Flexural Stress Rupture Research Articles

Stress Rupture in Tension and Flexure of a SiC‐Whisker‐Reinforced Silicon Nitride Composite at Elevated Temperatures

Stress rupture of a 20 vol% SiC whisker‐reinforced Si3N4 composite processed by gas pressure sintering was investigated by both tension and flexure methods. The stress exponents for the stress rupture decrease with increasing temperature. The fracture surfaces of both tensile and flexural stress rupture at 1000°C consist of mirror, mist, and hackle regions. The size of the mirror region increases with decreasing stress. Crack propagation is a mixture of intergranular and transgranular modes at 1000°C. Both tensile and flexural fracture surfaces under constant stress at 1200°C were characterized by a rough zone and a mirror zone; the size of the rough zone increased with decreasing stress. Creep crack growth occurred at 1200°C, which is a process of cavity nucleation, growth, and interlinkage in front of a crack. The transition of fracture mechanisms with temperature is discussed.

Flexural Stress Rupture and Creep of Selected Commercial Silicon Nitrides

Four commercial Si3N4 compositions were compared with regard to flexural stress rupture and creep in ambient air as functions of temperature from 1100° to 1400°C and stress from 200 to 350 MPa. One Si3N4, SN252, was found to be more resistant to time‐dependent deformation in both stepped‐temperature stress rupture tests and creep tests than a very similar Si3N4 composition and two other dissimilar Si3N4 compositions. Materials were compared on the bases of percent final strains, creep rates, and posttest microscope examinations. The latter revealed tensile face transverse cracking, and slow crack growth. The superior behavior of the SN252 Si3N4 was related to its microstructure.

Strength characterization of yttria-partially stabilized zirconia/alumina composite

The flexural strength of yttria-partially stabilized zirconia/alumina composite in the sintered and hot isostatically pressed condition (Super PSZ) was evaluated as a function of temperature (20–1300°C in air environment), applied stress and time. Failure was essentially governed by the presence of processing defects such as zirconia or alumina agglomerates. The sudden decrease in fracture strength at relatively low temperatures (400–600°C) is believed to be due to the stability of the tetragonal phase and relative decrease in the extent of the stress-induced martensitic phase transformation of the tetragonal to monoclinic phase. Flexural stress rupture testing at 300–1000°C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature. Stress rupture testing at 1000°C at low applied stress levels showed bending of specimens, indicating the onset of plasticity or viscous flow of the glassy phase and consequent degradation of material strength.

Strength characterization of MgO-partially stabilized zirconia

The flexural strength of MgO-partially stabilized zirconia was evaluated as a function of temperature (20–1300 °C in air environment), applied stress and time. The indentation-induced-flaw technique did not produce well-defined symmetrical cracks of controlled size, whose length (on the tensile surface) or depth (on the fracture face) can be measured unambiguously, and therefore it should not be used for measuring fracture toughness. The sudden decrease in fracture strength at moderately low temperatures (200–800 °C) is believed to be due to stability of the tetragonal phase and relative decrease in the extent of the stress-induced martensitic phase transformation of the tetragonal to monoclinic phase. Flexural stress rupture testing at 500–800 °C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature. Stress rupture testing at 1000 °C and above at low applied stress levels showed bending of specimens, indicating the onset of plasticity or viscous flow of the glassy phase and consequent degradation of material strength.

