2.7 THE CAD CONVERTER Z88X

2.7.1 OVERVIEW Z88X

The CAD converter Z88X works in two directions:

(I) You design your component in a CAD system and generate Z88 data. You cover in the CAD system your component with a FE mesh or a super-structure following certain rules which follow below, and add if necessary boundary conditions and material informations. Then make your CAD system generating a DXF file and start the CAD converter Z88X. The Z88 entry files are produced by Z88X and you can start with the FE analysis.

Windows:

Z88X, > Type Conversion > 4 from Z88X.DXF to Z88I1.TXT
Z88X, > Type Conversion > 5 from Z88X.DXF to Z88I* . TXT (default)
Z88X, > Type Conversion > 6 from Z88X.DXF to Z88NI.TXT

... and > Compute > Go

UNIX:

z88x -i1fx (Z88X.DXF to Z88I1.TXT, "I1 from X")
z88x -iafx ( Z88X.DXF to Z88I* . TXT, "I all from X", )
z88x -nifx (Z88X.DXF to Z88NI.TXT, "NI from X")

... or use the Z88-Commander with the proper option for Z88X

(II) Convert your Z88 entry files into CAD data. This is very interesting for Z88 data sets already existing, for controls, for completions of the FE structure, but also for plotting the FE structure by CAD program.

Windows:

Z88X, > Type Conversion > 1 from Z88I1.TXT to Z88X.DXF
Z88X, > Type Conversion > 2 from Z88I* . TXT to Z88X.DXF
Z88X, > Type Conversion > 3 from Z88NI.TXT to Z88X.DXF

... and > Compute > Go

UNIX:

z88x -i1tx (Z88I1.TXT to Z88X.DXF, "I1 to X")
z88x -iatx ( Z88I* . TXT to Z88X.DXF, "I all to X", )
z88x -nitx (Z88NI.TXT to Z88X.DXF, "NI to X")

... or use the Z88-Commander with the proper option for Z88X

Since the converter is completely compatible in both directions, you can execute the possibilities I and II in succession as you wish. You will not find any data loss!
That makes a most interesting variant:

(III) Mixed Operation, e.g.

- Component-and super-structural layout done in CAD program
- Conversion CAD ---> Z88
- Meshing in Z88
- Conversion Z88 ---> CAD
- Complete FE structure in CAD e.g. with not-mesh generator capable elements
- Conversion CAD ---> Z88
- Change e.g. material informations in Z88
- Conversion Z88 --- > CAD
- Installation of the boundary conditions in CAD
- Conversion CAD ---> Z88
- FE analysis in Z88
- Etcetera

Which CAD systems can cooperate with Z88 ?

Well, any CAD systems which can import (read) and export (write) DXF files. However, we cannot guarantee any success as some of the CAD guys are changing their DXF definitions from month to month. Z88 V12 has been intensively tested together with the different AutoCAD and AutoCAD LT versions for Windows of Autodesk, and AutoDesk's DXF guidelines have been regarded as the inventor of the DXF interface, according to AC1009 and AC1012. Choose AutoCAD R12 DXF, if in doubt!

The general philosophy of a CAD - FEA data interchange:

CAD files containe nondirectional informations. It is only a wild collections of lines, points and texts, stored in the order of its production to make things worse.

Basically, a FEA system needs topological information which most CAD systems cannot supply. The FEA system must know that these and those lines form a finite element and that these and those points are included in this element. This could be made on principle if one would design in the CAD system in a quite firmly predefined order. Experiments showed that, indeed, this is possible for very simple components, but it will not work for complex components. And, yes, this is what one wants to do in practice: FE analysis on complex structures !

These difficulties are known for a long time and appear at the data interchange of CAD - NC data likewise. As a proper work-around, integrated CAD - FEM systems do exist which are only to acquire at a very high price.

Another attempt enlarges (better: blows up) the CAD system by e.g. additional modules or macros to such an extend, that partly utilizable FEA data can be produced. This is done frequently. It bears the disadvantage that it neither works well for all CAD programs nor works quite exactly even for the same products of one CAD program manufacturer.

