Questions about 'reference stress'

[andorrax1]

Hi all,

Couple of questions here:

I've recently started working on a project concerning limit analysis and keep coming across the term 'reference stress' (Or 'nominal stress'). Most of the literature I've gone into so far give only vague hints as to what it actually is ( stress far away from the concentration point being the recurring theme - but then how far away ?). I've been reading through Peterson's book on stress concentration factors and while it is relatively straightforward for a lot of common defects, I feel it doesn't really go into details about why the nominal stress is defined the way it is for any given problem. ( I haven't gone through it in detail so I may be mistaken about this.)

A concept that occurred to me from my read of Peterson thus far is that we could perhaps define the reference stress as the stress that would be present at the same location as the stress concentration point if the defect causing the concentration were absent ( e.g. stress at the center of a hole in a plate if the hole were absent ). Would this be a valid way to go about things ? Are there any good references that talk about this ?

So TL,DR :
1. What exactly is the physical meaning of 'reference stress' ?
2. Could the reference stress be defined as stress that would be experienced at the same location as the stress concentration point if the defect causing the concentration were absent.

Thanks in advance !

 

[SWComposites]

The reference stress is the far field value that the Kt is defined/derived relative to. A proper Kt chart or method should define the ref stress.

 

[MOHR1951]

Have never recalled the term "reference stress",in all the literature I have read."Nominal stress" is by far the most commonly accepted term,and as SWC points out,this is simply the far field stress,with no stress concentration,at work.It took me a long time as a student to grasp what "nominal stress",was referring to,this was before the time of the usage of near and far field was popular.The engineering society,and particular,authors should strive to use a clear and standardized terminology.I still see a lot of engineering students,to this day,that are confused by terminology,and this from native English speakers,I can only imagine the nightmares created for foreign students whose native language is not English.True technical publisher's would catch glitches like this,but few publisher's with technical smarts exist.The professional societies could do most to help this problem,but I see no real interest,on their part.

 

[rb1957]

1) reference stress is the "datum" stress that the Kt calculation is based on.
2) i guess you could think of it that way, but it would be whatever stress the calc is based on. For example, some Kt are based on gross section stress (like the stress at a hole, based on Wt area) but some are based on net area (that'd be (W-D)t).

another day in paradise, or is paradise one day closer ?

 

[MOHR1951]

So,rb1957,the conditions you point out in your #2,set certain conditions on it.So,reference stress is really a misnomer.to say the least.I just checked my three favorite strength of materials text books,none even mention reference stress,by name,the most generic or preferred name appears to be nominal,here again,what does that mean ? If this were an ethnic studies text or a text book for some other social science course,these vague forms would be acceptable,but they should not be allowed in structural engineering texts.This is not to lay blame at the reader,or even his interpretation,but to the author,and as I said who ever has the task of doing the technical editing.Is it any wonder that a great deal of confusion exists in our student's minds ?

 

[rb1957]

reference stress is a typical parameter in fatigue calcs, and not a typical term for strength of materials. Every Kt calculation depends on how the baseline (or reference stress) is calculated, as in my example a gross section stress Kt is different to a net section Kt because it (the Kt) is referenced from a different stress. As such it is no misnomer, IMHO; it is merely a definition.

another day in paradise, or is paradise one day closer ?

 

[andorrax1]

1) reference stress is the "datum" stress that the Kt calculation is based on.
2) i guess you could think of it that way, but it would be whatever stress the calc is based on. For example, some Kt are based on gross section stress (like the stress at a hole, based on Wt area) but some are based on net area (that'd be (W-D)t).

Thanks for the reply rb1957. Although I'm still a bit lost since your explanation in #1 seems a bit circular - Kt is defined in terms of nominal stress and nominal stress is the stress that Kt is based on...

I wasn't aware that the term 'reference stress' wasn't used much. I suppose it's because the literature I started with for this project was from the 80s and they did refer to it as 'reference stress' at some points. Modern references seem to have shifted to 'nominal stress' now. TIL !

Also what exactly does the term 'far-field' stress mean ? Is it simply a value of stress sufficiently far away from the concentration zone ? Or is it something different ?

I'm sorry if these questions seem dumb but as many others have pointed out here - the term isn't all that well defined in literature.

 

[rb1957]

not circular ... Kt is based on a stress or, if you will, Kt is referenced to a stress.

it is a reasonably common term in fatigue calcs (Niu uses it).

nominal stress, and far field stress, are similar terms.

At the end of this the peak stress at a stress concentration is Kt*stress. When you look at a Kt derivation it will be associated with a stress (beit reference, far field, nominal, whatever). You need to use this stress to determine the stress spectrum. Eg the Kt at a hole can be based on either gross section or net section stress.

clear as mud ?

another day in paradise, or is paradise one day closer ?

 

[MOHR1951]

Another example of sloppy terminology.Design work grasps all of it's roots from strength of materials and basic structural analysis,they are the heart of the matter,without them we would have people running around "designing" stuff based on their feelings or what they felt was pleasing,a dangerous situation.The term nominal stress was used through out the 60's,and 70's in most major engineering universities,and not referenced stress,I know,been there,done that.