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ANSYS Workbench Fatigue Module

Ansys Fatigue Module 1

 

This tips & tricks article illustrates how to take into account a non-zero mean stress in an ANSYS Workbench fatigue analysis.


Compiling and Loading User Defined Functions using FLUENT 14.5 or 15.0

The Fluent solver is a general-purpose code. In order to customize the Fluent solver, users can use their own C-codes called user-defined functions (UDFs) to accomplish:

  • Special boundary conditions
  • Customized or solution dependent material properties
  • New physical models
  • Reaction rates
  • Source terms
  • Customized post-processing
  • Solving user-supplied partial differential equations
  • More …

UDFs can either be compiled or interpreted. Fluent is bundled with a C++ interpreter that can be used to interpret UDFs. To compile UDFs, you can use Microsoft Visual Studio (Express or Professional Editions) under Windows. Most Linux systems provide a C compiler as a standard feature.



Customize Logo in ANSYS Graphics

ANSYS Custom Logo
Show the World Who You Are! Proudly Display Your Company Image in the ANSYS® Interface!

Cyclic Symmetry Considerations in ANSYS® Workbench Mechanical

ANSYS Cyclic Symmetry 1

Figure 1: One Sector in a Cyclic Symmetry Analysis, Showing Low and High Boundary Selections

ANSYS Workbench Mechanical supports cyclic symmetry analysis, and makes it easy to simulate with a Symmetry branch in the Outline at the Model level. A user indicates the Low Boundary and the High Boundary for the Cyclic Region of interest. Analyses such as static structural and modal analysis can follow, with both of them performing the analysis as if an entire circular structure was present.

In the simplest cases, the Low Boundary and High Boundary for a Cyclic Region are selected on opposite side faces on one body. They can also be applied to opposite side faces on a multibody part as in the half-cylinders in Figure 1 above. In DesignModeler, parts that do not touch can be declared to form a multibody part, with the result that they will aid in creating Symmetry branches for cyclic symmetry. This article explores some of the details. 


Element Quality Plots in ANSYS® Mechanical (Workbench) 16.0

ANSYS 16 Element Plot 1

Figure 1: Colored Mesh Quality Plot in ANSYS Mechanical (Workbench) 16.0

ANSYS Mechanical (Workbench) 16.0 makes it easy to review element quality during the development of meshing controls, as well as in postprocessing results. Well-shaped elements yield superior results, and help reduce element shape errors during large displacement analysis, such as when using hyperelastic materials with substantial strain.

This article briefly reviews features in the Workbench Mechanical Outline for displaying element quality (1) in color in the Mesh branch, (2) using bar charts of element quality, and (3) plotting element quality in postprocessing after element shapes have been distorted by strain in a model.
. 

Three Element Quality Techniques
Workbench Mechanical 16.0 has a new feature for reviewing element quality at the Mesh branch, in order to review the consequences of meshing controls prior to solving a model. First, though, here is a view of mesh metrics as they have been measured in Workbench Mechanical in the last few versions of the software:
 

ANSYS 16 Element Plot 2

Figure 2: Mesh Metrics in a Coarse Tet Mesh

To get the above view of mesh Metrics, click the Mesh branch in the Outline, go to Details of “Mesh”, open the Statistics section of the Details, and use the drop-down list for Mesh Metric to select the metric of interest—in the above Figure 2, “Element Quality” has been chosen. Many choices are available:

 

   ANSYS 16 Element Plot 3a       ANSYS 16 Element Plot 3b

Figure 3: Mesh Metric Selection

 

As seen above in Figure 3, the full listing of Mesh Metrics in the Mesh branch in 16.0 is:

  • None
  • Element Quality
  • Aspect Ratio
  • Jacobian Ratio
  • Warping Factor
  • Parallel Deviation
  • Maximum Corner Angle
  • Skewness
  • Orthogonal Quality

After choosing a quality metric, a bar chart is generated showing how many elements fall into ten Element Metrics zones. If one of the ten bars in the chart is clicked by the user with the cursor, then the elements that fall into the zone of that Element Metric are displayed in the Graphics Window. In Figure 4 below, the bar with Element Quality around 0.5 has been clicked, and elements with that quality are shown in the Graphics Window, with the geometry shown translucent in order to give context to the element locations:
 

ANSYS 16 Element Plot 4

Figure 4: Elements Falling into a Mesh Matrics Zone


The second means of viewing element quality is to create color plots. What is new in 16.0 is the ability to produce color charts of a variety of element quality metrics. As seen in Figure 5 below, a mesh quality assessment can be shown in a color plot at the Mesh branch.
 

