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The fatigue crack growth of lone and small surface cracks has been investigated in air under four-point bending conditions at a load ratio of 0.1 for a PM Ni-based superalloy, Udimet 720. Cracks were found to initiate from surface...
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The fatigue crack growth of lone and small surface cracks has been investigated in air under four-point bending conditions at a load ratio of 0.1 for a PM Ni-based superalloy, Udimet 720. Cracks were found to initiate from surface pores despite an increased level of inclusions introduced by doping. Attention has been focused on small fatigue crack growth at maximum stresses beyond the 0.2 percent proof stress of the material. Typical small fatigue crack growth behaviour was found, i.e. growth rates for small fatigue cracks were faster than for long fatigue cracks at all fatigue stress intensity factor ranges investigated. A faceted-striated transition on the fatigue fracture surfaces has been found for both long and small fatigue crack growth. For long fatigue cracks, this transition appears to occur when the reversed plastic zone size is approximately equal to the grain size. For small fatigue cracks, the transition crack depth, d_0, was found to be 10-20 times the grain size and independent of stress range employed.
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Fatigue has been the subject of innumerable books, papers, and studies since the late 1800s and continues to be a major factor in component failure with an estimated 90% of all mechanical failures being attributed to fatigue (Diet...
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Fatigue has been the subject of innumerable books, papers, and studies since the late 1800s and continues to be a major factor in component failure with an estimated 90% of all mechanical failures being attributed to fatigue (Dieter, Mechanical metallurgy, 1976). The classic fatigue "thumbnail" often visible to the naked eye may allege fatigue, but microscopic striations-tiny ridges that bear immutable witness to cyclical loading-pronounce the verdict. As such, counting striations should provide the analyst with valuable information regarding the loading regimen, the time to failure, and insight into events that gave birth to fracture. This paper presents both pros and cons of striation counting and discusses striation count accuracy. It also presents practical techniques for use in fatigue striation counting based on laboratory experience and observation.
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The deterioration of adhesive bonds as a result of exposure to aggressive service environments is a problem for both metallic and composite bonded joints in aircraft structures. In this context airworthiness issues arising from th...
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The deterioration of adhesive bonds as a result of exposure to aggressive service environments is a problem for both metallic and composite bonded joints in aircraft structures. In this context airworthiness issues arising from the in service growth of small naturally occurring defects in adhesively bonded repairs has led to the need to develop a fracture mechanics based approach for setting the inspection intervals associated with the growth of such naturally occurring defects. This involves issues associated with both durability and damage tolerance that are associated with such small defects because it is well known that, for metals, the use of long crack data can lead to erroneous non-conservative estimates. As such a means for assessing the issues related to such small naturally occurring defects is urgently needed. In this context the Boeing Wedge test, and its related variants, plays a central role in not only qualitatively assessing environmental performance but also in ranking surface treatments. However, this method has the problem that the initial defect introduced during wedge testing may not be representative of the small initial defects that can arise in service. Thus the relative ranking of various surface treatments obtained using wedge tests may be misleading. To this end, the paper presents a simple fracture mechanics based method that has the potential to account for the discrepancy in the initial defect size. This approach resembles crack growth laws developed to account for the growth of both long and short cracks in metals, environmentally enhanced crack growth in metals and delamination growth in composites.
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Short crack growth mechanism and kinetics were studied under symmetrical push-pull straining in austenitic stainless 316L steel. Fatigue cracks initiated along strain localized areas and crack linking was the main mechanism of the...
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Short crack growth mechanism and kinetics were studied under symmetrical push-pull straining in austenitic stainless 316L steel. Fatigue cracks initiated along strain localized areas and crack linking was the main mechanism of the early crack growth. Two different regimes in short crack growth rate were identified. Based on the experimental data, the quantitative description of crack propagation rates was obtained for short and long cracks. The integration of this equation can be used to predict the fatigue life or residual life and to evaluate the number of cycles necessary to reach a given crack length.
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This technical brief is intended to warn failure analysts against assuming that dimples indicate overload fracture. Dimples simply mean that microvoid processes were active; these processes can occur in cyclic as well as monotonic...
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This technical brief is intended to warn failure analysts against assuming that dimples indicate overload fracture. Dimples simply mean that microvoid processes were active; these processes can occur in cyclic as well as monotonic fractures. The failure analyst must examine the entire fracture, the macroscopic as well as the microscopic features, to be confident of the cracking mode.
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摘要 :
This technical brief is intended to warn failure analysts against assuming that dimples indicate overload fracture. Dimples simply mean that microvoid processes were active; these processes can occur in cyclic as well as monotonic...
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This technical brief is intended to warn failure analysts against assuming that dimples indicate overload fracture. Dimples simply mean that microvoid processes were active; these processes can occur in cyclic as well as monotonic fractures. The failure analyst must examine the entire fracture, the macroscopic as well as the microscopic features, to be confident of the cracking mode.
