Help with if statements

Hello I am getting errors for a varaible I am defining in a if loop any help would be great.

The error is stopping it at no QbarA Variable defined

close all, clear all,clc
E1 = 40;
E2 = 1.8;
V12 = 0.25;
V21 = 0.01125;
G12 = 0.7;
t = 0.005;
z0 = -t*2;
z1 = -t;
z2 = 0;
z3 = t;
z4 = t*2;
theta = [0,15,90,-30];
Q11 = E1/(1-V12*V21);
Q12 = (V21*E2)/(1-V12*V21);
Q22 = E2/(1-V12*V21);
Q66 = G12;
for i = 1:4
    m = cosd(theta(i));
n = sind(theta(i));
end
Qbar11 = Q11.*m^4+2.*(Q12+2*Q66).*m^2.*n^2+Q22*n^4;
Qbar12 = (Q11+Q22-4*Q66)*m^2*n^2+Q12*(m^4+n^4);
Qbar16 = (Q11-Q12-2*Q66)*m^3*n+(Q12-Q22+2*Q66)*m*n^3;
Qbar22 = Q11.*n^4+2.*(Q12+2*Q66).*m^2.*n^2+Q22*m^4;
Qbar26 = (Q11-Q12-2*Q66)*m*n^3+(Q12-Q22+2*Q66)*m^3*n;
Qbar66 = (Q11+Q22-2*Q12-2*Q66)*m^2*n^2+Q66*(m^4+n^4);
Qbar = [Qbar11 Qbar12 Qbar16
    Qbar12 Qbar22 Qbar26
    Qbar16 Qbar26 Qbar66]
if i == 1
    QbarA = Qbar;
end
if i == 2
    QbarB = Qbar;
end
if i == 3
    QbarC = Qbar;
end
if i == 4
    QbarD = Qbar;
end
A = QbarA*t+QbarB*t+QbarC*t+QbarD*t 
B = 1/2*(QbarA*(z1^2-z0^2)+QbarB*(z2^2-z1^2)+QbarC*(z3^2-z2^2)+QbarD*(z4^2-z3^2))
D = 1/3*(QbarA*(z1^3-z0^3)+QbarB*(z2^3-z1^3)+QbarC*(z3^3-z2^3)+QbarD*(z4^3-z3^3))
Nx = 1200*4*t;
Ny = -600*4*t;
Nxy = 600*4*t;
My = 36/12;
Mx = -18/18;
Mxy = 0;
N = [Nx;Ny;Nxy]
M = [Mx;My;Mxy]

Output = [A,B;B,D]^-1*[N,M]

Answer 1

[Adam Drake]

"i" will always equal 4 because your for loop has no condition that will stop it from going to the end. Because "i" will always be 4 at the end of the loop, QbarA, QbarB, QbarC will never get set and therefore, when you get to the line where "A" is being set, you will get an error.

The following code is probably closer to what you want, but your output function will return an array mismatch error because A, B, and D are 3x3 arrays, N and M are 1x3 arrays. Would need background on your algorithm to help further.

clc, close all, clear variables

E1 = 40;
E2 = 1.8;
V12 = 0.25;
V21 = 0.01125;
G12 = 0.7;
t = 0.005;
z0 = -t*2;
z1 = -t;
z2 = 0;
z3 = t;
z4 = t*2;
theta = [0,15,90,-30];

Q11 = E1/(1-V12*V21);
Q12 = (V21*E2)/(1-V12*V21);
Q22 = E2/(1-V12*V21);
Q66 = G12;

Nx = 1200*4*t;
Ny = -600*4*t;
Nxy = 600*4*t;
My = 36/12;
Mx = -18/18;
Mxy = 0;
N = [Nx;Ny;Nxy];
M = [Mx;My;Mxy];

for i = 1:4
    m = cosd(theta(i));
    n = sind(theta(i));

    Qbar11 = Q11.*m^4+2.*(Q12+2*Q66).*m^2.*n^2+Q22*n^4;
    Qbar12 = (Q11+Q22-4*Q66)*m^2*n^2+Q12*(m^4+n^4);
    Qbar16 = (Q11-Q12-2*Q66)*m^3*n+(Q12-Q22+2*Q66)*m*n^3;
    Qbar22 = Q11.*n^4+2.*(Q12+2*Q66).*m^2.*n^2+Q22*m^4;
    Qbar26 = (Q11-Q12-2*Q66)*m*n^3+(Q12-Q22+2*Q66)*m^3*n;
    Qbar66 = (Q11+Q22-2*Q12-2*Q66)*m^2*n^2+Q66*(m^4+n^4);

