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GATE 2016 ME (ME2) – Question 48

Machine Design · Fatigue strength and S-N diagram · 2 marks · Multiple choice

In a structural member under fatigue loading, the minimum and maximum stresses developed at the critical point are 50 MPa and 150 MPa, respectively. The endurance, yield, and the ultimate strengths of the material are 200 MPa, 300 MPa and 400 MPa, respectively. The factor of safety using modified Goodman criterion is

  1. 3/2
  2. 8/5
  3. 12/7
  4. 2

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Correct answer: (D) 2

Explanation

Alternating stress is(150−50)/2=50MPa and mean stress is(150+50)/2=100MPa. Modified Goodman gives1/n=σa/Se+σm/Sut=50/200+100/400=0.50. Thus **n=2, D**. Yield safety300/150=2 is also consistent.