tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 65

High Cycle Fatigue: A Mechanics of Materials Perspective part 65. 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. | 626 Appendix I from Hawkyard et al. 12 and are assumed to decrease linearly from 5MPa m to at R and maintain values of 5 and at R 0 and R respectively. The stress intensity factor solution of Raju and Newman 13 for surface cracked smooth bars is utilized and assumptions are made concerning the geometry of the crack such that the stress intensity factor may be approximated as where a is the crack depth and D is the diameter of the bar. Due to the presence of the notch the applied stress is multiplied by a KT of 2 in order to determine AKHCF AKLCF acri ag HCF and ag LCF. This is an appropriate assumption when the crack is very short. However as the crack extends out of the notch it has been shown that the crack growth rate will approach that for KT 1 and a crack depth equaling the total depth of the crack and notch 14 . These assumptions using KT are conservative and will cause the model to underestimate the crack propagation life. However under the conditions of interest the propagation life is a small percentage of the total life. NUMERICAL RESULTS The numerical algorithm has been implemented on a personal computer and operates in one of two modes. In the first mode the user specifies aa am and n and the total life corresponding to 107 HCF cycles is calculated in terms of CCF load blocks N. In the second mode the user specifies am N and n and an iterative algorithm is invoked to determine aa such that the calculated value of N is within a specified tolerance of the requested value of N. Solution time increases with increasing N and increasing fraction of total life spent in crack propagation especially when the crack is actively growing in both HCF and LCF cycles. For the most computationally intensive cases considered the solution took no more than one to two minutes on a 100-MHz personal computer. Correlation with experiment A limited number of HCF-LCF tests on notched bars were conducted by Guedou and Rongvaux 9 . Ti-6Al-4V bars .