Another attempt does nothing in the CAD system. The FEA system, however, contains a kind of mini- or semi-CAD system, in order to process or rework the raw and totally useless CAD data into FEA data, but only by massive support of the operator. The disadvantage is here, that the operator must master two CAD systems, and the integrated semi-CAD system has not got the performance and power of the real CAD system.

At Z88 these difficulties are solved as follows:

1: FROM CAD SYSTEM TO Z88:

1.1 in the CAD system:

Remark: This point case 1.1 will be explained in greater detail in chapter 2.7.2. This is a summary.

(1) Design your component. Order and layers as you like.

(2) Define the FEA structure or the super structure by lines and points. Any order and layers, therefore unproblematic and fast.

(3) Number the nodes with the TEXT function on the layer Z88KNR. Any order, therefore unproblematic and fast.

(4) Write the element information with the TEXT function on the layer Z88EIO. Any order, therefore unproblematic and fast.

(5) Outline each element with the LINE function on the layer Z88NET. The only section with firm work rules and orders (because of the topological informations).

(6) Write general information, material information and control information for the stress processor Z88D on the Layer Z88GEN.

(7) Define the boundary conditions on the layer Z88RBD.

(8) Define the surface and pressure loads on the layer Z88FLA.

(9) Export or store your 3-D model or 2-D drawing under the name Z88X.DXF.

1.2 in Z88: Starts the CAD converter Z88X

You can choose depending on your input data whether

- A mesh generator file Z88NI.TXT or
- A file of the general structure data Z88I1.TXT or
- A complete Z88 data set with Z88I1.TXT, Z88I2.TXT, Z88I3.TXT and Z88I5.TXT (if needed)

is produced. Everything else runs automatically.

1.3 in Z88: Starts other Z88 modules

Check output files produced by Z88X once more with the Filechecker Z88V.

Run the FEM analysis by starting the different Z88 modules at your choice:

* Mesh Generator Z88N
* Plot Program Z88O
* Direct Cholesky Solver Z88F
* Sparse Matrix Iteration Solver Z88I1/Z88I2
* Sparse Matrix Solver with Fill-In Z88I1/Z88PAR
* Stress Processor Z88D
* Nodal Force Processor Z88E

2: FROM Z88 TO CAD PROGRAM

2.1 in Z88: Input files Z88xx.TXT

You have produced the input files

* Mesh generator file Z88NI.TXT or
* File of the general structure data Z88I1.TXT or
* complete Z88 data set with Z88I1.TXT, Z88I2.TXT , Z88I3.TXT and Z88I5.TXT (if needed)

either by an editor, a word processing program, EXCEL or an own routine or by modifying data files that came from the CAD converter Z88X.

2.2 in Z88: Launch CAD converter Z88X

Define which Z88 input files shall be converted. The DXF-file produced by Z88X is Z88X.DXF. If the input files contained polar- or cylindrical coordinates, they are converted into cartesian coordinates.

2.3 in the CAD system:

Import the DXF file Z88X.DXF. Save the loaded model or drawing under a valid CAD name (e.g. at AutoCAD name.DWG) and work with the drawing. You can switch off and switch on the different Z88-layers as you like.

2.7.2 Z88X IN DETAIL

Proceed in the following steps and reserve the following layers

Z88GEN: Layer for general information (1st input group in the mesh generator input file Z88NI.TXT and general structure data file Z88I1.TXT). Include further the material information (4th input group in the mesh generator input file Z88NI.TXT and general structure data file Z88I1.TXT). Add, if necessary, the data of the stress parameter Z88I3.TXT.

Z88KNR: Layer including the node numbers.

Z88EIO: Layer including the element information like element type and in the case of mesh generator input file Z88NI.TXT control information for the mesh generator.

Z88NET: Layer containing the mesh which was drawn or outlined in defined order.

Z88RBD: Layer containing the contents of the boundary conditions file Z88I2.TXT.

Z88FLA: Layer containing the surface and pressure loads as defined for Z88I5.TXT

A further layer, Z88PKT, is produced by Z88X if you convert from Z88 to CAD. It shows all nodes with a point marker so that one better recognizes the nodes. For the reverse step, from CAD to Z88, it is completely insignificant.

