Sample Problems > Applications > Stress

{ PLASTICITY.PDE

This is a moving-mesh model of large displacement plasticity.

It uses a pseudo-time evolution to relax into its final configuration.

The "time" behavior is not intended to model the actual temporal behavior of such a loaded bar.

The Elasticity is at first strongly diffused to avoid erratic hopping between elastic and plastic states.

Once the shape is established, we decrease the diffusion to focus in on the transition surface.

}

TITLE 'Beam, Shear-Loaded at End'

SELECT

regrid=off ! Don't allow mesh refinement. It confuses the elasticity transition

prefer_stability ! allow Newton steps as necessary for convergence

order=2 ! use quadratic FEM interpolation

VARIABLES

u(1e-4)

v(1e-3)

xm= move(x)

ym= move(y)

E(10e9)

DEFINITIONS

L=1.0

h=0.2

mu=0.3

s_pl=1e9 { Stress limit for elasticity }

E0 =200e9

C=E/(1-mu^2) G=E/(2*(1+mu))

uv=vector(u,v) uvm=magnitude( uv)

ex=dx(u) ey=dy(v) exy=dx(v)+ dy(u)

!>> Mises for rest elasticity determines where the elasticity switches.

C0=E0/(1-mu^2) G0=E0/(2*(1+mu))

sx0=(C0*(ex+ mu*ey)) sy0=(C0*(mu*ex+ ey)) sxy0=(G0*exy)

ez0=-mu*(sx0+ sy0)/E0

mises0=sqrt( 0.5*( (sx0-sy0)^2+ sx0^2+ sy0^2)+ 3*sxy0^2)

! E1 = SWAGE(mises0-s_pl, E0, E0/20, s_pl/10)

E1 = if(mises0 < s_pl) then E0 else E0/20

sx=C*(ex+ mu*ey) sy=C*(mu*ex+ ey) sxy=G*exy

ez=-mu*(sx+ sy)/E

p_ang=0.5* arctan( 2*sxy/(sx-sy) ) { Radians }

sxp0=(sx+sy)/2+ (sx-sy)/2*cos(2*p_ang)+ sxy*sin(2*p_ang)

syp0=(sx+sy)/2- (sx-sy)/2*cos(2*p_ang)- sxy*sin(2*p_ang)

{ Test for highest algebraic value: }

p_angl= if sxp0>syp0 then p_ang else p_ang+ pi/2

sxp= if sxp0>syp0 then sxp0 else syp0 { sx' }

syp= if sxp0>syp0 then syp0 else sxp0 { sy' }

p_angle=p_angl*180/pi { Degrees }

mises=sqrt( 0.5*( (sx-sy)^2+ sx^2+ sy^2)+ 3*sxy^2)

energy_d=(1/2)*( sx*ex+ sy*ey+ sxy*exy) { Energy density }

! report the plastic distribution

plastic= if mises0>1e9 then 1 else 0

! start out with a heavily smeared distribution of E,

! then gradually narrow the transition by reducing the smear coefficient

smear = (h/(t+2))^2

endload = -6e7*min(t,1)

endpoint = POINT(L,0)

INITIAL VALUES

E=E0

EQUATIONS

! instantaneous evolution of displacement

u: dx( sx)+ dy( sxy)=0

v: dx( sxy)+ dy( sy)=0

! mesh motion follows displacement

xm: dt(xm) = dt(u)

ym: dt(ym) = dt(v)

! relax the effective elasticity towards the computed value

E: dt(E) = smear*div(grad(E)) + 10*(E1 - E)

BOUNDARIES

region 'steel'

mesh_spacing = h/20*(1+3*x) ! denser mesh near mount

start 'outer' (0,-h/2)

LABEL 'bottom' ! label a piece of the boundary

load(u)=0 load(v)=0 line to (L,-h/2)

ENDLABEL 'bottom'

load(u)=0 load(v)=endload line to(endpoint) to (L,h/2)

LABEL 'top' ! label another piece of the boundary

load(u)=0 load(v)=0 line to (0,h/2)

ENDLABEL 'top'

value(u)=0 value(v)=0 velocity(xm)=0 velocity(ym)=0

mesh_spacing=h/40

line to close

FRONT (mises0-s_pl, s_pl/10)

resolve(E)

TIME 0 by 1e-5 to 5

MONITORS

for cycle=10

contour(E) painted as "Effective Elasticity"

contour(E1) painted as "Computed Elasticity"

contour( mises0) painted

contour(plastic) painted

PLOTS

for cycle=50

grid( x, y)

contour( u) contour(dt(u))

contour( v) contour(dt(v))

contour(E) painted as "Effective Elasticity"

contour(E1) painted as "Computed Elasticity"

contour( mises0) painted

elevation(mises0) on 'top'

contour( plastic) painted

contour( sxy) painted

elevation(y, h/2+v) on 'top' as "Validate Displacement (V::Y)" ! verify that mesh and displacement agree

elevation(y, -h/2+v) on 'bottom' as "Validate Displacement (V::Y)"

history( sxy) at (endpoint)

history( sxy) at (0,h/2)

history(endload)

history(dt(v)) at (endpoint)

history(deltat) LOG

history(smear)

END