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I am using my own fortran code to solve a large strain 2D elastic
problem using FEM in Updated Lagrangian framework and I am using incremental
formulation. It works well for 2D pull of a square block with 2 elements and I've
compared the results with a commercial FEA package. But while solving bending
of a cantilever beam the results starts to blow up and the elements distort
abruptly after certain time step. This happens only when the load or the
displacement given at the free end of the beam is large otherwise it works
fine.

Basically, I am supposed to solve a gradient visco-plasticity
problem using the formulation given by Niordson and Fleck (see the paper
attached herewith). I am facing the same problem when using the viscoplastic
formulation. The results start to blow up after few steps only.

Does anyone have any idea what could be the possible
problem?

Thanks in advance.