Testing to Failure

[preserved_killick]

Today was my first experience filling in holes in the hull from removing through-hulls. It all want pretty well and as planned (so far) except for squeeging out excess resin. The air warmed up a little sooner than I expected and the resin got a little thick & difficult to pull out. I'm not too concerned about this repair, but it did make me wonder if there's been any failure analysis done on epoxy & fiberglass that falls outside of "perfect application".

I come from a previous career as a mountain guide, and in climbing everything is tested to failure and measured endlessly. Not only is it tested in "standard" conditions, but with all sorts of variables mixed in. For example, knots are tied into ropes and pulled to failure when the knot is 100% tied, and tested at various stages of completion. Ever wonder how secure a Figure Eight Follow Through is when the tail is not followed through? How strong an ice screw is when not fully buried into the ice? Manufactures are willing to throw real-world mishaps into the test and publish the results.

With fiberglass repair, I often wonder what I'm sacrificing in strength if I have no choice but to do something that's not ideal. I wonder the strength difference in a high ratio resin to glass mix compared lower ratios. Differences in adhesion between fresh resin compared to partially cured resin? What is the differences in adhesion of a fiberglass patch with the different bevels to thickness. Difference in adhesion between a chemical bond and a mechanical bond.

I looked around the West website, and could only find absolute testing in perfect situations like this one:
http://www.westsystem.com/ss/west-syste ... ss-repair/

Anyone know of any published information?

[Tim]

I don't know about published testing of this nature, but even if some exists, it's unlikely to shed too much light on whatever situation one has at any given moment. To me, fiberglass is pretty straightforward: use the appropriate material, get it wet enough, and get the air and excess resin out during application, and it will be as strong as or stronger than necessary. The real world is never an ideal environment, but it's easy to ensure that the results are more than adequate.

Remember that your entire boat is probably well outside any hope of "ideal" laminate quality to begin with. They virtually all are. Fortunately, fiberglass is very strong, even when applied in less than perfect circumstances. There are limits to the adaptability of this material, but within reason it is strong enough that even when made weaker by imperfect application, it's still strong enough. So I'm not sure what a test of some predetermined "imperfect" laminate situation would tell, given that all laminating situations are imperfect by the nature of the application environment. Even today's high-tech resin infusion and other techniques rarely produce perfect laminates (perfect in terms of achieving fully 100% of the theoretical maximum strength that the materials combination should have).

One thing the real world does time and again is prove just how many variables it can throw at anything, however exhaustive previous testing of whatever product has been. There's always some other wrench that can be thrown in the works that someone didn't think of, or test for. Personally I don't find much comfort in testing results, and therefore doubt that laminate testing would help me out too much. My mind works in a practical and logical way, and I generally don't put much stock in numbers and engineering data other than to know that the basics of what I do have already been put to the real test over years of experience, mine and others'. Lots of carefully engineered and tested stuff has failed throughout history. We learn much more from real-world failures than anything ever done in a lab.

Know the basics, and strive to always learn and improve from there. What you've experienced is all part of the learning process. The next time you'll be quicker and avoid this problem, or will take other steps to prevent its recurrence. You sound confident enough in the effected repair, so good enough.

[Duncan]

Anyone know of any published information?

There's a lot of information on "testing to failure" that comes from more and more of use of composites in aerospace. This engineering is subject to rigorous specification and testing. A google re: "testing aerospace composites", or somethink like that, will turn up lots of published information.

Another place to look might be ocean racing, since those guys do a lot of destruction chasing the ideal strength-to-weight composite materials and techniques.

With fiberglass repair, I often wonder what I'm sacrificing in strength if I have no choice but to do something that's not ideal. I wonder the strength difference in a high ratio resin to glass mix compared lower ratios. Differences in adhesion between fresh resin compared to partially cured resin? What is the differences in adhesion of a fiberglass patch with the different bevels to thickness. Difference in adhesion between a chemical bond and a mechanical bond.

In the same spirit as Tim, I think that if you "follow the directions on the can", "use your common sense", and "ask the guys that know what they're doing", you'll probably avoid most of the mistakes commonly made. As far as "doing something that's not ideal", I think there are always choices and ways to avoid compromising (on the issues you mention).