Help with data from tensile test

[Huumah]

Hi

Just got the data from a tensile strength test on a axle we did in school.

Before it broke:
http://i.imgur.com/1ddrZ7j.png

The data :

https://www.dropbox.com/s/xf5m8m8gyco561n/data.txt

column 1: the displacement in mm

column 2: the force in kNI'm supposed to plot a stress strain diagram like this http://i.imgur.com/qdoHBse.pngMy MATLAB code:

    load data.txt

    %diameter 4 mm
    d=4;
    cross_section_area=pi*d^2/4;

    Length=140;

    displacement=data(:,1);
    force=data(:,2);

    for i=1:length(displacement)

        stress(i)=force(i)/cross_section_area;
    end
    stress

    for i=1:length(force)
        strain(i)=displacement(i)/Length;
    end
    strain
    plot(strain,stress)

I'm supposed to filter the original data a bit but I have tried but i don't get nothing simular to the desired graph. Can anyone help me

Edit : Its aluminum 6082 T6

 

[nvn]

Huumah: Did your rod break suddenly, right at the end of the rod? Perhaps you had a stress concentration, because you did not provide a large fillet at the rod ends?

Aside from that, something seems amiss in your rod dimensions. If the diameter of the rod shown in your photograph is 4 mm, then its length (the small portion) cannot be 140 mm. Check the dimensions of your rod carefully. What were the exact dimensions of the small portion of your rod, before applying force to it?

Furthermore, if your rod is really 4 mm in diameter, then if it is Al 6082-T6, it would break at approximately 3.7 kN, not the values listed in your data file. Something seems amiss.

By the way, although this will not change the shape of your plot, do not forget to convert the force to Newtons (N), by multiplying your force by 1000.

Also, in post 1, you said column 1 in your data file is displacement (mm). Then why does the value in column 1 virtually never change? There appear to be three columns in your data file. Therefore, what is column 3, in your data file?