Quantifying plasticity
Quantifying plasticity
(OP)
All,
When designing with FEA to a requirement of "no plastic deformation" or "no yielding" under proof, one time loading, how would you utilize the results of a nonlinear static analysis?
Would you look at plastic strains and keep them under 0.2%? That's what I usually do when working with steel, for example.
I have come across some colleagues who look at stress distributions after such analyses and they identify that most of the hot spots show stresses less than yield, without examining strains. Is this valid?
Is there some accepted quantified value of plastic strain that is typically allowed?
tg
When designing with FEA to a requirement of "no plastic deformation" or "no yielding" under proof, one time loading, how would you utilize the results of a nonlinear static analysis?
Would you look at plastic strains and keep them under 0.2%? That's what I usually do when working with steel, for example.
I have come across some colleagues who look at stress distributions after such analyses and they identify that most of the hot spots show stresses less than yield, without examining strains. Is this valid?
Is there some accepted quantified value of plastic strain that is typically allowed?
tg





RE: Quantifying plasticity
if you are referring to FAR25.305 the requirement is no "detrimental" yielding at limit (not quite the same as no yielding at limit).
if you are working to a company/customer spec, and you have any sort of holes (rivets, bolts) in your structure then you'll be limited to something like yield/3 ("something like" 'cause the Kt for a filled hole is less than 3 (for rivets) but an open hole (screws, bolts) is of course 3).
RE: Quantifying plasticity
This is for non-aerospace applications. I work on rail vehicles, and I suspect it's closer to your notion of "no detrimental yielding".
How do you confirm "no detrimental yielding" by NL static FEA in your industry?
tg
RE: Quantifying plasticity
the real test/confirmation for us is our static tests.
RE: Quantifying plasticity
RE: Quantifying plasticity
I would be very careful with this one....we might know (or think we do) what is safe, etc. but I'm sure any explanations would quickly become very difficult if it became a legal matter......
Ed.R.
RE: Quantifying plasticity
I was hoping, maybe simplistically, that the engineers who perform FEA have certain guidelines for evaluating the results of nonlinear static analyses, before performing redesign.
If so, do you evaluate strains? Or do you look at stress distributions? I am mostly referring to structures without explicitly modeled connections, so there are no holes. The only real stress raisers are corners, for example.
tg
RE: Quantifying plasticity
a stress peak at a corner ... how well modelled is the corner ? again, a sliver of high stress would probably be "clearly" acceptable.
but your spec raises the question. if it says "no yielding at limit/service loads" then that's what you have to work with; you know fty and fcy, so they'll limit your allowable stresses.
RE: Quantifying plasticity
Tara
http://tinyurl.com/4ydjg7m