Afatigue test was conducted in which the mean stress was 90 mpa (13050 psi), and the stress amplitude was 190 mpa (27560 psi). (a) compute the maximum stress level. mpa (b) compute the minimum stress level. mpa (c) compute the stress ratio. (d) compute the magnitude of the stress range. mpa

Mean stress = 90 MPa
Amplitude = 190 MPa

Part (a): Maximum stress level
Means tress = 1/2 (Max. stress+ Min. stress)
Then,
90 = 1/2(Max. stress + Min. stress)
180 = Max. stress + Min. Stress (1)
On the other hand,
Amplitude = 1/2(Max. stress - Min. stress)
Then,
190 = 1/2(Max. stress - Min. Stress)
380 = Max. stress - Min. stress (2)

Solving (1) and (2);
 Eqn (1) + (2)
180 = Max. stress + Min. stress
380 = Max. stress - Min. stress -

560 = 2 Min. stress
∴ Max. stress = 560/2 = 280 MPa

Part (b): Minimum stress level
Using equation (1)
180 = Max. stress + Min. stress
180 = 280 + Min. stress
Min. stress = 180 - 280 = -100 MPa

Part (c): Stress ratio
R = Min. stress/Max. stress = -100/280 = -0.357

Part (d): Stress range
Stress range = Max. stress - Min. stress = 280-(-100) = 380 MPa

Independent variable is something that is changed in a experiment.