长寿命服役结构疲劳内部破坏规律与强度评价

Towards a fundamental understanding of failure assessment and life prediction of mechanical products under extreme conditions, an attempt is made to resolve the micro-mechanisms of interior fatigue failure and strength assessment of long-term service structures, which is a common and basic issue facing engineering structures undergoing lower stress with longer lifetime.. In doing so, a multi-scale characterization of initial fatigue damage around interior micro-defects from very high cycle fatigue is firstly proposed to investigate the physical mechanisms of interior fatigue failure. Based on a new assumption of the similarity between micro-defects and notches, an in-situ fatigue test framework using specimens with machined holes is designed to track the formation and establish the evolution laws of fine granular area that is often observed on fracture surface on fatigued samples, by utilizing the current theory of cracking from notches. The aim is to finally develop a fatigue damage model that can describe the whole internal failure process. Then a cross-scale fatigue strength model from materials to structures is put forward, incorporating the developed micro-mechanisms of interior failure, to propose the solutions to fatigue strength assessment of long-term service structures.. The objective of this work is, to propose a micro-mechanism-based solution to fatigue strength assessment, via a scientific understanding of damage and fracture mechanisms for interior fatigue failure in the point view of both material and mechanical science, and thus to lay a fundamental basis for design against fatigue of engineering materials and components with long-term service requirement.

围绕构筑极端条件下机械产品失效评价与寿命预测的科学基础，以低应力长寿命服役结构面临的共性基础问题为突破口，尝试解决长寿命服役条件下材料内部疲劳破坏规律与结构强度评价难题。通过对超高周疲劳内部破坏微缺陷处的初始损伤进行多尺度的表征，研究内部疲劳破坏的物理本质；提出内部微缺陷与缺口的相似性假设，基于微孔处的原位疲劳试验研究和缺口疲劳断裂理论，研究内部疲劳破坏初始损伤细晶区的形成及演化规律，构建反映内部疲劳破坏全过程的损伤断裂模型；通过建立材料-结构跨尺度的疲劳强度分析模型，结合内部疲劳损伤断裂规律，提出长寿命服役结构疲劳强度评价方法。目标是从材料和力学角度获得内部疲劳破坏的损伤机制及断裂规律的科学认识，提出基于内部破坏物理本质评价疲劳强度的方法，为长周期运行工程材料及构件的抗疲劳设计奠定基础。

超高周疲劳研究对解决超长寿命服役条件下结构强度设计具有重要意义。项目围绕“内部缺陷处疲劳破坏的损伤机制及裂纹扩展规律”、“结构长寿命疲劳强度评价方法”两个科学问题，重点开展了超高周疲劳缺陷致裂机理、焊接接头的超高周疲劳行为、机械结构的防断设计方法和结构损伤的测量技术等4个方面的研究。研究取得的进展包括：（1）提出了“板条马氏体断裂形成位错胞”的细晶区机理模型，建立了基于微缺陷的超长寿命预测Z参量方法，揭示了超高周疲劳微缺陷致裂的物理本质；（2）澄清了取样方式、加载频率、试样尺寸、测试温度、临氢环境等对焊接接头超高周疲劳行为的影响规律，提出了疲劳强度评价模型，发现了焊接结构削弱系数与疲劳寿命的相关性，形成了焊接结构抗疲劳设计思路；（3）拓展了确定断裂阀值的裂纹闭合与裂纹扩展驱动力统一模型，提出了基于断裂参量的结构防断思路，使结构防断理念由“被动预防”走向“主动控制”；（4）建立了高精度的结构损伤及裂尖场测量新方法，提出了缺陷致裂的临界应变准则，提供了疲劳裂纹扩展闭合效应、过载机制等经典疲劳现象的科学证据，促使疲劳损伤测量与表征成为学科新方向。研究共发表学术论文24篇，其中在Acta Materialia、International Journal of Fatigue等本领域权威期刊发表SCI论文9篇，申请/授权国内外发明专利2项，获上海市技术发明一等奖、国际会议优秀论文各1项；项目负责人受邀作国内外学术会议邀请报告13次，入选为上海市“青年科技启明星”，并获得基金委优秀青年基金项目资助；培养毕业博士生2名、毕业硕士生3名，另有6名硕士生在读。