Minimum dimensions of beam for bending test

[Rogue 1]

I am working with composite sandwich panels (FRP skins, EPS core) and have built a rudimentary bending test machine with a hydraulic jack, force sensor and eventually a linear potentiometer for deflection.

The beam thickness is close to 4" and I can vary the width and length. They are currently 4" and 20".

The first test saw a force of ~400 lbf and deflection 1-2" (linear pot not hooked up yet) at failure, and I am curious if there is a dimensional guideline to make sure this is a good test of bending and not compression. This first test actually did look to compress the core EPS before any other failure modes although the force effector was a sharp sided block and looked to have too much of a concertation on the beam center.

I would like to eventually use these results to determine Young's Modulus for our different constructions, but haven't quite been able to make sense of the beam deflection equation to solve this question.
I'm actually a bit new to structure mechanics and am open to ideas of what to test for. Eventually I will be building a box structure to test but while getting started I've devised this test to quantify changes in base materials, adhesives and manufacturing parameters.

d_max = FL³ / 48EI

Thanks in advance!

 

[3DDave]

There is an arrangement for applying a pure moment to the sample - 4 point bending

https://en.wikipedia.org/wiki/Four-point_flexural_test

This will avoid local compression and shear contributions - you will need to measure curvature rather than simple displacement.

 

[SWComposites]

See ASTM D7249 and D7250.

 

[3DDave]

Good reference -found this because of it:

https://www.youtube.com/watch?v=Z4wOl8TEcf0

ASTM - D7249 (testing)