1. Fatigue Given the following 1) 2 standard deviation S-N curve, with equations for the two parts of the curve (for S in MPa) N = 428.8 x 10 9 S -3 (for N < 10 7 ) and N = 525.2 x 10 12 S -5 (for N ≥ 10 7 ), 2) sequential stress ranges and numbers of cycles in the table below: Numbers of cycles / year Stress range MPa 0.6x10 6 30 x10 6 60 25 Cycles and stress ranges a) Calculate the fatigue damage and fatigue life using the S-N curves and Miner’s rule in full. b) If both stresses are superimposed throughout every year and a peak of 60MPa stress is always accompanied by a peak in the 25MPa stress, recalculate the fatigue life using the bilinear curve. c) Approximately calculate the fatigue damage using the 3 rd /5 th power weighted stress method. 1 . 10 5 1 . 10 6 1 . 10 7 1 . 10 8 1 . 10 9 10 100 Cycles to failure Stress range (MPa)
Given the following 1) 2 standard deviation S-N curve,
with equations for the two parts of the curve (for S in MPa) N =
428.8 x 109 S-3 (for N < 107) and N = 525.2 x 1012 S-5 (for N
107),
2) sequential stress ranges and numbers of cycles in the table
below:
Numbers of cycles / year
Stress range MPa
0.6x106 30 x106
60 25
Cycles and stress ranges
a) Calculate the fatigue damage and fatigue life using the S-N
curves and Miners rule in full.
b) If both stresses are superimposed throughout every year and a
peak of 60MPa stress is always accompanied by a peak in the 25MPa
stress, recalculate the fatigue life using the bilinear curve.
c) Approximately calculate the fatigue damage using the 3rd/5th
power weighted stress method.
