FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

FEM - ANSYS Classic •

•

•

•

•

Lecture 1 - Introduction: – Introduction to FEM – ANSYS Basics – Analysis phases – Geometric modeling – The first model: Beam model Lecture 2 - Preprocessor: – Geometric modeling – Specification of Element type, Real Constants, Material, Mesh – Frame systems – Truss systems – Element tables Lecture 3 - Loads: – Boundary conditions/constraints/supports – Loads – Mesh attributes, meshing – Sections Lecture 4 – 2D plane models : – 2D Plane Solid systems – Geometric modeling – Postprocessing Lecture 5 – Analysis types: – Analysis types – Modal analysis – Buckling analysis

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

2

FEM - ANSYS Workbench/CAD •

Lecture 6 – 3D Solids: – 3D solid models – Booleans – Meshing issues

•

Lecture 7 – 3D Modeling: – Operate – Import CAD – Advanced topics

•

Lecture 8 – Analysis types: – Analysis types – Postprocessing – TimeHistProc

•

Lecture 9 – Workbench basics: – Workbench basics – Geometric modeling

•

Lecture 10 – Workbench analysis: – Workbench analysis types

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

3

Solution phases Analysis Type – specify the character of the problem Define Loads – apply loads to the element model Solve – run the solution process, e.g. for linear static systems solve (Gaussian elimination) for the unknown displacements: The global stiffness Unknown displacement vector ndof x 1

-1

matrix [K]: ndof = total number of nodes x number degrees of freedom per node

[K]{D} = {R} → {D} = [K] {R} Known global stiffness matrix ndof x ndof FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Known load vector ndof x 1 Loads

4

Solution Menu The Solution menu will be either “abridged” or “unabridged,” depending on the actions you took prior to this step in your ANSYS session. The abridged menu contains only those solution options that are valid and/or recommended for modal analyses.

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

5

Solution Menu • If you are using the GUI to perform a structural static, transient, modal, or buckling analysis, you have the choice of using abridged or unabridged Solution menus: – Unabridged Solution menus list all solution options, regardless of whether it is recommended, or even possible, for you to use them in the current analysis. (If it is not possible for you to use an option in the current analysis, the option is listed but is grayed out.) – Abridged Solution menus are simpler. They list only those options that apply to the type of analysis that you are performing. For example, if you are performing a static analysis, the Modal Cyclic Sym option does not appear on the abridged Solution menu. Only those options that are valid and/or recommended for the current analysis type appear. FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

6

Loading Programme for Lesson:

• • • • • • • • • • • • • • •

Types of loads Load step Substep Time tracking Application of loads Solid-model loads Finite element loads DOF Constraints Forces (Concentrated Loads) Surface Loads Pressure Loads on Beams Body Loads Misc. Loads Multiple load steps Applying loads

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

7

Types of loads • Structural: displacements, forces, pressures, temperatures (for thermal strain), gravity • Thermal: temperatures, heat flow rates, convections, internal heat generation, infinite surface • Magnetic: magnetic potentials, magnetic flux, magnetic current segments, source current density, infinite surface • Electric: electric potentials (voltage), electric current, electric charges, charge densities, infinite surface • Fluid: velocities, pressures

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

8

Types of loads • Loads are divided into six categories: – DOF constraints – forces (concentrated loads) – surface loads – body loads – inertia loads – coupled-field loads

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

9

Applying Displacement loads

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

10

Applying Force/Moment loads

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

11

Load step • A load step is simply a configuration of loads for which a solution is obtained. In a linear static or steady-state analysis, you can use different load steps to apply different sets of loads - wind load in the first load step, gravity load in the second load step, both loads and a different support condition in the third load step, and so on. In a transient analysis, multiple load steps apply different segments of the load history curve. FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

12

Application of loads • Most loads are applied either – on the solid model (on keypoints, lines, and areas) or – on the finite element model (on nodes and elements)

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

13

Solid-model loads •

Advantages –

–

•

Solid-model loads are independent of the finite element mesh. That is, you can change the element mesh without affecting the applied loads. This allows you to make mesh modifications and conduct mesh sensitivity studies without having to reapply loads each time. The solid model usually involves fewer entities than the finite element model. Therefore, selecting solid model entities and applying loads on them is much easier, especially with graphical picking.

Disadvantages –

– – –

Elements generated by ANSYS meshing commands are in the currently active element coordinate system. Nodes generated by meshing commands use the global Cartesian coordinate system. Therefore, the solid model and the finite element model may have different coordinate systems and loading directions. Solid-model loads are not very convenient in reduced analyses, where loads are applied at master degrees of freedom. (You can define master DOF only at nodes, not at keypoints.) Applying keypoint constraints can be tricky, especially when the constraint expansion option is used. (The expansion option allows you to expand a constraint specification to all nodes between two keypoints that are connected by a line.) You cannot display all solid-model loads.

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

14

Finite element loads • Advantages – Reduced analyses present no problems, because you can apply loads directly at master nodes. – There is no need to worry about constraint expansion. You can simply select all desired nodes and specify the appropriate constraints.

