Text: Building Better Products with FEA,
by V. Adams & A. Askenazi, (Read pp. 155-174)

Special Elements:

/title, Single DOF Oscillator - FEA HW 1 /prep7 !* ET,1,COMBIN14 ! Combination element = spring !* KEYOPT,1,2,2 ! 2D element, acting in UY only KEYOPT,1,3,0 !* ET,2,MASS21 ! Lumped Mass element !* KEYOPT,2,2,0 KEYOPT,2,3,2 ! 2D without rotary inertia !* R,1,1000,0.01,, ! Spring Stiffness = 1000 N/m !*              ! Damping, c = 0.01 N*s/m !* R,2,10,    ! Mass = 10 kg !* n,1     ! Create Nodes by Direct Definition n,2,,1 TYPE,1, ! Set Attributes for SPRING MAT,1, REAL,1, ESYS,0, !* e,1,2    ! Define Spring element TYPE,2,  ! Set Attributes for MASS MAT,2, REAL,2, ESYS,0, !* e,2      ! Define Mass element finish

Springs ("line" elements, i.e., point-to-point) axial or torsional

Two usages:

1. 3D, k acts along line from node-to-node
2. 1 DOF, k acts in only one DOF direction

Damper (dashpot, i.e., shock absorber) for dynamic analyses

Lumped Mass Include the mass of a component without actually modeling it in detail. (Its mass is at a single node point.)

Mass Moments of Inertia (of a complete model) Figure 4.52, p. 158

• available from Pro/E and from ANSYS
• gives volume, mass, c.g. location, 3x3 inertia tensor (matrix)
  

  ***** CENTROID, MASS, AND MASS MOMENTS OF INERTIA ***** CALCULATIONS ASSUME ELEMENT MASS AT ELEMENT CENTROID TOTAL MASS = 7.8668                MOM. OF INERTIA    MOM. OF INERTIA CENTROID      ABOUT ORIGIN       ABOUT CENTROID XC = 1.2500    IXX = .0000    IXX = .0000  YC = .00000    IYY = 16.38    IYY = 4.091  ZC = .00000    IZZ = 16.38    IZZ = 4.091                 IXY = .0000    IXY = .0000                 IYZ = .0000    IYZ = .0000                 IZX = .0000    IZX = .0000  *** MASS SUMMARY BY ELEMENT TYPE *** TYPE   MASS   1   7.86680

Rigid Elements (rigid links, MPC = multi-point constraints)

• Gives a "fixed" relative position of two or more nodes in one or more DOF directions.
• In ANSYS (FEA-2), we use coupled DOF’s (CDOF) or constraint equations (CEQN). In ANSYS 7.0, individual rigid elements MPC184 have been added.
• Useful to connect dissimilar meshes (ANSYS: CEINTF) and when element types are mixed (and different DOF’s must be controlled)

CONTACT Modeling (FEA-2) touch, slide, bounce, react between parts

NONLINEAR: ON/OFF + Friction

Gap elements (point-to-point) ANSYS: CONTAC12, 52

good for: aligned nodes, small sliding, can be a "hook"

Slide-line elements (point-to-line) contact curves CONTAC26

General contact elements (point-to surface) CONTAC48, 49

(surface-to-surface) CONTA169 - 174

simplest to use, compute intensive

Crack Tip Elements (for fracture mechanics) 1/4-point, singular

Part vs. Assembly Models (p. 167-171)

Assemblies: parts do not behave as a single, continuous entity

May involve changing contact or mechanisms

Component Contribution analysis:

Try not to start out with an assembly. Rather, isolate each part and handle the interaction between parts as boundary conditions on the one you are analyzing. The idea is: "Keep It Simple ... "

Transitional Meshing (mixing elements with different DOF’s)

Solid-to-shells or solid-to-beams or shell-to-beams

2D solids have UX, UY DOF’s, 2D beams have UX, UY, ROTZ DOF’s

Attaching different meshes:

by merging coincident nodes, overlapping elements, MPC’s, or "bonded-always" contact pairs

REMEMBER: Around the transition, results are suspect.