金属基复合材料中热残余应力的分析方法及其对复合材料组织和力学性能的影响

对目前金属基复合材料热残余应力的分析方法进行了概述,并对热残余应力对金属基复合材料的性能影响进行了分析,提出了目前在复合材料热残余应力的实验分析和理论计算中仍存在的问题以供材料科学工作者共同研究解决.     

金属基复合材料残余应力测定

分析了金属基复合中热残余应力的形成机理及表现特点，并对几种常用的测定金属基复合材料残余应力的方法进行了分析比较，普通Ｘ射线法虽然简便、经济、但穿透能力有限，所测值仅反映试样表面的残余应力状态；中子衍射的区域较大，测出的仅是残余应力平均值。Ｘ射线能量衍射法虽然穿透能力较强，能测出试样内部较深范围的残余应力，但由于产生衍射法穿透力强，分辩率高，能测出复合材料内部的残余应力场梯度，是测定金属基复合材料残     

2D-C/SiC复合材料的主泊松比演化行为

泊松比是材料及其结构力学性能分析的重要参数之一。本文旨在研究2D-C/SiC复合材料主泊松比的非线性演化行为。首先，基于Mini复合材料模型与正交层压板模型，考虑纤维的横观各向同性性质，建立了2D-C/SiC复合材料的热残余应力计算模型；其次，应用剪滞理论与经典层压板理论，考虑材料的损伤与热残余应力释放机制，建立了2D-C/SiC复合材料的主泊松比计算模型；最后，通过试验表征了材料的应变响应及泊松比演化规律，并对理论模型进行了分析验证。结果表明，2D-C/SiC复合材料内部热残余应力较大，拉伸损伤过程中的热残余应力释放是负泊松比产生的原因；应力-应变曲线及泊松比演化曲线的模型预测结果均与试验曲线吻合较好，表明了理论分析模型的准确性与合理性。     

颗粒增强金属基复合材料微屈服行为的细观力学计算

本文采用细观力学模型 ,计算了颗粒增强金属基复合材料的微屈服行为规律。计算模型是以有限元法、应力二阶矩的割线模量法、Eshelby等有效夹杂方法和双夹杂模型等为基础。计算结果表明在基体材料的微屈服规律符合Brown Lukens线性规律的情况下 ,颗粒增强金属基复合材料的σ 〈εp〉1 2 关系也近似呈线性符合Brown Lukens规律。同时计算了增强体颗粒的含量、热残余应力和位错密度等多方面因素对复合材料微屈服规律的影响。     

考虑纤维随机分布的复合材料热残余应力分析及其对横向力学性能的影响

建立了考虑纤维随机分布并包含界面的复合材料微观力学数值模型,模拟玻璃纤维/环氧复合材料固化过程中的热残余应力。通过与纤维周期性分布模型的计算结果进行对比,发现纤维分布形式会对复合材料的热残余应力产生重要影响,纤维随机分布情况下的最大热残余应力明显大于纤维周期性分布的情况下。研究了含热残余应力的复合材料在横向拉伸与压缩载荷下的损伤和破坏过程,结果表明:热残余应力的存在显著影响了复合材料的损伤起始位置和扩展路径,削弱了复合材料的横向拉伸和压缩强度。在横向拉伸载荷下,考虑热残余应力后,复合材料的强度有所下降,断裂应变显著降低;在横向压缩载荷下,考虑热残余应力后,复合材料的强度略有下降,但失效应变基本保持不变。由于热残余应力的影响,复合材料的横向拉伸和压缩强度分别下降了10.5%和5.2%。      

金属基复合材料的微屈服行为

金属基复合材料（MMC）的微屈服行为有其特殊性，主要表现在其体中的热残余应力水平和位错组态与宏观屈服阶段显著不同，因而表现出的力学行为也不同，本文综述了金属基复合材料微屈服行为的宏观表现和微观特性，并对其研究发展进行了概述，指出了有待深入研究的问题。     

短纤维增强金属基复合材料微屈服行为的计算机模拟

本文采用细观力学模型 ,根据Eshelby等效夹杂原理和双夹杂模型等 ,用计算机模拟的方法定量计算了短纤维增强金属基复合材料微屈服行为规律。计算结果表明在基体材料的微屈服规律符合Brown Lukens线性规律的情况下 ,金属基复合材料的σ - <εp>1 2 也近似符合Brown Lukens规律。同时计算了增强体短纤维的含量、形状、热残余应力和位错密度等多方面因素对复合材料微屈服规律的影响     

金属基复合材料界面残余应力的研究进展

综述了金属基复合材料界面残余应力的各种影响因素、残余应力的实验测试方法和理论分析方法及残余应力对复合材料宏观力学性能的影响,分析讨论了目前研究中存在的问题和不足之处,并指出了今后工作的重点与方向.     

