(Al2O3)f/Al复合材料在强界面结合下的疲劳损伤模式

在拉-拉载荷下测定了(Al₂O₃)_f/Al复合材料的疲劳寿命(S-N)曲线。通过夭折试验以及SEM疲劳断口和纵截面组织结构分析,研究了复合材料的疲劳损伤模式。研究结果表明,(Al₂O₃)_f/Al复合材料的疲劳极限为750MPa,远高于SCS-6碳化硅纤维增强钛基复合材料。该复合材料兼有钛基和树脂基纤维复合材料疲劳损伤的特点,高应力下由单个裂纹的起源和生长导致复合材料的失效;低应力下,疲劳损伤模式包括纤维劈裂、众多基体裂纹和单个基体裂纹的横向扩展。其中纤维劈裂是主控机制。其更高的疲劳极限可归因于低应力下纤维的纵向劈裂。     

两种Al2O3短纤维增强纯Al复合材料拉伸断裂过程与强化机理研究

选用国产Al₂O₄纤维与英国Saffil纤维怍为增强体,工业纯Al作为基体制成复合材料。在扫描电镜下,进行了微观断裂过程动态观察,并结合强度测定及断口分析探讨了不同情况下的断裂机理。     

SiC颗粒增强铝合金基梯度复合材料拉伸力学性能及其评价

对SiC颗粒增强铝合金基梯度复合材料的拉伸力学性能进行了研究。提出了一种新的材料力学性能评价指标——梯度系数K_x。理论研究表明,金属基梯度复合材料(MMGC)各组分性能的差异使其在拉伸变形时产生了三轴应力。实验及断口分析表明,由拉伸试验获得的材料平均力学性能指标弹性模量E和屈服强度σ_0.2近似符合ROM法则;此类材料的拉伸力学性能受SiC颗粒梯度分布方式的影响。外层为基体,芯部为高SiC含量的材料拉伸断口具有韧性断裂特征,能发挥其性能优势。反之,断口表现出脆性断裂特征,降低了材料的力学性能;受多种因素影响,裂纹穿过层间界面时,会引起断裂方式和扩展方向的改变。当梯度层间SiC颗粒体积分数相差较大时,材料会产生分层开裂;梯度系数K_σb和K_δ可反映材料的梯度分布方式和性能的优劣程度,梯度系数K_E则反映了材料性能的稳定性程度。     

SiC 纤维增强复合材料基体裂纹偏移机理的有限元分析

运用基于能量的裂纹偏移准则, 分别建立了两相和三相复合材料基体裂纹偏移/ 穿透的轴对称有限元模型, 考察了纤维体积分数、描述材料特性弹性失配的Dundurs 参数α和相对裂纹扩展长度a_d / a_p 对相对能量释放率Gd / Gp 的影响。将两相复合材料的有限元结果与He 等人的结果进行对比, 进一步考察了三相复合材料界面层厚度和Dundurs 参数α₁ 和α₂ 对G_d / G_p 的影响。分别将碳涂层SiC 纤维增强复合材料SiC/ C/ Ti-6A1-4V 和碳涂层陶瓷基增强复合材料SiC/ C/ SiC 运用于有限元分析中, 结果表明, 所建立的模型能够准确地预测和比较基体裂纹偏移的机理。     

二维石英纤维增强多孔Si3N4-SiO2 基复合材料的制备及其力学性能

采用浆料法结合真空浸渗工艺, 制备了二维(2D)石英纤维增强多孔Si₃N₄-SiO₂基复合材料。采用X射线衍射技术就粘结剂种类和氮化硅的加入对材料析晶性能的影响进行了分析, 采用扫描电镜对不同浸渗次数复合材料的截面和断口形貌进行了观察, 并采用裂纹偏转因子对复合材料断裂模式进行了分析。结果表明, 粘结剂中杂质Na+促进了石英析晶, 而Si₃N₄对石英析晶影响不大。增加浸渗次数虽不能有效提高复合材料强度, 但却使裂纹偏转因子变小, 断裂模式由韧性断裂向脆性断裂转变, 断口形貌由纤维成束拔出变为多级拔出。     

