THE EFFECT OF MEAN STRESS ON MIXED-MODE (I+III) FATIGUE THRESHOLDS

Mode I, mode III and mixed mode (I + III) fatigue crack growth thresho ld tests have been performed on a 3.5%NiCrMoV steel at a range of mean stresses corresponding to load ratios R = 0.06 to 0.5. Angled slit th ree point bend specimens were used for mixed mode (I + III) tests, cir cumferentially slit round bars for mode Ill tests and conventional thr ee point bend specimens for mode I tests. Test results were divided in to those specimens which had failed and those in which no extension of the initial slit had occurred. The results were compared with the res ponse predicted by two existing mixed-mode threshold models. It was fo und that a model based on the magnitude and direction of mode I crack opening gave results which were in better agreement with the experimen tal results than a model based on branch crack formation.