基于雨流计数法的高低频复合疲劳试验研究

船体固有频率与遭遇波浪频率及其倍频相接近时,波浪载荷极易引起船体结构产生持续的波激振动现象,对大型船舶结构疲劳损伤的影响达到40 %以上。因此,有必要针对波激振动引起的非线性垂向弯矩载荷特点,开展高低频复合工况下典型切口试件疲劳试验及累计损伤分析研究。基于非线性随机载荷的分析方法,采用闭合雨流计数法提取叠加应力历程中的多级循环载荷,并结合平均应力修正方法和疲劳极限以下SN曲线局部修正法,对叠加应力历程中小载荷的损伤效应展开研究。分析结果表明,高低频载荷叠加产生的附加损伤效应对疲劳寿命的影响显著,在恒定应力比下随平均应力的变化近似成二次函数关系。考虑高频小载荷的叠加应力历程疲劳寿命明显降低,可以通过修正累计损伤临界值或引入应力放大因子,近似的利用线性累积损伤理论预测实际叠加应力历程的寿命范围。

The Study of Multiple Frequencies Fatigue Test Based on Rainflow Counting Method

When the hull natural frequency is close to the encountered wave frequency and its frequency, the wave load can easily cause the hull structure to generate continuous wave-induced vibration, and the impact on the fatigue damage of the large ship structure reaches more than 40%. Therefore, it is necessary to carry out fatigue test and cumulative damage analysis of typical notched specimens under high and low frequency composite conditions for the characteristics of nonlinear vertical bending moment loads caused by wave-induced vibration. Based on the nonlinear random load analysis method, the closed rain flow counting method was used to extract the multi-stage cyclic loading in the superimposed stress history. Combined with the mean stress correction method and the local correction method of the SN curve below the fatigue limit, the small load damage in the superimposed stress history was analyzed. The analysis results show that the additional damage effect caused by the superposition of high and low frequency loads has a significant influence on the fatigue life, and it approximates the quadratic function relationship with the change of the average stress under the constant stress ratio. Considering the superimposed stress history of small loads with high frequency and small loads, the fatigue life is obviously reduced. The cumulative damage history can be approximated by using the linear cumulative damage theory by correcting the critical value of cumulative damage or introducing the stress amplification factor.