Mode II Macrocrack Initiation in Orthotropic Composite Viscoelastic Plate

Safe loads and initiation time for a straight macrocrack in viscoelastic orthotropic material that is intended to model a fiber composite plate under shear loads is investigated. The composite material is modeled by viscoelastic orthotropic medium. Determination of expression for crack shear displacement as function of time is based on the corresponding elastic solution and the method of operator continued fractions. Initiation time is obtained as a solution of integral equation for the incubation period. Numerical calculations are given for mode II macrocrack initiation.     

Stress Concentration in Microstructural Elements of Viscoelastic Composite Materials

We present solution of the problem of estimating the stress concentration in the constituents (matrix and inclusions) of a multiconstituent viscoelastic composite material, depending on the shape of inclusions, properties of the matrix and the whole composite. The matrix material is isotropic and viscoelastic. A wide range of properties of inclusions (e.g., pores, solid and viscous particles) is considered. To solve this problem, we use the method of integral transformations. __________ Translated from Problemy Prochnosti, No. 5, pp. 138 – 149, September – October, 2005.     

3D Analyses of the Stability Loss of the Circular Solid Cylinder Made from Viscoelastic Composite Material

The 3D approach was employed for investigations of the stability loss of the solid circular cylinder made from viscoelastic composite material. This approach is based on investigations of the evolution of the initial infinitesimal imperfections of the cylinder within the scope of 3D geometrically nonlinear field equations of the theory of viscoelasticity for anisotropic bodies. The numerical results of the critical forces and critical time are presented and discussed. To illustrate the importance of the results obtained using the 3D approach, these results are compared with the corresponding ones obtained by employing various approximate beam theories. The viscoelasticity properties of the cylinder's material are described by the fractional-exponential operator. The numerical results and their discussion are presented for the case where the cylinder is made of a uni-directional fibrous viscoelastic composite material. In particular, it is established that the difference between the critical times obtained by employing 3D and third order refined beam theories becomes more non-negligible if the values of the external compressive force are close to the critical compressive force which is obtained at t = � (t denotes a time).     

3D FEM Analysis of the Buckling Delamination of a Rectangular Viscoelastic Composite Plate with an Embedded Rectangular Crack Under Two-Axial Compression

In Akbarov, Yahnioglu and Karatas (2010) a buckling delamination problem for a rectangular viscoelastic composite plate with a band and edge cracks was investigated under uniaxial compression of the plate. In the present study this investigation is developed for the case where the mentioned rectangular plate contains an embedded rectangular crack and in addition it is assumed that the plate is subjected to two-axial compression.
It is supposed that all end surfaces of the considered plate are simply supported and that these ends are subjected to uniformly distributed normal compressive forces with intensity p₁ and p₃ which act along the O_x1 and O_x3 axes, respectively. Moreover, we assume that the plate contains a rectangular embedded crack, the edge-surfaces of which have initial infinitesimal imperfections before the loading.
The evolution of these initial imperfections with time under two-axial compression of the plate is studied within the framework of the three-dimensional geometrically nonlinear field equations of the theory of viscoelasticity for anisotropic bodies.
For determination of the values of the critical force or of the critical time, as well as those of the buckling delamination mode of the considered plate, the initial imperfection criterion is used.
For the solution to the corresponding boundary-value problems, boundary form perturbation techniques; the Laplace transform; the Schapery method for obtaining the numerical inverse Laplace transform of the sought values; and the 3D FEM are used.
The numerical results of the critical force and critical time, as well as of the buckling delamination modes are presented and discussed.     

粘弹性材料的微振子模型研究

针对利用粘弹性材料进行结构主被动控制的需要,建立了一套确定微子模型各参数的方法,并且与标准流变模型,分数导数模型以及试验结果进行比较,算例表明确定微振子模型各参数的方法有效,同时也表明微振子模型不但能准确描述粘弹性材料的本构关系,而且能与有限元方法相融合,建立二阶线性系统,能很方便地利用线性系统控制理论进行控制设计。     

Intersonic Crack Propagation in an Orthotropic Material

Intersonic crack propagation is found to exhibit essentially the same features in orthotropic and isotropic materials, provided that the crack propagates along a plane of elastic symmetry. Thus the stress and strain singularity at the crack edge is weaker than the inverse square root singularity in the sub-Rayleigh case, except at one distinct velocity. The energy flux into the process region is determined by using the Barenblatt model. It depends on the crack velocity and on the size of the process region, approaching zero with this size. This revised version was published online in July 2006 with corrections to the Cover Date.     

