Use the default settings for contacts, and for the NLADAPTIVE and NLMESH commands, to solve the problem. If you encounter difficulties, consider the following:
For cases with mesh distortion using the mesh-quality-based criterion (NLADAPTIVE,,,MESH), check the mesh quality at every substep of the solution. For cases requiring refinement (position-based, (NLADAPTIVE,,,BOX), energy-based (NLADAPTIVE,,,ENERGY), or contact-based (NLADAPTIVE,,,CONTACT) criterion), checking the mesh quality at every substep of the solution is not recommended; it is better to check mesh quality based on your refinement requirements for the specific time period.
For 2D cases, check mesh quality with a maximum corner angle of approximately 155 to 165 degrees. For 3D tetrahedral cases, check mesh quality with a skewness (NLADAPTIVE,,,,SKEWNESS) between 0.92 to 0.96 for SOLID285, and between 0.94 to 0.98 for SOLID187.
If solution divergence occurs, check the stability of materials and contact pairs to ensure that they have optimal settings and are not causing the divergence. Comparing analysis runs with and without nonlinear mesh adaptivity is not necessarily a like-to-like comparison, as even one remeshing can change the path of nonlinear deformation and make the final results difficult to compare.
Nonlinear mesh adaptivity cannot automatically resolve problems caused by model instabilities or bifurcations. It is also more sensitive to such perturbations after a refinement due to remeshing, resulting in divergence and/or unrepeatable solutions.
If severe deformation occurs at every substep on a global scale for the nonlinear adaptive component (as most elements in the component tend to become distorted), it is better to reduce the minimum time-step so that more remeshing can repair the large distortion.
Remeshing controls (NLMESH) may require adjustment depending on the model and physical deformation pattern:
If deformation occurs on a large scale at every substep for the nonlinear adaptive component (as most elements in the component tend to become distorted), increasing the number of sculpting layers (NLMESH,NLAY) helps to reduce the number of attempted remeshings, as more potential seed elements can be identified for remeshing at the same time.
If a new mesh cannot be generated, increase the number of sculpting layers (NLMESH,NLAY) until the remeshing domain is the whole component. Doing so reduces constraints on the new mesh.
In cases when the initial mesh is graded for the component activated in nonlinear adaptivity, retain the mesh-sizing gradient (that is, do not issue NLMESH,GRAD,0). If the initial mesh is not graded (that is, has a uniform element size throughout the nonlinear adaptive component), it is good practice not to retain the mesh-sizing gradient (NLMESH,GRAD,0) and use a global sizing ratio (NLMESH,SRAT) close to but less than 1; doing so accommodates the change in element shape caused by large deformation.
For all mesh-size gradient options, each remesh may include a small degree of size refining or coarsening. Many remeshings occurring in same location may therefore introduce an accumulating size-refining/-coarsening pattern. In such cases, adjust the global sizing ratio (NLMESH,SRAT) to compensate.
Too great of a global sizing-ratio change (NLMESH,SRAT) in the remeshed region(s) can adversely affect the target mesh quality. The boundary between the remeshing region(s) and the untouched region(s) must remain unchanged, so a global sizing-ratio change adds constraints to the remeshing operation.
Avoid high thresholds for the boundary angle (NLMESH,BDRA) and edge-angle (NLMESH,AEDG). In most cases, the default threshold values are acceptable. If solution divergence occurs due to an inadequate new mesh quality, try increasing the threshold values by a few degrees. Understand that the loss of features due to the larger angles (especially at contact boundaries) may affect mapping, cause rebalancing difficulties, or generate errors immediately after remeshing.
For a nonlinear adaptive component using SOLID187 elements with hyperelastic material and contact, it may be necessary to increase the contact normal penalty-stiffness factor (FKN), or to decrease the penetration-tolerance factor (FTOLN), for the respective contiguous pairs to achieve stable convergence. To further aid convergence in such cases, you can also try increasing the minimum number of substeps to be taken (NSUBST,,,
NSBMN), or decreasing the maximum time-step size (DELTIM,,,DTMAX).SOLID285 is preferred for large deformation with hyperelastic materials. SOLID187 is preferred for large deformation with elastoplastic materials.
