Question

Question 13 1 pts An aluminium alloy experiences the loading below: Given its fatigue life data in the graph below, approximately how long will the aluminium last if the pattern above is repeated every 10secs. Express your answer in hours. 100 450 50 40 250 30 20 140 Min. Median Max 80 10 No. cycles 10 specimens at each level 40 10 Cycles to failure

Answer 1

In the loading pattern given, 140 MPa stress cycle occurs for 1 sec, 100 MPa stress cycle occurs for 2 sec and 90 MPa stress cycle occurs for 7 sec in the total 10 sec interval.

Stress range in 140 MPa stress cycle = 280 MPa

Stress range in 100 MPa stress cycle = 200 MPa

Stress range in 90 MPa stress cycle = 180 MPa

Probability of occurrence of 280 MPa stress range = 1/10

Probability of occurrence of 200 MPa stress range = 2/10

Probability of occurrence of 180 MPa stress range = 7/10

Effective stress range acting on member in one interval of 10 sec = 280 x 1/10 + 200 x 2/10 + 180 x 7/10 = 194 MPa

100 450 50 40 250 30 20 140 Min Median Max 80 10 No. cycles 10 specimens at each level 10 10 10 10 10 Cycles to failure IS O 40 360

Number of cycles to failure = 2 x 10⁴

Time required for failure = 2 x 10⁴ x 10 = 2 x 10⁵ sec = 55.56 hours