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Working with Elements

Module 4. Working with Elements. 4. Working with Elements. Most preprocessing — geometry creation, meshing, and even loading — is usually done with the help of solid modeling entities. All postprocessing is done at the node and element level, a majority of it with nodal quantities.

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Working with Elements

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  1. Module 4 Working with Elements

  2. 4. Working with Elements • Most preprocessing — geometry creation, meshing, and even loading — is usually done with the help of solid modeling entities. • All postprocessing is done at the node and element level, a majority of it with nodal quantities. • But there are several situations where you may need to work directly with elements: • Special types of surface loads, such as a pressure that is not normal to the surface • Postprocessing of beams and element-specific data • Composite (layered) elements • Link and combination elements such as spars, springs, and convection links October 30, 2001 Inventory #001571 4-2

  3. ...Working with Elements • In this chapter, we will present the following element-specific topics: A. Element Coordinate System B. Surface Effect Elements C. Element Table D. Workshop October 30, 2001 Inventory #001571 4-3

  4. Working with ElementsA. Element Coordinate System • Every element in a model has an element coordinate system associated with it. • Its purpose is to orient the following quantities: • orthotropic material properties (EX, KXX, etc.) • strains and stresses calculated during solution (EPX, SX, SY, etc.) • Just like MAT (material), TYPE (element type), and REAL (real constant set), the element c.s. is a fourth element attribute called ESYS. October 30, 2001 Inventory #001571 4-4

  5. J I X L Y Z I X J O N P M Y Yg Z X K I Zg Xg J Working with Elements...Element Coordinate System • The default ESYS orientation depends on the element type. For example: • Line elements are generally oriented with element X from node I to node J. • Shells usually default to: • element X from node I to node J • element Y perpendicular to element X in the plane of the element • element Z normal to X-Y by right-hand rule • Solids default to global Cartesian. K October 30, 2001 Inventory #001571 4-5

  6. Yg X Zg Xg Working with Elements...Element Coordinate System • The default ESYS is sufficient for most cases, but there are a few situations where you might need to change it. Example: • The material property direction of a solid element, representing fiberglass for example, may not be parallel to global Cartesian. • To orient tangential pressures applied on surface effect elements. October 30, 2001 Inventory #001571 4-6

  7. Working with Elements...Element Coordinate System • To change the default ESYS, you need to align it with a specified local coordinate system (CSYS 11 or greater). • The procedure is as follows: 1. Define a local coordinate system with the appropriate orientation. • Location is generally arbitrary. • Utility Menu > WorkPlane > Local Coordinate Systems > Create Local CS October 30, 2001 Inventory #001571 4-7

  8. Working with Elements...Element Coordinate System 2. Select the desired elements. 3. Modify the ESYS attribute of all selected elements to the local system number defined in step 1. • Preprocessor > -Modeling- Move/Modify > -Elements- Modify Attrib • Or EMODIF command (e.g, emodif,all,esys,11) 4. Reactivate all elements and switch back to previous coordinate system (CSYS). October 30, 2001 Inventory #001571 4-8

  9. Shell Element Normals A related issue is the normal orientation of shell elements. Defines the top and bottom faces. Determined by the node order (I-J-K-L) in which the element is defined, which in turn is determined by the underlying area. Sometimes you may need to "flip" the normal direction of some elements to match the rest of the model. Element plot in Powergraphics mode uses different colors for the top and bottom faces. Working with Elements...Element Coordinate System October 30, 2001 Inventory #001571 4-9

  10. Working with Elements...Element Coordinate System • Two ways to flip shell element normals: • Reverse the underlying area • Or reverse the elements themselves • To reverse the underlying area: • Preprocessor > -Modeling- Move/Modify > -Areas- Area Normals • Then pick the area with the "correct" orientation. ANSYS will scan all areas and reverse those that don't match the correct one. The attached elements are also reversed (by default). October 30, 2001 Inventory #001571 4-10

  11. Working with Elements...Element Coordinate System • To reverse the shell elements directly: • Preprocessor > -Modeling- Move/Modify > -Elements- Shell Normals • Or issue ENORM,P • Then pick an element with the "correct" orientation October 30, 2001 Inventory #001571 4-11

