陶瓷金属扩散焊接的残余应力及其缓和措施

采用热弹塑性有限元法分析了热压氧化硅和Ｋ－５００合金的散焊接引起的残余应力状态。比较了方棒试件三维残余就力和圆棒试件二维轴对残余应力的特征，探讨了不同过渡层的影响。结果表明，在靠近连接界面际近的陶恣外表面存在轴向最大拉伸应力，它是导致接头开裂和强度降低的关键因素。     

填充环氧树脂界面残余应力研究——Ⅳ.界面残余应力形成的分子机理

本文针对橡胶改性环氧树脂体系,从微观分子运动角度对界面残余应力形成原因进行了初步探索,发现复合材料界面残余应力的生成是界面处分子运动不平衡所导致的。同时界面残余应力有效体积是一个重要参数,它和材料温度以及界面键合强弱有关。     

烧结陶瓷涂层残余热应力的力学模型

以界面剪应力作为唯一的变形协调方式，用平面应力法建立了陶瓷涂层刀片横截面应力分布的力学模型。涂层中的正应力分别沿Ｘ轴和Ｚ轴呈线性增加，在界面中心处达到最大值，剪应力的分布与ｘ轴无关，而沿ｚ轴呈二次曲线变化。     

陶瓷涂层的残余应力评价及其尺寸效应

当一个平直长条试样镀上单面涂层后,往往因为膨胀系数不匹配而在涂层和基体内形成残余应力并产生弯曲变形,通过变形后的曲率半径以及涂层与基体的厚度参数可以计算出涂层内的残余应力,这种方法也称为Stoney方法,相应的弯曲变形样品定义为Ⅰ型涂层,而不产生弯曲变形涂层构件定义为Ⅱ型涂层。由于残余应力不是材料常数,用Stoney方法测到的Ⅰ型涂层残余应力并不代表Ⅱ型涂层的残余应力,而实际工程中绝大部分涂层属于Ⅱ型。如果能建立Ⅰ型涂层残余应力与Ⅱ型涂层之间的关系,就可以通过Stoney方法评价任意构件涂层的残余应力。本研究基于相对法导出了两种类型之间的关系以及Ⅱ型涂层残余应力计算公式,测定了金属基体上对称热障涂层的残余应力。结果表明,该方法简单易行,解决了现实中各种无弯曲变形的实际构件的残余应力评估。     

不同界面形态聚晶金刚石复合片热残余应力分析

聚晶金刚石复合片（Polycrystalline Diamond Compact,简称PDC）钻头因具有极高的耐磨性、抗冲击韧性以及锐利的切削刃,在地质和石油钻探中受到了广泛的应用。但目前国内外油田钻探中所使用的复合片有80%的非正常失效断裂是由于PDC的残余应力致裂及界面结构问题引起的,热残余应力是造成PDC非正常失效的主要因素。文章根据PDC制造过程中的热力学工艺条件,对平面界面及其他几种典型不规则界面的PDC热残余应力作详尽的数值计算和分析比较,对PDC热残余应力的分布规律作深入研究。研究结果表明,界面形态与热残余应力的大小及其分布规律密切相关,不同界面形态热残余应力大小和分布均不相同,并提出了优化PDC界面结构、有效降低界面残余应力的措施。这对提高PDC生产合格率及使用寿命具有重要意义。     

注塑件残余应力影响因素的模拟实验研究

在分析注塑过程中残余应力产生机理和原因的基础上，应用软件模拟实验方法，研究了不同成型工艺条件对聚苯乙烯平板注塑件残余应力的影响，分别给出了沿厚度方向的残余应力和固化压力分布。研究发现：残余应力在厚向上呈拉-压-拉（表层-次表层-中间层）分布，并随熔体温度和保压压力升高而增大，固化压力是决定注塑件厚向残余应力分布的一个主要因素。     

强界面陶瓷层状复合材料优化设计的最佳层厚比探讨

针对提高材料整体强度的目标，通过对材料中应力状态与几何结构因素关系的分析，给出了强界面层状材料优化设计的最佳层厚比原则。通过实验研究，探讨了A12O3—14．31％ZrO2／A12O3—39．17％ZrO2(质量分数，下同)层状材料的强度、断裂功和弹性模量等随着材料中残余应力的存在而发生变化的规律。A12O3—14．31％ZrO2／A12O3—39．17％ZrO23层材料的最佳层厚比理论计算值为4．14。实验结果表明，最佳层厚比在2．49～4．36之间。在最佳层厚比处，抗拉强度、弹性模量、断裂功和Vickers硬度均达到最佳值。实验进一步印证了理论分析的正确性。     

