craigd
Every time steel is stressed to its elastic limit it loses some of its elasticity.If a tensile sample is stressed to its elastic limit and the load is repeatedly removed and reapplied the elasticity in the steel is ultimately eliminated, the net result being that the ultimate strength of the sample under test and the elastic limit occur at the same point on the stress/strain curve,the net result is immediate fracture.This phenomenon is best illustrated by bending back and forth the limb of a paper clip.The limb gets progressively stiffer to bend until fracture occurs.This represents the ultimate strength/yield point of the cliphas been reached.
In terms of a gun barrel it is more liable to crack every time a bulge is removed[ie yield point is raised].I would point out that it is acceptable that barrels bulged in proof can be hammered down,rebored as required and resubmitted to proof.Many repaired barrels pass proof because the yield point of the barrel steel was increased during the repair proceedure.
