Effects of initial cracks and firing environment on cannon fatigue life

Abstract

Abstract A case-study description is given of laboratory fatigue life tests of a US Army 155 mm inner diameter cannon tube, performed in the early 1990s. Measured fatigue lives and results from stress and fracture mechanics analyses are used to determine the effects of service conditions on the safe fatigue life of the cannon tube. Fatigue failure in the laboratory tests occurred at nearly the same number of load cycles at the tube inner diameter and at a notch on the tube outer diameter, so the different effects on life at the two locations are considered. The description of the life test results and the related mechanics analyses include: measured initial crack sizes for different firing environment and analysis of the effect of initial crack size on life; solid mechanics calculations of local applied and residual stresses at the locations of fatigue crack growth; fracture mechanics assessment of fatigue life including effects of initial crack size, applied firing stresses, residual overstrain stresses, and stress concentrations; a log normal statistical analysis of safe fatigue life for various combinations of test results. The tests and analyses, combined with other related work, show that [i] the use of fatigue intensity factor in a stress — life plot gives a consistent description of fatigue life over a broad range of test variables, including cylinder configuration, initial crack size, applied and residual loading, and material yield strength; [ii] the fatigue intensity factor method can be used to differentiate between fatigue scatter and abnormal fatigue life behavior and to focus on the cause of the abnormal behavior; [iii] a larger than expected initial crack size of 0.05 mm was identified by metallography and found to be the cause of a significant decrease in calculated log-normal safe fatigue life.