1st step: Design your component in the CAD system as usual. You do not need to maintain a definite order and you can use any layers. It is highly recommended to put symbols on one layer, edges on another layer, dimensions on a third layer, invisible lines and center lines on a forth layer and so on. This enables you to remove all unnecessary information in the next step.

2nd step: Plan your mesh subdivision, that means suitable finite element types and their distribution. Subdivide the FE structure or the super structure into elements by lines, insert all points which are not yet existing (for example intersection points or end-points of lines are usable). Any order and layer. However, it is recommended not to use the Z88-layers like Z88NET, Z88GEN, Z88PKT, Z88KNR, Z88EIO and Z88RBD. Better define any new layer for this or use already available layers from step 1.

3rd step: Define the Z88-Layer Z88KNR and make it the active layer. Catch or trap every FE node, which were already defined in the 1st step by your construction or have been completed in the 2nd step, and number them. Write to every node P blank node-number e.g. P 33, with the TEXT function of the CAD program. Be very careful to snap exactly the node and attach the number exactly to the node's location. Take your time ! With the snap modes of AutoCAD (intersection point, end-point, point etc.) this works well. Choose any order of the work consequence as you like, you can well number the node 1 (P 1), then the node 99 (P 99) and then node 21 (P21). However, the numbering of the nodes must make sense and must be meaningfull for a FE analysis. You define which node in node 99 and which other node reads 21. Bad node numbering can cause heavy (but not really necessary) storage needs and computing times. Consult a good FEA book for this aspects.

4th step: Define the Layer Z88EIO and make it the active layer. Write the element information with the TEXT function anywhere (of course, it looks nicer with the element infos placed in middle of the respective finite element or super element). The order of the work consequence is up to you. You can describe element 1 first, step to the attaching element 17 and then proceed with element 8. However, your element choice and description must make sense for a FE analysis. The following information have to be written:

For all finite element types from 1 to 20 (not 16 and 17):

FE Element number Element type

Write into one line, separate each item by at least one blank.

Example: An Isoparametric Serendipity Plane Stress Element No.7 is supposed to get the element no. 23. Write e.g. into the middle of the element with the TEXT function FE 23 7

For super-elements 2-dimensional No. 7, 8, 11, 12 and 20

SE
Element number
Super-element type
Type of the finite elements to be produced by meshing
Subdivision in local x direction
Type of subdivision in local x direction
Subdivision in local y direction
Type of subdivision in local y direction

Write into one line, separate each item by at least one blank.

Example: Sudivide an Isoparametric Serendipity Plane Stress Element with 12 nodes ( Element type 11) used as super-element into finite elements of type 7, i.e. Isoparametric Serendipity Plane Stress Elements with 8 nodes (Element type 7). Subdivide in local x direction three times equidistantly and subdivide in local y direction 5 times ascending geometrically. The super element is supposed to have the number 31. Write e.g. into the middle of the element with the TEXT function: SE 31 11 7 3 E 5 L (e or E for equidistant is equivalent)

For super-elements 3-dimensional Hexahedrons No.10

SE
Element number
Super-element type
Type of the finite elements to be produced by meshing
Subdivision in local x direction
Type of subdivision in local x direction
Subdivision in local y direction
Type of subdivision in local y direction
Subdivision in local z direction
Type of subdivision in local z direction

Write into one line, separate each item by at least one blank.

Example: Subdivide an Isoparametric Serendipity Hexahedron with 20 nodes (Element type 10) as super element into finite elements of the type Isoparametric Hexahedrons with 8 nodes (Element type 1). Subdivide equidistantly three times in local x direction, 5 times ascending geometrically in local y direction and subdivide equidistantly 4 times in local z direction. The super element is supposed to have the number 19. Write e.g. into the middle of the element with the TEXT function:

SE 19 10 1 3 E 5 L 4 E (e or E for equidistant is equivalent)

5th step: Define the Layer Z88NET and make it the active layer You need concentration for this step, because a firm and rigid work consequence must now be kept because of the topological information. One of the most important information, the coincidence, is defined in this step, that means which elements are defined or outlined by which nodes. Choose a proper color which differates well from the colors used till now and remove all superfluous information by switching off unused layers.

