On Reliability Evaluation of a Structure for Fatigue under Service Loads with Rain Flow Method and Fatigue Limit Diagram
Masaru ZAKO, Tetsusei KURASHIKI and Hiroaki NAKAI
Abstract:A service load like random load has been analyzed by the rain flow method extensively. When the reliability of a structure is estimated, mean stresses on fatigue life are probably ignored because it is not easy to measure. The purpose of this study is to propose a practical estimation method of a structural reliability for fatigue under service loads by Miner's law. In addition, the fatigue tests to obtain S-N curve is carried out under constant amplitude loading. However, the each stress ratio of cyclic stress counted with rain flow method is different. In the proposed method, the effect of stress ratio has been considered by fatigue limit diagram with failure probability. On this diagram, the increment of cumulative cycle ratio ‡™Dpf for each cyclic stress can be calculated with failure probability by a computer program. The expected life Npf is defined as the period when Dpf reaches 1.0. If we assume the ergodicity of the sampling time T, Npf is T/ Dpf. It gives fatigue life distribution, when a structure is under service loads. In conclusion, we have got a reliability curve of the structure for fatigue life. This curve can be obtained with service load or design stress easily by using our developed program. This proposed method is practical and the industrial applicability is wide. Key Words:Service loads fatigue, Rain flow method, Mean stress, Fatigue limit diagram, Cumulative cycle ratio, Reliability