that should be covered by a constitutive equation for polymer fibres are discussed.     

In-situ tensile tests under SEM and X-ray computed micro-tomography aimed at studying a self-healing matrix composite submitted to different thermomechanical cycles

Ceramic matrix composite (CMC) based on SiC fibres and matrix is gradually introduced in aeronautical application, mostly in hottest parts of engines. Three dimensional (3D) structured materials are good candidates for complex parts such as turbine blades. Material is submitted to mechanical stresses at high temperatures in oxidizing and corrosive environments for long durations. During thermomechanical cycles, damage, oxidation and healing-phenomenon occur and develop in the material. X-ray computed micro-tomography (μCT) and tensile test under scanning electron microscopy (SEM) are experimental means to study these phenomenons. These techniques are implemented for the understanding of the behaviour of the oxide (solid or liquid) in the crack of the material. The influence of the oxide in the crack was analyzed during tensile test under SEM or μCT. The observation allows to determine the influence of the oxide on the reclosure of the crack during the unloading.     

Stress-strain behavior and tensile dilatometry of glass bead-filled polypropylene and polyamide 6

The mechanical behavior of multiphase materials is closely related to the interfacial adhesion between their various components. There is considerable interest in the development of simple experimental techniques for characterization of interfacial debonding during mechanical loading. Probably the best known method is tensile dilatometry, in which the onset and progression of debonding are related to the volume of microvoids generated in the material as it undergoes mechanical loading. Several authors have suggested that equivalent information can also be extracted from stress-strain data generated during a simple constant strain rate test. In practice, however, the transition between the initially well-bonded and the debonded state is obscured by the strain-induced softening of the matrix, which is usually observed in the same strain range as the debonding. In this work the filler/matrix debonding in polypropylene and polyamide 6 filled with up to 50 vol % of glass beads is examined using both tensile dilatometry and an analysis of tensile stress-strain curves. It was found that, depending on the level of adhesion, either a complete or partial debonding occurs in the strain range studied (0–8%). It appears that the volume change due to debonding is a small part of the total volume strain recorded. Therefore, the accuracy of the tensile dilatometry is not sufficient to detect the onset of debonding. The loss of stiffness of the composite, particularly when compared to the loss of stiffness of the matrix offers a more promising way to follow the debonding process. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 653–665, 1997     

Creep damage resistance of ceramic–matrix composites

The tensile creep and creep fracture properties in air at 1300 °C are documented for two ceramic fibre-reinforced ceramic–matrix composites (CFCMCs). These recently developed materials were produced with woven bundles of Hi-Nicalon™ fibres reinforcing either A1₂O₃ or enhanced SiBC matrices, allowing data comparisons to be made with similar CFCMCs having different fibre–matrix combinations. The results confirm that the longitudinal fibres govern the rates of strain accumulation and crack growth, but the fracture characteristics are determined by fibre failure caused by oxygen penetration as matrix cracks develop. The analysis then suggests that carbon fibre-reinforced doloma–matrix composites could offer a combination of creep-resistant fibres and creep damage-resistant matrices suitable for long-term load-bearing service in high-temperature oxidizing environments.