Analysis of high temperature fatigue crack growth behavior (original) (raw)
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High temperature fatigue crack growth in two metals under constant and variable amplitude loading
International Journal of Fatigue, 2002
Results are presented for fatigue crack growth at elevated temperatures during both constant amplitude and variable amplitude loading. A careful experimental investigation is presented to show that the concept of the effective stress-intensity factor range ⌬K eff I can be used to eliminate the load ratio effect on fatigue data and produce one single set of crack growth data. The fatigue crack propagation data corrected for crack closure is then used for comparison of mean crack propagation rates for variable amplitude loads with simple time histories. It is concluded that measured crack closure can not fully explain the discrepancies between measured and predicted propagation rates. A reduction factor fitted to the experimental data could be used to successfully correlate the results.
Mechanics and mechanisms of fatigue damage and crack growth in advanced materials
International Journal of Solids and Structures, 2000
The mechanisms of fatigue-crack propagation in ceramics and intermetallics are examined through a comparison of cyclic crack-growth behavior in ductile and brittle materials. Crack growth is considered to be a mutual competition between intrinsic mechanisms of crack advance ahead of the crack tip, which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip, which impede it. In this paper, we examine and model the widely diering nature of these mechanisms, with emphasis on behavior in ceramics at ambient and elevated temperatures, and compare their speci®c dependencies upon the alternating and maximum driving forces (e.g., DK and K max ), thereby providing a useful distinction of the process of fatigue-crack propagation in these dierent classes of materials. Published by Elsevier Science Ltd.
Fatigue Crack Growth in High Temperature Alloys
Superalloys 1984 (Fifth International Symposium), 1984
Fatigue crack growth rates of a variety of high temperature alloys (nickel-base, iron-base) were investigated at various temperatures in air and in vacuum. Growth rates at intermediate cyclic stress intensity ranges and also in the threshold regime were studied. At room temperature the materials show a rather similar crack growth behaviour in the Paris regime. Differences in the threshold regime can be explained by microstructural and closure effects. Even in vacuum the temperature dependence of the Paris regime cannot be explained only by the temperature dependence of Young's modulus. Oxidation predominantly influences the crack growth rates at elevated temperatures. Under these conditions crack growth in the threshold regime is governed by mechanisms similar to those at room temperature but additionally by oxidation induced mechanisms (crack branching, oxide induced closure) occur.
A general model for crack growth from initial defect in Very-High-Cycle Fatigue
Procedia Structural Integrity
During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Multiple mechanisms controlling fatigue crack growth
Fatigue & Fracture of …, 2003
Recognizing that fatigue is a two-parameter problem requiring two load parameters to define cyclic loads unambiguously, a unified approach has been developed to account for crack growth behaviour in terms of K and K max. Since both driving forces govern the crack growth rate, any analysis based on either K or K max will provide only partial information about the fatigue behavior of materials. It is shown that K-K max plots and the associated crack growth trajectory maps reflect the basic mechanisms that contribute to crack growth in a material. These plots also provide a convenient basis to recognize the changes in the micromechanisms that can occur as a function of load ratio or crack growth rate, or both. Taking examples from the literature, crack growth trajectory maps are provided showing such changes in the governing mechanisms of crack growth. It is shown that the K-K max approach is not an alternative to crack closure models, but it reflects the intrinsic material behaviour that must be understood before reliable crack prediction models can be developed. Keywords K-K max parameters; crack growth mechanisms; environmental effects; fatigue crack growth; multiple mechanisms; unified approach.
