I need to create a cicle in which, for a specific value, it solves one set of equation or another. It doesn't compute.

Lcz = 10; %Contact lenght [mm]
delta0 = 25; %Softening Lenght [mm]
delta_c = 50; %Decohesion Lenght [mm]
sigma0 = 120; %Softening Tension [MPa]
E = 70000; %Young's Modulus [MPa]
h = 6; %Section Height [mm]
b = 25; %Section Width [mm]
I = (Lcz*h^3)/12; %Inertia Momentum [mm^4]
L = 200; %Beam Lenght [mm]
l == (L-Lcz); %Crack Tip [mm]

Unrecognized function or variable 'l'.

P = 100; %Shear Force at the tip [N]
A = (2*Lcz*sigma0)/(E*I*delta0);
B = -(2*Lcz*sigma0)/(E*I*(delta_c-delta0));
D = (Lcz*sigma0*delta_c)/(E*I*(delta_c-delta0));
syms v(s) 
for delta = 40
    if delta <= delta0
        c1 = v(0) == 0;
        v1 = diff(v,s);
        v2 = diff(v,s,2);
        v3 = diff(v,s,3);
        c2 = v1(0) == 0; 
        c3 = v(l) == delta0/2;
        c4 = v1(l) == 0;
        v(s) = dsolve(diff(v,s,4) == -A*v,c1,c2,c3,c4);
        
    elseif (delta > delta0) && (delta <= delta_c)
        c1 = v(0) == 0;
        v1 = diff(v,s);
        v2 = diff(v,s,2);
        v3 = diff(v,s,3);
        c2 = v1(0) == 0; 
        c3 = v(l) == delta0/2;
        c4 = v1(l) == 0;
        v(s) = dsolve(diff(v,s,4) == (-D-B*v),c1,c2,c3,c4);
        
    end
end
s = [0:10:L];
displacements = v(s)/(E*I)
fplot(s,displacements,"b -o");
xlabel('Beam Position');
ylabel('Displacements');

[EDOARDO GELMI]

I put the variable in a symbolic representation but i doesn't work anyway

Answer 1

[Torsten]

Works for me. Maybe you use an older MATLAB release ?

Lcz = 10; %Contact lenght [mm]
delta0 = 25; %Softening Lenght [mm]
delta_c = 50; %Decohesion Lenght [mm]
sigma0 = 120; %Softening Tension [MPa]
E = 70000; %Young's Modulus [MPa]
h = 6; %Section Height [mm]
b = 25; %Section Width [mm]
I = (Lcz*h^3)/12; %Inertia Momentum [mm^4]
L = 200; %Beam Lenght [mm]
l = (L-Lcz); %Crack Tip [mm]
P = 100; %Shear Force at the tip [N]
A = (2*Lcz*sigma0)/(E*I*delta0);
B = -(2*Lcz*sigma0)/(E*I*(delta_c-delta0));
D = (Lcz*sigma0*delta_c)/(E*I*(delta_c-delta0));
syms v(s) 
for delta = 40
    if delta <= delta0
        c1 = v(0) == 0;
        v1 = diff(v,s);
        v2 = diff(v,s,2);
        v3 = diff(v,s,3);
        c2 = v1(0) == 0; 
        c3 = v(l) == delta0/2;
        c4 = v1(l) == 0;
        v(s) = dsolve(diff(v,s,4) == -A*v,c1,c2,c3,c4);
        
    elseif (delta > delta0) && (delta <= delta_c)
        c1 = v(0) == 0;
        v1 = diff(v,s);
        v2 = diff(v,s,2);
        v3 = diff(v,s,3);
        c2 = v1(0) == 0; 
        c3 = v(l) == delta0/2;
        c4 = v1(l) == 0;
        v(s) = dsolve(diff(v,s,4) == (-D-B*v),c1,c2,c3,c4);
        
    end
end
figure(1)
displacements = v/(E*I);
fplot(displacements,[0 L])
xlabel('Beam Position');
ylabel('Displacements');

●

figure(2)
displacements = matlabFunction(v/(E*I));
s = 0:10:L;
plot(s,displacements(s),'b -o')
xlabel('Beam Position');
ylabel('Displacements');

[EDOARDO GELMI]

The version isn't the latest but it's quite recent, i try to fix just like you did and i'll let you know

Thanks!

[EDOARDO GELMI]

Now it works, thank you so much. Can you explain what was wrong about my code?

[Torsten]

l = (L-Lcz); %Crack Tip [mm]
fplot(displacements,[0 L])

instead of

l == (L-Lcz); %Crack Tip [mm]
fplot(s,displacements,"b -o");

[EDOARDO GELMI]

Thank you so much, now i understand the issue