tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 14

High Cycle Fatigue: A Mechanics of Materials Perspective part 14. The nomenclature used in this book may differ somewhat from what is considered standard or common usage. In such instances, this has been noted in a footnote. Additionally, units of measurement are not standard in many cases. While technical publications typically adhere to SI units these days, much of the work published by the engine manufacturers in the United States is presented using English units (pounds, inches, for example), because these are the units used as standard practice in that industry. The graphs and calculations came in those units and no attempt was made to convert. | 116 Introduction and Background predictions the RFL model described above was used. The baseline tests with Nf 106 cycles were selected as LCF tests that would typically be available from test programs in addition to HCF properties. The RFL model treats run-out and failure tests with 2D scatter in both life LCF regime and the endurance stress HCF regime . An advantage of the RFL model is that the 1D scatter assumptions are not required. The RFL model predictions with the baseline step or staircase results are given in Figure . If only the step tests are used in combination with the LCF tests a tighter fatigue limit variation is obtained with more error in life than for the staircase plus LCF tests where fatigue limit variation dominates the life error term. In Figure the average and lower bound HCF limits for all approaches are summarized. If a 1D scatter in stress s is used similar lower bound predictions are obtained for the step and staircase tests. However a 1D scatter in life results in a different lower bound as noted before. For the cases using Figure . Random fatigue limit model predictions using baseline tests Nf 106 cycles step or staircase approaches. Accelerated Test Techniques 117 HCF Limits for Ti-6Al-4V at 75 F and R - 7 S OT average Smax ksi lower bound Smax ksi 1 L Analysis with 1D scatter Analysis with 2D scatter _j Step Staircase Staircase Nf 106 Nf 106 All BAA s scatter s scatter Nf LCF step LCF single load scatter staircase data Figure . Summary of predicted average and lower bound HCF limits for Ti-6Al-4V at 75 F and R . the 2D scatter a similar lower bound is observed for either step staircase or all data when combined with the baseline LCF data. These predictions are seen to be different than those obtained with the 1D scatter assumptions. Another important feature that distinguishes the RFL model from conventional least squares fitting LSF is the manner in which run-out tests are handled. As .