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P A S Reed - One of the best experts on this subject based on the ideXlab platform.
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a numerical study of the effects of shot peening on the Short Crack growth behaviour in notched geometries under bending fatigue tests
International Journal of Fatigue, 2017Co-Authors: Mithila Achintha, Binyan He, P A S ReedAbstract:Abstract The current paper presents a numerical analysis of the effects of shot peening on Short Crack growth in a low pressure (LP) steam turbine material, FV448. The fatigue behaviour of this material has been experimentally evaluated using a U-notched specimen (representing the fir tree root geometry of the turbine blade) under 3-point bend tests. Two different shot peening intensities were considered in this study: an industrially applied shot peening process and a less intense shot peening process. In the modelling work, a 2-D finite element (FE) model with static Short Cracks has been developed, incorporating both compressive residual stress and strain hardening distribution effects caused by shot peening. Both linear-elastic (LEFM) and elasto-plastic (EPFM) fracture mechanics were used to characterise the Crack driving force in the un-peened and shot-peened conditions, taking into account the effects of stress redistribution caused by residual stress relaxation and Crack opening. The stress intensity factor used in the LEFM approach was calculated using the weight function method, and the equivalent stress intensity factor used in the EPFM approach was calculated from the J-integral, which was evaluated using the Cracked FE model. These results could explain the mechanism of (experimentally observed) retardation of Crack growth through the shot-peening-affected layer and also quantified this influence on fatigue life. The relative contributions of compressive residual stresses and strain hardening were assessed by investigating them separately. The sub-surface compressive residual stress distribution produced by shot peening could effectively reduce Crack propagation but the strain hardening distribution, in contrast, can accelerate it. However, strain hardening is expected to hinder the Crack initiation process by restricting the plastic deformation during cyclic loading. Predictions of the fatigue life of the shot-peened notched specimens were made based on this numerical analysis. Acceptable results were obtained using both the LEFM and EPFM approaches and the difference between them is discussed.
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effects of microstructure on room temperature fatigue Crack initiation and Short Crack propagation in udimet 720li ni base superalloy
International Journal of Fatigue, 2008Co-Authors: H T Pang, P A S ReedAbstract:Abstract An assessment of the effects of microstructure on room temperature fatigue Crack initiation and Short Crack propagation in a Ni-base superalloy is presented. The assessment was carried out on microstructural variants of U720Li, including as-received U720Li, U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20 Hz sinusoidal cycling waveform on plain bend bars. Tests were conducted in 3-point bend under load control with an R-ratio of 0.1. A maximum load of 95% σy was used in all tests. Room temperature fatigue Crack initiation was noted to occur due to slip band Cracking and from porosity on or just beneath the surface in all materials. Crack propagation was noted to be highly faceted (due to planar slip band Cracking) immediately after Crack initiation followed by a transition to a flatter Stage II type Crack path as Crack length increases. U720Li-LP was noted to show the longest fatigue lifetime, followed by U720Li-LG while U720Li shows the Shortest life. The longer lifetime of U720Li-LP was linked to a higher resistance to both fatigue Crack initiation and Short Crack propagation. U720Li and U720Li-LG show approximately similar Crack initiation resistance although U720Li-LG showed slightly improved Short Crack growth resistance. The observations have been rationalised in terms of the microstructural characteristics of the materials, and it is believed that larger grain size, larger coherent γ′ precipitate size and higher volume fractions of both coherent and primary γ′ precipitates will improve overall fatigue lifetimes in PM Ni-base alloys which exhibit planar slip characteristics at room temperature.
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microstructure effects on high temperature fatigue Crack initiation and Short Crack growth in turbine disc nickel base superalloy udimet 720li
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2007Co-Authors: H T Pang, P A S ReedAbstract:Abstract An assessment of the effects of microstructure on fatigue Crack initiation and Short Crack growth in a turbine disc nickel-base superalloy at 650 °C in air is presented. U720Li and microstructural variants of U720Li, i.e. U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent γ′ precipitate variant) have been assessed by uninterrupted and replicated Short Crack tests in polished U-notch specimens using a 1-1-1-1 trapezoidal loading cycle at nominal stress levels ranging between 700 and 850 MPa (calculated in the unCracked ligament). Crack initiation was primarily due to porosity on or near the surface but also due to grain boundary oxidation. Initial transgranular Crack growth across four to six grains in air was noted at Short Crack lengths before oxidation-assisted intergranular Crack growth modes were established at larger Crack lengths. At a nominal applied stress of 840 MPa, U720Li and U720Li-LP show similar fatigue lifetimes while U720Li-LG demonstrates a significantly improved fatigue lifetime, particularly when lifetimes are compared on a local strain range basis. A larger grain size gave the most significant performance benefits in terms of overall fatigue lifetime under these test conditions.
