O metodě konečných prvkůLect_14.ppt

M. Okrouhlík

Ústav termomechaniky, AV ČR, Praha

Plzeň, 2010

Finite element analysisTransient tasks

Numerical solutions in detail

www.it.cas.cz

E:\edu_mkp_liberec_2\pdf_jpg_my_old_texts\

Numerical_methods_in_computational_mechanics\Num_methods_in_CM.pdf

Metoda centrálních diferencí

function [disn,veln,accn]=cedif(dis,diss,vel,acc,xm,xmt,xk,xc,p,h)% central difference method

% dis,vel,acc ............... displacements, velocities, accelerations% at the begining of time step ... at time t% diss ...................... displacements at time t - h% disn,veln,accn ...........corresponding quantities at the end % of time step ... at tine t + h% xm ........................ mass matrix% xmt ....................... effective mass matrix% xk ........................ stiffness matrix% xc ........................ damping matrix% p ......................... loading vector at the end of time step% h ......................... time step

% constantsa0=1/(h*h);a1=1/(2*h);a2=2*a0;a3=1/a2;

% effective loading vectorr=p-(xk-a2*xm)*dis-(a0*xm-a1*xc)*diss;% solve system of equations for displacements using lu decomposition of xmt disn=xmt\r;

% new velocities and accelerationsaccn=a0*(diss - 2*dis + disn);veln=a1*(-diss + disn);% end of cedif

SUBROUTINE CEDIF(XM,XM1,XK,DISS,DIS,DISN,VEL,ACC,P,R, 1 IMAX,NBAND,NDIM,H,EPS,IER,A0,A1,A2,A3,IMORE)C : : : : : : : DIMENSION XM(NDIM,NBAND),XM1(NDIM,NBAND),XK(NDIM,NBAND), 1 DISS(IMAX),DIS(IMAX),DISN(IMAX),VEL(IMAX),ACC(IMAX), 2 P(IMAX),R(IMAX)CC *** CASOVA INTEGRACE METODOU CENTRALNICH DIFERENCI ***CC SYSTEMU POHYBOVYCH ROVNIC TVARUC [M]{ACC}+[K]{DIS}={P} (TJ. SYSTEM BEZ TLUMENI)C SYMETRICKE PASOVE MATICE(MATICE HMOTNOSTI(POZITIVNE DEFINITNI),C REDUKOVANA EFEKTIVNI MATICE HMOTNOSTI,MATICE TUHOSTI)C JSOU usporne ULOZENYC V OBDELNIKOVYCH POLICH XM,XM1,XK O DIMENZICH NDIM*NBAND.C V PAMETI JE ULOZENA JEN HORNI POLOVINA PASU (VCETNE DIAG.).CC POZADOVANE PODPROGRAMY:C MAVBA ... NASOBENI SYMETRICKE PASOVE MATICEC VEKTOREM ZPRAVAC GRE ... RESENI SOUSTAVY LIN. ALGEBRAICKYCH ROVNICC (GAUSSOVA ELIMINACE)CC PARAMETRY PROCEDURY:CC XM(NDIM,NBAND) ...... HORNI POLOVINA PASU MATICE HMOTNOSTIC XM1(NDIM,NBAND) ..... HORNI POLOVINA PASU REDUKOVANEC EFEKTIVNI MATICE HMOTNOSTIC XK(NDIM,NBAND) ...... HORNI POLOVINA PASU MATICE TUHOSTIC DISS(IMAX) .......... POSUVY V CASE T-HC DIS (IMAX) POSUVY V CASE TC DISN(IMAX) .......... POSUVY V CASE T+HC VEL(IMAX) ........... RYCHLOSTI V CASE TC ACC(IMAX) ........... ZRYCHLENI V CASE TC P(IMAX) ............. VEKTOR ZATEZUJICICH SIL (V CASE T)C R(IMAX) ............. VEKTOR EFEKTIVNICH ZATEZUJICICH SILC IMAX ................ POCET NEZNAMYCHC NBAND ............... POLOVICNI SIRKA PASU (VCETNE DIAG.)C NDIM ................ RADKOVY ROZMER MATIC V HLAVNIM PROGRAMUC H ................... INTEGRACNI KROKC EPS ................. TOLERANCE (VIZ PROCEDURA GRE)C IER ................. CHYBOVY PARAMETR (VIZ PROCEDURA GRE)C A0,A1,A2,A3 ......... KONSTANTY METODY CENTRALNICH DIFERENCIC IMORE ............... = 0 POCITA POUZE POSUVY V CASE T+HC = JINA HODNOTA POCITA TAKE RYCHLOSTIC A ZRYCHLENI V CASE T+HCC **************************************************************

