Effective method for fatigue crack arrest in structural steels based on artificial creation of crack closure effect (original) (raw)
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Procedia Structural Integrity, 2018
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.
Fatigue crack closure: a review of the physical phenomena
Fatigue & Fracture of Engineering Materials & Structures, 2017
induced, roughness-induced and oxide-induced crack closures are reviewed. Special attention is devoted to the physical origin, the consequences for the experimental determination and the prediction of the effective crack driving force for fatigue crack propagation. Plasticity-induced crack closure under plane stress and plane strain conditions require, in principle, a different explanation; however, both types are predictable. This is even the case in the transition region from the plane strain to the plane stress state and all types of loading conditions including constant and variable amplitude loading, the short crack case or the transition from small-scale to large-scale yielding. In contrast, the prediction of roughness-induced and oxide-induced closures is not as straightforward. Keywords fatigue crack growth; oxide-induced crack closure; plasticity-induced crack closure; roughness-induced crack closure; threshold.
A new method for the analysis and assesment of fatigue crack closure. II: experimental study
Theoretical and Applied Fracture Mechanics, 1992
This second paper of the two-paper series describes a comprehensive experimental verification of the response of the Rigid-Insert Crack Closure Model (RICC) and the related CMOD-Compliance Closure Assessment Technique (CCCA) to variations in the material elastic properties, crack surface roughness, specimen dimensions, crack size, and loading conditions. The experimental procedures are particularly designed to ensure a "uniform" K-increasing crack wake which provides consistent crack closure data. The experiments are performed using two different materials, two selections of specimen sizes, at different crack lengths and stress intensity factors.
International Journal of Fatigue, 2006
Crack growth rates of small fatigue cracks in a high strength steel tested in 3.5% NaCl solution with cathodic protection are analysed in the Paris regime through the comparison with the corresponding results obtained in air or in high vacuum. Environmental effects in the saline solution are due to hydrogen produced by cathodic polarisation, which causes intergranular and transgranular brittle fracture surfaces. By comparison to fatigue crack growth rates obtained in air, it could be concluded that hydrogen effects are negligible at low DK and then increase with DK. But in fact, when compared to results obtained in a non-active media such as high vacuum, hydrogen effects in the saline solution are very high at low DK and decrease slightly when DK increases. In air, adsorption of the different gaseous species as well as hydrogen effects due to water vapour dissociation strongly enhance crack growth rate compared to vacuum especially for low DK. Consequently, a comparison with fatigue crack growth results obtained in air does not allow to quantify properly environmental effects due to another active environment such as the saline solution with cathodic protection considered in the present work.
Method of arresting crack growth for application at a narrow working space
Mechanical Engineering Journal, 2014
The many fracture accidents in engineering applications are related to fatigue crack initiation and growth. Therefore, the inspection of fatigue crack initiation and the arrest of crack growth are very important for the safe operation of a machine and the maintenance of a structure. Non destructive inspection has been developed in order to prevent a fracture accident resulting from the cracks or defects. When a crack initiates in machine equipment, continuous observation of the crack is carried out and the crack growth is arrested by some method. To arrest the crack growth, stop-holes are drilled at the crack tips in a case of initiation of a small number of cracks. Drilling the stop-hole is an effective method to prevent crack growth because it reduces stress concentration at the crack tip. Application of residual stress around the stop-holes and making a plastic hardening zone around the crack tips are effective to arrest crack growth. Applying a patch for repairing cracked parts is more effective than drilling stop-holes. In practical cases, patches are applied to repair cracked parts in airplane bodies, bridges, etc. However those methods cannot be used in some cases, because of the conditions of working space, for example. In this paper, an example of the application of a crack arrester at a car facility is introduced. Then, the effectiveness of the crack arrester is discussed. When many cracks are initiated and working space is very small, the crack arrester is a good tool for repairing the cracked parts. It is necessary to develop a method for stopping crack growth in order to replace cracked equipment, because new equipment should be designed carefully for preventing crack initiation again.
