Creep of Hi-Nicalon S Ceramic Fiber Tows at 800 deg C in Air and in Silicic Acid-Saturated Steam

Robertson, Scott J.

doi:10.21236/ad1003576

Cited by 4 publications

(7 citation statements)

References 107 publications

(231 reference statements)

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“…The outliers at 800° and 900°C have measured strengths about 20% lower than predicted strengths (Figures B and B). Strength decreases of this magnitude have also been observed during static fatigue of Hi‐Nicalon™‐S fiber tows in steam, and are currently under further investigation in extensive studies of Hi‐Nicalon™‐S in silicic acid saturated steam . Another SiC strength degradation mechanism that is additional to effects of scale residual stress is inferred to be active during oxidation in H 2 O containing environments.…”

Section: Resultsmentioning

confidence: 78%

Model for SiC fiber strength after oxidation in dry and wet air

Hay

Mogilevsky

2018

J Am Ceram Soc

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The strengths of oxidized SiC fibers were modeled from the effects of SiO 2 scale residual stress on fracture. Surface tractions from scale residual stress were determined for SiC surface flaws. The residual stress was the sum of the growth stress from oxidation volume expansion, thermal stress from SiO 2 -SiC thermal expansion mismatch, and stress from phase transformations in crystallized scale. The partial relaxation of tensile residual stress from scale cracking was also calculated. Scale thicknesses were determined using Deal-Grove oxidation kinetics for glass and crystalline scales. Kolmogorov-Johnson-Mehl-Avrami (KJMA) kinetics was used to determine scale crystallization rates. Strengths of fibers with glass and with crystalline scales formed by oxidation in dry and wet air between 600°and 1400°C were modeled. The effects of partially crystallized scales were calculated using Weibull statistical methods. Modeled strengths were compared with measurements. Slight strength increases after glass scale formation, large decreases that accompany scale crystallization, and some differences between dry and wet air oxidation were accurately modeled. This suggests that under some conditions the scale residual stress dominates the changes in strength after SiC fiber oxidation. However, modeled strengths were significantly higher than those measured for some fibers oxidized in wet air, which suggests another degradation mechanism is active for these conditions. Modeling assumptions and implications for SiC fiber strength after oxidation for long times are discussed. K E Y W O R D S fibers, modeling/model, oxidation, silicon carbide, strength Abbreviations: A, Deal-Grove oxidation parameter; a, Radius of a penny-shaped surface crack; minor axis of ellipse shaped flaw; B, Deal-Grove parabolic oxidation parameter (parabolic); B C Deal-Grove parabolic oxidation parameter for crystalline SiO 2 scale; c, Major axis of ellipse shaped flaw; c 1 , , SiC fiber fracture toughness (Critical stress intensity factor); K R , Residual stress intensity factor; k V , Pre-exponential factor for Newtonian viscosity; κ, Cooling rate; Λ, Fraction of original thermal strain energy remaining in a cracked scale; λ, Crack spacing; m, Weibull modulus; n Time growth exponent for scale crystallization; P Pressure across SiC-SiO 2 interface imposed by residual stress; Q V Activation energy for Newtonian SiO 2 viscosity; Q f , Activation energy for KJMA SiO 2 crystallization kinetics; Θ A collection of terms used in the elasticity solution for calculating thermal residual stress ; R, Gas constant; r f , Final SiC fiber radius after oxidation; r i , Initial SiC fiber radius before oxidation; r, Risk of failure; σ AV , Average fiber strength; σ GΖ SiO2 , Axial growth stress in SiO 2 scale; σ Gθ SiO2 Hoop growth stress in SiO 2 scale; σ o , Weibull characteristic fiber strength; σ TZ SiO2 , Axial thermal stress in SiO 2 scale; σ Z SiC Axial stress in SiC fiber; σ Z SiO2 Axial stress in SiO 2 scale ; T, Temperature; T L , Lock-in temperature for thermal...

