Question

2. The fatigue data for a steel alloy are given as follows:

STRESS AMPLITUDE (ksi)	CYCLES TO FAILURE
70.0	4 x 103
64.0	104
56.0	4 x 104
50.0	105
42.0	4 x 105
36.0	106
36.0	5 x 106
36.0	107
36.0	108

1. Make an S-N plot (stress amplitude versus log of cycles to failure) using these data.
2. What is the fatigue limit (endurance limit) for this alloy?
3. Determine fatigue lifetimes at stress amplitudes of 58 ksi and 46 ksi.
4. Estimate fatigue strengths at 5 x 10⁴ and 2 x 10⁵ cycles.

Use the semilog graph attached. Label the axes with titles and proper units.

Answer 1

a). S N curve is plotted.

b). Fatigue or endurance limit = 36ksi.

c). Fatigue life times at stress amplitudes of 58ksi = 2 × 10^4 cycles.

At 46ksi = 2 × 10^5 cycles.

d). Fatigue strength at 5× 10^4 cycles = 54ksi.

at 2× 10^5 cycles = 45.5ksi.

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