Analysis of pre-crack kinking and delamination in terms of SLB test of honeycomb sandwich structure

The flatwise tension(FWT) and single leg bending(SLB) tests were used to investigate the fracture behavior of honeycomb sandwich specimens.In the FWT test,only the interlaminar delamination was observed.The test results show that the interfacial peel strength is higher than the interlaminar peel strength.In terms of SLB experiment,a new fracture mode was found,namely IKP(initiation of interlaminar delamination,kinking into facesheet and propagation of interlaminar delamination).The virtual crack closure technique was applied to separate the values of Mode I and Mode II components of the strain energy release rate in SLB experiment.The finite element analysis result shows that the Mode I strain energy release rate is higher than the Mode II strain energy release rate.To simulate the failure of SLB test of honeycomb sandwich specimens,a new computational model based on the Tsai-Hill failure criterion and the cohesive zone model is proposed.In comparison with experimental results,it can be concluded that the computational model can validly simulate the IKP of a honeycomb sandwich structures with reasonable accuracy.     

大豆蛋白复合纤维/亚麻混纺纱的活性染料染色性能

采用活性黄B-4RFN对大豆蛋白复合纤维/亚麻混纺纱进行染色,并研究其染色性能.在对染料固色率的影响因素(温度、碱剂用量、促染剂用量、时间)进行分析的基础上,利用正交试验确定了大豆蛋白复合纤维/亚麻混纺纱用活性黄B-4RFN进行染色的优化工艺,即染料用量2%owf.、纯碱15g/L、氯化钠50g/L,温度70℃,固色时间50 min.性能测试结果表明:活性黄B-4RFN上染大豆蛋白复合纤维/亚麻混纺纱的固色率为49.96%,且染色产品具有较高的牢度性能,可以满足一般染色产品对色牢度的要求,具有工艺可行性.     

复合材料单向板断裂韧性的光弹性实验研究

结合金属材料三点弯曲试验方法,采用光弹贴片法实测纤维增强复合材料的断裂韧性。简要推导光弹条纹参数与裂尖应力强度因子的关系,并以玻璃纤维增强环氧单向板为实例探讨复合材料断裂韧性测试方法。实验表明,利用光弹图像可以实时观测试件裂纹扩展过程,并可结合P-V曲线判断开裂点,进而得到材料的断裂韧性。     

Size effect of deformation nanotwin bundles on their strengthening and toughening in heterogeneous nanostructured Cu

Herein, we fabricated a heterogeneous nanostructured Cu with deformation nanotwin bundles(NTBs) embedded in a matrix of nanograins by means of dynamic plastic deformation at liquid nitrogen temperature. We conducted fracture mechanical measurements to investigate the effect of longitudinal length of the NTBs on their strengthening and toughening. Results suggest that an increase in the NTB length had a marginal influence on the tensile strength; however, it remarkably promoted both fracture initiation toughness and crack growth toughness. Longer NTBs are more effective not only in intrinsically enhancing crack tip plasticity by suppressing the strain localization and void nucleation but also in serving as crack bridges behind the crack front to extrinsically resist the fracture by shielding the crack tip.     

铝合金摩擦搅拌焊接焊区的I/II型复合断裂研究

采用二维计算机视觉-数字图像比对技术,进行裂纹稳定扩展实时表面位移测量,定量分析了铝合金2024-T351母材及摩擦搅拌焊接后不均匀、各向异性的焊区的I/II型复合裂纹稳定扩展的断裂性能、临界COD及其分量CODI和CODII.并研究了母材及焊区裂纹的扩展路径.结果表明(1)距裂纹尖端后某一固定距离的临界COD可反映焊区的断裂韧性,(2)临界COD值反映复合裂纹的扩展受I型或II型裂纹扩展主导,(3)焊区的临界COD值对应地为母材的85%,(4)用数字图像比对技术分析I/II型裂纹的断裂性能是切实可行的一种新方法.     