  12. Working with Elements...Element Coordinate System • Demo: • Enter PREP7 and resume pipe.db (shell mesh of an IGES model; note that some shells are reversed) • Move/Modify > Shell Normals (or issue ENORM,P) • pick one of the purple elements • Replot elements and note the correct orientation October 30, 2001 Inventory #001571 4-12

  13. Working with ElementsB. Surface Effect Elements • How would you apply a pressure load that is: • tangential to the surface, such as a shear load? • spatially varying over the surface, such as a bolt load? • oriented at an angle to the surface, such as ice load on a rooftop? • Surface effectelements provide an effective way to do this. October 30, 2001 Inventory #001571 4-13

  14. Working with Elements...Surface Effect Elements • Characteristics: • They overlay the surface of an underlying mesh like a “skin.” • They act as a conduit for surface loads. • Creating them is very easy: • Select nodes on the surface of interest. • Activate the appropriate element type • Issue ESURF (or Preprocessor > Create > Elements > Surf Effect > ...). • Select all nodes. October 30, 2001 Inventory #001571 4-14

  15. Working with Elements...Surface Effect Elements • Available for both 2-D and 3-D models: • SURF151 & 153 are line elements (thermal and structural) meant for edges of 2-D models. • SURF152 & 154 are area elements (thermal and structural) meant for surfaces of 3-D models. • We will discuss only SURF154 in this section, but you can apply the same concepts to the other elements. SURF154 as depicted in the Elements Reference manual October 30, 2001 Inventory #001571 4-15

  16. Working with Elements...Surface Effect Elements • SURF154 uses different element face numbers to accept different types of loads. • The face number is a field in the "Apply PRES on elems" dialog (Solution > -Loads- Apply > Pressures > On Elements), as shown below. Or the LKEY field on the SFE command: • SFE, ELEM, LKEY, PRES, , VAL1, VAL2, VAL3, VAL4 October 30, 2001 Inventory #001571 4-16

  17. Working with Elements...Surface Effect Elements • Suppose you apply a pressure of magnitude 1000 on a set of SURF154 elements. Its orientation depends on which element face is used. Face 1: • Normal pressure. • Positive value acts into the element (along element -Z). • Example: sfe,all,1,pres,,1000 (after selecting desired elements). October 30, 2001 Inventory #001571 4-17

  18. Working with Elements...Surface Effect Elements Faces 2 & 3: • Tangential pressures, along element X & Y respectively. • Example: sfe,eflat,3,pres,,1000 sfe,eslope,2,pres,,1000 (eflat and eslope are components made of elements) October 30, 2001 Inventory #001571 4-18

  19. Xg Working with Elements...Surface Effect Elements Face 4: • Normal pressure, tapered. Magnitude = P1 + XgP2 + YgP3 + ZgP4 • P1-P4 are specified VAL1-VAL4 on SFE command. • Xg,Yg, Zg are the global Cartesian locations of the element's integration points. • P2,P3,P4 are the slopes in global X,Y,Z respectively and default to P1 if left blank. • Positive value acts into the element (along element -Z). P2 P1 Xg=0 October 30, 2001 Inventory #001571 4-19

  20. Working with Elements...Surface Effect Elements Face 4 (cont'd): • For example, to apply a tapered pressure of 200 to 1000 in the global X direction, with X values ranging from -2 to +2: • Slope P2 = (1000-200)/4 = 200; P3 = 0; P4 = 0 • P1 is the value at Xg=0, calculated as P1 = 2(200) + 200 = 600 • sfe,eflat,4,pres,,600,200,0,0 October 30, 2001 Inventory #001571 4-20

  21. Working with Elements...Surface Effect Elements Face 5 • Vector-oriented pressure of magnitude P1. • Direction = • P2,P3,P4 now represent the direction cosines of the vector and have no effect on the magnitude. • Example: sfe,eflat,5,pres,,1000,-1,-1,0 defines a pressure at angle of 45° in the X-Y plane. October 30, 2001 Inventory #001571 4-21

