Apparent Enhanced Fatigue Resistance under Cyclic Tensile Loading for a HIPed Silicon Nitride

Cyclic tensile loading tests of a commercial HIPed silicon nitride at elevated temperatures have indicated apparent "enhanced" fatigue resistance compared to static tensile loading tests under similar test conditions. At 1150°C, stress rupture results plotted as maximum stress versus time to failure did not show significant differences in failure behavior between static, dynamic, or cyclic loading conditions, with all failures originating from preexisting defects (slow crack growth failures). At 1260°C, the stress rupture results showed pronounced differences between static, dynamic, and cyclic loading conditions. Failures at low static stresses (<175 MPa) originated from environmentally assisted (oxidation) and generalized creep damage, while failures at similar times but much greater (up to 2 x) cyclic stresses originated from preexisting defects (slow crack growth failures). At 1370°C, stress rupture results did not show as pronounced differences between static, dynamic, and cyclic loading conditions, with most failures originating from environmentally assisted (oxidation) and generalized creep damage.     

Comparison of the Creep and Creep Rupture Performance of Two HIPed Silicon Nitride Ceramics

Measurements of the tensile creep and creep rupture behavior were used to evaluate the long-term mechanical reliability of a commercially available and a developmental hot isostatically pressed (HIPed) silicon nitride. Measurements were conducted at 1260° and 1370°C utilizing button–head tensile specimens. The stress and temperature sensitivities of the secondary creep rates were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The stress and temperature dependencies of creep rupture life were determined by continuing individual creep tests to specimen failure. Creep deformation in both materials was associated with cavitation at multigrain junctions. Two-grain cavitation was also observed in the commercial material. Failure in both materials resulted from the evolution of an extensive damage zone. The failure times were uniquely related to the creep rates, suggesting that the zone growth was constrained by the bulk creep response. The fact that the creep and creep rupture behaviors of the developmental silicon nitride were significantly improved compared to those of the commercial material was attributed to the absence of cavitation along two-grain junctions in the developmental material.     

Tensile Stress Relaxation of Alumina at High Temperature

The development of a tensile testing methodology for ceramics which enables a stress vs strain-rate response to be measured at high temperature is described. The test involves a carefully controled stress relaxation test at constant total strain using an experimental procedure and phenomenological analysis previously developed for metallic materials. It is demonstrated here with preliminary tests on alumina at 1050° and 1150°C. This offers, with further development, the possibility of establishing design stresses associated with low strain-rate behavior for structural applications. The results demonstrate that data covering four decades of strain rate may be generated in tests lasting a few hours. The inelastic strain consists of substantial anelastic recoverable strain in addition to a permanent creep strain.     

Tensile Creep and Rupture of Silicon Nitride

We have characterized the tensile creep, rupture lifetime, and cavitation behavior of a commercial, gas-pressure-sintered silicon nitride in the temperature range 1150° to 1400°C and stress range 70 to 400 MPa. Individual creep curves generally show primary, secondary, and tertiary creep. The majority of the primary creep is not recoverable. The best representation of the data is one where the creep rate depends exponentially on stress, rather than on the traditional power law. This representation also removes the need to break the data into high and low stress regimes. Cavitation of the interstitial silicate phase accompanies creep under all conditions, and accounts for nearly all of the measured strain. These observations are consistent with a model where creep proceeds by the redistribution of silicate phase from cavitating interstitial pockets, accommodated by grain-boundary sliding of silicon nitride.     

