Measuring compressive strength
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.
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Small Particles and Bibliography
Here is the info I alluded to yesterday but waited to disclose until I had the price and talked to the company along with the bibliography of sources I've used lately.
The information I found on small particles involved strengthening epoxy with particles much smaller than silica fume aka 20nm particles. The particles are so small that unlike carbon black or silica fume, they <B>DO NOT</B> cause an increase in viscosity!
Nanoparticles provide better fracture toughness, better compressive/tensile modulus by a factor of 100 in some cases, lower shrinkage, lower coefficient of thermal expansion, less heat distortion and better surface finish and abrasion resistance. These SiO2 nanoparticles come wetted in epoxy so there is no problem with mixing or agglomeration.
These SiO2 particles are made by Nanoresins, a spinoff of Hanse-Chemie in Germany. They cost 25$ to 35$ dollars a pound and are added in quantities of no more than 10% by weight. The URL of the company is www.nanoresins.com. I have attached to this post the datasheet sent to me by the leader of the U.S. operation, Dr. Oliver Pyrlik, Technical Sales Director Nanocomposites, hanse chemie USA inc. They have graciously offered me some samples which I may accept when I can deal with them.
I may have confused some people by saying hard data in my last post but what I meant is that I wanted to find the price and talk to the company before raising hopes here. I haven't done any lab work and can't until I stop sitting here posting and go finish building my shop :)
I am approaching this problem with the idea of working smarter, not harder. I'm thinking that if we design our epoxy mix to use safe additives and admixtures and the smallest practical aggregates for the strength needed then we will likely produce parts with a nicer finish than even the commercially built machines while requiring a minimum of work. Judging by Walter's results from the composite made with just sand, it sounds like it might be reasonably easy to come up with a viable composite without huge aggregates.
<h4>Bibliography of materials I have used in my posts:</h4>
Materials Science and Engineering An Introduction
Third Edition
William D. Callister Jr.
John Wiley & Sons, Inc.
Copyright 1994
ISBN 0-471-58128-3
This is the book whose chapter 17 got me thinking about small particle reinforcement. The technique is still so cutting edge that there is little data 13 years after this book was published.
Introduction to Mechanics of Solids
Egor P. Popov
Prentice Hall
Englewood Cliffs, NJ
Copyright 1968
This book is out of print but a xerox authorized by the publisher surved as the text for most of my engineering mechanics class some years ago. I found a used copy of the book at Powells books in Portland. I've been using chapter 13 to study the process of working out deflections with Lagrangian methods using techniques like Castigliano's Theorem.
ASTM Standard D695
Standard Test Method for Compressive Properties of Rigid Plastics
Annual Book of ASTM Standards 1985 edition
This is the ASTM standard for doing compressive tests of reinforced plastics. I have used it as guidance in my quest for an improvised testing machine to determine the properties of our materials.
Polymer Composites from Nano to Macro Scale
Frederich Klaus
Springer
Copyright 1995
ISBN 0-387-24176-0
This is the book that led me to the above discussion on 20nm sol-gel silica reinforced epoxy as mentioned above. Most of the book is about unrelated things but the 10 or so pages starting from pg 92 were fascinating and said such things like "100 fold increase in modulus" with the introduction of the nano-particles. They pointed out using cure temperatures of up to 200 C. They also listed the use of CAA, Cobalt Acetylacetonate from Sachem Chemicals, and N,N Dimethyl Benzamine as catalysts in addition to the epoxy's normal hardener in the 1% to 1.5% by weight range and indicated approximately 5% strength improvements for the CAA. CAA is even safe!
Marks Standard Handbook for Mechanical Engineers
1987 Edition
McGraw Hill Inc.
I've used this for looking up things like moments and Accepted young's moduli and other misc data for back of the envelope calculations.
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Shopmaninc/US Composites Epoxy
Hi Walter et. al
I called US Composites today and they told me that their low viscosity slow curing epoxy is Reichhold 37-606. Since this is the hardener and the specs are given for an epoxy on page 3 of the data sheet, one would assume that Reichhold 37-140 is the epoxy corresponding to US Composites 63556x series epoxy where I use the x to deisignate the part number changes due to different sized containers.
This epoxy doesn't look like anything special. Data sheet is attached.