Weak solutions under finite elements

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SUMMARY

Finite element methods yield weak solutions to partial differential equations (PDEs), which are only valid in integral form. If a classical (strong) solution exists, it is also a weak solution; however, weak solutions may exhibit discontinuities and can differ significantly from classical solutions locally while remaining accurate globally. Numerical errors in finite element software, such as those caused by hourglassing and shear locking, can lead to discrepancies between weak and strong solutions. Accurate modeling is crucial to minimize errors stemming from discretization and boundary conditions.

PREREQUISITES
  • Understanding of weak and strong solutions in the context of PDEs
  • Familiarity with finite element methods (FEM) and their applications
  • Knowledge of numerical errors in FEM, including hourglassing and shear locking
  • Experience with software tools for finite element analysis, such as ANSYS or Abaqus
NEXT STEPS
  • Research "Finite Element Procedures" by K.J. Bathe for convergence and error analysis
  • Study "Concepts and Applications of Finite Element Analysis" by R.D. Cook for practical applications
  • Explore techniques to mitigate hourglassing and shear locking in finite element models
  • Learn about the impact of boundary conditions on finite element analysis accuracy
USEFUL FOR

Engineers, researchers, and students involved in finite element analysis, particularly those focusing on numerical methods and error minimization in structural simulations.

feynman1
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Finite elements give weak solutions, that is, the solutions to a PDE are only correct in its integral form. Is it possible that in finite element software, the solution differs a lot from the analytic one locally while it's exact in its integral form (globally)?
 
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By "analytic" do you mean "classical", as in: satisfying the differential form of the PDE?

A classical solution need not exist, and weak solutions may admit discontinuities. In that case, a weak solution would be very far from "differentiable", and the answer to your question would be "yes".

On the other hand, if a classical solution exists, then it is also a weak solution. If, moreover, weak solutions are unique, then the classical solution and the weak solution are the same, and course the answer to your question would be "no".

(This all assumes that the software accurately reproduces the weak solution, including its possible discontinuities.)
 
S.G. Janssens said:
By "analytic" do you mean "classical", as in: satisfying the differential form of the PDE?

A classical solution need not exist, and weak solutions may admit discontinuities. In that case, a weak solution would be very far from "differentiable", and the answer to your question would be "yes".

On the other hand, if a classical solution exists, then it is also a weak solution. If, moreover, weak solutions are unique, then the classical solution and the weak solution are the same, and course the answer to your question would be "no".

(This all assumes that the software accurately reproduces the weak solution, including its possible discontinuities.)
Sorry there was a typo in my original post, should change 'analytic' to 'strong'.
Let's not consider discontinuities.
If a classical/strong solution exists, then why is it also a weak solution?
Yes, this post considers numerical errors by software. So it's asking whether software numerical solutions can give weak solutions differing a lot from the strong one locally while it's accurate in its integral form (globally).
 
There are things like hourglassing and shear locking.
 
caz said:
There are things like hourglassing and shear locking.
I use quadratic elements and U-P nearly impressible algorithm, will these still happen?
 
I’m not so much into FEM theory but you may find the answer in "Finite Element Procedures" by K.J. Bathe or "Concepts and Applications of Finite Element Analysis" by R.D. Cook. These two books cover convergence and error problems in detail.

In practice, I wouldn’t worry about that. There’s always an error (at least a few percent discrepancy from actual behavior of the structure) but it’s mainly caused by discretization, inaccurate material properties, boundary conditions that do not fully represent real-life supports and so on. Basically, largest part of this total error is caused by imprecise modeling.

When it comes to hourglassing and locking, let’s summarize them shortly:
- hourglassing - occurs when bending is analyzed using first order elements with reduced integration
- shear locking - occurs when bending is analyzed using first order elements with full integration
- volumetric locking - occurs when incompressible or nearly incompressible materials are analyzed, especially using elements with full integration
 
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