Q: I have used a temperature dependent BISO table for my material, and activated the "Modified Rational Polynomial Creep Equation" of Table
2.4-6. Thank you for the feedback; I didn't realize that I should not use BKIN
for the plastic deformation in this combined effects model.
A: Since you are using explicit creep, you *can* use BKIN or BISO. The
choice will probably be dictated by cyclic loading applications (inclusion
or omission of the Bauschinger effect).
As a side note, although isotropic hardening is also used for finite
strain applications because of the expansion rather than shifting of the
yield surface, I am not sure how suitable finite strain is with creep. I am
not saying that BISO shouldn't be used with creep but, rather, that if the
application involves finite (large) strains, I am not sure how valid the
relationship would be by treatment of plastic (rate-independent and
rate-dependent) in a decoupled fashion. Both explicit and implicit (5.6)
creep handle plastic and creep strain terms independently, although they are
evaluated at the (current) stress state. Unified theories try to treat all
inelastic strains together in a coupled fashion, although I've come across
so many different models (from customers) that it can get tricky to figure
out which is most applicable (based on strain rates, temperatures, type of
cyclic loading, etc.). Anyways, I bring this up simply because I don't know
enough about it whether it is a concern or it but just wanted to point it
Since you are using 5.5, explicit creep is the only option available.
(Implicit creep was introduced at 5.6) From your problem description, it
does not seem like you are dealing with finite strain for the initial load,
and it seems that you are looking at relaxation effects separately. So
maybe the above paragraph was too much rambling, but I just wanted to point
it out, anyway.
Q: I am using ANSYS 5.5 -- sounds like 5.6 would be better.
A: If your characteristic relaxation times are much shorter than the period
of interest, implicit creep would definitely be much more efficient. To me,
solving problems with explicit creep is really like solving it in
Q: -- I don't understand why there is a comment on "creep constant defined through TBDATA" and its temperature dependence. I have a spatially varying temperature... is not creep within each element calculated with respect to that element's temperature? Is the built-in creep model that I am using (Table 2.5-6, ANSYS 5.5) appropriate for a spatially varying temperature in a structure during creep analysis? Does a spatially varying temperature in structure analysis of creep require a temperature dependent creep constant table as implemented in ANSYS 5.6 -- I hope not.
A: In 5.6, implicit creep includes the provision to include
temperature-dependent constants. This means that temperature can be
included in two ways:
As usual, I probably digressed too much, but in answer to your question,
at 5.5, there is no temperature-dependent creep constants allowed. In 5.6,
for implicit creep, you can include them, but you don't have to. It all
depends on your creep equation and the material constants you have
- usually, via the Arrhenius eqn which is the term w/ the activation
energy: exp(-Q/RT). This is just like explicit creep, and it includes the
temperature-dependency as part of the equation.
- For implicit creep, all constants C1-Cx can also be
temperature-dependent. This feature could've been included for a number of
reasons, but I'm guessing that it has been included for more simple models
(strain- or time-hardening terms only) which don't include the Arrhenius
function but require temperature-dependency, such as a common equation of
d(ecr)/dt = C1 * sigma(C2) * epsilon(C3)
where C1, C2, or C3 can be temperature-dependent since temperature is not
explicitly included in the above equation.
Q: in 5.5, with explicit creep, the creep and plasticity are treated sequentially within each iteration, if you do them separately you can see the potential for inaccuracies if the time step is too large, this is one of the reasons why the crplim has to be small and explicit creep analysis takes so long
with implicit creep, the calculations are done simultaneously (within the same subroutine) which can partially explain the 20X+ solution speed increase that you see between using implicit vs explicit (for the same material law)
A: In a separate posting by Carlos (above), it seemed that it was implied
that explicit creep is slower than implicit creep because of the treatment
of the plastic strain (plastic and creep calculations done sequentially or
simultaneously). I probably misread this or the intent of the author, but I
don't believe this to be true. Even if rate-independent plasticity was not
included in the calculations, if the time domain of interest is much longer
than the characteristic relaxation times, the solution using the explicit
creep method will take much longer than the implicit creep method (i.e., it
doesn't matter if you include plasticity or not). This is because creep is
defined by the strain rate d(ecr)/dt. How one determines this calculation
(dt) can be either forward- or backward-Euler method. That time integration
term is what tends to make explicit creep longer (what I referred to as
solving the problem in 'real time') since forward-difference method is only
conditionally stable (we're assuming the slope of the function is the same
at time=t+dt and time=t, so dt has to be small for explicit creep).
The treatment of plastic and creep strain terms (calculations done
sequentially or simutaneously) probably also affects the overall solution
time, but not as much as the fact that explicit creep requires much smaller
time steps. The treatment of the plastic and strain terms affect accuracy,
Anyways, as usual, feel free to correct any of my misinformation since I am
known to spread them like wildfire. :) I am definitely not a material
science person, so I often obtain this information off of the back of cereal