the neutral axis

General Discussion
101

Bert,

In regards to this Neutral Surface of Infinite Thickness and the placement of internal springboards,

would you recommend limiting the size (width, length) or shape of the springboard in order to effectively place it well within or as close as possible to the NSIT? From your experience, can a small spring board (or boards) effectively placed do more good than a larger one that may come close to the NSIT in some areas but is way off in other areas?

Also, further up you have remarked that of the two general construction designs we are contrasting here (horizontal springboard compsand vs. no stringer compsand) that one is more fit for small wave tricks and the other for gouges and powerful cutties, etc. It would seem to me that a board designed to bend a ways before springback would only work in smaller waves, but could produce some radical results under the right conditions. Plus, foot placement isn’t as much of a issue on a 5’6" slop biscut either. Hollow or very light stringerless boards have the ability to rapidly react and adjust to forces exerted both by the rider and by the water, but these adjustments need to happen more quickly and without much deformation. In such a design a springboard is just added weight that won’t carry any load until your skins and lam have failed and your board is snapping. Am I on the right track? Thanks a million Bert.

Tranny

P.S. if anyone thinks that NSIT is a lame acronym or description for what we are talking about

feel free to kill it. I’m just a newbie, no ego no enemy

102

Did you never see adhesive failure in the bondline? Depends on which resin is used with which foam, and the artisan’s craft.

Wouldn’t it be swell for the resin to resist buckling in compression as well as the fiber resists fracture in tension.

Perhaps the bending loads could be supported by a tough, springy rail that encompasses the neutral surface.

test article

on the seventh day, there was rest

103

Ah, day after tomorrow is my seventh day. Taking a break from the weekend of 47 degree water for some hot powder turns and time well spent with a few international coeds. Can’t wait…

Wouldn’t it be swell for the resin to resist buckling in compression as well as the fiber resists fracture in tension?

That would be swell. When we put resin and fiber on decks and bottoms and all over rails and then expect them to share stress loads equally and efficiently and provide impact resistance, we’re joking ourselves.

Perhaps the bending loads could be supported by a tough, springy rail that encompasses the neutral surface.

Sounds like this could be an equation for more efficient load sharing between deck, bottom and rail. How would you attach your rail? Springy composition?

I think load sharing is a concept that we can begin relating to our discussion of

tension, compression and the neutral surface. As you have pointed out test_article, in most boards being made today (whether they’re made poopie or sailboardie) we use the same materials, all over the board, and we get a bunch of weak links where load is focused or the materials don’t perform well with the type of load being applied. So we’ve got load sharing between materials and structural load sharing to think about. Maybe your springy rail could add a little of both?

Tranny

104

Here’s a springy laminated timber rail for you: http://www.olosurfer.com/flexy.wmv

I know that I have posted this clip before, just a flex obsessed tinkerer for the past ten years, probably needing sine wave therapy !

:slight_smile:

105

Hi like one of the other newbies said Im a long time listener first time flapper, I liked that so Ill flap away…With reference to the springer concept, I cant stop imagining a lovely springy

prestressed h stringer, locked inside a stiff sandwich construction.

As the majority of the movement of a stressed surfboard is along its length

why not free up the nose and tail some how, I was even thinking square noses

so all that movement doesn’t get trapped into the nose triangle. As bert has said

before, sheer movement is a bad thing, and I imagine by creating sheermovment at the

nose and tail, which are relatively small areas you are lessening sheer stresses elsewhere.

As that was it id just like to say all the talk here is really inspireing…thanks

106

hi, i don/t know if this link has been posted before but the whole of the site is quite interesting as well, pete

www.oceankayaks.com/Sandcore.html

107

oops, should have been ,

oneoceankayaks.com/Sandcore.html

108

Site looks good so far however you have to leave off the L

http://oneoceankayaks.com/Sandcore.htm

109

thanks Dan, pete

110

I found the part about the styrofoam cores to be very interesting. It reenforced a lot of what I’ve been thinking about how things fit together.

111

Hey All,

Been away for new year. Nice waves - unheard of in summer on my bit of coast. Trust the new year finds you all well.

test article,

Can’t say I’ve ever seen adhesive failure - unless the glue-up poor or the wrong adhesive was used. Because getting both things right mean the glue-up is stronger than the materials it’s joining. It’d be pretty weird if we found an adhesive that does everything :slight_smile: Right material in the right place for the job is, I think, the point of compsand boards.

Tranny,

I think you are getting on the right track, here. To me thrick looks to be using the right combination of materials to produce the right physical properties in the right places. Part of that is strong components (materials, adhesives, etc), part of it is flex, part of it is diffusing load. Have you read the What is flex? thread? You’ll find some interesting stuff in there. Considering the NP/NS on its’ own is pointless - just a conceptual or aesthetic thing. Using it important. I mentionned before that we ideally want this in the centre layer of the board and that less under, more over will help with this AND reduce weight. We have also talked about locating springers there. Besides producing “spring back” what do you think a springer does? Helps spread the load, IMHO.

Ned,

Interesting thoughts! Want to share anything more concrete? Or would you like to digest it a bit more first.

