短纤维增强金属基复合材料拉伸变形的屈服行为的理论研究

运用空间轴对称弹塑性有限元方法的混合律模型,推导出应力应变分配系数的解析表达式。并由此提出了一种新的定义复合材料比例极限和屈服强度的方法,进而研究了材料参数（纤维长径比、纤维体积分数、纤维根间距和基体应变硬化指数）对短纤维金属基复合材料拉伸变形行为的影响。     

颗粒增强金属基复合材料的屈服行为

运用空间轴对称弹塑性有限元方法和混合律模型，给出了应力应变分配系数与复合材料的弹性模量，屈服强度以及切线模量之间的定量关系式，并由此提出了一种新的定义颗粒增强金属基复合材料比例极限和屈服行为的方法，进而研究了颗粒形状（球体，正圆柱体以及椭球体）和材料结构参数（颗粒体积分数和颗粒根间距）对颗粒增强金属基复合材料拉伸变形行为的影响。研究表明，通过研究应力应变分析系数及其二阶导数来确定复合材料屈服行为的方法不仅适用于短纤维增强金属基复合材料，而且也适用于颗粒增强金属基复合材料。该方法可以较好地反映出颗粒形状和材料结构参数对复合材料屈服行为的影响，预测的比例极限与已发表的实验结果吻合较好。     

Thermal residual stresses and stress distributions under tensile and compressive loadings of short fiber reinforced metal matrix composites

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真空压力浸渗制备CF/Al复合材料热收缩与残余应力数值模拟与实验研究

针对连续碳纤维增强铝基复合材料(CF/Al复合材料),采用细观力学数值模拟与热性能试验结合的方法,研究了真空压力浸渗制备过程中的热收缩行为和热残余应力分布。结果表明,复合材料的横向热收缩应变量远大于轴向热收缩应变量,且具有横观各向同性,纤维随机分布的单胞有限元模型能够准确地预测复合材料轴向与横向热收缩行为曲线;复合材料制备完成后纤维和基体合金分别处于压应力和拉应力状态,基体和纤维的横向残余应力均小于其轴向残余应力,且均表现出横观各向同性;基体合金在轴向残余拉应力作用下会出现不同程度的损伤现象,特别是纤维间距较小部位过高的残余应力会引发界面的局部失效,从而不利于发挥复合材料承载性能,减少纤维局部偏聚是进一步改善提高复合材料力学性能的重要技术手段。     

Relationship Between the Depth of Drilling and Residual Strain Relief in Fiber Reinforced Composite Materials

The mechanical properties of fiber are quite different from matrix in composite laminates, and parameters, such as the angle of ply, and the temperature and pressure during composite forming, would play an important role in residual stress in the composite. The residual strains between layers depend on the forming parameters and their mechanical properties. In this article, grating rosette, Moiré interferometry, and step drilling-hole method are used to measure the relief strains when drilling each single laminate step by step, and find some relationship between the depth of drilling and residual strain relief in fiber reinforced composite material.     

短纤维增强金属基复合材料拉伸应力场的有限元数值分析

运用空间轴对称弹塑性有限元方法,研究了短纤维增强金属基复合材料拉伸应力场分布。研究表明,基体和纤维的应力分布及基体塑性行为具有明显的不均匀性,材料参数（纤维长径比,纤维体积分数,纤维根间距和基体应变硬化指数）以不同方式通过影响应力传递基体约束变形和基体应变硬化进而影响应力场分布。     

Rolling consolidation of metal matrix composites

Transverse rolling of metal matrix composite precursor wires is proposed as a consolidation technique for making sheets. Rolling in the transverse direction to the fiber orientation is analytically shown to be feasible, and longitudinal rolling results in fiber breakage. Plasticity analysis is conducted using Hill's general yield criterion for an isotropic materials and the associated Levy-Miles equations modified for plane strain conditions. The slab method is used to calculate the stresses in the material, and the effects of rolling parameters on the principal stress ratio are investigated.At the microscopic level, an elastic-plastic finite element formulation and a computation procedure are presented. Individual fibers are modeled to determine the stress state around each fiber. The principal stress ratio is suggested as a parameter that determines the tendency for void formation due to debonding and fiber breakage; finite element analysis is used to determine the effects of the principal stress ratio on the fiber-matrix interfacial stresses in the micromechanics model. The analysis determines the deformed mesh, plastic zone propagation and the stresses at the interface as a function of volume fraction and principal stress ratio. Interfacial stresses are assumed to be responsible for debonding during the deformation of metal matrix composites. This assumption and the results of the analysis provide guidelines for defining, the level of the biaxial stress field in the plane transverse to the fibers during rolling that will minimize interfacial fiber-matrix stresses.     

