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From: li pan <li76pan@ya...>  20051222 17:58:54

Hi Nachiket, Hey Nachiket, I implemented the algorithm of K.J.Bathe from his book << Finite element method >>, chapter 6, 1990. The algorithm is called Total Lagrange Formulation. I haven't calculated material tangent modulus. Because I only need stiffness matrix and second Piolar stress tensor. There must be something wrong in my implemented algorithm. I have to check it. Or you could give me some advice. I'm trying to implement an algorithm for large deformation, which fill the condition of Viscoelasticity. best pan  Nachiket Gokhale <gokhalen@...> wrote: > On Wed, 20051221 at 12:38 0600, Roy Stogner > wrote: > > On Wed, 21 Dec 2005, li pan wrote: > > > > > I used Newton iteration to solve a nonlinear > elastic > > > problem. This method is embedded in a Newmark > implicit > > > time integration. At the first several timestep > the > > > Newton iteration convergenced very well (10 > steps). > > > > Did you see quadratic convergence? > > I agree with everything that Roy said. > > If it is not too much trouble, could you post the > constitutive relations > that you are using?. That is, (strain energy > function (W), the second > PiolaKirchhoff tensor (S), and the material tangent > modulus (C_{IJKL}). > I ask this just to make sure that you are assembling > the right > consistent tangent matrix. > > Nachiket. > > > > > > > > The first thing you ought to check > > if Newton's method is behaving oddly is whether > you get quadratic > > convergence as you approach the root  if not, > then (assuming your > > linear solves are accurate enough) your Jacobian > (i.e. your system > > matrix, not the geometric Jacobians) is probably > inconsistent with > > your residual function. If that happens, your > iteration can still > > converge, but the region of convergence may be > smaller. > > > > Other possible problems: > > > > You may be starting from a bad initial guess. > This has happened to me > > before, when I left junk in a solution vector. > Depending on whether > > you're solving for the next timestep's solution or > for the > > delta_solution, a relatively safe initial guess > might be the previous > > timestep's solution or zero. > > > > The nonlinearities in your problem may be so bad > that the region of > > covnergence is too small for you to hit with your > initial guess. In > > that case you'll want to use a slower but safer > method (successive > > approximation, perhaps, or continuation) to get > close to the root > > followed by Newton to eliminate the last of your > error. > > > > > The error could be numerical calculation of > inversing matrix, > > > because I used simple Gauss elimination. I will > apply the method of > > > Petsc. > > > > We've had some problems before with LASPACK not > converging, but Gauss > > elimination isn't one of them  I often switch to > complete LU > > factorization as a preconditioner when I'm trying > to make sure that > > the linear solver isn't causing me problems. > Gauss is bad because > > it's slow, not because it's inaccurate. > >  > > Roy Stogner > > > > > > >  > > This SF.net email is sponsored by: Splunk Inc. Do > you grep through log files > > for problems? Stop! Download the new AJAX search > engine that makes > > searching your log files as easy as surfing the > web. DOWNLOAD SPLUNK! > > > http://ads.osdn.com/?ad_id=7637&alloc_id=16865&op=click > > _______________________________________________ > > Libmeshusers mailing list > > Libmeshusers@... > > > https://lists.sourceforge.net/lists/listinfo/libmeshusers >  > > > > >  > This SF.net email is sponsored by: Splunk Inc. Do > you grep through log files > for problems? Stop! Download the new AJAX search > engine that makes > searching your log files as easy as surfing the > web. DOWNLOAD SPLUNK! > http://ads.osdn.com/?ad_id=7637&alloc_id=16865&op=click > _______________________________________________ > Libmeshusers mailing list > Libmeshusers@... > https://lists.sourceforge.net/lists/listinfo/libmeshusers > __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com 