Creep in Interlaminar Shear of a Nextel™720/aluminosilicate Composite at 1100°C in Air and in Steam

Creep in interlaminar shear of an oxide–oxide ceramic composite was evaluated at 1100°C in air and in steam. Composite consists of a porous aluminosilicate matrix reinforced with mullite/alumina (Nextel^?720) fibers, has no interface between fibers and matrix, and relies on the porous matrix for flaw tolerance. The interlaminar shear strength was 7.6 MPa. Creep behavior was examined for shear stresses of 2–6 MPa. Creep run‐out of 100 h was not achieved. Larger creep strains and higher creep strain rates were produced in steam. However, steam had a beneficial effect on creep lifetimes. Composite microstructure, damage, and failure mechanisms were investigated.     

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

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

Creep mechanisms and microstructure evolution of Nextel™ 610 fiber in air and steam

Creep rates of Nextel™ 610 alumina fibers were measured at 1100 °C and 100–500 MPa in air and steam. Steam increased creep rates and reduced fiber lifetimes. Fiber microstructures were characterized by TEM. The small amounts of grain growth, fiber-axis grain elongation, and pore growth that occur during creep were quantified. To separate the effects of stress and temperature on microstructural evolution, grain growth and elongation were also quantified for fibers heat-treated for 1–100 h in air at 1100–1500 °C. Grain growth laws were determined. The contributions of pore growth and grain elongation to creep strain were quantified. Grain elongation accounts for a large fraction of the strain during creep in air, but little in steam. Pore growth was more pronounced in steam, but does not create significant creep strain. Creep and failure mechanisms consistent with the observed microstructural changes are discussed.     

Tensile creep behavior of three-dimensional four-step braided SiC/SiC composite at elevated temperature

This article presents experimental results for tensile creep deformation and rupture behavior of three-dimensional four-step braided SiC/SiC composites at 1100 °C and 1300 °C in air. The creep behavior at 1300 °C exhibited a long transient creep regime and the creep rate decreased continuously with time. The creep behavior at 1100 °C exhibited an apparent steady-rate regime and the creep deformation was smaller than that at 1300 °C. However, the creep rupture time at both temperatures showed little difference. The mechanisms controlling creep deformation and rupture behavior were analyzed.     

Static fatigue of Hi-Nicalon™-S fiber at elevated temperature in air,steam, and silicic acid-saturated steam

A facility for testing SiC fiber tows in static fatigue and creep at elevated temperatures in air and steam was developed. Static fatigue of Hi-Nicalon™-S fibers was investigated at 800°C-1100°C at applied stresses between 115 and 1250 MPa in air, in Si(OH)₄(g)-saturated steam, and in unsaturated steam. Fibers tested in Si(OH)₄(g)-saturated steam and in air had silica scales throughout the test sections, but those tested in unsaturated steam did not develop scales near the steam injection point. Fiber lifetimes in static fatigue were shortest in unsaturated steam, intermediate in Si(OH)₄(g)-saturated steam, and longest in air. Failure strains did not exceed 0.3%. Steady-state strain rates and static fatigue lifetimes are modelled empirically by the Monkman-Grant relationship. Failure mechanisms are discussed.     

Effects of prior aging at 288°C in air and in argon environments on creep response of PMR‐15 neat resin

The creep behavior of PMR‐15 neat resin, a polyimide thermoset polymer, aged in air and in argon environments at 288°C for up to 1000 h was evaluated. Creep tests were performed at 288°C at creep stress levels of 10 and 20 MPa. Creep periods of at least 25‐h in duration were followed by 50‐h periods of recovery at zero stress. Prior isothermal aging increased the elastic modulus and significantly decreased the polymer's capacity to accumulate creep strain. The aging environment had little influence on creep and recovery behaviors. However, aging in air dramatically degraded the tensile strength of the material. Dynamic mechanical analysis revealed an increase in the glass transition temperature from ∼330°C to ∼336°C after 1000 h in argon or in air at 288°C. The rise in the glass transition temperature with aging time is attributed to an increase in the crosslink density of the PMR‐15 polyimide. Increase in the crosslink density due to aging in both air and argon environments is likely behind the changes in the elastic modulus and the decreased capacity for inelastic straining. A visibly damaged surface layer of ∼0.16 mm thickness was observed in specimens aged in air for 1000 h. Results indicate that the unoxidized core material governs the overall mechanical response, whereas the oxidized surface layer causes a decrease in tensile strength by acting as a crack initiation site and promoting early failures. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Scheelite coatings on SiC fiber: Effect of coating temperature and atmosphere

Scheelite coating was deposited on SiC fiber tows from various liquid-phase precursors followed by heat treatments between 900 °C and 1100 °C in different atmospheres. The tensile strength was fully retained for the coated fibers treated at 900 °C in vacuum. Subsequent heat treatment at 1100 °C in Ar had little effect on the fiber strength, which is explained by the excepted good thermal stability between the scheelite coating and SiC fiber. However, larger strength degradation and poor spool ability of coated fibers prepared in Ar/air were found. Assisted oxidation of SiC fiber by calcium salts is suggested to be responsible for the much larger strength degradation of fibers prepared in Ar/air.     