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Section: Resultsmentioning

confidence: 78%

Model for SiC fiber strength after oxidation in dry and wet air

Hay

Mogilevsky

2018

J Am Ceram Soc

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“…Figure B shows a schematic of the experimental facility for static fatigue and creep testing ceramic fiber tows at elevated temperature in steam saturated with Si(OH) 4 (g), as well as in air and in unsaturated steam. A detailed description of this facility is given elsewhere …”

Section: Methodsmentioning

confidence: 99%

“…The effective gauge lengths ( γ ) are summarized in Table . Detailed discussion of the temperature profiles, the effective gauge lengths and strain calculations is provided elsewhere . Run‐out was defined as survival of 100 hours at stress.…”

Section: Methodsmentioning

confidence: 99%

“…A detailed description of this facility is given elsewhere. 76,77 Fiber tows were heated to test temperatures at 1°C/s, then held at test temperature for 45 minutes prior to testing. Test temperatures of 800°C, 900°C, 1000°C, and 1100°C were used.…”

Section: Methodsmentioning

confidence: 99%

“…Detailed discussion of the temperature profiles, the effective gauge lengths and strain calculations is provided elsewhere. 76 Run-out was defined as survival of 100 hours at stress. A total of 81 static fatigue experiments were run; they are listed in Table 2.…”

Section: Methodsmentioning

confidence: 99%

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

et al. 2019

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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.

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Subcritical crack growth models for static fatigue of Hi‐Nicalon^TM‐S SiC fiber in air and steam

et al. 2021

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Three subcritical crack growth (SCG) laws were used to model strain-rates and failure times for static fatigue of Hi-Nicalon TM -S SiC fiber tows in air and Si(OH) 4 (g)saturated steam. Models were fit to tow failure times (t f ) and steady-state strain rates (ἑ) for brittle creep measured at 700 to 1100°C under initial applied stresses (σ A ) of 260 to 1260 MPa. A power law, a reaction-rate law, and a bond-energy law were used to describe SCG that caused sequential filament failure, and ultimately tow failure.Two versions of each model were developed. One allowed access of chemisorbed species to flaws throughout the fiber (mode 1) and another only allowed access to flaws at the SiC-SiO 2 interface (mode 2). The stress increase on intact filaments as others fractured and as filaments oxidized, and the increase in stress intensity geometric factors (Y) as crack size increased were incorporated in the models. The fits to data were compared for the different models by using both simple regression analysis and orthogonal distance regression (ODR) analysis. Faster convergence and more consistent results were achieved using ODR analysis. Regression analyses found parameters for all models with similar error in data fits, so validity of a model could not be distinguished by regression analysis alone. For all models, the stress dependence of SCG rates was much stronger in steam than in air, and for most models activation energies were between 300 and 420 kJ/mol, regardless of environment. For the steam environment, the bond-length parameter (δ) for the bond-energy model was very close to the lattice parameter of β-SiC (.436 nm), but in air it was significantly lower at 0.25-0.26 nm, but still larger than the Si-C bond length of 0.189 nm. Other factors suggest that either a bond-energy based law or a modified version of a reaction-rate law are the best choices for a SCG law. Filament strength distributions initially described by Weibull distributions could not be described by such distributions after application of the models. SCG mechanisms are discussed.

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Creep of Hi-Nicalon S Ceramic Fiber Tows at 800 deg C in Air and in Silicic Acid-Saturated Steam

Cited by 4 publications

References 107 publications

Model for SiC fiber strength after oxidation in dry and wet air

Model for SiC fiber strength after oxidation in dry and wet air

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

Subcritical crack growth models for static fatigue of Hi‐Nicalon^TM‐S SiC fiber in air and steam

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Creep of Hi-Nicalon S Ceramic Fiber Tows at 800 deg C in Air and in Silicic Acid-Saturated Steam

Cited by 4 publications

References 107 publications

Model for SiC fiber strength after oxidation in dry and wet air

Model for SiC fiber strength after oxidation in dry and wet air

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

Subcritical crack growth models for static fatigue of Hi‐NicalonTM‐S SiC fiber in air and steam

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Subcritical crack growth models for static fatigue of Hi‐Nicalon^TM‐S SiC fiber in air and steam