硅酸铝纤维增强铝基复合材料的疲劳断裂特征

采用压力铸造法, 制得Al₂O₃?SiO₂短纤维增强的铝合金复合材料, 对其弯曲疲劳性能进行了测试, 并详细观察了疲劳裂纹的形成及扩展方式。结果表明: Al₂O₃?SiO₂f/ ZL 108复合材料存在 多种疲劳源; 疲劳裂纹的扩展是通过主裂纹与裂尖前方孔洞的相互联接而进行的, 是不连续的, 沿着纤维及渣球密集的路径扩展; 疲劳过程中主裂纹的形成消耗了大部分的疲劳寿命, 一旦主裂纹形成就快速扩展瞬间断裂。该复合材料的断裂宏观上是脆性的, 但微观上显示出塑性的特征。     

界面断裂

本文分析了复合材料的界面断裂问题,阐述了有关界面裂纹的基本概念,并对界面裂纹的力学模型进行了比较和归纳,最后讨论了用断裂力学参量G_c和K_c评估界面结合强度的有效性及一般原则。     

层状Ti3SiC2陶瓷的组织结构及力学性能

利用热压烧结TiH₂,Si和C粉获得了致密度大于98%的层状Ti₃SiC₂陶瓷。利用压痕法,在不同的载荷下测定了材料的维氏硬度, 发现其硬度值随载荷的增加而降低,在最大载荷30kg时,硬度值为4GPa。压痕对角线没有发现径向裂纹的出现。 这归因于多重能量吸收机制——颗粒的层裂、裂纹的扩展、颗粒的变形等。利用三点弯曲法和单边切口梁法测定了材料的强度和韧性分别为270MPa和6.8MPa·m1/2。Ti₃SiC₂材料的断口表现出明显的层状性质,大颗粒易于发生层裂和穿晶断裂,小颗粒易被拔出。当裂纹沿平行于Ti₃SiC₂基面的方向扩展造成颗粒的层裂,当裂纹沿垂直于基面的方向扩展时,裂纹穿过颗粒的同时,在颗粒内部发生偏转,使裂纹的扩展路径增加。裂纹的扩展路径类似人们根据仿生结构设计的层状复合材料。裂纹在颗粒内的多次偏转、裂纹钉扎以及颗粒的层裂和拔出等是材料韧性提高的主要原因。此外,在室温下得到的荷载-位移曲线,说明Ti₃SiC₂材料不象其它陶瓷材料的脆性断裂,而是具有金属一样的塑性。     

Al2O3短纤维/Al-5.5Zn复合材料裂纹形核及扩展的TEM动态拉伸观察

利用透射电镜(TEM)动态拉伸技术研究了外力作用下Al₂O₃短纤维增强Al-5.5Zn基复合材料的细观力学行为。结果表明,该材料中裂纹形核及扩展主要是在基体中或纤维/基体界面处进行。导致这一结果的原因在于纤维、基体之间强度差异太大。实验中还发现,卸载时,微裂纹有闭合现象。     

基于虚拟裂纹闭合技术的应变能释放率分析

基于虚拟裂纹闭合技术(VCCT)，建立了复合材料层合板层间裂纹尖端的应变能释放率(SERR)三维有限元计算模型。该模型考虑了裂纹尖端大转动和离散单元形状变化对应变能释放率计算的影响,修正了裂纹尖端应变能释放率的计算方法。利用该模型计算了裂纹长度为15 mm和35 mm时纯Ⅰ型和纯Ⅱ型的应变能释放率，纯Ⅰ型应变能释放率分别为 207 J/m2和 253 J/m2；纯Ⅱ型应变能释放率分别为 758 J / m 2和 1040 J / m2；计算值与试验值吻合得很好。同时，该模型计算了混合型不同比值 R=(G_Ⅱ/G_Ⅰ+G_Ⅱ)的长裂纹层合板层间断裂过程的应变能释放率，其中Ⅰ型和Ⅱ型应变能释放率计算值与试验平均值的最大误差为 11.4%，最小误差为 0.4%。该模型能有效计算裂纹尖端的应变能释放率。     

Analysis of the experimental flexural behaviour of a concrete beam grid reinforced with C-FRP bars