Select the LINE command and select the proper snap options e.g. points, intersection points and, if necessary, end-points.

Start at the first element. For Z88 the first element is the element with which you start now, that means the one which you have chosen for your first element (SE 1 or FE 1). Select the node you want to be the first node of this element (this can be e.g. globally the node 150) and draw a line to the node which shall be the second node of this element (this can be e.g. globally the node 67). From there, draw a line to the third node of this element (this can be e.g. globally the node 45). Connect all required nodes with lines and draw at last a line to the starting point, the first node, then quit the LINE function.

Then you do the same with the second element. Remember: You determine with this order which of the elements will be the real second element now. In the previous 4th step you have only defined what kind of element the second element is. You determine here how the element is defined topologically.

The third element follows and so on. If you should make a mistake at the outlining of an element then delete all previous lines of this element (e.g. with an UNDO function) and start again at the first point of the questionable element. But if you notice now just outlining element 17 that you have made a mistake at element 9 , then you must delete all lines of the elements 9 to 17 and restart with element 9.

For your comfort, you must keep the following outline orders which partly differ from the orders shown at the element descriptions when entering the coincidence by hand. Z88X then sorts internally correctly.

Example: The coincidence for the element type 7 is as follows in the element description : First the corner nodes, then the middle nodes, reads 1-2-3-4-5-6-7-8. The coincidence list must look like this in the Z88 input files. However, for Z88X' use for comfortably outlining the elements the order is 1-5-2-6-3-7-4-8-1 (left picture) respectively A-B-C-D-E-F-G-H-A (right picture):

Following the CAD outline orders for all elements but No. 16 and No.17 (because these tetrahedrons can only machine- generated, nearly impossible by hand):

Element No.7 and No.20 :1 - 5 - 2 - 6 - 3 - 7 - 4 - 8 - 1

Element No.8: 1 - 5 - 2 - 6 - 3 - 7 - 4 - 8 - 1

Element No.11: 1 - 5 - 6 - 2 - 7 - 8 - 3 - 9 - 10 - 4 - 11 - 12 - 1

Element No.12: 1 - 5 - 6 - 2 - 7 - 8 - 3 - 9 - 10 - 4 - 11 - 12 - 1

Element No. 2, 4, 5, 9, 13: Line from node 1 to node 2

Element No.3, 14, 15 and 18 :1 - 4 - 2 - 5 - 3 - 6 - 1

Element No.6: 1 - 2 - 3 - 1

Element No.19: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 -10 - 11 - 12 - 13 - 14 - 15 - 16 - 1

Element No.1:

Upper plane: 1 - 2 - 3 - 4 - 1, quit LINE function
Lower plane: 5 - 6 -7 - 8 - 5, quit LINE function
1 - 5, quit LINE function
2 - 6, quit LINE function
3 - 7, quit LINE function
4 - 8, quit LINE function

Element No.10:

Upper plane: 1 - 9 - 2 - 10 - 3 - 11 - 4 -12 - 1, quit LINE function
Lower plane: 5 - 13 - 6 - 14 - 7 - 15 - 8 - 16 - 5, quit LINE function
1 - 17 - 5, quit LINE function
2 - 18 - 6, quit LINE function
3 - 19 - 7, quit LINE function
4 - 20 - 8, quit LINE function

6th step: Define the layer Z88GEN and switch it active. Write with the TEXT function into a free space (well into any place of your drawing):

6.1 general information, i.e. the first input group of the general structure data Z88I1.TXT or the mesh generator file Z88NI.TXT,

In case of Z88I1.TXT (i.e. FE mesh):

Z88I1.TXT
Dimension of the structure
Number of nodes
Number of finite elements
Number of degrees of freedom DOF
Number of material information lines
Coordinate flag (0 or 1)
Beam flag (0 or 1)
Plate flag (0 or 1)
Surface and pressure load flag (0 or 1)

Write into one line, separate each item by at least one blank. Definitely write in the layer Z88GEN.