  22. Working with Elements...Surface Effect Elements Face 5 (cont'd): • The magnitude of vector-oriented pressure also depends on KEYOPT(11). • KEYOPT(11)=0 (default) and 1 applies pressure on the projected area of surface elements. • Useful for bolt loading (or pin loading). • Example: sfe,ecurv,5,pres,,1000,0,-1,0 defines a bolt load on the curved surface, as shown by a POST1 contour plot below. October 30, 2001 Inventory #001571 4-22

  23. Working with Elements...Surface Effect Elements • KEYOPT(11)=2 applies pressure on the full area. • Useful for inclined surfaces (such as a roof top) or wind loads. • Example: sfe,eslope,5,pres,,1000,0,-1,0 defines uniform, full pressure on all faces of an inclined surface, as shown below. October 30, 2001 Inventory #001571 4-23

  24. Working with Elements...Surface Effect Elements • Other applications of surface effect elements: • Torque loading of solid elements • Apply tangential pressure with the help of a modified element coordinate system. • Useful for shafts and bolts • The thermal versions (SURF151 & 152) have many capabilities, including: • Heat and mass transfer to 1-D pipe element FLUID116 • Easy calculation of heat loss (or gain) across a surface • Radiation October 30, 2001 Inventory #001571 4-24

  25. Working with Elements...Surface Effect Elements • Demo: • Enter PREP7 and resume boltload.db • Add SURF154 as element type 1 (show options) • Select bottom area of hole and nodes attached to it • Activate TYPE 1 and issue ESURF to create surface elements • Select everything and plot elements • Select TYPE 1 elements and apply pressure on face 5: • value= 1000, val2= 0, val3= -1, val4= 0 • Select everything, save db, and solve • Plot and/or animate SY stresses in POST1 • Stay in POST1 (for next demo) October 30, 2001 Inventory #001571 4-25

  26. Working with ElementsC. Element Table • The element table, available in POST1 (the general postprocessor), serves two functions: • Performing arithmetic operations among results data • Accessing results data that are not directly available (such as for certain line elements) • It works somewhat like a spreadsheet: • Each row represents an element • Each column consists of data for the elements October 30, 2001 Inventory #001571 4-26

  27. Working with Elements...Element Table • A typical element table session consists of the following steps: • First select the desired elements • Load results data into the table • List or plot the data • Perform desired arithmetic operations • Element table functions are located in the GUI under General Postproc > Element Table. October 30, 2001 Inventory #001571 4-27

  28. Working with Elements...Element Table • Select the desired elements • In most cases, you only need to use the element table for a subset of the elements in the model. • Therefore, the first step is to select the desired elements: • by attributes (MAT, TYPE, REAL, or ESYS) • by their nodes • by attached solid modeling entities • by results • etc. October 30, 2001 Inventory #001571 4-28

  29. Working with Elements...Element Table • Loading results data into the table • Identify the data item you want to load and assign a name (label) to it. The label is used for listings, plots, and operations. • Use the ETABLE command (e.g, etable,evolume,volu) • Or Element Table > Define Table… October 30, 2001 Inventory #001571 4-29

  30. Working with Elements...Element Table • Some types of data can only be identified by sequence numbers, which are documented in the Elements Reference manual. • For example, the effective pressure on face 5 of a SURF154 element is stored as sequence number “SMISC 17.” To load it into the element table: etable,p5,smisc,17 Refer to your Basic Analysis Procedures Guide (available on-line) for details of the “sequence number method.” October 30, 2001 Inventory #001571 4-30

  31. Working with Elements...Element Table • Listing or plotting element table data • Element Table > List Elem Table… (or PRETAB command) October 30, 2001 Inventory #001571 4-31

  32. Working with Elements...Element Table • Element Table > Plot Elem Table… (or PLETAB,name) October 30, 2001 Inventory #001571 4-32

  33. Working with Elements...Element Table • The default is to not average the data at the nodes, i.e, there is one color per element. To get a smooth contour plot, activate the averaging key. October 30, 2001 Inventory #001571 4-33

