Mechanical Engineering 101

[GeneWeber]

Hi Folks,

I could use some basic mechanical engineering help. I’d like to understand how to calculate the failure strength and bending characteristics of a composite “beam” when it is used in a cantilevered application, and also in an application where it is supported at both ends. (My apology if I accidentally use any terms improperly, I’m a CS, EE guy).

The composite beam would be constructed of a core material for which I know compressive and shear strength. The beam would be faced with carbon fiber/epoxy laminate for which I believe I can get compression and tension strength numbers. Are these characteristics, in addition to the physical dimensions of the beam sufficient to calculate the beam’s bending characteristics and breaking point? I would think the adhesion (peel strength?) of the laminate to the core would also be important. What else do I need to know? What are the formulas to do the calculations once I have all the needed information? If you want to point me to a primer text on this subject, that would be great.

Thanks & Best Regards,

Gene

[C. Beaty]

Gene, as I’m sure you know, the basic formulas for beams are contained in “Marks” but that won’t answer all your questions about composite structures.

A good primer is “Laminar Aircraft Structures” by Alex Strojnix, a professor of physics at Arizona State. EAA may stock this book or you can contact the author directly. Other titles in the series are “Laminar Aircraft Design “and” Laminar Aircraft Technologies, all worth the money.

With composite beams, the usual core materials, foams; offer negligible shear strength so shear webs must be incorporated. Honeycomb and balsa cores can carry considerable shear loads.

[Dean_Dolph]

Gene, I'm not sure this guy can help you with the mechanical side of things but when it comes to composites you would be hard pressed to find anyone more knowledgable. His name is Gary Hunter and I posted info about him once before on Norm's conference.

He is the owner of Gary Hunter Composites based in Houston TX. and is a former Shell answer man for composites. His job at that time was to take calls from anyone around the world, including hobbists, who needed help with composites. He has personal building experience by constructing a Longeze and is the crew chief for Bruce Bohannon's Exxon Tiger time to altitude record holder. He is also a EAA technical advisor and gives seminars at the major EAA Fy-ins.

He went into business for himself when Shell downsized a couple of years ago. I'm not sure how busy he is these days but his email address is gluegaru@earthlink.net and home phone is 281-277-7767.

Don't tell him I sent ya!

[GeneWeber]

Hi Chuck,

Thanks. “Marks’ Standard Handbook For Mechanical Engineers” has been mention before on Norm's forum. I considered purchasing a copy, but it is $295 new at Amazon (I'll look for it used ;) ). I’ll gladly part with the $$ if it’s a book that is comprehendible by a neophyte to ME. I’d hate to spend the money just to have it sit on my shelf and make me appear smart. I did find the Strojnik books on line, and they are pretty reasonable.

UPDATE: Sorry, that was a CD ROM version for $295. The paper version is $118 new on Amazon. (I’ll still probably look at used).

The core being considered is Dupont Korex honeycomb. Here is the link to the datasheet in case you’re interested. Korex Datasheet (http://www.hexcelcomposites.com/NR/rdonlyres/eialhbg72x6wg3rln2qhhibu4u525kjvgxbcmxowzbz52prq4k wke4hr2r4b7saozx47a2725yyc7qsqeayr7lp7drb/HexWeb_Korex.pdf)

Hi Dean,

Thanks. You had given me Gary Hunter’s contact information about a year or so ago when I asked a question about Kevlar. That’s probably the posing you’re thinking of. I did email him, and he got right back to me and was helpful. I think I posted his email. Perhaps it’s time to email him again.

Regards,

Gene

[C. Beaty]

Gene, I find “Marks” nearly indispensable. It’s a compilation of charts, tables, formulas, etc. If you can find an earlier edition at an affordable price, go for it. Basic mechanical engineering doesn’t change.

I am somewhat familiar with DuPont plastic honeycomb cores. Difficult to shape if you need something other stock thickness. One way is to fill with water, freeze and saw or machine to shape while still frozen.

Balsa is an easier core material to work with. Grain must run crosswise to flanges. Density is about 10 lb./ft^3 and shear strength across the grain is very high.

Aircraft Spruce sells 2 x 4 planks that can be sawn crosswise.

Marine suppliers sell scrim backed, vertical grain balsa sheets in various thicknesses.

[PW_Plack]

Dean,

"...when it comes to composites you would be hard pressed..."

Only if you're laminated. Sorry...couldn't resist!

Carry on. I appreciate reading and learning from you guys.

[raghu]

I realize I am treading dangerous ground when I contradict master Beaty, but here I go:

In a sandwich beam the facings ( fibre glass, carbon fibre, etc.) take the bending loads while the core material ( foam, balsa, etc.) transfers the shear loads from one facings to another so that both facings act through a common neutral axis ( typically the centroid of the core) . I suppose an approximation would be to ignore the core and calculate the bending stress with the usual My/I formula, and calculate the shear strength ignoring the facings and just considering the core. I cannot remember the formulas from the top of my head but essentialy it is a variation of the usual bending equation my/I with the the addition the various youngs moduli ( core and facings).

Here are a couple of links that may give more information-

howard.engr.siu.edu/staff2/abrate/NSFATE/chapte~2.pdf

www.msm.cam.ac.uk/mmc/research/steelsheet/ sandwichbase/principlesofsandwiches.htm

In terms of books, the Strojnik books are very good and worth a read ( I have all three of them) however, as far as I can recall they dont give the specific formula you require though he talks of the various failing modes- buckling, wrinkling etc.. You can find all this with a design procedure in Bhrun's book on aerospace structures ( an incredible reference for design info. albeit poorly organized) and alternatively also in Martin Hollman's book on composite construction ( Usualy his books promise a lot more than they deliver and I do not recommend them, but this one does have the specific formula with a worked example in it.)

