Working with ElementsModule 4

4. Working with ElementsMost 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 surfacePostprocessing of beams and element-specific dataComposite (layered) elementsLink and combination elements such as spars, springs, and convection links

...Working with ElementsIn this chapter, we will present the following element-specific topics:A. Element Coordinate SystemB. Surface Effect ElementsC. Element TableD. Workshop

Working with ElementsA. Element Coordinate SystemEvery 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.

Working with Elements...Element Coordinate SystemThe 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 Jelement Y perpendicular to element X in the plane of the elementelement Z normal to X-Y by right-hand ruleSolids default to global Cartesian.K

Working with Elements...Element Coordinate SystemThe 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.

Working with Elements...Element Coordinate SystemTo 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

Working with Elements...Element Coordinate System2.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 AttribOr EMODIF command (e.g, emodif,all,esys,11)4.Reactivate all elements and switch back to previous coordinate system (CSYS).

Working with Elements...Element Coordinate SystemShell Element NormalsA 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 SystemTwo ways to flip shell element normals:Reverse the underlying areaOr reverse the elements themselvesTo reverse the underlying area:Preprocessor > -Modeling- Move/Modify > -Areas- Area NormalsThen 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).

Working with Elements...Element Coordinate SystemTo reverse the shell elements directly:Preprocessor > -Modeling- Move/Modify > -Elements- Shell NormalsOr issue ENORM,PThen pick an element with the "correct" orientation

Working with Elements...Element Coordinate SystemDemo: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 elementsReplot elements and note the correct orientation

Working with ElementsB. Surface Effect ElementsHow 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 effect elements provide an effective way to do this.

Working with Elements...Surface Effect ElementsCharacteristics: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 typeIssue ESURF (or Preprocessor > Create > Elements > Surf Effect > ...).Select all nodes.

Working with Elements...Surface Effect ElementsAvailable 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

Working with Elements...Surface Effect ElementsSURF154 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

Working with Elements...Surface Effect ElementsSuppose 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).

Working with Elements...Surface Effect ElementsFaces 2 & 3:Tangential pressures, along element X & Y respectively.Example:sfe,eflat,3,pres,,1000sfe,eslope,2,pres,,1000(eflat and eslope are components made of elements)

Working with Elements...Surface Effect ElementsFace 4:Normal pressure, tapered. Magnitude = P1 + XgP2 + YgP3 + ZgP4P1-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).P2Xg=0P1

Working with Elements...Surface Effect ElementsFace 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 = 0P1 is the value at Xg=0, calculated as P1 = 2(200) + 200 = 600sfe,eflat,4,pres,,600,200,0,0

Working with Elements...Surface Effect ElementsFace 5Vector-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.

Working with Elements...Surface Effect ElementsFace 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.

Working with Elements...Surface Effect ElementsKEYOPT(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.

Working with Elements...Surface Effect ElementsOther applications of surface effect elements:Torque loading of solid elementsApply tangential pressure with the help of a modified element coordinate system.Useful for shafts and boltsThe thermal versions (SURF151 & 152) have many capabilities, including:Heat and mass transfer to 1-D pipe element FLUID116Easy calculation of heat loss (or gain) across a surfaceRadiation

Working with Elements...Surface Effect ElementsDemo:Enter PREP7 and resume boltload.dbAdd SURF154 as element type 1 (show options)Select bottom area of hole and nodes attached to itActivate TYPE 1 and issue ESURF to create surface elementsSelect everything and plot elementsSelect TYPE 1 elements and apply pressure on face 5:value= 1000, val2= 0, val3= -1, val4= 0Select everything, save db, and solvePlot and/or animate SY stresses in POST1Stay in POST1 (for next demo)

Working with ElementsC. Element TableThe element table, available in POST1 (the general postprocessor), serves two functions:Performing arithmetic operations among results dataAccessing results data that are not directly available (such as for certain line elements)It works somewhat like a spreadsheet:Each row represents an elementEach column consists of data for the elements

Element No.

Item 1

Item 2

Item 3

1

2

3

Working with Elements...Element TableA typical element table session consists of the following steps:First select the desired elementsLoad results data into the tableList or plot the dataPerform desired arithmetic operationsElement table functions are located in the GUI under General Postproc > Element Table.

Working with Elements...Element TableSelect the desired elementsIn 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 nodesby attached solid modeling entitiesby resultsetc.

Working with Elements...Element TableLoading results data into the tableIdentify 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

Working with Elements...Element TableSome 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,17Refer to your Basic Analysis Procedures Guide (available on-line) for details of the sequence number method.

Working with Elements...Element TableListing or plotting element table dataElement Table > List Elem Table (or PRETAB command)

Working with Elements...Element TableElement Table > Plot Elem Table (or PLETAB,name)

Working with Elements...Element TableThe 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.

Working with Elements...Element TableNote 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 elements 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.

Working with Elements...Element TablePerforming operationsSomewhat 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 elementsEtc.

Working with Elements...Element TableExample:

Working with Elements...Element TableIf 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.

Working with Elements...Element TableUpdating the TableIf 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 TableSince 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.

Working with Elements...Element TableSaving 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 TableOr ETABLE,ERASETo delete one item (column) in the element table:Use the Delete button in the Element Table Data dialog.Or ETABLE,name,ERASE

Working with Elements...Element TableDemo:Continue with previous "bolthole" demoSelect TYPE 1 elements (SURF154s) and all nodes attached to themBring 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 itSelect 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 themStore 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 1000sSelect everything and plot SY stresses, etc. Compare with load step 1 results.

Working with ElementsD. WorkshopThis workshop consists of the following problem:W3. Bolt TorquePlease refer to your Workshop Supplement for instructions.