Micro and macro residual stress due to machining

[mikycampo]

In a manufacturing chain, Macro-scale residual stresses are introduced in a speciman by heat treatment, welding or other manufacturing process occuring before machining.

These residual stresses are self-equilibrated before starting material layers removal in machining. When material has been removed, the macro residual stresses are not more self equilibrated and produce distortion in the specimen.

Machining process also induces in specimens micro scale residual stresses due to thermo-mechanical effects (Morrow's law).

These stresses are limited to 0.1 mm to 1 mm under machined surface.

Can these stresses also produce distortion in the machined speciman? or these micro scale stresses influence the fatigue behavior of the specimen, and only macro scale residual stresses carry to distortion?

Thanks
Michele

 

[tomwalz]

Let me restate to see if I understand it.

Heat treatment, welding or other manufacturing processes create stresses that distort parts.

If you let them sit they adjust to and don’t distort any more as long as you leave them alone.

When you machine the stresses come back into play and distort the parts as material is removed. In addition machining creates tresses at or immediately below the surface.

Question: Can micro scale stresses from machining cause distortion or is it just macro scale stresses?

If this restatement is correct then my answer would be that machining can cause distortion.

I would add that I don’t think it I that simple.

I still remember trying to get a very complex, $10,000 part approved. We delivered first thing in the morning. The part was in spec when we started measuring. As the plant warmed up the part grew and twisted. Basically they had changed their minds and were trying to weasel out. Still we could never get a consistent set of measurements.

I don’t think I would divide them into macro and micro except for concept training. I would prefer to consider stress overall.

Stress can affect fatigue and part size and shape.

I would also consider stress relief. A round part machined from rolled plate will have different stresses than the same part machined from pipe. Cast has different stresses than rolled.

How you machine a part can add or relieve surface stress.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com