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Use a displacement-controlled loading scheme, since damage is often associated with a reduction in load carrying capacity, see the Cracking of a Notched Beam example in the Geomechanics Module and Nonlinear Structural Materials Module Application Libraries.
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Better convergence is often obtained when the Nonlinear method is set to Constant (Newton) with a Damping factor equal to 1, and a Jacobian update on every iteration.
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Reduce the smallest and/or largest allowable step size in an Auxiliary sweep, or restrict the time steps in a time-dependent study.
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Introducing a soft spot where a crack is expected can make the localization of strains more stable, see the Brittle Damage in Uniaxial Tension example in the Geomechanics Module Application Library and Nonlinear Structural Materials Module Application Libraries. Alternatively, a random spatial distribution of the material parameters could be employed to obtain a more stable solution.
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The size of the biggest mesh element h should not exceed 2EGf/σts2, where E is the Young’s modulus, Gf is the fracture energy per unit area, and σts is the tensile strength. Larger values of h will cause a snap-back of the stress-strain curve at the material point level.
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1
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Initialization. The crack phase field, displacement field and state variables are known at step n.
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2
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Update state variables. Update internal state variables used by the phase field model with values from step n.
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3
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Solve for the Crack Phase Field. Compute the crack phase field variable in a Newton step, with the displacement field frozen at step n.
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4
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Solve for the Displacement field. Compute displacement field variables in a Newton step with the updated crack phase field.
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The default solver will suggest the above single-pass algorithm for the Phase field damage model when it is feasible to perform the operator split. Cases where this is not possible include when some multiphysics couplings are present in the model and when a segregated contact algorithm has to be used.
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