# Gloss vs Sanded: Not just Cosmetics? *PIC*

Kevin: Not sure I agree with the conclusion. I think the stress diagram explains why cracks propagate. Regardless of the thickness of the “gel” the crack will reach the laminate. The thickness may define the time it takes to get to the laminate not if or whether it will get to the laminate. By the way…did your fin experiment work? Newbs

Kevin: Not sure I agree with the conclusion. I think the stress diagram > explains why cracks propagate. Regardless of the thickness of the > “gel” the crack will reach the laminate. The thickness may > define the time it takes to get to the laminate not if or whether it will > get to the laminate.>>> By the way…did your fin experiment work?>>> Newbs Here’s Inglis formula, (it is crude but handy) st = s ( 1 + 2 (L/r)^1/2 ) where, st is the stress in the region of the crack tip s is the stress in the material far from the crack L is the depth of the crack r is the radius of the tip of the crack and (L/r)^1/2 is the square root of L/r So lets consider two (hypothetical) cases, where only the depth of the crack changes, that is the s and r really don’t change. st1 = s ( 1 + 2 (L1/r)^1/2 ) and st2 = s ( 1 + 2 (L2/r)^1/2 ) then, after a little algebra st1/st2 = ( (r^1/2)/2 + (L1)^1/2 )/( (r^1/2)/2 + (L2)^1/2 ) But r is very small compared to L (the square roots may not be, but what the hell… ) So roughly, we have st1/st2 ~ (L1/L2)^1/2 Lets say, I really slop on the gel coat, say it winds up being 1/32 in thick on one board (L1 = 1/32 in), and on my next board (same shape etc…) it winds up being only 1/64 in thick (L2 = 1/64 in). Then, st1/st2 ~ ((1/32)/(1/64))^1/2 = (64/32)^1/2 = (2)^1/2 = 1.4 or the stress at the tip of a crack in board 1 is possibly 40% greater than in board 2. (These are crude ball park number to show the kind of differences.) Now the ‘thin’ gel coat board crack will hit the laminate much sooner than the thick gel coat board and stop, having at its tip a given stress. (The mechanics of fracture change at the laminate layer.) But the thick one progresses and eventually reaches the laminate, but when it does it will have a much greater stress concentration at the tip, and is more likely, or at least you increase the likelyhood of the crack progressing right into the laminate. … About my fin… Its coming along fine. Regretably, the fellow who owns my shaping house (basically a shack with two rooms) is in the process of selling the place. Actually, the shack is worthless, its the land that has value. And the value of land in my little corner of the world has gone through the roof. The locals are all taking part in the windfall (I think out of the fear that it might be temporary) - its a real feeding frenzy. Virtually all of the towns in the area seem to be in the process of re-zoning commercial to residential, and want no part of me… ‘waste land’ is now ‘rustic’. There won’t be an industry left, just a lot of police, mega-homes and fences. So at the moment all extra projects have been slowed down a bit, as I am trying to put together some alternatives. My main thrust has been finishing and selling the blanks which I have… and there’s just one left now. Nevertheless, the fin was laid up, cut and sanded, and I hope to pour the silicone sometime next week. My final board will have priority though. Still this hasn’t stoped me from consider another solution… next post.

The mechanism governing fracture in composites varies, but the following is believed to describe what is happening in fiberglass. It should be pointed out that this mechanism is in the glass/resin layer. (The gel and gloss coat, if sufficiently thick, fractures via the mechanism as illustrated in the prior post.) Basically the crack progresses through the material until it is in the vicinity of a fiber. The stress concentration ahead of the crack is very high, see prior post, and the weak interface between glass-fiber and resin breaks prior to the arrival of the crack. When the crack finally hits the fractured area, it stops (or has an increased likelihood of stopping) because of the geometry of the fractured area, i.e. you might say that the radius of the tip of the crack has been sufficiently increased to reduce the stress concentration, see prior posts for Inglis equation. In fact what appears to happen in fiberglass is that the fracture, once it hits a fiber, changes direction and travels along the fiber until it hits another fiber headed in another direction. All this fracturing is why dings appear to be white, or whitish. Curiously, and this may be hard to believe, the strength difference of cracked fiberglass is not much different from the strength of uncracked fiberglass, … uncracked fiberglass is more resilient or pliable though.

but 40% of a very small number is still a very small number. In the big picture, I’m not sure that the gel and/or gloss was intended to provide that much additional strength to the laminate. It’s more a filler to smooth out the irregularities of the lam and to make it shine…

but 40% of a very small number is still a very small number. In the big > picture, I’m not sure that the gel and/or gloss was intended to provide > that much additional strength to the laminate. It’s more a filler to > smooth out the irregularities of the lam and to make it shine… I agree about gel/gloss adding little strength to the structure. But a 40% increase in stress is kind of hefty (if its true) and will definately make an existing crack migrate that much farther. Once the crack does hit the laminate layer things do change however, see Composite post in this thread. But at the laminate layer you still have to deal with the increased stress concentration. The point is that by using excess resin you are amplifying the stress concentrations at the laminate layer unnecessarily. The resin doesn’t seem to play any great role in strengthening the structure, but it does play an important role in toughing the composite (See Composite and Definitions post. Tough and strong are not the same thing; tough refering to the ability of the material to handle fractures, strong refering to the ultimate breaking stress.) All this doesn’t make your point any less significant however, the gel coat definately works the way you mention. But if what I have said is true, then using as little resin as possible (in general) will make for a stronger (and lighter!) board. So heavy does not necessarily equate with strength in surfboards… less is more. Kevin

I agree about gel/gloss adding little strength to the structure. But a 40% > increase in stress is kind of hefty (if its true) and will definately make > an existing crack migrate that much farther. Once the crack does hit the > laminate layer things do change however, see Composite post in this > thread. But at the laminate layer you still have to deal with the > increased stress concentration. The point is that by using excess resin > you are amplifying the stress concentrations at the laminate layer > unnecessarily.>>> The resin doesn’t seem to play any great role in strengthening the > structure, but it does play an important role in toughing the composite > (See Composite and Definitions post. Tough and strong are not the same > thing; tough refering to the ability of the material to handle fractures, > strong refering to the ultimate breaking stress.)>>> All this doesn’t make your point any less significant however, the gel > coat definately works the way you mention. But if what I have said is > true, then using as little resin as possible (in general) will make for a > stronger (and lighter!) board.>>> So heavy does not necessarily equate with strength in surfboards… less > is more.>>> Kevin That`s right, Kevin! For instance, instead of a thick, double six ounce, think a lean resin lamination using 3, four ounce layers… higher glass content, less resin.

That`s right, Kevin! For instance, instead of a thick, double six ounce, > think a lean resin lamination using 3, four ounce layers… higher glass > content, less resin. This IS the next logical step! … and it shall be so on my next board. Thank you, Kevin