Impact Users Manual

This chapter is intended for the user more than the programmer. Reference for the Fembic indata program is also included

Installation

Impact is a Java program which means that there is no compilation of sourcecode or similar to be done. However, there are some programs you need to install to be able to run Impact and to see the results.

Start by downloading the program files of impact from http://sourceforge.net/projects/impact

The file is a .zip file and must be untarred using the command tar -xvf filename.zip if you are running Linux. For Windows users the Winzip program will handle the expansion.

You will now have a directory for impact (highly originally called impact) where some files and directories will reside. They should be:

• README
• jama (dir) - The directory containing classes for matrix algebra
• run (dir) - All the source code for Impact
• Impact.gid (dir) - Configuration files for the GID pre/post processor
• manual_Users (dir) - Users manual
• manual_Programmers (dir) - Programmers manual and program installation
• examples (dir) - All the example problems for Impact (.in)

A good Java engine is the Sun version which can be found at http://java.sun.com/j2se/1.4/download.html. You can take either the Runtime environment or the Software Development Kit.

There are several alternative Java engines. IBM has one which is very fast and is also recommended.

After installation, you can run the solver by going to the Impact directory (cd Impact) and then writing java run.Impact examples/xxxxx.in where xxxxx is the name of your indatafile that you want to run with the solver.

Impact will create two outdatafiles:

• xxxxx.in.flavia.res
• xxxxx.in.flavia.msh

These files can be read by the GID pre-and postprocessor.

Processor

The recommended pre and postprocessor is GID. It is freely available as a limited edition. You will need to download a version later than 6.2.0d since Impact uses features that are currently being implemented. You can download GID from http://gid.cimne.upc.es

This is how you should set up and use GID for Preprocessing:

1. Run the installation file for GID and install the program.
2. If you haven't installed Impact, proceed to do this.
3. Look in the GID directory for a subdirectory called problemtypes and go there
4. Make a new subdirectory called Impact
5. Now copy the directory Impact.gid from where you installed Impact, making sure all files come with it
6. The directory structure should now be GiD/problemtypes/Impact/Impact.gid/some files
7. If you now start GiD, you should find Impact as an option under the DATA menu.
8. GiD can now export indata files to Impact via the File->Export->CalculationFile menu

Impact also has a built in pre- and postprocessor which is under development. They are accessable from the ImpactGUI.

Solving problems with impact

The solution process is made in three stages:

1. Creation of a model using a pre-processor or direct writing of the Fembic indata file
2. Solution using the Impact program
3. Presentation of the results using a post-processor and the result files from the solution

It is simplest to run Impact and the built in pre- and postprocessors from the GUI. To do that, just run the ImpactGUI.bat file in this directory if you are a Windows user or make the ImpactGUI.sh runnable (chmod 777 ImpactGUI.sh) and run that with ./ImpactGUI if you are a Linux/Unix/Mac user. Alternatively, just write bash ImpactGUI.sh to start.

Creating the indata file

The model can be created by the help of a pre-processor. A good one is called GID and is freely available for small models. it can be downloaded from here in versions for both unix and windows. Be sure to get a version higher than 6.2.0 due to advanced features used by Impact.

After installation of GID, you should copy the directory Impact/Impact and all its contents into the GID subdirectory called GID/problemfiles. This will install an extra option on the GID preprocessor menu and configure GID for Impact file format.

Before starting to create models with GID, you should set GID in the Impact mode so that your model will be tailored for Impact. This is done by selecting Data -> Problem type -> Impact -> Impact. This will give you a range of options under the Data menu which you can use to set material, boundary conditions etc.

After the model is created, it should be exported to a file. This is preferably done using the File -> Export -> Calculation file feature. It is of course also possible to write the complete file by hand using a favourite ASCII editor. The syntax of a Fembic file is explained in a later chapter.

Summary of how you should use GID for Pre-processing (creating models for Impact)

1. Start by selecting Impact as your solver by Data->Problemtype->Impact->Impact
2. Fill in the problem datas under Data->Problem Data->...
3. Create a model and mesh it (read the GiD manual for how to do this)
4. Set materials on all elements using Data->Materials
5. Set boundary conditions on the nodes using Data->Conditions
6. You can now export the indata file via Files->Export->Calculation file

Solving

The solution of the problem is initiated from the GUI or by writing java run.Impact file at the command prompt, where file is the name of the indata file. In the case of a Fembic file, make sure it ends with .in because otherwise Impact will not recognise the format. It is also important that you are placed in the impact directory at the time of execution. A java engine must also be installed on your system before execution.