ANSYS 16 Element Plot 5

Figure 5: Colored Element Quality Plot at the Mesh Branch in v16.0

 

Note first that in order to get the color plots of element quality, in the Statistics section of Details of “Mesh”, the Mesh Metric must be set to “None”, as seen in Figure 5. In the Display Style section of Details of “Mesh” a number of choices are available:
   

ANSYS 16 Element Plot 6a     ANSYS 16 Element Plot 6b

Figure 6: Display Style Selection for Element Quality Type

 

Element coloring is controlled by the “Display Style” drop-down list, as seen in Figure 6 above. Body Color does not show an element quality assessment. The quality plot choices are:

  • Element Quality
  • Aspect Ratio
  • Jacobian Ratio
  • Warping Factor
  • Parallel Deviation
  • Maximum Corner Angle
  • Skewness
  • Orthogonal Quality

Note that some choices for color plots of element quality are not applicable to tetrahedral elements, such as “Parallel Deviation”, which produces a blank or null result.

The third type of element quality assessment that is available is in postprocessing. The elements in their deformed shape are assessed, at a solution time that can be set by the user. The plots are from the User Defined Results list.
 

ANSYS 16 Element Plot 7

Figure 7: Postprocessing Element Shape Quality with User Defined Results

 

In Figure 7 above, note that several element shape quality assessments can be chosen from the listing of User Defined Results. They include:

  • Element Quality
  • Aspect Ratio
  • Jacobian Ratio
  • Warping Factor
  • Parallel Deviation
  • Maximum Corner Angle
  • Skewness

Note that some choices, such as Parallel Deviation, are not applicable to tetrahedral elements, and will produce a null result. As postprocessing results, the element shapes are assessed in the element deformed condition, with element deformation a consequence of strain in the model. The time of the plot produced will default to “Last”, but can be set by a user to any preferred time value. Figure 8 below illustrates.

 

ANSYS 16 Element Plot 8

Figure 8: Mesh Element Quality for Deformed Elements at Time=2.0

 

In Figure 8, the Element Quality shape assessment has been performed on element shapes as deformed by the solution. In the Tabular Data for this solution result, note the difference in the Minimum and Maximum values at Time values of 1.0 and 2.0. This difference is a result of changing loads on the model between times of 1.0 and 2.0.

Conclusions
Workbench Mechanical 16.0 offers three ways to view element shape quality assessments.

While preprocessing, color plots of element quality can be performed by choosing Mesh branch details Display Style drop-down list values, while keeping the Mesh Metric choice set to “None”.

At the Mesh branch, a user can go to Details and use the Statistics >Mesh Metric choice to produce bar charts of the range of a number of element shape assessments.

When evaluating Solution results, User Defined Results can be created for several element shape assessments, which are applied to the deformed elements, yielding colored plots.

In the colored plots, values in the Legend can be user-adjusted, and other review tools can be employed, such as Probe, Min and Max, and the usual contour controls.


Export the Deformed Geometry Shape from an ANSYS® Model

ANSYS APDL command language provides the ability to export the deformed geometry from a completed analysis.  This can be accomplished in Mechanical APDL with command language or in Workbench Mechanical via inserted Command Snippets.  Often times, it is useful to export the deformed geometry from an analysis for further post-processing with perhaps a drafter, another department, or a vendor that will need to design other components to fit in, on, or around the deformed shape (e.g., compressed rubber bushing in a steel housing).  This can be quite valuable in the sense that it reduces the design cycle, time-to-market, or even costs affiliated with prototyping in a trial-and-error type process.  This article discusses and presents the command language that is available for an analyst to perform the exportation to an Initial Graphics Exchange Specification (IGES) file format, whether on a 2D or 3D model.  Each set of commands is presented below (4 total); instructions then follow for using any of them in Mechanical APDL or Workbench Mechanical.


