A video presentation by Yachting Monthly on testing the strength of various chains, and what size is safe for your boat. ... read more
Video Transcript Welcome to Atlantic Engineering in beautiful Birkenhead for Yachting Monthly’s test of anchor chains. So here is our lab, here are samples of chains and here is our expert Vyv Cox to talk us through it.
We have here 13 samples of chains which have been obtained from North Wales juggleries and from some specialist suppliers. At this end of the table we have grade 30 chains which is the lower strength, is commonly used for anchoring. The four at this end are grade 40 chains which are slightly more expensive but should be rather stronger than the grade 30. The intention is I have already measured in the dimensions of these chains to check that they comply with the specifications and from now we shall be testing them in a tensile test machine which will determine their ultimate tensile strength.
What you have here is at Denison tensile testing machine capable of pulling up to 50 tons. It consists of the upper jaws here and the lower jaws here. We clamp the chain in between these jaws, this table then move down and we can measure the strength at which the chain fails on this panel over here. We can also determine the rate at which the table is moving down again off that panel there and the whole machine is driven hydraulically.
The load is now beginning to come on the chain, the rate is being displayed in the lower screen. it takes just a little bit of adjustment but we are working on 140mm a minute. The load there measured in kilo newtons is now just past 20 which is just over 2 metric tons. This is a grade 30 chain, the specification for that is over 30 kilo newtons. But once it passes 30 we can confidently say that the chain is well within specifications. The rate is still climbing at a good rate which means we are not yet approaching the ultimate tensile strength. Once you reach an area close to the UTS, the rate will decline and eventually stop. The pull on the chain will continue until it eventually fractures. 40 kilo newtons is 4 tons so this is considerably above the specifications for this. The rate is just beginning to decline a little bit now, that seems to be the maximum, the machine is still pulling the chain. Oh it’s moved on up again, this would seem to be a particularly strong example of 5th grade 30 chain.
All of these have now been pulled in the tensile tester. We’ve recorded all the results and we can observe that there are three different fracture types and I’ll talk about those now.
This is a very typical tensile failure in a ductile metal. The fracture is known as a cup and cone type, so we have a cone shape there and a cup type there. There is a lot of elongation, the appearance of the fracture is somewhat known as woody and that is a very typical ductile fracture.
This is a fracture that is taking place immediately adjacent to the weld, it’s also a cup and cone type. That is a cone shape there and there is the cup there. This is the weld here so the fracture is taking place in the area that is being heated during the welding operation.
In this example the fracture is taking place immediately at the weld. When we examined these faces with the magnifying glass we can see that they are marks there consistent with where the chain was cropped during the manufacturing process. This suggests that the welding is incomplete, we have what’s known as poor penetration and the metal has never been welded correctly when it was manufactured as a chain.
Another factor that’s important in the testing is that it has highlighted differences in the quality of the galvanizing. Galvanizing applied to steel is not a coating as with a coat of paint but when it’s done correctly is a series of intermetallic compounds developed between zinc and the iron, with a steadily increasing zinc content as we approach the surface. This means that when the steel is deformed, the galvanizing should not flake off but should remain adhered to the surface. Here is a good example, this has undergone 4.5 tons of force and the galvanizing has perfectly adhered to the surface of the steel. Conversely we look at this one and here the galvanizing has flaked away in several areas such as here the galvanizing will fall off and it reveals the steel substrate that’s beneath. So this has not been galvanized correctly. read less