As a student we conducted tensile tests on steel specimens. The purpose of one test was to demonstrate that the yield point of steel can increased by reversal of stress. The experiment showed that if a steel specimen is stressed to the yield point and the load is removed and then a load is reapplied the yield strength increases.At the same time elongation is slightly reduced but the ultimate strength is not changed.You can demonstrate this phenomenon by taking a paper clip and bending it back and forth. You will note that it gets progressively stiffer to bend the clip untill it finally fractures. This is the point when yield and ultimate strength are one and the same.It follows that the elongation is zero at fracture.
My understanding of the British rules of proof is that barrels submitted for proof that are rejected due to small bulge, may be repaired by hammering down the bulge. After reboring these barrels may be submitted for reproof. Provided they pass proof,they are approved for use by the gun maker.
