重畳ランダム荷重下におけるアルミニウム合金溶接継手の疲労寿命推定 : 2次元レインフロウ法の適用

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In some structures and machines, e. g., railway bridges, aircraft, high speed ships, gas turbines, and so on, engineering components experience characteristic load cycles in which secondary load waves with relatively small amplitudes and short periods are superimposed on primary load waves with large amplitudes and long periods. As those load waves generally have broad banded spectra and large irregularity factors, ordinary fatigue life estimation procedures for narrow banded waves, such as simple Rayleigh approximation, can hardly be applied. In such cases, it is necessary to analyze the amplitude distribution characteristic of each load wave. Furthermore, as the primary load waves can act on the secondary load waves as mean loads, mean stress effects should be considered in fatigue life estimation. In this study, fatigue tests were carried out on welded T-joints of JIS A5083P-O aluminium alloy under constant amplitude and superimposed random load waves. A 10 mm thick rib plate was attached to a 10 mm thick main plate by automatic MIG welding. After the welding, weld toes were ground with a pencil grinder. Each random wave was generated by superimposing a secondary zero-mean random process having a specified power spectral density on a primary wave. Both broad banded and narrow banded spectra were used for the secondary random waves, and constant amplitude pulsating trapezoidal waves, which simply simulated the. GAG (Ground-Air-Ground) loads, and constant DC (Direct Current) components were used as the primary waves. For fatigue life estimation, three types of wave count methods were comparatively examined, namely, the range count method, the 1-dimensional rainflow method and the 2-dimensional rainflow method, with combined to the modified Miner's law. And each stress range obtained by the 2-dimensional rainflow method was corrected by the modified Goodman's line and the Gerber's curve according to the mean stress value prior to the application of the modified Miner's law. It was found that the range count method is not suitable to those kinds of superimposed random waves because of its incapability of counting the maximum range during each GAG load cycle. Moreover, the 1-dimensional rainflow method is still insufficient as the mean stress effects are not taken into consideration. From the life estimation with the 2-dimensional rainflow method and the modified Goodman's correction, it was concluded that the mean stress effects of primary waves on secondary waves are larger in the case of GAG than DC. Considering this, a life estimation procedure was proposed, in which σ_u and σ_T were properly used in the modified Goodman's correction according to the primary wave type (GAG or DC), and gave the most consistent and acceptable estimation results.
社団法人日本船舶海洋工学会の論文

重畳ランダム荷重下におけるアルミニウム合金溶接継手の疲労寿命推定 : 2次元レインフロウ法の適用

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