High Temperature Crack Growth Research Articles

アルミナ‐スピネルセラミックスの高温延性とクラック成長

アルミナ‐スピネルセラミックスの高温延性とクラック成長

High temperature crack growth in silicon nitride under static and cyclic loading: Short-crack behavior and brittle-ductile transition

Crack propagation in Si 3N 4 at elevated temperatures was investigated using controlled surface flaws. Crack growth was generally slower under cyclic than static loading conditions. Concomitantly, there was a tendency for crack growth rate to initially decelerate despite an increasing driving force, exhibiting the so-called short-crack behavior. This tendency became more pronounced at higher temperatures, lower stress intensity factors, and larger cyclic stress variations. A corresponding transition in the crack profile, from a sharp to a blunt crack, was observed. These phenomena are attributed to evolutions of crack-wake shielding. Specifically, with rising temperature and stress cycling, grain fraction is lowered, triple points are separated and massive grain pullout is triggered. This mode of pullout mechanism is qualitatively distinct from that operating at lower temperatures, and the transition is believed to occur when the slip length of the grain boundary reaches the average half-length of the grain. This picture is supported by a fracture mechanics estimation of crack growth rate, crack opening displacement and characteristic length of the R-curve based on the pullout mechanism.

Influence of a damage zone on high temperature crack growth in brittle materials

High temperature crack growth in ceramics often occurs by the nucleation, growth and coalescence of cavities in a region ahead of the crack tip known as the damage zone. Models describing this type of behaviour generally assume that the presence of cavities does not affect the stress distribution ahead of the crack tip. In this study, a crack growth simulation has been developed which incorporates the effects of cavity nucleation, growth and coalescence on the stress field ahead of the crack. Cavity growth dominated both by grain boundary diffusion and by surface diffusion has been modelled. The models follow both the transient effects which occur following initial loading and the development of a steady-state regime under conditions of constant applied stress intensity factor K I. In general, the wedging action due to cavity growth reduces the stress field near the crack tip. However, this is largely compensated for by an increase in the damage zone size, as a result of load transfer from the crack tip to the end of the damage zone. We have therefore demonstrated that the established analytical models which do not account for stress redistribution give a much better description of steady-state crack growth behavior than one would expect.

Damage mechanics approach for predicting high-temperature crack growth under mixed-mode loading conditions

Recent studies of the factors that control high-temperature crack growth under mixed-mode loading conditions are reviewed. In particular, measurements of crack growth direction and crack growth rate in specially designed specimens under mode I, mode II and mixed-mode loading conditions are discussed. Attempts have been made with the aid of finite element results to determine how both the crack path and the crack growth rate are influenced by the nature of loading. Discrepancies as to the ability of the von Mises equivalent stress in correlating the experimental data have been observed. A damage mechanics approach is proposed that brings mixed-mode crack growth data into better agreement when plotted against an effective value of C ∗ . This approach is also shown to correlate mixed-mode stress field calculations with reported observations of crack growth direction. The results demonstrate the importance of addressing damage mechanics ahead of the crack tip for accurate predictions of high-temperature crack growth under mixed-mode loading conditions.

Shear-driven high-temperature fatigue crack growth in polycrystalline alumina

High-temperature crack growth behavior of a polycrystalline alumina was examined under Mode-I tension-tension cyclic loading. Locally at the crack tip, the fatigue crack was found to advance is shear by fractional sliding of grains on alternating sets of planes of the maximum shear. Evidence of a shear-driven crack growth was given in terms of topological and morphological analyses of the fatigue crack surface, grain sliding, frictional debris, and temperature-dependence of fatigue crack growth kinetics. Based on experimental observations, a new model of fatigue crack growth by alternating shear was proposed.

Investigations of high temperature deformation and crack growth under variable load histories

Time dependent deformation and crack growth behavior under variable load conditions are investigated in this study. Experimental observations of load/unload effects in cracked creeping bodies are first discussed. Then a detailed analysis of a typical test result is presented. The potential of integral parameters, including the T∗-integral, to characterize this complex response, is shown.

High temperature corrosion and crack growth of SiCSiC at variable oxygen partial pressures

Abstract Thermal gravimetric analysis (TGA) and subcritical crack growth measurements of chemical-vapor-infiltrated SiC matrix reinforced with Nicalon fibres and with a 1 μm thick C fiber-matrix interface have been conducted at 1100 °C over O 2 Ar mixtures ranging from 0.25% to 20.0% O 2 . The TGA and interface recession measurements both gave linear reaction kinetics for O 2 concentrations of 2.0% or less and a reaction order of unity. Subcritical crack growth measurements demonstrated that the crack velocity, in the stress-intensity-independent stage II regime, increases with increasing O 2 /Ar ratio. Also, the transition from stage II to the stress-intensity-dependent stage III regime is shifted to lower stress intensities with increasing O 2 /Ar ratio. A time-dependent crack growth model that incorporates creep of the bridging SiC fibers and the removal of the C interfacial layer by oxidation successfully explains the subcritical crack growth characteristics.

