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<div class="">On Sep 12, 2018, at 4:23 PM, Matthew Knepley <<a href="mailto:knepley@gmail.com" class="">knepley@gmail.com</a>> wrote:</div>
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<div dir="ltr" class="">On Wed, Sep 12, 2018 at 5:16 PM Smith, Barry F. <<a href="mailto:bsmith@mcs.anl.gov" class="">bsmith@mcs.anl.gov</a>> wrote:<br class="">
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The function norm given by the -snes_monitor is the function norm for the DVI problem (which depends on where the solution is constrained) while FormFunction() norm you compute is for the original unconstrained problem.<br class="">
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I am still not sure why you solve this as a DVI. If you solve it without the constraints do you get the "wrong" answer?<br class="">
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<div class="">If this is a variational crack problem, you can solve it unconstrained if you use a quadratic penalty term. However,</div>
<div class="">that induces spurious long range communication between crack tips. If you bound the phase field between 0 and 1,</div>
<div class="">you can use a linear penalty which has physical behavior.</div>
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<div>Not really, you still need the growth constraint from iteration to iteration or you could get healing in unloading cycles, or cack path that change over time. A classical and easy to reproduce examples is to look at a crack tip with two small inclusion
at different directions and different orientation relative to the tip. As the loading increase, you could have a crack that goes from tip to inclusion 1, then later from tip to inclusion 2 without going through inclusion 1...</div>
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<div>@Josh: are you alternating solves in displacement and phase field or trying to implement a monolithic approach? Convergence of monolithic approaches is elusive, and they do not necessarily give you physical path (that is paths that can be interpolated
along a path of monotonically decreasing energy)</div>
<div>With a staggered (alternate directions) approach, there is no excuse for not using SNESVI. As you mentioned, since you are propagating a crack tip, most of the constraints are saturated and the active set changes very little from step to step. In my experience,
SNESVI is very fast in this situation (2-3 iterations), and its cost still negligible compared to the cost of the computation of the equilibrium displacement.</div>
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<div>In addition, once you can handle growth through SNESVI, you have no excuse not to use a linear damage dissipation potential, which is physically much reasonable (finite support often damage localisation, existence of an elastic limit, and correct nucleation
in a wide range of situation as shown in my recent paper (Tanné et al JMPS 2018)</div>
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<div>My implementation of phase field fracture is available at <a href="https://bitbucket.org/bourdin/mef90-sieve" class="">
https://bitbucket.org/bourdin/mef90-sieve</a> It is still based on an old petsc because I can never find the time to catch up with the release, and because I needed some form of I/O in “natural ordering” that only came in recently.</div>
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<div>Feel free to ping me directly if you need more details</div>
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<div>Blaise Bourdin</div>
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<div class=""> Matt</div>
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Barry<br class="">
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> On Sep 12, 2018, at 3:32 PM, Josh L <<a href="mailto:ysjosh.lo@gmail.com" target="_blank" class="">ysjosh.lo@gmail.com</a>> wrote:<br class="">
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> Hi,<br class="">
><span class="Apple-converted-space"> </span><br class="">
> my solution is mostly very close to 1. only for a very small area where solution goes from 0 to 1(a smeared crack).<span class="Apple-converted-space"> </span><br class="">
><span class="Apple-converted-space"> </span><br class="">
> I set -snes_atol 1e-7 and it is converging.<br class="">
><span class="Apple-converted-space"> </span><br class="">
> I've noticed the following:<br class="">
><span class="Apple-converted-space"> </span><br class="">
> There is a difference between the function norm.<br class="">
><span class="Apple-converted-space"> </span><br class="">
> I calculate the function norm in FormFunction, so every time it is called it gives the function norm<br class="">
> , and the result is different from the function norm given by -snes_monitor if i set<span class="Apple-converted-space"> </span><br class="">
> <span class="Apple-converted-space"> </span><br class="">
> SNESSetType(snes,SNESVINEWTONRSLS,ierr)<br class="">
> SNESVISetVariableBounds(snes,xl,xu,ierr)<br class="">
><span class="Apple-converted-space"> </span><br class="">