Nonlinear mesh adaptivity is an advanced feature based, and dependent upon, several fundamental underlying technologies. Any limitation of those technologies can affect nonlinear mesh adaptivity and could be amplified by meshing changes and meshing sensitivity (such as limitations of the equation solvers, materials, or contacts).
Using the mesh-quality-based criterion (NLADAPTIVE,,,MESH) with small-deflection effects (NLGEOM,OFF) is not recommended, as only the initial mesh shape distortion is handled.
For problems with multiple load steps, operations referencing node numbers on the initial mesh (in remeshing regions) but scoped across the load steps are not allowed. Such operations include applying body-force loads (BF), defining degrees of freedom at nodes (D), or deleting the applied boundary conditions or loads, in a subsequent load step after remeshing. See Initial-Mesh Loading and Constraint for supported scenarios.
Example 2.7: How Remeshing in the First Load Step Changes Node and Element Numbers
Any remeshing in the first load step changes the node and element numbers, and connectivity, so the command operations defined on the initial mesh node and element numbers in the second load step will no longer work. The following input demonstrates this scenario:
Nonlinear adaptivity in a transient analysis is recommended for low- or moderate-speed dynamic applications, or for quasi-static applications. Due to the mass redistribution from remeshing, it is not recommended for impact problems or high-speed dynamic applications.
In case the solution is interrupted or stopped, ensure that the combined results file exists at all times during the remeshing solution (DMPOPTION,RST,YES,ALL).
Alternatively, you can combine the results files after the solution (COMBINE).
For contact-based cohesive zone problems in a nonlinear adaptivity framework:
The standard projection-based method (KEYOPT(4) = 3) is recommended for contact detection.
If solution divergence occurs, try decreasing the number of checks of the CZM criterion (NLADAPTIVE,,,CONTACT,CZM) and increasing the number of sculpting layers (NLMESH,NLAY) for mesh refinement. You can also add stricter mesh-quality criterion to improve distorted elements.
To further aid convergence, try increasing the minimum number of substeps to be taken (NSUBST,,,
NSBMN), or decreasing the maximum time-step size (DELTIM,,,DTMAX).If the standard settings lead to over-refinement, try increasing the value in the CZM criterion for more localized refinement, decreasing the number of checks of the CONTACT,CZM criteria, and increasing the element-refinement ratio.
The debonding behavior of CZM models is generally mesh-dependent. The criterion based on relative change in energy released (NLADAPTIVE,,,CONTACT,CZM,
VAL1) can change the onset of debonding through mesh refinement.For models with more than one nonlinear adaptivity component defined, and/or large models, add a nonlinear-adaptivity solid-element component (such as a mesh-quality criterion) in the region of the CZM contact surface. Doing so helps to maintain good element quality throughout the lifecycle of the analysis.
CZM-based coarsening remeshing is activated via NLADAPTIVE,,,CONTACT,CZM only. When only coarsening remeshing (without refinement) is required, specify a very high
VAL1refinement value (such as 10000) and do not specifyVAL2.
Increasing the number of layers (NLMESH,NLAY with
VAL2) for coarsening remeshing with energy-based, location-based or contact-based criteria does not increase the number of coarsened elements in the same way as it does for refinement. To avoid losing solution quality in unselected elements, coarsening is restricted to the selected seed elements and a small transition zone .A high size ratio (
VAL3on NLMESH,SRAT) may lead to worse mesh quality in the transition zones. A combination with mesh-quality-based criteria may improve the mesh quality but may result in a smaller element size in previously coarsened regions.For solutions involving element-removal-based-criterion:
The normal to target nodal detection method (KEYOPT(4) = 2) is recommended for most contact pairs associated with the bodies involved in element removal.
Orphaned elements occurring during solution due to the adaptive element removal procedure are not automatically removed and may lead to solver pivot warnings during solution.
Depending on the material behavior (such as hyperelasticity) and model physics, sudden instability and contact status changes may happen at the element removal substep. You should ensure that there is no element removal for a few subsequent substeps if convergence difficulties are encountered or if the FEA topology appears unphysical.
If “CMDB” errors are seen at an element removal substep, it is likely due to topological issues in the FEA geometry at that substep. This is common when too many elements are suddenly removed from the model.
Since elements can be removed anywhere in the model at any time during solution, always check the solution history carefully at each substep to ensure the simulation makes sense.