4 Point Bend Specimen & Test Fixture

I am writing a blog series about designing a partially 3d printed test machine and using it to analyze the mechanical properties of 3D printed materials. You can read about the project at the link at the bottom, any feedback or fresh ideas will be welcome!
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The rectangle is a test specimen specifically designed for a 4 point bend loading on my machine. The part 'should' require 200 lbs to break when printed at 100% infill.
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The test fixture design is also included here to reduce