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# Maximum damaging angle

## Maximum damaging angle

(OP)
Can someone help me understand the maximum damaging angle with respect to Fatigue behavior of metals. I my opinion it means that in a multi-axial state of stress the max fatigue damage occurs at this angle if an equivalent uniaxial stress is applied at it.

### RE: Maximum damaging angle

orientation of principal stress ?

### RE: Maximum damaging angle

(OP)
I would like to know the significance of max damage angle, and how to interpret it in physical terms.

### RE: Maximum damaging angle

I'm guessing you're looking at a sheet/plate with various loadings. each will have it's own principal stress, and so it's own direction for maximum fatigue damage, yes? (fatigue damage being related to principal stress).

A simple, conservative, approach is to take the principal stress from each loading and determine fatigue from this. This is conservative as it is taking the worst (most damaging) stress from each loadcase, but these won't typically align.

So if you wanted to be super-precise then you'd work in global directions and use Mohr's circle to determine the normal stress, producing fatigue damage, for each load case. Then figure out the direction that maximises fatigue damage.

Fatigue and crack growth are treated the same.

### RE: Maximum damaging angle

Suggest the OP add a sketch to help us figure out what he's talking about? I took the term as meaning the angle between peak tensile stress and the principle (long) axis of a flaw/crack. I.e. worst case is tension applied at 90 degrees to said flaw, and perhaps best case is a -90 degrees (ie. compression)...

### RE: Maximum damaging angle

(OP)
Hi rb1957,

You got it right, this is what I was looking for. So If I understand correctly, max damage angle would simply mean that if I resolve all the stresses ( normal & shear ) along this angle, then the resultant stress would produce max fatigue damage (perpendicular to this angle).

yep

### RE: Maximum damaging angle

(OP)
thanks..!!

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