If you are running some of the example problems supplied, you need to add the path to the examples directory. The syntax then becomes: java run.Impact examples/file where file applies as above.

If all goes well, you should now see the indata file being parsed by impact and the solution process initiated. Each time results are written, a notice will be written to the screen and you will see that execution is in progress. A solution can take considerable time, so be patient.

Reading the results

The results are printed to the flavia.res and flavia.msh files. They will end up in the same directory as your sourcefile. These are tailor made for the GID and the built in post processor.

Start by firing up GID and switch to post processing mode. Next read in the result file flavia.res. The mesh (flavia.msh) file will be read automatically. You should now see the model on the screen.

Press ctrl-d to set the timestep for deformation. Go from the top of the menu, starting by selecting deformation and then time analysis. Select timestep 0, magnification factor 1.0 and then press apply.

Next press ctrl-v and select the results , time analysis and contour fill. Finally, select gausspointstress and apply.

Finally, press ctrl-m. You should now see the results as an animation. There are plenty of ways to view your results, but I refer to the GID users manual for that.

Summary of how you should use GiD for Post-processing (looking at the results)

1. Fire up GID and switch to post-processing mode.
2. Open the xxxxx.in.flavia.res file. If all goes well, you should be able to see your model.
3. Press ctrl-d to set the timestep for deformation.
4. Go from the top of the menu, starting by selecting deformation and then time analysis.
5. Select timestep 0, magnification factor 1.0 and then press apply.
6. Next press ctrl-v and select the results , time analysis and contour fill.
7. Finally, select gausspointstress and apply.
8. Next press ctrl-m to get a nice animation!

Alternatively, the built in postprocessor is directly accessable from the GUI. Just open the resultfile and select the time you want from the left column and you should see the model. Rotation, moving and zooming is done by holding down any of the mouse buttons while moving the mouse.

Contact handling in Impact

• Contact_Triangle (CT)
• Contact_Line (CL)

The CT is used to sense contact between nodes and surfaces and the CL senses contact against other CL elements. Together, these two elements can be used to enable contact detection for most cases and models. Both of them are classified as elements which means that they can directly be part of a model mesh as all elements. The user can for example model a wall or a complex rigid contact surface with them.

Since they only have the sole purpose of sensing contact, the have no stiffness at all. This means that if they are used on their own in the model, the nodes connecting them should be fixed by constraints to prevent them from drifting when in contact. It also means that the user can use them in combination with ordinary elements to provide contact sensing where this is not default.

One example where this is useful is when a body has been meshed using solid elements, for example an engine block in a car. This body can then be "dressed" on the outside with a second mesh of contact elements to provide the contact sensitivity against other elements in the car. Any contact sensing inside the engine block is not needed and valuable calculation time can then be saved with this approach.

Some elements have contact sensing as default. Examples of these are:

• Shell_C0_3
• Shell_BT_4
• Rod_2
• Beam_2

When any of these elements is created, one or several contact elements are created by default. These are embedded inside the element and share the element nodes. The rod and beam elements use the Contact_Line element to sense contact. The Shell elements use the Contact_Triangle element to sense contact against the surface and optionally Contact_Line elements at the edges to sense contact against other edges.

The contact elements drain quite a bit of computing resources and as the number of elements increase, so does the amount of computing power since the increase is more than linear. Therefore, some of the elements have options to reduce the contact resolution. This means that the contact sensing will be less accurate during large deformation of the elements, but the solution will run faster. For this reason, contact sensing has also not been implemented in the solid elements since the user can best minimise the amount of calculations needed, by distributing the contact element where they are needed.

The details of how contact sensing is implemented is explained in the programming manual.

Fembic Indata Format

The fembic indata format is the default indata format for Impact. It is designed to be simple to read and understand, and is written in free format which means that you can type as you wish and do not have to put data at specific locations in the file. A file which is written in Fembic should have a name which ends with .in and be in ASCII format.

Comments may be included on any line but must be preceded by a # character. Every text written behind this sign will be ignored and the parser will continue on the next line instead.

The General syntax for this manual is that letters in bold are required input for the command. Input in normal writing are optional.

Block Structure

A fembic file is structured in blocks. Each block starts with a keyword, followed by data related to that block. The blocks can come in any order and may be repeated. A block must start on a new line using any of the keywords. Each keyword and the related data will now be described.

