【作者】 刘洋；

【导师】 胡平； 朱祎国；

【作者基本信息】 大连理工大学 ， 固体力学， 2018， 博士

【摘要】 提升先进材料的加工与服役性能对发展汽车工业与航空航天事业有着重要的作用。在汽车工业中,铝合金与高强钢因为其优质的比强度而被广泛使用。用于加工制造铝合金和高强钢的温成型技术与热冲压技术,可显著增加汽车结构件的强度并减少回弹效应。在航空工业中,钛合金因为其高比强度、低密度、优质的抗低温和抗高温以及防腐蚀性能,近年来被广泛用于制造超音速飞机的结构件。针对结构件的不同性能需求,探究材料微观结构对于结构性能的影响,是汽车工业与航天航空事业发展的重要组成部分。在材料制造成型和载荷服役的过程中,微观结构是增强材料性能的关键。然而当前的显微技术并不足以捕捉材料在高温时或不同载荷条件下的内部实时力学响应和微观结构的改变。近年来,许多学者采用晶体塑性有限元在晶体学尺度中预测材料的微观结构和热力学响应。本文提出了一个新型的基于物理学基础的晶体塑性模型,并将该模型与位错密度、温度场和损伤机制相耦合,模拟了铝合金与高强钢在不同载荷条件下的力学响应。为了得到钛合金结构件的响应,该晶体塑性模型又与多尺度算法相结合,模拟并预测了材料在微观尺度与宏观尺度下的裂纹萌生位置。本文的主要工作包括:1.采用热激活原理提出了在单晶晶体塑性中可同时用于有限应变和小应变框架的剪切应变率方程;分析了位错密度强化律中单晶晶粒内部林位错和碎片位错的钉扎与演化机制,并耦合滑移剪应变控制的损伤机制进行力学响应计算,以此描述在后颈缩区的应力损伤软化行为;使用Newton-Raphson方法和向前Euler方法推导了隐式与显式的应力更新计算流程。2.阐述了当前的多尺度算法在晶体塑性分析中的应用,并介绍了计算钛合金多尺度响应的基于特征应变的均匀化方法。该方法采用影响函数和系数张量使微观结构性质与宏观力学响应相关联。多子域的划分方式既可提高局部计算精度又能捕捉晶粒内部应力或应变的不均匀分布。3.构造了基于有限变形框架下的晶体塑性有限元模型,分析了铝合金和高强钢代表性体积单元的网格尺寸、网格类型和晶粒数目对于材料宏观和微观应力分布的影响;通过应变率与温度跳跃实验测定了热激活原理参数,并采用多目标优化方法识别了其它的物理学材料参数。该模型只需要一组材料参数即可实时捕捉该材料在不同温度梯度与应变率条件下的力学行为。4.在有限变形框架下,验证了不同温度与应变率下晶体塑性模型的数值模拟结果可以很好地吻合铝合金和高强钢的热力拉伸与剪切的实验数据;同时,还确认了该晶体塑性模型可精确捕捉材料的应变强化率,应力损伤软化及断裂应变,并可以实时跟踪延性损伤的演化过程;分析了该模型中位错密度分布和晶粒取向差分布对于断裂路径的影响,由此发现局部位错密度增长和较大的晶粒取向差都与损伤萌生直接相关;通过将晶体塑性模拟结果与7075铝合金的实验观测结果相比较得知,不同的初始晶粒取向差分布导致了不同的断裂形貌与颈缩路径;分析了 7075铝合金的局部取向差分布,从而获得了不同断裂形貌下促进孔洞生长与融合的临界取向差分布范围。5.提出了在小应变框架下从晶粒尺度跨越到结构尺度的高效多尺度晶体塑性模型,并使用该模型分析了钛合金的宏观与微观裂纹萌生,大大提高了大尺度晶体塑性模型计算的效率;将特征应变的降阶均匀化方法与基于位错密度的晶体塑性相结合,分析了密排六方和体心立方晶体的力学响应。与传统晶体塑性模型的计算方法相比,该均匀化方法在小变形框架下可以精确捕捉局部和总体的应力应变响应。通过对钛合金疲劳裂纹萌生的实验现象进行分析,本文提出了一个新型疲劳裂纹萌生判定方法。在多尺度晶体塑性模型中应用该方法所得的数值模拟结果与实验测量结果进行对比,验证了该判定方法的正确性。最后,在宏观结构件上进行高效多尺度晶体塑性模拟,捕捉了微观与宏观尺度上裂纹萌生位点的空间分布。更多还原