CC VEKTOR EFEKTIVNICH ZATEZUJICICH SIL V CASE TC {R}={P}-[K]{DIS}+A2*[M]{DIS}-A0*[M]{DISS}C CALL MAVBA(XK,DIS,VEL,IMAX,NDIM,NBAND) CALL MAVBA(XM,DIS,ACC,IMAX,NDIM,NBAND) CALL MAVBA(XM,DISS, R ,IMAX,NDIM,NBAND)C DO 10 I=1,IMAX10 R(I)=P(I)-VEL(I)+A2*ACC(I)-A0*R(I)CC POSUVY V CASE T+HC CALL GRE(XM1,R,DISN,IMAX,NDIM,NBAND,DET,EPS,IER,2)C IF(IMORE .EQ. 0) GO TO 30CC VYPOCTI TAKE RYCHLOSTI A ZRYCHLENIC DO 20 I=1,IMAX VEL(I)=A1*(-DISS(I)+DISN(I))20 ACC(I)=A0*(DISS(I)-2.*DIS(I)+DISN(I))C30 DO 40 I=1,IMAX DISS(I)=DIS(I)40 DIS(I)=DISN(I) RETURN END

Metoda lineárního zrychlení

Newmark

function [disn,veln,accn]=newmd(beta,gama,dis,vel,acc,xm,xd,xk,p,h)

% Newmark integration method

%

% beta, gama ................ coefficients

% dis,vel,acc ............... displacements, velocities, accelerations

% at the begining of time step

% disn,veln,accn ............ corresponding quantities at the end

% of time step

% xm,xd ..................... mass and damping matrices

% xk ........................ effective rigidity matrix

% p ......................... loading vector at the end of time step

% h ......................... time step

%

% constants

a1=1/(beta*h*h);

a2=1/(beta*h);

a3=1/(2*beta)-1;

a4=(1-gama)*h;

a5=gama*h;

a1d=gama/(beta*h);

a2d=gama/beta-1;

a3d=0.5*h*(gama/beta-2);

% effective loading vector

r=p+xm*(a1*dis+a2*vel+a3*acc)+xd*(a1d*dis+a2d*vel+a3d*acc);

% solve system of equations for displacements using lu decomposition of xk

disn=xk\r;

% new velocities and accelerations

accn=a1*(disn-dis)-a2*vel-a3*acc;

veln=vel+a4*acc+a5*accn;