Commands

Command	Contact_Triangle
Block	Elements
Description	This is a 3 node contact element. It has no other purpose than detecting if other nodes are about to penetrate it's surface. If this is the case, the element will repel the node with a resulting reaction force onto itself. This element is useful to model contact surfaces. It is also used in most of the other finite elements to handle contact detection.
Syntax	nr nodes = [node1,node2,node3] T = thickness factor = factor friction = friction
Options	nr The element number. Must be a unique number in the model, i.e. another element cannot have the same number. node1,2,.. Nodes are defined in a counter clockwise direction as shown in the figure. Results from the element can be local stresses and strains. The picture shows the local axes where the x-axis direction are primarily defined by node 1 and 2. The z-axis is normal to the shell surface. The local y-axis is defined to be octagonal to the x-axis and z-axis. thickness is the thickness of the contact element. The thickness is assumed to be constant over the element width. Nodes outside the thickness are not assumed to be in contact. The contact zone extends to half the thickness on each side of the element. factor The repelling force to be used when a node penetrates. The force will increase linearly as the node intrudes further. friction The friction coefficient to be used. Usually between 0.2 and 0.8 depending on material and condition. Only useful if contact is enabled. If not set, friction is disabled.
Example	1 nodes = [113,118,110] t = 1.0 factor = 100 friction = 0.2
See also	Elements, Nodes, Shell_C0_3, Shell_BT_4, Contact_Line

Command	Shell_BT_4
Block	Elements
Description	This is a 4 node shell element based on the classical Belytchko-Tsai formulation. It is a very robust design which has become the workhorse in explicit finite element simulations. The element has only one integration point which means that the result are only calculated in one point which is situated in the middle of the element. The advantage is that the element is very fast. The drawback is that it is sensitive to hourglassing. To prevent this, there is an additional compensation built in to the formulation, called hourglass control.
Syntax	nr nodes = [node1,node2,node3,node4] T = thickness material = elmaterial NIP = noip PIP = nopip SHEAR_FACTOR = shearfactor HOURGLASS = hglass MHC = mhc OOPHC = oophc RHC = rhc LOAD = loadname FACTOR = c_factor CONTACT = c_type FRICTION = friction THINNING = thinning
Options	nr The element number. Must be a unique number in the model, i.e. another element cannot have the same number. node1,2,.. Nodes are defined in a counter clockwise direction as shown in the figure. Results from the element can be local stresses and strains. The picture shows the local axes where the x-axis direction are primarily defined by node 1 and 2. The z-axis is normal to the shell surface. The local y-axis is defined primarily by node 1 and 4. thickness is the thickness of the shell. The thickness is assumed to be constant over the element width. elmaterial The name of the material that the shell element uses. This name must be defined under the material block. noip the number of integration points through the thickness of the element. All from 1 up to 5 integration points are possible. A minimum of three integration points are recommended. nopip the number of the integration point which results will be printed in the result file. Can be anything from 1 up to NIP. If nothing is specified, it will be the middle point in the shell thickness, for example if NIP = 5, PIP will be equal to 3. shearfactor the arbitrary parameter used to enforce the Kirchhoff normality condition as the shell become thin. Default value is 1.0. hglass a switch to enable or disable hourglass control on the element. It can be either ON or OFF. Default is ON. mhc the Membrane Hourglass Control factor. This factor is multiplied with the calculated hourglass forces acting in the shell membrane plane. Default is 0.1. oophc the Out Of Plane Hourglass Control factor. This factor is multiplied with the calculated hourglass forces acting out of the membrane plane, causing ex. twist of the element. Default is 0.1. rhc the Rotational Hourglass Control factor. This factor is multiplied with the calculated hourglass moments. Default is 0.1. loadname Name of a load defined under the load block. Pressure onto the shell element is defined this way. c_factor The contact factor. This is the reaction force at full penetration of contact node. c_type Contact type. Can be OFF to disable contact. Default contact type is BASIC which means that two Contact_Triangle elements will represent the surface. This works fine for small deformations of the element. ADVANCED will use four Contact_Triangle elements and be able to handle self contact within the element itself, but at a cost of calculation time. EDGE enables edge contact sensitivity along the edges of the element together with the standard surface contact sensitivity. ADVANCED_EDGE does the latter but together with the advanced contact surface model which uses four Contact_Triangle elements. friction The friction coefficient to be used. Usually between 0.2 and 0.8 depending on material and condition. Only useful if contact is enabled. If not set, friction is disabled. thinning Determines if the shell should reduce thickness at large strains. Useful in pressing simulations. Default is ON. It can be disabled by setting equal to OFF.
Example	1 nodes = [113,118,110,106] nip = 5 t = 1.0 material = steel load = pres
See also	Elements, Materials, Nodes, Shell_C0_3

