Why do we use avg shear stress and not max shear stress?

[chad mcelroy]

Hello everyone, I've been messing around with FEA and I'm trying to match my simulation with hand calculations. While doing this it occurred to me that i really don't understand why we use avg shear stress instead of maximum shear stress when determining the safety factor of a structural member i.e. a pin

 

[PhanthomJay]

Hello everyone, I've been messing around with FEA and I'm trying to match my simulation with hand calculations. While doing this it occurred to me that i really don't understand why we use avg shear stress instead of maximum shear stress when determining the safety factor of a structural member i.e. a pin

When a bolt or pin is in direct shear, average V/A shear stress is used. Maximum VQ/It shear stresses are used for beams subject to shear and bending. But often in a Wide Flange beam average shear stress is used by using the area of the web and not the flanges, and you get close to the same result as the max shear stress formula. Even the max shear stress hand calculation is an approximation, because there are variations in shear stress not only top to bottom, but often left to right as well,as FEA might show. The safety factor has it covered. Nothing tops the good old fashioned hand calcs.

 

[chad mcelroy]

PhanthomJay thank you for replying. I did a double shear test on 3/8 in rod of C1215 steel. I've attached the image I think. On the top and bottom of the rod the surface has noticeably yielded as if it has been cut into. are contact stresses a factor as well as avg shear stress. I'm trying to accurately account for all stresses in this type of application. Also do you know of any literature or online resources that I could use to learn more advanced theories of ductile fractures.

Thank you

 

[PhanthomJay]

PhanthomJay thank you for replying. I did a double shear test on 3/8 in rod of C1215 steel. I've attached the image I think. On the top and bottom of the rod the surface has noticeably yielded as if it has been cut into. are contact stresses a factor as well as avg shear stress. I'm trying to accurately account for all stresses in this type of application. Also do you know of any literature or online resources that I could use to learn more advanced theories of ductile fractures.

Thank you

besides checking for shear stresses in bolts or rods (using average V/A shear stress calculations when bolt or rod is subject to shear and not bending), you must also check for bearing stresses in the bolt and plate at the contact area between the bolt/rod and the connected plate hole wall. Most codes allow for some deformation and yielding at the bearing contact areas. I haven't done much research in ductile fracture theories. Shear tear out in the plate must also be checked for holes near edges.

 

[chad mcelroy]

Thank you for your help