Fatigue Life Evaluation of an Actual Structure under the Irregular Loading Using an Acceleration Test

Abstract

Various types of random loading have been used in the fatigue tests and in the fracture mechanics determination of structures; Under service conditions, the fatigue load varies randomly, and hence, the overall uncertainty of fatigue life predictions increases. A fatigue test was carried out on a specimen to evaluate the fatigue life of an actual structure. To apply the test results obtained from the specimen to an actual structure, the loaded state and the constraint condition of the specimen must be the same as those of an actual structure. However, the loaded state and constraint conditions of a specimen cannot be the same as those of an actual structure, which are complicated. In order to reduce these differences, an actual structure test was carried out for a wide range of frequencies to obtain a fatigue life curve. In this study, ten sets of accelerated test units attached with an unbalanced mass were prepared. Also, the acceleration history on the vibration of an actual structure was acquired. Rainflow counting was used on the acceleration history, and the life curve return formula was assumed. The measure of damage is simply the cycle ratio based on the assumptions of constant work absorption per cycle, and characteristic amount of work absorbed at failure. The return formula that cumulative damage satisfied '1(100%)' was acquired in a feedback process by the Miner's rule, which is the linear cumulative damage theory. A conservative fatigue life curve was determined by the return formula to have been presumed by each set. The fatigue life of an actual structure under regular RPM condition was calculated based on these conservative fatigue life curves.