> The function norm calculated in FromFunction is NOT reducing, however, the function norm given by -snes_monitor is reducing<br class="">
> They are the same if I just use regular SNES without setting variable bounds.<br class="">
><span class="Apple-converted-space"> </span><br class="">
><span class="Apple-converted-space"> </span><br class="">
> Thanks,<br class="">
> Josh<br class="">
><span class="Apple-converted-space"> </span><br class="">
> 2018-09-12 12:02 GMT-05:00 Smith, Barry F. <<a href="mailto:bsmith@mcs.anl.gov" target="_blank" class="">bsmith@mcs.anl.gov</a>>:<br class="">
><span class="Apple-converted-space"> </span><br class="">
> You have too tight a convergence tolerance for your problem. You can't expect to get more than 1.e-12 as the minimum residual norm or even less.<span class="Apple-converted-space"> </span><br class="">
><span class="Apple-converted-space"> </span><br class="">
> How close is your solution to 1 and -1?<br class="">
><span class="Apple-converted-space"> </span><br class="">
> If you really need much higher convergence you can try ./configure --with-precision=__float128<span class="Apple-converted-space"> </span><br class="">
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><span class="Apple-converted-space"> </span><br class="">
> Barry<br class="">
><span class="Apple-converted-space"> </span><br class="">
> > On Sep 11, 2018, at 11:53 PM, Josh L <<a href="mailto:ysjosh.lo@gmail.com" target="_blank" class="">ysjosh.lo@gmail.com</a>> wrote:<br class="">
> ><span class="Apple-converted-space"> </span><br class="">
> > Yes, I initialize all u_i to 1.0<br class="">
> ><span class="Apple-converted-space"> </span><br class="">
> > <span class="Apple-converted-space"> </span><br class="">
> ><span class="Apple-converted-space"> </span><br class="">
> > 2018-09-11 23:37 GMT-05:00 Smith, Barry F. <<a href="mailto:bsmith@mcs.anl.gov" target="_blank" class="">bsmith@mcs.anl.gov</a>>:<br class="">
> ><span class="Apple-converted-space"> </span><br class="">
> > Do you start with initial conditions of 0 <= u_i <= 1 ?<br class="">
> ><span class="Apple-converted-space"> </span><br class="">
> > Run with -snes_monitor -snes_converged_reason -ksp_monitor_true_residual -info -snes_linesearch_monitor and send all the output<br class="">
> ><span class="Apple-converted-space"> </span><br class="">
> > Barry<br class="">
> ><span class="Apple-converted-space"> </span><br class="">
> ><span class="Apple-converted-space"> </span><br class="">
> > > On Sep 11, 2018, at 11:33 PM, Josh L <<a href="mailto:ysjosh.lo@gmail.com" target="_blank" class="">ysjosh.lo@gmail.com</a>> wrote:<br class="">
> > ><span class="Apple-converted-space"> </span><br class="">
> > > Hi,<br class="">
> > ><span class="Apple-converted-space"> </span><br class="">
> > > I am using SNES to solve an nonlinear equation f(u), and I know all the u_i should be 0 and 1.<br class="">
> > ><span class="Apple-converted-space"> </span><br class="">
> > > First, I use SNES without constraint, and it converges.<br class="">
> > ><span class="Apple-converted-space"> </span><br class="">
> > > But, If I set<span class="Apple-converted-space"> </span><br class="">
> > > SNESSetType(snes,SNESVINEWTONRSLS,ierr)<br class="">
> > > SNESVISetVariableBounds(snes,xl,xu,ierr)<br class="">
> > ><span class="Apple-converted-space"> </span><br class="">
> > > where xl and xu is vector, and xl_i=0 and xu_i=1<br class="">
> > ><span class="Apple-converted-space"> </span><br class="">
> > > then SNES fails to converge, because linesearch fails(snes reason = -6), and the norm of residual is not reducing(the norm of incremental solution is reducing)<br class="">
> > ><span class="Apple-converted-space"> </span><br class="">
> > > The reason to add constraint is that I want to implement some irreversibility.<br class="">
> > ><span class="Apple-converted-space"> </span><br class="">
> > ><span class="Apple-converted-space"> </span><br class="">
> > > Thanks,<br class="">
> > > Josh <span class="Apple-converted-space"> </span><br class="">
> > > <span class="Apple-converted-space"> </span><br class="">
> ><span class="Apple-converted-space"> </span><br class="">
> ><span class="Apple-converted-space"> </span><br class="">
><span class="Apple-converted-space"> </span><br class="">
><span class="Apple-converted-space"> </span><br class="">
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<div class="">What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br class="">
-- Norbert Wiener</div>
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<div class=""><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank" class="">https://www.cse.buffalo.edu/~knepley/</a></div>
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-- </div>
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Department of Mathematics and Center for Computation & Technology</div>
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Tel. +1 (225) 578 1612, Fax +1 (225) 578 4276 <a href="http://www.math.lsu.edu/~bourdin" class="">
http://www.math.lsu.edu/~bourdin</a></div>
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