Subscript indices must either be real positive integers or logicals.

clear;
clc;
% For Graphite-Eboxy 
S_1m = 204000; % maximum Sigma 1 (psi)
S_2m = 9050; % maximum Sigma 2  (psi)
S_12m = 11500; % maximum Tau 12  (psi)
St_1m = 0.00888; % maximum normal strain 1 
St_2m = 0.00725; % maximum normal strain 2 
St_12m = 0.00725; % maximum shear strain 12 

% Initial Modulus :
E_1o = 23E6; % (psi)
E_2o = 1.25E6; % (psi)
G_12o = 0.975E6; % (psi)

% Initial Vlaue for Secant Modulus (0.9 * Initial Modulus):
E_1s = E_1o; % (psi)
E_2s = 0.9*E_2o; % (psi)
G_12s = 0.9*G_12o; % (psi)

% Material constant:
B1 = 0; C1 = 1; D1 = 0;
B2 = 0; C2 = 1; D2 = 0;
B3 = 0.05139; C3 = 0.067339; D3 = -0.003119;

%Fiber Orientation:
Theta = 0;
C = cosd(Theta);
S = sind(Theta);

%Total Strain Energy (psi): 
U_1m = 0.5 * S_1m * St_1m ;
U_2m = 0.5 * S_2m * St_2m ;
U_12m = 0.5 * S_12m * St_12m ;

% Tau = Tau _xy First trial:

t_xy = 0 ;
S_x = 0;
s_y = 0;
for i = 0:0.01:200  % Ry range
    
    a1 = C^2 + i*S^2;
    a2 = S^2 + i*C^2;
    a12 = S*C*(i - 1);
    t1 = 2 * t_xy * S * C;
    t2 = -t1;
    t12 = t_xy *( C^2 - S^2);
     
    Error = 1;
    U_p_total_o = 0;
    
    S_x =0;
    S_y = 0;
    while Error > 0.00000001 %To calculate the plastic strain energy
        
       C_1 = 2 * E_1s * U_1m ;
       C_2 = 2 * E_2s * U_2m ;
       C_12 = 2 * G_12s * U_12m ;
       
       b = (2*a1*t1/C_1) + (2*a2*t2/C_2)+(2*a12*t12/C_12);
       a = (a1^2/C_1) + (a2^2/C_2)+(a12^2/C_12);
       c = -1 + ((t1^2/C_1)+(t2^2/C_2)+(t12^2/C_12));
       d = sqrt (b^2 - 4*a*c) ; 
       
       % For Sigma X, Y, and txy
       S_x = (-b+d)/(2*a);  
       S_y = i * S_x ; 
        
       
       % For Sigma 1, 2, and t12
       S_1 = S_x * C^2 + S_y * S^2 + t_xy *(2 *C * S);
       S_2 = S_x * S^2 + S_y * C^2 - t_xy *(2 * C * S);
       t_12 = (-S_x * C * S) + (S_y * C * S) + t_xy * (C^2 - S^2); 
       
       % For plastic strain energy:
       U_p1 = ((S_1)^2 / 2 )*((E_1o-E_1s)/(E_1o*E_1s));
       U_p2 = ((S_2)^2 / 2 )*((E_2o-E_2s)/(E_2o*E_2s));
       U_p12 = ((t_12)^2 / 2 )*((G_12o-G_12s)/(G_12o*G_12s));
       U_p_total_n = U_p1 + U_p2 +   U_p12 ;
       
       
       %For New Secant modulus:
       E_1s = E_1o*(1- B1*(U_p_total_n)^C1 + D1*(U_p_total_n) );
       E_2s = E_2o*(1- B2*(U_p_total_n)^C2 + D2*(U_p_total_n) );
       G_12s = G_12o*(1- B3*(U_p_total_n)^C3 + D3*(U_p_total_n) );
       
       %Error Calculation: 
       
       Error = (( U_p_total_n -  U_p_total_o )/ U_p_total_n); 
       
        U_p_total_o = U_p_total_n ;  
       
    end
 
  
    S_x(i) = S_x ;
    S_y(i) = i * S_x ; 
   
end 

Array indices must be positive integers or logical values.

plot (S_x, S_y);

The error came from the last two line ( S_x(i) = S_x; and S_y(i) = S_x * i; ) I'm trying to save all the output result from For Loop but the error: "Subscript indices must either be real positive integers or logicals" and plot it, any help please ??

