Hints, tips and FAQ

Note: Some items on this page may be specific to Zencrack v7.7.

This page lists general issues. For issues specific to the finite element interfaces, please refer to the Support / F.E. interfaces option on the left hand menu.

My cracked mesh has inside out elements. Why?

The rep file reports that crack front nodes are "closed" but a displaced mesh plot clearly shows the crack is open. Why?

What steps can I take to ensure that my cracked model is "correct"?

How can I visualise the calculated crack growth profiles?

How can I generate a vs N data from the analysis results?

How can I create crack growth animations?

How can I ensure that the crack region is loaded in the cracked model when I apply centrifugal load?

What units should I use for my crack growth data?

The boundary supports are missing from the crack region in my cracked model. Why?

Energy release rate values have been processed after the f.e. analysis but there are no stress intensity values. Why?

The evaluation version of Zencrack has limits on the job size. Are there any limits in the full version?

If it is the initial cracked mesh:

• the initial crack may be too distorted e.g. large or small edge ratios or one large and one small edge ratio
• the uncracked element(s) may be too distorted
• you may have requested a combination of *MAPPING options that does not work well together.

If it is a cracked mesh part way into a growth analysis:

• the crack may have grown past the acceptable size limit in the crack-block and distortion is too great
• relaxation of the region surrounding the crack-blocks may not be able to remove excessive distortion outside the crack-blocks.

This is probably due to the tolerance value of TOLOPEN. When considering the crack face opening displacements, Zencrack has a tolerance value below which a status of "closed" is defined. By default this is 0.0. Sometimes there can be a small amout of noise in "zero" values and small positive values really should mean "closed" (this is more the case for complex crack geometries than simple one-sided mode I problems).

To trap these small values, the TOLOPEN tolerance can be changed. There are two ways to do this:

• locally for the job - by adding a line to the zcr file as indicated in the above warning:
  *CONTROLS,TOLOPEN=value
• globally for all analyses - by editing the value of TOLOPEN in the file "Zencrack folder"\crack\tol.dat; this file contains defaults for a variety of tolerances.

Then if the value of TOLOPEN is changed, for example, to 1.0e-6, a warning similar to this may appear in the rep file:

 ***WARNING
 There are   86 crack face nodal positions with opening displacement
 between 0 and the tolerance value TOLopen = 1.000000E-06.
 These nodes are treated as being closed. Please check that the value
 of TOLopen is appropriate for this analysis. TOLopen can be reset in the zcr
 file by using:
 *CONTROLS,TOLOPEN=value

The consequence of a "closed" crack front node is that a -ve sign is applied to the G and K values to indicate the closure. In a growth analysis there will be no growth for the node (an exception may be if there is a superposition load system). See page section 5.3 of the v7.7 User Manual for further discussion.

Use the "3dmesh" utility program. This utility program reads the .shp file from a growth analysis and generates a mesh consisting of beam elements to represent the growth profiles. The initial open part of the crack face is also represented. This mesh can be read into a pre-processor to visualise the crack growth.

This utility is executed using "runzcr77 3dmesh". Refer to section 2 of the v7.7 Utility Programs Manual for more information.

Use the "process" utility program. This utility program allows generation of a csv file for results data at:

• one or more crack front nodes
• a parametric distance along each crack front
• at intersection of each crack front with a plane.

The csv file output by this utility can be imported into a spreadsheet and used to generate a variety of plots, including a vs N.

This utility is executed using "runzcr77 process". Refer to section 3 of the Utility Programs Manual for more information.

The "da" data generated by this program is obtained by summing the differences in nodal coordinates of a node from one crack profile to the next. Due to the shifting and re-distribution of nodes along a crack front during crack growth, this may produce misleading results for nodes on the "interior" of a crack front if the "node" method is selected. It is recommended that the node method is only used for surface node positions (and "deepest" node positions in symmetry models).

Animation options in post-processors are geared towards producing animations from results in a single f.e. results file. In this case we need to produce an animation by taking one snapshot from each of several results files.

• Firstly you need to save f.e. results files from each f.e. analysis of a crack growth run. This can be done using keyword SAVE and requesting the appropriate output file e.g. ODB=YES, T16=YES, DB=YES.
• Run the growth analysis.
• Load each of the result files into your post-processor in turn. For each one set the same viewpoint, scale factors etc. and save a screen image e.g. "print" to a .tiff or .png file. The options for this depend upon the post-processor you are using.
• Use an animation package (such as Animation Shop from JASC) to import the sequence of still images and create an animation file from them.

For the Abaqus interface you need to define an element set that contains all the elements in the uncracked mesh and apply the load to that element set. When Zencrack generates the cracked mesh the element set is updated to include the new crack-block elements and so the load gets applied to the entire crack region.

For the Ansys interface there is no special requirement.

Zencrack does not have any knowledge about the consistent units that are used for the finite element analysis. Therefore it is the user's responsibility to ensure that the crack growth data is in units that are consistent with the f.e. data. For example, if the f.e. model is defined with millimetre coordinates and MPa stress units, the crack growth data should be in terms of:

stress intensities : MPa mm^0.5
da/dn : mm per cycle

The greatest scope for error lies in conversion of the Paris and Walker constant, C, if those growth models are being used.

A unit conversion utility is supplied with the program documentation and is also available here.

In Zencrack there are special requirements for the way that boundary conditions are specified on the elements that are replaced by crack-blocks. These requirements are stated for different finite element interfaces in sub-sections of section 2 of the Interface Manuals. Check that your uncracked mesh satisfies these requirements.

The Young's modulus and Poisson ratio were not available to Zencrack. Ensure that the *MATERIAL keyword is used.

For version 7.4 and later:

• Maximum number of elements in the cracked mesh = no limit^*
• Maximum number of nodes in the cracked mesh = no limit^*
• Maximum number of crack-blocks allowed in an analysis = 999
• Maximum number of finite element analyses in a crack growth run = 999

* - Subject to available memory and format restrictions - see section 11.2 of the v7.7 User Manual.

For versions 7.3e and 7.3f:

• Maximum number of elements in the cracked mesh (during generation) = 75000
• Maximum number of nodes in the cracked mesh (during generation) = 375000
• Maximum number of crack-blocks allowed in an analysis = 999
• Maximum number of finite element analyses in a crack growth run = 999

For versions up to and including 7.3d:

• Maximum number of elements in the cracked mesh (during generation) = 50000
• Maximum number of nodes in the cracked mesh (during generation) = 250000
• Maximum number of crack-blocks allowed in an analysis = 999
• Maximum number of finite element analyses in a crack growth run = 999