Simulation of crack growth in composite material shell structures

This paper implements a domain integral energy method for modelling crack growth in composite material shell structures using the finite element method. Volume integral expressions to evaluate the dynamic energy release rate in a through‐thickness three‐dimensional crack are derived. Using the domain integral, the energy release rate computation is implemented in the DYNA3D explicit non‐linear dynamic finite element analysis program wherein crack propagation is modelled by releasing the constraints between initially constrained node pairs. The implementation enables the program to either determine the energy resistance response for the material (provided experimental data is available) or predict the rate of crack propagation in shell structures. The numerical implementation was verified by simulating mode I and mode III slow crack growth problems in semi‐infinite transversely isotropic media, for which analytic solutions are available. Oscillations of energy following the release of nodal constraints as the crack propagates in discrete increments were suppressed using light mass proportional damping and a moving averaging scheme. Copyright © 2004 John Wiley & Sons, Ltd.     

挤压SiCw／LD2复合材料的疲劳裂纹扩展

本文测量了挤压19％(Vol)SiCw/LD2复合材料疲劳裂纹扩展的门槛值ΔK_(th)和扩展速率da/dN。并在扫描电镜下观察了疲劳断口形貌。结果表明,复合材料在门槛值附近和中速扩展区的疲劳裂纹扩展抗力高于基体材料LD2合金。复合材料纵向试样于170℃时效后的疲劳裂纹扩展抗力高于150℃时效,疲劳裂纹扩展扩展过程包括不在一个面内的微裂纹长大和联接这些微裂纹的(Ⅰ+Ⅱ)复合型裂纹的扩展。与裂纹相垂直的晶须有效地阻碍了微裂纹的长大。     

低碳钢解理裂缝源的微观分析

本文通过对低碳钢光滑拉伸试样及缺口弯曲试样在低温下断裂的断口表面形貌及其纵剖面金相所进行的SEM观察,揭示了解理裂缝萌生及扩展的随机性,分析了解理断裂过程的控制因素。建立了“解理裂缝源”的概念以与“解理初裂缝”相区别,据此并结合显微组织特点,指出了低碳钢解理断裂的微观机制。     

Application of viscoelastic fracture criteria to progressive crack propagation in polymer matrix composites

With the view of comparing local and global viscoelastic fracture criteria, an extension of Christensen's criterion to composite materials with stiff elastic fibers is proposed. Several versions of this criterion are compared with Schapery's local approach of the same phenomenon. The asymptotic version of Christensen's criterion for rapid crack propagation is found suitable for the material investigated, at room temperature. Dissipation in the specimen has two main sources: the undamaged material on one hand, and the damaged material inside the `failure zone' close to the crack tip on the other. The respective roles of these two kinds of dissipation are assessed.     

微胶囊自修复复合材料断裂力学实验研究

测试了微胶囊自修复复合材料的基体和胶囊壳材料的断裂韧性,并分析了夹层模拟试件和含模拟胶囊试件的断裂特性,研究了胶囊壳材料与基体材料断裂韧性的匹配关系.结果表明:当基体材料与胶囊壳材料的断裂韧性差值小于107754.45 Pa·m1/2时,裂纹扩展时胶囊壳能够破裂.     

裂尖塑性区内方向应变能裂纹扩展归一化准则

裂纹的起始扩展总是沿着裂纹的半径方向,在塑性区内半径方向的应变能能反映出材料的抵抗断裂的能力。对同一材料,不管它处于哪一种裂纹形式,它的断裂韧性参数是一个常量。本文引入了裂纹顶端临界扩展本征区。认为裂纹顶端存在一个决定裂纹扩展本征区,裂纹扩展是因为本征区的应力应变状态或损伤状态达到材料的断裂韧性才发生的。依此针对Ⅰ-Ⅱ...     

金属材料疲劳裂纹扩展速率Paris模型中材料常数的相关性

金属材料疲劳裂纹扩展速率是工程应用中进行损伤容限设计、疲劳寿命评估的一项重要力学性能指标。Paris模型是疲劳裂纹扩展速率最常用的表达形式,该模型认为疲劳裂纹扩展速率随裂纹前缘应力强度因子范围的变化呈幂函数关系,涉及到两个材料常数C和m。本研究借助部分文献中的试验数据,分析合金钢、铜合金、钛合金、铝合金等不同金属材料疲劳裂纹扩展速率Paris模型中材料常数C和m的关系。结果表明:Paris模型中材料常数C和m的关系可用m=alnC+b线性模型来描述,且不受试样取样方向、焊缝位置、试验环境等因素影响;但不同应力比对该模型的线性度有一定影响,尤其是当应力比为负值时,影响较为显著;不同金属材料常数C和m线性模型的斜率并不相等,其影响因素有待于进一步系统研究。分析结果可为金属材料在工程中的疲劳设计与应用提供参考。     

煤体爆破裂纹扩展速度试验研究

为了进一步研究煤体爆破裂纹扩展规律,以煤体爆破裂纹扩展速度为研究内容,选取0.14 mm漆包铜线和0.7 mm铅笔芯作为探针传感器材料,利用爆速仪改进的裂纹扩展测试系统对煤体内部和表面裂纹扩展速度进行测试。试验结果表明:煤体爆破裂纹扩展速度变化规律与煤体的力学性质、波阻抗及初始裂纹数量等因素有关。在初始裂纹的导向作用下,煤体中的爆破裂纹扩展速度大致经历先迅速衰减、然后近似匀速、最后缓慢衰减3个阶段。     