Harmonic Analysis Face Rotation Measurement in ANSYS® Workbench Mechanical

ANSYS Harmonic Face Rotation 1 

Figure 1: Harmonic Load on the End of a Cantilevered Plate

Introduction

ANSYS Workbench Mechanical can perform harmonic response analysis, by either modal superposition, or by full harmonic analysis. Measurement of face rotation is sometimes of interest to customers. If a remote point is associated with a face of interest, rotation of that remote point can be measured in an APDL Commands Object, and reported to the user, with the option of creating an output parameter from the rotation value.

In Figure 1 above, the far end of a plate has been fixed. The nearer end has harmonic loads applied at two edges, one pointing up, and one pointing down. The objective is to excite the plate at frequencies around a mode of vibration that has the twisting motion implied by these forces.

End face rotation will be measured, using a Remote Point that is associated with the face, and set to “Deformable” so that the behavior of the model is not influenced. Associating a remote point with a face meshed with a large number of nodes can increase solution time and memory requirement—setting a pinball radius can reduce this.

Recent features in ANSYS that make it easier to find amplitude results in harmonic analysis are illustrated here. Settings that measure face rotation in harmonic analysis will be reviewed.




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Performing EKILL Element Death in an ANSYS Workbench Mechanical Model

ANSYS Rotation When running structural models in ANSYS Workbench Mechanical, users sometimes want to perform element birth and death between load steps. Although the Mechanical APDL interface permits birth and death, it is not directly supported in the Workbench interface.


PIPE16 Circumferential Stress in ANSYS® Mechanical APDL

 ANSYS Pipe16

Figure 1: Principal Stress 1 in a PIPE16 Element, when the Only Load is Internal Pressure

 

Introduction
ANSYS includes a pipe element PIPE16 in its list of Archived Elements. This element is still of interest to a variety of users. When internal pressure is applied to a PIPE16 element, and no other load, the circumferential (hoop) stresses of a cylinder are expected. Experimentation shows that the stress in the results file is based on the circumferential stress of a thick-wall cylinder, and is presented for the outside diameter, not the inside diameter.

This article presents a simple example that illustrates the result.


Quick Tip on Insufficient Storage in ANSYS® Workbench

ANSYS Storage

Figure 1: Temporary Storage during Solve in Workbench Mechanical

 

Cleaning Up Hard Drive Storage in Workbench Mechanical

Here is a simple tip concerning loss of hard drive storage space. As seen in Figure 1 above, a temporary _ProjectScratch folder is created when Workbench Mechanical solves. With large models and certain solvers, very large temporary files can be created, such as file.LN09 and file.osav in the directory listing above. If a user stops solution prematurely or the solver is otherwise killed, these files may not be cleaned up, not even when data is cleared, the model is saved and Workbench is shut down. This can result in fatal errors in subsequent solves when the free hard drive space is consumed, usually at the end of a substep, with users left baffled as to what is wrong. This difficulty will be seen more often on solid state hard drives used for speed but lacking terabytes of storage.

Once a model is saved on the hard drive and Workbench is shut down, users can employ Windows Explorer to remove an unwanted _ProjectScratch folder on the hard drive, eliminating the lost hard drive space problem.


Retrieve Beam Reaction Forces In ANSYS® Random Vibration Analysis

 ANSYS Beam Reaction Forces 1

ANSYS Workbench (WB) Mechanical provides the ability to perform random vibration analyses, which are sometimes referred to as power spectral density (PSD) analyses, or more simply, spectrum analyses.  PSD is statistical in nature and as a result of the solution technique, many results that would typically be available in WB Mechanical for a classic static structural analysis, are not natively available in WB for a PSD analysis.  However, some can still be retrieved from the WB Mechanical solver via inserted Command Snippets.  This document discusses and presents the command language that is available for an analyst to extract force reaction results for beam connections used in WB Mechanical in a PSD analysis.  Two primary approaches are given:  one to obtain beam force reaction results in the global Cartesian coordinate system and another to obtain the results in each beam’s local element coordinate system.  There is also discussion given to obtain results at various confidence intervals (1-sigma, 2-sigma, 3-sigma).  