可计及温度与层状结构影响的超高温陶瓷基复合材料涂层残余热应力理论表征模型

目的创建可计及温度与层状结构共同影响的超高温陶瓷基复合材料涂层与基体层因热不匹配导致的残余热应力的理论表征模型。方法基于经典的层合板理论与超高温陶瓷基复合材料热物理性能参数对温度的敏感性研究,引入温度和层状结构对涂层与基体层所受残余热应力的影响,形成各层残余热应力温度相关性的理论表征方法,并以ZrB_2-SiC复合材料涂层为例,利用该理论方法系统地研究了各种控制机制对残余热应力的影响及其随温度的演化规律。结果超高温陶瓷基复合材料涂层与基体层所受的残余热应力随着温度的变化而变化,涂层热膨胀系数与基体层热膨胀系数差别越大,变化幅度越大。当涂层材料热膨胀系数大于基体层材料热膨胀系数时,涂层材料遭受残余拉应力,基体层材料遭受残余压应力;随着涂层厚度的增加,涂层所受拉应力减小,而基体层所受压应力增大;当涂层材料热膨胀系数小于基体层材料热膨胀系数时,涂层材料遭受残余压应力,基体层材料遭受残余拉应力;随着涂层厚度的增加,涂层所受压应力减小,而基体层所受拉应力增大。低温下,各层所受残余热应力对层厚与每层材料组成的变化比较敏感,随着温度的升高,敏感性降低。结论对于涂层材料,应设计涂层材料的热膨胀系数小于基体层材料的热膨胀系数,使涂层遭受残余压应力,这不仅能够降低材料表面产生裂纹的危险,同时可以抑制表面已有缺陷的扩展。同时应当设计相对较小的涂层厚度,以增大涂层所受的残余压应力,降低基体层所受的残余拉应力,有效提高整体材料在不同温度下的强度性能。     

等离子喷焊低温马氏体相变合金粉末的组织与性能

为减小喷焊过程中热收缩形成的拉伸应力,采用等离子喷焊的方法,在Q235钢基体上进行堆焊,制备出具有残余压缩应力的低温马氏体相变合金耐磨复合涂层。利用金相显微镜(OM)、SEM、EDS、XRD、X射线残余应力测试仪、显微硬度仪和摩擦磨损试验机等,对喷焊层金属的微观组织、成分和力学性能进行了分析和研究。结果表明:合适的工艺参数下,能够得到与基体呈冶金结合的无缺陷喷焊层组织;喷焊层的组织主要为马氏体和少量残余奥氏体组成;喷焊层获得较为理想的残余压缩应力,最大残余压缩应力可达到-351.2 MPa,平均残余奥氏体的质量分数约为10.18%;和基体材料相比较,等离子喷焊层的硬度提高2.5倍,耐磨性提高47.22倍。     

Evaluation of residual stress development in FRP-metal hybrids using fiber Bragg grating sensors

This paper presents experimental measurement methods for the determination and evaluation of process related thermal residual stresses in fiber metal laminates. A cure monitoring system with fiber Bragg grating (FBG) sensors is used to measure the in-plane strains during processing of carbon fiber reinforced plastic (CFRP)-steel laminates. The simultaneous measurement captures the thermal expansion during the heating stages, the cure shrinkage, and the cooling thermal shrinkage. The results enable the characterization of the co-cure bonding process and the stress transfer between the metal and FRP-layers during the creation process. The residual strains, which are used for calculation of the residual stresses, are recorded at room temperature after manufacturing. In addition, an advanced method using FBG-sensors and the deflection of asymmetric hybrid specimens is developed to validate the gained residual stress data. Asymmetrical specimens are created by removing selected layers after cure. Quantitative evaluation is achieved by determination of their curvature and measuring the strain changes with the embedded FBG-sensors. For validation, the methods were successfully demonstrated on two different curing cycles with different resulting residual stress levels. The simultaneous strain measurement enables the investigation of stress development and delivers more in-depth process knowledge for further optimization of the manufacturing process.     