Example: 3-dimensional FE structure with 150 nodes, 89 finite elements, 450 degrees of freedom, 5 material information lines. Input with cartesian coordinates, structure contains neither beams No.2 nor beams No.13. Thus Z88I1.TXT 3 150 89 450 5 0 0 0 0

In case of Z88NI.TXT (i.e. super structure):

Z88NI.TXT
Dimension of the structure
Number of nodes
Number of super element
Number of degrees of freedom DOF
Number of material information lines
Coordinate flag (0 or 1)
Beam flag (must here be 0!)
Plate flag (0 or 1)
Surface and pressure load flag (0 or 1)
Trap radius header flag (most 0)

Write into one line, separate each item by at least one blank. Definitely write in the layer Z88GEN.

Example: 2-dimensional super-structure with 37 nodes, 7 super elements, 74 degrees of freedom, one material information line. Cartesian coordinates, no beams (anyway forbidden in the mesh generator file), no plates, use default for trap radius. Thus Z88NI.TXT 2 37 7 74 1 0 0 0 0 0

6.2 Material information lines:

For every material information one separate line:

MAT
Number of the material information
This material information starts with element no. abc inclusively
This material information ends with element no. xyz inclusively
Young's Modulus
Poisson's Ratio
Integration order (from 1 to 4)
Cross section value (e.g. for plane stress elements thickness, for trusses cross section area)

... And if beams (but not plates !) are defined in addition:

Second moment of inertia yy (bending around yy axis)
Max. distance from neutral axis yy
Second moment of inertia zz (bending around zz axis)
Max. distance from neutral axis zz
Second moment of area (torsion)
Second modulus (torsion)

... And if plates (but not beams !) are defined in addition:

area load

Write into one line, separate each item by at least one blank. Make sure to write in the layer Z88GEN.

Example: The structure has 34 super elements type 7 with varying thickness: Elements 1 to 11 have thickness 10 mm, elements 12 to 28 have 15 mm and elements 29 to 34 have 18 mm. Material steel. Integration order shall be 2.

MAT 1 1 11 206000. 0.3 2 10.
MAT 2 12 28 206000. 0.3 2 15.
MAT 3 29 34 206000. 0.3 2 18.

6.3 Stress parameters:

The input line of the stress parameter file Z88I3.TXT

Z88I3.TXT
Integration order (0 to 4)
KFLAG (0 or 1)
Von Mises stresses (0 or 1)

Write into one line, separate each item by at least one blank. Make sure to write in the layer Z88GEN.

Example: The structure uses finite elements type 7. The stress calculation is supposed to be carried out in 3*3 Gauss points per element, stresses are supposed to be calculated in addition radially and tangentially. Compute von Mises stresses, too. Thus Z88I3.TXT 3 1 1

7th step: Define the Layer Z88RBD and activate it. Write with the TEXT function into a free space (well into any place of your drawing):

7.1 number of the boundary conditions, i.e. the first input group of the boundary condition file Z88I2.TXT

Z88I2.TXT Number of the boundary conditions

Write into one line, separate each item by at least one blank. Make sure to write in the layer Z88RBD.

Example: The structure has 10 boundary conditions, e.g. two loads and eight constraints i.e. support reactions. Thus Z88I2.TXT 10

7.2 Boundary conditions, the second input group of the boundary condition file Z88I2.TXT

RBD
Number of the boundary condition
node number
Degree of freedom
Header flag force/displacement (1 or 2)
Value

Write into one line, separate each item by at least one blank. Make sure to write in the layer Z88RBD.