High-temperature stress rupture tests for sintered silicon nitride

Silicon nitride is being extensively evaluated for use in making structural components exposed to high temperatures. Strength degradation during use, which governs the durability of silicon nitride, is generally caused by subcritical crack growth from a pre-existing flaw. One method for making a direct evaluation of subcritical crack growth is to measure the crack growth velocity as a function of the applied stress intensity. A stress rupture test for examining delayed fracture under a constant stress is usually conducted to evaluate strength degradation due to subcritical crack growth from a natural flaw. At high temperatures, however, other strength degradation mechanisms such as oxidation and cavity formation can also affect delayed fracture. It is therefore important in stress rupture tests to identify the fracture origins. Several studies [1-5] have been reported on the fracture origins of silicon nitride in stress rupture tests in air at high temperatures. Quinn et al. [1] examined the delayed fracture surface of hot-pressed silicon nitride; they reported that a short time-to-failure test of 1 h). Oxidation pits found in delayed fracture origins of silicon nitride were also reported for stress rupture tests in air [3]. Thus, flaw generation during testing needs to be minimized in order to evaluate the effect of subcritical crack growth. In the present study, stress rupture tests of sintered silicon nitride at high temperature were conducted in a nitrogen atmosphere to minimize flaw generation due to passive oxidation, which forms an oxide surface layer, and active oxidation, which causes the surface decomposition. The material used in this study was sintered silicon nitride having a density of 3.08 g cm -3 . This material was fabricated by sintering a compact of powder mixtures of 88 wt % S i 3 N 4 , 8 Wt % Y 2 0 3 and 4 wt % A120 3 in a nitrogen atmosphere under a pressure of 0.1 MPa at 1700 °C for 1 h. Specimens prepared for stress rupture tests were ground to the dimensions 2 mm × 8 mm x 35 mm. Fig. 1 shows a schematic diagram of the apparatus used for three-point bending stress rupture tests. The load was applied on the test specimen using a deadweight assembly with a cantilever arm. The experimental set-up was equipped with a microswitch to cut off power to the furnace and the timer at the instant specimen failure occurred. After the specimen was installed in the three-point bending fixture made of silicon carbide, the temperature was raised using graphite heating elements. Nitrogen gas was introduced into the furnace through an inlet located in the side, near the bottom. The load was applied on the specimen after reaching the test temperature of 1000 °C at a heating rate of approximately 300 ° C h 1 . Details of the design, construction and operation of the furnaces as well as the fixtures were described in [6]. Fourteen flexural stress rupture specimens of sintered silicon nitride were tested. The applied stress was varied from 296 to 508 MPa. Complete results are given in Table I. Eight specimens were fractured during the tests and tests for six specimens were interrupted before fracture occurred. The time-to-failure ranged from 0.0007 to 24.7 h. Fracture of specimen no. 1 occurred just after loading,

Effect of thermal and stress exposure on grain boundaries of silicon nitride

Abstract The development of silicon nitride materials for high-temperature applications requires an understanding of the grain boundary chemistry. Analytical electron microscopy was used to evaluate the chemical composition of the grain boundaries before and after flexural stress rupture testing and various heat treatments for silicon nitride having strontia and yttria sintering aids. At an elevated temperature the grain boundary pockets were found to under go phase separation with the grain boundaries becoming enriched in strontia, and remaining non-crystalline. In this system creep appears to occur by a solution/reprecipitation mechanism. Viscoelastic flow may also be a strong contributor.

Fracture of hot-pressed alumina and SiC-whisker-reinforced alumina composite

The flexural strength of hot-pressed alumina and SiC-whisker-reinforced alumina composite were evaluated as a function of temperature (20 to 1400° C in air environment), applied stress and time. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A temperature-independent region of fast fracture (catastrophic crack extension) existed up to 800° C, in which the failure mode was a mixture of transgranular and intergranular crack propagation. In this region, the alumina composite showed significantly higher fracture strength and toughness compared to polycrystalline alumina. Above 800° C, both materials (alumina and alumina composite) displayed a decreasing fracture strength due to the presence of subcritical or slow crack growth which occurred intergranularly. Flexural stress rupture evaluation in the temperature range 600 to 1200° C has identified the stress levels for time-dependent and time-independent failures.

Delayed failure of a commercial vitreous bonded alumina

The static fatigue resistance of a commercial low-cost alumina is evaluated. Flexural stress rupture results are compared to fracture mechanics crack growth testing. The original goal of this work was to develop satisfactory experimental procedures for the two test methods prior to more extensive testing on high-performance ceramics such as silicon nitride. The static fatigue trends measured by the two methods are comparable at 1000° C. The results are consistent with a model of static fatigue involving microcrack growth, coalescence and fracture due to stress corrosion. Creep deformations are very small, suggesting that creep fracture is not the mechanism of failure. A refined method for double torsion testing is presented.