New method for stress determination based on digital image correlation data

Accurate material characterisation of anisotropic yielding behaviour for modern sheet metal requires the testing under multiaxial stress conditions like in the shear test or plane strain test. While most approaches include time-consuming finite element simulations for evaluation of inhomogeneous stress distributions, this paper shows a new method to determine stress data based directly on digital image correlation data. This accurate and efficient semi-analytical method can be used in direct stress analysis or inverse material parameter identification schemes as well. The methodology will be described and exemplarily results of the plane strain test and the shear test will be discussed.     

Residual stresses and strains in orthogonal metal cutting

The finite element method is used to simulate and analyze the orthogonal metal cutting process under plane strain conditions, with focus on the residual stress and strain fields in the finished workpiece. Various modeling options have been employed. The frictional interaction along the tool-chip interface is modeled with a modified Coulomb friction law. Chip separation is modeled by the nodal release technique based on a critical stress criterion. Temperature-dependent material properties and a range of tool rake angle and friction coefficient values are considered. It is found that while thermal cooling increases the residual stress level, the effects of the rake angle and the friction coefficient are nonlinear and depend on the range of these parameters. The predicted residual stress results compare well with experimental observations available in the literature.     

一种结构焊接变形数值分析方法

采用结构力学理论与方法,对结构焊接变形进行了有限元数值模拟.将焊缝等效为残余应力与残余应变,其中,将T形接头视为板条梁,采用热弹塑性有限元分析方法分析了焊后残余应力分布特征和残余应变分布.根据虚功原理将焊缝残余应力等效加载到结构上与焊缝邻近的节点处,采用位移法将残余应变等效加载到结构相应部位.然后采用结构有限元分析方法...     

Modeling the Influence of Process Parameters and Additional Heat Sources on Residual Stresses in Laser Cladding

In laser cladding thermal contraction of the initially liquid coating during cooling causes residual stresses and possibly cracks. Preweld or postweld heating using inductors can reduce the thermal strain difference between coating and substrate and thus reduce the resulting stress. The aim of this work is to better understand the influence of various thermometallurgical and mechanical phenomena on stress evolution and to optimize the induction-assisted laser cladding process to get crack-free coatings of hard materials at high feed rates. First, an analytical one-dimensional model is used to visualize the most important features of stress evolution for a Stellite coating on a steel substrate. For more accurate studies, laser cladding is simulated including the powder-beam interaction, the powder catchment by the melt pool, and the self-consistent calculation of temperature field and bead shape. A three-dimensional finite element model and the required equivalent heat sources are derived from the results and used for the transient thermomechanical analysis, taking into account phase transformations and the elastic-plastic material behavior with strain hardening. Results are presented for the influence of process parameters such as feed rate, heat input, and inductor size on the residual stresses at a single bead of Stellite coatings on steel.     

随焊加载冷源或位移控制载荷的无应力焊接方法

应用一维杆模型论证了随焊加载冷源或位移控制载荷实现无应力焊接的原理. 应用数值模拟探讨了其实现方式，对7种加载方案下杆件中心点应力、塑性应变等物理量的时程分布进行了数值模拟对比研究，认为任意时刻外加位移控制载荷产生的机械应变和热输入满足一定条件时焊后残余应力为零. 随焊降低残余应力的工艺在焊件局部区域产生的变形速率与该局部区域的热膨胀或收缩变形量速率大小相同时，降低残余应力的效果达到最优. 应用模型导出的原理对焊前预拉伸、焊前温差拉伸、动态温差拉伸、低应力无变形焊接(LSND)等工艺的原理和参数设置依据进行了讨论. 结果表明，使焊件内部在冷却时产生足够大的拉伸塑性应变，是降低焊接残余应力的有效途径.     