    Qbar = [Qbar11 Qbar12 Qbar16
            Qbar12 Qbar22 Qbar26
            Qbar16 Qbar26 Qbar66];

    A = Qbar*t+Qbar*t+Qbar*t+Qbar*t;
    B = 1/2*(Qbar*(z1^2-z0^2)+Qbar*(z2^2-z1^2)+Qbar*(z3^2-z2^2)+Qbar*(z4^2-z3^2));
    D = 1/3*(Qbar*(z1^3-z0^3)+Qbar*(z2^3-z1^3)+Qbar*(z3^3-z2^3)+Qbar*(z4^3-z3^3));
    
    Output = [A,B;B,D]^-1.*[N,M]
end

[Robert Fields]

The code is for a composite Laminate Analysis. I am trying to calculate the midplane stresses. The layers of the composits are 0,15,90,-30 So I need the Qbar of each of those angles added up for A B and D

[Robert Fields]

You led me in the right direction with changing the for loop to contain everything thank you my new code looks like this and is doing how I hope.

close all, clear all, clc
E1 = 40*10^6; %Msi
E2 = 1.8*10^6; %Msi
V12 = 0.25;
V21 = E2/E1*V12;
G12 = 0.7*10^6; %Msi
t = 0.005; %in/ply
Xt = 230; %ksi
Xc = -180; %ksi
Yt = 7; %ksi
Yc = -325; %ksi
S = 10; %ksi
z0 = -t*2;
z1 = -t;
z2 = 0;
z3 = t;
z4 = t*2;
% After performing the calculation with to find out the build up it was
% determined it would be build up 3.
theta = [0,15,90,-30];
Q11 = E1/(1-V12*V21);
Q12 = (V21*E2)/(1-V12*V21);
Q22 = E2/(1-V12*V21);
Q66 = G12;
for i = 1:4
    m = cosd(theta(i));
    n = sind(theta(i));

Qbar11 = Q11.*m^4+2.*(Q12+2*Q66).*m^2.*n^2+Q22*n^4;
Qbar12 = (Q11+Q22-4*Q66)*m^2*n^2+Q12*(m^4+n^4);
Qbar16 = (Q11-Q12-2*Q66)*m^3*n+(Q12-Q22+2*Q66)*m*n^3;
Qbar22 = Q11.*n^4+2.*(Q12+2*Q66).*m^2.*n^2+Q22*m^4;
Qbar26 = (Q11-Q12-2*Q66)*m*n^3+(Q12-Q22+2*Q66)*m^3*n;
Qbar66 = (Q11+Q22-2*Q12-2*Q66)*m^2*n^2+Q66*(m^4+n^4);
Qbar = [Qbar11 Qbar12 Qbar16
    Qbar12 Qbar22 Qbar26
    Qbar16 Qbar26 Qbar66] 
%Displayed in this order Qbar0, Qbar15, Qbar90, Qbar-30
if i == 1
    QbarA = Qbar;
elseif i == 2
    QbarB = Qbar;
elseif i == 3
    QbarC = Qbar;
elseif i == 4
    QbarD = Qbar;
end
end
%Question 1
A = QbarA*t+QbarB*t+QbarC*t+QbarD*t %lb/in
B = 1/2*(QbarA*(z1^2-z0^2)+QbarB*(z2^2-z1^2)+QbarC*(z3^2-z2^2)+QbarD*(z4^2-z3^2))%lb
D = 1/3*(QbarA*(z1^3-z0^3)+QbarB*(z2^3-z1^3)+QbarC*(z3^3-z2^3)+QbarD*(z4^3-z3^3))%lbin
%Question 2
Nx = 1200*4*t;
Ny = -600*4*t;
Nxy = 600*4*t;
My = 36/12;
Mx = -18/18;
Mxy = 0;
N = [Nx;Ny;Nxy] %lb/in
M = [Mx;My;Mxy] %lbin/in
Output = [A,B;B,D]^-1*[N;M]