Command	Shell_C0_3
Block	Elements
Description	This is a 3 node shell element based on the classical C0 formulation by Belytchko et. al. It complement the BT_4 shell and is common in explicit finite element simulations. The element has only one integration point which means that the result are only calculated in one point which is situated in the middle of the element. The advantage is that the element is very fast. Unlike the BT_4 element, this element does not need hourglass control. Triangulars are however by it's nature stiffer than the BT_4 element which means it should be used with care.
Syntax	nr [node1,node2,node3] T = thickness material = elmaterial NIP = noip PIP = nopip LOAD = loadname FACTOR = c_factor CONTACT = c_type FRICTION = friction THINNING = thinning
Options	nr The element number. Must be a unique number in the model, i.e. another element cannot have the same number. node1,2,.. Nodes are defined in a counter clockwise direction as shown in the figure. Results from the element can be local stresses and strains. The picture shows the local axes where the x-axis direction are primarily defined by node 1 and 2. The z-axis is normal to the shell surface. The local y-axis is defined to be octagonal to the x-axis and z-axis. thickness is the thickness of the shell. The thickness is assumed to be constant over the element width. elmaterial The name of the material that the shell element uses. This name must be defined under the material block. noip the number of integration points through the thickness of the element. All from 1 up to 5 integration points are possible. A minimum of three integration points are recommended. nopip the number of the integration point which results will be printed in the result file. Can be anyting from 1 up to NIP. If nothing is specified, it will be the middle point in the shell thickness, for example if NIP = 5, PIP will be equal to 3. loadname Name of a load defined under the load block. Pressure onto the shell element is defined this way. c_factor The contact factor. This is the reaction force at full penetration of contact node. c_type Parameter to set contact sensitivity type. Can be set to OFF to disable contact. If unspecified, contact is enabled for surface only. If set to EDGE, the surface contact will be complemented with edge contact sensitivity. friction The friction coefficient to be used. Usually between 0.2 and 0.8 depending on material and condition. Only useful if contact is enabled. If not set, friction is disabled. thinning Determines if the shell should reduce thickness at large strains. Useful in pressing simulations. Default is ON. It can be disabled by setting equal to OFF.
Example	1 nodes = [113,118,110] nip = 5 t = 1.0 material = steel load = pres
See also	Elements, Materials, Nodes, Shell_BT_4

Command	Solid_Iso_6
Block	Elements
Description	This is a simple isoparametric solid element with eight integration points as showed in the figure. The element is available with either one integration point which is then situated in the middle of the element, or eight integration points. The benefit of having eight points are the the element will be more stable since hourglass modes cannot occur. At the time of writing, there is no hourglass control algorithm implemented which enables a stable use of one integration point which means that this configuration is currently not recommended. The results from the element are stresses and strains in global directions.
Syntax	nr [node1,node2,node3,node4, node5,node6,node7,node8] material= elmaterial NIP=noip
Options	nr The element number. Must be a unique number in the model, i.e. another element cannot have the same number. node1,2,.. Nodes are defined as shown in the figure where the local axes are shown as well. Results from the element can be local stresses and strains. Other types of solid elements can be created by collapsing the element edges. This is achieved by assigning the same node number to the original nodes on the cube element. For example, the wedge would be achieved by assigning node nr 1 on position 1 and 2 for the solid element. In general, the collapse of elements are not as good as rewriting them from scratch. They are however possible to use, but will not give optimal results, so be aware of this when using them. noip the number of integration points in the element; can either be 1 or 8. This will also change the result files since results are calculated in each integration point and the data from each point will then be printed. elmaterial The name of the material that the rod element uses. This name must be defined under the material block.
Example	1 nodes = [23,24,34,42,65,76,89,33] material = steel nip = 8 2 nodes = [23,23,34,42,65,65,89,33] material = steel nip = 8
See also	Elements, Materials, Nodes

Other Indata and Outdata Formats

Impact is designed to handle other indata and outdata formats. At the time of writing, there are no additional formats supported but the process of extending Impact is documented in the programmers manual for those who would like to.