This paper describes a relationship between fatigue failure and the input energy from results of forced vibration experiment using straight pipe. In Japan, mechanical structures installed in nuclear power plants such as piping and equipment are usually designed statically in the elastic region. Although these mechanical structures have a sufficient seismic safety margin, understanding the ultimate strength is very important in order to improve the seismic safety reliability in an unexpected severe earthquake such as the Great Kobe Earthquake (1995) and the Niigataken Chuetsu-oki Earthquake (2007) in Japan. A rational design method is also being required in accordance with a revision of seismic standard in Japan. In this study, the ultimate strength of a mechanical structure is investigated from a viewpoint of the energy balance equation that is one of valid methods for the structural calculation. A main feature of the energy balance equation is that explains accumulated information of motion. Therefore the energy balance is adequate for an investigation into the influence of cumulative load. Authors have already confirmed a relationship between fatigue failure and the input energy from results of experiments using simple single degree of freedom models in a previous study. In this paper, straight pipe models are adopted instead of simple single degree of freedom models, and the relationship between fatigue failure and the input energy is investigated. The straight pipe model is made of stainless steel, and has natural frequency of approximately 21 Hertz. The investigation is implemented by forced vibration experiments that lead the experimental model to fatigue failure. In the experiment, colored random waves whose predominant frequencies are similar to natural frequency of the experimental model are input. The experimental model vibrates in the elasto-plastic region due to the colored random wave input, and cracks finally. As a result of the experiment, it is confirmed that the input energy for failure increase with an increase of time for failure. In other words, much input energy for failure is needed in case of small input level. This tendency is same as the result of the previous study using simple single degree of freedom model. Therefore it is expected that any actual piping satisfy this tendency, and time for failure can be expected by using energy balance.

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