Upfront Modal Analysis
Prior to any PSD analysis, there must first be a modal analysis, of which the solution feeds into the PSD analysis.  This connectivity between the systems is first set up on the WB project page, which is illustrated in Figure 1.

 

ANSYS Beam Reaction Forces 1

Figure 1:  Modal Analysis Feeding PSD Analysis

The modal analysis can be performed as usual.  However, in the Output Controls of the Modal analysis, the option to output nodal forces must be set to YES (Figure 2), thereby populating the results files with the necessary data to later extract beam forces from the PSD analysis.  The other output options are at the discretion of the analyst, depending on what other results are required in post-processing.  

 

 ANSYS Beam Reaction Forces 2

Figure 2:  Turning On Nodal Force Output

 

Identifying Beam Connections for Post-Processing
Beam connections are created in WB Mechanical under the Connections branch.  However, the beam elements are not actually created and assigned element numbers until the solve process.  Therefore, it is not possible to know upfront what number is assigned to each beam element and, as shown later, the beam element numbering scheme is pertinent to post-processing the results.  To address this, it is necessary to insert a Command Snippet under each beam connection. 

Right Mouse Button on Each Beam → Insert → Commands

For each Command Snippet, the following command should be inserted:

BEAM1=_bid

In the above command line, the numeral 1 increments for each beam.  As the solver creates a beam, a parameter _bid exists with a unique numerical value that represents that beam.  _bid is overwritten once the solver begins creating the next beam.  Therefore, the above command is intended to define a numerically unique parameter (e.g., BEAM1) for each given beam that will not be overwritten and can be used for traceability later in post-processing.  See Figure 3 below to illustrate the unique sequential numbering of each beam.

 

ANSYS Beam Reaction Forces 3

Figure 3:  Numbering Beam Connections

 

Extracting Beam Forces From PSD Analysis (Global Cartesian Coordinates)
After a modal analysis has been setup and connected to a PSD analysis, the procedure to extract force reaction results from beam connections is fairly straightforward.  The results obtained using this approach will give results in the global Cartesian coordinate system for the 1-sigma confidence interval.  Discussion is given later as to how to obtain results in each beam’s local coordinate system.  To proceed in WB Mechanical, insert a Command Snippet in the Solution branch of the PSD model. 

Right Mouse Button on Solution →Insert →Commands

The analyst can then copy/paste the following commands into the Command Snippet, which will execute immediately after the solution is done.  An example is illustrated in Figure 4.  Notice the second command line (NUMBEAMS) where the analyst must specify the total number of beams being dithered (e.g., 3 in this example).


SET,3,1         ! Specify the third load step, which is the 1-sigma confidence interval
NUMBEAMS=3      ! Analyst should specify total number of beams
*DO,i,1,NUMBEAMS      ! Begin DO loop to process through each beam connection individually
  ! Insert comment lines, which will echo in the solver output for viewing ease
  /COM,*********************************************
  /COM,The following results are for Beam %i%
  /COM,*********************************************
  ESEL,,TYPE,,BEAM%i% !Select beam by type, where TYPE is parameter value assigned to BEAM%i%
  PRESOL,F            !Print the element force results for the selected beam
*ENDDO          ! Conclude the DO loop


 

ANSYS Beam Reaction Forces 4

Figure 4:  Inserting Command Snippet in WB Mechanical

Once solved, the results will then be printed node-by-node, in the global Cartesian coordinate system, for each beam element in the Solver Output, which is available from the Solution Information branch.  An example is illustrated below in Figure 5.  Because the leading command was SET,3,1, the results are for the 1-sigma confidence interval.  There is discussion later for obtaining results at other confidence intervals.

