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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...
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Haritos, G., Nicholas, T., and Lanning, D., “Notch Size Effects in HCF Behavior of Ti-6Al-4V”, Int. Nicholas, T., Barber, J.P., and Bertke, R.S., “Impact Damage on Titanium Leading Edges from Small Hard Objects”, Experimental Mechanics pp. Birkbeck, J.C., “Effects of FOD on the Fatigue Crack Initiation of Ballistically Impacted Ti- 6Al-4V Simulated Engine Blades”, Ph.D. and Ritchie, R.O., “Influence of...
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FRACTURE MECHANICS CONSIDERATIONS. The implications of the shape of the Haigh diagram, as described by the modified Jasper formulation, for example, can be examined in terms of the fracture mechanics parameter related to the threshold for crack growth, namely K th . The FLS, which can be defined as the maximum stress under which a crack will not initiate and...
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Figure 8.17. Haigh diagram for smooth and notched specimen data compared with fracture mechanics-based computations of allowable stresses based on initial flaw size a i. Looking at the same general problem of relating a Haigh diagram to a threshold plot, Nicholas and Zuiker [7] evaluated behavior under combined LCF–HCF loading. Using another set of data and addressing the problem from...
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Figure 8.27. by finding the best value of the exponent m using a least squares fit to a straight line representation of all of the lines in Figure 8.26. The resulting model has a value of m = 034 and is shown in Figure 8.28. Here, all of the lines are brought together in a single band, but the degree...
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a value of K th = 24 MPa. For the constant K max , increasing K min test, a value of K th = 22 MPa. By comparison, for the identical material, Ti-6Al-4V, small crack growth behavior from naturally initiated and FOD-induced small cracks was not observed below a K of 29 MPa. The concern for history effects in determining...
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Figure 8.43. value for a normal distribution or some other measure of spread in the distribution curve is needed for a robust probabilistic design as indicated in the figure. Again, both the mean and the spread of the distribution of material strength and FODed strength are important for design purposes. (This representation is purely conceptual so the amplitude and shape...
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This may provide a measurable improvement in the endurance limit or the fatigue limit strength at a large cycle count.. As noted above, the effect of residual compressive stress is to move the mean stress in the negative direction. For a smooth bar, this simply shifts the Haigh diagram, a plot of alternating stress against mean stress, to the left....
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Marked changes in the slope of the S–N curves were observed in Ti-6-7/WQ owing to pronounced lifetime improvements in the HCF regime while finite life was only slightly improved.. These results serve to illustrate the complex nature of the effect of surface treatments on fatigue behavior, particularly in the HCF regime, and the dependence of such behavior on the characteristics...
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Since the residual stresses are compressive, the computed K res is negative. While it should be recognized that K res being negative has no formal meaning, a negative K res is used to represent the contribution of the compressive loading across the crack surfaces. This, in turn, is added to the K of the applied loading. In the reported investigation,...
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The residual stress state that results from plastic straining will depend on the cylinder geometry, material strain-hardening characteristics, unloading behavior of the material, and the level of strain hardening imposed. In that article, the authors point out that the computed stress intensity is the sum of the contributions due to the internal pressure loading, the residual stresses, and the internal...
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These results indicate that 15.1% of the accidents were due to axle failures.. This is consistent with the results quoted by Lardner [5], who indicated that 18% of the accidents were due to axle failures at a much earlier date.. Apparently, the earliest documented research on fatigue of materials was conducted by Albert [21], who performed repetitive tension tests on...
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A report [53] summarizing the results of the investigations was published in 1849. The report contains the first systematic investigation of the effect of moving loads on railroad bridge structures and fatigue of rails. Whether the substance of metal which has been exposed for a long period to per- cussion and vibrations, undergoes any change in the arrangement of its...
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Only after the initial stages of the HCF Program were underway did it become known that the aerodynamic driver was the downstream stationary airfoils and an effective redesign occurred.. By the late 1980s Headquarters Tactical Air Command became concerned enough with HCF failures and fleet impacts, that the Director of Logistics sent a letter to the Air Force Systems Command’s...
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Blend limits for FOD and criteria for removal or repair should be based on the FOD analysis required in the preceding paragraph. Material allowables for material which has been repaired, such as by blending or welding, should be adjusted as necessary to account for any degradation of the fatigue limit due to the blend or repair operation. The effect of...
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Engine components should not fail due to HCF or a combination of HCF and LCF when subject to the maximum attainable combined steady-state and vibratory stresses at a rate above that stated in A.4.13.3.. Low-order crossings and all known drivers within two stages either upstream or down- stream of the subject component should have probabilistic design margins for final design...