Creep crack growth in power plant materials

Economic considerations have made it desirable to extend the 30 to 40 year operating life of power plants by another 10 to 20 years. Crack growth at elevated temperatures is an important consideration in estimating the remaining life, determining operating conditions and deciding inspection criteria and intervals for power plant materials. This paper presents an overview of high-temperature crack growth phenomenon in such materials. The focus is on various techniques used for characterizing creep crack growth (CCG) and creep-fatigue crack growth (CFCG) in high-temperature materials. The collection of data, their analysis and the interpretation of results is discussed in detail, especially for CFCG laboratory testing. The discussion is primarily focussed on creep-ductile materials such as those used in power plant applications. Special considerations for elevated temperature crack growth in weldments are also presented. Finally, the application of these concepts to the life prediction of power plant components is also discussed.

High temperature crack growth behavior in a precipitate-hardened nickel base superalloy under constant K I conditions

Nickel base superalloys are typically used in high temperature components of gas turbines. A new precipitate (ppt)-hardened cast superalloy has been developed to satisfy the needs for structural component applications in advanced aircraft engines. These applications require an adequate understanding of crack extension mechanisms due to creep deformation and damage in the crack tip region in response to sustained or slowly varying loads. In this study, high temperature crack growth behavior in the nickel base superalloy under constant K[sub I] (Mode I) condition was investigated. The experiments were performed at 650C in air.

Comparison of High‐Temperature Crack Growth in SiC‐Whisker‐Reinforced Mullite and Si3N4

Crack growth behavior under creep conditions was studied in SiC‐whisker‐reinforced mullite and silicon nitride. Tests of four‐point bend specimens with indentation cracks were periodically interrupted to observe the creep behavior. At each interruption the bulk creep strain of the specimen, the growth of the indentation cracks, and the nucleation and growth of creep‐induced cracks were measured. A strong linear correlation was observed in both materials between the crack growth rate and the creep strain rate. For a given strain rate, cracks in the silicon nitride composite propagated at velocities about an order of magnitude greater than those in the mullite composite. On the other hand, for similar nominal stresses, creep rates in the silicon nitride composites were about an order of magnitude less than with the mullite composite.

Standardization of procedures for the high-temperature crack growth testing for FBR materials in Japan

To develop a standard procedure for testing high temperature crack growth behavior of LMFBR materials, a cooperative study was performed using type 304 stainless steel and FEM simulation. J- and J′-integrals (the same as C∗) were selected as parameters to characterize crack growth behavior. Based on the evaluation of the experimental data and FEM analysis, a standard testing procedure was recommended.

An Analysis of High-Temperature Crack Growth in Type 308 Cb Stainless Steel and Its Weldment

Creep crack growth behavior of type 308 Cb austenitic stainless steel and its weldment has been studied in the temperature range of 875 to 1075 K under plane stress condition. The creep crack growth (CCG) rate has been correlated with parameters like stress intensity factor, K, net section stress and energy rate line integral C*. Among these parameters C* appears to describe well the CCG rate in both the cases of base metal and weldment. The rupture time tr of the cracked material has been found to be related to the energy rate line integral C*s in the steady-state condition, given by a relation of the type C*s · tr = constant.

Fatigue Crack Growth Behavior of Alloy 800H at Elevated Temperature

High temperature crack growth behavior is investigated over a wide range of R-ratios, frequencies, and temperatures in Alloy 800H. It is found that high R-ratio, low frequency, or high temperature can enhance creep damage and thus induce an intergranular crack growth mode. At low frequencies, the nonlinear fracture mechanics parameter, C*, is found to correlate time-dependent fatigue crack growth rate well if the applied mean stress is used in calculating C*. On the other hand, the Paris crack growth law using Keff is proven to be an adequate expression to use when fatigue (time-independent) damage dominates. These conclusions correlate well with damage mechanisms observed from sample fracture surfaces.

