A bad break

[milkman]

I stored that boat for about 1.5 years. It was in the rafters of my garage. Oddly enough, I was looking at the serial number plates of all 4 Preludes I own and they're all 2008 vintage--even the new one I just got from Appomattox River Company. I have a feeling that Esquif pumped out a lot of Preludes that year. I did see one advertised by a Maryland dealer that is a 2011. I would have bought it, but the dealer doesn't ship out of state.

No wood gunnels on my boats--just the stock ones. It's true this could be a matter of aging plastic, but I have kayaker friends who have kayaks just as old and older that are still paddling them. And I've also seen many kayaker crack kayaks that were less than a year old.

My feeling is that boat manufacturers should all do everything they can to make their boats as tough as possible. We're all rough on our boats. That's the nature of whitewater paddling. I have yet to paddle a river that doesn't have rocks.

[NickParker]

http://www.gruppofrattura.it/ocs/index. ... /7791/5228

"...It can be seen that annealing for 24 hrs increased the fracture energy of HDPE by about 2.5 times..."

In other words, annealing HDPE can increase its toughness by 250%.

Here's the recipe:

- gradually heat to 250F over a 2 hour period
- hold at 250F for one hour
- cool at 10F per hour (25 hours)
- do all this in an inert environment (nitrogen atmosphere)
- the hull will get flexible, so build molds to keep it from deforming until it cools

Obviously that's an expensive proposition, with a small market, so don't be holding your breath for a hull annealing business to pop up in your neighborhood.

Slowly cooling the hulls after they are molded will have a similar effect, but there's only so far the manufacturers can go with that before it becomes cost prohibitive.

[the great gonzo]

There are, unlike in Vinyl (PVC), no softeners used in PE, therefore it does not get brittle due to outgassing of additives unlike PVC. It does however get brittle when exposed to UV (sunlight for extended periods of time.
The Prelude does not have an inherent cracking problem due to its shape, the prove of this ois all the Pyranha made preludes that are all at least about 7 years old and that are still going strong.

While I agree that wood gunnes do stiffen out a boat more than vinyl or molded in gunnels, my anecdotal experience through mainttaining the Esprit fleet of canoes did not indicate that boats with wood gunnels are more prone to crack than one without them. The one scenatio where a wood gunnel would make a PE boat more prone to crack is a hard impact on the sidewall near the gunnel. But the majority of the cracks that I have seen are on the bottom of the boat, usually right next to or under the saddle, where the transition from the unsupported area to the area supported by the bulkhead saddle causes a significant stress raiser.
One interesting thing is that I have never seen a crack in the hull of one of the old Dagger Quakes or Aftershocks, and a number of them are still around and paddled hard. These boats did not have bulkhead saddles, they came with the old Daggers saddle console thatwas bolted in. The big different between that and the current crop of bulkhead saddles is that the old console was not clamped down by the thwarts which stiffens the bottom and creates stress raisers, but held in by 4 bolts that went through the sidewall of the boat. It was freefloating vertically, allowing the bottom of the boat to flex more evenly, without creating any stress raisers. Something to think about.

TGG!

[milkman]

That's an interesting observation about the bulkhead saddle. My sample of three Preludes may confirm that. The 5-year-old Prelude's saddle was no longer attached at the bottom of the canoe. It came loose after about a year or two. To secure the seat, I put in one of the Mohawk metal cylinders they give you with their outfitting into the foam and attached it to the thwart. That boat survived lots of going right over rocks. Ultimately it broke under my knee pad, where I know it also took some very hard hits over the years--I felt every one of them. What's more, that Prelude's saddle was like a Mohawk saddle in that the channel for water to go from one side to the other was an half moon opening of the foam at the bottom of the boat. Both the other Preludes had firmly glued seats and were solid all along the bottom of the hull with plastic pipes creating the water channels. Both those boats broke right under the saddle.

[Paddle Power]

Great word use Nick, I had to look up annealing!

Also a good observation by the great gonzo on the free floating saddles. Unfortunately I never found those Dagger consoles comfortable but I like the possibilities that free floating saddles create (not being tied down to a thwart opens up outfitting possibilities).

[DougB]

Oddly enough, I was looking at the serial number plates of all 4 Preludes I own and they're all 2008 vintage--even the new one I just got...

One of the guys I paddle with recently got a new Prelude with a 2008 serial number. It cracked 4th time out on a class II rapid - on an obvious rock but one I've purposely plowed into/over with my S Fly & Ledge countless times. The Prelude is surprisingly light weight. Thin plastic flexes nice, but there's always a limit to the give. Boat was replaced with a freshly minted 2013. We'll see how it does.

[milkman]

Wonder if it still has that new boat smell? Definitely let us know how that holds up.

[NickParker]

When the hull is taken out of the hot mold, the inner and outer surfaces of the plastic cool/shrink at a faster rate than the center. Because of that differential in shrinkage, internal stresses build up as the part cools. Those stresses are significant, enough to change the mechanical properties of the material.

