On the use of critical distance theories for the prediction of the high cycle fatigue limit stress in notched Ti–6Al–4V

Methods are investigated for predicting the high cycle fatigue (HCF) lives of notched cylindrical Ti–6Al–4V specimens using critical distance concepts that employ the stress distribution in the vicinity of the notch. Cylindrical fatigue specimens had circumferential V-notches with a range of elastic stress concentration factors (kt=1.97–4.07). Notched and unnotched specimens were cycled to failure using a step-loading technique to generate points on a Haigh (Goodman) diagram for a constant fatigue life of 106 cycles. Finite element solutions were generated to provide stress distributions for the notched gage sections. The stress distributions were used in the search for a critical distance over which the quantities of mean stress, stress range, or elastic strain energy may contribute to the fatigue process and can be correlated to similar quantities from smooth, unnotched specimens. If the decrease in the local stress ratio at the notch root for high applied stress ratio is accounted for in the analysis, trends independent of applied stress ratio were found in the calculated critical distances. Predictions based upon the results gave accuracy to within 12% of the experimental fatigue limit stresses and illustrate the method has promise for use in fatigue design of Ti–6Al–4V components.