What gives a ski its strength?

[bhs]

Is it all the parts, top sheet, bottom sheet and core working together? Or does the strength and flex characteristics come from the way the top sheet is laid up alone?

[Jordan]

combination of all of the components in the lamination.

[Horton]

@bhs read this https://en.m.wikipedia.org/wiki/Sandwich_panel & https://en.m.wikipedia.org/wiki/Sandwich-structured_composite

[grab2go]

A picture might be worth a 1000 words... In the top picture the wood strips are stacked on top of one another and allowed to slide relative to one another when placed in a 3 point bending test fixture. In the bottom picture they are bonded to together. Notice the difference in the load and deflection of each.

 

 

[BraceMaker]

Anytime you separate two layers of composite around a space you've made an I-beam.

 

I do wonder however, does the core of a modern ski bond to the carbon around it? Do those cores tend to "shear" off the inner face of the ski?

 

What about around penetrations into the core for the inserts/fin boxes etc?

 

The sandwich structured composite sheets (Mastercraft floors c. 1992?) are interesting because the core is almost more a space filler than a structural component, however the discussion of the cores of skis seems to imply the core foam gives a structural change to flex pattern as well.

 

[AdamCord]

A sandwich structure carries the loads in the outer layers of the sandwich. In the case of high end skis, that would be the carbon fiber layers. When a ski bends while you ski, the top surface is compressed (called compression), while the bottom layer is stretched (called tension). This means that like the image above with the wood strips, the top and bottom layers will try and slide past each other.

 

What keeps them from sliding too far is the core. This means that the core is loaded in what's called Shear, where the top and bottom are pulled in different directions. So as a ski builder/designer, what we care about with regards to the core is making sure that the carbon fiber bonds well to the core, and what the Shear Stiffness (called shear modulus) is of the core material. The higher the shear stiffness, the less the top and bottom layers of the ski can slip past each other, and the stiffer the ski will be.

 

In practice we don't often change core materials to adjust the flex though, because it's too expensive and less predictable (core materials must be purchased in large quantities so changing it all the time is tough). What we can change is the amount/type/direction of carbon in the top and bottom layers, and also the thickness of the ski. A thicker ski will be stiffer.

[Edbrazil]

In the Back When (~1965), I did an engineering school project on composite water skis. I made up a pair of jumper skis that had Aluminum tops (6061T6) and a foam core. Foam was 2-part liquid that was foamed in place. Skis turned out OK. Took them to Florida on Spring Break, and they skied well. Then tried them on the ramp, and they failed on the 2nd jump. Not the Aluminum sheets, but the foam core. The foam just had to adhere to the Al and hold the shear stress, but the landing shock did it in. Nice clean failure in the ski's forebody. Later on, I got a special shock-recorder cube and fastened on a wood jumper ski. There was an inked ball inside, and the size of the blob it made was related to how many gs experienced. Device typically used for putting in shipping crates.

 

When I used it for a few jumps and then reviewed the blob pattern, it apparently was out-of-range, as in something like 400 gs. Realized just how much stress on on jump skis. No wonder that they now use a composite design like an airplane wing, with honeycomb Aluminum as the core.

 

Project got me a good mark, anyway. I did more than just construct skis, but did math analysis, which also included the forces, accelerations, and velocities acting on a skier. So, I got the BSME degree and went off to Real Work.

 

Saucier and EP did a better job on ski design, and wooden jump skis went out of use after a few short years.

[bigskieridaho]

Good for you @Edbrazil When I did a report on straight snow skis vs. shaped the instructor hated it and gave me a bad grade in college. At least it sounds like 1965 was cool for that ha ha.

[eleeski]

The skins do carry most of the load in a modern waterski.

 

The largest loads are along the length of the ski so many skis use unitdirectional cloth to specifically carry this load.

 

Other loads are impact damage, binding retention and underfoot crushing. The skins carry most of this as well.

 

The core has to be strong enough to resist crushing and the shear loads. A good bond to the core is critical.

 

Polyurethane cores tended to break down over time and repeated flexing and the core itself would fail causing the ski to break.

 

Honeycomb is difficult to bond with the skins. It is available in lots of densities and materials. Light honeycomb can fail in shear or get crushed easily. It can fill with water if damage to the ski or porous construction allows water in. Aluminum honeycombs corrode with water (especially salt water) and time. When done right it is quite strong.

 

PVC cores are very tough. They bond well with epoxy. They survive multiple stress cycles. They are easy to shape. They have reasonable shear strength and accept stringers or reinforcements. I'm using them now.

 

Other foam cores exist but my experience is limited.

 

Wood cores are used in snow skis a lot. But snow skis are relatively heavy. Wood is nature's composite, reasonably light and strong. It is somewhat inconsistent. It moves with time. Moisture may affect the resin or core bond. It's no longer common in waterskis.

 

As @AdamCord noted, the bottom of the ski is in tension while the top is in compression. Carbon is extremely strong in tension but just OK in compression. Clever design (extra reinforcement, composite spars, material mixing and exotic materials like boron) takes care of the compressive issues.

 

Note that a waterski deflects just millimeters while a snow ski deflects centimeters so construction will be quite different.

 

@Edbrazil My first ski was also an engineering class project! My ski also skied well enough to get me a good grade but failed as soon as I tried to break it. Pour foam is horrible stuff. So is superlight honeycomb. Good story Ed!

 

Most of my (many!) ski failures have been failures of the top skin. A few delaminations from the core, a few core degradation failures and a few environmental damages. See where it failed and add more layers. If it doesn't fail, remove some weight until it does!

 

Eric

 

[grab2go]

@AdamCord I have always been curious about the bond between the upper and lower facesheets on the top edges of a slalom. Is that a butt joint or is there fiber overlap? Is that area designed by combined torsion and bending or damage tolerance?

 

[eleeski]

@grab2go Some skis are "cap" molded where the edges do carry load and transfer the loads from top to bottom skins.

 

Most let the core transfer most of the load. On those skis, the edges are just there for ding protection. Some had extra material to allow for custom tuning with a file.

 

If your edge is dinged, repair it as best you can and watch the ski to make sure it is still structurally sound.

 

Eric