-doug

112

That is a great link! I’ve been questioning the comment made by the author: that in a sandwich with a weak core, “The skins experience very little stress because the core deforms easily.” It seems to me that for any given amount of flex (strain), a material will accept a certain load (stress) no matter what it is attached to. But maybe the author is saying that since the core shears easily it will strain tons in shear before the skins even start to bend(strain), therefore disallowing them [the skins] to accept any load. But, if the outer shell doesn’t bend that contains the core, how can the core be straining that much? I feel confused. It seems to me that no matter what, the skins always will take most of the load. Core materials resist strain and share load once the skins (and therefore the whole sandwich panel) becomes flexed, but I don’t see how a higher modulus of elasticity in a core will help the skins carry more load themselves. I’m feeling more confused. I’m in a flat spin… which strains first the chicken or the egg??? Does a stiffer core really enhance the effective properties of the skins, or does it just help share the load better?

Help Dan? Anyone?

I think the author is right on when they spoke about matching the moduli of your materials in a sandwich. Seems optimal for sharing load between cores and skins, resins and cloths, etc.

P.S. is there a string specifically dedicated to the concept of load sharing? I searched it and the best I got was one of Berts famous strings “shear load, snap resistance. Greg?” Lots of info there. I’d like to get a little more specific on where people think flex load is focused in a composite/sandwich surfboard (w/ and w/out perimeter stringers) and how it can be shared between the different structural components. Anyone have any links for me…?

Cheers & Happy New Years people,

T.

113

Sorry doug nothing concrte yet, just a few morw crazy ideas! Im just gona put my two pence worth in here, (im English), I may be way off the mark on some things I say, and If I am then Id like to be put right anyway.

I think that a very important concideration when using sandwhich construction is the distance between the laminates, the reason a compsand is so strong is that tension and compression is happening on both sides of the lamiate, what im saying is that you dont just have one NA but 3 because other than the one in the core materal you also have one in each of the sandwhich skins! I go back to what bert said ‘sheer is a bad thing’. So in a compsand you have sheer happening all over the place.

A stringerless construction method I have been thinking about is to have a compleatly flat deck that is that is very strong possibly with carbon fiber, this will stop short of the rails, then the hole thing would be laminated as usual (using epoxy on EPS of course!) but the under side would be very light with flexable resin. No sandwhiches!

What I would aim to achive by this would be

  1. A single NA near the deck side (as the deck would be so much stronger than the bottom)

  2. Less compression on the deck side ( because the deck would be flat encoraging it to bend and the flexable resin on the bottom would allow it to).

  3. Good board response to the rider, because of a hard deck.

Please im open to get riped apart with this… food for the sharks

115

Strong, rigid, longitudinally straight deck, soft resilient hull…a worthy approach. Since I don’t know how the rails are being handled, here’s some input.

By compsand, I take it we mean two skins in the deck each separated by an appropriately deep shear web and carried out to the rails, maybe including the rails. How the deck is structurally tied to the hull brings trannydogooder’s points above into play.

We need to avoid what is called ‘stress concentration’, i.e., abrupt transitions or inclusions from highly-loaded, strong regions through or into those that are much less strong. The key word is ‘abrupt’. Gradual changes in load-carrying ability are much better tolerated. Some guidelines to accomplish this include avoidance of square corners (radius corners; ‘inside’ corners are more affected than ‘outside’), smoothly blending thicker section with thinner sections (maybe the upper and lower skins in the compsand could gradually come together at the panel edges where they will meet (?) the single skinned bottom), staggering all joints and scarfing joints to spread the load into a larger bondline.

If the deck/rail combination is strong enough, it could compensate for the soft bottom when the board happens to be upside down under a falling ax.

test_article

Swaylocks promotes the team approach.

116

tranny,

Nah, mate. The skins handle most of the stress. A static structure which is bent downwards experiences compression underneath and tension on top. The forces are primarily experienced on the outermost surfaces. Very little stress is experienced beneath these surfaces (some, but not a lot)… Normally. The core cannot flex until the whole structure flexes. And that means the skins will flex too.

I think by “matching” he was talking about using the right material in the right place. Simple MoE/MoR comparison isn’t enough. I don’t think it’s something where you can just look at one isolated mechanical property and say “that’s it”. It’s more of a wholistic thing. Tho, of course, mechanical properties have their part to play in that.

ned,

From a purist perspective there’s only going to be one NP in a given structure. Because that’s the point where all forces equal out. I understand what you mean. The skin sort of hasd an NP between its surfaces and I think thats a valid way of examining it (looking at the skins as structures on their own). But I think the load the skins can handle is so far above the rest of the structure that it becomes a moot point.

How near the deck would your NP be on this theoretical board? And how are you going to work that out? :smiley: Your bottom materials are going to need to be very tension bearing :stuck_out_tongue:

test_article,

Stress concentration is interesting :slight_smile: You are, of course, spot on IMHO. Know what happs to the NP/NS when you have stress concentration? It abruptly changes, “stepping” up or down.