【Abstract】 The processing and service performance of advanced material have an important influence on the development of both vehicle engineering and aerospace engineering.The warm forming and hot stamping technique have been widely used in the manufacturing of structural components in vehicle engineering.Aluminum alloy and boron steel are the mostly applicable materials in these techniques to obtain higher strength and to reduce spring back.In aerospace engineering,the titanium alloy is generally used in the structural components of the engineering of hypersonic aircraft because of its high strength,low density,the unique resistance of high or low temperature and corrosion.The material microstructure is the key to increase the forming limit in the manufacturing and the service performance in the hypersonic loading.Under high temperature and different loading conditions,the current microscope technique is not sufficient to track the real-time mechanical responses and the micro structural change in the material.The crystal plasticity finite element method gives a prediction of the microstructural and thermo-mechanical responses in the perspective of the crystallography.This thesis proposed an advanced physical-based crystal plasticity method coupled with dislocation density,temperature field and damage mechanism to model the material behavior under different loading and environmental conditions.The main work including:1.In the single crystal plasticity,the thermal activation mechanism is introduced to propose the energy-based shear strain rate function for both finite and small strain frameworks.Dislocation density based hardening rule is considered to inform the forest and debris dislocation pinning mechanism inside the grain.The damage mechanism controlled by shear strain is coupled in the computation of mechanical responses to describe the stress softening during the post-necking region.Based on the Newton-Raphson method and forward Euler methods,implicit and explicit calculation procedures are derived for the stress updating in the advanced constitutive model.2.The introduction of the current multi-scale method application in crystal plasticity and the eigen-strain-based homogenization method used in the structural-scale simulation.The influence functions and the coefficient tensors are applied to obtain the macroscopic responses informed by the microstructural properties.The multi-part method could be used to increase the accuracy of describing stress or strain distributions inside the grains.3.In the representative volume element,the mesh size,mesh type and grain number are discussed for aluminum alloy,boron steel and titanium alloy.Based on the optimization method,a single set of physical-based parameters could be calibrated to capture the mechanical responses for one material at different temperatures and strain rates.The thermal activated material parameters are captured by using the strain rate and temperature jump tests.4.In the thermal tensile and shear tests of aluminum alloy and boron steel,reasonable agreement is obtained between experimental and numerical results for different specimens,temperature conditions and materials.This approach simultaneously captures the strain hardening rate,damage softening,non-linear post-necking and fracture strain.Microstructural effects on ductile fracture are tracked and investigated including dislocation density and crystallographic orientation.The results show that local dislocation density rise is associated with damage initiation.Different fracture morphologies and necking paths are caused by distinct initial misorientation distributions in comparison with experimental observation of 7075 aluminum alloy.Local misorientations are investigated and critical misorientation ranges are computed for promoting void growth in zigzag and straight fracture morphologies.Schmid factor is computed as not necessary variable to trigger void growth.5.In the small strain framework,the proposed approach is used to investigate the prediction of crack nucleation in titanium alloys in the context of a multiscale computational methodology that allows the effective upscaling such as grain level information to the structural scales.The eigenstrain-based reduced-order homogenization(EHM)approach has been extended to account for the presence of HCP(primary a phase)and BCC(β phase)grains,within which the deformation process is modeled using a dislocation density-based crystal plasticity formulation.The generalized crystal plasticity EHM implementation has been thoroughly verified to assess the accuracy of the reduced order model in capturing local and global behavior compared with direct CPFE simulations.A new and simple microstructural fatigue crack nucleation metric is proposed,numerically verified and validated against experimental observations in Ti-6242S subjected to pure fatigue and dwell fatigue.The proposed fatigue nucleation metric tracks the total sessile dislocation density and models the effect of dislocation pile-up near the hard-soft grain boundary that ultimately leads to facet formation.A structural scale study is performed to characterize the spatial distribution of fatigue crack nucleation sites in the context of a large structural component analysis.更多还原