Multilinear Kinematic Plasticity created from a stress-strain graph

I was sent a stress-strain graph of a brittle metal tested in an Instron machine. It looked like this.

I have matlab and downloaded Grabit, a utility that makes digitizing data from a graph very easy. Here is what that looked like:

Now that I had the curve digitized, I could convert the Stress and Strain into True Stress and Plastic Strain so I could enter values in a material model using Multilinear Kinematic Plasticity. The formulas to convert are shown below:

E = 13,000 ksi

True Strain = ln(1+Strain)   where ln is the natural logarithm function.

True Stress = Stress(1+Strain)

Elastic Strain = True Stress/E

Plastic Strain = True Strain - Elastic Strain

I used a spreadsheet to correct the offset in the origin of the graph to put it back on zero.

The first two columns came from the matlab Grabit utility.

The last two columns are copy paste into the Multlinear Plasticity material model, except for the first line.

The first first line is the yield stress. This was the material model used for the thread fracture analysis.

 

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