碳纤维／铜—5%锡基复合材料的弯曲性能

测定了具有不同碳纤维的短碳纤维增强铜-5％锡基复合材料的弯曲强度值,实验表明:随碳纤维体积含量的增大,弯曲强度上升,并近似符合σ=94.5-421V_f+1557V_f~2(MPa)拟合式。与粉末治金铁基摩擦材料的弯曲性能对比表明:复合材料的弯曲性能较好。对碳/铜复合材料的弯曲断口作了扫描电镜观察,断口呈“部分拔出型”。最后,结合弯曲强度实验结果及断口观察,初步分析了该复合材料的断裂过程。     

On the Mechanism of Energy Dissipation in a Fatigue Crack

Experimental studies of specimens with a Mode-I edge crack in bending vibration have demonstrated that energy dissipation in the fatigue crack is mainly due to an elastoplastic zone at the crack tip. The absolute level of energy dissipation in the crack is unambiguously governed by the stress intensity factor range and is independent of specimen dimensions and crack location.     

Crack Tip Shielding or Anti-shielding due to Smooth and Discontinuous Material Inhomogeneities

This paper describes a theoretical model and related computational methods for examining the influence of inhomogeneous material properties on the crack driving force in elastic and elastic-plastic materials. Following the configurational forces approach, the crack tip shielding or anti-shielding due to smooth (e.g. graded layer) and discontinuous (e.g. bimaterial interface) distributions in material properties are derived. Computational post-processing methods are described to evaluate these inhomogeneity effects. The utility of the theoretical model and computational methods is demonstrated by examining a bimaterial interface perpendicular to a crack in elastic and elastic-plastic compact tension specimens.     

Time-Dependent Deformation and Damage Growth in a Rubber-Toughened Fiber Composite

Time-dependent deformation and damage growth was studied in arubber-toughened carbon fiber composite. Cyclic creep/recovery loading wasperformed on unidirectional off-axis coupons to derive the transverse andshear moduli. Constant stress-rate experiments were also performed usingmodal acoustic emission monitoring. This monitoring provides indirectevidence of what is believed to be the primary damage mechanisms in thematerial studied, matrix cracking and fiber/matrix debonding.Three significant findings from this study are emphasized in this paper.The first is the viscoelastic and viscoplastic behavior of the material withgrowing damage. Although a complete characterization has not beenperformed, emphasis is on material behavior believed to be not previouslyreported in the literature. The second is a new Damage Effect Study thatquickly identifies the material parameters in a nonlinear viscoelasticconstitutive theory that are affected by damage. This study can assist instreamlining the characterization process. Finally, a simplified materialmodel for the microstructure is developed based on AE monitoring of damagegrowth. This results in damage evolution equations based on AE data andviscoelastic fracture mechanics. The damage equations correlate AE data fordifferent loading histories. The use of direct monitoring to develop damageevolution equations and the Damage Effect Study reflect a new approach tocharacterization testing of time-dependent materials.     

Modelling on crack propagation behaviours at concrete matrix‐aggregate interface

A numerical model is proposed to simulate crack propagation at concrete matrix‐aggregate interface. One single aggregate surrounded by concrete matrix is taken to demonstrate the behaviours of crack penetration into concrete matrix and crack growth along the interface. Influences of side‐edge constraint, aggregate direction, and interface fracture energy on the crack propagation behaviours are respectively investigated. The results show that, tensile constraint on the side edge, a smaller angle between tensile axis and aggregate, and higher fracture energy lead to a higher rupture strength of the interface. Once the interface crack starts to grow, it propagates to the two ends of aggregate major axis drastically and further penetrates into the matrix. Nevertheless, these factors have no appreciable influence on crack propagation path. By mapping interface crack into major axis, ordinary crack is generated. Using the above simplification, modelling of multiple crack propagation in concrete is efficiently achieved.     

On a thermo-mechanical effect and criterion of crack propagation

A thermo-mechanical effect from partial conversion of fracture work into heat energy during crack propagation is considered with a simple mathematical model. It is assumed that the heat production zone in the vicinity of the crack tip is very small. Thus, the crack propagation process can be viewed as propagation of the crack in elastic material with a point thermal heat source fixed at the tip of the crack. This thermal heat source generates its own temperature and stress fields around the crack tip. As shown in this paper it also generates a negative stress intensity factor that specifies fracture mode I and has to be accounted for in the energetic fracture criterion. The model developed may help to explain many experimental observations such as the increase in the specific surface energy that accompanies an increase in the crack speed and why fracture mode I has a special role in crack propagation phenomena.