Maybe they mean special metal alloys used to make leaf springs? Sounds like a test to failure machine to me, but more context would be helpful

because springs would be difficult to test, given that they are designed to be resilient, to deform. If testing steel, it can relatively safely be assumed that the material is intended NOT to yield too much, so you can grip it between two sets of jaws and pull it till it breaks. Try doing the same with a spring! You'd need a very long test bed, or some means of correlating from tests carried out on short specimens cut from the original, etc.