Fracture of flash oxidized, yttria-doped sintered reaction-bonded silicon nitride

The oxidation behaviour of a slip cast, yttria-doped, sintered reaction-bonded silicon nitride after “flash oxidation’ was investigated. It was found that both the static oxidation resistance and flexural stress rupture life (creep deformation) were improved at 1000° C in air compared to those of the same material without flash oxidation. Stress rupture data at high temperatures (1000 to 1200° C) are presented to indicate applied stress levels for oxidation-dependent and independent failures.

Static fatigue and creep resistance of a commercial sialon

The static fatigue and creep resistances of a commercial sialon material were evaluated by elevated temperature flexural stress rupture testing in air. The material is a ceramic alloy with a solid solution β′ sialon phase, Si6-z Al z O z N8-z , with a substitution levelz less than 1. The utilization of yttria sintering aids and post-sintering heat treatments leads to a fully crystalline grain-boundary phase. Creep and static fatigue resistances were excellent at temperatures up to 1200° C.

Strength characterization of Yttria/alumina-doped sintered silicon nitride

The flexural strength of yttria/alumina-doped sintered silicon nitride (Ford Material—RM 20) was measured as a function of temperature (20 to 1400°C), applied stress and time. Flexural stress rupture testing at 800 and 1000°C indicated that the material can sustain 344 MPa and 276 MPa, respectively, without failure, for a limited time (≤ 100 h). The RM 20 material was susceptible to both oxidation and early stages of creep deformation at temperatures above 1000°C and displayed extensive creep deformation and degradation in strength above 1300°C.

Mechanical property evaluation at elevated temperature of sintered β-silicon carbide

The mechanical properties at room and elevated temperature of sintered β-silicon carbide were evaluated. Testing included room temperature flexural strength, flexural stress rupture at 1200°C, and stepped temperature stress rupture (STSR) experiments. Fractographic examination identified the strength limiting flaw populations. Properties measured on this material are typical of commercial grades of sintered silicon carbide.

Phenomenology of fracture in sintered alpha silicon carbide

Crack propagation mechanisms in a sintered alpha silicon carbide were studied as a function of initial flaw size, temperature, loading rate and applied stress. Surface cracks of controlled size were introduced using the microhardness indentation-induced-flaw (IIF) technique. At room temperature, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that processing flaws of 20 to 40μm were strength controlling. The flexural strength was found to be independent of temperature (20 to 1400° C) and the fracture faces did not show the presence of subcritical crack growth (SCG). Preliminary results from stress rate testing also failed to show the presence of SCG in tests made at 1200° C in air. In contrast, flexural stress rupture tests carried out at 1200 and 1300° C in air using precracked specimens indicated the materials susceptibility to time-dependent deformation and showed the presence of SCG. Fractographic evidence for transgranular crack propagation during fast fracture (catastrophic failure) and intergranular crack propagation during SCG is presented.

ChemInform Abstract: UNIAXIAL TENSILE AND FLEXURAL STRESS RUPTURE STRENGTH OF HOT‐PRESSED SILICON NITRIDE

Chemischer InformationsdienstVolume 13, Issue 17 Physical Inorganic Chemistry ChemInform Abstract: UNIAXIAL TENSILE AND FLEXURAL STRESS RUPTURE STRENGTH OF HOT-PRESSED SILICON NITRIDE R. K. GOVILA, R. K. GOVILASearch for more papers by this author R. K. GOVILA, R. K. GOVILASearch for more papers by this author First published: April 27, 1982 https://doi.org/10.1002/chin.198217008AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume13, Issue17April 27, 1982 RelatedInformation

Uniaxial Tensile and Flexural Stress Rupture Strength of Hot‐Pressed Si3N4

Uniaxial tensile stress rupture testing of hot‐pressed silicon nitride NC‐132 was carried out at 1000°, 1200°, and 1300°C in air at various applied stress levels and the corresponding times‐to‐failure were measured. Fracture occurred from both surface and internally initiated flaws. Fractographic evidence for failure‐initiation sites and the associated presence of subcritical (slow) crack growth are presented. Limited flexural stress rupture tests at 1200°C in air were also carried out and the data are compared. The uniaxial tensile and flexural stress rupture tests are much more sensitive in revealing the time‐dependent crack‐growth behavior than flexural stress rate tests.