asme B31.3 Appendix P question (stress range) 1

[r77]

Hi all!

There is something I can't grasp about stress range calculation according to the asme B31.3 appendix P. It says :
<<The operating stress range is the range of STRESS between any two operating conditions..>>.
This is a different definition than that given in 319.2.3 (b) :
<<the algebraic difference between STRAINS in the extreme displacement conditions [...] produces a corresponding stress differential : the displacement stress range>>

So the code permits to evaluate the OPERATING stress range instead of the DISPLACEMENT stress range. Yet, apart from the inclusion of axial stress, the formula is the same :
Se=((|Sa|+Sb)^2+4St^2)^0.5, and also Sb and St are evaluated by the same formulae in 319.4.4.

can someone shed o bit of light on the following points?

1. What's the difference between the two approaches?
2. Let's suppose I have two operating conditions 1 & 2. For both I know N,Mi,Mo,Mt. Can I say that (neglecting for the moment the SIFs) :

Sa= (N1-N2)/A
Sb= ((Mi1-Mi2)^2)+(Mo1-Mo2)^2)^0.5)/Z
St= (Mt1-Mt2)/2Z
and the OPERATING stress range is :
Se=((|Sa|+Sb)^2+4St^2)^0.5

Thank you for any help!

 

[BigInch]

I'll take a stab at it.

302.3.5 is to evaluate maximum stresses due to sustained loads and stresses from displacement strains, thermal and imposed. Allowable Displacement Stress Range, Sa is is evaluated in accordance with eq. 1a & 1b

The displacement stresses (range) for operating condition extremes is used to evaluate flexibility (see 319.2.3 a&b) 319.4.4 for systems primarily stressed in bending ...
Where systems are flexible enough, the axial stresses should be insignificant, hence they are not directly included in the flexibility case formula and permissible additive stresses shall be as specified in para.302.3.5(d).

However if you have calculated the axial stresses anyway, then you can evaluate the already obtained sum by the alternate method, which shifts the allowable to compensate for the shift in (assumed neutral) position to the entire range, pushed there by the axial stresses.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[r77]

My doubts are not related to allowable stress(in this sense the appendix P is clear, Sa=1.25f(Sc+Sh)), but to the definition of <<operating stress range>> Se. The standard gives no formula, it just refers to the same formula used for the displacement stress range (319.4.4).
Can operating stress range be calculated as I mentioned in my previous post, as the difference between moments in two operating conditions?

 

[BigInch]

That is the same, except of course for the inclusion of axial stress. The difference of moments between one end of the range and the other end of the range. I didn't want to guess what cases your subscripts refer to.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[r77]

bigHinch,
Of course, subscripts 1 & 2 refer to Operating case 1 and Operating case 2
I definitely agree with you, the definition of operating stress range is numerically the same than displacement stress range, apart from axial stress inclusion.
A big difference seems to be, on the other hand, the allowable stress Sa, that for operating stress range should be higher, infact :
Operating stress range allowable = 1.25f(Sc+Sh) (P1b)
Displacement stress range allowable= f(1.25*Sc+0.25*Sh)(02.3.5 d)
the first is always greater than the second
If this is the case, and given the equality of calculated stress as previously stated, the <<alternative rules to evaluate the stress range>> (quote from P300 paragraph) of appendix P are more permissive and easy to be satisfied than those from chap II regarding displacement stress range

Or do I miss something?

 

[BigInch]

It's one case or the other. Whichever case gives you the max, use that one. It's not the range of two cases, since it is calculating the deviation from the "as built" temperature.

For example, as built temp to coldest temperature condition, or as built temperature to the hottest condition, one being 100 degrees higher, the other 100 degrees lower than as built temperature, would give you the same flexibility range from norm, even though one was on the expansive side (+ moments) and one on the contraction side (-moments). It's checking the swing or deviation from the norm, not the highest to lowest range.


Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[BigInch]

I think what I meant to say was the largest absolute value of any operating case.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso