Question for Jacques (plywood composite strength)

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jacquesmm
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Re: Question for Jacques (plywood composite strength)

Post by jacquesmm » Sat Mar 30, 2019 11:06 am

Greene is very good reading but this discussion reminds me of something.
Many years ago, I did read somewhere that Boeing had tested laminates as produced in the shop and depending on the crew and time of the day, the sample properties varied by a factor of 5. I could not believe it but since I was at that time employed by a large company and had to test some new products anyway, I made the workers fabricate a large number of samples over several weeks. They were sent to a lab and the results varied by a factor of almost 2! Take out extremes, you still have a 1.5 variation between laminators with the Monday morning sample probably the worse.
All that was with consistent materials.
That is not all: do your calculations and then decide if you will use one layer of DB120 or 2? That will be your choice, your 5 decimal precision is blown away.
Then think that some of the builders will produce an excellent job done with quality materials and others will be sloppy. You will introduce a large safety factor.

Considering all that, the Dave Gerr book is more than enough for those who want to calculate their own panels.

For those who want to learn more about composites, Read Eric Greene:
http://ericgreeneassociates.com/publications.html
His book is very complete and you will see where some of the ideas I post here come from.
Another book that I love is by Burt Rutan. Many years ago, he published a shop manual. It was included in some of his plans but never formally published. I have a copy.
Then there is this:
http://macnaughtongroup.com/macnaughton ... e_book.htm
You can use his sheated strip/epoxy/glass scantlings for epoxy plywood.


Jacques Mertens - Designer
http://bateau.com

fallguy1000
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Re: Question for Jacques (plywood composite strength)

Post by fallguy1000 » Sat Mar 30, 2019 11:16 am

My neighbor has a Rutan Var Ez in his pole barn. He got so nervous about laminating variances; he scrapped the project.

My project has been plagued with laminating issues. My best results are max vac pressure, but this causes microvoids.

I honestly have not been able to dial in wet bagging and am now convinced infusion is the only way to fly. This is perhaps straying a bit...

Some of the better work has been the hand laminating. Sure there is air here n there..

Anyhow, the part that bothers me the most is boat building is a bit artsy.
My boat build is here -------->

viewtopic.php?f=12&t=62495

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jacquesmm
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Re: Question for Jacques (plywood composite strength)

Post by jacquesmm » Sat Mar 30, 2019 12:06 pm

fallguy1000 wrote:
Sat Mar 30, 2019 11:16 am
My neighbor has a Rutan Var Ez in his pole barn. He got so nervous about laminating variances; he scrapped the project.

My project has been plagued with laminating issues. My best results are max vac pressure, but this causes microvoids.

I honestly have not been able to dial in wet bagging and am now convinced infusion is the only way to fly. This is perhaps straying a bit...

Some of the better work has been the hand laminating. Sure there is air here n there..

Anyhow, the part that bothers me the most is boat building is a bit artsy.
I agree. I did my share of infusion and vacuum bagging, went to practical seminars from the Composite Fabricators Assoc, tested methods in production and in the BBC lab (our shop). I decided not to specify it for my designs. It can always be done as an option but I prefer a good reliable hand layup than a so and so infused part. Compared to the total weight and "strength" of a boat, the differences are extremely small. Let's stick to the KISS principle.
If somebody knows what he is doing, infusion is great. It is very clean and there is less sanding.
Jacques Mertens - Designer
http://bateau.com

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