Delayed failure of ceramic matrix composites in tension at elevated temperatures

Ultimate tensile strength of five different continuous fiber-reinforced ceramic matrix composites (CMCs), including SiC_f/BSAS (two dimensional (2D), 2 types), SiC_f/MAS (2D), SiC_f/SiC (2D), and C_f/SiC (2D, 2 types), was determined as a function of test rate at 1100–1200 °C in air. All five CMCs exhibited a significant dependency of ultimate tensile strength on test rate such that the ultimate tensile strength decreased with decreasing test rate. The dependency of ultimate tensile strength on test rate, the applicability of preload technique, and the predictability of life from one loading configuration (constant stress-rate loading) to another (constant stress loading) all suggested that the overall, phenomenological delayed failure of the CMCs would be governed by a power-law type of slow crack growth.     

Experimental and numerical study on creep behaviors of 2D twill woven quartz fiber/silica matrix composites

This study investigates the creep deformation, damage, and rupture behaviors of 2D woven SiO₂/SiO₂ composites via experimental and numerical methods. In situ monotonic tensile tests and creep tests were conducted at 900 °C using a self-designed experimental system and digital image correlation. The tested specimens were characterized by X-ray computed tomography and scanning electron microscopy to conduct quantitative analyses and fracture observations. The obtained creep strain–time curves consist of primary and secondary stages, similar to the creep strain–time curves of most ceramic matrix composites. The matrix at the intersection of fiber bundles cracked under tensile loading. During subsequent creep loading, the propagation of matrix cracks, interfacial debonding, and fiber breakage in longitudinal fiber bundles were observed. At the mesoscale, the creep rupture entails a mechanism analogous to that observed in the monotonic tensile tests. Overall, the SiO₂/SiO₂ composites employed in this study exhibit excellent potential for long-term operation under mechanical loads at high temperatures. Next, a micromechanics-based creep model was proposed to simulate the creep behavior of the composites. In this model, the primary creep law and rule of mixtures were combined to describe the stress redistribution of various constituents and predict the deformation of the composites. In addition, the rupture life was predicted based on the global load-sharing model, two-parameter Weibull model, and shear lag model. The degradation of the matrix modulus and fiber strength was also considered to improve the accuracy of the simulation. The predicted results were in good agreement with the experimental data.     

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 Creep and Creep Rupture Behavior of Monolithic and SiC-Whisker-Reinforced Silicon Nitride Ceramics

The tensile creep and creep rupture behavior of silicon nitride was investigated at 1200° to 1350°C using hotpressed materials with and without SiC whiskers. Stable steady-state creep was observed under low applied stresses at 1200°C. Accelerated creep regimes, which were absent below 1300°C, were identified above that temperature. The appearance of accelerated creep at the higher temperatures is attributable to formation of microcracks throughout a specimen. The whisker-reinforced material exhibited better creep resistance than the monolith at 1200°C; however, the superiority disappeared above 1300°C. Considerably high values, 3 to 5, were obtained for the creep exponent in the overall temperature range. The exponent tended to decrease with decreasing applied stress at 1200°C. The primary creep mechanism was considered cavitationenhanced creep. Specimen lifetimes followed the Monkman–Grant relationship except for fractures with large accelerated creep regimes. The creep rupture behavior is discussed in association with cavity formation and crack coalescence.     

Fiber Strength After Grain Growth in Nextel™ 610 Alumina Fiber

Nextel^? 610 alumina fiber tows were heat‐treated at 1100°C–1500°C for 1 to 100 h in air. Tensile strengths and Weibull moduli were measured for 30 filaments after each heat‐treatment. 3‐D grain size and orientation distributions were described using oblate ellipsoids. The number of grains in a 1 inch gauge length and grains with the largest major and minor ellipsoid‐axes were determined from these distributions. The grain with the largest K_EFF for mixed‐mode fracture was also determined, using the maximum energy release rate criteria from grain‐size and orientation distributions. Grain‐size dependence of tensile strength and Weibull modulus was evaluated. Strength had no obvious dependence on grain size for fibers with average major‐axes smaller than 0.25 μm. For fibers with larger grains, grain‐size dependence may involve flaws originating from clumps of grains, rather than a single grain. Possible relationships between strength and grain‐size and other causes of strength degradation after heat‐treatment are discussed.     