High-Temperature Tensile Behavior of a Boron Nitride-Coated Silicon Carbide-Fiber Glass-Ceramic Composite

Tensile properties of a cross-ply glass-ceramic composite were investigated by conducting fracture, creep, and fatigue experiments at both room temperature and high temperatures in air. The composite consisted of a barium magnesium aluminosilicate (BMAS) glass-ceramic matrix reinforced with SiC fibers with a SiC/BN coating. The material exhibited retention of most tensile properties up to 1200°C. Monotonic tensile fracture tests produced ultimate strengths of 230–300 MPa with failure strains of ∼1%, and no degradation in ultimate strength was observed at 1100° and 1200°C. In creep experiments at 1100°C, nominal steady-state creep rates in the 10⁻⁹ s⁻¹ range were established after a period of transient creep. Tensile stress rupture experiments at 1100° and 1200°C lasted longer than one year at stress levels above the corresponding proportional limit stresses for those temperatures. Tensile fatigue experiments were conducted in which the maximum applied stress was slightly greater than the proportional limit stress of the matrix, and, in these experiments, the composite survived 10⁵ cycles without fracture at temperatures up to 1200°C. Microscopic damage mechanisms were investigated by TEM, and microstructural observations of tested samples were correlated with the mechanical response. The SiC/ BN fiber coatings effectively inhibited diffusion and reaction at the interface during high-temperature testing. The BN layer also provided a weak interfacial bond that resulted in damage-tolerant fracture behavior. However, oxidation of near-surface SiC fibers occurred during prolonged exposure at high temperatures, and limited oxidation at fiber interfaces was observed when samples were dynamically loaded above the proportional limit stress, creating micro-cracks along which oxygen could diffuse into the interior of the composite.     

Thermomechanical Fatigue Behavior of a Silicon Carbide Fiber-Reinforced Calcium Aluminosilicate Composite

Isothermal fatigue and in-phase thermomechanical fatigue (TMF) tests were performed on a unidirectional, continuous-fiber, Nicalon®-reinforced calcium aluminosilicate glass-ceramic composite ([O]_16, SiC/CAS-II). Monotonic tensile tests were performed at 1100°C (2012°F) and 100 MPa/s (14.5 ksi/s) to determine the material's ultimate strength (σ_ult) and proportional limit (σ_pl). Isothermal fatigue tests at 1100°C employed two loading profiles, a triangular waveform with ramp times of 60 s and a similar profile with a superimposed 60-s hold time at σ_max. All fatigue tests used a σ_max of 100 MPa (40% of σ_pl), R = 0.1. TMF loading profiles were identical to the isothermal loading profiles, but the temperature was cycled between 500° and 1100°C (932° and 2012°F). All fatigued specimens reached run-out (1000 cycles) and were tested in tension at 1100°C immediately following the fatigue tests. Residual modulus, residual strength, cyclic stress-strain modulus, and strain accumulation were all examined as possible damage indicators. Strain accumulation allowed for the greatest distinction to be made among the types of tests performed. Fiber and matrix stress analyses and creep data for this material suggest that matrix creep is the primary source of damage for the fatigue loading histories investigated.     

Experimental Assessment of Plasticity of Nanocrystalline 1.7 mol% Yttria Tetragonal Zirconia Polycrystals

High temperature mechanical behavior of nanocrystalline 1.7 mol% (3 wt%) yttria tetragonal zirconia polycrystals (nc-YTZP) was characterized by compression creep tests. The hot isostatically pressed nc-YTZP with mean grain size of 120 nm was subjected to grain growth to obtain grain sizes in the range of 120–310 nm. Direct measurements of the creep parameters were performed in the temperature range 1150°–1300°C and stress range 5–400 MPa. The strain rates at 1150°C ranged between 2 × 10⁻⁷ and 9 × 10⁻⁵ s⁻¹ when increasing the stress from 15 to 400 MPa. Values of the stress exponent, n =2.0±0.3, and the activation energy, Q =630±40 kJ/mol, were obtained for all test conditions. A value of the grain size exponent, p =1.5±0.3, was obtained at 1150°C in the stress range studied. Detailed microstructural observations revealed the absence of glassy phase at the grain boundaries. The creep parameters were compared with those from the literature, and the results were discussed in terms of the model recently developed by the authors, with a reasonable agreement.     

