High Temperature Crack Growth Behavior Research Articles

Characterizing high temperature crack growth behaviour under mixed environmental, creep and fatigue conditions

Components in high temperature plant could undergo failure due to combinations of fatigue, creep or oxidation/corrosion depending on the loading, temperature and environmental conditions. A novel and robust approach for a progressive failure modelling is presented in this paper which for the first time attempts to combine these failure mechanisms as time or cycle dependent processes. In this study, a combined multiaxial inter/transgranular crack growth model at the meso-scale level was proposed to conveniently deal with the various failure scenarios that may exist in plant components. The simulated crack under the combinations of time dependent creep and oxidation mainly propagated along grain boundaries initiating from the notch surface, exhibiting an irregular shapes with crack branching. Whereas under fatigue/oxidation condition, the crack grew in a transgranular manner. Furthermore, the role of creep, fatigue and oxidation on the failure life was dependent on the applied duration period at peak loads. Cracks were prone to nucleate in transgranular and then propagate in intergranular. There existed competitions between creep, fatigue and oxidation damage. Finally, the failure modes due to different damage mechanisms and loading conditions in the cases of creep-fatigue-oxidation were proposed. The calculated failure modes corresponded with those observed in engineering alloys.

Effect of intermittent unloading or reversed loading on high temperature crack growth behaviour of Grade 91 steel under constant tensile load

As a part of the efforts for developing a reliable assessment procedure for crack growth in high temperature components, crack growth tests at various loading conditions were performed on Grade 91 steel. 1T compact tension specimens of 20 mm thickness were kept under constant tensile load at 600°C, but periodically unloaded or reversely loaded to compressive side to observe these effects on deformation behaviour as well as crack growth behaviour. It was found that periodical reversed loading accelerates crack growth due to re-acceleration of inelastic deformation during load holding, but its extent was not as large as predicted by creep J-integral in a conventional way. The predictions were improved by introducing an additional parameter to take account of creep damage recovery which was caused by the excursion to compressive load.

High Temperature Crack Growth Behavior of 304 Stainless Steel under Creep and Fatigue Loading

The crack growth behavior in a 304 stainless steel has been investigated at 538°C in air environment. Compact tension specimens were subjected to fatigue, creep and creep-fatigue loading. The combined effects on crack growth rates of load level and hold time have been examined. Stress intensity factors are found to correlate crack growth rates reasonably well for fatigue crack growth. Creep crack growth rates are found to correlate with stress intensity factor and C*(t). Crack growth rates under hold time cycles are successfully correlated with C*(t)avg under various load levels and hold times. Crack growth under creep-fatigue loading has been simulated by elastic-plastic-steady state creep finite element analyses. The results of analysis show that fatigue loading interrupts stress relaxation around the crack tip during hold time and causes stress reinstatement, thereby giving rise to accelerated crack growth compared with crack growth under static loading. Analysis of hold time crack growth based on the cyclic stress-strain response yields crack closure during unloading, and creep deformation during hold time tends to lower the closure load.

Analysis of high temperature fatigue crack growth behavior

High temperature fatigue crack growth has been examined in the light of the new concepts developed by the authors. We observe that the high temperature crack growth behavior can be explained using the two intrinsic parameters ΔK and Kmax, without invoking crack closure concepts. The two-parameter requirement implies that two driving forces are required simultaneously to cause fatigue cracks to grow. This results in two thresholds that must be exceeded to initiate the growth. Of the two, the cyclic threshold part ΔK∗th is related to the cyclic plasticity, while the static threshold K∗max is related to the breaking of the crack tip bonds. It is experimentally observed that the latter is relatively more sensitive to temperature, crack tip environment and slip mode. With increasing test temperature, the cycle-dependent damage process becomes more time-dependent, with the effect that crack growth is dominated by Kmax. Thus, in all such fracture processes, whether it is an overload fracture or subcritical crack growth involving stress corrosion, sustained load, creep, fatigue or combinations thereof, Kmax (or an equivalent non-linear parameter such as Jmax) remains as one essential driving force contributing to the final material separation. Under fatigue conditions, cyclic amplitude ΔK (or an equivalent non-linear parameter like ΔJ) becomes the second necessary driving force needed to induce the characteristic cyclic damage for crack growth. Cyclic damage then reduces the role of Kmax required for crack growth at the expense of ΔK.

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.

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.

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.

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 behaviour in Nimonic PE16 and Alloy 718

AbstractThe high-temperature crack-growth behaviour of Nimonic PE16 has been studied at 650°C under cyclic, static, and combined loads, and the results are compared with those of Alloy 718. Crack-growth rates in vacuum under continuous cycling are the same in both alloys, but the rates differ significantly in air. A 1min hold has no effect on the growth rate offatiguecracks in Nimonic PE16, but has a large effect in Alloy 718 in air. This difference is due to the difference in their time-dependent crack-growth behaviour. In Alloy 718, time-dependent crack growth occurs readily at low stress intensities. However, in Nimonic PE16, high stress intensities are required to induce crack growth. Furthermore, the crack-growth rates in the two alloys differ by nearly three orders ofmagnitude. The higher growth rate in Alloy 718 is due largely to environmental effects. Time-dependent crack growth in Nimonic PE16, however, is mostly due to creep, and environment has a negligible effect on the growth. The results ind...