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PostPosted: August 22nd, 2007, 9:17 am 
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Location: Belleville, ON
OK folks,

I tracked down the data from Epoxyworks with the strengths for various cedar glass epoxy layups. They were done with West epoxy, in 12" squares and then cured fo 60 days, and weighed carefully before being tested on an MTS machine.

The deflection data is how far the sample was flexed at the point of failure.

The table I have attached is the same data as in Epoxyworks #10, pg 5. But I've also added a couple calculated columns (shaded) and sorted the data slightly differently.

Image

The things I think are of key importance are the 3/16" strip and single 6 oz glass layup and dual 4 oz glass layup as compared to the 1/4" strip and single layer of 6 oz... The strengths of the two 6 oz combinations is so close as to be negligible in difference. However the 3/16" strips give a significantly lighter weight for the same strength. Using two layers of 4 oz with 3/16" strips gives greater strength and still weighs slightly less than the "standard" 1/4" and 6oz.
Food for thought for builders.

If you plan on using your hull lightly then 3/16" and 4 oz is as strong as and significantly lighter than 1/4" and 4 oz.

Further 3/16" inch thickness seems to be the sweet spot in terms of strip thickness for strength and weight. Even for higher strengths requirements. Its only when the requirements become "extreme" in strength or stiffness, that thicker strips gain anything and that requires going to 3/8" strips to exceed the capability of the 3/16" strips.

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PostPosted: August 22nd, 2007, 2:58 pm 
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Very interesting indeed!!! ....and somewhat counterintuitive from the point of view of beam strength. I would have expected the changes in beam depth to be the dominant factor. It would be interesting to know just how the samples failed. It almost seems as though the cedar was failing in shear and the 'glass/epoxy remained well within their strength limits. If the failure was indeed in shear then perhaps we ought to be looking closely at the shear strength of the various species suitable for strip construction.

bob

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PostPosted: August 22nd, 2007, 3:31 pm 
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I think you're right in that in the mid-range of thicknesses the failures are in shear within the cedar, or between the cedar and the glass. That is the parts acted more independently in the 1/4 and 5/16 thick strips. But more like a complete system with the 3/16" and 3/8" strips. Maybe even better with 1/8" strips. But I've never gotten around to testing that out.

Also the glass used in the testing was E-glass not the higher modulus and strength S-glass. S-glass is more costly (usually) and harder to get, but definitely improves the properties over E-glass. I found that with legwork and a willingness to shop around I could get S-glass in larger quantities for about the same price per yard as E-glass in small lengths.

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PostPosted: August 23rd, 2007, 8:48 am 
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Rapt,

Some further questions.

Where the table lists "1 layer n oz." does this mean a single layer on one side of the cedar or a single layer on each side? My immediate assumption was that it meant a single layer on each side which would be typical for building a canoe but on reflection realized that Gougeon Bros. markets for a lot of uses, some of which could be interested in results from samples with glass on only one side. If it means a single layer on one side only then canoe builders would be interested only in the "2 layers ? oz." data.

Does the article mention whether the 'glass weave was filled to a level surface? The answer to that would be of interest to those, myself included, who are interested in the weight of a layup as done by the pros at Gougeon Bros. A builder could be disappointed if he or she assumed that it meant "filled to a level surface" and wound up with a boat much heavier than anticipated.

A question more of curiosity than utility: Does the article mention the rate at which the load was applied? The answer would indicate whether we are looking at strengths for transient loads such as striking a submerged obstacle or sustained loads such as supporting a seat directly on the shell rather than at the gunwale.

I looked at the EpoxyWorks pages on the West System site and found that back issues are available only to #12 :cry:

b

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PostPosted: August 23rd, 2007, 9:26 am 
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The layups were intended to meet boatbuilding requirements.

As such 1 layer x oz means, one layer on each side. They don't specify if its filled weave or not. So that's not so clear.

But I do know that I can achieve between 50 and 55% resin by weight layups without too much difficulty. I pre-weigh my epoxy based on the glass weight, for 50% ratio before I start. I apply it in smallish batches with a squeegee only. I set aside an additional amount (10% of the 50% weight) in case I need it to finish. I've only ever used all of it once.

In my vacuum bagging slightly better than 50% was fairly easily achieved in smallish parts (about 1 sq ft.) Again pre-weighed at 50% but some was taken off in the bleed ply layers.

The rate was of force ramp was to quote the article of "fairly short duration"... Which I take to mean it was dynamic as would be seen in striking an object or spanning waves etc... Failures were reported as being loud and dramatic, bangs and puffs of dust...

The website provides some back issues and online information. The Number 10 issue (Winter 1998) was largely about strip construction of boats. The website only shows going back to Issue 12.

However in issue 23 there is an article on cost and weight of panels than might be of interest. As well as one on making strongbacks (for molds, but also applicable to boat building.

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