Role of Oxidation in the Time-Dependent Failure Behavior of Hot Isostatically Pressed Silicon Nitride at 1370°C

Dynamic fatigue studies were conducted on a hot isostatically pressed silicon nitride in ambient air and inert (argon or nitrogen) environments using four-point flexure at 1370°C. Specimens tested in ambient air exhibited a stressing rate dependence with decreased flexure strength with decreased stressing rates. All fracture surfaces of specimens tested in ambient air possessed a sweeping stress-oxidation damage zone that originated at the tensile side of each bend bar. In addition to this stress-oxidation damage, creep damage (e.g., cavitation) was concurrently observed in the specimens tested at the slower stressing rates, which appeared to further weaken the material. However, tests conducted in argon or nitrogen revealed flexure strength to be independent of the stressing rate. Creep damage was present at the slower stressing rates, but no stress-oxidation damage was evident similar to that observed on the specimens tested in ambient air. By decoupling the effects of oxidation and creep, it was evident that the former contributed to the formation of a detrimental stress-oxidation damage zone which significantly reduced the strength of this material at 1370°C.     

Creep and Creep Rupture of an Advanced Silicon Nitride Ceramic

Creep and creep rupture behavior of an advanced silicon nitride ceramic were systematically characterized in the temperature range 1150° to 1300°C using uniaxial tensile creep tests. Absence of tertiary creep and the order-of-magnitude breaks in both creep rate and rupture lifetime at certain threshold combinations of stress and temperature were two characteristic features of the creep behavior observed. Thermal annealing was found to have enhanced both subsequent creep resistance and creep rupture life. The stress exponent (n) and the activation energy (Q) defined in the Norton relation were found to be 12.6 and 1645 kJ/mol for the material investigated. Both values appear to fall in the general range of those reported for other but similar types of Si₃N₄ ceramic materials. The stress exponent, m , equivalent to the slope of the Larson–Miller equation was found to be in the range 13 to 14.4, and that defined as p in the Monkman–Grant relation to be 0.91, based on the available experimental data. The values of m , n , and p obtained above approximately support the interrelationship of the three exponents given by p = m/n.     

Tensile Creep Behavior of a Vitreous-Bonded Aluminum Oxide under Static and Cyclic Loading

Creep deformation and rupture behavior of a vitreousbonded aluminum oxide was investigated under uniaxial static and cyclic tensile loadings at 1000°, 1100°, and 1175°C. The material was more creep resistant, i.e., having lower creep strain rates, under cyclic loading compared to that under static loading. For the same maximum applied stress, the ratio of steady-state creep rate under static loading to that under cyclic loading at 1100°C was approximately 100. However, the value of this ratio decreased to about 10 when the testing temperature was raised to 1175°C or lowered to 1000°C. Under static loading the material had more propensity to develop creep damage in the form of micro- and macrocracks, leading to early failure, whereas under cyclic loading the creep damage was more uniformly distributed in the form of cavities confined to the multigrain junctions. Viscous bridging by the grain boundary second phase may be the primary contributor to the lower creep deformation rate and improved lifetime under cyclic loading.     

Cyclic-Fatigue Behavior of SiC/SiC Composites at Room and High Temperatures

Tension-tension cyclic-fatigue tests of a two-dimensional-woven-SiC-fiber-SiC-matrix composite (SiC/SiC) prepared by chemical vapor infiltration (CVI) were conducted in air at room temperature and in argon at 1000°C. The cyclic-fatigue limit (10⁷ cycles) at room temperature was ∼160 MPa, which was ∼80% of the monotonic tensile strength of the composite. However, the fatigue limit at 1000°C was only 75 MPa, which was 30% of the tensile strength of the composite. No difference was observed in cyclic-fatigue life at room temperature and at 1000°C at stresses >180 MPa; however, cyclic-fatigue life decreased at 1000°C at stresses < 180 MPa. The fracture mode changed from fracture in 0° and 90° bundles at high stresses to fracture mainly in 0° bundles at low stresses. Fiber-pullout length at 1000°C was longer than that at room temperature, and, in cyclic fatigue, it was longer than that in monotonic tension. The decrease in the fatigue limit at 1000°C was concluded to be possibly attributed to creep of fibers and the reduction of the sliding resistance of the interface between the matrix and the fibers.     