50 Years of Controversy on Fatigue Crack Closure
Procedia Structural Integrity, 2024
In this article the 50 years of observations, implications and debates related to fatigue crack closure are discussed. New insights related to plasticity, oxide, and roughness induced crack closures and their role in shielding effects of the fatigue crack tip are reexamined. Supporting evidence for these insights comes from the lack of Kth dependence on R in a high vacuum (with partial pressure of 10-5 Pa or less). The presented new critical chemical-mechanical analyses are based on experimental results reported in the literature that demonstrate the marginal R-ratio effect on Kth of long cracks in vacuum for both planar/wavy slip alloys but show R-dependence in the lab air and in chemical environment. The latter is due to the formation of viscous nature of the oxide, which forms in humid air at the newly expose fresh fracture surfaces. It is demonstrated that the dominant factor related to the experimentally observed R-ratio effects on fatigue crack growth (FCG) behavior (on several alloys) in not related to crack closure but the access of the environment to the crack tip region that affects fatigue damage. In chemical environments, our viewpoint is supported by a critical analysis of corrosion processes that found that there is insufficient time for most metallic species to form ions, hydrolyze, and transform into hard phases at the crack tip before closure. Therefore, when crack flanks contact occurs, most of the oxidized metallic species will exist as aquo-complexes, gel, or colloids, that have insufficient shear strength to wedge crack faces during unloading. Dislocation-based models have indicated that the crack tip shielding effect from a single asperity is small. The roughness induced crack closure has been suggested as a mechanical obstruction in the wake of the crack during cyclic unloading for planar slip alloys at the threshold region, the emphasis on the access of the environment to the crack tip for an environmental damage was not considered.
Fatigue crack growth mechanisms in steels
International Journal of Fatigue, 2003
Fatigue crack growth behavior of structural steels is examined by using the Unified Approach developed by the authors. In this approach, fatigue requires two-load parameters involving maximum stress intensity, K max , and stress intensity amplitude, ̅K. For a fatigue crack to grow, both K max and ̅K must exceed their respective threshold values. Similarly, for any other crack growth rate, two limiting values, K max * and ̅K * are required to enforce the growth rate. The variation of these two critical values forms the crack growth trajectory map, which is defined by plotting ̅K * vs. K max * as a function of crack growth rate. In this trajectory map, the line defined by ̅K * = K max * represents pure fatigue crack growth behavior induced by cyclic strains. It is shown that this line provides a reference norm for defining deviations in the trajectory resulting from environmental and/or monotonic fracture modes superimposed on fatigue. Using this approach, we have examined the crack growth behavior of many structural steels. The trajectory maps of these steels show deviations due to superimposed environmental effects. These effects vary with grain size, yield strength, microstructure, and chemistry. In addition, for a given material, changes in the trajectory paths occur because of changing crack growth mechanisms. Analysis of material behavior using trajectory maps provides a clear understanding of the relative magnitude of cyclic and environmental damage and how the microstructure, chemistry, and crack tip plasticity affect fatigue crack growth behavior of steels.
Growth of cracks in steel under repeated thermal shock in a chemically controlled environment
Repeated thermal shock loading is common in many industrial situations including the operation of pressure equipment found in thermal power stations. Thermal shock can produce a very high stress level near the exposed surface that eventually may lead to crack nucleation. Further crack growth under the influence of repeated thermal shock is a very complex phenomenon due to both the transient nature of the highly nonlinear thermal stresses and the strong influence of the environment. This paper describes an experimental analysis of crack growth in heated carbon steel specimens exposed to repeated thermal shocks using cold water. Analysis of the effect of steady state primary loads on the growth of the cracks is isolated using a unique test rig design. Environmental effects due to the aqueous nature of the testing environment are found to be a major contributor to the crack growth kinetics.