Hughes Spruce Goose

[congo]

I'm not all to sure of epoxy. I've seen boats well made using traditional methods last well over a century when epoxy compsite hulls have been known to begin falling apart within a decade.

In the classic yacht industry anyone worth their salt will use the same glue Hagar mentioned. It's been around for over a century, will create a bond stronger than the wood it's holding together and it's long term effects are well known.

On the other hand, a wood epoxy composite is only just below kevlar and carbon fibre in terms of strenght to weight. Provided it's done right and long term results do not have to be guarenteed.

You also have to be sure with epoxy that the resin and the hardener are exactly 50/50 and that it's very well mixed or strengh is significantly undermined.

I'm suprised though that epoxy was used on the Spruce Goose. I would have thought she was cold moulded or something like that.

Ok, some misconceptions here again Woodlouse.

Classic yacht design only requires glueing of the spars, traditional boats are built without glue of any sort, relying on metal fastenings and natural fibre caulking for hull integrity.

Epoxy is superior to any of the traditional glues used in the past and has made lightweight timber composite contruction possible. A wood epoxy joint will always fail in the timber, not the glue joint, as you say is the case for other glues, (which I can dispute from personal experience with the materials in question, I have seen resourcinol joints open up, probably due to the critical conditions required for a good bond using resourcinol).

Also, as you say, wood/epoxy composite is only just below the strength of epoxy/carbon/kevlar composites.....TRUE, but aren't those the strongest materials that man has yet developed for practical contruction purposes?

Epoxy has a couple of achilles heels, one is UV light, and the other is heat degradation. It's no surprise then that the Space Shuttle program has had so much trouble with the insulating tiles falling off the Shuttle, they are glued with a special epoxy and no other substance has been found to match or better this use!

Contruction grade epoxies are typically mixed at ratios in the order of 5:1 resin/hardener, and this ratio is definitely critical. Epoxy bonds are dependent on molecular bonds, any unbonded molecules serve only to weaken the structure. Mixing epoxy accurately and thoroughly is the one critical requirement in it's use. Some epoxies are mixed by weight, others by volume, brand dependent, and there is typically, (for boatbuilding applications), a 10% margin of error before the structure is compromised by errors in mixing. Good work practices dictate that there is an area in the workshop with a dedicated bench for epoxy mixing, and a responsible person in the workshop is selected who mixes the glue, and he is responsible for the quality control on that job. It's dangerous to distract this person while he is mixing, lest an error creep into the work. Special epoxy pumps are also available that dispense epoxy resin and hardener on a single pump lever with an adjustable mix ratio, these pumps are highly recommended as then anyone in the workshop can dispense the liquid epoxy accurately.

I hope I've cleared some of this up for you, as more and more epoxy/composites are replacing metal components in aircraft all the time, and for good reasons.

[Hagar]

This might have been pointed out earlier in the thread but I think the main advantage of epoxy resin adhesives over traditional glues is their gap filling properties. A perfectly formed joint using older conventional wood glues can be just as strong if not stronger than with one of the modern adhesives.  Most modern boats & glider wings using this form of laminated construction also use a glass fibre cloth covering to seal it from the elements. The glass fibre is bonded to the wood & becomes part of the whole structure making it extremely strong for its weight. I don't know when glass fibre cloth was first invented or how long it's been used for this purpose.

An extra consideration with boats or water planes is water saturation which will affect most forms of construction over a period of time to varying degrees. While epoxy resin is water resistant the wood isn't so unless the epoxy is bonded properly with the wood, deterioration will take place over a period of time. This would not have been considered so important during WWII as most aircraft only had a limited service life.

[Woodlouse2002]

[quote]Cold moulded is epoxy composite construction Woodlouse.

If an epoxy boat is falling apart it's not because of the epoxy, but rather other factors of design loading or lack of maintenance (epoxy is UV sensitive and must be kept painted if exposed to sunlight).

Here is an example of the durability of epoxy:

The Australian Gliding Federation was concerned about the aging Australian glider fleet. Most sailplanes were imported into Australia in the 1970's and were epoxy composite construction. These aircraft have been used privately and in clubs for 4 decades. No prior research had been done on the fatigue and possible lifespan of the material and it's implications on air safety. A matter of some realistic concern!

A common epoxy composite 2 seat sailplane, I believe it was a Janus, was set upon a specially designed stress testing rig at the Melbourne Institute of Technology.

The aircraft was shaken violently up and down for months, I believe several repairs had to be made to the test rig during the trials. Eventually, a small stress crack appeared near the spar root I believe, (I just can't remember the details accurately), and the cause was a

[congo]

The question is a little unfair Woodlouse, considering the fact that production has only been happening for about 40 years on aircraft, and less on boats. (with the exception of some experimental craft).

I don't need converting, I've owned a traditional wooden vessel myself for twenty years, there's not a drop of epoxy in it. But I've also used epoxy extensively and I know it's strengths and weaknesses.

I really am only just realising that epoxy was around for quite a long time, and I was amazed at the spruce goose using the material. Until just recently, the oldest use I had heard of was in early FRP glider construction, around mid to late fifties, and I have seen those gliders still in regular service here in Australia.

As I said before, problems associated with epoxy construction are design and maintenance related, (though I should have included quality control in this list).

You will find the epoxy sailplane manuals state that ALL surfaces are to be painted WHITE and that a silicone free wax polish be applied at all times. Now, if someone paints the elevator blue or red, then he shouldn't be surprised when it starts to de-laminate. Silicones in polish penetrate the paint/gelcoat finish and actually permeate the laminate, this is dangerous, insidiously so, because if at some stage a repair is required, the repaired structure could have bond failure due to the introduced silicone.

If paint is allowed to wear thin, or colors are used, UV light breaks down the epoxy in no time, severely weaking the structure. These problems are known are they are merely a maintenance issue.

Quality control is important, and as an example I'll cite a well known problem in FRP boat manufacturing. I also have been wondering if this problem is what you may be mistakenly thinking of when you are bagging the use of epoxy.

FRP (Fibreglass Reinforced Plastic) boat construction predominately uses Polyester resins and not epoxies, Polyesters are cheaper and relatively easier for novices to use, (without critical mixing ratios).

[Woodlouse2002]

The question is a little unfair Woodlouse, considering the fact that production has only been happening for about 40 years on aircraft, and less on boats. (with the exception of some experimental craft).

This is precisely my point. Epoxy Composites have not been around nearly long enough to be completely trustworthy as the long term effects are simply not known.

As for the issues of FRP (GRP?) and osmosis, that is a completely different kettle of fish and is what you get for owning a tupperware boat.

[congo]

Well, I guess epoxy in aircraft construction could be likened to aluminium in aircraft construction with the advent of WW2 fighter planes before alloy was 50 years old. Except perhaps that alloy has a known and relatively short fatigue life.

[congo]

Actually Woodlouse, you've made me reflect on