Answer 1

[Scott MacKenzie]

You've got some funny things going on with the S_x and S_y variables. I made a small change: building up the S_x and S_y values in each pass in vectors xResult and yResult. This, I think, is what you are after:

% For Graphite-Eboxy 
S_1m = 204000; % maximum Sigma 1 (psi)
S_2m = 9050; % maximum Sigma 2  (psi)
S_12m = 11500; % maximum Tau 12  (psi)
St_1m = 0.00888; % maximum normal strain 1 
St_2m = 0.00725; % maximum normal strain 2 
St_12m = 0.00725; % maximum shear strain 12 
% Initial Modulus :
E_1o = 23E6; % (psi)
E_2o = 1.25E6; % (psi)
G_12o = 0.975E6; % (psi)
% Initial Vlaue for Secant Modulus (0.9 * Initial Modulus):
E_1s = E_1o; % (psi)
E_2s = 0.9*E_2o; % (psi)
G_12s = 0.9*G_12o; % (psi)
% Material constant:
B1 = 0; C1 = 1; D1 = 0;
B2 = 0; C2 = 1; D2 = 0;
B3 = 0.05139; C3 = 0.067339; D3 = -0.003119;
%Fiber Orientation:
Theta = 0;
C = cosd(Theta);
S = sind(Theta);
%Total Strain Energy (psi): 
U_1m = 0.5 * S_1m * St_1m ;
U_2m = 0.5 * S_2m * St_2m ;
U_12m = 0.5 * S_12m * St_12m ;
% Tau = Tau _xy First trial:
t_xy = 0 ;

% S_x = 0;
% s_y = 0;
xResult = [];
yResult = [];
for i = 0:0.01:200  % Ry range
    
    a1 = C^2 + i*S^2;
    a2 = S^2 + i*C^2;
    a12 = S*C*(i - 1);
    t1 = 2 * t_xy * S * C;
    t2 = -t1;
    t12 = t_xy *( C^2 - S^2);
    
    Error = 1;
    U_p_total_o = 0;
    
    S_x =0;
    S_y = 0;
    while Error > 0.00000001 %To calculate the plastic strain energy
        
        C_1 = 2 * E_1s * U_1m ;
        C_2 = 2 * E_2s * U_2m ;
        C_12 = 2 * G_12s * U_12m ;
        
        b = (2*a1*t1/C_1) + (2*a2*t2/C_2)+(2*a12*t12/C_12);
        a = (a1^2/C_1) + (a2^2/C_2)+(a12^2/C_12);
        c = -1 + ((t1^2/C_1)+(t2^2/C_2)+(t12^2/C_12));
        d = sqrt (b^2 - 4*a*c) ; 
        
        % For Sigma X, Y, and txy
        S_x = (-b+d)/(2*a);  
        S_y = i * S_x ; 
        
        
        % For Sigma 1, 2, and t12
        S_1 = S_x * C^2 + S_y * S^2 + t_xy *(2 *C * S);
        S_2 = S_x * S^2 + S_y * C^2 - t_xy *(2 * C * S);
        t_12 = (-S_x * C * S) + (S_y * C * S) + t_xy * (C^2 - S^2); 
        
        % For plastic strain energy:
        U_p1 = ((S_1)^2 / 2 )*((E_1o-E_1s)/(E_1o*E_1s));
        U_p2 = ((S_2)^2 / 2 )*((E_2o-E_2s)/(E_2o*E_2s));
        U_p12 = ((t_12)^2 / 2 )*((G_12o-G_12s)/(G_12o*G_12s));
        U_p_total_n = U_p1 + U_p2 +   U_p12 ;
        
        
        %For New Secant modulus:
        E_1s = E_1o*(1- B1*(U_p_total_n)^C1 + D1*(U_p_total_n) );
        E_2s = E_2o*(1- B2*(U_p_total_n)^C2 + D2*(U_p_total_n) );
        G_12s = G_12o*(1- B3*(U_p_total_n)^C3 + D3*(U_p_total_n) );
        
        %Error Calculation: 
        
        Error = (( U_p_total_n -  U_p_total_o )/ U_p_total_n); 
        
        U_p_total_o = U_p_total_n ;  
        
    end
    
    
    xResult = [xResult, S_x];
    yResult = [yResult, i*S_y]; 
    
end 
plot (xResult, yResult);

[Ibrahim Odeh]

Thank you Scott, it is working perfectly

[Scott MacKenzie]

You're welcome. A pleasure to help.

Answer 2

[AndresVar]

In matlab arrays are indexed starting at 1 so S_x(0) is an error.

You can use another array to store the 'range' values

range = 0:0.1:20;
for idx = 1:length(range)
    i = range(idx);
    ..
    S_x(idx) = ..
end

Answer 3

[Image Analyst]

This is one of the most FA'd FAQ.

See a thorough discussion in the FAQ:

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22