Let us start to have a look to the fatigue limit (also known as fatigue strength). For constant amplitude loading this is the stress amplitude that indicates the transition from infinite life to finite life. If the stress amplitude is smaller than the fatigue limit, the fatigue life will be infinite. At least, for constant amplitude loading it will be.
So if you want to design for infinite life, just keep all stress amplitudes below the fatigue limit.
That looks pretty straightforward, but there are some issues:
- Is there really such a thing as the fatigue limit?
- How to obtain the fatigue limit?
- How to handle variable amplitude loading?