This video describes how to account for the effect that mean stresses have on fatigue lifetimes and how to calculate factors of safety for infinite life.
If the mean stress is zero you are intersecting the stress amplitude axis where all lines converge...so the factor of safety is just equal to the stress amplitude divided by the endurance strength. Thank you for asking this question, because the Gerber formula does not work when the mean is zero!!
The walker approach is more conservative at low mean stress, but less conservative at high mean stress. So, creating a fatigue failure map using it is a bit more complicated, and leads easily to confusion. Fatigue is really important, but also very phenomenological. Adding another parameter, gamma, to create a family of curves is hard to swallow. When I teach design, I always try to use the simplest possible approach...but I will consider this and consider making a video on it. I presume you must use it. As to SWT, I do know that strain-life is really the better approach, but it gets very hard to quantify and is especially hard to incorporate into a machine design course.
What a gem of a video! Thank you very much for the clear, concise explanation.
Man, this video is seriously great and is the first one that solves the mystery of the mean stress. Thanks!
Thanks...more is coming...I just need a bit of a break from all this. Fatigue is vital, but very phenomenological, so it can be a bit hard to digest!
Hello.Nice video!!But I have a question. How to calculate safety factor using Gerber theory, when my load cycle is symmetrical (mean stress is zero)?
If the mean stress is zero you are intersecting the stress amplitude axis where all lines converge...so the factor of safety is just equal to the stress amplitude divided by the endurance strength. Thank you for asking this question, because the Gerber formula does not work when the mean is zero!!
Nice video
Can you add something related to how to calculate walker stress
The walker approach is more conservative at low mean stress, but less conservative at high mean stress. So, creating a fatigue failure map using it is a bit more complicated, and leads easily to confusion. Fatigue is really important, but also very phenomenological. Adding another parameter, gamma, to create a family of curves is hard to swallow. When I teach design, I always try to use the simplest possible approach...but I will consider this and consider making a video on it. I presume you must use it. As to SWT, I do know that strain-life is really the better approach, but it gets very hard to quantify and is especially hard to incorporate into a machine design course.
Hi