Effects of particle size on residual stresses of metal matrix composites

A finite element analysis was carried out on the development of residual stresses during the cooling process from the fabrication temperature in the SiCp reinforced AI matrix composites. In the simulation, the two-dimensional and random distribution multi-particle unit cell model and plane strain conditions were used. By incorporating the Taylor-based nonlocal plasticity theory, the effect of particle size on the nature, magnitude and distribution of residual stresses of the composites was studied. The magnitude thermal-stress-induced plastic deformation during cooling was also calculated. The results show similarities in the patterns of thermal residual stress and strain distributions for all ranges of particle size. However, they show differences in magnitude of thermal residual stress as a result of strain gradient effect. The average thermal residual stress increases with decreasing particle size, and the residual plastic strain decreases with decreasing particle size.     

中子衍射在残余应力分析中的应用

中子衍射应力测量原理基于布拉格方程,通过取样测量体积内样品晶格的晶面间距进行精确表征,获取材料微观晶格的形变信息,可以实现晶体材料深层内部弹性应变的直接无损测量。测量值通常与样品测量位置和外加温度或力学载荷相关,用于评估实际部件整体结构和变形参数。本研究介绍中子衍射的基本原理,辊压法调控搅拌摩擦焊的残余应力和变形、管道焊接残余应力、大尺寸工程部件、近表面材料的残余应力测量分析、低温下飞机组件材料、原位热处理焊接材料,以及热老化和蠕变过程中内应力定量分析等典型应用案例。     

纤维增强金属基复合材料中轴向热残余应力分析

采用ANSYS有限元分析软件,通过显微复合材料模型,系统地分析了纤维直径、TiC 中间层厚度、纤维体积分数和制备温度等因素对SiC纤维增强Cu基复合材料轴向热残余应力的影响特征.结果表明,纤维直径的变化并不影响热应力的人小,但直径越小应力梯度越大;TiC中间层的厚度变化对自身及基体中的应力影响较小:但制备温度的变化对TiC层中的热应力影响较大;纤维体积分数的变化对纤维和基体中的热应力也有着非常显著的影响.     

金属基复合材料界面性能对残余应力的影响

采用ANSYS有限元软件计算涂层法制备的SCS-6 SiC/Ti-6Al-4V复合材料内热残余应力,分析了界面性能对热残余应力的影响。结果表明:较高的热膨胀系数(CTE)导致界面层产生高的应力梯度,使环向残余应力由低热膨胀系数时的压应力转变为较高的热膨胀系数时的拉应力;界面层弹性模量的增加,使得纤维和界面层内径向残余压应力明显增加,但对基体中的残余应力影响并不大;界面层厚度的变化对基体中径向残余应力影响不大,但随着界面层厚度增加,基体中残余应力有所减小。     

Role of interfacial and matrix creep during thermal cycling of continuous fiber reinforced metal–matrix composites

A uni-dimensional micro-mechanical model for thermal cycling of continuous fiber reinforced metal–matrix composites is developed. The model treats the fiber and matrix as thermo-elastic and thermo-elasto-plastic-creeping solids, respectively, and allows the operation of multiple matrix creep mechanisms at various stages of deformation through the use of unified creep laws. It also incorporates the effect of interfacial sliding by an interface-diffusion-controlled diffusional creep mechanism proposed earlier (Funn and Dutta, Acta mater., 1999, 47, 149). The results of thermal cycling simulations based on a graphite fiber reinforced pure aluminum–matrix composite were compared with experimental data on a P100 graphite–6061 Al composite. The model successfully captured all the important features of the observed strain responses of the composite for different experimental conditions, such as the observed heating/cooling rate dependence, strain hysteresis, residual permanent strain at the end of a cycle, as well as both intrusion and protrusion of the fiber-ends relative to the matrix at the completion of cycling. The analysis showed that the dominant deformation mechanism operative in the matrix changes continually during thermal cycling due to continuous stress and temperature revision. Based on these results, a framework for the construction of a transient deformation mechanism map for thermal excursions of continuous fiber composites is proposed.