广义喷丸法生成的梯度纳晶金属的断裂行为的建模与模拟研究

Surface nanocrystallization through various shot peening methods has become the most widely used approach to generate gradient nanocrystalline (GNC) metals. However, the extremely high intensity and high coverage adopted in the shot peening process may cause surface damage and early failure of the treated metal parts. Motivated by this situation, the fracture behavior of GNC processed by generalized shot peening method is systematically investigated in this project. Numerical simulations are carried out to investigate the mechanisms behind the fracture behavior from both macroscopic and microscopic viewpoints. Specifically, we intend to (1) determine the stress fields near the crack tip in GNC metals and establish appropriate initiation criteria for the crack propagation, (2) investigate the influence of surface roughness on the stress distribution, and the effect of re-shot peening with weak intensity on the surface roughness, (3) simulate the interactions between the crack and the defects such as grain boundaries, dislocations, and twin boundaries, and (4) develop a novel numerical scheme to simulate the crack growth and/or blunting in GNC metals considering grain boundary mechanisms, grain interior plasticity and damage evolution. This project is believed to contribute to the improvement of toughness and reliability of the GNC metals.

通过喷丸处理使表面纳米化是制备梯度纳晶金属的常用方法，但改方法采用的过度喷丸容易造成疲劳断裂和早期失效。本项目针对这一现状，以喷丸引起的表层微裂纹和表面粗糙度增加为着眼点，在梯度纳晶金属断裂失效这一重要却未充分研究的领域开展研究。项目主要采用数值模拟手段从宏观和微观两个角度研究广义喷丸法制备的梯度纳晶金属的断裂行为。研究内容包括：（1）求解梯度纳晶金属中裂尖附近的应力场，建立适合梯度纳晶金属的起裂判据；（2）研究表面形貌对表层应力分布的影响，探讨通过二次弱喷丸改善表面粗糙度及应力分布的途径；（3）模拟梯度纳晶铜中裂纹与晶界、位错、挛晶等微缺陷间的相互作用；（4）兼顾晶内和晶界主导机制，结合晶界演化、晶内塑性、损伤演化发展新的数值方法来模拟梯度纳晶中裂纹扩展过程。本项目旨在为提高梯度纳晶金属的服役可靠性提供理论指引和技术储备。

喷丸表面纳米化是制备梯度纳晶金属的常用方法，该方法具有成本低、适应性广、操作灵活等优点，但所涉及的过度喷丸容易造成损伤断裂和早期失效。本项目针对这一现状，研究了喷丸处理参数和表面完整性关系，以及喷丸法生成的梯度纳晶金属的断裂行为，旨在为提高梯度纳晶金属的制备完整性和服役可靠性提供理论指引和技术储备。项目主要采用有限元和分子动力学数值模拟方法研究喷丸处理有效性及梯度纳晶金属的断裂行为。完成的研究内容包括：（1）建立了高效模拟喷丸过程的周期胞元有限元模型并模拟了二次喷丸、喷砂除锈等过程；（2）模拟了梯度纳晶铜在单轴载荷下的宏观应力应变场和微观结构演化，研究了变形伴随的位错运动、晶界迁移以及晶粒转动机制；（3）计算了含边裂纹和中心裂纹的梯度纳晶铜中的裂尖J积分并研究了其与材料、几何及残余应力参数间的关系；（4）模拟了梯度纳晶铜中裂纹扩展和钝化过程，并探究了裂纹与晶界、位错、堆垛层错等微结构间的相互作用。