-doug

117

doug im really under the impression that the bottom of the board is the compression side

and the top is the tension side

the forces of feet of rider and wave make a board want to fold outward toward the bottom and then spring back to nuetral

whick explains (when you stress a compsand)

splits in the deck

and creases in the bottom

118

silly,

My understanding is the deck is under compression, the bottom under tension.

My logic:

Place your board on two chairs, one under the nose and one under the tail, bottom down (of course).

Now stand on the deck. The middle flexes towards the floor, the nose and tail towards the roof. This is roughly what happens when you are standing on a board in the water and will serve for discussion purposes.

As the ends flex upwards they come closer togethor. This puts pressure on the materials of the deck as though the ends were being crushed togethor, compressing the deck.

At the same time the bottom is being stretched like elastic pulled around a curved object, elongating the bottom and placing it into tension.

If I have something seriously wrong and someone can explain why I am eager to hear!

-doug

119

yep pretty standard understanding of they things are i guess

and up until now i have thought that way

but i think the rules are changing

okay

how about both sides are at times both the compression and tension sides

cuz the boards flex both way

and to what degrees depends on the riders stance and weight

if you have a wide stance with X amount of force in each foot

pushing down on the the end(tail) and roughly middle of the board.

even pressure underneath from the water

surely at times in the turn or whatever

the point between the feet is a centre point to a tensioned force

because the water underneath the feet will give to the direct pressure of the feet

the load would be distributed out a fair bit ,but the forces would still be there

like this

okat so then the rider hits the lip and the lips force is stronger than your foot pressure(unless your Kong) so then the deck becomes the compression side

my flexi fish failed on the deck with a lengthwiswe split from landing an aerial

to me that indicates tension

and bert said Beaus board split as well on the deck

i dunno i may be way off.im not an engineer

more an artist really

120

Hey Paul!

Howzit? Hope you have swell over there.

yep pretty standard understanding of they things are i guess

and up until now i have thought that way

but i think the rules are changing

I don’t think so :slight_smile: Bending is a combination of tension and compression. The inside curve of the bent object is under compression, and the outside curve is under tension.

Think about this. It’s all fine and well to say that is the way things work in static illustration x. And they do. But other forces can come into play. Structure cross-section impacts force/stress diffusion and thus flex and where how much compression and tension lies. And as you mention below counter flex in the other direction also does. IMHO it’s a mistake to think of it in a static application - but that’s a useful place to start… And everyone has to start somewhere. I’m happy to continue complicating this until I am out of my depth :smiley:

okay

how about both sides are at times both the compression and tension sides

cuz the boards flex both way

Bingo! You just hit the nail on the head! On a wooden bow flex and return is in one direction only - because of the string. What happens when the string breaks? The bow explodes into splinters much of the time. There are also other differences between the two. But as a conceptual tool it can be useful.

Twang a ruler off the edge of a table or desk. It doesn’t go on forever. In fact it stops pretty quickly. Why? Flex back in the opposite direction is only a fraction of the original flex because in the “flex/counter flex/counter flex/counter flex/counter flex/counter flex” pattern each flex is smaller than its’ parent, until there’s no visible movement. I believe this is what you are talking about in a board. But because counter-flex is so small the bottom only ever receives a fraction of the compression of the deck and the deck a fraction of the tension of the bottom.

and to what degrees depends on the riders stance and weight

if you have a wide stance with X amount of force in each foot

pushing down on the the end(tail) and roughly middle of the board.

Yeah. And other factors. Draw an axis through the centre of the riders body mass to a point centred between their feet and you have a rough focal point for the downward force (simplified). That force spreads out between the feet but is roughly centred there. Other things like stiffness of the board also come into play (esp with spreading the force).

even pressure underneath from the water

Um. I don’t think that varies in and of itself (water) enough to matter - water compresses very poorly. Pressure from under the board is more closely tied to low/high pressure planing speed and weight on the deck. As I understand it.

surely at times in the turn or whatever

the point between the feet is a centre point to a tensioned force

because the water underneath the feet will give to the direct pressure of the feet

the load would be distributed out a fair bit ,but the forces would still be there

Urm… The ends of the board would have to flex down, if that were true. Since centrifical force holds the rider onto the board during the turn I think acceleration is at play somewhat and compression on the deck increases. But the board will flex in the other direction after acceleration ceases/slows and the deck come under some small amount of tension, with the rest of what you say being true at that time.

okat so then the rider hits the lip and the lips force is stronger than your foot pressure(unless your Kong) so then the deck becomes the compression side

Yep. And when you go aerial it twangs back with the reverse force, becoming momentarily under tension.

my flexi fish failed on the deck with a lengthwiswe split from landing an aerial

to me that indicates tension

and bert said Beaus board split as well on the deck

Why does it indicate tension to you? To me it suggests too much sudden donward force rupturing the supporting skin structure.

i dunno i may be way off.im not an engineer

more an artist really

The world needs both! We need both! Even if you are wrong you and various lurkers and posters will still learn something from it. And I don’t think you are wrong, though I may disagree with you on some points. Which is what we are discussing and clarifying.

i like 2ft onshore

Paul

hehe! Still love it!

-doug