Fatigue life assessment of waste steel reused as tsunami buoy keel structures: A case study

Waste steel from used ship propeller shafts is reused for the keel structures of InaTEWS buoys. Because of the application of waste material, fatigue life assessment is critical. The purpose of this study is to assess the mechanical and fatigue properties of the waste material and to estimate the fatigue life of the keel structure as a result of sea wave loading. Material tests, fatigue tests and model tests were performed to obtain the parameters required for the estimation of the fatigue life, together with application of the spectral analysis method, including the effects of spectral band width. Chemical and tensile tests identified the material as low-carbon steel, with mechanical properties comparable to AISI 1035 steel. The fatigue tests resulted in an S-N curve (NS^m = K) with m = 7.7 and K = 3.2×10²⁴, showing a lower fatigue strength than AISI 1035 steel. The observed reduction in fatigue strength is ascribed to the previous use of the shaft. The calculated fatigue life based on the experimental S-N curve and the observed in-situ wave data is approximately 9.5 years, with a safety factor of 5.