Environmental interactions in high-temperature fatigue crack growth of Ti-1100
Metallurgical Transactions A, 1993
The crack growth behavior of Ti-1100 is investigated for loading frequencies ranging from 30 to 0.0031 Hz at temperature levels extending from 23°C to 650°C in both air and vacuum environments. Two types of time-dependent damage mechanisms have been identified: oxidation and creep effects. It is concluded that the effect of oxidation on the crack growth acceleration is rapidly developed and only weakly dependent on total cycle time. Creep effects, on the other hand, are dominant at low frequencies in both air and vacuum and are loading rate dependent. The degree of contribution of each of these two damage modes during the steady state growth region has been phenomenologically determined by examining the frequency dependence on the exponent and coefficient parameters of the Paris-type crack growth equation. It is found that these parameters are largely determined by the extent of the viscoplastic response of the crack tip region below a specific, environment-sensitive transition loading frequency. Furthermore, the physical mechanisms involved in the environment-affected damage are identified with the nature of crack tip plastic work input as a function of loading frequency. The influence of frequency and environment on the anomalous appearance of pronounced stage I/stage II knee regions is also discussed with respect to closure levels and creep transient response.
Elevated-Temperature Crack Growth
Fatigue and Fracture, 1996
The purpose of this program was to extend the work performed in the base program (CR 182247) into the regime of timedependent crack growth under isothermal and thermal mechanical fatigue (TMF) loading, where creep deformation also influences the crack growth behavior. The investigation was performed in a two-year, six-task, combined experimental and analytical program. The path-independem integrals for application to time-dependent crack growth were critically reviewed. The crack growth was simulated using a finite element method. The palh-indepemdcnt integrals were computed from the results of finite-element analyses. The ability of these integrals to correlate experimental crack growth data were evaluated under various loading and temperature conditions. The results indicate that some of these integrals are viable parameters for crack growth predicition at elevated temperatures.
Mechanics and mechanisms of fatigue crack growth in metals: a review
This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Energy Research and Development Administration, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. ..
International Journal of Fatigue
The fatigue crack growth rate (FCGR) curve of metallic alloys is usually divided into three regions. Region II is often referred to as the Paris regime and is usually modelled with a power law relationship with a single exponent. Regions I and III are located at the beginning and end of the FCGR curve, respectively, and are frequently modelled with asymptotic relationships. In this paper we hypothesize that fatigue crack growth is governed by power law behaviour at all crack lengths and all stress intensity factor ranges (ΔK). To accommodate for the changes in the FCGR slope at regions I-III mathematical pivot points are introduced in the Paris equation. Power law behaviour with the presence of pivot points enables direct fitting of the crack length vs. cycles (a-N) curve to obtain the FCGR as a function of ΔK. This novel approach is applicable to small and long crack growth curves and results in accurate multilinear FCGR curves that are suitable for reconstruction of the measured aN curves. The method is subsequently applied to i) different alloys to show local changes in the FCGR curve for changes in alloy composition and heat treatments, ii) naturally increasing ΔK testing of microstructurally small cracks to obtain accurate small crack FCGR data. The comparison with accurate long crack data shows that small cracks are faster, but the transition from region I to region II occurs at a specific fatigue crack growth rate which results in an apparent shift in ΔK at the transition. iii) Long cracks, which show that the FCGR increases with maximum stress for a given ΔK and stress ratio when the maximum stress approaches the yield stress. The maximum stress phenomenon becomes important in the case of fatigue testing, where the initial crack lengths are usually small and maximum stresses are high. It is concluded that for long cracks the phenomenon explains why the Paris equation is applicable rather at low maximum stress, while the Frost-Dugdale equation is more applicable at high maximum stress.
Fatigue & Fracture of Engineering Materials and Structures, 1980
An analytical model to represent and predict the influence of frequency on the elevated temperature fatigue crack growth behavior of structural alloys is described. This model was formulated by considering the time dependence of stress and strain rate in the crack tip region due to creep deformation. Fatigue crack growth rate data were generated on ASTM grade A470 class 8 (CrMoV) steel at 538°C (1000°F) for several frequencies to evaluate the model. Based on these results and data taken from the literature on 304 stainless steel, it was concluded that the proposed model is capable of accurately representing and predicting the effect of frequency over several orders of magnitude on the fatigue crack growth rates at elevated temperature.