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elevated temperature Short Crack fatigue behaviour in near eutectic al si alloys
International Journal of Fatigue, 2003Co-Authors: M R Joyce, C M Styles, P A S ReedAbstract:This paper considers two candidate automotive piston alloys and highlights the influence of microstructural features on fatigue behaviour. Fatigue initiation and subsequent Short Crack growth was assessed at 20, 200 and 350 °C. It is shown that both temperature and test frequency have a strong influence on the fatigue performance of the materials tested. The microstructure was quantitatively characterised in terms of the primary Si distribution. Together with post failure analysis, this allowed identification of critical microstructural features affecting both fatigue Crack initiation and early growth. Large primary Si particles were found to act as preferential initiation sites by Cracking or decohesion (dependent on test temperature) and are also sought out preferentially during Short Crack growth.
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Short Crack initiation and growth at 600 c in notched specimens of inconel718
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2003Co-Authors: Thomas Connolley, P A S Reed, M J StarinkAbstract:The natural initiation and growth of Short Cracks in Inconel®718 U-notch specimens has been studied at 600 °C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross-sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site Crack initiation occurred in the notch root. Many of the Cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for Crack initiation by carbide expansion during oxidation is discussed. Surface Short Crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed Short-Crack growth rates were approximately constant across a wide range of Crack lengths. However, there was a transition to rapid, accelerating Crack growth once Cracks reached several hundred micrometers in length. This rapid propagation in the latter stages of the fatigue life was assisted by Crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life.
Yongxiang Zhao - One of the best experts on this subject based on the ideXlab platform.
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experimental research on dominant effective Short fatigue Crack behavior for railway lz50 axle steel
International Journal of Fatigue, 2012Co-Authors: Bing Yang, Yongxiang ZhaoAbstract:Abstract Behavior of dominant effective Short fatigue Crack, the Crack results in the final failure of specimen, was experimentally investigated by a replica technique with seven smooth hourglass shaped specimens of railway LZ50 axle steel. Character of two-stages, i.e. the micro-structural Short Crack stage and the physical Short Crack stage, was revealed for the Crack initiation and growth. Most importantly, the Crack growth rate exhibited decelerations twice in micro-structural Short Crack stage. This behavior was related to the ferrite grain boundary firstly and then to the pearlite banded structure. The boundary appeared a barrier because there were pearlites around with significant higher micro-hardness values. Also because of the higher hardness values, the layered pearlite banded structure acted as another barrier for the Crack to overcome. In physical Short Crack stage, the Crack propagated with a decreasing resistance of micro-structural barriers as the Crack length increased. Hence, the process of Crack initiation and growth are subjected to the competition between the intrinsic resistances from the barriers and the increasing driving force from the growing Crack size. Finally, a new Short Crack growth model is presented to describe the periodic influence of micro-structural barriers. Description to the test results of LZ50 steel has indicated the availability and reasonability of present model.
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Surface Rolling Effect on Effective Short Fatigue Cracks Density for Railway LZ50 Axle Steel
Advanced Materials Research, 2010Co-Authors: Bing Yang, Yongxiang ZhaoAbstract:Surface rolling effect on effective Short fatigue Cracks density, which reflect the affecting capacity on the initiation firstly and then growth of the dominant Short Crack result finally in specimen failure, is experimentally studied by a replica technique. Two groups of smooth hourglass shaped specimens of LZ50 axle steel with/without rolled surfaces were tested. The Crack density of surface rolled specimens was much lower than that of the other group. This indicates surface rolling technology having the effect of hardening surface layer material to introduce compressive residual stresses. The effect appears to restrain the Short Crack nucleation and propagation and then, to extend the fatigue life.
S J Zhao - One of the best experts on this subject based on the ideXlab platform.
