Date: Thu, 14 Dec 2000 19:33:17 -0500 From: Blake Mantel Subject: Re: COZY: Micro from below - Hard shelling "Marc J. Zeitlin" wrote: > Blake Mantel wrote: > >> What is the current consensus on hard shelling anyway? > > Some of us like it, and some of us think it's the spawn of the devil. > There's no concensus. > > >> And has any lab testing been done? > > No, just anecdotal evidence in both directions, but nothing substantial. > There are no problems that can be linked directly to hard-shelling, but > there are certainly potential problems with it. > Useful, eh? > Marc J. Zeitlin Hello Listers the world over! I dug this little gem back out of my library files, and I am looking for some input and feedback. The reason for this is that I am going to do my senior engineering project on this topic. An analysis of structural strengths when manufactured with the standard and the "hard shell" processes. There was no definite data ever collected that i could find, just anecdotal evidence... I have Martin Hollman's "Composite Aircraft Design" book which explains some procedures and destructive testing techniques I would like to hear of some additional styles/tests/etc. I would like to work with 6" X 6" or so test pieces to minimize the materials used and have access to an Instron 4204, computerized tensile strength testing machine. What information would you find valuable in the proving or disproving the safety of hard shelling? How would you design the test pieces and testing methods? At the end of this project I will make my report available to everyone for use. Thank You, Blake Cozy Mk IV Plans #0008 -- CUM CATAPULTAE PROSCRIPTAE ERUNT TUM SOLI PROSCRIPTI CATAPULTAS HABEBUNT. (When catapults are outlawed, only outlaws will have catapults....) Triumph Tiger Motorcycle page at: http://pages.cthome.net/bmantel/ Date: Thu, 14 Dec 2000 21:04:51 -0500 From: Carl Denk Subject: Re: COZY: Micro from below - Hard shelling Structurally, I can quickly think of several modes of failure that are pertinent, there may be more, (with and without hardshelling, but this is a start: Bond of the glass to the foam Tensile - load applied perpendicular to flat surface, bot peal strength, and with load appied at the center of a symetric area. Shear - Load applied parallel to the surface, both parallel, 45 degrees, and perpendicular to the direction of major fibers if there is a difference in the weave like UNI. Also combinations of the above loading, to simulate pealing and shearing. Compression - Bond various number of plies to both sides of foam, samples short enough act like a short column (crushing of the material), and others long enough to act like a long column (sample buckles in middle). My own opinion: Spend the time to hot wire accurate cores and don't hardshell. Hardshelling is kind of a cheating way to fix inaccurate foam surface, can only serve to add weight. The lightest weight can't be when there is a coating of micro, and then applying a second layer (or straight epoxy) with the glass. Although its likely that structurally adequate IF the hardshell is prepared properly, say sanding with coarse sandpaper, this means every fractional inch can not be the as cured micro surface. Also note that during normal glassing, some of the micro does fill the glass weave, and this is lighter than pure epoxy. Also most of the filling on top the glass can and should be dry micro, which usually is lighter weight than the slurry called for on the foam. Too dry of micro applied to the foam will not properly wet out the foam, causing poor bond. Date: Fri, 15 Dec 2000 18:17:02 -0500 From: "Marc J. Zeitlin" Subject: Re: COZY: Micro from below - Hard shelling Michael Caine (who is on the COZY list, but sent this from an email address that isn't on the list, so it didn't get through) wrote: >When I was considering hard shelling for the fuselage on my Long-EZ, I made >up some small test pieces of PVC foam, about 3x3x0.7 inches. On one side I >did a standard 2-ply BID layup using the standard wet micro process. On the >other side I did the hardshelling, sanding and layup approach. I >intentionally left a little glass overhanging one edge on each side. I then >pull tested the samples to see which side "won". In 3 out of 5 cases the >hardshelled side gave up the ghost first, while in the other two the >standard layup quit first. ALL of the samples failed at the foam/micro >interface (the glass/micro interface NEVER failed). After examining the >pieces, I found that the wet micro seemed to penetrate the foam more deeply, >and hence got more of a "bite" into the foam. > >What I concluded from this little experiment was: (1) working the micro >deeper into the foam (wet or dry) gave a stronger bond, and (2) the >micro/foam interface is ALWAYS the weak link, no matter what process you >use. > >Nothing quantitative here, just some simple seat of the pants tests. My >basic recommendation is that anyone trying out a new or different process >should do some testing first. It's usually easy and quick, and in the end >means a lot more than any number of opinions (IMHO). Hey, that's why >they're called "experimental" airplanes, right?