Cutting the crack plane into the mesh

Improvements have been made in the calculations related to cutting the crack plane into the mesh as the crack grows. Specifically, there was previously a chance that as a crack grows past a corner the internal representation of the crack plane may not have extended fully through the mesh. This resulted in an inability to cut the crack plane fully into the model and the failure of the growth run. This process is now improved and there is significantly less chance of the crack plane failing to cut fully through the model as a crack reaches a corner and grows past it.

Improvements in creation of ring elements

The generation of ring elements is improved to reduce distortion where:

• the free surface is curved,
• the crack front meets the surface at a small angle,
• there is a local change in curvature along the crack front close to the free surface.

In some cases this means that an analysis is able to continue where previously it may have failed.

Extra checks for inside-out crack front elements (#1)

Parabolic elements with quarter-point nodes are numerically more sensitive to becoming inside-out due to distortion in their overall shape than elements with midside nodes. Such distortions may arise as a crack passes a chamfer or corner in a model. In order to reduce the possibility of the analysis stopping, the remeshing process detects inside-out parabolic crack front elements and, if there are any, converts all collapsed elements for that crack front to have radial nodes at the midside position for that analysis step. The elements are then more robust though it is possible that there may still be inside-out issues. The possibility to change crack front element topology is the default behaviour and can be turned off via an option in the GUI ini file.

Extra checks for inside-out crack front elements (#2)

In the final stage of ring creation for a surface breaking crack front some nodes on the end planes of the rings are adjusted to also be on the exterior surface of the model. As a crack passes a corner the positioning of such nodes may introduce inside-out elements in the rings. If there are no inside-out elements in the rings before this final adjustment, the program reinstates those valid ring elements. The possibility to reinstate the valid elements is the default behaviour and can be turned off via an option in the GUI ini file.

Trimming the ends of the crack front

In crack growth analysis there may sometimes be undesirable effects introduced in the crack profile shape from growth calculations related to the nodes on the ends of the crack (i.e. crack front nodes on the surface of the model). In particular this can occur when there has been a sudden change in geometry at the free surface e.g. at a sharp corner. A new option for remeshing, RemeshTrimEnds, is added to the [REMESHING] section of the zencrack_gui.ini file. This option allows the crack growth increment calculated for the end nodes from the integration scheme to be rejected. A new representation of the profile is generated by linear or parabolic extension of the next two or three corner nodes on the crack front. This can have beneficial effects in terms of retaining a meaningful crack profile. The default behaviour is that this option is off - it must be activated by changing the setting in the GUI .ini file.