Engineering (material science) Fatigue life prediction using integral

[Pipsqueakalchemist]

Homework Statement

I have question and solutions pictures below

Relevant Equations

Pictures of equations posted below

So for this question, I understand the math but just wanted to be clear on a few things. So I had this question on my midterm but instead of tensile and compressive stresses, it was tensile and tensile stress. I initially thought that the delta sigma in the integral was the maximum stress so in the example 100 MPa. But I believe my professor said that if it's tensile and compression then delta sigma is just equal to the tensile stress, but if it's tensile and tensile then the delta sigma would be max tensile - min tensile. And of course to find critical crack length I would use the largest tensile stress. I just wanted to make sure that this was correct because I ask my professor awhile ago and can't be 100% sure if this was correct, so please can someone confirm if I'm correct.

 

[Chestermiller]

In your example, in my judgment, $\Delta \sigma$ should be 150 MPa.