Questions about failed schooner fittingJan 1, 2003
I read Peter Kalajian's piece on jury-rigging a schooner backstay ("Saving the Rig," Issue No. 92, September/October 1998) with considerable interest. While most of your readers, doubtless, focused on the very interesting sea-going elements of the story, my attention was arrested by the discussion of the failure mode in the closed spelter socket pictured at the close of the article. In my view, a larger discussion of the socket failure was warranted.
The importance of this incident and its technical or forensic analysis is, I believe, of consummate importance. The global scope of applications where socketed assemblies are utilized in critical marine work is of such magnitude as to warrant amplified discussion. Any failure in this domain is, therefore, of the greatest possible interest to everyone in the industry.
Let me make a few suppositions of my own. First and foremost, I'm a bit dubious as to whether the degree of concavity depicted in the photo on page 55 was singularly sufficient to cause the wire rope to pull out.
Second, I'd like to hear more regarding the extent to which not just the strands but the individual wires were splayed, broomed, and thoroughly cleaned prior to the socket pour. Synthetic socketing involves scrupulous attention to the details of wire rope preparation in order for the load transfer mechanics to work as intended. Each wire is bonded individually, of course, but also is held through the frictional effect of the grains within the resin compound in and around the interstices created by the wires within the rope broom.
The object, then, is for the wires within the resin agent to act much like rebar in concrete structures in order to contribute to the wedge action that takes place in the lower third of the socket cone. Preparation and cleanliness are critical to the success of this operation, without which even perfectly conical socket internals will produce poor, even catastrophic, results.
Joel Altus is a marine rigger based in Seattle.Pete Kalajian responds,
Sea Education Association received a verbal report from the technical department of Crosby-Lauglin, manufacturers of the socket. The report notes that the cleaning could have been done a bit better, but that there were traces of epoxy left on the strands to indicate the bonding was good. The structure of the brooming wire was as per specs. There was one broken strand, but the primary reason for the failure was the shape of the interior cone of the socket.
That conclusion was based on the fact that the annular rings of epoxy at the socket interior surface had not sheared, indicating that the epoxy plug did not move down the socket. Without this movement, the socket could not have developed the necessary compressive force on the wire/epoxy bundle to prevent pullout.
We follow the specifications for cleaning and brooming meticulously and employ a carbon tetrachloride-based brake cleaner to clean each individual strand. Although field conditions can never be as ideal as laboratory conditions, we feel that our crew pay close attention to getting the best possible pour, because they're going to be at sea with that piece of wire, and, surely, enlightened self-interest coupled with good training and oversight are good quality controls.
We had a strength test done on the other end of the same wire, the socket of which was poured at the same time by the same person. The wire showed signs of breaking at 57 tons. This socket was conical in section.
We share Mr. Altus's concern that this information be widely disseminated. To this end we contacted the Coast Guard and have tried to foster as much discussion as possible about this issue.
Still, the best thing a rigger can do to prevent this from happening again is to follow the manufacturer's instructions closely and inspect the socket with a critical eye. If it's conical in section with flat interior sides, there should be no problem.