Original code (with disps removed):
K1 = 0;
a = 5;
n = 30;
P0 = 100;
c = 3;
for i = 1:30
s = cos(((2*i-1)*pi)/(2*n));
x = s*a;
P = P0*(exp((-0.5)*(x/c)^2))*(1-(x/c)^2);
K1 = K1+(sqrt(pi*a)*(1/n)*P*(1+s));
figure(1)
plot(x,K1)
grid
title('K1 as a function of crack length')
end
You're plotting 30 lines, but each line contains only one point and the lines don't have any data marker. A single point with no marker doesn't show up. Also, each line replaces the previous line plotted.
To fix these things, you could include a data marker in your plot calls and call hold on to keep all the lines:
K1 = 0;
a = 5;
n = 30;
P0 = 100;
c = 3;
for i = 1:30
s = cos(((2*i-1)*pi)/(2*n));
x = s*a;
P = P0*(exp((-0.5)*(x/c)^2))*(1-(x/c)^2);
K1 = K1+(sqrt(pi*a)*(1/n)*P*(1+s));
figure(1)
plot(x,K1,'.') % '.' data marker
hold on % use hold on to preserve existing lines
grid
title('K1 as a function of crack length')
end
However, it's better (and I imagine more like what's intended) to do a vectorized calculation for x and K1 and plot them all at once:
% K1 = 0; % no longer needed
a = 5;
n = 30;
P0 = 100;
c = 3;
i = 1:30;
s = cos(((2*i-1)*pi)/(2*n));
x = s*a;
P = P0*(exp((-0.5)*(x/c).^2)).*(1-(x/c).^2); % use element-wise operations (.*, .^) to calculate all values at once
K1 = cumsum(sqrt(pi*a)*(1/n)*P.*(1+s)); % use cumsum (cumulative sum) to add the sequence of K1 values
figure() % (making a new figure this time)
plot(x,K1) % no data marker (but you could still use one if you want)
grid
title('K1 as a function of crack length')
