Prediction of fatigue life and its probability distribution of notched friction welded joints under variable-amplitude loading

In the present study, fatigue tests under constant-amplitude loading (CAL) and variable-amplitude loading (VAL) are carried out on the notched frictio n welded (FW) joints of 45 (0.45%C) carbon steel, which is similar to the A ISI 1045 steel. Statistical and regression analysis on the fatigue test, re sults under CAL shows that the fatigue life of 45 steel notched EW joints w ith small size can be expressed as the function of the equivalent stress am plitude, and the values of the fatigue resistant coefficient and threshold are thus obtained. From the probability distribution of fatigue resistant c oefficient and threshold, the expressions of the fatigue life with given su rvivability, i.e., the so-called P-S-N curves of 45 steel notched FW joints are obtained, which can cover the whole life region, in particular, the lo ng life range and are used to determine the probability distribution of fat igue strength of 45 steel notched FW joints. Furthermore, the so-obtained e xpressions for P-S-N curves and Miner's rule are directly applied to predic t the fatigue life and its probability distribution of 45 steel notched FW joints considering that 45 steel is of non-continuous strain-hardening char acteristic and thus nil overloading effect. The prediction procedures for f atigue life with given survivability of 45 steel notched FW joints under VA L by using Miner's rule and the expressions of P-S-N curves are outlined. T he fatigue life and the critical cumulative fatigue damage of 45 steel notc hed FW joints under VAL predicted by using the above procedures follow the log-normal distribution, and checked by test results under one type of prog rammed block loading and the statistical analysis. The procedures mentioned above may be applicable in predicting the probability distribution of fati gue life of the notched elements of metals with non-continuous strain-harde ning characteristic, including the notched friction welded joints under VAL . (C) 2000 Elsevier Science Ltd. All rights reserved.