% end of newmd

SUBROUTINE NEWM2(BETA,GAMA,DIS,VEL,ACC,F,NF,CKR,CMR,NDIM,

1 NBAND,H,EPS,IER,DISS,VELS,ACCS,P,G)

C

DIMENSION DIS(NF),VEL(NF),ACC(NF),F(NF),CKR(NDIM,NBAND),

1 CMR(NDIM,NBAND),DISS(NF),VELS(NF),ACCS(NF),

2 P(NF),G(NF)

C

C ***DIMENZIE VEKTOROV DISS,VELS,ACCS,G,P SA ROVNAJU***

C ***(2*POCTU UZLOV) DISKRETIZOVANEJ OBLASTI ***

C

C ***TATO PROCEDURA INTEGRUJE SUSTAVU POHYBOVYCH ROVNIC***

C ***NEWMARKOVOU METODOU S PARAMETRAMI BETA,GAMA ***

C

C ***VYZNAM PARAMETROV:***

C BETA,GAMA.......... PARAMETRE NEWMARKOVEJ METODY

C DIS(NF) .......... NA VSTUPE-VEKTOR POSUVOV V CASE T

C NA VYSTUPE-VEKTOR POSUVOV V CASE T+H

C VEL(NF) .......... NA VSTUPE-VEKTOR RYCHLOSTI V CASE T

C NA VYSTUPE-VEKTOR RYCHLOSTI V CASE T+H

C ACC(NF) .......... NA VSTUPE-VEKTOR ZRYCHLENI V CASE T

C NA VYSTUPE-VEKTOR ZRYCHLENI V CASE T+H

C NF .......... POCET NEZNAMYCH JEDNEHO TYPU

C CKR(NF,NBAND) ..... OBDELNIKOVA MATICE,OBSAHUJICI HORNI

C TROJUHELNIKOVU CAST (VCETNE DIAGONALY)

C EFEKTIVNI REDUKOVANE MATICE TUHOSTI.

C T.J. [KRED]=[K]+A1*[M] PO PRUCHODU

C GAUSSOVOU PROCEUROU GRE S KEY=1

C CMR(NF,NBAND) ..... OBDLZNIKOVA MATICA,KTORA OBSAHUJE HORNU

C TROJUHOLNIKOVU CAST (VCITANE DIAGONALY)

C SYMETRICKEJ,PASOVEJ,POZIT. DEFINITNEJ

C MATICE HMOTNOSTI.

C NBAND .......... POLOVICNA SIRKA PASU VCITANE DIAGONALY

C NDIM .......... RIADKOVY ROZMER MATIC CKR,CMR,DEKLAROVANY

C V HLAVNOM PROGRAME.

C F(NF) .......... VEKTOR ZATAZENIA V CASE (T+H)

C H .......... INTEGRACNY KROK

C DISS(NF),VELS(NF),ACCS(NF) ..... HODNOTY Z PREDCHOZIHO KROKU

C P(NF),G(NF) ....... PRACOVNI VEKTORY

A1=1./(BETA*H*H)

A2=1./(BETA*H)

A3=1./(BETA*2.)-1.

A4=(1.-GAMA)*H

A5=GAMA*H

DO 10 I=1,NF

DISS(I)=DIS(I)

VELS(I)=VEL(I)

10 ACCS(I)=ACC(I)

DO 40 I=1,NF

40 G(I)=A1*DISS(I)+A2*VELS(I)+A3*ACCS(I)

C WRITE(1,900) A1,A2,A3,A4,A5

C900 FORMAT(" NEWM1 ",5G15.6)

C WRITE(6,9010) BETA,GAMA,H,EPS,NF,NBAND,NDIM,IER

C9010 FORMAT(" BETA,GAMA,H,EPS:",4G15.6/

C 1 " NF,NBAND,NDIM,IER:",4I6)

C WRITE(1,901) G

C901 FORMAT(" G PRED MAVBA"/(5G15.6))

CALL MAVBA(CMR,G,P,NF,NDIM,NBAND)

C DO 45 I=1,NF

C IF(P(I) .LT. 1.E-7) P(I)=0.

C45 CONTINUE

C WRITE(1,902) P

C902 FORMAT(" P PO MAVBA"/(5G15.6))

DO 50 I=1,NF

50 G(I)=F(I)+P(I)

C WRITE(1,903) G

C903 FORMAT(" VEKTOR G PRED GRE"/(5G15.6))

C WRITE(6,919)

C919 FORMAT(" CKR PRED GRE")

C DO 920 I=1,NF

C WRITE(6,904) I,(CKR(I,J),J=1,NBAND)

C904 FORMAT(I6/(5G15.6))

C920 CONTINUE

C

IER=0

DET=0.

CALL GRE(CKR,G,DIS,NF,NDIM,NBAND,DET,EPS,IER,2)

C WRITE(6,906) DIS

C906 FORMAT(" DIS PO GRE"/(5G15.6))

C

DO 60 I=1,NF

60 ACC(I)=A1*(DIS(I)-DISS(I))-A2*VELS(I)-A3*ACCS(I)

C

DO 70 I=1,NF

70 VEL(I)=VELS(I)+A4*ACCS(I)+A5*ACC(I)

RETURN

END

Houbolt method

FINITE ELEMENT PROCEDURES

Klaus-Jürgen Bathe

Professor of Mechanical Engineering

Massachusetts Institute of Technology

1996 by Prentice-Hall, Inc.

