Tuesday, February 1, 2011

More Bulkhead Repairs

After returning from the nationals we identified another small crack in the bulkhead/deck joint but this time it was much closer to the centre line of the boat. Upon closer inspection the crack occurred at the ply drop of a unidirectional layer from two layers down to one layer, yet still with a -+45 deg carbon cloth patch covering the ply drop.

The crack is clearly a compression failure and as can be seen even propagates past the carbon fibre material and into the bog covered fibreglass cloth each side.

To me this suggests that only 1 layer of carbon uni at 50mm wide is not sufficient to carry the loads running through this part of the hull. While considering that i suspected the load in this bulkhead/deck joint to diminish as the distance from the side stay increased (spreading the load into the adjacent bulkhead and deck material). This decrease is either non-existent or smaller than i anticipated. Also it is clear the failure occurred at the ply drop, which is a known stress concentration point. In future i would recommend that ply drops in a similar situation should be tapered (e.g. cut the end of the strip to become a point) as this should spread the stress concentration over a longer distance and prevent a straight line that is perpendicular to the load path being formed.

Also noted was that when i peeled up the damaged area that the epoxy did not stick particularly well the to the bog/filler that was next to the laminate e.g. from the old paint and filler. In future i think that needs to be sanded back further and more attention paid to make sure that all "stop putty" is removed with acetone before laminating.

I suspect that the reason this is happening is for two reasons, both involving the stiffness of the structure.

1. As mentioned earlier, stiffer structures generally experience higher peak shock loads as the structure deflects less when loaded (due to being stiffer) the time over which the load is applied and reacted to is reduced e.g. a shorter impact time. This gives a higher peak acceleration to the overall structure, which for a given mass gives a higher force (F=ma). Therefore a stiffer boat with stiffer rigging is going to suffer higher accelerations when slamming on waves. The next question is whether my new boat is actually any stiffer than other boats out there at the moment. I feel that i could be but its rather hard to prove or test and therefore hard to judge with this has a significant effect or not.

2. The second reason i feel this has occurred is due to the relative stiffness of the laminates used in and around the bulkhead/deck joint. Woven fibreglass cloth laminates have an elastic modulus (stiffness) which is drastically lower than carbon fibre fabric or unidirectional tapes as per below.

Young's or Elastic Modulus:

Woven E-glass (0/90 deg) = 25 GPa
Woven E-glass (-+45 deg) = 12.3 GPa
Woven Carbon (0/90 deg) = 70 GPa
Woven Carbon (-+45 deg) = 17 GPa
Uni Carbon (0 deg) = 135 GPa

Source: (http://www.performance-composites.com/carbonfibre/mechanicalproperties_2.asp) 01/02/2011

Now assuming that the E-glass fabric each side of carbon fibre strips is at 0/90 degrees then the unidirectional carbon is approx 5.5 times stiffer then the glass next to it. I feel that it is this reason that very little load is being transferred into the deck or bulkhead but instead travelling along the much stiffer carbon. This is because as the carbon compresses and strains the glass next to it strains an equal amount but because of the low stiffness it only corresponds to a low stress level and therefore contributes to very little load carry ability of the structure.

If this fibreglass is in fact not on an 0/90 degree angle the situation only gets worse until the angle reaches -+45 degrees when the stiffness difference becomes approximately 11 times...

With this now in mind i need to consider how i can make this carbon fibre laminate strong enough to solely carry the rigging loads and allow it to transfer to the fore stay without just moving the problem further along the boat.

Since the failure occurred in a single ply of unidirectional carbon yet not in the section with a double ply it would be okay to assume that this would be sufficient. Though considering this appears to be quite highly stressed and it is unlikely that the boat has experienced its highest loads that it ever will. With this in mind and also the effects of fatigue to be considered (although UD carbon has an excellent fatigue life) it would be a good idea to increase the factor of safety. Therefore i will try to locate the previous ply drops and rejoin them to give a 3 ply laminate to carry from side stay to centre line on the deck on both sides.

From here i will need to try to "dissipate" this load to the foredeck and the "V" beam underneath the deck. To do this i plan to arrange a patch or two of carbon cloth at different fibre orientations to try and spread it over a large enough area in the E-glass deck to carry the load.

Otherwise the next step would be to run carbon all the way along the deck to the fore stay mount. Though this would be rather undesirable as quite a large area would then need to be faired in and re-painted, which would be quite a major repair.

I will try to document the repair process as much as possible when i undertake it later on in the week.

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