Mechanical APDL Structural Analysis Guide

nsel,all

prrfor

finish

Example 9.2: Simple Example of a Linear Perturbation Modal Analysis

/prep7

mp,ex,1,10! Simple material

mp,nuxy,1,0.3

mp,dense,1,0.5

n,1,

n,2,1

n,3,1,1

n,4,0,1

n,5,2,0

n,6,2,1

n,7,3,0

n,8,3,1

et,1,182,,,1

e,1,2,3,4

e,2,5,6,3,

e,7,8,6,5

allsel,all

d,1,all

d,4,ux

f,8,fy,-2.0! Apply a lateral point load at node 8

dlist

finish

/solution

antype,static! Static analysis

nlgeom,on! Large deformation and large strain

rescontrol,,all,1! Get restart files for all substeps

nsub,5,5,5! Use fixed time steps

solve

finish

!! Begin modal linear perturbation analysis

/solu

/com FIRST PHASE OF LINEAR PERTURBATION

antype,,restart,1,3,perturb! Restart at 1st loadstep, 3rd substep,

! from the base nonlinear static analysis

perturb,modal

solve,elform! Execute 1st phase of linear perturbation, recovering Kt of NLGEOM,on

!!

/com SECOND PHASE OF LINEAR PERTURBATION

dlist,all! Displacement boundary conditions are not deleted

flist,all! Lateral point load at node 8 has been deleted internally at SOLVE,elform

f,8,fy,-4.0! Apply new load for downstream analysis,

f,8,fx,-2.0! (downstream analysis is not shown in this simple example)

! This newly applied point load does not affect modal solution

flist

modopt,lanb,3! Solve for lowest 3 modes by using block Lanczos

mxpand,3,,,yes! Expand mode the same time

outres,esol,all

solve! Execute 2nd phase of linear perturbation: modal analysis

dlist! Verify perturbation loads again

flist

fini

/post1

file,,rstp! Use *.rstp file to review results from linear perturbation

set,list! It should list 3 eigen-modes

set,last! List stresses of the 3rd mode

esel,s,elem,,1

etable,seqv,s,eqv

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Linear Perturbation Analysis