亚麻增强热塑性树脂基复合材料的开发

利用亚麻纤维的可纺性,与热塑性聚丙烯(Polypropylene,PP)纤维通过捻合形成PP包覆亚麻的混合纱结构,将所得混合纱线进行平纹布的织造,选取5层作为铺层数,在热压机上进行热压复合,制得亚麻/PP热塑性树脂基复合材料,并对其拉伸性能进行测试,得出其经向拉伸强度高于纬向,分别为69.34 MPa和61.76 MPa.     

包覆工艺制备C—SiC—B4C—TiB2复合材料的组织与性能

以鳞片石墨,B4C,SiC,TiO2为原料,利用包覆工艺在不同热压温度下制备了W（C）=50％的C—SiC—B4C—TiB2复合材料,并详细研究了热压温度对复合材料显微组织和性能的影响规律．结果表明,当热压温度高于1850℃时,复合材料由C,SiC,B4C和TiB2这四相组成;复合材料的体积密度、抗折强度和断裂韧性均随着热压温度的升高而增加．2000oC热压时,复合材料的体积密度、气孔率、抗折强度和断裂韧性分别达到2．41g／cm^3,3．42％,176MPa和6．1MPa·m^1/2;热压温度升高,复合材料的碳相和陶瓷相逐渐致密,碳相最终形成了在陶瓷基体上镶嵌的直径为40μm橄榄球状和条状这两种形貌．碳／陶瓷相的弱界面分层诱导韧化和第二相TiB2与陶瓷基体之间热膨胀系数不匹配所致的残余应力使变形过程中微裂纹的扩展路径发展变化,使复合材料的韧性提高．     

Mechanical Properties and Erosive Wear Resistance of Zirconia Toughened Al2O3-TiC Ceramics

The effect of ZrO2, content on the fracture toughness, flexural strength and Vickers hardness as well as the erosive wear resistan properties of zirconia toughened Al2O3-TiC ceramic composites has been investigated. The results showed that the improvement in fracture toughness and flexure strength of composites with the content of zirconia less than 50% and 10% in mass fraction respectively,is primarily attributed to stress-induced transformation toughening by the analyse of X-ray diffraction. The dependance of erosion wear resistance on the attak angle and the content of ZrO2, panicles of the composites was also revealed. It is found that the erosion rate of the composites has a sharp rise at a attack angle over 65°. This phenomenon is due to a brittle response to the erosion test by microsmictural observation on eroded surface.     

Fabrication and Mechanical Properties of TiC／TiAl Composites

TiC/ TiAl composites with different TiC content were fabricated by rapid heating technique of spark plasma sintering ( SPS ). The effect of TiC purticles on microstructure and mechanical properties of TiAl matrix was imestigated. The results indicate that grain sizes of TiAl matrix decrease and mechanical properties are improved because of the addition of TiC particles. The composites display a 26.8% increase in bending strength when 10wt% TiC is added and 43.8% improvement in fracture toughness when 5ut% TiC is added compared to values of TiC-free materials. Grain-refinement and dispersion-strengthening were the main strengthening mechanism. The improvement of fracture toughness was due to the deflexion of TiC particles to the crack.     

Rock fracture under anti-plane shear (Mode III) loading

Anti-plane punch-through shear test and anti-plane four-point bending test are used to study the crack initiation and propagation under anti-plane shear (Mode III) loading. The tensile and shear stresses at the crack tip are calcualted by finite element method. The results show that under Mode III loading the maximum principal stress σ₁ at crack tip is smaller or a little larger than the maximum shear stress τ_max. Since the tensile strength of brittle rock is much lower than its shear strength, σ₁ is easy to reach its critical value before τ_max reaches its critical value and thus results in Mode I fracture. The fracture trajectory is helicoid and the normal direction of tangential plane with the fractured helicoid is along the predicted direction of the maximum principal stress at the notch tip. It is further proved that Mode I instead of Mode III fracture occurs in brittle rock under Mode III loading. The fracture mode depending on the fracture mechanism must be distinguished from the loading form. Foundation item: Project (50374073) supported by the National Natural Science Foundation of China; project (2002032256) supported by the Postdoctor Science Foundation of China     

