Experimental research on dynamic mechanical properties of PZT ceramic under hydrostatic pressure

An experimental technique for initially applied hydrostatic pressure in specimens subjected to axial impact has been developed to study the dynamic mechanical properties of materials. The technique was employed for the purpose of examining the dynamic mechanical properties of lead zirconate titanate (PZT) at zero to 15 MPa hydrostatic pressures. Experimental results unambiguously exhibit the ductile behavior of PZT when hydrostatic pressure is involved. The compressive strength is demonstrated sensitive to the initial hydrostatic pressure and the strain-rate. The fracture modes are analyzed by means of scanning electron microscopy (SEM). Moreover, a failure criterion based on Mohr-Coulomb failure theory is suggested to explain the brittle and ductile failure of PZT.     

Assessment of energy dissipation during mixed-mode delamination growth using cohesive zone models

The use of cohesive elements to simulate delamination growth involves modeling the inelastic region existing ahead of the crack tip. Recent numerical and experimental findings indicate that the mixed-mode ratio varies at each material point within the inelastic region ahead of the crack tip during crack propagation, even for those specimens whose mixed-mode ratio is expected to be constant. Although the local variation of the mode mixity may adversely affect the predicted numerical results, most existing formulations do not take it into account. In this work, the mode-decomposed J-integral is implemented as a finite element post-processing tool to obtain the strain energy release rates and the mixed-mode ratio of the inelastic region as a whole, allowing the assessment of crack propagation in terms of energy dissipation and mixed-mode ratio computation. Different cohesive elements are assessed with this method.     

Synthesis, crystal growth and characterization of organic material: N-Bromosuccinimide (NBS) for NLO applications

The organic nonlinear optical crystal of N-Bromosuccinimide (NBS) was grown by slow cooling solution growth technique using methanol as solvent. Single crystal and powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectral analyses were carried out to confirm the NBS crystals. The ultra violet (UV)-Visible and photoluminescence spectral studies were carried out, the green band at 2.02 eV can be attributed to radiative recombination between deep donors and shallow acceptors. The second harmonic generation (SHG) behaviour of NBS was tested by Kurtz-Perry powder technique. The hardness behaviour, laser damage threshold and dielectric characteristics of NBS crystals were studied.     

基于小波内聚力模型的界面裂纹扩展

利用区间B样条小波良好的局部化性能,将内聚力模型（CZM）引入小波有限元法（WFEM）数值分析中,以区间B样条小波尺度函数作为插值函数,构造小波内聚力界面单元,推导了小波内聚力界面单元刚度矩阵,基于虚拟裂纹闭合技术（VCCT）计算界面裂纹应变能释放率（SERR）,采用β-Κ断裂准则,实现界面裂纹扩展准静态分析。将WFEM和传统有限元法（CFEM） 的SERR数值分析结果与理论解进行比较,结果表明:采用WFEM和CFEM计算的SERR分别为96.60 J/m2 和 101.43 J/m2,2种方法的SERR数值解与理论解相对误差分别为1.85%和3.06%,这明确表明WFEM在计算界面裂纹扩展方面能用较少单元和节点数获得较高的计算精度和效率。在此基础上,探讨了界面裂纹初始长度和双材料弹性模量比对界面裂纹扩展的影响,分析结果表明:界面裂纹尖端等效应力随界面裂纹初始长度的增加而增加;双材料弹性模量比相差越大,界面裂纹越易于扩展,且裂纹扩展长度也越大,因此可通过调节双材料弹性模量比来延缓界面裂纹扩展。      

A micromechanical model for a viscoelastic cohesive zone

A micromechanical model for a viscoelastic cohesive zone is formulated herein. Care has been taken in the construction of a physically-based continuum mechanics model of the damaged region ahead of the crack tip. The homogenization of the cohesive forces encountered in this region results in a damage dependent traction-displacement law which is both single integral and internal variable-type. An incrementalized form of this traction-displacement law has been integrated numerically and placed within an implicit finite element program designed to predict crack propagation in viscoelastic media. This research concludes with several example problems on the response of this model for various displacement boundary conditions.     

