超载基础上的短时间保载所产生的塑性区对疲劳裂纹扩展率的影响(英文)

Effect of Plastic Zone Size Induced by a Single Dwell Overload on the Fatigue Crack Growth Rate under Cyclic Loading

潜水器所受变幅循环载荷中包含了疲劳和室温蠕变两种载荷模式。由于变幅载荷的复杂性,要预报结构的寿命需要对载荷进行合理的简化。其中一个典型的简化载荷模式就是在循环载荷的基础上具有单个超载并在超载基础上进行短时间保载。以往的试验研究表明,超载基础上的短时间保载会造成裂纹扩展的迟滞,但目前无可靠的理论模型能够预报这一现象。文章针对这一载荷模式,提出了一个基于裂纹尖端单向压缩塑性区变化的简化预报模型,给出了超载和保载共同产生的塑性区大小以及裂纹闭合大小的计算方法,并以常用潜水器材料TC4 ELI为例进行了演示。其中,裂纹扩展率模型及蠕变模型参数均通过试验获得。计算结果显示,该模型能够合理地反映保载塑性区造成的裂纹扩展迟滞效应。

Creep and fatigue are involved in the loading of deep manned submersible, which is a rather complex variable amplitude pattern. Reasonable assumptions should be made before calculating the room temperature dwell fatigue life. A typical simplified loading type is the cyclic loading with one period of overloaded dwell time. Retardation effects are observed in the prior experimental research while no proper prediction equations were proposed to explain the phenomenon. In this paper, a new prediction model for the load pattern of cyclic loading with a single dwell overload is proposed. Retardation is explained by the increase of monotonic compressive plastic zone size. The calculation methods of the total monotonic plastic zone size in the vicinity of crack tip, due to the overload and the dwell time are provided and the corresponding variation of crack opening level based on the total monotonic plastic zone can be obtained. Tests are conducted to obtain the parameters used in the crack growth models and creep models together with tensile test results of the material TC4 ELI, which is as an example to calculate the retardation effects of overload and dwell time. It is observed that the model can reasonably reflect the retardation effects due to compressive plastic zone in the vicinity of the crack tip induced by dwell time.