A Simon & Schuster Company

Englewood Cliffs, New Jersey 07632

ISBN 0-13-301458-4

Houbolt procedure in Matlabfunction [disn,veln,accn] = ...

houbolt(dis,vel,acc, diss,vels,accs, disss, velss, accss, xm, xd, xkh, p, h);

% Houbolt integration method

% disn, veln, accn new displacement, velocities, acceleration at t+h

% dis, vel, acc displacement, velocities, acceleration at t

% diss, vels, accs displacement, velocities, acceleration at t-h

% disss, velss, accss displacement, velocities, acceleration at t-2*h

% xm, xd mass and damping matrices

% xkh Houbolt effective stiffness matrix

% p loading vector at time t+h

% h integration step

% effective loading vector

r=p+xm*(5*dis-4*diss+disss)/h^2+xd*(3*dis-1.5*diss+disss/3)/h;

% new displacements

disn = xkh\r;

% new velocities and accelerations

veln = (11*disn/6 - 3*dis + 3*diss/2 - disss/3)/h;

accn = (2*disn - 5*dis + 4*diss - disss)/h^2;

% end of houbolt

Let’s recall dispersion

1D continuum is a non-dispersivemedium, it has infinite number of frequencies, phase velocity is constantregardless of frequency.

Discretized model is dispersive,it has finite number of frequencies,velocity depends on frequency, spectrum is bounded,there are cut-offs.

overestimated consistentFrequency (velocity) is with mass matrix.

underestimated diagonal

L1 element

Dispersive properties of a uniform mesh(plane stress) assembled of equilateral elements, full integrationconsistent and diagonal mass matrices

wavenumberHow many elements to a wavelenght

Direction of wave propagation

Dispersion effects depend alsoon the direction of wave propagation

Artificial (false) anisotropy

Time space discretization errors

Correct determination of time step

lmin … length of the smallest elementc … speed of propagationtmin = lmin/c … time through the smallest elementhmts … how many time steps to go through the smallest elementh = tmin/hmts … suitable time step

hmts < 1 … high frequency components are filtered out, implicit domainhmts = 1 … stability limit for explicit methods … 2/omegamax,

where omegamax = max(eig(M,K))hmts = 2 … my choicehmts > 2 … the high frequency components, which are wrong, due to time

and space dispersion, are integrated ‘correctly’

LS Dyna usesh = 0.9*tmin as a default

Newmark and central differences vs. analytical solution

Rázové zatížení tenké tyče

Newmark vs. Houbolt _1

0 0.5 1-0.5

0

0.5

1

1.5eps t = 1.6

0 0.5 10.4

0.6

0.8

1

1.2

1.4dis

0 0.5 1-0.5

0

0.5

1

1.5vel

L1 cons 100 elem Houbolt (red)0 0.5 1

-100

-50

0

50

100acc

Newmark (green), h= 0.005, gamma=0.5

Newmark vs. Houbolt _2

0 100 200 300 400-0.01

0

0.01

0.02

0.03

history of displacement (in time steps)

Newmark (green), h= 0.005, gamma=0.5

0 100 200 300 400-1

-0.5

0

0.5

1

history of velocity (in time steps)0 100 200 300 400

-100

-50

0

50

100

history of acceleration (in time steps)

0 100 200 300 400-3

-2

-1

0

1node = 101 L1 cons 100 elem Houbolt (red)

history of strains (in time steps)

Newmark vs. Houbolt _3

0 50 100 150 200 250 300 3500

100

200

300L1 cons 100 elem Houbolt (red) Newmark (green), h= 0.005, gamma=0.5

kine

tic e

nerg

y

0 50 100 150 200 250 300 3500

100

200

pote

ntia

l ene

rgy

0 50 100 150 200 250 300 3500

200

400

sum

of e

nerg

ies

J:\edu_mkp_liberec_2\tele_bk_2009_09_15_education_cd_jaderna_2005\11_computational aspects\A simple 1D study in Matlab_3.doc

part 1 part 2

S

L = length of rod

P(t)1 2 kmax

A simple 1D study in Matlab

To clarify the basic ideas of finite element (FE) computation, the relations between mechanical quantities and to show the behaviour of elastic stress waves in rods a simple FE program, utilizing 1D rod elements, was concieved. The program is based on assumptions of small displacement theory and linear elastic material response.

