Study on Fatigue Strength of Welded Joints Subject to Intermittently Whipping Superimposed Wave Load

The stochastic characteristics of hull vibration superimposed stress waveform experienced by 6500 TEU container ship’s deck longitudinal is examined by Non-Linear Hydro-Elasticity Analyses. It is found that the stress waveform’s characteristics can be simplified so that the slamming impact occurs once in every 4 to 5 waves and the impact stress range is comparable to the wave stress range. Based on these results, the whipping superimposed stress waveform which are applied in fatigue tests are chosen. An electric exciter-driven plate bending vibration fatigue testing machine, which can apply various hull vibration superimposed stress waveform in high speed, is newly developed. Performing the whipping superimposed fatigue tests of welded joints using this machine, the validity of rainflow cycle counting for whipping superimposed loadings is examined. It is found that rainflow counting led to conservative estimates of fatigue lives, and the fatigue damage up to the failure of intermittently superimposed cases is larger than that of constantly superimposed cases under conditions chosen.