T=readmatrix('tensile1_270_23.txt');
D=T(:,1);
F=T(:,2)*1000;
B=25;
A=(12.6*4.1);
strain=(D/B)+3.1710e-05;
stress=F/A+1.78295;
[idx val] = max(stress); % max value of stress in data
Thresh = val; % chose it as threshold
[TF S] = ischange(stress,'linear','Threshold',Thresh); % extract first occurance of abrupt change in data
idx = find(TF);
E = stress(idx(1))./strain(idx(1)) % Elastic (youngs) modulus
Yield_sig = stress(idx(1)) % yield stress MPa
Offset = strain(idx(1))*0.2 % offset distance @ 20% strain
plot(strain,stress,'LineWidth',2)
hold on
plot(strain(1:idx(1))+Offset,stress(1:idx(1)),'r--')
grid on
x=xlabel('strain, \epsilon');
y=ylabel('stress, \sigma (MPa)');
you can try this approach