Example: The structure shall be a truss- framework. Node 1 shall be fixed in Y and Z, node 2 fixed in X and Z. Nodes 7 and 8 have a load of 30,000 N each in Z direction, pointing down. Node 19 is fixed in X and Z and node 20 is fixed in Y and Z. Thus

RBD | 1 | 1 | 2 | 2 | 0
RBD | 2 | 1 | 3 | 2 | 0
RBD | 3 | 2 | 1 | 2 | 0
RBD | 4 | 2 | 3 | 2 | 0
RBD | 5 | 7 | 3 | 1 | -30000
RBD | 6 | 8 | 3 | 1 | -30000
RBD | 7 | 19 | 1 | 2 | 0
RBD | 8 | 19 | 3 | 2 | 0
RBD | 9 | 20 | 2 | 2 | 0
RBD | 10 | 20 | 3 | 2 | 0

8th step: if surface and pressure loads are defined: create the layer Z88FLA and activate it. Write with the TEXT function into a free space (well into any place of your drawing):

8.1 Number of surface and pressure loads

i.e. the first input group of the surface and pressure loads file Z88I5.TXT

Z88I5.TXT number of surface and pressure loads

Write into one line, separate each item by at least one blank. Make sure to write in the layer Z88FLA.

Example: The structure features 12 surface loads. Thus: Z88I5.TXT 12

8.2 Surface and pressure loads

i.e. the second input group of the surface and pressure loads file Z88I5.TXT

FLA number of the surface and pressure load

The following entries depend from the element type with surface and pressure load:

>> Plain stress element No.7 and 14 and Torus elements No.8 and 15:

Element number with surface load

Pressure, positive if poiting towards the edge

Tangential shear, positive in local r-direction

3 nodes of the loaded edge

Example: The plain stress element 97 is the third element with surface load. The load should be applied onto the edge defined by the corner nodes 5 and 13 and by the mid node 51. One surface load is applied normally to the edge with 100 N/mm and the other surface load is applied tangentially and positive in local r direction with 300 N/mm (defined by the two corner nodes). Thus: FLA 3 97 100. 300. 5 13 51

>> Hexahedron No.1:

Element number with surface and pressure load

Pressure, positive if poiting towards the surface

Tangential shear, positive in local r direction

Tangential shear, positive in local s direction

4 nodes of the loaded surface

######

Example: The hexahedron 356 is the 34th element with surface loads. The load should be applied onto the surface defined by the corner nodes 51, 34, 99 und 12 .The first surface load is pressure with 100 N/mm. The second surface load is applied tangentially and positive in local r direction with 200 N/mm. The third surface load is applied tangentially and positive in local s direction with 300 N/mm . Thus

FLA 34 356 100. 200. 300. 51 34 99 12

>> Hexahedron No.10:

Element number with surface and pressure load

Pressure, positive if poiting towards the surface

Tangential shear, positive in local r direction

Tangential shear, positive in local s direction

4 nodes of the loaded surface

>> Plate elements No.18, 19 and 20:

Element number with pressure load

Pressure, positive if poiting towards the surface

(It is easier to enter the pressure loads for plate elements directly into Z88I1.TXT than via Z88I5.TXT)

Separate each item by at least one blank. Make sure to write in the layer Z88FLA.

9th step: Export (store) your model or drawing under the name Z88X.DXF in the DXF file format. For precision of decimal positions take the default value which the CAD program suggests. Take care that you export directly into the Z88-directory or you must copy the file Z88X.DXF by hand into the Z88-directory, because the CAD converter Z88X expects the input and output files in the same directory, where Z88X is located.

You may launch the CAD converter Z88X then.

Note: If you want to convert Z88 text files as Z88X.DXF to CAD, you can choose the text size which applies to all texts like node numbers, element numbers etc. This is very important from time to time because there is no possibility in e.g. AutoCAD to change the text size globally afterward. From time to time you must make some trys untill you have found the suitable text size for the respective Z88 file. Simply call Z88X once more with another text size.

Windows: In Z88X: File > Textsize

UNIX: z88x -i1tx | -iatx | -nitx | -i1fx | -iafx | -nifx -ts number

Caution, valuable note: Use the Z88X keywords "P number, FE values, SE values, MAT, RBD, Z88NI.TXT, Z88I1.TXT, Z88I2.TXT and Z88I3.TXT" only where they are really needed. Take care that they do not appear in other drawing items ! Otherwise Z88X cannot interpret the DXF file properly and will flag error messages!