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Three different methodologies for characterizing fatigue crack closure are compared using constant amplitude fatigue crack growth simulations. The adjusted compliance ratio (ACR), crack wake influence (CW1), and conventional Elber...
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Three different methodologies for characterizing fatigue crack closure are compared using constant amplitude fatigue crack growth simulations. The adjusted compliance ratio (ACR), crack wake influence (CW1), and conventional Elber crack opening load approaches were considered, with the CWI approach reported in the literature generalized to allow a crack surface traction distribution. The ACR methodology consistently indicated lesser levels of crack closure or shielding, which were insensitive to the applied stress level, when compared with the CWI and opening load approaches for R > 0. Closure levels using the Elber and CWI methodologies compared well for lower values of applied stress. For R = -1, the three methodologies resulted in similar levels of crack tip shielding. No evidence was observed that applied loading below the conventional crack opening load becomes more significant in the near-threshold regime.
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Two stochastic models of fatigue crack growth under constant amplitude cyclic loading are proposed and studied by using the stochastic averaging method and total probability theorem to account for high frequency component and low ...
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Two stochastic models of fatigue crack growth under constant amplitude cyclic loading are proposed and studied by using the stochastic averaging method and total probability theorem to account for high frequency component and low frequency component, respectively, of the irregular nature of fatigue crack growth observed in Virkler's experiment. Particular attention is paid to the prediction performance of the models under a change in the initial crack length. It is shown that the models proposed in the present paper yields better agreement with experimental data than other models available in literature.
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The role of crack initiation and short crack growth on fatigue life in the very high cycle fatigue regime (VHCF) is investigated for three heat treatments of the wrought magnesium alloy WE43. As-received (T5) WE43 with a relativel...
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The role of crack initiation and short crack growth on fatigue life in the very high cycle fatigue regime (VHCF) is investigated for three heat treatments of the wrought magnesium alloy WE43. As-received (T5) WE43 with a relatively fine grain size was solution treated and aged to produce precipitation strengthened coarse-grained microstructures in the underaged and peak-aged (T6) conditions. Ultrasonic axial fatigue tests with a cyclic frequency of 20 kHz were conducted using smooth specimens. Heat treatment was shown to have a strong effect on fatigue strength, with the fine-grained, strain-hardened T5 condition exhibiting much higher values than the coarse-grained conditions. No significant difference in fatigue strength was observed between the underaged and peak-aged microstructures of equivalent grain size. Crack initiation and short crack growth behaviors in each condition were investigated to determine if one of these behaviors dominated the VHCF lives. It was found that average short crack growth rates for the three conditions were similar and had no clear dependence on microstructural condition. Crack initiation was shown to occur through cyclic slip deformation in particularly large and favorably oriented grains in each condition. Subsurface crack initiation was observed at low stresses and high lifetimes in the coarse-grained conditions, but not in the fine grained T5 condition. Crack growth rates in vacuum were investigated using a unique combination of ultrasonic fatigue instrumentation and scanning electron microscopy (UFSEM) in order to simulate subsurface crack propagation. Environment was shown to have a significant effect on crack growth rate, with rates in vacuum nearly two orders of magnitude lower than in laboratory air.
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In this paper, the recently proposed UniGrow model modification is further improved and a modeling framework of the proposed model is developed to account for both the short and long crack growth behavior. The modeling approach in...
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In this paper, the recently proposed UniGrow model modification is further improved and a modeling framework of the proposed model is developed to account for both the short and long crack growth behavior. The modeling approach incorporates the integration of short and long propagation into the UniGrow crack growth model to address the short and long crack growth in a unified manner. A systematic study is performed to further validate the proposed modeling approach and assess its prediction capabilities and shortcomings to predict crack growth behavior of short and long cracks. The proposed crack growth model is assessed by first comparing the predicted fatigue crack growth results with long crack growth data sets of 7010-T7, 2024-T3, 2324-T3, 7050-17 and 7075-T6 aluminum alloys at four different R ratios; The results are further assessed by comparison of short and long crack growth predictions to both short and long crack growth data sets of 2090-T8E41, LC9cs aluminum alloys and Ti-6Al-4V titanium alloys at two different R-ratios. The model showed good correlation with long crack data sets of 7010-T7, 2024-T3, 2324-T3, 7050-T7 and 7075-T6 and predicted crack growth results matched well with the nonlinearity of four material data sets at four different R-ratios. The results revealed that even though the proposed model was in good agreement with long crack data sets of 2090-T8E41, LC9cs aluminum alloys and Ti-6Al-4V titanium alloys at two different R-ratios, the model did not correlate well with the short crack data sets of these three materials and to accurately account for the variability of short crack growth.
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