基于累积塑性破坏的船体板低周疲劳裂纹扩展寿命研究

船体板的总体断裂破坏往往是低周疲劳破坏与累积塑性破坏两种破坏模式耦合作用的结果，故在船体板低周疲劳裂纹扩展寿命评估中，其基于累积塑性应变的船体板低周疲劳裂纹扩展寿命分析能够更为符合实际地评估船体板的总体断裂承载能力。船体板低周疲劳裂纹扩展寿命由宏观可检测裂纹扩展到临界裂纹而发生破坏这段区间的寿命。船体在实际航行中受到多次波浪外载作用而使其进入塑性变形不断累积或不断反复的破坏过程，并最终导致低周疲劳裂纹的萌生及扩展而使结构破坏，其破坏形式分别对应于增量塑性变形破坏（或棘轮效应）或交变塑性变形破坏（或低周疲劳）。局部塑性变形的累积会加剧低周疲劳裂纹不断扩展，因而基于累积塑性破坏研究船体板低周疲劳扩展寿命更为合理。文中以船体板单次循环载荷后塑性应变大小为基础，依据累积递增塑性破坏过程及弹塑性理论，计算经过N次变幅循环载荷后船体板累积塑性应变值，结合循环应力—应变曲线获得相应的稳定的迟滞回线，确定裂纹尖端应力应变曲线及确定相关塑性参量并依据选取的断裂判据判定裂纹扩展。建立循环载荷下基于累积递增塑性破坏的船体板低周疲劳裂纹扩展寿命的计算模型考虑应力比对此裂纹扩展寿命计算模型的影响。由该方法计算出的疲劳裂纹扩展寿命将对正确预估船舶结构的低周疲劳强度从而提高船舶安全性有重要意义。

Research on Low-cycle-fatigue Crack Propagation Life for Ship Plate Based on Accumulative Plastic Damage

The fracture failures of ship plates under high in-plane cyclic loading are generally the coupling result of low-cycle-fatigue (LCF) damage and accumulative plastic damage. Therefore, an analytical model for predicting the LCF crack propagation life of plates was proposed taking account of accumulative plastic damage. The propagation of LCF crack is considered to be associated with ac-cumulation of plastic strain that could be expressed as a part of the area of the saturated hysteresis loop. The LCF crack propagation life was analyzed by using the elasto-plastic fracture mechanics ap-proach accounting for accumulative plastic damage. In this study, the focus is to get the accumula-tive plastic strain after N time’s constant amplitude cyclic loading, stress-strain curve at crack tip and relevant plastic parameters of the plate under high in-plane cyclic loading. A cyclic plasticity model combined with the fracture criterion that is presented by Chaboche is implemented, and then the LCF crack propagation prediction can be realized by discontinuous crack propagation assump-tion. In order to evaluate the performance of the approach, a numerical model was generated based on accumulative plastic strain, allowing for the influence of the stress ratio. It has been found that high level stress ratio increases the propagation of fatigue crack significantly. For the validation of the present analysis, the results of this paper were compared with those from available experimental da-ta of the reference. A good coincidence was found between the predictions of the paper and the ex-isting experimental results. Results reveal a strong dependence of LCF crack propagation on accumu-lated plasticity, and show that the analytical model of the paper is capable of predicting LCF crack propagation life of plates.