The Interpretation of a Failure of an Ordnance Structure

Abstract

Abstract For the analysis of a failure of a war model of a 240-mm howitzer at half pressure, a fragment was furnished the laboratory. The problem was divided into three phases: First, the origin of failure; second, the mechanism by which the failure developed in the structure; third, the conditions responsible for the failure. Emphasis is placed on determining the spot at which the failure started. A method of reading the rupture flow lines found on the surface of the fragments is described. The arrow points found in torn metal surfaces are shown as pointing to the origin of rupture. These arrow points and other surface characteristics furnish the time scale by which priority as to time may be established for tears or for ruptures which appear to be independent. Once the beginning of a failure is found, however, the laboratory investigation then becomes simple as it may be confined to a small area. In this investigation the heat-affected zone of a weld was disclosed by acid etching. The design called for no welding at this point. Microscopic investigation showed that weld metal of an inferior quality was present, and that cracks existed in the heat-affected zone of the weld. A strip of metal of high Brinell hardness was found just outside the weld metal. The failure progressed from old cracks in this region of high hardness on the exterior surface of the gun. The tension and impact properties and the macroetch characteristics of the metal were determined and their influence upon the failure estimated. Finally, manufacturing records were checked and showed that a machine error on the surface in question had been repaired by welding.