 

 ANSYS Beam Reaction Forces 5

 

Figure 5:  Beam Force Reaction Results in Solver Output (global Cartesian coordinates)


Extracting Beam Forces From PSD Analysis (Local Beam Coordinates)
Similar to the previous option, the procedure again is fairly straightforward.  The results obtained using this approach will give results in each beam’s local coordinate system for the 1-sigma confidence interval.  Specifically, the results will contain only the force along each beam’s axis (i.e., no side/shear forces will be provided); by WB Mechanical convention, X is the local coordinate axis defined for a beam’s axis.  To proceed in WB Mechanical, insert a Command Snippet in the Solution branch of the PSD model. 

Right Mouse Button on Solution → Insert → Commands

The analyst can then copy/paste the following commands into the Command Snippet, which will execute immediately after the solution is done.  An example is illustrated in Figure 6.  Notice the second command line (NUMBEAMS) where the analyst must specify the total number of beams being dithered (e.g., 3 in this example).


SET,3,1         ! Specify the third load step, which is the 1-sigma confidence interval
NUMBEAMS=3      ! Analyst should specify total number of beams
*DO,i,1,NUMBEAMS      ! Begin DO loop to process through each beam connection individually
  ! Insert comment lines, which will echo in the solver output for viewing ease
  /COM,*********************************************
  /COM,The following results are for Beam %i%
  /COM,*********************************************
  ESEL,,TYPE,,BEAM%i% !Select beam by type, where TYPE is parameter value assigned to BEAM%i%
  PRESOL,SMISC,1      !Print the element force results in local X for node I
  PRESOL,SMISC,14     !Print the element force results in local X for node J
*ENDDO          ! Conclude the DO loop


 

ANSYS Beam Reaction Forces 6 

 

Figure 6:  Inserting Command Snippet in WB Mechanical

 

Once solved, the results will then be printed element-by-element, in the Solver Output, which is available from the Solution Information branch.  The results will be broken down as SMIS1, which are the results at node I, and also SMIS14, which are the results at node J of each beam.  An example is illustrated below in Figure 7.  Because the leading command was SET,3,1, the results are for the 1-sigma confidence interval.  There is discussion later for obtaining results at other confidence intervals.

 

ANSYS Beam Reaction Forces 7

 

Figure 7:  Beam Force Reaction Results in Solver Output (local beam coordinates)

 

Obtaining Results at Other Confidence Intervals
A PSD analysis is statistical in nature.  Therefore, the results are also statistical, to include the beam force reaction results.  The above methods use the following command, which tells the post-processor that results are desired for the 1-sigma confidence interval. 

SET,3,1

Results can also be obtained at the 2-sigma confidence interval:

SET,3,1,2

Or at the 3-sigma confidence interval:

SET,3,1,3

Troubleshooting
The only likely issue that may arise from using these techniques is a situation where the analyst has solved the model already and then later inserted the Command Snippet to the Solution branch of the PSD analysis.  At this point, selecting Solve or Evaluate All Results will not update the Solver Output file since a full re-solve is not taking place.  Therefore, the analyst will not be able to see the results in the Solver Output file.  To address this, the PSD model needs a full re-solve.  Do so by:

Right Mouse Button on PSD Solution → Clear Generated Data

The analyst can then re-solve the PSD model to update the Solver Output file with the beam force reaction results.

Conclusions
A PSD analysis is a statistical type of analysis.  The initial discussion provided herein gave results in the 1-sigma confidence interval.  This means that the reported results are not expected to be exceeded 68.3% of the time for the given input acceleration spectrum.  Additional discussion was then given to illustrate how to obtain results at other confidence intervals; 2-sigma (95.4% confidence) and 3-sigma (99.7% confidence). 

It was also illustrated how to obtain reaction forces from beam connections either in the global Cartesian coordinate system or each beam’s local coordinate system (but restricted to axial results).  If force reactions are desired in all three coordinate directions, but aligned differently than the global Cartesian coordinate system, the RSYS command allows the analyst to rotate results into some other defined coordinate system before printing to the solver output file.  The analyst may reference the ANSYS Help Menu for further discussion related to the use of this command.


