Layman's Explanation of Dispersion Hardening
Larry,
I worked out that a 4.25 inch square E/G beam 48 inches long, with the 16GPa strength from my last calculation, should have enough strength to be within .004 deflection at the midpoint and weight about 60 lbs along with 50 lbs of z axis load for 110 lbs total.
As for the Russian abstract, that was interesting. Maybe not $43.00 worth of interesting but. . . Putting this in closer to layman's terms: as particle size decreases in graphite from about 150 times the width of a hair to the width of 1/20th of a hair, deflection in a beam made from the material goes down 33% for the same load and the ultimate load carried by the beam before it breaks is 10 times what it was before.
The mechanism is this. Imagine a block of epoxy as a bunch of tinkertoys. Each round hub has spokes coming out in several directions and the pattern of a bunch of tinker toys put together at the same angles is repetitive but, like the work of a child, it contains mistakes. These mistakes are called dislocations. If the structure contained no dislocations, aka was a perfect crystal, it would be very strong. When the epoxy is bent, these mistakes in the tinkertoy structure try to move in order to allow the structure to flex. In the tinkertoys, imagine a few missing spokes and spokes that are just barely pushed into the hubs. The effect of adding the tiny particles is like placing the tinkertoy structure into a bucket of pingpong balls. Anywhere you grab the structure and try to bend it, the hubs and spokes hit pingpong balls before they move very far. This effect is called dislocation pinning generally and in this case dispersion hardening. If bowling balls were used instead of the pingpong balls, the sticks in the tinkertoys would be too short and you would get some regions filled with globs of tinkertoys and other regions filled with bowling balls but no regions where large portions of the structure of tinkertoys was connected to itself.
In short, small particles cause keep the molecules of material from sliding past each other on the atomic scale. I would suspect that once a crack decided to form in the material that it would probably just keep going unless it hit a larger aggregate particle which was strong enough to stop it. That's why high strength concrete with pozzolans also has aggregate. Both macro and micro reinforcement are beneficial.
I do expect that the micro reinforcement is probably not needed at the level required to make the stuff turn into vaseline. If it's truly thixotropic however, vibration at the right frequency should make it thin right out.
P.S. I've written the first version of my program for doing crude beam calculations. It's in java so it is portable but I am still figuring out a way that it can be run without a hassle since it is the wrong dialect of java to run from a web browser right now.
Compressive Testing Costs: Ouch
I called plti as mentioned in my earlier post and inquired about budgetary pricing for compressive strength and modulus tests. Tests are $175 for strength and $175 for modulus and require 5 samples :drowning:
I think I'm going to hit the library and see if we can improvise something. We should be able to get strain with a dial gage so if we can come up with a good way to measure pressure, we can test it ourselves.