Critical Projection Cycle Counting Exploration Method for Fatigue Life Prediction of Small-Bore Connection Due to Multiaxial Piping Vibration

Abstract This study presents a method to reliably calculate the critical fatigue life prediction and identify the critical fatigue orientation of a small-bore connection (SBC) due to multiaxial piping vibration. This method was named “Critical Projection Cycle Counting Exploration (CPCCE)” method. As the previously presented method does, this method also requires just inputting the measured strain data into a spreadsheet after setting Young’s modulus, Poisson’s ratio and a fatigue curve with data points until 1011 cycles. The strain data consist of two bending strains and a torsional strain around an SBC measured by a strain gauge holder. Although the previous method could not obtain the critical accumulated fatigue damage from some strain data in a single calculation, the CPCCE method, which was added an “exploration” function to the previous one, could reliably obtain the critical accumulated fatigue damage from any strain data. The critical fatigue life predictions of 39 SBCs could be successfully calculated by the CPCCE method, out of more than 200 SBCs in real plants where the strains have been measured by strain gauge holders. The 39 SBCs were extracted because the maximum combined stress of those were greater than 10MPa. The calculation results were sufficiently conservative and relatively realistic. The accuracy of the measurement by the strain gauge holder was also confirmed by mockup testing and the reason for the high accuracy is explained in this paper.