An irreversible cohesive zone model for interface fatigue crack growth simulation

Fatigue crack growth (FCG) along an interface is studied. Instead of using the Paris equation, the actual process of material separation during FCG is described by the use of an irreversible constitutive equation for the cyclic interface traction-separation behavior within the cohesive zone model (CZM) approach. In contrast to past development of CZMs, the traction-separation behavior does not follows a predefined path. The model definition, its predicted cyclic material separation behavior and application to a numerical study of interface FCG in double-cantilever beam, end-loaded split and mixed-mode beam specimens are reported.     

A cohesive zone model for predicting delamination suppression in z-pinned laminates

This paper presents a cohesive zone model based finite element analysis of delamination resistance of z-pin reinforced double cantilever beam (DCB). The main difference between this and existing cohesive zone models is that each z-pin bridging force is governed by a traction-separation law derived from a meso-mechanical model of the pin pullout process, which is independent of the fracture toughness of unreinforced laminate. Therefore, two different traction-separation laws are used: one representing the toughness of unreinforced laminate and the other the enhanced delamination toughness owing to the pin bridging action. This approach can account for the large scale bridging effect and avoid using concentrated pin forces, thus removing the mesh dependency and permitting more accurate analysis solution. Computations were performed using a simplified unit strip model. Predicted delamination growth and load vs. displacement relation are in excellent agreement with the prediction by a complete model, and both models are in good agreement with test measured load vs. displacement relation. For a pinned DCB specimen, the unit strip model can reduce the computing time by 85%.     

Improved stability of plasma-sprayed dicalcium silicate/zirconia composite coating

Dicalcium silicate/zirconia composite coatings were produced on Ti-6Al-4V substrates using atmospheric plasma spraying. Different weight ratios of zirconia (50 wt.%, 70 wt.%, 90 wt.%) were mechanically blended with dicalcium silicate (C₂S) powders as feedstocks. The composite coatings were immersed in a simulated body fluid (SBF) and a Tris-HCl solution for the in vitro appraisement of stability and long-term performance in a biological environment. The ion concentration changes of Ca, Si, and P in SBF and Tris-HCl solution were monitored using inductively-coupled plasma atomic emission spectroscopy (ICP-AES). Compared to the pure C₂S coating, our results show that the dissolution rate of the composite coatings is effectively reduced and the stability is improved by the addition of zirconia. The high content of zirconia in the coatings ensures the long-term performance in biological environment, while dissolution of C₂S in the coatings results in a higher Ca ion concentration in SBF and rapid precipitation of bone-like apatite on the composite coating surfaces indicating good bioconductivity of the coatings.     

Work of fracture and cohesive stress distribution resulting from triaxiality dependent cohesive zone model

A new law has been proposed to describe the distribution of the cohesive forces present within the internally structured nonlinear zone that precedes the leading edge of a moving crack contained within a nonelastic solid. The nonlinear effects are modeled by the narrow strips emanating from the crack front and endowed with a certain internal structure (unlike the classic models of Barenblatt and Dugdale). The bulk of the material, though, is assumed to behave as linear elastic solid. Mathematical form the law resembles somewhat the Planck's formula used to explain radiation given off by a perfectly black body at very short wavelength of the visible light spectrum. With Sneddon's integral transformations employed and properly modified, the quantities essential in the Nonlinear Mechanics of Fracture have been quantified. In particular another so-called `ubiquitous eta factor' is discussed and related to the material microstructure by means of a certain transcendental equation. The eta-factor enters the formula for the specific work of fracture measured with specimens of various geometrical and loading configurations, and so far is known only empirically. Both the stationary and quasi-static crack problems are discussed. It has been shown that the variations in the microstructural parameters strongly affect the process zone along with the associated work of separation. The other important factors that influence the cohesive stress distribution and all the resulting fracture parameters, specifically those that are responsible for a ductile-to-brittle transition of fracture mode, are the characteristics of the state of stress induced in the vicinity of the crack front. These 3D effects are best represented by the triaxiality parameter, defined as the ratio of the mean stress to the von Mises effective stress     

Assessment of debond simulation and cohesive zone length in a bonded composite joint

