[structural] evaluation per FFS-1
For those who evaluate stresses with the criteria of the Norm API 579 /
ASME FFS-1: for decades we have been comparing the secondary stresses with
the limit of the larger of 3 S or 2 Sy. Now I found in the 2016 edition of
this Norm that this limit has changed into the smaller of this value and
UTS (part 14). It is strange, since it is known that secondary stresses are
relaxed by shakedown with small plastic deformation in the initial cycles.
Does anyone knows the reason of this change? ASME Section VIII Division 2
still uses 2 Sy or 3 S. Could it be because of non ductile materials? If
so, a ductile material could have a 2 Sy limit?
Andre R. Beim
TRESCA Eng., Brasil
www.tresca.com.br
On Feb 23, 2017, at 2:46 PM, André Beim - TRESCA wrote:
Does anyone knows the reason of this change? ASME Section VIII Division 2
still uses 2 Sy or 3 S. Could it be because of non ductile materials? If
so, a ductile material could have a 2 Sy limit?
I don't know the reason, but the 2Sy limit is a shakedown criterion. The primary stress criterion (S) for Div 1 is mixed: the lesser of UTS/3.5 (Div 1) or ⅝ Sy. The ⅝ Sy criterion ordinarily applies to low yield materials like 300 series stainless. I've always heard that it's required to limits permanent deformation, particularly in the hydro test.
The conventional wisdom is that the former UTS/4 (now UTS/3.5) criterion covered the lack of a mandatory fatigue analysis requirement in Div 1. I suspect the corresponding Div 2 criterion plays the same role within the fatigue screening provision in Div2. The 2 Sy limit is a shakedown criterion that derives from elastic/perfectly plastic behavior. The 3S criterion has been around for a long time and I'm practically certain (some day I'll work it out) that it's equivalent to a shakedown criterion for materials with an allowable stress based on yield. Here are a couple of papers you might want to review
http://www.odonnellconsulting.com/wp-content/uploads/fatigue_design.pdf
- B. F. Langer, “Design of Pressure Vessels for Low–Cycle Fatigue,” ASME J. Basic Eng. 84, 389–402 (1962).
http://web.ornl.gov/info/reports/1983/3445605994528.pdf
Actually I hate Div 2. It's pedantic, badly written and built on the questionable pretense that analysis techniques (and user skills) are more accurate than is actually the case.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/members/chrisw/
Chris,
Thanks for the information. This second article has the original "ASME best
fit fatigue curves".
You should change your opinion on Div 2: it accepts design by analysis and
has the criteria for evaluation of FEA stresses. There are a lot of
components that are acceptable by analysis but not by rules. I heard people
say that Div 2 is more modern, and closer to european norms.
Andre R. Beim
TRESCA Eng., Brasil
www.tresca.com.br
On Fri, Feb 24, 2017 at 1:02 AM, Christopher Wright chrisw@skypoint.com
wrote:
On Feb 23, 2017, at 2:46 PM, André Beim - TRESCA wrote:
Does anyone knows the reason of this change? ASME Section VIII Division 2
still uses 2 Sy or 3 S. Could it be because of non ductile materials? If
so, a ductile material could have a 2 Sy limit?
I don't know the reason, but the 2Sy limit is a shakedown criterion. The
primary stress criterion (S) for Div 1 is mixed: the lesser of UTS/3.5
(Div 1) or ⅝ Sy. The ⅝ Sy criterion ordinarily applies to low yield
materials like 300 series stainless. I've always heard that it's required
to limits permanent deformation, particularly in the hydro test.
The conventional wisdom is that the former UTS/4 (now UTS/3.5) criterion
covered the lack of a mandatory fatigue analysis requirement in Div 1. I
suspect the corresponding Div 2 criterion plays the same role within the
fatigue screening provision in Div2. The 2 Sy limit is a shakedown
criterion that derives from elastic/perfectly plastic behavior. The 3S
criterion has been around for a long time and I'm practically certain (some
day I'll work it out) that it's equivalent to a shakedown criterion for
materials with an allowable stress based on yield. Here are a couple of
papers you might want to review
http://www.odonnellconsulting.com/wp-content/uploads/fatigue_design.pdf
- B. F. Langer, “Design of Pressure Vessels for Low–Cycle Fatigue,” ASME
J. Basic Eng. 84, 389–402 (1962).
http://web.ornl.gov/info/reports/1983/3445605994528.pdf
Actually I hate Div 2. It's pedantic, badly written and built on the
questionable pretense that analysis techniques (and user skills) are more
accurate than is actually the case.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/members/chrisw/
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On Feb 24, 2017, at 8:24 AM, André Beim - TRESCA wrote:
Thanks for the information. This second article has the original
"ASME best
fit fatigue curves".
My total favorite (which I meant to include) is "Criteria of the ASME
Boiler and Pressure Vessel Code for Design by Analysis in Sections
III and VIII Division 2." It's been around for a while and I don't
have a better reference for it, but it's a true classic, I think by
B.F. Langer. It's the best explanation I've ever seen of Code
allowables. Following closely is 'Design of Pressure Vessels for Low
Cycle Fatigue,' again by Langer.
You should change your opinion on Div 2:
I'll stick to my guns. The first time I picked up a copy of Div 2 was
sometime before 1975 when it was pretty much a rewrite of the Nuclear
Code. It was a good piece of work, then. However the latest edition
of Div2 is wordy, obtuse and pedantic. Those characteristics alone
make it damn near unusable for the majority of working engineers who
deal with Code vessels. What's needed is an appendix explaining how
to implement the Div 1 U-2(g) requirement with advanced analysis,
particularly secondary and peak stress criteria, and addressing in
fundamental requirements for support loading and fatigue limits.
Much of Div 2 is redundant--rules for openings, flanges, closures
like floatingheads, expansion joints and clamps, for example-- are
just lifted bodily out of Div 1. The rules for shells and head are
also lifted from Div 1 and unaccountably buggered up with that
obscure exponential notation which makes exactly no difference--
obviously someone looking to impress us all with his advanced degree.
The ellipsoidal and torispherical head rules are unaccountably mixed
into an iterative process with a with a stepwise approach that seems
aimed at a particularly dull engineering undergraduate. Granted, the
fatigue provisions are important, probably even useful, although not
terribly precise. Fatigue is a major contributor to vessel failures,
after abuse, but fatigue analysis is yesterday's news--nicely covered
by Langer 40 years ago, and easy to incorporate, at the level of
understanding of most of the vessel designers I've ever encountered.
I personally think that having 3 codes (counting Section III) is
intolerable, not to say stark, staring nuts, particularly since the
real need of the industry is to incorporate a relatively few
additional provisions: fatigue, supports and attachments and perhaps
creep into Div 1. That could've been done so very easily, but now
ASME has a 3 headed monster they can't support, with a difference in
basic design margin of about 15% of the cost of steel plate--the very
cheapest thing that goes in a vessel.
I heard people say that Div 2 is more modern, and closer to
european norms.
I've heard that too. As you can see I disagree.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/members/chrisw/