高温后混凝土断裂全过程的裂缝扩展阻力曲线

采用楔入劈拉法试验,设置20～600 ℃共10组温度,试件尺寸统一采用230 mm×200 mm×200 mm,初始缝高比为0.4.根据试验得到的荷载-开口位移曲线（P-CMOD）求得应力强度因子曲线（K曲线）.基于虚拟裂缝黏聚应力,对高温后混凝土断裂全过程的裂缝扩展阻力曲线进行求解.将混凝土断裂全过程划分为4个部分,裂缝黏聚应力呈Petersson双线性软化曲线分布.计算表明,在某一特定温度下,裂缝扩展阻力曲线随裂缝扩展长度的增加而增长;阻力曲线随温度的上升呈下降趋势.对应力强度因子曲线与裂缝扩展阻力曲线进行裂缝扩展的稳定性分析.结果表明,当裂缝扩展阻力曲线位于应力强度因子曲线上方时,裂缝稳定发展;反之,裂缝将失稳发展.该判据可以等同于双K断裂准则,起裂韧度、失稳韧度是裂缝扩展阻力曲线上的2个关键点.     

螺杆泵挠性轴疲劳寿命可靠性分析

针对螺杆泵挠性轴在使用中频繁发生断裂这一问题, 用概率分析方法有针对性地给出其安全使用概率与寿命之间的关系。在计算挠性轴疲劳裂纹扩展寿命过程中, 考虑几种主要参数的不确定性因素:初始裂纹长度、工况载荷、断裂韧性、裂纹扩展速率等, 针对随机参数的分布特征, 对其进行模拟实际操作工况抽样, 应用MonteCarlo 数值计算方法计算挠性轴寿命, 得出可靠度随时间变化规律曲线。通过对计算结果的分析, 找出挠性轴设计中存在的问题, 并提出相应改进意见。     

新型 Al_2O_3 基陶瓷刀具材料颗粒弥散增韧与晶须增韧的协同作用

研制成功了一种新型陶瓷刀具材料——ＳｉＣ晶须（ＳｉＣｗ）增韧和ＳｉＣ颗粒（ＳｉＣｐ）弥散增韧Ａｌ２Ｏ３陶瓷刀具ＪＸ－２－Ｉ,该材料具有良好的抗弯强度和断裂韧性,同Ａ（Ａｌ２Ｏ３）,ＡＰ（Ａｌ２Ｏ３／ＳｉＣｐ）和ＡＷ（Ａｌ２Ｏ３／ＳｉＣｗ）等材料相比较表明,在ＪＸ－２－Ｉ材料中具有明显的增韧叠加效应,详细研究了晶须增韧和颗粒弥散增韧的协同作用,建立了理论模型并进行了实验验证．     

Phase separation morphology and mode II interlaminar fracture toughness of bismaleimide laminates toughened by thermoplastics with triphenylphosphine oxide group

Toughness improvement of bismaleimide (BMI) resin is very important for its application in composite materials. Blending with thermoplastic polymer is usually used to increase the toughness of BMI matrix. In this work we prepared two thermoplastic polymers with polar triphenylphosphine oxide group in the polymer backbone. The synthesized thermoplastics with different polarities were investigated by several physicochemical methods. Then through scanning electronic microscopy we observed the phase separation morphology of BMI blends at different doping concentrations of thermoplastics. Additionally mode II interlaminar fracture toughness G _IIC of BMI laminates toughened with thermoplastics by ex-situ method was examined. The results showed that thermoplastic with strong polarity would bind tightly with BMI during curing and the phase-separation structure might be fixed at the primary stage; while secondary phase separation could happen in a relatively weak polarity system. It indicates that by regulating the polarity of thermoplastic, we may control the phase separation morphologies of blending system and the mechanical properties of composite.     

