Find Ultimate Tensile Strength (max/peak of graph) given Stress and Strain Data

I have included the .xlsx file for the stress and strain values required to determine the Ultimate Tensile Stress (UTS).

I need to determine each UTS which is the maximum value of the curve and locate it on the graph. I need to plot all five UTS points on the graphs.

I have tried to use max, islocalmax, and findpeaks but I can not get the code to work.

Here is my code thus far

% READS DATA FILE
filename = 'TensileMEX-4545XYZ.xlsx';
data = xlsread(filename);
S1 = data(:,1:2);
S2 = data(:,4:5);
S3 = data(:,7:8);
S4 = data(:,10:11);
S5 = data(:,13:14);
matrix = [S1 S2 S3 S4 S5]; %Required to condense data to Nx10 matrix

matrix(matrix < 0.015) = NaN; %Ignores Values less than 0.015

% PLOTS STRESS-STRAIN CURVES
figure(1)
plot(matrix(:,1), matrix(:,2),'r')
hold on 
plot(matrix(:,3), matrix(:,4),'k')
plot(matrix(:,5), matrix(:,6),'g')
plot(matrix(:,7), matrix(:,8),'b')
plot(matrix(:,9), matrix(:,10),'m')

    title('Data Set 1, ME2016 HW3, Camila Gill')
    xlabel('Strain [%]')
    ylabel('Stress [MPa]')

% LOCATES UTS FOR EACH SAMPLE
[UStrs, UStrn] = findpeaks(matrix(:,1),matrix(:,2));

The last line is the most recent attempt, but the following error occur

>>Error using findpeaks

>>Expected X to be finite

Thank you for any help.

Answer 1

[Star Strider]

This line:

matrix(matrix < 0.015) = NaN; %Ignores Values less than 0.015

is inserting NaN values into ‘matrix’, and that is causing problems with findpeaks. The stress data are all significantly greater than that, so why not just delete that line?

[Camila Gill]

The values replaced by the NaN do not coorespond to a single/multiple lines for all the values.

Can I do this?

matrix(matrix < 0.015) = 0; %Ignores Values less than 0.015

[Star Strider]

You can, however the reason for doing that (or assigning it as NaN) is not obvious to me. The values you want (and that findpeaks returns) are significantly in excess of that value, so I seriously doubt that findpeaks will even consider it.

I would just eliminate that line. That should solve the problem.