Fatigue precracking of spin-burst toughness specimens

Abstract

A hydrostatic pressure-cycling technique developed to permit controlled fatigue precracking of large spin-burst fracture-toughness specimens (disks in excess of 12 in. in diameter and 3 in. long) is described. The procedure involves localized internal-pressure cycling of the specimens by varying the hydrostatic pressure within a small “notched” hole. The technique is capable of generating hydrostatic pressures in excess of 40,000 psi at an operating frequency of one pressure cycle per second. In addition, the cyclic stresses are independent of specimen size, provided thick-wall pressure-vessel conditions exist. An ultrasonic flaw-detection technique used to measure the extent of fatigue-crack growth during cyclic loading is also described. The pertiment data associated with the fatigue precracking of eight alloy steel (σys ≈ 85,000 psi) spin-burst specimens approximately 13 in. in diameter and 10 in. long are presented. The effectiveness of pressure cycling and the accuracy of the crack-monitoring technique are discussed. At a cyclic load range of 8000 to 40,000 psi, fatigue cracks on the order of 0.3 in. long were developed in approximately 60,000 cycles (17 hr). The ultrasonic crack-length-measurement technique was found to exhibit a sensitivity of 0.030 in.