Army Cannon Fatigue Life Evaluation: Crack Initiation, Fracture Mechanics, andNDI

摘要

Laboratory hydrostatic test results and fracture mechanics analysis from cannonfatigue life investigations are used (1) to demonstrate the large changes in total fatigue life caused by the initial crack size and the levels of applied and residual stress at the cannon bore, and (2) to compare ultrasonic nondestructive insection (NDI) measurements of crack growth and cannon fatigue life measurements with fracture mechanics-based crack growth and life analyses. The cannon of this investigation have 120-mm inner diameter, a nominal 75-mm wall thickness. and are made from ASTM A723 Ni-Cr-Mo-V steel, quenched and tempered to 1120 MPa yield strength. Problem Solution: The methods of testing and analysis are believed to be generally applicable to problems in fatigue liie evaluation of pressure vessels. particilary to gain an understanding of the effects of crack size, applied and residual siress, and vessel configtnticn on the service fatigue life. Proceccres and result are presented which show the importance of initial crack size obtained from NDI methods in determining the fatigue lifetime of cannons and other pressure vessels. Unresolved Problem: An unresolved problem will be presented for possible solution, that being the need for an NDI methed to accurately measure initial crack sizes at a cannon bore. Characterization of the thermally-induced cracking at a connon bore is a difficult problem, since a network of closely-spaced, very short (--0.1mm) cracks is produced in a layer that is also severely thermally altered from that of the base material.