  34. Working with Elements...Element Table • Note that there is only one value per element for each of the data items: the average or effective value for that element. For example, if you load nodal displacements UX, the average UX of the element’s nodes is stored per element. • Other ways to review element table data: • You can map the data onto a path and obtain a path plot or listing. • You can load three scalar quantities (such as element X-pressure, Y-pressure, and Z-pressure) and combine them to obtain a vector plot. • See the PLVECT command for details. October 30, 2001 Inventory #001571 4-34

  35. Working with Elements...Element Table • Performing operations • Somewhat like a spreadsheet, you can do arithmetic operations among the element table columns. • Many operations are available: • Add two columns (SADD) • Multiply (SMULT) • Compare and store the maximum or minimum of two columns (SMAX or SMIN) • Sum each column (SSUM — useful for calculating the total volume of the selected set of elements • Etc. October 30, 2001 Inventory #001571 4-35

  36. Working with Elements...Element Table • Example: October 30, 2001 Inventory #001571 4-36

  37. Working with Elements...Element Table • If a desired operation is not available, you can “upload” an element table item to an array parameter and use array operations. • Use the *VGET command (or Utility Menu > Parameters > Get Array Data…) • Then use one of the array operations: *VOPER, *VSCFUN, etc. • For example, statistical measures such as mean and standard deviation cannot be calculated at the element table level, but are available for array parameters. October 30, 2001 Inventory #001571 4-37

  38. Updating the Table If you read in results for a different load step or from a different results file, the element table is not automatically updated. Issue ETABLE,REFL (or click the Update button in the Element Table Data dialog) to “refill” or update the table. The column header identifies items as “current” or “previous” depending on the status of the data items. Working with Elements...Element Table October 30, 2001 Inventory #001571 4-38

  39. Working with Elements...Element Table • Since there is no automatic update of the element table, you can easily compare the results between two load steps or two different analyses, as follows: • Read in load step 1, and store a result item in the element table as A1 (for example). • Then read in load step 2 and store the same result item as A2. • Now you can compare A1 and A2 and store the maximum or minimum in a third column, or do other desired operations. • This scheme can be extended to compare different results files for the same model. October 30, 2001 Inventory #001571 4-39

  40. Working with Elements...Element Table Saving and Deleting the Table: • To save the element table along with the ANSYS database: • Save the database while still in POST1 (Toolbar > SAVE_DB or SAVE command). • Or use the "Save everything" option when exiting ANSYS (Toolbar > QUIT or /EXIT,ALL command). • To delete the entire element table: • Element Table > Erase Table… • Or ETABLE,ERASE • To delete one item (column) in the element table: • Use the Delete button in the Element Table Data dialog. • Or ETABLE,name,ERASE October 30, 2001 Inventory #001571 4-40

  41. Working with Elements...Element Table • Demo: • Continue with previous "bolthole" demo • Select TYPE 1 elements (SURF154s) and all nodes attached to them • Bring up help for SURF154 and show the "Output Definitions" and "Item and Sequence Numbers" tables. Note that P5EFF (effective pressure on face 5) is SMISC 17. • Store SMISC,17 as P5 in the element table, plot it with and without averaging, then list it • Select everything, return to PREP7, and set KEYOPT(11)=2 for SURF154 (Options > K11 = full area w/tan). This changes face 5 pressures to "full" area instead of projected area. • Switch to Solution, type in ANTYPE,,RESTART (to continue with load step 2), and solve. • In POST1, read in load step 2, then select TYPE 1 elements (SURF154s) and all nodes attached to them • Store SMISC,17 as P5A in the element table. (Notice that P5 from the previous load step still exists.) • List the element table… P5A column should show all 1000s • Select everything and plot SY stresses, etc. Compare with load step 1 results. October 30, 2001 Inventory #001571 4-41

  42. Working with ElementsD. Workshop • This workshop consists of the following problem: W3. Bolt Torque Please refer to your Workshop Supplement for instructions. October 30, 2001 Inventory #001571 4-42

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