# Extracting data on boundary

**URL:** https://community.freefem.org/t/extracting-data-on-boundary/1469
**Category:** General Discussion
**Created:** [February 3, 2022, 5:30pm UTC](https://community.freefem.org/t/extracting-data-on-boundary/1469 "2022-02-03T17:30:25Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![Ajinks](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/ajinks/32/816_2.png) [@Ajinks](https://community.freefem.org/u/Ajinks)
#### Post date: [February 3, 2022, 5:30pm UTC](https://community.freefem.org/t/extracting-data-on-boundary/1469/1 "2022-02-03T17:30:26Z")

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Dear all,

I am solving a simple mechanical problem. I am able to solve the problem. I want to get the reactions at the fixed boundary. I tried but was not able to get the values correct. Can anyone tell me how to get reactions/stresses on the nodes?

```auto
// Solving simple mechanical problem

//Generating 2D mesh

int lower = 1;
int upper = 2;
int right = 3;
int left = 4; 

border B(t=0,1){ x=0+t; y=0; label= lower; }
border S1(t=0,1){x= 1;y=0+t; label= right;}
border T(t=0,1){ x=1-t; y=1; label= upper; }
border S2(t=0,1){x= 0;y=1-t; label= left;}

int n=2;

plot(B(n) + S1(n) +T(n) +S2(n), wait=true);
mesh Omega = buildmesh(B(n) + S1(n) +T(n) +S2(n) );
plot(Omega,fill=1, cmm="Discritized Mesh",wait=true);
//---------------------------------------------------------------------------
// Finite element function space
fespace Displacement(Omega, P1); // linear shape functions 
Displacement ux, uy, delux, deluy;

fespace Stress(Omega, P0); // values to store
Stress sigmaxx, sigmayy, sigmaxy, Tx, Ty, Fx; // To be checked

// Material Properties
real lambda = 121.15e3;
real mu = 80.77e3; 
real E= mu*(3*lambda+2*mu)/(lambda+mu);
real nuu = lambda/2/(lambda+mu);

// Constitutive realations
real sqrt2=sqrt(2.);
macro epsilon(ux,uy) [dx(ux),dy(uy),(dy(ux)+dx(uy))/sqrt2] //EOM
macro div(ux,uy) (dx(ux)+dy(uy)) //EOM
macro C E/(1-nuu^2)*[[1, nuu, 0],
                     [nuu, 1, 0],
                     [0, 0, (1-nuu)/2]] //EOM
macro Grad(s) [dx(s),dy(s)] //EOM

macro sigma(ux,uy) lambda*div(ux,uy)*[1.0, 1.0, 0.0]+ 2*mu*epsilon(ux,uy) //EOM  

macro sigma1(ux,uy) [lambda*div(ux,uy)+2*mu*epsilon(ux,uy)[0],lambda*div(ux,uy)+2*mu*epsilon(ux,uy)[1], 
                     2*mu*epsilon(ux,uy)[0]] //EOM

// Applying loads

real ue=0.5;

problem mechanical([ux, uy],[delux, deluy]) =
        int2d(Omega)((C*epsilon(delux,deluy))'*epsilon(ux,uy)) 
      + on(left,ux=0,uy=0)
      + on(right,ux=ue)
;

//--------------------------------------------------------------------------------------
// solving problem

mechanical;

sigmaxx = sigma1(ux,uy)[0];
sigmayy = sigma1(ux,uy)[1];
sigmaxy = sigma1(ux,uy)[2];

plot(sigmaxx, cmm="Stresses", wait=true, fill=1);

Tx =0;
Ty =0;

Tx = int1d(Omega,left)(sigmaxx*N.x + sigmaxy*N.y);
Ty = sigmaxy*N.x + sigmayy*N.y;

plot(Tx, cmm="traction-x", wait=true, value=1, fill=1);
plot(Ty, cmm="traction-y", wait=true, value=1, fill=1);

Fx = int1d(Omega,left)(sigmaxx*N.x + sigmaxy*N.y);
//Fx = int1d(Omega,left)(sigmaxx*N.x + sigmaxy*N.y);

ofstream gg("data.dat");
gg << Tx[] <<endl;

real sum=0;

ofstream ff("values.dat");

for(int i=0;i<100;i++) {
	// x, y, Sxx, Syy, Sxy
		real yline = 0. + 1*i/100.;
		real xline = 1*i/100.;
        sum = sum + Fx(xline,yline);
	}

    ff << "Net reaction ::" << sum <<endl;

plot(sigmaxx, cmm="Stresses", wait=true, fill=1);

cout<< "Solution Completed" << endl;

```

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<div class="post-metadata">

### Author: ![vlaude](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/vlaude/32/939_2.png) [@vlaude](https://community.freefem.org/u/vlaude)
#### Post date: [February 6, 2022, 2:07pm UTC](https://community.freefem.org/t/extracting-data-on-boundary/1469/2 "2022-02-06T14:07:24Z")

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Lines 75 and 81 you assign a scalar value from `int1d(...)` to finite element space variables `Tx` and `Fx`. Is it intentional?  
Furthermore you use P0 elements for stresses and forces, so they have constant value inside each element (only one degree of freedom).