碳纳米管增强镁基复合材料热残余应力的有限元分析

为了探寻Ni层厚度对镀镍碳纳米管增强AZ91D镁基复合材料(Ni-CNTs/AZ91D)中热残余应力的影响, 在实验基础上, 建立不同Ni层厚度时Ni-CNTs/AZ91D复合材料的有限元模型, 模拟了Ni-CNTs/AZ91D复合材料中热残余应力的分布。研究发现: 在碳纳米管表面镀镍能够明显降低Ni-CNTs/AZ91D复合材料中的热残余应力。Ni-CNTs/AZ91D复合材料中, 热残余应力在Ni层厚度为6nm时最小; Ni层厚度由2nm增至6nm时, 热残余应力随着Ni层厚度的增加而减小; 当Ni层厚度超过6 nm时热残余应力随着Ni层厚度的增加而增大。复合材料中热残余应力的最大值随碳纳米管表面Ni层厚度的增加向Ni层与基体的界面移动。      

三维机织复合材料残余应力/应变多尺度分析及工艺参数优化

为预测三维机织复合材料工艺引入的残余应力/应变,提出工艺制度优化方案,建立了一种工艺过程分析的多尺度模型.通过建立纤维尺度及纱线尺度代表体元(RVE),计算了成型过程中纤维纱线及三维机织复合材料的模量演化历程.考虑固化过程中树脂的化学收缩效应,在纱线尺度上开展热-化学-力学耦合分析,预测了细观残余应力-应变及其演化规律...     

Modelling of cyclic fatigue stress for life prediction of structural ceramics

The stress models of a cyclic fatigue test for structural ceramics were developed using the theory of fracture mechanics to predict the accurate life cycles of the specimens. Four kinds of models were tried to obtain the representative stresses corresponding to the cyclic stresses applied to the alumina specimens. Crack-growth exponents of 21.81, 22.15, 24.57 and 24.43 were obtained from the arithmetic mean stress model, the integrated stress model, the maximum stress model, and the equivalent static stress model, respectively. It is considered that the equivalent static stress model is the most reasonable and gives the most accurate value of the crack-growth exponent from the view point of the theoretical background for developing the model.     

湿度对瓦楞纸板纵向压缩承载性能的影响

目的 瓦楞纸板由于其材料的特殊性,在运输过程中会受到环境湿度的影响,因此,首先研究标准环境下的平台应力模型,然后引入湿度因素对模型进行修正.方法 采用试验及理论建模的方法建立基于环境湿度的纵向压缩平台应力理论模型.通过实验手段探讨瓦楞原纸极限应力随环境湿度的变化规律,建立其随环境湿度的特征曲线,然后引入标准环境下的平台应力理论模型中,建立任意环境湿度下的平台应力理论模型.结果 与未考虑湿度建立的瓦楞纸板平台应力相比,考虑湿度建立的理论模型与实验结果有较好的一致性,验证了模型的准确性.结论 建立任意湿度下的理论模型对于平台应力有很好的预测作用,在实际应用中对瓦楞纸板的选择以及包装设计有很大的指导作用.     

考虑相变的TC4钛合金流动应力研究

为完善TC4钛合金的塑性成形理论,必须建立准确的流动应力模型,而TC4钛合金的流动应力不仅与温度、应变速率和应变量相关,也受到βα+β相变的影响.本文通过等温实验和DSC实验研究了900℃时TC4钛合金的等温与连续冷却过程中的相变动力学,建立了相变动力学模型;通过Gleeble 3800热模拟机进行热压缩实验,根据实测的TC4钛合金在高温β区与低温α+β区的流动应力曲线,建立了流动应力模型;将相变动力学模型与流动应力模型结合,建立了考虑相变的流动应力模型;最后将TC4钛合金在900℃等温不同时间后进行压缩变形,对比实验得到的流动应力曲线与模型计算得到的流动应力曲线,验证了该模型的准确性.     

A State-Space Model of Fatigue Crack Growth

This paper proposes a nonlinear dynamic model of fatigue crack growth in the state-space setting based on the crack closure concept under cyclic stress excitation of variable amplitude and random loading. The model state variables are the crack length and the crack opening stress. The state-space model is capable of capturing the effects of stress overload and underload on crack retardation and acceleration, and the model predictions are in fair agreement with experimental data on the 7075-T6 aluminum alloy. Furthermore, the state-space model recursively computes the crack opening stress via a simple functional relationship and does not require a stacked array of peaks and valleys of stress history for its execution; therefore, savings in both computation time and memory requirement are significant. As such, the state space model is suitable for real-time damage monitoring and control in operating machinery. This revised version was published online in July 2006 with corrections to the Cover Date.     

