Estimating Yield Strength of Work Hardened Metals

[DrewEven]

I'm running into an issue, such that I need to be able to predict the yield strength of a cold worked part. The part is a hollow tube, seam welded from 3/16" SAE 1020 flat stock, which has a punching op. after welding and plating. I'm running Hertzian equations to predict the compressionial stress for a given loading scenario that we have on the slots(cylinder on flat plate), but I'm predicting quite large failure on the softer 1020 tube, but in fact there is no failure there, I supspect because the slot is highly work hardened in that area.

So, my question is this...Is there a good way to theoretically calculate the local yield strength in the slot area, or should I simply try to take the hardness on that slot (easier said than done) and try to correlate that to a yield strength?

Any help is extremely appreciated...

Thanks! - Drew

 

[TVP]

The answer to your question is basically no. Grade 1020 flat rolled steel that is subsequently formed and welded into a tube does not have one value for yield strength-- there will be a range depending on the prior thermomechanical processing (hot-rolled vs. cold-rolled, annealed after cold rolling, etc.), the roll pass schedule during tube forming, and whether or not the tube is subsequently processed (normalized? drawn over mandrel?). If you have some history of with this type of tube, you may be able to correlate tensile properties to hardness. For example, if the tube has used a consistent raw material, is always formed the same way, and undergoes the same post-welding treatment, and you have historical data on the tensile properties and hardness, then you can estimate the yield strength in the slot area based on local hardness measurements.

Keep in mind that Hertzian contact stresses are much different than tensile stresses when it comes to failure prediction. Contact stresses are a type of "surface pressure", and materials can tolerate higher surface pressures than tensile stresses before failure/fracture.

 

[DrewEven]

That makes good sense. I think maybe the best way to calculate the surface hardness in the slot area is with a micro-hardness tester.

As for the "surface pressure" generated from the line contact, what sort of relationship does a materials ability to resist surface pressure have with it's tensile or hardness properties? This seems to make sense, but I can't really find any analytical theory on failure prediction with Hertzian contact equations either...

Drew

 

[TVP]

Increased tensile strength = increased hardness = increased resistance to contact stresses. The best treatment of this that I have seen is in the German standard VDI 2230 for analysis of bolted joints. The term used in the standard is limiting surface pressure to describe the stress that occurs in the "bearing area" of a joint (under the bolt head). Values approximate 0.9*UTS for various Al and steel alloys.