The Influence of Substructures on Tensile Creep of MgO Single Crystals

Tensile creep tests were performed at 1400°C and a stress of 44.1 MN/m² on <110> oriented MgO single crystals which had been prestrained at 1800°C in tension to produce various dislocation substructures. Several substructures, ranging from blocky (somewhat diffuse) subgrains to random dislocation arrays, were developed by varying the tensile strain rate at 1800°C. Introducing substructure had no significant effect on the early stages of primary creep (to ∼1% strain), but increased the minimum creep rate compared to that in pristine (substructure-free) material. A precreep substructure of blocky subgrains was more creep resistant than one consisting of random dislocation arrays.     

Static and Cyclic Fatigue of Alumina at High Temperatures

Fatigue behavior of alumina at 1200°C was investigated. Uniaxial tensile tests were conducted in both static and cyclic loading. A variety of loading wave forms were applied during the cyclic tests. Cyclic lifetime is found to be cycle shape dependent and controlled by the duration of the hold time at the maximum tensile stress in a cycle. Cyclic loading with a higher strain rate and a short duration of maximum stress during each cycle provides a beneficial effect on lifetime in comparison to static loading at the same maximum stress. The time to failure for cyclic loading with a longer hold time at maximum stress is very close to the static loading lifetime. Viscous boundary phase may be the primary contributor to the improved cyclic fatigue resistance for cyclic loading with a short duration of maximum stress.     

Effect of Creep Damage on the Tensile Creep Behavior of a Siliconized Silicon Carbide

The tensile creep behavior of a siliconized silicon carbide was investigated in air, under applied stresses of 103 to 172 MPa for the temperature range of 1100° to 1200°C. At 1100°C, the steady-state stress exponent for creep was approximately 4 under applied stresses less than the threshold for creep damage (132 MPa). At applied stresses greater than the threshold stress for creep damage, the stress exponent increased to approximately 10. The activation energy for steady-state creep at 103 MPa was approximately 175 kJ/mol for the temperature range of 1100° to 1200°C. Under applied stresses of 137 and 172 MPa, the activation energy for creep increased to 210 and 350 kJ/mol, respectively, for the same temperature range. Creep deformation in the siliconized silicon carbide below the threshold stress for creep damage was determined to be controlled by dislocation processes in the silicon phase. At applied stresses above the threshold stress for creep damage, creep damage enhanced the rate of deformation, resulting in an increased stress exponent and activation energy for creep. The contribution of creep damage to the deformation process was shown to increase the stress exponent from 4 to 10.     

High-Temperature Fatigue Behavior of Polycrystalline Alumina

The strength and fatigue behavior of a 99.5% polycrystalline alumina were measured as a function of temperature. Both the strength and fatigue behavior remained essentially constant up to 500°C; from 800° to 1100°C the strength and fatigue resistance decreased markedly and at >1100°C macroscopic creep was observed. It is believed that the decrease in strength and fatigue resistance is caused by a grain-boundary glassy phase enhancing subcritical crack growth. Proof-testing at room temperature was effective in improving the strength distributions at both room temperature and 1000°C; however, at 1000°C it was not effective, due to crack growth during the proof test. The good agreement between proof-test results and fracture-mechanics theory indicates that the same flaws control the strength at room temperature and at high temperatures.     

