The test was devised in order for the average owner to determine whether the shank of his Rocna anchor was made to the standard originally desired by the designer, around 800 megapascals, or to the far lower strength of many production anchors, around 400 megapascals. In this case the anchor shank was a later version in which the strength is close to the original design intention, at 742 MPa.
DIY Brinell test (is your shank made from good steel?)
The Brinell test is a simple method of determining the hardness, and therefore the strength, of a metal. A small ball bearing is pressed into the surface of the test metal where it produces an indent. In the standard test the diameter of the indent is measured and the hardness is determined from tables. In the comparative test two indents are produced using the same load, one in the specimen and one in a metal of known hardness, allowing the diameters to be compared.
I carried out a comparative test using the head of a metric 8.8 bolt as the standard. 8.8 steel has an ultimate tensile strength (UTS) of 800 megapascals (Mpa) and a yield strength (YS) that is 80% of that figure, 640 Mpa, similar to the Rocna shank as designed. The load was applied using a vice, sandwiching the ball between the bolt head and the anchor shank. I used a cut sample but the shank could be used almost as easily, in which case a small area of galvanising needs to be ground off. I found it easier to grease the ball to hold it in place. Neither the exact load nor the ball size is critical. I leaned fairly heavily on the vice handle and left the load on for about 10 seconds.
The photograph shows the results. On the right, blue arrows, are the indents produced on the shank and bolt head. These are of similar size, as would be expected. On the left, red arrows, are the indents produced in the bolt head and a spacer that I estimate to have similar properties to Q420 steel. The indent produced in the spacer is obviously larger, signifying softer steel. The ball bearing I used is shown at the right; these are readily available at, for example, cycle shops.
