Butt welded shaft fatigue strength

[user71]

How can the tensile fatigue limit be estimated in a 1.25 inch shaft that has been butt welded assuming it has been properly prepared and welded? I've looked in British Standard 7608 and a couple other machine design books and do not see any examples of that kind of connection in a shaft, only for plating tht is butt welded.
Thanks
Larry

 

[desertfox]

Hi user71

Have a look at these sites:-

http://www.gowelding.com/weld/fatigue/fatigue.htm

http://www.staff.ncl.ac.uk/s.j.bull/mmm373/WELDMECH/sld016.htm

 

[metengr]

How can the tensile fatigue limit be estimated in a 1.25 inch shaft that has been butt welded assuming it has been properly prepared and welded?

First off, I would determine the fatigue strength for the base material of the shaft, assuming no butt weld. You can search for this information or if it is carbon or low alloy steel, you can use a rough correlation that the high cycle fatigue strength will be 0.5 x ultimate tensile strength of the steel.

Once you have the fatigue strength of the base material with no weld, you now have to establish an appropriate knock-down factor because you have placed a metallurgical notch in the original shaft base material. The heat from welding alters the original microstructure of the base metal unless the shaft is re-heat treated after welding, which can be done but presents concerns for distortion.

If you have a full penetration butt weld with the weld crown blend ground flush with no stress risers, I would use a knock down factor of 0.85 x the fatigue strength.

How did I arrive at this? The 0.85 is a common factor used for castings versus wrought material. In this case, the weld metal is cast so this is where I would start regarding a knock-down factor for welds.

 

[PriamEngineering]

BS5400 1999 issue 2 page 53 has the exact setup you are looking for. It's class D.

http://www.priamengineering.co.uk