Fatigue Failure Criteria - Mean and Alternating von Mises Stress - Example 1

CORRECT way to find alternating and mean von Mises stresses (textbooks are WRONG).
Main Video: Fatigue Failure Criteria in Just Over 10 Minutes
• Fatigue FAILURE CRITER...
Example 2: • Fatigue Failure Criter...
Fatigue Failure Criteria - Von Mises Stress Equation for Given Normal and Shearing Stress
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Marin Factors for Endurance Limits in Just Over 10 Minutes
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Example 1: • Marin Factors - Evalua...
Example 2: • Marin Factors - Size F...
Fatigue Stress Concentrations in Just Over 10 Minutes
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Example 1: • Fatigue Stress Concent...
Example 2: • Fatigue Stress Concent...
Example 3: • Fatigue Stress Concent...
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[@canberkgunduz3487]

In this video you use min and max von-mises stresses to obtain mean and alternating von-mises stresses while also saying that the formulas you wrote in orange are wrong however in playlist "Mechanical Engineering Design I" video 61 you derive the formulations using the orange colored von-mises stresses. What is the reason behind this contradiction?

[@fondofbunny]

Do you have a reference that supports this way of computing Von Mises mean and alternating stresses? Shigley, Norton, Juvinal and Marshek all use the methods you identify as incorrect (at least in the editions I own.).

[@fondofbunny]

If the Mod-Goodman and Gerber theories were tested/compared to empirical data using the method of computing Von Mises mean and alternating in the textbooks, I would be hesitant to change the method of computation. Your methods result in a much higher safety factor. I am leaning toward sticking to a more conservative approach unless you have journal publications that back this up.

[@LessBoringLectures]

If you are currently an engineering student, my recommendation is that you follow what you have in your books, otherwise you'll mostly likely get into trouble with your instructor (if they are stubborn, that is). The explanation is mechanics-based and simplified with an example here, but the mathematical proof can be developed with non-numerical values if you follow the same simple steps I shared here; no need for journal papers to back up such a simple concept: no serious journal would publish such a simple and unoriginal "study" (mistaken textbook concepts and equations happen more often that you might think). It might help to expand on it by saying that negative normal bending stresses would result in values of von-Mises stresses that are oriented perpendicular to the planes where the principal stresses that consider the positive normal bending stresses exist.

[@fondofbunny]

@@LessBoringLectures I guess I'm just one of those stubborn instructors. Your argument for ignoring 3 other reputable sources is not convincing. Sure, I follow the logic of your presentation of the stress calculations in the video, but if the Modified Goodman Line and others were developed based on a different interpretation of the mean and alternating Von Mises stress, I will stick with the original interpretation. (Unless you have a source showing that your methods have been empirically verified or that you have sources showing the text books have misrepresented the original work by Goodman and others.)

[@ecokecok1376]

Do you still believe the textbook solutions are wrong ?

[@kurniawijayakusuma9193]

sir, can you give the literature you used to get the equation of Von mises, fracture and true stresses?

[@rafaymalik9705]

Why did you not use Kf in your calculations, while determining the Von Mises Stresses?

[@Christian-gm2zi]

Kf is a stress concentration factor. If you're analyzing a bar/rod, then there are no discontinuities (notches/holes/shoulders). So in that case we can assume Kf=1 and Kfs=1

[@bayikancanozturk]

thank you for giving both estimation methods!

[@jameshughes100]

Well the "wrong" version got my answer correct