Finally, it is not that the calculations that are tricky ( high school algebra), however the variation in quality intrinsic to the hand layup procedure mandates that you would have to test a few samples and just use the calculations only as a guide ( with sufficient safety factor) to design.

cheers
Raghu..

[eruttan]

I can find no referance to a "Bhrun" with an aerospace book on the web. Can you give more information?

I also cant find LAMINAR AIRCRAFT DESIGN by Alex Strojnik
I can find a LOW POWER LAMINAR AIRCRAFT DESIGN by Alex Strojnik
http://www.aircraftspruce.com/catalog/bvpages/strojnik.php
Are these the same as you mentioned chuck?

Thanks

[raghu]

Well I mispelled, here is the title,
Analysis and Design of Flight Vehicle Structures
by E. F. Bruhn

This book has been out of print for a while but you can still find it used. Also, in the context of this discussion if you are looking primarily for composite information, Bruhn is not the best source( loads of design data for metal), however it does have a section on composite design and some generic design procedures but you will not find composite material properties data ( though lots of metal data though). I mentioned it because the formulas Gene wanted are there. The Hollman books are better for composite specific information.

cheers
Raghu..

[eruttan]

any info on the Strojnik books?

[C. Beaty]

In a sandwich beam the facings ( fibre glass, carbon fibre, etc) take the bending loads….

I think you misspoke Raghu. Perfectly supported facings (flanges, sparcaps, etc), are subjected only to axial loads.

Yes, Eric, those are the Strojnix books to which I was referring. The Aircraft Spruce prices look high though. I’ll bet you can find them used.

[eruttan]

Great price from the source

http://www.sailplanehomebuilders.com/ads-misc.htm

"Low Power Laminar Aircraft Design (aerodynamics), "Low Power Laminar Aircraft Technology", "Low Power Laminar Aircraft Structures". $27 ea. plus postage and handling $2, Priority
$4. Overseas $4 (Airmail $10). Cirila Strojnik, 2337 E. Manhatton, Tempe, AZ 85282

[GeneWeber]

Hello Raghu,

Thanks for the links and additions suggestions.
Finally, it is not that the calculations that are tricky ( high school algebra), however the variation in quality intrinsic to the hand layup procedure mandates that you would have to test a few samples and just use the calculations only as a guide ( with sufficient safety factor) to design.
Yes, agreed. The calculations are to make some intelligent choices as to what material combinations and physical dimensions look promising and are worth testing.

Regards,

Gene

[GeneWeber]

Hi Chuck & Raghu,

I purchased Mark’s used for $75. Cool book, which I’m sure I will refer to many times. I couldn’t find a copy of “Analysis and Design of Flight Vehicle Structures” for less than $140 (used), but I did find that the PA InterLibrary loan program lets me borrow books from various colleges through my local library, so I got a one month loan from Penn State. The section on “Sandwich Construction and Design” should be very helpful. “Low Power Laminar Aircraft Design” is on order.

Thanks for the excellent recommendations.

Regards,

Gene

[Vance]

Hi Gene, I am a destructive testing enthusiast. It gets a little tricky with composits because they are more affected by the enviroment and the passage of time. Designing a test regimen takes some thought but I have found it to be time well spent. Thank You, Vance

[eruttan]

It gets a little tricky with composites because they are more affected by the environment and the passage of time

Other than temperature extremes melting epoxy or UV not blocked with the correct paint I understand composites are immune to environmental factors. Is this correct or did I miss something?

[Dean_Dolph]

Eric, see my original reply to Gene at the start of this thread for the guy, Gary Hunter, who can answer these types of questions.

He has the data from the test bars made from the various Shell epoxies and resins. I believer these test bars were subjected to tensile, and compression tests before and after environmental/aging test chamber exposure, but I don't think they were tested in torsion. I also believe they were all flat bars and not composite structures but the test data should give one an idea of what to expect.

For those that will be at the EAA SWRFI in New Braunfels TX. 5/14 & 5/15, Gary will probably give a seminar on composites.

[Vance]

It has been my experance that all materials change over time. In my limited experance with composits the change is more dificult to predict. I have often traced the challanges to improper manufacture.

I am a 4130 welded tube enthusiast but even this material has arived at my shop work hardened from its jorney across country. It is also very sensitive to poor welding processes.

I have had experance with kevlar changing over time even though it was inside a building with a fairly stable tempeture.

When I raced motorcycles we replaced the helmets on a regular basis per the manufacture's recomendation. Before you say that was so they could sell more helmets they used to replace them at no cost. Bell Motorsports said that both the liner and the shell changed over time.

That doesn't mean that all compisits are unstable, only that it is something to consider when you are betting your life on a part continuing to meet design specifications.

Fatigue life (cycle life) is a very important consideration for all parts made of every material.

I only mention this aspect of composits because my realitive inexperance makes me afraid of the unknown. I have not had good luck finding people who are composit enthusists shareing the pitfalls.

Thank You, Vance

[GeneWeber]

Hi Folks,

I did exchange email with Gary Hunter about using honeycomb core. In summary, he said that people who typically use that core use a prepreg AND an adhesive cloth. The cloth is adhesive embedded in a thin scrim fabric, and is placed between the prepreg and honeycomb. It apparently wicks the epoxy up the edges of the honeycomb structure to make a solid bond that will resist delamination.

Regards,

Gene