• Disadvantages – Any modification of the finite element mesh invalidates the loads, requiring you to delete the previous loads and re-apply them on the new mesh. – Applying loads by graphical picking is inconvenient, unless only a few nodes or elements are involved. FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

15

DOF Constraints

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

16

DOF Constraints • A DOF constraint fixes a degree of freedom (DOF) to a known value. Examples of constraints are specified displacements and symmetry boundary conditions in a structural analysis, prescribed temperatures in a thermal analysis, and flux-parallel boundary conditions FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

17

DOF Constraints • Applying Symmetry or Antisymmetry Boundary Conditions • Transferring Constraints – To transfer constraints that have been applied to the solid model to the corresponding finite element model

• Resetting Constraints – By default, if you repeat a DOF constraint on the same degree of freedom, the new specification replaces the previous one. You can change this default to add (for accumulation) or ignore

• Scaling Constraint Values FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

18

Forces (Concentrated Loads)

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

19

Forces (Concentrated Loads) • A force is a concentrated load applied at a node in the model. Examples are forces and moments in a structural analysis, heat flow rates in a thermal analysis, and current segments in a magnetic field analysis

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

20

Forces (Concentrated Loads) • Repeating a Force – By default, if you repeat a force at the same degree of freedom, the new specification replaces the previous one. You can change this default to add (for accumulation) or ignore

• Scaling Force Values • Transferring Forces

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

21

Surface Loads

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

22

Surface Loads • A surface load is a distributed load applied over a surface. Examples are pressures in a structural analysis and convections and heat fluxes in a thermal analysis

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

23

Surface Loads • Specifying a Gradient Slope • Repeating a Surface Load – By default, if you repeat a surface load at the same surface, the new specification replaces the previous one.

• Transferring Surface Loads • Using Surface Effect Elements to Apply Loads – to apply a surface load that the element type you are using does not accept. For example, you may need to apply uniform tangential (or any non-normal or directed) pressures on structural solid elements, radiation specifications on thermal solid elements, etc. FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

24

Pressure Loads on Beams Solution > Define Loads > Apply > Structural > Pressure > On Beams Select the line

Enter 10

Press OK to finish FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

25

Body Loads

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

26

Body Loads • A body load is a volumetric or field load. Examples are temperatures and fluences in a structural analysis, heat generation rates in a thermal analysis, and current densities in a magnetic field analysis

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

27

Body Loads • Specifying Body Loads for Elements • Specifying Body Loads for Keypoints • Specifying Body Loads on Lines, Areas and Volumes • Specifying a Uniform Body Load • Repeating a Body Load Specification – By default, if you repeat a body load at the same node or same element, the new specification replaces the previous one.

• Transferring Body Loads • Scaling Body Load Values FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

28

Misc. Loads •

Inertia Loads – Inertia loads are those attributable to the inertia (mass matrix) of a body, such as gravitational acceleration, angular velocity, and angular acceleration. You use them mainly in a structural analysis

•

Coupled-Field Loads – Coupled-field loads are simply a special case of one of the above loads, where results from one analysis are used as loads in another analysis. For example, you can apply magnetic forces calculated in a magnetic field analysis as force loads in a structural analysis

• •

Axisymmetric Loads and Reactions Loads to Which the DOF Offers No Resistance – If an applied load acts on a DOF which offers no resistance to it (i.e. perfectly zero stiffness), the ANSYS program ignores the load.

•

Initial Stress Loading – Initial stress loading is only allowed in a static or full transient analysis (the analysis can be linear or nonlinear). Initial stresses can be applied only in the first load step of an analysis.

• •

Applying Loads Using TABLE Type Array Parameters Graphing or Listing the Boundary Condition Functions

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

29

Solvers •

Several methods of solving the system of simultaneous equations are available in the ANSYS program: – – – – – –

•

•

sparse direct solution frontal direct solution Jacobi Conjugate Gradient (JCG) solution Incomplete Cholesky Conjugate Gradient (ICCG) solution Preconditioned Conjugate Gradient (PCG) solution Automatic iterative solver option (ITER)

The sparse direct solver is the default solver for all analyses, except for electromagnetic analyses, analyses that include both p-elements and constraint equations, spectrum analyses, and substructuring analyses (which each use the frontal direct solver by default). In addition to these solvers, the Parallel Performance for ANSYS add-on product includes two multiprocessor solvers: – Algebraic Multigrid (AMG) solution – Distributed Domain Solver (DDS)

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

30

Solvers

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

31

Example 0110 – Cantilever beam

Objective: Display the moment curve Tasks: Obtain values in intermediate points? Create an element table? Display the moment curve? Topics: Element type, pressure load, Element table/output, list

FEM – ANSYS Classic Computational Mechanics, AAU, Esbjerg

Loads

E = 210000N/mm2 n = 0.3 L = 1000mm a = 10mm b = 10mm p = 10N/mm

32