桑皮纤维／棉纤维混纺纱线拉伸性能的研究

为了研究桑皮纤维／棉纤维混纺纱线中桑皮纤维质量分数对混纺纱线拉伸性能的影响,纺制了不同混纺比的桑皮纤维／棉纤维混纺纱线,在YG061电子单纱强力仪上进行了拉伸性能测试,并对测试结果进行了分析和比较。结果表明,桑皮纤维／棉纤维混纺纱线的断裂强度随着桑皮纤维质量分数的增加而线性增加,当质量分数达到一定程度后,混纺纱断裂强度随着桑皮纤维质量分数的增加而降低。混纺纱线的断裂伸长率随着桑皮纤维质量分数的增加,有先增加后降低的趋势。总之,桑皮纤维质量分数在20％～30％范围内,混纺纱的拉伸性能比较优良。     

Fabrication and mechanical properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiAl composites

Al₂O₃/TiAl composites were successfully fabricated by hot-press-assisted exothermic dispersion method with elemental powder mixtures of Ti, Al TiO₂ and Nb₂O₅, and the microstructure and mechanical properties were investigated. The results indicate the fine Al₂O₃ particles tend to disperse on the grain boundaries. The grain size of TiAl matrix decreases and the hardness increases with increasing Nb₂O₅ content. The bending strength and fracture toughness reach to a maximum when Nb₂O₅ content is 6 wt%, under 642 MPa and 6.69 MPa·m^1/2, respectively. Based on the fractography and the observation of crack propagation path, it is concluded that the strengthening and toughening of such composites at room temperature can be attributed to the refinement of the TiAl matrix, the deflection behavior in the crack propagation and the dispersion of Al₂O₃ particles.     

纤维素纤维增强高韧性水泥基复合材料的拉伸力学性能

研究了3种纤维掺量UF纤维素纤维增强水泥基复合材料(UF-ECC)的单轴拉伸性能及纤维掺量对ECC力学性能的影响规律,探讨了裂缝扩展宽度与轴拉应力的关系,分析了ECC的拉伸断裂能与特征长度.结果表明:当纤维体积掺量由0.3%提高到0.6%时,UF-ECC的最大拉应变提高183%,断裂能提高419%,特征长度提高281%.ECC复合材料的最大拉应变是聚丙烯纤维混凝土的8~20倍.UF纤维素纤维具有良好的阻裂增韧效用,显著提高了ECC的变形能力,ECC在单轴拉伸荷载下能实现应变硬化和多重裂纹初裂.     

楔形翼缘连接板腹板连接节点断裂性能

为定量确定采用楔形翼缘连接板的钢框架结构梁柱腹板连接节点焊缝应力强度因子与初始缺陷深度之间的函数关系,采用断裂力学与有限元分析相结合的方法.判断焊缝开裂的依据是一型应力强度因子,应力强度因子通过J积分的方法求得.通过有限元计算研究初始裂缝深度、梁截面尺寸、柱截面尺寸、梁柱长度和翼缘连接板伸出柱边缘的长度对腹板连接节点焊缝应力强度因子的影响.采用正交设计法进行研究方案设计,根据参数分析结果归纳出应力强度因子计算公式.结果表明,梁下翼缘焊缝比上翼缘更容易开裂,采用楔形翼缘连接板可以明显改善焊缝边缘的断裂性能,应力强度因子与梁截面参数是增函数关系,与柱截面参数是减函数关系.     

UHTCC与钢材界面的剪切型断裂试验研究

为了将超高韧性水泥基复合材料(UHTCC)应用于大跨径钢箱梁桥,研究双材料界面的力学性能. 采用无切口单边对称加载复合试件,研究UHTCC与钢材界面剪切型裂缝扩展过程. 利用数字图像相关法(DIC),验证该方法用于定量测定双材料界面剪切型断裂韧度的可行性,探究不同界面处理方式对复合试件界面剪切型断裂韧度的影响. 试验结果表明,采用UHTCC-钢材无切口单边对称加载复合试件,结合DIC技术可以实现界面纯剪切型断裂韧度的定量测试;不同界面处理方式对UHTCC与钢材界面剪切型断裂韧度的影响均较小;UHTCC与钢材界面具有较高的剪切断裂韧度,抗剪切性能良好.     

低掺量S-P混杂纤维增强增韧的作用研究

研究并讨论了钢纤维与聚丙烯纤维混杂对高性能混凝土力学性能的影响。研究结果表明,在较低掺量下(总体积率0.9％),混杂纤维混凝土的抗压、抗拉强度、断裂性能和抗弯韧性得到了较显著的提高,使混凝土的破坏具有预征兆性,并在混凝土材料初裂后呈现优越的应变硬化行为,体现了两者的混杂效应。