High Temperature Tensile Creep of Magnesium Oxide Single Crystals

The creep behavior and the dislocation substructure developed during creep were investigated for 〈011〉 oriented MgO single crystals creep tested in tension. Creep deformation was studied over stress and temperature ranges of 29.0 to 86.2 MN/m² and 1200 to 1500°C, and the minimum creep rate, ε, was found to obey the relation:

where σ = applied tensile stress, k = the Boltzmann constant, T = absolute temperature, n = 3.8 to 4.5, and A = ll × 10⁻² (MN/m²)^-4 s^-1. Dislocation substructures developed during creep were studied by transmission electron microscopy and etch pitting techniques. At 1400°C, the dislocation density, ρ , at 0.10 tensile creep strain depended on applied stress as ρασ ^2.1. Numerous dislocation loops and long straight dislocations were present, but subboundaries were seldom observed. The results are discussed in terms of two possible operative creep mechanisms: (1) a recovery process based on annealing out of dislocation dipoles and loops, and (2) dislocation glide limited by atmospheres of charged defects surrounding dislocations.     

Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

The creep behavior of a commercial grade of Si₃N₄ was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si₃N₄. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si₃N₄ did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.     

High‐Temperature Compression Deformation Behavior of Fine‐Grained Carbon‐Bonded Alumina

Carbon‐bonded alumina with 33 wt% residual carbon was tested in compression at room temperature and at temperatures between 700°C and 1500°C in quasi‐static tests, creep tests, and stress relaxation tests. Therefore, a new high‐temperature test set up with inert gas chamber and inductive heating was used. The tests were accomplished by investigations of microstructure and Young's modulus. At room temperature, the results exhibit a pronounced hysteresis for the first loading cycle, which almost completely disappeared in subsequent cycles. The creep tests showed characteristic curves for compression whereas primary and secondary (stationary) creep occurred. Above 1000°C, a strong increase in creep rate was detected, whereas almost no creep was observed below this temperature. All creep curves were approximated with the models of logarithmic and Andrade creep. The activation energy for creep was found to be 263 kJ/mol above 1150°C. The resistance against stress relaxation showed an anomaly with a minimum between 1000°C to 1200°C and a maximum between 1300°C and 1400°C.     

Influence of Stress Ratio on the Elevated-Temperature Fatigue of a Silicon Carbide Fiber-Reinforced Silicon Nitride Composite

The influence of stress ratio on the tensile fatigue behavior of a unidirectional SiC-fiber/Si₃N₄-matrix composite was investigated at 1200°C. Tensile stress ratios of 0.1, 0.3, and 0.5 were examined. Fatigue testing was conducted in air, at a sinusoidal loading frequency of 10 Hz. For peak fatigue stresses below the proportional limit of the composite (approximately 195 MPa at 1200°C) specimens survived 5 × 10⁶ cycles, independent of stress ratio. At peak stresses above the proportional limit, fatigue failures were observed; fatigue life decreased significantly as the stress ratio was lowered from 0.5 to 0.1. Creep appears to be the predominant damage mechanism which occurs during fatigue below the proportional limit. Both mechanical cycle-by-cycle fatigue damage and creep contribute to specimen failure at peak stresses above the proportional limit.     

Effect of frequency upon fatigue strength of a short glass fiber reinforced polyamide 6: A superposition method based on cyclic creep parameters

The fatigue behavior of a conditioned short glass‐fiber reinforced polyamide 6 was studied and the effect of the cyclic frequency investigated. Load controlled fatigue tests were performed, and the strains and surface temperature of specimens were recorded continuously. The number of cycles to failure was found to be dependent upon cyclic creep rate, as is typical for short glass fiber reinforced polyamides in the conditioned state. A strong reduction of fatigue strength was observed for increasing cyclic load frequency. This was mainly related to the specimen temperature increase due to hysteretic self heating and its effect on the cyclic creep speed. A frequency superposition method is proposed, expressing the relationship between temperature rise, applied stress, and cyclic creep speed in terms of a parameter derived from the Larson–Miller steady creep parameter. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers