CHAPTER 5 USE OF STL FILE FOR FINITE ELEMENT ANALYSIS

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CHAPTER 5 USE OF STL FILE FOR FINITE ELEMENT ANALYSIS 5.1 Introduction: Most CAD software in the market can generate STL files, and these are generally used for prototyping and rendering purposes.

1 CHAPTER 5 USE OF STL FILE FOR FINITE ELEMENT ANALYSIS 5.1 Introduction: Most CAD software in the market can generate STL files, and these are generally used for prototyping and rendering purposes. These files represent a triangulated boundary of a solid. Algorithms for the generation of STL triangulation are highly efficient, and the solid can be approximated very precisely. The STL file format is composed of an extensive list of triangle facets and the surface normals. These facets are composed of the coordinates of the three vertices of the triangle, in addition to the coordinates of the normal oriented to the exterior of the solid. This triangulation is built to minimize a geometric approximation criterion that is related to the real boundary of the solid. In any FEA analysis discritisation of a solid model plays a very important role and research is done in the area of effective remeshing of STL file generated from solid model when the aspect ratio is very high, for example the solid model of a cylinder which is converted into STL file using any commercial software. So for remeshing mainly STL file is considered which is developed from boundary represented solid model. But in this chapter an attempt is made to use a triangulated STL file which is already perfectly meshed and originally developed from point cloud data for finite element analysis. Thus in this process the time and efforts required for remeshing can be avoided. Finite element analysis divides the given region into finite size elements. Basically four types of elements are used to discretise the given continuum. They are point elements, line elements, area elements, and volume elements. So one has to choose an appropriate type of element and this selection is mainly dependent upon the application. The point cloud data obtained from either CMM or Laser Scanner is the data of coordinates of various points lying on a surface. This is converted in to small triangular planer surfaces, when STL file is created. Thus STL file converts the point cloud data in to a well stitched area patch without gaps and overlaps, creating a surface model for which triangulated area element being appropriate. 5.2 Classification of elements depending upon shape of the element: The commonly used area elements in finite element analysis are triangles, or quadrilaterals. Triangular elements are preferred as they can 133

negotiate curve boundaries also. As STL file gives triangular patches program developed creates triangular elements. the Further classification of element depending upon direction of load.

2 negotiate curve boundaries also. As STL file gives triangular patches program developed creates triangular elements. the Further classification of element depending upon direction of load. Loads acting in case of area elements can be in-plane loads, or normal loads. If an element is oriented in x-y plane, and if the force acting is in x-y direction it is called as in-plane load, while if the load is in z direction it is called as normal load. The mathematical treatment to estimate stresses for both cases is totally different. If a plate is hinged at one edge and a force parallel to plate is applied at other edge, it is in plane stress. If the ends of a tube are fixed, and if the tube is heated, due to thermal expansion a compressive stress is induced in tube. This is the case of in-plane stress. While the stress induced in boiler shell due to internal pressure, which acts normal to shell, hoop stress (tensile stress) is induced in the shell. This is the case of normal load. Plain Element (a) Fig. 5.1 Shell Element (b) The vertices of elements are called as nodes in finite element analysis. In STL file the triangles are created by joining the three points of which x, y, and z coordinates are known, as such the various points present in point cloud data are nothing but nodes for the purpose of finite element analysis, and the triangular facets created are the triangular elements for the purpose of finite element analysis. Preprocessing, Solution, and Post processing are the three major steps involved in any commercial finite element analysis software. 134

3 (A). Steps in Preprocessing are: a. Defining the type of analysis to be carried out (structural, thermal, fluid, fluid dynamics, electro-magnetic etc.) b. Defining type of element, (point, line, area, volume etc.) Further classification of element such as triangular or quadrilateral. Further classification of element such as plane stress, plane strain, shell etc. c. If required defining real constants such as thickness in case of shell elements. d. Defining material by specifying required material properties, such as modules of elasticity, Poisons ratio etc. e. Creation of model f. Meshing the model g. Applying boundary conditions, such as constraints, and external forces etc. The above mentioned steps are carried out manually and need a skilled interpretation and consumes more time. (B). Solution: In this step a set of linear simultaneous equations is formulated, and then these equations are solved using suitable solver. The number of equations created is product of number of nodes and degree of freedom available per node. The result of solution of equations is stored in a separate data base. This process is carried out with the click of mouse when using commercial software. (C). Post processing: In this step using the results obtained from solution of linear simultaneous equations, further calculations are carried out and the required results are obtained. The results thus obtained are then presented in graphical form, or tabulated form as requested. This process is also carried through software interactively. 5.3 Use of db.log file: This is a database file and it mainly records the different commands used in ANSYS after opening a file. This file is similar to the list command used in AUTOCAD. Hence this db.log file records all the commands records in a particular sequence. In ANSYS major parameters required to define are the type of analysis, type of element, material selection, real constant if required, model creation, meshing parameters and load 135

conditions like force acting on nodes and displacement of nodes. All these parameters get recorded in a sequence in db.log file and can be opened in notepad. The file can be edited if required.

4 conditions like force acting on nodes and displacement of nodes. All these parameters get recorded in a sequence in db.log file and can be opened in notepad. The file can be edited if required. This modified file can be reopened in ANSYS with all the new changes. But initially the parameter selection is done manually. Using this concept a program is now developed to replace the manual task of preprocessing using following methodology: Parameters assumed are as below: 1. The type of analysis : Structural 2. Type of element : Area element 3. Further classification of : Triangular plane stress analysis element 4. Material properties : (suitable for Steel) Modules of elasticity = 2*10 5 N/mm 2. Poisons ratio = 0.3 All above parameters can be changed if required through software. Analysis type, type of element, its real constants, and material used can be modified through software as they are the physical properties of a physical component 5. Creating model: The file "temp4.dat" is the main input to create model. The file "temp4.dat" consists of the x, y and z coordinates of the vertices of triangles written in sequence as given below: A sample "temp4.dat" file: Node no X Y E E E E E E E E E E E E-02 Z Table 5.1 temp4.dat file 136

5 The first three lines of this sample data file denote the three vertices of the first triangular facet, while the next three lines denote the three vertices of the second triangular facet, and so on. In terminology of finite element analysis the file denotes 12 nodes, the first three nodes denoting element no. 1, while the next three nodes denoting element no.2 and so on. A db.log file as shown below is created by AN SYS using above sample file. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, , , , , , , , , , , , , , , , , , , , , , , , , ,,, ,,, ,,, ,,, ,,, ,,, ,, ,, ,, , , , FITEM,2, 1 FITEM,2, 2 FITEM,2, 3 E,P51X FLST,2,3,1 FITEM,2, 4 FITEM,2, 5 FITEM,2, 6 E,P51X FLST,2,3,1 FITEM,2, 7 FITEM,2, 8 FITEM,2, 9 E.P51X FLST,2,3,1 FITEM,2, 10 FITEM,2, 11 FITEM,2, 12 E,P51X 137

6 When the program "analysis" is executed a data base log (db.log) file is created, using a temp4.dat" as input and "application.log" as output. This log file is used as input to "ANSYS" software which creates the model, with above mentioned parameters, which can be modified if required. Boundary conditions are applied by user as required, completing preprocessing process. Solution is done by using solve command, and then post-processor displays the required results. 5.4 Quality factor: STL triangulation cannot be used directly in the finite element method (FEM), mainly because it requires specific characteristics in the geometrical description of the domain to be computed. In FEM, the geometric and functional support is provided by the elements (triangles or others), and they must have a specific shape, i.e. the proper size and the proper quality factor for the error estimator of the calculation to be as high as desired [57,58]. Here, for a surface triangle e, its quality (in the isotropic case) is defined where Ae is the area of the triangle and Li is the length of the ith edge. Elements with Qe larger than 0.7 are considered to be of a good quality while elements with a value lower that 0.3 indicate poor-quality elements. This quality factor is bounded between 0 (for all kinds of degenerated triangles) and 1 (for equilateral triangles). The finite element analysis software such as ANSYS has an inbuilt facility to check aspect ratio, or quality factor of an element. In general quality factor can be looked upon as ratio of area of triangle to the perimeter of triangle. Mathematically Quality factor has been defined as: QFACTOR = 4*1.7320*AREA/(E12+E22+E3 2 ) where E1,E2,E3 denotes lengths of three edges of triangle. A program name "anaval" has been developed to estimate the quality factor using following methodology. The file temp4.dat is the input to this program. For each geometry, the following information is provided through temp4.dat: 1. initial number of points, 2. the initial number of triangles. The output of this program is a text file named "anarepo.txt" which reports the element number, its x, y and z coordinates, and quality factor (if it is less than 0.3, total number of elements and number of bad elements). The lengths of three edges are calculated by using distance formula d x = ^(x 2 -x l f+(y 2 -y l ) 2 +(z 2 -z l ) 2 138

7 And area of triangle is calculated by using the formula A = ^]sx(s-a)x(s-b)x(s-c) triangle while s = (a+b+c)/2 where a,b,c denotes the three sides of 5.5 Aspect Ratio: If quality factor is less than 0.3 is not recommended however AN SYS help file states: "Aspect ratio is computed and tested for all except Emag or FLOTRAN elements. This shape measure has been reported in finite element literature for decades, and is one of the easiest ones to understand. Some analysts want to be warned about high aspect ratio so they can verify that the creation of any stretched elements was intentional. Many other analysts routinely ignore it". Unless elements are so stretched that numeric round off could become a factor (aspect ratio > 1000), aspect ratio alone has little correlation with analysis accuracy. Finite element meshes should be tailored to the physics of the given problem; i.e., fine in the direction of rapidly changing field gradients, relatively coarse in directions with less rapidly changing fields. Sometimes this calls for elements having aspect ratios of 10, 100, or in extreme cases (Examples include shell or thin coating analyses using solid elements, thermal shock "skin" stress analyses, and fluid boundary layer analyses.) Attempts to artificially restrict aspect ratio could compromise analysis quality in some cases. The aspect ratio for a triangle is computed in the following manner, using only the corner nodes of the element. A line is constructed from one node of the element to the midpoint of the opposite edge, and another through the midpoints of the other 2 edges. In general, these lines are not perpendicular to each other or to any of the element edges. 1. Rectangles are constructed centered about each of these 2 lines, with edges passing through the element edge midpoints and the triangle apex. 2. This construction is repeated using each of the other 2 corners as the apex. 3. The aspect ratio of the triangle is the ratio of the longer side to the shorter side of whichever of the 6 rectangles is most stretched, divided by the square root of

8 The best possible triangle aspect ratio, for an equilateral triangle, is 1, but it can vary upto 20 which is accepted by ANSYS. Flowchart ANAVAL, for: f Start J Read temp4.dat file Create Anarepo.txt file a = sqrt{(x 2 -x 1 ) / +(y 2 -y 1 )'+(z 2 -z 1 )') b = sqrt((x3-x 2 ) 2 +(y 3 -y2):+(z3-z 2 ) 2 ) c = sqrt((x r x 3 r+(yry 3 ) 2 +(zi-z 3 ) 2 ) peri = (a+b+c) s = peri/2 area = sqrt(s*(s-a)*(s*b)*(s-c) qfactor = 4*1.7320*AREA/(a 2 +b 2 +c 2 ) If QFACTOR < 0.3 then Write to ANAREPO.txt file X1.Y1.Z1 X2.Y2.Z2 X3.Y3.Z3 QFACTOR "BAD ELEMENT" ( Stop ) ( End Fig. 5.2 ) Annexure 5.1 gives program listing on "ANAVAL". 140

9 5.6 Case Study In this case study mainly problem is handled in two parts: 1. Testing of STL file to check how many bad quality elements are their in the file along with their quality factors. For this the program used is "ANAVAL" for the calculation of quality factor of an element. 2. Generation of db.log file so that this file can be opened into ANSYS for further analysis. For the automatic creation of db.log file "ANALYSIS" program is developed. To test above two programs the data used is organized point cloud data obtained originally from laser data. The laser data after operated under Row-Column is converted into organized data having pitch = 1mm and step over distance = 1mm. Then this data is triangulated using Matrix method and finally STL file is generated. The "temp4.dat" file is tested under Euler's formula to check its topology and then it is operated under "ANAVAL" for the calculation of quality factor or aspect ratio. To check the quality factor basic input required is temp4.dat file which is developed during triangulation process. This temp4.dat file mainly contains the information like serial number of points / nodes along with their x, y, and z co-ordinates. All these nodes are arranged in a proper sequence so that first 3 nodes form the first triangle, the next three nodes second triangle and so on. The point cloud data as shown in fig 5.5 (a) is exported in the ANSYS by creating its db.log file using plane element. Basic inputs like type of analysis, type of element, material properties are included in the db.log file, and then this file is opened in ANSYS. The boundary conditions such as restrictions and external forces are applied on the model created. In the example considered here, the left edge is restricted to move in X direction, while on every node of right side edge a force of magnitude ION is applied. Fig. 5.5 (b) shows the displacement of nodes in X direction. Fig. 5.5 (c) shows the stress acting in nodes in X direction. Similar methodology is followed on the same file, but this time the type of element is changed from plane type to shell element. Left and right edges are restricted to move in all three directions that is x, y, z direction, and a pressure of 100N/mm 2 is applied on the selected nodes near center portion. The thickness of the shell is taken to be uniform and equal to 1 mm over entire region. 141

10 The figures 5.6 shows displacement in X, Y, Z direction, stresses acting in x,y,z direction using shell elements. and The "temp4.dat" file is operated under "ANALYSIS" program to create dblog file that is data base log file. Before using analysis program user has to decide the type of element to be used. The choice is in between 2D plane stress shell element. In this case study both types of elements are used. Suitable boundary conditions, and external forces are applied, And finally displacement values of all nodes and stress acting at the nodes in x, y and z directions are obtained. Point cloud data used: RC lxl Pitch: 1 mm Step over distance: 1 mm Number of points: 6056 Number of triangles / elements: asc Views of meshed geometry and diagrams of stress - strain analysis are shown in following figures: Fig. 5.4 Point cloud data after processing under Row-Column method and then opened in Imageware 142

11 Plain Element Fig. 5.5 (a) Meshing and applying boundary conditions like displacement of nod and forces acting on nodes. 5.5 (b) Displacement of nodes in X direction ANSYS Fig. 5.5 (c) Stress acting on nodes in X direction 143

12 Shell Element Fig. 5.6 (a) Meshing and applying boundary conditions like displacement of nod and forces acting on nodes. Fig. 5.6 (b) Displacement of nodes in X direction Fig. 5.6 (c) Displacement of nodes in Y direction 144

13 Fig. 5.6 (d) Displacement of nodes in Z direction Fig. 5.6 (e) Stress acting on nodes in X direction Fig. 5.6 (f) Stress acting on nodes in Y direction 145

14 Fig. 5.6 (g) Stress acting on nodes in Z direction 5.7 Conclusion: STL file developed form the above discussed point cloud data is tested through ANAVAL program to apply the quality check for every triangulated element. The report generated through "ANAVAL" program is given below: Total number of elements: Total number of bad elements: 394 The quality of all good elements (11408) is within acceptable range from 0.3 tol, the average value being bad elements contribute only 3.3% of total elements. Depending upon the results obtained from this program, the point cloud data file can be improved by deleting some points or by making the data coarser especially when handled data is a row-column type data. In general the program developed "anaval" help to check the aspect ratio, which may be helpful for making necessary corrections, and the second program "analysis" creates a log file, which reduces the time required for building of FEA MODEL to a large extent. 146

Modelling Flat Spring Performance Using FEA

Modelling Flat Spring Performance Using FEA Blessing O Fatola, Patrick Keogh and Ben Hicks Department of Mechanical Engineering, University of Corresponding author bf223@bath.ac.uk Abstract. This paper