Polyethylene is inherently flexible (ductile), so you would expect it to show signs of ductile fracturing -- where the material is stretched and deformed along the edges of fractures. The cracks I've seen in polyethylene hulls don't look that way, they have sharp clean edges. I've seen one Ledge that had a "spider" fracture, with multiple sharp edged cracks radiating from the impact point. Those are classic signs of brittle fractures, where an impact has caused a sudden release of the internal stresses that were already in the material. In essence, the plastic was already trying to pull itself apart, the impact just gave it enough additional energy to separate.

If you get rid of those internal stresses, it will take more impact energy to cause a fracture. In the conditions tested in that research paper I linked to in my last post, they found that annealing (stress relieving) gained a 250% increase in crack resistance.

It should be perfectly feasible for manufacturers to offer "super tough" versions of their HDPE hulls (made tougher by either annealing, or very slow cooling coming off the mold), but they would cost more.

It's sort of like whisky. If you want the fresh stuff you can get it cheap, but if you want the good stuff you have to give it some time, and be willing to pay more.

[milkman]

The cracks I've seen in polyethylene hulls don't look that way, they have sharp clean edges.

Interesting observation. All three boats have sharp, clean cracks. What would I pay for a 2.5X increase in crack resistance? Easily another $200 or $300 per boat. It would be cool if manufacturers backed that with a 3 year warranty.

[the great gonzo]

The annealing is a nice idea, but I think the long cycle time would make this proposition pretty much cost prohibitive to any canoe manufacturer. As far as I know from some kayak manufacturers the cycle time for a boat is well under one hour. With the annealing we are looking at a day. And that in an inert athmospere. I would imagine that the price difference between a regular hull and an ennealed hull would be in the same percentage range as is is between a regular 8 year whisky and the 18 odr 24 year premium one !
Let's face it, the overall failure rate of PE is low, and I don't know too many paddlers who would be willing to pay a huge premium for an annealed hull. I don't think that 2-300 bucks would cover that, considering the necessary capital investment for the sealed oven and the ongoing cost for heat and gas for the process.
But if enough folks are willing to fork over a big pile of cash my way, then let me know and I'll start a hull annealing business... !

Also, I am not sure if the breakage resistance of the hull is really 2.5 times higher. If you read the abstract and look at ther data in detail, then you will find that the load where the fracture initiation occurs is about 150 for the as molded and about 190 for the annealed sample, an increase of about 27%. The differnce is in the energy absorbed in the propagation phase. The way I am interpreting this is that the crack will probably start at relatively similar impact loads, with the big difference being the length of the crack. Smaller crack with the annealed hull, and due to the higher ductility probably a little less likely to develop cracks in areas where stress raisers like edges of saddles are present.
An interesting article here on the JK blog that gives a little insight in the importance of cycle time. Appears that, before JK was molding boats in house, they had problems with molders cutting corners and trying to shorten the cycle time, leading to brittle boats. http://jacksonkayak.com/blog/2009/04/30 ... c-jackson/" onclick="window.open(this.href);return false;

TGG!

[NickParker]

I think there are a lot of disgruntled Ledge owners out there who would disagree that PE hulls have a low fracture rate. Off the cuff you would think that the heavy Ledge would be tougher than the light Ledge, due to the thicker material. But we know that is not so.

The center of the thicker material holds on to its heat longer than thinner material. If the cooling cycle wasn't properly adjusted for the thicker material, you would expect the internal stresses to be higher, and the material more brittle.

Annealing is the only way to fix brittleness in "cold" hulls. I agree that it is cost prohibitive, especially for such a small market.

The time to address brittleness is when the hot hull is pulled out of the mold. If it is cooled too fast it is going to be brittle. Ideally they would be cooled at a controlled rate for 24 hrs, but that also is a costly proposition. However it's not necessary to go to the full 24hrs or have precise control of the rate, to see improvements. There is a middle ground.

[the great gonzo]

Sure, owners of certain boats may agree, but the fact is that, looking at all the boats and all the manufacturers the indication is that overall the breakage rate is low and thus the process of rotomolding PE boats is well established and controlled. Higher breakage rates for any given boat or manufacturer indicate that the problem is isolated to either the material, thickness or manufacturing process of said boat and/or manufacturer.
Historically, brittleness in PE boats seems to mostly have been to either using not the appropriate grade of HDPE or due to manufacturing proccess issues, usually undercooking.

TGG!

[NickParker]

The breakage rate as measured by returns to the manufacturers may indeed be low. The number of unreported breaks welded by the owner of the boat, his buddy, or the guy in town who offers that service, is a much higher number.

Looking at the disparity between the materials properties of polyethylene and the nature of the fractures, it's easy to see that there are still processing issues that are making the hulls more brittle than they should be.

I've done some CNC machining of various thicknesses (up to 3/4") of large HDPE sheet stock. The sheets arrive from the plastics manufacturer nice and flat. But when you cut away one side of the sheet you find that it is only flat because the internal stresses are balanced between the two sides of the sheet. Remove thickness from one side and it warps, badly, so there are obviously internal stresses at play. That's with stock that undergoes a mild annealing process when it is manufactured (a continous assembly line type process, not the full annealing process described earlier).

In comparison, hulls are rotomolded in less controlled conditions, don't undergo any annealing, and are cooled rather quickly. I think the resulting internal stresses are a major factor in brittle fractures of those hulls.

[JimW]

Also, I am not sure if the breakage resistance of the hull is really 2.5 times higher. If you read the abstract and look at ther data in detail, then you will find that the load where the fracture initiation occurs is about 150 for the as molded and about 190 for the annealed sample, an increase of about 27%. The difference is in the energy absorbed in the propagation phase.

Interpreting this data is not easy if you don't do it regularly, I'm an engineer rather than a materials scientist, which means I should be able to understand it, but don't look at it often enough!

The claim is that the annealed material is 2.5x tougher, i.e. it can absorb that much more energy before failure. Charpy testing is about through thickness fracture, not fracture along sheet material, you are not looking at the propagation of a crack but the progress of the sharp edge of the test pendulum through the material.

Now the key thing is that it is about energy, not just load. Energy is load x distance. By absorbing 2.5x the energy to fracture that is a combination of load and distance. In this case you identify that load goes up from 150N to 190N but missed the point that material can now deflect further under that load before fracturing - at 190N it can deflect 97% further before it breaks, presumably at 150N it could deflect 150% further before breaking (this is where I get a bit unsure due to unfamiliarity). This is what ductility is, the ability of a material to bend and stretch without breaking, not necessarily to withstand a higher load, but to deform more before it breaks.

In short an annealed hull would tend to pick up more dents but fewer cracks than a non-annealed hull. That's toughness, rather than strength. The ultimate strength of most meterials doesn't change when annealed, although the yield strength might.

Annealing is very common in all kinds of metalwork, in an annealed state a metal can be bent or stretched much further to form into useful products without fracturing. It is especially useful for working materials like copper, brass and aluminium, aluminium in particular is quite brittle in it's non-annealed state and will quickly crack when worked. It also work hardens, that is when subjected to blows, say from a panel beaters hammer, the molecules get jiggles back into a non-annealed state - it is therefore non unusual during complex cold forming processes for something like aluminium to be re-annealed over and over again to ensure that whilst being hammered into shape it is as ductile as possible to avoid cracking it.

Annealed aluminium can become very pliable initially, sometimes you can literally form it between your fingers. The concern I would have with annealed HDPE, is over how pliable it might become - would it still hold it's shape properly or could you dent it if you pick it up wrong? Oh yeah, and the cost of increasing the cycle time by maybe 20x (i.e. cutting output by 95%....)

[the great gonzo]

Jim, you are right about the Charpy test and the energy absorbed.

Looking at the big picture of cracking PE boats is however that, except for the well known cracking issues with certain particular boats, where the spider cracks and long S-shaped cracks definitely show a brittleness issue, most cracks IMHO not due to brittleness. I have seen and welded many cracked boats and here are some of my observations.
One of the biggest, if not the single biggest contributor to cracks in boats are built in stress raisers and stiffeners. Virtually all the cracks that were not brittleness related (and even most of them), occurred in areas either under or right next to where a stiffener, seat or saddle caused a stress raiser that prevented the boats hull from flexing properly to allow it to absorb the impact. On Canoes, they are usually right under or next to the bulkhead saddles that are held down by pressure and usually glued in, too. As I mentionned in my previous post, I have never seen a crack in a Dagger boat with a console saddle that as not attached to the thwart, but free floating.
The vast majority of cracks that I have seen in Kayaks are either under or right next to the stiffening foam pillars or under the seat, which acts as a stiffener, too.
Most cracks that I have seen in other, unsupported places were very short and localized and accompagnied by significant plastic deformation, indicating hard impact on a very sharp object.

I have also fixed many huge dents in PE boats. One kayak that had the nose caved in by a good 8 inches after a hard piton. No crack whatsoever, all that was needed was some heat and gentle pressure to get the nose back into shape. I have also had the bow of my old Pyranha Prelude completeluy caved in after a hard Piton, no crack, heat and gentle pressure got it back into shape. in the stern I had probably close to half a dozen dents that needed to be removed using the above mentionned technique, without it ever cracking. So plenty of ductility here. also no stress raisers in these areas due to no outfitting, the hull can flex as needed to absorb the energy of the impact.

It is however well understood in the industry that undercooking PE causes brittleness.

I think if boats are properly proberly cooked, using the the proper grade of plastic, then brittleness is not an issue and annealing not necessary. The one area where there is room for improvement to make boats more crack resistant is in minimizing stress raisers and in designing the outfitting in a way that it allows the hull to adequately flex to absorb impacts.

My point is essentialy thatin the case of rotomolded PE boats, annealing would cure an issue that does not occurr if process and material are up to spec.

TGG!