FATIGUE LIFE MEASUREMENTS AND ANALYSIS FOR OVERSTRAINED TUBES WITH EVACUATOR HOLES

Sections of cannon tubes with inner radius of 53, 60, and 78 mm were c ycled from near zero to 100-300 MPa internal pressure until fatigue fa ilure occurred. The failure locations were along 2-mm holes cut throug h the cannon wall at a 30-deg angle to the tribe axis, for the purpose of evacuating combustion gases from the cannon after firing. The cann ons had various amounts of autofrettage by overstraining, including 0, 30, 50, and 100 percent. The amount of overstrain affected both the i nitiation position of the fatigue crack along the evacuator hole and t he measured fatigue life. Increasing the amount of overstrain moved th e crack initiation from the tube inner radius toward a mid-wall positi on and significantly increased fatigue life. Fracture mechanics and so lid mechanics-based calculations of fatigue life were performed for co mparison with the measured lives. The calculations gave a goad descrip tion of the measured life, taking account of tube configuration, appli ed pressure, amount of overstrain, stress concentration of rite hole, crack size and shape, material fatigue crack rate behavior and yield s trength, and pressure in the hole and on the crack surfaces. As with m easured fatigue life, the calculated life was significantly affected b y the amount of autofrettage of the tube. The ratio of outer to inner radius of the tube and the presence of pressure in the evacuator hole also had substantial effects on the calculated fatigue life.