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Figure 4.21. Figure 4.22. Figure 4.23. HCF thresholds at R = 01 on a Kitagawa type diagram.. Figure 4.24. The horizontal line in each figure represents the experimentally determined endurance limit of the uncracked specimen corresponding to 10 7 cycles. It can be seen in both figures that the data obtained using LCF at R = 01 (circles) show what...
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They examined LCF–HCF interactions at negative values of R under both smooth bar conditions, where LCF generated cracks are difficult to detect and may not even exist, and under notch fatigue, where cracks were deliberately introduced and detected. Their investigation explored the nature of the crack initiation, threshold crack propagation, and any associated load-history effects when a specimen is initially...
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LCF–HCF nomenclature. Combined LCF–HCF loading has caused some confusion over the years because of nomenclature and the differences that occur when dealing with actual usage versus mathematical formulations involving simple linear summation concepts. Consider the schematic of Figure 4.39 that shows the major cycles (LCF) with a hold or dwell time in between them during which HCF cycles can occur....
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the LCF cycles can be considered to be an underload on the baseline HCF cycles. The results, based on the use of a number of values of OLR, are presented in Figure 4.51. OLR = 1 represents the case where there is no overload in the baseline LCF–HCF cycle. The results show that as OLR increases, the retardation effect of...
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Values of k t for U-shaped notch.. FATIGUE NOTCH FACTOR. The fatigue behavior of a notched component is not necessarily governed solely by the maximum stress at the notch root. In general, the fatigue life or fatigue strength is greater in a notched component based solely on the stress at the notch root compared to a smooth bar with that...
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When notches act like cracks, the mechanism leading to a fatigue limit is the growth of small cracks from the notch tip which may become non-propagating cracks. The criterion for the fatigue limit is the onset of crack propagation from an arrested crack and not crack initiation. This is a material-based limit according to Miller [15] rather than a limit...
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Mean stress (ksi). Figure 5.19. Smooth and notch fatigue data from [22] and methods for representing the notch curve.. So, not heeding my own advice, I offer the following as a method for fitting notch data based on the results shown in Figure 5.19. Following the concepts presented above that were put forth by Bell and Benham [18], the modification...
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Figure 5.25. What makes this approach work so well is the fact that G F turns out to be a constant for a large body of data. It is also shown in [29] that the fatigue notch factor can be obtained from the following equation.. In the work of Nisitani and Endo [16], the stress field ahead of an elliptical...
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This formula is based on the assumption that the fatigue ratio is a function of the creep rupture strength only. The empirical relationship is of the form. From these, the fatigue strength of a smooth bar at R. 1 and R = 0 can be determined at any temperature in the creep regime. At that temperature, the Haigh diagram is...
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One rather ironic example of a fretting-fatigue failure is that experienced while con- ducting fretting-fatigue experiments in the laboratory. Figure 6.6 is a schematic of the upper part of a dovetail fixture used to conduct fretting-fatigue experiments [5]. The fixture is free to rotate because it is held by a pin which, in turn, is held in a clevis which...
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Figure 6.16. Figure 6.17. In Figure 6.16, it is clearly seen that for = 10, the local contact stresses are dominant up to a crack length of approximately a/b = 005 and, as mentioned above, there is a significant decrease in K up to that crack length. Figure 6.18, on the other hand, shows that the far-field stress. Finally, the...
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Of greatest interest in the determination of the FLS for fretting fatigue is the maximum value of the axial stress, xx , at the interface as shown in Figure 6.21, for example, at the edge of contact. [31] the value of this stress is taken as the sum of the contributions due to the contact shear load, the normal pressure,...
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Because of the steep stress gradients that are present in a contact region, parameters that are used for notch fatigue (see Chapter 5) should also have merit in fretting-fatigue life prediction.. Parameters used for notches such as a modified SWT, Walker equivalent stress, modified shear stress range, or Findley parameter have been tried in an attempt to consolidate data from...
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Figure 6.42. Cross-section of a fretting fatigue cracked specimen prior to fracture [47].. Figure 6.43. Cross-section of failed fretting fatigue sample showing crack orientation [52].. Their criterion for initiation of a crack is related to the direction of maximum shear stress range, but propagation is governed by the direction of the maximum tangential stress range around the crack tip. In...
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b is the semi-width of the overall contact and a is the semi-width of the central flat part.. These numerical findings are consistent with those of the previously cited references where larger local values of COF are deduced from combined experimental and numerical findings. Thus, the value of COF becomes an important aspect in the analysis of fretting-fatigue conditions and,...
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the damage depths found in service vary significantly (see Appendix G). Whether the damage should be blended out or the component be removed from service and replaced is very difficult to ascertain in many cases. Another item of concern is the determination of whether damage depth is a good indicator of the actual remaining fatigue life of an FOD impacted...
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SCOPE OF THE FOD PROBLEM. Fan and compressor blades at the front end of jet engines are the components that receive the majority of damage, particularly at the leading edge of the airfoil. Due to the high frequency vibratory stresses that can be present in the fan and compressor sections associated with normal engine operation, it is not uncommon for...
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Figure 7.20. The energy plotted in the figure represents the input kinetic energy only. There was no information obtained experimentally about the residual velocity of the spheres. At any given energy level, the figure shows that there is a lot of scatter in the permanent depth.. Careful examination of the individual points revealed that at the highest energy level, obtained...
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This finding is consistent with that discussed in the previous section for impact on leading edge geometries. Of greater significance is the observation that ballistic impacts to the same depth as pendulum or quasi-static indentations are more severe in terms of the resulting fatigue limit strength reduction. This is especially true for the deeper indents corresponding to ballistic impacts at...
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Figure 7.40. Predicted and experimental k f for FOD tests in the axial specimen geometry with a 50 impact angle.. ~20° Impact angle. Figure 7.41. For FOD, as will be shown later, changes in the residual stress states, and material damage, as severity of the impact increases, make it even more difficult to extend the approaches described herein.. ~20° Impact...