Estimation method of the equivalent cumulative strain of elbow piping subjected to in-plane repetitive bending displacement load using cumulative strain damage rule that considers multiaxial stress level

Elbow piping subjected to in-plane repetitive bending displacement load suffers reduction in real rupture ductility in multiaxial stress field and the accumulation of cumulative strain. As a result, it lowers low-cycle fatigue life significantly more than the values from the optimum fatigue curve obtained by uniaxial tensile-compression small-scale test or Manson equation. This paper outlined the elbow piping test under the in-plane repetitive bending displacement load that became basis for evaluation method development and the analysis based on finite element method. The author proposed a method to combine the extended modified universal slope method as an empirical expression and the cumulative damage rule considering multiaxial stress level, thus enable the direct estimation of the value of cumulative strain that is difficult to calculate in good accuracy according to the general-purpose structural analysis code. This paper described this author's method and its evaluation results. In addition, this paper described the tolerance possessed by the ASME design fatigue curve, based on the concept of cumulative damage rule considering multiaxial stress level. The scope covered in this paper is a repetition area of about 60 to 300 times, which is the number of repetitions for the seismic design of the primary piping of light water reactors in Japan. (A.O.)