11

11, **

0

kkkl

ES

21

12,

6**0 mmm

Slq1

element length

q2

local matrices 2 by 2

global matrix imax by imax

imax = kmax + 1

Compare central differences vs. Newmark

J:\edu_mkp_liberec_2\mtl_prog\matlab_prog_2010\mtlstep\barl1nc% barl1nc is a matlab program calculating the propagation of a strees

% wave in a bar. The previous version is linbadh.m

% Newmark and central difference methods are compared

% strains, displacements, velocities and accelerations

% along the bar are ploted for a given time

%

% time history of displacements, velocities and accelerations

% of a chosen mode is recorded

%

% main variables

% xm, xk, xd.............. global mass, rigidity and damping matrices

% xme,xke ................ local mass and rigidity matrices

% imax ................... number of generalized displacements

% lmax ................... number of local degrees of freedom

% kmax ................... number of elements

% h ...................... step of integration

% ityp ................... type of loading

% = 1 .... Heaviside pulse

% = 2 .... rectangular pulse

% = 3 .... sine pulse

% timp ................... length of pulse

% delta .................. type of mass matrix

% = 2/3 .... consistent

% = 1 ...... diagonal

% = 0.8 .... improved

% gama ................... Newmark artificial damping parametr

% tinc ................... time increment for plotting

% tmax ................... maximum time

% ck, cm ................. damping coeficients

kmax=100 %input('number of elements=? (max 100) ');

if kmax>200 , disp('linbad err. kmax>100'); break; end

h=0.005; %input('integration step=? ');

timp=0.25; %input('length of pulse=? ');

delta= 1; %input('mass matrix'(2/3-cons) or (1-diag) or (0.8-improved)');

ityp=menu('Type of loading','heavi','rect','sin');

gama=0.5; %input('gama for Newmark ');

ck=0; %input('damping coef. for rigidity matrix ');

cm=0; %input('damping coef. for mass matrix ');

tinc=0.1; %input('time increment for plotting ');

tmax=0.5; %input('tmax=? ');

Play with

mass matrix formulation,

time step

number of elements

conditional and unconditional stability

numerical dumping

Compare Houbolt vs. Newmark

J:\edu_mkp_liberec_2\mtl_prog\matlab_prog_2010\mtlstep\barl1nhe4.m

% barl1nhe

% semantics: BAR - L1 1D elements - Newmark - Houbolt - Energy

%

% This is a matlab program calculating the propagation

% of stress waves in a bar modelled by L1 elements

% for three types of loading pulse.

% Different types of right-hand side constraint are assumed.

% Strains, displacements, velocities and accelerations

% along the bar are ploted for a given time.

% Strains, displacements, velocities and accelerations

% for a given node are ploted as a function of time.

%

% Newmark and Houbolt methods are compared.

%

% Also potential kinetic and total energies are computed

% at each time step and plotted at the end.

%

% main variables

% xm, xk, xd.............. global mass, rigidity and damping matrices

% xme,xke ................ local mass and rigidity matrices

% imax ................... number of generalized displacements

% lmax ................... number of local degrees of freedom

% kmax ................... number of elements

% h ...................... step of integration

% ityp ................... type of loading

% = 1 .... Heaviside pulse

% = 2 .... rectangular pulse

% = 3 .... sine pulse

% timp ................... length of pulse

% delta .................. type of mass matrix

% = 2/3 .... consistent

% = 1 ...... diagonal

% = 0.8 .... improved

% gama ................... Newmark artificial damping parametr

% tinc ................... time increment for plotting

% tmax ................... maximum time

% ck, cm ................. damping coeficients

Play with

mass matrix formulation,

time step

number of elements

conditional and unconditional stability

numerical dumping