In recent years the use of composite materials as reinforcement in concrete beams has increased whenever durability is the main controlling parameter. Many studies have been performed on the behaviour of concrete beams but few have analysed the structural behaviour of plane elements like slabs or grids. The paper analyses the experimental flexural behaviour of a concrete beam grid reinforced with CFRP bars. An one-third scale experimental model of a real concrete floor, was tested in bending. The experimental results show the flexural behaviour in terms of cracking phases and deflection until the failure. The experimental model was affected by the cracking of concrete even under low loads. The experimental model is analysed both by elastic theory of orthotropic plate and a FEM code. An approximate method of calculation, based on an orthotropic plate model, is developed that considers modified bending rigidities for orthogonal directions.     

DCB-specimen loaded with uneven bending moments

A double cantilever beam specimen loaded with uneven bending moments (DCB-UBM) is proposed for mixed mode fracture mechanics characterisation of adhesive joints, laminates and multilayers. A linear elastic fracture mechanics analysis gives the energy release rate and mode mixity analytically for both isotropic and orthotropic materials. By varying the ratio between the two applied moments, the crack tip stress state can be varied from pure mode I to pure mode II for the same specimen geometry. The specimen allows stable crack growth. A special test fixture is developed to create uneven bending moments. As a preliminary example, the DCB-UBM specimen was used for characterising fracture of adhesive joints between two laminates of thermoset glass fibre reinforced plastic.     

Stress analysis of the double cantilever beam specimen

A higher-order plate theory which includes transverse shear deformation and a thickness-stretch mode is utilized to analyze a complete double cantilever beam specimen. Homogeneous, orthotropic materials are considered. The beam is divided into a section along the crack and a second section along the uncracked region. Complete continuity of inplane force resultant, transverse shear force resultant, bending moment, and displacements are satisfied across the boundary between the two sections. This analysis allows one to obtain an approximate distribution of the interlaminar normal stress ahead of the crack. The effect of specimen geometry on energy release rate is investigated numerically. Consideration is also given to the average stress criterion as an alternative to a fracture mechanics approach for characterizing interlaminar peel strength.     

Effect of bending moment on the dynamic fracture of a beam or plate under tensile loading

The dynamic fracture response of a long beam of brittle elastic material under tensile loading is studied. If the magnitude of the applied loading is increased to a critical value, a crack is assumed to propagate across the beam cross section. As an extension of previous work, an induced bending moment generated during fracture is incorporated into the analysis and this improved formulation is presented. The crack length, crack tip speed, axial force and bending moment on the fracturing section are determined as functions of time after crack initiation. It is found that the bending moment has a significant effect on the fracture process in that it tends to retard fracture and causes a drastic change in the slope of the loading curve for large crack depths. Finally, by appropriate change of the elastic modulus, the results may be applied to plane strain fracture of a plate in pure tensile loading.     

陶瓷薄基板材料裂纹预制与断裂韧性评价

预制长度可控的裂纹以及原位观察裂纹扩展是研究陶瓷薄基板抗断裂行为的两大重点。本研究提出应变诱导法, 通过将基板与黄铜梁粘结形成复合体, 利用黄铜梁弯曲变形带动侧面陶瓷薄板受拉侧拉伸变形产生可控裂纹。在工具显微镜下对复合体进行四点弯曲, 原位观察样品的裂纹扩展情况, 通过调节黄铜梁宽度来控制初始裂纹长度, 在初始裂纹萌发后继续加载, 使裂纹达到测试断裂韧性的标准长度。将这种测试方法与块体材料断裂韧性的测试标准进行了对比, 结果表明: 采用该方法预制裂纹后测试断裂韧性具有简易性和可靠性。应变诱导法预制裂纹成功率高, 裂纹萌发位置及长度可控, 且操作方便, 可推广应用于超薄玻璃等其他超薄脆性材料的断裂韧性评价和分析裂纹扩展阻力。     

On modelling cracks in orthotropic plates under biaxial loading: synthesis and summary

The model of a crack with a process zone is considered and generalized to orthotropic materials. It is assumed that a material in the process zone satisfies a strength condition of arbitrary form. Based on the crack model, the fracture of an orthotropic cracked plate under biaxial loading is studied. The crack is directed along one of the anisotropy axes with external loads being applied in parallel and perpendicularly to it. The influence of the biaxiality of external loading on the critical state of the cracked plate is analysed within the framework of the critical crack opening displacement and critical J ‐integral criteria. Numerical solution is obtained using the Mises‐Hill and Gol’denblat‐Kopnov strength criteria. Theoretical results are compared with experimental data obtained by testing specimens made of structural metals.     

