Builders' Forum Archives
Re: stress crack
Posted by Terry Mcadams on Sep 10, 2004
So anyway, getting back to Michelle's question, epoxy is really brittle - the thinner it is, the more brittle and fragile it is, which is why we reinforce it with stuff like fiberglass, kevlar and plywood. Otherwise the stuff would readily break or shatter under loads and impacts. What it does do, even after we reinforce it, is develop fine, hairline cracks at areas of the boat that get high stress loads that causes the brittle epoxy to flex, like the areas around the rudder post and the foot brace bolt holes.
These "stress cracks" are not limited to plywood-core boats like CLC designs, but can appear even in the thick, heavily reinforced fiberglass hulls of large ocean-going yachts.
In my experience (more so with yachts than with plywood/glass kayaks), the cracks usually are not indicative of imminent failure of the cracked area, but are a signal that more reinforcement should eventually be added to the cracked area. There are exceptions, like areas that would cause major damage or injury if the glass failed.
Another source of cracks in plywood core boats is the cracking of the plywood, which may crack the fiberglass on it, but not always. This can happen in places where the plywood is stressed during construction, like a curved deck, or unsupported, like the flatter after deck of the LT Cheasapeakes. This type of cracking can be minimized by using more fiberglass on both sides of the plywood and by placement of structural supports like bulkheads and deck beams.
Hope this helps.
In Response to: Re: stress crack by Michelle Moran on Sep 10, 2004