A mean stress correction model for tensile and compressive mean stress fatigue loadings

A new mean stress fatigue model based on the distortional strain energy is proposed to account for the mean stress effects on fatigue life. The proposed model is compared with the Morrow and the Smith‐Watson‐Topper (SWT) mean stress correction models using a number of experimental data sets for one cast iron, two steels and two aluminium alloys under tensile and compressive mean stress loadings. It is found that both the proposed mean stress correction model and the SWT model yield similar results and provide very good correlation for positive mean stress data and moderate negative mean stress data. For high compressive mean stresses, the proposed model shows reasonably good correlations, while the SWT model fails to correlate the fatigue data. The Morrow model was found to give poor correlations for all fatigue data analysed by yielding conservative results for compressive mean stresses and non‐conservative results for tensile mean stresses.     

粘着应力拉伸过程数值模拟及粘着应力模型

为了精确计算粘性介质压力成形过程中粘性介质/板材界面粘性附着作用,分析了压力、剪切速率、温度等因素对粘着应力的影响,提出了粘着应力计算模型.将提出的粘着应力模型引入有限元分析软件中,对粘着应力拉伸过程进行了数值模拟,并将计算结果与试验结果进行对比,验证提出模型的可靠性.结果表明:采用建立的模型预测的试样伸长量及应变分布与实验测量结果具有较好的一致性.说明建立的粘着应力模型可以准确反映板材/粘性介质界面粘着应力大小,为精确模拟粘性介质压力成形过程提供了模型.     

Dual analysis of plane stress problems by commonly based finite elements

Finite element models for linear elastic plane stress problems which provide, alternatively, a completely compatible displacement field or a precisely equilibrated stress field are developed. The basis for both models is a biquintic interpolation polynomial representing Airy's stress function over a triangular region. The polynomial coefficients are modified and grouped to establish compatibility while retaining equilibrium within each element. Nodal kinematic parameters are selected and matched to the stress function for the compatible (modified stiffness) model, while nodal stress function parameters are chosen for the equilibrium (modified flexibility) model. Constraints on the global freedoms, enforced by Lagrange multipliers, are introduced to augment nodal connectivity in establishing interelement compatibility in the ‘stiffness’ model and uniform stress transmission in the ‘flexibility’ model. Appropriate boundary conditions are formed for each model. Numerical solutions are obtained and assessed.     

Effects of residual stress on toughening of brittle polycrystals

The effects of residual stress on toughening of brittle polycrystalline materials, in the absence of microcracking, were investigated by considering the mode I stress intensity factor reduction at the tip of a stationary crack under combined applied and residual stress loading. Toughness enhancement associated with a number of model singular and non-singular residual stress fields was evaluated. The singular residual stress fields were used to model grain-sized thermal expansion anisotropy due to grain-orientation differences in a polycrystal. The numerical results indicate that residual stress can significantly toughen a stationary crack against initiation. For the same average value of residual stress, toughness enhancement due to singular residual stress fields is more substantial than that due to non-singular residual stress fields. Sample toughness enhancement results are presented for a single-phase polycrystal failing by intergranular fracture.     

桥式射频MEMS开关上电极薄膜的残余应力改进模型

桥式射频(RF)微机电系统(MEMS)开关的上电极薄膜残余应力影响开关的响应时间、下拉电压等性能参数,降低了开关的可靠性.在分析传统拉伸线模型的基础上,提出了一种改进的残余应力静力学模型,将残余应力扩展到压应力的情况,并可应用于非对称均布载荷.选取了一组典型的开关参数,计算了双轴残余应力对两端固支MEMS薄膜均布载荷下拉挠度的影响.结果表明,在选取的参数条件下,拉应力增大到1 MPa时,薄膜的最大挠度减少7.95%,压应力增大到-1 MPa时,薄膜的最大挠度增加9.47%,与传统模型的计算结果相比,改进模型消除了非线性叠加产生的约0.3%的误差.采用迭代方法计算下拉电压,对两组实验测试数据与模型计算结果进行了验证,模型计算结果与实验数据基本相符.     

Higher-order strain/higher-order stress gradient models derived from a discrete microstructure, with application to fracture

Higher-order gradient models are derived from a discrete particle structure. A general set of constitutive equations is found in which zeroth and higher-order stress terms are related to zeroth and higher-order strain terms. As special cases, a model with only higher-order strain terms is considered as well as a model with both higher-order stress and higher-order strain terms. The model without higher-order stress is found to be unstable. The model with higher-order stress, on the other hand, is stable. The model with a higher-order stress term and a higher-order strain term can successfully be used to model softening phenomena.