喷丸强化零件细观材料力学表征及多轴疲劳寿命预测

The shot peening is an important manufacturing step for improving the fatigue property and structural reliability of high performance mechanical parts. Recent experiments revealed, however, that the fatigue life may reduce due to the surface treatment. If this influence cannot be quantified, unpredictable catastrophic consequences may take place during the service due to the surface hardening. The present proposal is separately investigating three different effects from the shot peening: Distortions of the subsurface layer, the surface roughness and the residual stresses. The subsurface material is taken from shot peened specimens and tested with help of the instrumented indentation as well as the microscopy tension. The material’s behavior is investigated and modeled by developing a cyclic plasticity including martensitic phase transformation and depending on material distortions. The surface damage induced by shot peening is investigated experimentally and computationally. By using special specimens effects of the surface topography are correlated with fatigue life and peening parameters. From the phenomenological continuum damage mechanics the fatigue damage evolution is related to the surface roughness. Based on the notched panel testing mechanical behavior and deformation mechanism of the treatment affected subsurface layer are investigated. With help of meso-scale material testing and finite element modeling the project is quantifying effects of material distortions and surface damage from peening to low cyclic fatigue life prediction and, furthermore, figuring out the cyclic damage evolution rules of the affected surface layer of the mechanical parts. The inter-dependence among distortion, surface damage and the LCF life of the peening affected layer is studied for computing variations of residual stresses in different geometrical parts under various temperature and mechanical loading conditions. The final goal of the project is solving the key issues in creating multi-axial low cycle fatigue design criteria and assessment methods for surface hardened mechanical parts under complex loading conditions.

喷丸强化处理是改善高性能零件疲劳寿命及可靠性的重要工序。然而有研究发现，在低循环载荷条件下喷丸工艺可能降低零件的疲劳寿命，从而可能导致机械系统在服役期间出现不可预知的灾难性事故。本项目分解研究喷丸处理零件表面改性层细观力学行为、表面形貌表征和残余应力演化规律。通过扫描电镜原位拉伸仪测量改性层畸变材料变形，运用纳米压痕技术并结合有限元计算分析量化改性层材料畸变和改性层材料力学性能的关联，建立改性层材料细观力学模型，进而推演残余应力和载荷条件的函数关系，量化材料畸变对疲劳损伤及其疲劳寿命的影响。通过制备特殊试件，运用唯像损伤力学的概念研究喷丸表面形貌对材料疲劳损伤演化的影响，揭示表面形貌、残余应力和零件几何形状及机械载荷的映射关联。建立喷丸工艺、细观材料力学性能和零件表面完整性分析方法，探索喷丸强化零件多轴疲劳寿命预测及其强度设计理论，建立高性能强化处理零件多轴低循环疲劳寿命评价方法。

表面强化处理是提高关键零部件疲劳寿命及可靠性的重要工序。然而，在表面强化材料低周疲劳寿命研究过程中发现，表面强化处理可能降低其疲劳寿命，从而导致零件寿命设计问题。如何评估表面强化处理对零件材料疲劳行为的本质影响，须对强化材料力学行为进行机理和定量描述研究。.本项目系统开展了对喷丸强化处理镍基高温合金材料的实验和计算研究，借助均匀预压缩变形试验、纳米压痕测试和有限元计算等研究方法，分别获得不同预应变下材料的应力应变关系，建立塑性变形和纳米压痕硬度、应力应变关系之间的幂律关系。通过对表面强化材料改性层不同深度位置的材料进行纳米压痕试验，根据前述研究结果，鉴别表面强化改性层材料不同深度位置的力学性能分布。结合有限元，建立了纳米压痕测试有限元计算模型，模拟不同预应变材料纳米压痕试验，可以通过计算准确的模拟实验结果。该研究为改性层材料力学性能评价提供方法，为评价表面强化材料疲劳损伤力学行为提供基础。.在研究材料力学性能的基础上，进一步开展了强化材料多轴疲劳损伤和寿命评估研究，运用唯像损伤力学的概念研究喷丸表面形貌对材料疲劳损伤演化的影响，揭示表面形貌、残余应力和零件几何形状及机械载荷的映射关联。建立喷丸工艺、细观材料力学性能和零件表面完整性分析方法，探索喷丸强化零件多轴疲劳寿命预测及其强度设计理论，提出了基于平均应力修正和强化材料本构模型的强化零件疲劳寿命预测方法，建立了高性能强化处理零件多轴低循环疲劳寿命评价方法。