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Secondary stress validity
2

Secondary stress validity

Secondary stress validity

(OP)
In some threads primary and secondary stresses problem is raised. I would like to ask everybody who has experience
in this subject to telling something more about it.
For example thermal stress (from expansion) are treated as
a secondary stress - is it always correct? In fact if any part is heated it induce thermal stress and that are real stress (stress occuring in reality). Secondary stress are not so important like primary stress? When stress can be qualified as a primary and when as a secondary stress?
Any examples from different analysis will be very advisable to develop this subject. Interesting publication title in this subject will be also valuable.

RE: Secondary stress validity

(OP)
Some additional comments may extent subject above.
For example according to EN 1993-1-6:2007, section 4.2.2.2, page 21, "The primary stresses should be taken as the stress
system required for equilibrium with the imposed loading".
I would like to ask for some examples which stresses will be in equilibrium with imposed loading and which not. Similarly according to EN 13445-3:2002, section C2, pages 551-552, primary stress: "stress which satisfies the laws of equilibrium of applied loads (pressure, forces and moments)", secondary stress: "stress developed  by constraints due to geometric discontinuities, by the use of materials of different elastic modulii under external loads, or by constraints due to differential thermal expansions".  

RE: Secondary stress validity

In pressure vessel design codes they give examples of primary/secondary and peak stress classification. In general secondary stresses are self limiting, ie. by thermal expansion where the strain is limited to the strain due to thermal expansion. Primary stresses are load determined.

corus

RE: Secondary stress validity

In the context of stress categorization (primary vs. secondary vs. peak), the question that you must always ask yourself is "What Failure Mode am I protecting against?"  Primary stresses that exceed a threshold of, say, yield, will lead to a plastic collapse situation.  Secondary stresses, on the other hand, will not result in such a failure, regardless of their magnitude.

RE: Secondary stress validity

Can u suggest an article where (primary vs. secondary vs. peak)is explained in detail.  

RE: Secondary stress validity

ASME Section VIII, Division 2, Part 5 would be a good place to start.  I would also look at the document titled "Criteria of the ASME Boiler and Pressure Vessel Code for Design by Analysis in Sections III and VIII, Division 2".  There will also be a good tutorial that explains Part 5 (and will have some detail on stress categorization) at the 2009 ASME PVP Conference.  (http://www.asmeconferences.org/pvp09)

RE: Secondary stress validity

(OP)
Bagarbilla - primary vs secondary vs peak are described in EN 13445. I am looking something more about it. I think that
ASME code mentioned by TGS4 will be more detailed.

RE: Secondary stress validity

Pawel27, the wording you quoted from EN13445 is a complete and meaningful definition, however I understand that, without examples, its implications may be difficult to grasp.
The classical example of a primary stress that is determined by the laws of equilibrium:
-you know that in a shell under pressure the stress across the thickness is not constant (all three components vary). The variation is normally negligible for thin shells, but may be not in thick shells. Now from the definitions you can determine that the constant average (membrane) portion of stress is primary, as it satisfies the equilibrium, and the variable portion is secondary because it is due to an internal constraint of the structure (I note here that the wording of EN13445 is somewhat incomplete, as this is not a geometric discontinuity; that wording should be completed by 'geometric discontinuities and self- constraint as in ASME VIII)
Another classical example:
-in the junction between a shell and a head (take a hemispherical one for simplicity) it is known that membrane and bending stresses develop in the junction, additive to those due to pressure in the two parts when they are thought as separate, that are due to the different deflection of the two structures under the same pressure. Those additional stresses are secondary, as they are 'developed  by constraints due to geometric discontinuities.
Don't want to confuse you, but there are examples of thermal stresses that are classified as primary. The first that comes to mind is the stress developed in a pipe support by the thermal expansion of the pipe: if, as in the general practice, the support is analyzed separately from the piping, then the force exerted on the support, whatever is its origin, will fall into the category 'stress which satisfies the laws of equilibrium of applied loads'.
Again don't want to confuse you more, but can't resist to proposing here a different definition of primary and secondary stresses that I love:
-a primary stress is any of the stress distributions that can satisfy the equations of equilibrium in the theory of elasticity (so a primary stress is somewhat arbitrary, as there are generally infinite solutions to the equations of equilibrium, the designer can choose the distribution he likes among that infinity)
-a secondary stress is the balance, with respect to the chosen distribution of primary stress, that is necessary to satisfy the equations of compatibility as defined in the theory of elasticity

prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes for fun rides
http://www.levitans.com : Air bearing pads

