船舶结构多轴高周疲劳评估的时域和频域方法研究

Ship structures often produce multiaxial stresses in some welded joints detail, because of random wave loading. So it prone to occur multiaxial fatigue failure. To assess their fatigue life, the goal of this project is to provide alternative methods in the time and frequency domain which are computationally more efficient, and can be applied directly in the ship structure fields. Based on the stresses response amplitude and phase per unit wave amplitude for each wave frequency, the short term time traces of all stress components can be obtained. Consequently, the multiaxial fatigue assessment methods can be applied in the ship structures, and short term fatigue damage can be got. Considering the shear stress and the normal stress in the critical plane separately based on IIW rule, each of fatigue damage will be calculated by uniaxial fatigue method in the frequency domain. The damage combination method of both will be studied through the fatigue calculation method in time domain. Fatigue tests of the welded specimen under tension and torsion are carried out to verify the validity of the proposed methods. The multiaxial fatigue assessment methods are applied to calculate structural fatigue damage in short-term sea state, then the damage during the entire service time could be achieved combined with the damage in sea state of each short-term. Based on the above methods, the author will develop a post-processing procedure in ANSYS software to achieve rapid assessment the damage due to multiaxial fatigue of ship structures. The proposed method can provides technical support for the design and analyze of ship structure.

船舶结构频繁地遭受到随机波浪的作用，其结构的局部经常处于多轴应力状态，易于发生多轴疲劳破坏。本课题拟以船舶结构的多轴疲劳问题为研究对象，开展时域和频域分析方法研究。根据单位波幅波浪载荷作用下船舶结构的应力响应幅值和相位，得到短期海况下各个应力分量的时间历程，从而使多轴疲劳分析方法可用于船舶结构。根据IIW规范，以临界面方法为基础，将剪应力和正应力分开来考虑，分别计算疲劳损伤，结合时域疲劳计算方法，研究两者的损伤组合。进行拉扭联合作用下焊接试件的疲劳试验，验证所提出方法的正确性。将多轴疲劳时域和频域评估方法应用于短期海况下的结构疲劳损伤计算，联合各个短期海况的损伤得到整个服役期内结构的损伤。根据此方法开发基于ANSYS有限元软件的后处理程序，实现船舶结构多轴高周疲劳的快速评估。研究结果可为船舶结构的设计和分析提供技术支持。

船舶与海洋工程结构频繁地遭受多种随机载荷作用，结构的局部经常处于多轴应力状态，易发生多轴疲劳破坏。以随机载荷作用下焊接结构的多轴疲劳问题为研究对象，开展频域疲劳分析方法研究，给出了多轴高周疲劳分析的频域方法，主要研究内容如下：.(1) 研究了影响多轴疲劳损伤计算的主要因素，确定了合理的疲劳损伤计算参量。结果表明剪正应力比对多轴疲劳的寿命分析有很大影响；对于比例加载情况可以将多轴疲劳问题转化为单周疲劳，而对于非比例加载情况使用单轴疲劳计算方法会给出偏于危险的结果。.(2) 研究了6种不同规范中的多轴疲劳分析方法，使用焊接结构多轴疲劳试验数据验证了规范方法的有效性。结果表明规范方法在比例加载情况下给出的结果较保守，而在非比例加载情况下会给出部分偏于危险的结果。.(3) 构造了参数化的双峰矩形功率谱密度函数，通过数值模拟方法将功率谱密度函数转化为时间序列，研究了8种单轴疲劳频域分析方法的计算精度。结果表明Benasciutti –Tovo方法、Park方法的计算精度较高，并且Park方法和Dirlik方法的雨流范围概率密度函数和模拟方法符合较好。.(4) 基于最大主应力和相互作用方程方法，建立了两种多轴疲劳频域分析方法。选用海上风机支撑结构为分析对象，对以上两种频域分析方法进行了验证。结果表明计算得到的正应力功率谱密度函数和时域计算结果符合较好，基于主应力的近似方法在峰值部位的响应略高于时域分析结果。