喷丸强化过程的数值模拟与疲劳裂纹扩展行为研究

发布时间：2018-05-15 20:07

本文选题：疲劳裂纹扩展 + 应力比　；参考：《浙江工业大学》2016年博士论文

【摘要】：疲劳问题一直以来都是工业上备受关注的问题之一。金属零部件和工程结构疲劳断裂不仅造成巨大的经济损失,甚至会危及到人员的生命安全。研究材料的疲劳裂纹扩展行为具有重大的工程实用价值和社会经济意义。疲劳易启裂于零部件的表面,为了抑制表面裂纹的萌生和扩展,通常对材料的表面进行强化处理。喷丸是一种在工业生产中应用极为广泛的表面强化工艺,通过向金属材料的表层注入残余压应力并优化其组织结构,显著提高零部件的抗疲劳和抗应力腐蚀等性能。近年来,随着有限元技术和电子计算机的飞速发展,通过数值模拟研究喷丸强化机制和优化喷丸过程参数,已成为当前喷丸强化技术进一步发展的要求和潮流。通过单丸喷丸模型研究模型参数(单元尺寸、摩擦系数、分析步长和弹丸材料性质)对数值结果的影响。从受喷区域的动态应力演化和残余应力分布两个方面评估对称胞元模型预测的残余应力。根据对称胞元模型的模拟方法,对预应力喷丸强化过程进行建模研究,发现随着喷丸速度的增大,喷丸强化所致残余压应力对预加拉应力愈加敏感。根据真实的喷丸强化工艺和基于受喷表面凹坑分布的统计分析,提出随机概率喷丸模型。利用该模型模拟二次喷丸强化过程,结果表明二次喷丸强化能够有效强化靶材表层的残余压应力并减小受喷表面粗糙度。基于激光冲击波压力的时空分布模型和能够表征材料流动应力Arrhenius和non-Arrhenius特征的统一本构模型,建立三维单点激光喷丸高导无氧(OFHC)铜的有限元模型。通过该模型定量研究激光功率密度和激光脉冲半高宽对喷丸强化效果的交互影响以及应力波在靶材内部的传播特性。单点多次激光喷丸强化的残余应力趋于饱和,这主要与靶材的硬化历史有关。双面激光喷丸强化的残余应力对靶材的厚度十分敏感,这主要与双面喷丸过程中冲击波的相互作用有关。为了研究喷丸的组织结构强化机制,采用位错密度演化模型定量预测喷丸强化过程中受喷区域细化的晶粒尺寸和增加的位错密度。在弹丸或激光束的冲击载荷下,靶材表层的晶粒尺寸显著减小,位错密度显著增加,并且细化的晶粒尺寸和增加的位错密度随着重复喷丸次数的增加而逐渐趋于饱和。此外,建立晶体塑性本构模型,模拟单晶铜在冲击载荷下的变形行为。采用非标准紧凑拉伸试样对2024-T4铝合金进行常幅和变幅载荷下的疲劳裂纹扩展试验。在常幅载荷下,用名义应力强度因子幅表征的疲劳裂纹扩展行为呈现出明显的应力比效应。基于疲劳启裂和裂纹扩展的统一模型能够很好地预测不同应力比下疲劳裂纹扩展速率。单个拉伸过载会产生显著的疲劳裂纹扩展迟滞现象。当裂纹扩展出过载影响区后,疲劳裂纹扩展行为将恢复到常幅载荷下的状态。高-低变幅载荷工况引起的疲劳裂纹扩展迟滞行为主要与两个顺序加载历史参数有关。变幅载荷下的疲劳裂纹扩展行为能够采用Wheeler模型进行合理地表征。
[Abstract]:Fatigue problem has always been one of the most important issues in industry. Fatigue fracture of metal parts and engineering structures not only causes huge economic losses, but also endangers the safety of people. Fatigue crack propagation behavior of materials is of great engineering practical value and socioeconomic significance. Fatigue crack is easy to crack in zero. In order to suppress the initiation and expansion of surface cracks, the surface of the material is usually strengthened. The shot peening is a widely used surface hardening process in industrial production. By injecting the residual compressive stress into the surface of the metal material and optimizing its microstructure, the anti fatigue and anti stress corrosion of the parts can be significantly improved. In recent years, with the rapid development of finite element technology and electronic computer, the study of shot peening mechanism and optimizing the parameters of shot peening process by numerical simulation has become the demand and trend of the further development of shot peening technology. The influence of the properties of the projectile material on the numerical results. The residual stress predicted by the symmetric cell model is evaluated from two aspects of the dynamic stress evolution and the distribution of the residual stress in the sprayed region. The residual pressure stress is more sensitive to the preloading stress. A random shot peening model is proposed based on the actual shot peening process and the statistical analysis based on the surface pits distribution on the sprayed surface. The model is used to simulate the two shot blasting process. The results show that the two shot peening intensifying can effectively strengthen the residual compressive stress on the surface of the target and reduce the stress. The surface roughness. Based on the spatio-temporal distribution model of the laser shock wave pressure and the unified constitutive model which can characterize the flow stress Arrhenius and non-Arrhenius characteristics of the material, a finite element model of the three dimensional single point laser shot peening high conductivity oxygen free (OFHC) copper is established. The model is used to study the laser power density and the laser pulse half width and width. The interaction of the effect of bolus intensification and the propagation characteristics of the stress wave inside the target. The residual stress intensification of the single point laser shot peening tends to saturate, which is mainly related to the history of the target hardening. The residual stress of the double-sided laser shot peening is very sensitive to the thickness of the target, which is mainly related to the interaction of the shock waves during the double shot peening process. In order to study the strengthening mechanism of the tissue structure of the shot peening, the grain size and the increased dislocation density of the sprayed zone are predicted by the dislocation density evolution model. The grain size of the target surface decreases remarkably under the impact load of the projectile or laser beam, and the dislocation density increases significantly, and is refined. The grain size and the increase of dislocation density gradually become saturated with the increase of repeated shot blasting times. In addition, the plastic constitutive model is established to simulate the deformation behavior of single crystal copper under the impact load. The fatigue crack propagation test of 2024-T4 aluminum alloy under constant amplitude and amplitude loading is carried out by non standard compact tensile specimens. Under load, the fatigue crack propagation behavior characterized by nominal stress intensity factor amplitude shows obvious stress ratio effect. A unified model based on fatigue crack initiation and crack propagation can well predict fatigue crack propagation rate under different stress ratios. A single tensile overload will produce significant fatigue crack growth retardation. The fatigue crack propagation behavior will be restored to the state of the constant amplitude load after the display of the overloading area. The fatigue crack growth retardation caused by the high low amplitude load condition is mainly related to the two historical parameters. The fatigue crack propagation behavior under the variable amplitude load can be reasonably characterized by the Wheeler model.

【学位授予单位】：浙江工业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG668

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