Further developments in testing and analysis of the plate twist test for in-plane shear modulus measurements

The plate twist test for testing the in-plane shear modulus of composite materials is analysed. In the proposed test set-up the loads are not applied to the corners of the plate. This makes the test easier to perform than the original plate twist test. Based on a strength of materials approach, an analytical expression is found for the correction factor that accounts for the shifted position of the loading points. The analysis is made without making the assumption of equal bending stiffness in the diagonal directions of the plate, and can thus be applied for off-axis tests on orthotropic or lower symmetry composite materials.     

Fracture mechanics analysis of geometrically nonlinear shear deformable plates

This paper presents boundary integral equations for fracture mechanics analysis of geometrically nonlinear shear deformable plates. A radial basis function and dual reciprocity method are utilized to evaluate the derivative terms and the domain integrals that appear in the formulations, respectively. Numerical examples of the clamped and simply supported plates containing a center crack subjected to uniform transversal loadings are presented. Displacement extrapolation technique is used to compute the stress intensity factors (SIFs). Stress intensity factors of mode I for plate bending and membrane problems are presented. The normalized stress intensity factors in membrane significantly increase after few increments of the load while the normalized stress intensity factors in bending decrease. Less displacement and rotational constraints in cracked plates under uniform transversal loadings will raise the stress intensity factors. The bending stress intensity factors of a central crack in clamped square plate were found to be the highest values compared to those for clamped non-square plates.     

Mixed-mode fracture in biaxial stress state: Application of the diametral-compression (Brazilian disk) test

Mixed-mode fracture of soda-lime glass was studied using a diametral-compression test that features disk specimens with symmetric through-cracks. The test enables one to study fracture under pure mode I loading, pure mode II loading, or any combination of mode I and mode II loading by a simple alignment of the crack relative to the diameter of compression loading. The disk specimens were precracked with the aid of both chevron notches and water-assisted subcritical crack growth. The directions of noncoplanar crack extensions and the relative magnitudes of mode I and mode II stress-intensity factors for mixed-mode fracture under inert conditions were compared to the predictions of three different mixed-mode fracture theories. None of the theories was completely adequate to explain the experimental observations, but a maximum hoop stress criterion modified to include second order, nonsingular term in the series solution for the crack-tip region stress gave reasonable agreement with the experimental results.     

Crack closure effect on stress intensity factors of an axially and a circumferentially cracked cylindrical shell

Crack-face closure occurs when a shell or plate containing a through-the-thickness crack is subjected to a bending load, which leads to lower stress-intensity factors than those expected from non-closure assumption. This article presents a theoretical analysis of the effect of crack-face closure on the stress intensity factors of an axially and a circumferentially cracked cylindrical shell subjected to bending moment respectively. The presented analysis extends the shallow shell theories of Delale and Erdogan by incorporating the effect of crack-face closure. In keeping consistent with the shear deformation shell theory, crack-face closure is modeled by a line contact at the compressive edges of the crack face. The unknown contact force is computed by solving a mixed-boundary value problem iteratively, i.e. along the crack length, either the normal displacement of the crack face at the compressive edges is equal to zero or the contact pressure is equal to zero. The results show that the distribution of the contact force along the crack is generally nonuniform. Furthermore, it is found that, similar to the case of spherical shells, crack closure may occur over the full length or only some segments of the crack in cylindrical shells, depending on the geometry of the shell and the nature (direction) of applied bending load. Comparisons of the stress intensity factor ratios between the closure solutions and the non-closure solutions reveal that the crack-face closure influences significantly the magnitude of the stress intensity factors and it tends to reduce the maximum stress intensity factor. The closure effect of crack face on the stress intensity factors is highest when the shell radius becomes very large for a given crack length and shell thickness.