Melt spinning of poly(L‐lactic acid) and its biodegradability

The melt spinning and melt drawing of poly(L ‐lactic acid) (PLLA) were carried out with a melt‐spinning machine, and the mechanical properties, structure, and biodegradability of PLLA fiber were investigated. PLLA fiber with a tensile strength of 0.81 GPa was successfully obtained through two steps of drawing at a draw ratio of 18 in hot water. This fiber had enough tensile strength for common engineering use. The fiber could be degraded under controlled composting conditions at 70°C for 1 week. In scanning electron microscopy observations of the fiber, a regular pattern of cracks running along the vertical direction to the fiber axis was clearly observed. This suggested that the PLLA fiber built up a highly ordered structure arranged along the direction of the fiber axis. After the fiber was left to lie in the ground for 1 year, however, the surface of the fiber was still smooth, and the tensile strength did not decrease much. This PLLA fiber could not be hydrolyzed after 1 month of steeping in a buffer solution at 37°C, but it was rapidly hydrolyzed at more than 60°C. It was suggested that the degradation (hydrolysis) rate of PLLA depended on the glass‐transition temperature. Upon hydrolysis at 80°C for 48 h, a regular crack along the vertical direction to the fiber axis was found that was very similar to that observed in degradation under composting conditions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2118–2124, 2005     

Fabrication of nearly stoichiometric polycrystalline SiC fibers with excellent high‐temperature stability up to 1900°C

Due to the extensive applications of SiC fiber‐reinforced composite materials in the fields of aviation, aerospace, and nuclear power, there are increasing demands for SiC fibers with both excellent mechanical performance and high‐temperature stability. In this work, nearly stoichiometric polycrystalline SiC fibers were fabricated using amorphous Si–C–Al–O fibers with excess carbon and oxygen (C/Si = 1.34, O content: 7.74 wt%). The nearly stoichiometric composition (C/Si = 1.05) of the product fibers was achieved by thermal decomposition of the starting fibers. The fibers were well‐crystallized with grain sizes of ~200 nm due to sintering at a high temperature of 1900°C. The fibers exhibited a high tensile strength and a high elastic modulus and were composed of SiC grains with twins and stacking‐faults, exhibiting intragranular fracture behavior. Furthermore, the fibers maintained their original tensile strength after being maintained at 1800°C for 5 hour or at 1900°C for 1 hour under an inert atmosphere, and they exhibited a high strength retention (97%) after exposure at 1300°C for 1 hour under air. The high‐temperature stability and creep resistance of the fibers were comparable to that of commercial Hi‐Nicalon S and Tyranno SA fibers.     

Microstructural and mechanical characterization of a mullite fiber

The effect of thermal exposure on a mullite fiber was analyzed. This type of mullite fiber, consisting of γ-Al₂O₃ and amorphous SiO₂, was developed for high-temperature applications. Heat treatments at temperatures ranging from 900 °C to 1500 °C for 1h were performed in air. Investigations showed that the tensile strength of the initial fiber was about 1.60 GPa. And the elastic modulus was about 133.51 GPa. The bundles’ strength decreased at 900 °C slightly after thermal treatment, then increased and got a maximum at 1100 °C with 1.65 GPa. At above 1100 °C, the strength degraded sharply due to the mullite phase transformation and the exaggerated grain growth. At 1300 °C, the phase reaction almost finished with a tensile strength of 0.86 GPa. And the strength retention was only 47.50%. When the heat-treated temperature got to 1500 °C, the density of surface defects in the fiber surged, making it too fragile and weak to go through the tensile tests.     

