Hello, I am currently investigating the compressive behavior of an origami structure made from 3D printed PLA. I am confused on how to determine the yield strength from the graph. What method should I use? I have read ASTM D695 standard and I'm confused on how much offset should I apply to determine yield.
Attached below is the compressive curve obtained from experiment and what ASTM D695 says on determining yield strength.
Both ASTM D694-15 and ISO 604:2002 define compressive yield strength (or stress at yield) as the first stress at which an increase in strain occurs without an increase in stress.
The stress-strain curve you plotted indicates a distinct yield point, which corresponds to the value of slightly below 1800 (unit unknown) located at a strain of ~2.7 (presumably %). However, a small curved region (so-called toe region) is seen in the initial part of the graph, so the zero-strain point must be constructed if the strain at yield is also to be determined.
At first you need to linearize the stress-strain plot and reaclculate the zero-strain.
You can roughly construct the point where stress is maximum and look at your data to find out the stress value where strain starts to change before cold drawing or breaking.
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