RE: Secondary stress validity

(OP)
Prex, the last definition of primary and secondary stress
seems to be reasonable. It is coincide with my feeling.
Unfortunately according to this definition - stress above yield limit at the place of nodal force will be secondary. Meanwhile according to EN 13445-3:2002 loads like pressure, forces and moments cause primary stress (section C.2.3).
Eurocode definition seems to not cover all practical
cases. I would like to ask once more what about importance of primary and secondary stress? The name "secondary stresses" indicates that they are less important? Personally I would be very carerul with neglecting them.
I know some people who treat secondary stress as not so important. Example: stress caused by thermal expansion of two different materials joined together are classified as a secondary stress. Is it possible that due to different thermal expansion, materials will be rupture at the place of weld? If yes, classification of thermal stress as a secondary in this case seems to be not correct. I am not sure whether thermal stress in example above are self - limiting (i.e.secondary stress). Maybe there will be rupture?

RE: Secondary stress validity

Of course EN-13445 doesn't imply in any way that pressure, forces and moments will cause primary stresses only! The meaning of that wording is simply that primary stresses are those stresses that are required to satisfy the laws of equilibrium to equilibrate the applied loads (formed of course by pressure, forces and moments).
And by no means a calculated stress that is above yield is classified as such as a secondary stress!
What makes the difference is the behavior of the structure under each type of load, or, from another point of view, the type of failure that each type of load will cause when increased (as also suggested by another poster above).
Of course the classification is not a matter of importance, though one could say that secondary stresses are, in many normal and frequent situations, less dangerous than primaries, to the point that ASME VIII Div.1 explicitly recognizes that significant secondary stresses will exist in any structure, but they need not be determined under the coverage of that code.
What you need to understand is that:
-a calculated secondary stress may be above yield, but this is of course only due to the use of elastic analysis: no stress above yield may exist in a real (ideal) material
-the type of failure that a secondary stress may cause is ratcheting or incremental collapse: at every load cycle the plastic deformation is incremented (if the elastic stress is above yield) and when a sufficiently high deformation is accumulated a failure will occur.
In your example of two different materials joined together, if the joint is correct, there will never be rupture at the first heat up, whatever is the temperature up to an upper limit determined by material properties; you need to cool down and heat up a certain number of times to experience a failure.

prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes for fun rides
http://www.levitans.com : Air bearing pads

RE: Secondary stress validity

Hi,
I like Prex pointing in the direction that EN13445 Design By Analysis was the first Code (now the latest edition of the ASME seems to go in that direction too) to recognize the "Direct Route" (or "Inelastic Route") not only as valid but moreover preferable.
In fact, as far as the modern numerical-calculation methods allow you to calculate the full stress-state of a part, categorization of stresses becomes, by nature, almost meaningless in practice.
Of course, the effort needed to practice the Direct Route is considerable, as elasto-plastic non-linear analyses are required, the most time- and resources-demanding is the "Check against progressive plastic collapse" (the check against ratcheting phenomena, or the check for shakedown). In addition, there is also, at least with the first approved release of EN13445, a strong discrepancy between the combined-stress criteria used for the plastic constitutive law of the material in the Code, and in the current FEM programs (the "eternal question" Von Mises vs. Trescà-Guest...). Last but not least, also the use of the deviatoric maps is all but intuitive...

However, to return to the original point: in my opinion, the categorization route described in EN13445 is here for compatibility with ASME BPVC VIII-2, but in order to better understand it you'd better drop EN and examine the original ASME Code instead.

May I also suggest to read some very interesting - free - publications as regards EN13445 "DBA":

- PED Joint Research Center - 97/23/EC "Design by Analysis: the DBA Manual"; can be found at www.mpa-lifetech.de/EPERCTP/ped/jrc/jrc_design.html

- RWTüV, B.Gorsitzke: Tü154: "Berechnung der Ermüdungslebensdauer für Druckbehälter nach Europäischer Norm DIN EN 13445, Teil 3"

Regards

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