Creep properties of melt infiltrated SiC/SiC composites with Sylramic™-iBN and Hi-Nicalon™-S fibers

Isothermal tensile creep tests were conducted on 2D woven and laminated, 0/90 balanced melt infiltration (MI) SiC/SiC composites at stress levels from 48 to 138 MPa and temperatures to 1400°C in air. Effects of fiber architecture and fiber types on creep properties, influence of accumulated creep strain on in-plane tensile properties, and the dominant constituent controlling the creep behavior and creep rupture properties of these composites were investigated. In addition, the creep parameters of both composites were determined. Results indicate that in 2D woven MI SiC/SiC composites with Sylramic™-iBN or Hi-Nicalon™-S fibers, creep is controlled by chemical vapor infiltration (CVI) SiC matrix, whereas in 2D laminated MI SiC/SiC composites with Hi-Nicalon™-S fibers, creep is controlled by the fiber. Both types of composites exhibit significant variation in creep behavior and rupture life at a constant temperature and stress, predominantly due to local variation in microstructural inhomogeneity and stress raisers. In both types of composites at temperatures >1350°C, residual silicon present in SiC matrix to reacts with SiC fibers and fiber coating causing premature creep rupture. Using the creep parameters generated, the creep behaviors of the composites have been modeled and factors influencing creep durability are discussed.     

Thermal exposure effects on the long‐term behavior of a mullite fiber at high temperature

The behavior of an oxide fiber at elevated temperatures was analyzed before and after thermal exposures. The material studied was a mullite fiber developed for high‐temperature applications, CeraFib 75. Heat treatments were performed at temperatures ranging from 1200°C to 1400°C for 25 hours. Quantitative high‐temperature X‐ray analysis and creep tests at 1200°C were carried out to analyze the effect of previous heat treatment on the thermal stability of the fibers. The as‐received fibers presented a metastable microstructure of mullite grains with traces of alumina. Starting at 1200°C, grain growth and phase transformations occurred, including the initial formation of mullite, followed by the dissociation of the previous alumina‐rich mullite phase. The observed transformations are continuous and occur until the mullite phase reaches a state near the stoichiometric 3/2 mullite. Only the fibers previously heat treated at 1400°C did not show further changes when exposed again to 1200°C. Overall, the heat treatments increased the fiber stability and creep resistance but reduced the tensile strength. Changes observed in the creep strain vs. time curves of the fibers were related to the observed microstructural transformations. Based on these results, the chemical composition of the stable mullite fiber is suggested.     

Tensile creep and rupture of 2D-woven SiC/SiC composites for high temperature applications

The tensile creep and rupture behavior of 2D-woven SiC fiber-reinforced SiC matrix composites with potential for advanced high temperature structural applications was determined in air at 1315 °C. The results are compared to similar SiC/SiC data in the literature in order to understand the underlying creep and rupture mechanisms. Focus was placed on three different near-stoichiometric SiC fiber-types and three SiC-based matrix systems produced by different process routes. In general, the creep and rupture properties of the tested composites were primarily dictated by the creep resistance of the fiber-type, with the Sylramic-iBN fiber typically showing the best behavior. However, the type of matrix did have an effect on the composite creep and rupture lives due to load-sharing differences for the different matrix types and due to stoichiometry in the case of chemical vapor infiltration SiC matrices.     

Structural analysis during creep process of ultrahigh‐strength polyethylene fiber

Structural changes during the creep process of ultrahigh‐strength polyethylene fiber (UHSPE) were investigated using X‐ray and the solid‐state NMR techniques. As the creep strain increases, the quantity of the amorphous phase area estimated by the ¹³C‐NMR method increases until the final creep rupture. On the other hand, the amorphous quantity estimated by the X‐ray method does not change noticeably. To explain this contrast, we proposed a new model that illustrates how the defects such as chain ends incorporated into the crystalline phase are excluded from the crystallite and agglomerate to generate a new amorphous area, which has a size hardly detected by the X‐ray method. These small amorphous areas are considered to cause a decrease in the tensile strength and the successive final creep rupture. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 312–320, 2001     

Tensile creep properties and damage mechanisms of 2D-SiCf/SiC composites reinforced with low-oxygen high-carbon type SiC fiber

Tensile creep properties of 2D-SiC_f/SiC composites reinforced with low-oxygen high-carbon type SiC fibers were studied in vacuum at 1300°C∼1430°C. The fracture morphology was observed by scanning electron microscopy and the damage of fiber in 2D-SiC_f/SiC composites was characterized by nanoindentation. Moreover, the microstructure of the composite was investigated by high-resolution transmission electron microscopy. The results show that rupture time is much shortened and steady-state creep rate increase three orders of magnitude when creep temperature is higher than 1400°C. There are two different creep damage mechanisms due to the decrease of interfacial bonding strength at high temperature. The amorphous SiO_xC_y phase in the fibers can crystallize into SiC and C and the SiC grain grows in the fiber. The microstructural changes lead to the decrease of fiber strength and degrade the creep properties of the composite above 1400°C.