Originally Posted by Stumble
I wouldn't use a titanium shackles for anchor
chain unless you are also using titanium chain. There is no advantage to having a shackle substantially stronger than the chain it's attached to. Just use a shackle rated for the chain you have.
I've a few questions & thoughts on using Titanium for anchor
shackles, if you'd be so kind.
The precursor to my queries, being a bit of a read, such that those new to materials, engineering, & metal failure modes, can grasp understand what I'm asking & why.
Regarding Titanium anchor shackles. I'd first want to do some more study on them, but there IS a Big temptation to use them in that application, for me at least.
The reason being, that even when you buy a premium HT (high tensile) galvanized shackle for your ground tackle, which has a theoretical breaking strength of several times more than your G4/G43 chain. In severe storms (read hurricane
strength), shackles seem to often be one part of the ground tackle system, which fail.
And they do so, because the pin in the shackle, which passes through the end of the chain, is unsupported for almost all of it's length.
Now were the hole in the chain large enough so that you could insert a compression
sleeve overtop of the shackle's cross pin. And the crosspin were actually a cross bolt, as in some HT Crosby variants.
So, then, after doing this, you torqued the cross bolt down to the proper spec for it's size, & inserted a safety
Then the cross bolt (pin) would be being stressed in a different mode than is a simple cross pin.
One where it is/would be able to exercise a much greater percentage of it's theoretical/published strength because of this.
For when the pin in a shackle passes through a link of chain, all of the load on the shackle's cross pin, bears exclusively on the tiny section of metal which contacts the rounded over profile of the inside of the chain's link.
So, then, the bending moment, & strain on the shackle's pin are huge, due to this point loading.
Also, if one looks at how the strengths of shackles are rated. Such numbers are arrived at with a specified percentage of the cross pin being supported by the material which it's shackled though. And said percentage of the shackle's cross pin/bolt which is supported, is a large fraction of the gap which it is spanning.
Which, in the test, the pin passes through a straight hole, in a thick piece of metal. So that the load on the shackle pin is spread fairly equally across far more/most of the cross pin's length.
Thus rendering it significantly stronger than in a scenario where it passes through a link of chain.
Basically, in the test, the shackle's pin passes through a straight hole through a piece of metal, which is then pulled upon in order to load things up in order to conduct the testing (measurement).
So that the load exerted on the shackle's pin, is spread out over a FAR greater area, than is the case when it's slotted through a piece of anchor chain. And there's far less unsupported length of the pin, available to bend.
Or to simplify things, which is easier to bend?
- A 1/4" piece of steel
stock, resting atop a flat, 1/2" wide, unbendable rail.
- A 1/4" piece of steel
stock, resting atop a 1/2" diameter, round, unbendable rail.
Obviously the one on top of the round surface is easier to bend. And that, more or less, is what the cross pin of an anchor shackle is subject to where it passes through a link of chain.
And with steel, in order to get it to bend. Generally speaking, load wise. You've already exceeded 1/2 of the metal's ultimate strength.
So from there, it's load carrying ability goes downhill rapidly. And if the steel in such a scenario is an anchor shackle...
Most of this information, was distilled in another thread on here, which had to do with the performance of a Fortress
anchor during a hurricane
. And what, in the survivor's account worked, & what failed.
With inputs from Fortress
anchors, & some other industry experts, on some of the why's behind same.
Not that the majority of the shackles failed, necessarily. But of those which survived for inspection
, many were significantly deformed. With the attendent loss of strength that accompanies such.
Thus my interest in Titanium shackles for anchoring
. Assuming that they're of the proper grade of alloy, & they're well manufactured. As well as their not being subject to being too brittle for the job. Either due to the nature of the material, their manufacturing process, or from work hardening/cycling.