Strength Test of a Coiled Ring

The Test
Recently a discussion arose on The Arador Armour Library about forge welded rings. Discussion then turned to whether welding was even necessary - would not a ring that had been wound twice (as forge welded rings appear to be wound 1-1/2 to 2 times), heated to glowing, and then flattened be strong even if the coils did not weld together?

An assertion was made that this was so, and that not only would it be strong, but that a coiled ring that had been heated and flattened would be stronger than a coiled ring that had not been heated or flattened (untreated).

I doubted this assertion, so I put it to the test.


The picture below shows the test setup. I took a segment of 3/16" drill rod, and clamped it in my vice. I then threaded the test rings over this rod. The test ring was attached to the weight vehicle (the bucket) by a twisted segment of wire. In actuality, during the test progress I had to support the end of the drill rod with my hand as it began to bend. This should have had no effect on the load bearing capability of the ring, nor its failure.


The Test Setup

The image below shows the first test ring. This ring is a coil of 18GA mild steel wire, with a 3/8" ID. It has been wound approximately 2 times, as you can see. It has not been heated, and it has not been flattened. Note that I positioned the cut ends facing upward. This was consistant on all rings tested.


The unheated, unflattened ring coil

The ring was then put under load. I accomplished this by adding rings to the bucket. I added a clump of rings, handful by handful, until the ring failed. Below you can see the failed ring.


The unheated, unflattened ring coil, after failure

The images below show the amount of rings needed to cause the unheated, unflattened ring coil to fail. The tape measure is approximately 2" deep in rings.

Next, I constructed a coiled ring, identical to the first coiled ring, except that I heated this ring to yellow heat, dropped it in my flattening fixture, and quickly flattened it before it could cool. The images below show either side of the resulting ring:


Heated and flattened ring, either side

I then attached the bucket, with the same amount of rings that caused the unflattened, unheated ring to fail. The heated, flattened coil did not fail, and supported the load easily:


The heated, flattened ring supports the load that failed the unheated, unflattened ring

Then constructed a third ring, that had been coiled approximately 2 times and flattened, but not heated at all. It also supported the load easily. Note that the ring ends are spread apart slightly. This occured during flattening. Before flattening, the ring ends matched up (2 complete revolutions of coil).


The unheated, but flattened, ring coil supporting the load that failed the unheated, unflattened ring.

Next, I continued adding rings to the weight vehicle until the unheated, flattened ring coil failed:


The unheated, but flattened ring coil, after failure

Here you can see the amount of rings (roughly 5" deep)needed to cause failure of the unheated, but flattened, ring coil. This is over twice the amount of weight needed to fail the unheated, unflattened ring coil:


The amount of rings needed to fail the unheated, but flattened ring coil

Then I attached the heated and flattened ring coil to the weight vehicle, with the same amount of weight that failed the unheated, flattened ring coil. This heated, flattened ring also failed, under the same load.


Heated AND flattened ring coil failed under the same load as required to fail the unheated and flattened ring coil

Conclusions:
From the test, we know absolutely that a flattened ring of 2 coils will support roughly twice the weight of an unflattened ring of 2 coils before failure. Thus we can deduce that flattening the ring coil does impart pull-apart strength under the above test conditions.

However, because the ring coil that was both flattened and heated failed under the same load as the ring coil that was only flattened, we cannot deduce that a flattened ring which has been heated is stronger than a flattened coil that has not.

In other words, heating of the flattened ring coil does not make it stronger. In fact, it is possible that heating the flattened ring coil makes it weaker. However, this test did not check that possibility. Instead, as the above description shows, a load was found which would fail the unheated flattened coil, and that same load was applied to the heated flattened coil. It failed too, but it is possible that the heated ring might have failed under less weight load.

However, given that the deformed shape of both the heated and unheated flattened failed rings are roughly the same, it is my guesstimate that the rings are roughly equivalent in strength.

This result does not surprise me, as all of the rings were made of annealed black wire. Heating one of the rings would likely only anneal it further since I did not follow up with a quench - that was not one of the original conditions. However, it is unlikely quenching would add significant hardness, either, as the wire is low carbon steel.


Steven E. Sheldon
July 29, 2000

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