I am trying to use a kb factor in the Marin Factor calculation to determine an endurance limit for a hollow rectangular tube in bending only. The kb is based on a diameter but there is equivalent diameter equations. The diametrical equivalent for a rectangular cross section is .808(bh)^0.5. But I am uncertain if this is for a solid rectangular section or if this also applies to a hollow section. I'm not sure if I should take the above equation .808(bh)^0.5 and subtract the 95% stress area by using equation .05bh. Thereby the the equivalent diameter would be .808(bh)^0.5-.05bh

The diameter section is based on solid bar. The calculation you give, .807(bt)^.5, is based on making a 5% area of outer fiber equivalence. I'm not sure if the thickness is 2.5% that this would be equivalent.

Sometimes, no one replys because they do not understand the question. Endurance limit is a material property. If you know the material your rectangular tube is made from, go to an ASM Handbook and look up the endurance limit for that material. To find a round tube section that is equivalent to a rectangular section (you might choose to assume they are both the same thickness), find the bending moment of intertia equation for each section and set them equal to each other, and solve for the radius of the circular section. You will get a different result depending on which direction you are bending the rectangular section - moment applied in the major or minor axis direction. If this doesn't help, please consider revising your question. Please also be aware that most of us do not have a reference at hand that has the Marin Factor analysis described in it. Good luck!

Does anyone knows this questions? Sorry, I know this is a really old thread, but, anyone found the answer to this question? I know the .808(bh)^.5 is for solid rectangular section for sure, but let's say I have a hollow section, what should I do in this case for Marin Factor.