注射过程中残余应力演变的模拟与分析

采用线性黏弹性模型，模拟计算了平板形状制品残余应力在壁厚方向和流动方向上的分布及其在成型中的变化过程。根据模拟结果，深入讨论了注射成型过程中残余应力的形成机理和演变情况。结果表明：脱模时制品表层有着较大的拉应力，表层以下存在着一个应力低谷，在制品中心区域存在着抛物线形的拉应力分布；残余应力沿壁厚分布的形状在流动方向上基本相同，但是流动末端的应力值稍大于流动入口处。     

颗粒增强复相陶瓷残余应力和增韧机制分析

对颗粒增强复相陶瓷在增强颗粒密排分布条件下材料内残余应力的大小和分布进行了理论分析，探讨了αｐ〉αｍ和αｐ〈αｍ情况下的强化增韧机制以及在这两种情况下所存在的增韧机制的差别。研究结果表明，残余应力不仅与基体和颗粒的物理参量有关，而且还与所在点位置和增强颗粒体积含量有关。颗粒增强复相陶瓷在αｐ〉αｍ和αｐ〈αｍ情况下，都可能达到强化增韧的效果。在αｐ〈αｍ情况下，主要增韧机制是裂纹偏转和残余应力场强     

材料种类对夹芯注射制品残余应力的影响

采用Moldflow公司MPI软件中的Co-injection分析模块,对夹芯注射成型过程进行动态模拟分析；揭示不同材料种类组合对夹芯注射成型过程中残余应力的影响规律。结果发现,夹芯注射体系中两种物料相容性的好坏对芯/壳层界面处残余应力的分布有较大影响,芯层为结晶型材料时会导致残余应力有较大的跃迁。     

Mechanical behavior and failure mechanisms of boron carbide based three-layered laminates with weak interfaces

The effect of weak interfaces on failure mechanisms of a three-layered composite was studied. Three-layered B₄C/B₄C–C_nanofibers laminates have been produced using a hot pressing technique. The laminates were designed with thick (∼2.6 mm) outer layers of B₄C and a thin (∼90 μm) center layer of B₄C–70 wt.% C_nanofibers. Based on the difference in coefficients of thermal expansion and Young's moduli of the pure B₄C and B₄C–70 wt.% C_nanofibers layers, it was estimated that low tensile thermal residual stress with a magnitude of 11.3 ± 2.5 MPa was developed in the thick B₄C outer layers, and compressive residual stress with a magnitude of 455.7 ± 5 MPa was developed in the thin central B₄C–70 wt.% C_nanofibers layer. The apparent fracture toughness of laminates was measured and based on the estimated fracture toughness values, a threshold stress was calculated.     

Cracking of Laminates Subjected to Biaxial Tensile Stresses

During the processing of laminar ceramic, biaxial residual stresses can arise due to differential thermal contraction between unlike layers. A tensile stress can cause preexisting flaws to extend across the layer and into the adjacent layers and then tunnel until they meet either another crack or a free surface. A previous analysis has shown that for a given residual stress there is a critical layer thickness, below which no tunnel cracks will exist, regardless of initial flaw size. Here, the previous analysis was modified to take into account the crack extension into adjacent layers. To determine the validity of the analysis, laminates composed of alternating layers of zirconia and alumina/zirconia were fabricated by a sequential centrifugation technique. The composition of the alumina/zirconia layer was varied to change the biaxial, tensile stresses in the zirconia layer. Observations were then made to determine the critical layer thickness for tunnel cracks and their extension into the adjacent layers. These observations were compared to the theoretical predictions.     

变刚度铺放改善单钉双剪螺栓连接拉伸强度

沿最大主应力方向的变刚度铺放可有效提高复合材料构件的强度。以与孔周围设定区域内,单元格上流场速度矢方向与有限元得到的最大主应力方向之间的差异最小为优化目标,优化有势流场构造参数,从而得到优化的变刚度铺层轨迹。该方法得到的变刚度铺层能最大程度地使纤维主方向与层合板在该点处的最大主应力方向一致。研究结果表明,变刚度铺放不仅可提高层合板的拉伸强度,还可使试件在达到极限载荷后的承载能力下降速度平缓,从而降低了发生瞬时性损坏的概率。     

Experimental observation and finite element method modeling on scratch-induced delamination of multilayer polymeric structures

Scratches that result in delamination are common in multilayer polymeric laminates and coatings. In this study, the adhesive failure among a set of model double-layer epoxy coatings was experimentally investigated and numerically analyzed using the finite element method modeling based on the maximum principal stress criterion. The adhesive failure on the model epoxy coatings was generated using an ASTM-standard linearly increasing normal load scratch test. The parametric study reveals that delamination may initiate at locations underneath both scratch shoulder and behind scratch tip during scratching. It is also found that the magnitude and direction of peak tensile maximum principal stress developed at the interface are affected by both the laminate thickness and the material parameters of each layer. The parametric analysis shows that the onset of delamination can be delayed by possessing a softer base layer, a top or base layer with a higher yield stress, a base layer with a lower strain-hardening slope, and a lower surface coefficient of friction. The Mode I delamination at the interface will become dominant in a multilayer system when the base layer has a higher modulus and a lower strain hardening slope. The usefulness of the present study for determining the delamination resistance of multilayer polymeric laminates and coatings is discussed.     

