Geof et. al,

I hit the library today and pulled ASTM D695 which is the offical test for compressive modulus and strength. This test specifies crushing a .5 x .5 x 2 inch specimen in a press and recording the force. It also specifies doing the test on 5 identical samples. ASTM D695 references this interesting document from the world war II era at NASA http://ntrs.nasa.gov/archive/nasa/ca...1993090554.pdf

I believe that you are suggesting testing a small beam. The equation for deflection at the center of the beam you described is FL^3/(48EI). For a .5 x .5 in. square ASTM specimen has I=h^4/12=.00521.

Unfortunately, the Epoxy Granite composite has different moduli in compression and tension unlike steel so a beam bending test will give you more of an average between the values than the compression value for E/G.

As for yugami's suggestion: If the pressure gage on a press is remotely accurate then that pretty much solves the problem. I was originally thinking from the perspective of how to do the gaging at minimal cost starting from the position of having nothing but a dial indicator.

Measuring the pressure with the gage and the deflection with a dial test indicator and plotting the results would work fine and probably represents the lowest cost home shop solution.

Martin,

You are dead right about the ACME screws as they are the way that the old style presses created their force. I seem to be missing something however because the actual force applied by the press is related to the deflection and young's modulus of the item being squished. Thus crushing a steel bar and then crushing the E/G bar seems unlikely to me to tell you anything about the pressure on the E/G bar.

Hmm. :idea:
If you placed the specimen on a simply supported bar that would deflect by a few thousandths at max force and crushed the whole thing in the press keeping track of the deflection of this member with a dial gage then you'd have a jury rigged strain gage sitting under the specimen and have something although you would want to make damn sure that you measured everything really carefully. The h^4 term in the moment and L^3 term in the deflection mean that the beam is sensitive to conditions.

It seems that without some kind of pressure measurement system, all you can measure is displacement with the acme screws unless you turn them with a torque wrench so you can figure out what the torque was and thus what the current stress is. I think the problem of relating acme screw torque to force is harder than the one we set out to solve but it's probably due to my own limited experience.

Finally, using brunog's numbers for compressive strength from the NIST report, it looks like pressure at failure is about 21ksi leading to about 5000lbf required on the press to fracture the ASTM specimen.

P.S. I learned some other interesting stuff in the library today about small particle reinforcement, but I'll leave everybody in suspense until I have some hard data in a day or two.