Rotation Convention in ANSYS®

ANSYS Rotation

In a variety of situations when using ANSYS software, whether through the Mechanical APDL interface, or the Workbench…


Separating ANSYS DB Database File Contents from RST Results File Contents

ANSYS DB RST Files

When users deal with large Mechanical APDL models, the DB database files and the RST results files can become very large. Even if the RST results file contains only one solution, a very large database size can result when node-and-element data is included by default.


Single-Frame Restarts inside ANSYS® Workbench Mechanical

ANSYS Restarts 1 

Figure 1: Single-Frame Restart Example in Workbench Mechanical

 

Introduction
When running Workbench Mechanical, a user might like to have APDL commands execute between load steps, going to the /POST1 processor to examine a model, make the kinds of changes permitted between load steps according to results examined, then go back to the /SOLU solution processor and continue with SOLVE for a next load step. A single-frame restart can make this possible.

Although no longer documented, it is still possible to perform single-frame restarts inside ANSYS. Current documentation covers the multiframe restart, which lets a user restart an analysis from any saved load step or substep. Modifications between load steps might perform EKILL or EALIVE commands that are based on element results after a SOLVE. Other creative modifications could include material attribute changes (MPCHG) or Real constant adjustments (RMODIF).

Setup for a single-frame restart inside an APDL commands object in a Workbench Mechanical environment will be explored here. Actual model changes are up to the user.



Thermal Contact Settings in ANSYS Mechanical (Workbench) 15.0

ANSYS Thermal Contact Settings 1

Figure 1: Thermal Contact Model in ANSYS Mechanical (Workbench) 15.0

Introduction

ANSYS Mechanical (Workbench) makes it relatively easy to set up contact pairs. Manual tweaking of contact pair details can improve contact element performance, and the accuracy of results. This document considers some effects of contact pair settings on thermal results. Imperfection in thermal contact behavior can be inspected in plots of thermal flux in directions perpendicular to the contact elements.

A series of tests were done on contact between two 2D blocks in contact. Low-order quad-shaped 2D thermal elements were employed. One block is a thermal insulator, and the other is a thermal conductor. In Figure 1 above, a temperature boundary condition of 200C is applied to the left end of the model at a thermal insulator in a two-block model. A convective load is placed on the right end at a thermal conductor, with convection coefficient h=50W/m2 and an ambient temperature of 22C. Thermal results are affected by settings for the contact pair between the two blocks.

In the present set of tests, apparently best results can be seen if the Details for the contact pair have “Detection Method” set to the non-default value of “Nodal–Projected Normal from Contact”. Results can be better if the Contact side in a contact pair is on the body with the lower thermal conductivity, and with a finer mesh.


Using *TREAD in ANSYS® Mechanical APDL to Read External Data

ANSYS External Data 1a  ANSYS External Data 1b   ANSYS External Data 1c

Figure 1: 2-Column Data in a Spreadsheet, in CSV File Format, and in a Table Array in ANSYS

Introduction
Users sometimes want to use either the *VREAD command or the *TREAD command to get data into ANSYS Mechanical APDL, for a variety of potential uses. The most common data reading needs include numbers from a two-column 2D table, or from a multiple-column 2D table. More complex data structures are possible, but not discussed in this document.

The *VREAD and *TREAD commands read numerical data from an ASCII text file. *VREAD, like *VWRITE, requires a format statement to follow immediately. *VREAD loads data into a numerical Array and requires a file with data in a fixed format specified by the format statement. This offers little flexibility. The *TREAD command can read data from a Comma Separated Variable (CSV) file, without a format statement. *TREAD loads data into a Table Array, and is flexible in whether the data is in floating point or exponential format (e.g. 0.123e-2 versus 0.00123), plus, a variety of numbers of digits of accuracy is supported. A CSV file is easily written from an EXCEL spreadsheet, or by other software. Users might use *TREAD to enter CSV file data, even if the ultimate use for the data was not in a Table Array.

This article illustrates use of the *TREAD command, and differences in where data is placed in a Table Array when a 2-column CSV file is read, versus a multi-column CSV file. Movement of data from a Table Array into a numerical ARRAY is also illustrated.