High temperature crack growth in copper under cyclic loading conditions: Kim, Y.-S. and Nix, W.D. Scr. Metall. Mater. Jan. 1990 24, (1), 213–218

High temperature crack growth in copper under cyclic loading conditions: Kim, Y.-S. and Nix, W.D. Scr. Metall. Mater. Jan. 1990 24, (1), 213–218

Environmental Effects on High Temperature Crack Growth

Environmental Effects on High Temperature Crack Growth

High temperature crack growth in copper under cyclic loading conditions

In the present work, the fatigue crack growth behavior of pure Cu at a high temperature has been characterized. The fatigue crack growth data have been analyzed using fracture mechanics parameters and the fatigue fracture surfaces were examined to relate the appearance of the fracture surface to the operative crack propagation mechanism

Mechanisms of high temperature crack growth in nickel aluminide

AbstractNickel aluminides, based on Ni3Al, exhibit high strength at elevated temperature. However, they may be prone to low ductility intergranular crack growth in the presence of oxygen. It has been found empirically that the presence of chromium improves high temperature tensile ductility. A detailed study of oxidation and crack growth mechanisms in the absence of chromium has provided evidence to suggest that fracture is controlled by the internal diffusion of oxygen along grain boundaries in front of the crack tip. The current work considers the effect of chromium on crack growth and oxidation at crack tips in nickel aluminide. Some of the major results of a more extensive study are summarised, showing crack growth rates as a function of stress intensity and temperature, together with characterisation of the role of chromium in microstructure and crack tip oxidation, using a variety of analytical techniques.MST/1141

Mechanisms of high temperature crack growth in nickel aluminide

Nickel aluminides, based on Ni3Al, exhibit high strength at elevated temperature. However, they may be prone to low ductility intergranular crack growth in the presence of oxygen. It has been found empirically that the presence of chromium improves high temperature tensile ductility. A detailed study of oxidation and crack growth mechanisms in the absence of chromium has provided evidence to suggest that fracture is controlled by the internal diffusion of oxygen along grain boundaries in front of the crack tip. The current work considers the effect of chromium on crack growth and oxidation at crack tips in nickel aluminide. Some of the major results of a more extensive study are summarised, showing crack growth rates as a function of stress intensity and temperature, together with characterisation of the role of chromium in microstructure and crack tip oxidation, using a variety of analytical techniques.MST/1141

Effect of cyclic frequency on the fatigue life of ASME SA-106-B piping steel in PWR environments: Terell, J.B.J. Mater. Eng. Sept. 1988, 10, (3), 193–203

The application of several fracture mechanics data correlation parameters to predicting the crack propagation life of turbine engine hot section materials was evaluated. A survey was conducted to determine the conditions where conventional fracture mechanics approaches may not be adequate to characterize cracking behaviour. Both conventional linear and non-linear fracture mechanics analyses were considered. Isothermal and thermal-mechanical (variable temperature) crack propagation tests were performed in Hastelloy-X, and B-1900 + Hf materials. The crack growth data were reduced using the stress intensity factor, the strain intensity factor and the J-integral. None of these three parameters successfully correlated the crack growth data. All three parameters showed strain range effects and significant scatter between the various testing conditions (in-phase, out-of-phase, isothermal). The parameter which showed the most effectiveness in correlating high temperature and variable temperature crack growth was a modified stress intensity factor (ΔKσ) computed using the measured load, the closing bending moment caused by the increase compliance with crack length and with the effective opening stress. The ΔKσ was shown to rationalize the effect of mode of testing (stress vs strain controlled) and the mode of fracture. Furthermore, the isothermal data at Tmin and Tmax where shown to provide an upper and lower bounds for the thermal-mechanical fatigue data if plotted in terms of ΔKσ.

A study of high temperature crack growth in nickel-aluminide

The mechanism of high-temperature intergranular embrittlement in a nickel-aluminide intermetallic alloy has been investigated. Crack propagation rates were measured as a function of stressintensity in four-point bend specimens loaded in air at 500–760°C. Cracking and oxidation processes were examined using a variety of analytical techniques, including scanning, transmission and scanning-transmission electron microscopy, secondary-ion mass spectroscopy, and Auger-electron spectroscopy. Failure of the bend specimens occurred by stable intergranular crack growth at rates of up to 10 −4 ms −1 at 760°C. An activation energy for the process was estimated at 110 kJ mol −1, which is consistent with interfacial oxygen diffusion as the rate determining step. The crack and oxidation product morphology suggests that crack growth took place by a discontinuous step-wise mechanism, in which oxygen penetrates the grain boundary ahead of a stationary crack tip until the boundary is sufficiently embrittled to fracture locally under the prevailing stress intensity. The crack jumps forward into fresh unembrittled material and arrests, upon which the process is repeated. Mechanisms of embrittlement are discussed. Subsequent oxidation of the crack faces produces a multi-layered infill comprising a central conglomerate layer of unoxidised nickel, flanked by aluminium oxide containing sub-micron nickel particles.