Finite element methods combined with cohesive elements were used to simulate progressive failure behaviour in a bonded double cantilever beam configuration. The introduced cohesive zone was represented by three cases. Responses of both global load–displacement and local cohesive traction–separation were investigated. An unexpected finding was that the overall cohesive traction stiffness was much less than the assumed input value. In addition, the local nodal separation moment was identified. Consequently, correct cohesive zone lengths were obtained using the extracted traction profile along the cohesive zone path at this moment. Information of the global load–displacement profile, traction stiffness, and cohesive zone length induced by the three zone cases was explored. Moreover, the study can explain why very small cohesive zone lengths are generated numerically, as compared to theoretical solutions. Recommendations on the application of the numerical model with cohesive elements to practical experimental analysis were suggested.     

Evaluation of interfacial strength between fiber and matrix based on cohesive zone modeling

This paper presents a measurement technique of interfacial strength considering non-rigid bonding on a fiber/matrix interface modeled as a cohesive surface. By focusing on the stress concentration near a fiber crack obtained from a single-fiber fragmentation test, the stress contours in matrix observed by photoelasticity can be related to the interfacial strength by defining a characteristic length. An equation expressing the relationship between the characteristic length on the stress contour and the interfacial strength was derived, and validated using finite element analysis. The primary advantage of proposed measurement technique is that only a single fiber crack, which usually occurs within elastic deformation of matrix, is required for the evaluation of interfacial strength, whereas saturated fiber fragmentation is necessary in the conventional method. Herein, a sample application was demonstrated using a single carbon fiber and epoxy specimen, and an average interfacial strength of 23.8 MPa was successfully obtained.     

On the definitions of effective stress and deformation gradient for use in MD: Hill’s macro-homogeneity and the virial theorem

The continuum notions of effective deformation gradient and effective stress for homogenization problems with large deformations are reviewed. The “local” problem to be homogenized can include inertia effects to allow for a link between continuum homogenization and the estimation of average properties for particle ensembles via molecular dynamics. The focus of this paper is on the role played by boundary conditions in: defining a meaningful space average of deformation, defining a meaningful space average of stress, and establishing a connection between the idea of effective stress from micro-mechanics and that based on the virial theorem.     

A cohesive model of fatigue crack growth

We investigate the use of cohesive theories of fracture, in conjunction with the explicit resolution of the near-tip plastic fields and the enforcement of closure as a contact constraint, for the purpose of fatigue-life prediction. An important characteristic of the cohesive laws considered here is that they exhibit unloading-reloading hysteresis. This feature has the important consequence of preventing shakedown and allowing for steady crack growth. Our calculations demonstrate that the theory is capable of a unified treatment of long cracks under constant-amplitude loading, short cracks and the effect of overloads, without ad hoc corrections or tuning.     

An analysis of crack growth in thin-sheet metal via a cohesive zone model

A cohesive zone model (CZM) is applied to crack growth in thin sheet metal. CZM parameters are determined from results of global measurements and micromechanical damage models. Crack propagation in constrained center-cracked panels is analyzed to verify the choice of CZM parameters. Special attention is paid to the interaction between buckling and crack growth and to crack link-up in multi-site damaged specimens. The good agreement found between the predicted and experimental data demonstrates that the approach is attractive in investigation of structural integrity of thin-walled structures and does not require assumptions regarding the geometry and size dependence of crack growth parameters.     

Suhl Instability as a Mechanism of the Catastrophic Relaxation in the Superfluid 3He–B

In order to explain catastrophic relaxation, bulk mechanism based on Suhl instability (J. Phys. Chem. Solids 1, 209, 1957) is studied. It is shown, that at sufficiently low temperatures homogeneous precession of spin becomes unstable in the whole region of tipping angles of spin 0≤β≤π. In comparison with the previous publication of Surovtsev and Fomin (J. Exp. Theor. Phys. Lett. 83, 410, 2006) the leading zero temperature increments for the angles θ ₀≃104°≤β≤π are found. Estimation of the temperature of transition to the unstable state for the angle of 105°, that corresponds to the region of tipping angles in homogeneously precessing domain (HPD), is made.     