Local residual stress evolution of highly irregular thermally grown oxide layer in thermal barrier coatings

Local residual stress in thermally grown oxide (TGO) layers is the primary cause of failure of thermal barrier coating (TBC) systems, especially TBCs prepared by air plasma spray (APS) with a highly irregular TGO. Herein, the distribution of residual stress and the evolution of the irregular TGO layer in APS TBCs were investigated as a function of oxidation time. The stress was measured from cross-sectional micrographs and converted to the actual stress inside the coatings before sectioning. The TGO exhibited significant inhomogeneity at different locations. Stress conversion occurred across the TGO thickness; the layer near the yttria-stabilised zirconia (YSZ) component exhibited compressive stress, whereas that along the bond coat was under tensile stress. The evolution of the compressive stress is also discussed. These analyses may provide a better understanding of the mechanism of APS TBCs.     

Ceramic laminates with improved mechanical reliability by tailoring the porosity of the constituting layers

Two different ceramic laminates composed of porous alumina and alumina/zirconia layers were designed and produced in the present work. The two symmetrical architectures were selected whose fundamental difference is the presence on the surface of a porous layer in the first and a compact alumina/zirconia composite layer in the second. The residual stress profile and corresponding fracture toughness were tailored to promote the stable growth of surface defects prior to final failure to increase the mechanical reliability of the material. The laminates were realized by stacking together different green laminae (containing specific pore former content) in a specific order, thermo-compression and co-sintering. The results point out an important reduction of the strength scatter and a clear insensitivity to surface damage. It is also shown that the mechanical performances are strictly related to the specific architecture of the laminate, this allowing to tailor a priori the mechanical performances of the composite.     

Progressive failure analysis of carbon-fibre/epoxy composites laminates to study the effect of stitch densities under in-plane tensile loading

Through thickness reinforced stitched laminates with different stitch densities (0.11 and 0.028?mm^?2) were studied in order to analyse effects on laminate behaviour, under in-plane tensile loading based on continuum mechanics. Multi-layered stitched laminates with the stacking sequence [+45/90/?45/0₂/+45/90₂/?45/0]_s were modelled on a lamina-wise basis to analyse the macroscopic damage and local stress–strain constitutive behaviour. Interfaces between lamina and stitch yarns were assumed to be perfectly glued and were modelled by the contact capability. Discretisation procedures using the principle of virtual work were applied in addition to discretisation of the contact traction. Progressive failure analysis with Puck’s failure criteria was conducted to characterise the failure behaviour of the laminate. This analysis showed that reinforcement density is one of the key factors affecting strength, stiffness and crack propagation in composite laminates. By suppressing the damage initiation, densely stitched laminates showed 15.2% higher in-plane stiffness than moderately stitched laminates. The results obtained by the finite element technique are consistent with the experimental results.     

界面破坏对割口单向叠层板的应力集中及强度的影响

基于剪滞理论,建立一种计及界面损伤的分层剪滞模型,分析了含割口的单向叠层板在拉伸荷载作用下的应力重新分布问题,据此可求得界面损伤区长度和割口前缘完整纤维的应力集中因子.在此基础上,采用细观统计破坏理论,研究了割口单向叠层板的拉伸破坏机理和强度,定量获得了残余拉伸强度与界面剪切强度的关系,所得结果与现有实验吻合较好.结果表明,应力集中和强度与割口长度及界面剪切强度有关;适宜的界面黏结,具有较高的残余拉伸强度.     

Crack propagation and residual stress in laminated Si3N4/BN composite structures

The level of residual stress and crack propagation in a new generation of laminates, based on silicon nitride (Si₃N₄) layer and a mixture of boron nitride (BN) and alumina (Al₂O₃) interlayer, was presented. The structure consists of alternated concentric rings of Si₃N₄ separated by the weak BN interlayer possessing no planes of easy crack propagation and fracture resistance much larger than that of any classical planar laminates. The results on direction of crack propagation and residual stress in relation to inter-layer composition, the number of layers, and their thickness are investigated and reported. The effect of residual stress on crack propagation was studied by using Vicksrs intentation. The highest compressive residual stress of ∼170 MPa was found in samples with five layers possessing an average layer thickness of ∼310 × 10⁻⁶ m.     

聚合物基复合材料残余应力的研究进展

本文综述了聚合物基复合材料残余应力的分类、形成原因和层间残余应力对复合材料性能的影响.阐述了残余应力的实验测量方法和理论计算方法,并介绍了减小残余应力的方法.