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fatigue Short Crack propagation behavior of selective laser melted inconel 718 alloy by in situ sem study influence of orientation and temperature
International Journal of Fatigue, 2020Co-Authors: H L Zhai, L Zuo, W J Zhang, Shaoshi Rui, Qinan Han, J S Jiang, G F Chen, Guian Qian, S J ZhaoAbstract:Abstract The influence of orientation and temperature on fatigue Short Crack propagation behavior of Inconel 718 alloy was studied at both 25 °C and 650 °C by in-situ fatigue testing under scanning electron microscope. The fatigue Crack growth rates (FCGR) showed evident dependence on orientation at both temperatures, following: XZ
W Skrotzki - One of the best experts on this subject based on the ideXlab platform.
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initiation and propagation of Short Cracks in austenitic ferritic duplex steel
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2013Co-Authors: M Scharnweber, W. Tirschler, C G Oertel, W SkrotzkiAbstract:Abstract Austenitic–ferritic duplex stainless steels are widely used for applications, where both excellent strength and high corrosion resistance are required. In the current study, high cycle fatigue experiments were performed on the austenitic–ferritic duplex stainless steel 1.4462 in order to investigate Short Crack initiation and propagation. It is shown that the most frequent Crack initiation sites are grain boundaries between ferritic grains followed by slip bands in austenite and phase boundaries. Taking into account the elastic anisotropy of the austenitic phase, the surprisingly high fraction of Cracks formed along slip bands in austenite can be explained with the texture and microstructure of the material. Analyzing the correlation between Crack propagation rate and Crack length, it is found that in both phases, the transition Crack length between the regimes of microstructurally and mechanically Short Cracks correlates with the mean grain elongation under the average angle between Crack path and loading axis. Additionally, the dependence of the Crack propagation rate on the distance between Crack tip and opposing grain or phase boundary, i.e. the barrier effect of these boundaries for Short Crack propagation, could be measured. The results obtained expand the knowledge about Short Crack behavior in the investigated material, which is a key parameter for the lifetime in the high cycle fatigue regime.
Bing Yang - One of the best experts on this subject based on the ideXlab platform.
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experimental research on dominant effective Short fatigue Crack behavior for railway lz50 axle steel
International Journal of Fatigue, 2012Co-Authors: Bing Yang, Yongxiang ZhaoAbstract:Abstract Behavior of dominant effective Short fatigue Crack, the Crack results in the final failure of specimen, was experimentally investigated by a replica technique with seven smooth hourglass shaped specimens of railway LZ50 axle steel. Character of two-stages, i.e. the micro-structural Short Crack stage and the physical Short Crack stage, was revealed for the Crack initiation and growth. Most importantly, the Crack growth rate exhibited decelerations twice in micro-structural Short Crack stage. This behavior was related to the ferrite grain boundary firstly and then to the pearlite banded structure. The boundary appeared a barrier because there were pearlites around with significant higher micro-hardness values. Also because of the higher hardness values, the layered pearlite banded structure acted as another barrier for the Crack to overcome. In physical Short Crack stage, the Crack propagated with a decreasing resistance of micro-structural barriers as the Crack length increased. Hence, the process of Crack initiation and growth are subjected to the competition between the intrinsic resistances from the barriers and the increasing driving force from the growing Crack size. Finally, a new Short Crack growth model is presented to describe the periodic influence of micro-structural barriers. Description to the test results of LZ50 steel has indicated the availability and reasonability of present model.
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Surface Rolling Effect on Effective Short Fatigue Cracks Density for Railway LZ50 Axle Steel
Advanced Materials Research, 2010Co-Authors: Bing Yang, Yongxiang ZhaoAbstract:Surface rolling effect on effective Short fatigue Cracks density, which reflect the affecting capacity on the initiation firstly and then growth of the dominant Short Crack result finally in specimen failure, is experimentally studied by a replica technique. Two groups of smooth hourglass shaped specimens of LZ50 axle steel with/without rolled surfaces were tested. The Crack density of surface rolled specimens was much lower than that of the other group. This indicates surface rolling technology having the effect of hardening surface layer material to introduce compressive residual stresses. The effect appears to restrain the Short Crack nucleation and propagation and then, to extend the fatigue life.