On the determination of the cohesive zone parameters for the modeling of micro-ductile crack growth in thick specimens

The paper deals with the determination of the cohesive zone parameters (separation energy, , and cohesive strength, T _max) for the 3D finite element modeling of the micro-ductile crack growth in thick, smooth-sided compact tension specimens made of a low-strength steel. Since the cohesive zone parameters depend, in general, on the local constraint conditions around the crack tip, their values will vary along the crack front and with crack extension. The experimental determination of the separation energy via automated fracture surface analysis is not accurate enough. The basic idea is, therefore, to estimate the cohesive zone parameters, and T _max, by fitting the simulated distribution of the local crack extension values along the crack front to the experimental data of a multi-specimen J _IC-test. Furthermore, the influence of the cohesive zone parameters on the crack growth behavior is investigated. The point of crack growth initiation is determined only by the magnitude of . Both and T _max affect the crack growth rate (or the crack growth resistance), but the influence of the cohesive strength is much stronger than that of the separation energy. It turns out that T _max as well as vary along the crack front. In the center of the specimen, where plane strain conditions prevail, the separation energy is lower and the cohesive strength is higher than at the side-surface.     

A single-domain dual-boundary-element formulation incorporating a cohesive zone model for elastostatic cracks

A cracked elastostatic structure is artificially divided into subdomains of simpler topology such that the well-developed classic dual integral equations can be applied appropriately to each domain. Applying the continuity and equilibrium conditions along artificial boundaries and properties of the integral kernels a single-domain dual-boundary-integral equation formulation is derived for a cracked elastic structure. A cohesive zone model is used to model the crack tip processes and is coupled with the single-domain dual-boundary-integral equation formulation; the resulting nonlinear equations are solved using the iterative method of successive-over-relaxation. The constitutive law used for a crack includes three parts: a law relating cohesive force to crack displacement difference when a crack is opening, a characterization of tangential interaction between crack surfaces when the crack surfaces are in contact, and a maximum principal stress criterion of crack advance. Incorporation of local unloading effect of the cohesive zone material has enabled a simulation of fracture with initial damage, partial development of the failure process zone at structural instability and multiple crack interaction. Some of the features of the method are demonstrated by considering three examples. The first problem is a single-edge-cracked specimen that exhibits a snap-back instability. The second example is the development of wing cracks from an angled crack under compression. The last example demonstrates the capability to consider mixed-mode crack growth and interaction of cracks. Thus, the problem of crack growth has been reduced to the determination of the cohesive model for the fracture process. This revised version was published online in July 2006 with corrections to the Cover Date.     

Growth and characterization of glycinium oxalate (GOX) single crystals

Glycinium oxalate (GOX) single crystals were grown by slow cooling solution growth method. The X-ray diffraction and Fourier transform infra red studies confirm the grown crystal. The hardness values of GOX are found to be higher than glycine. The increase in hardness may be due to the C-H---O bonding. The UV-Visible studies show that GOX crystals can be used for nonlinear applications. The dielectric measurement indicates that the GOX crystals have domains of varying sizes and varying relaxation time. The SHG output of GOX was 210 mV at given pulse energy of 5 mJ/s and KDP was 240 mV.     

Experimental investigations of a closed-loop oscillating heat pipe

Results are given of experimental investigations of an oscillating heat pipe (OHP) made in the form of a closed-loop coil of a copper capillary tube with an inside diameter of 2 mm, 4.5 m long, and filled with water in an amount of 50% of internal volume. The starting characteristics of OHP are studied in the range of heat loads from 30 to 100 W under conditions of cooling by way of natural and forced air convection. The pattern of temperature pulsations in the zones of heating, heat transport, and cooling is investigated. It is found that temperature pulsations exhibit a chaotic pattern. In cooling of an OHP by way of natural convection, the increase in heat load is accompanied by an increase in the maximal temperature of the heating zone with a simultaneous decrease in the nonuniformity of the temperature field. When an OHP is cooled by way of forced convection, a decrease in the maximal temperature of the heating zone is observed; however, this is accompanied by an increase in the amplitude of temperature pulsations and in the nonuniformity of the temperature field.