Effects of porosity on the measured fracture energy of brittle materials

Although it is known that growing cracks will interact with pores, it is unclear whether the magnitude of this effect is sufficient to influence the fracture energy. To study this, experiments have been carried out where cracks have been grown through simple distributions of pores in poly(methyl methacrylate). These show that the applied force required to grow the crack between two pores can be greater than that required to grow the crack in the pore-free material. Direct observation during crack growth shows that this increase in applied force is associated with the crack front becoming curved. Based on these observations, the effect of equiaxed pores on the fracture energy of brittle materials has been quantitatively described. The analysis predicts how the relative fracture energy should be influenced by the pore volume fraction, and that it should be independent of the size of the pores or the fracture energy of the matrix. These predictions give good agreement with experimental measurements in different ceramic materials, in which the microstructure of the matrix surrounding the pores does not change with pore volume fraction.     

纳米接触过程中黏着规律的变化

本文应用大规模分子动力学方法, 模拟了两种具有不同粗糙形貌的、刚性球形探头与弹性平面基体之间的纳米尺度接触, 计算了探头与基体之间的拉离力和黏着功, 研究了接触过程中界面黏着力随载荷的变化规律, 分析了接触界面原子的法向应力分布. 研究发现, 原子级光滑接触的黏着力随着载荷的增大而线性增大, 而原子级粗糙接触的黏着力-载荷曲线分为以不同斜率增长的两个阶段. 相比于原子级光滑探头, 原子级粗糙探头与基体之间具有较小的拉离力和黏着功, 却在接触过程中形成了较大的黏着力. 因此, 拉离力和黏着功不能表征出纳米接触过程中原子吸引作用对界面法向力的贡献大小.     

光纤涂敷层对光纤裂缝传感器分布式监测能力的影响

基于混凝土模型试验,对涂敷层的传感光纤的裂缝复用能力进行了定量分析,得到了传感光纤和工程结构裂缝夹角在30°,45°,60°时的复用能力与裂缝宽度的关系曲线。对由软性材料构成的光纤涂敷层来说,光纤的复用能力随涂敷层厚度的增加而增大,在与裂缝夹角成45°时,二次涂敷粗光纤的复用能力约为细光纤的5倍,表明光纤敷层对光纤裂缝监测的复用能力具有较大影响。对这种影响的机理进行了理论分析和试验研究,发现当混凝土开裂时,光纤沿混凝土界面的滑移是光纤涂敷层影响光纤裂缝复用能力的主要原因,在光纤与裂缝夹角成45°时,这一影响占60%以上。     

Experimental investigation of mesoscale crack front triple line

The aim of this paper is to investigate the mesoscale behavior and structure of an adhesive near the fracture front of an asymmetric joint consisting of carbon fiber/epoxy resin composites bonded with a relatively soft, epoxy adhesive. A single cantilever beam fracture test at constant separation rate gave steady-state crack propagation, details of which were followed by digital image correlation (DIC). A deformed, triple line region was found between the adhesive, air, and the composite, somewhat resembling a “wetting ridge,” as found with a liquid meniscus in contact with a soft solid. Importantly, the partially separated bondline layer took part in (non-unidirectional) load transfer between adherends (and thus energy dissipation), contrary to common assumptions where the separated bondline is assumed no longer to play a structural role. A simple model, based on the Flamant contact mechanics approach, is proposed and compared with both a finite element solution and experimental data extracted from image correlation. The model points out the importance of two length scales: process zone extent and adhesive thickness, both being known to affect global properties of bonded structures.     

软X射线测量用电阻式薄膜量热计

介绍了一种用于软X射线辐射能量测量的电阻式薄膜量热计。利用电流的欧姆热效应对薄膜量热计的灵敏度进行了标定。在有基底薄膜的标定过程中,采用一维热扩散模型,考虑了金属薄膜向基底的传导热损失。利用电阻式薄膜量热计对聚龙一号装置钨丝阵Z箍缩产生的软X射线进行了测量,并与平响应X射线二极管(XRD)探测器的测量结果进行了比较。实验结果表明,电阻式薄膜量热计测量的软X射线辐射能量和辐射功率与平响应XRD探测器结果在测量不确定度范围内合理地一致。     

The surface cracking behavior in air plasma sprayed thermal barrier coating system incorporating interface roughness effect

The objective of this work is to understand the effect of interface roughness on the strain energy release rate and surface cracking behavior in air plasma sprayed thermal barrier coating system. This is achieved by a parameter investigation of the interfacial shapes, in which the extended finite element method (XFEM) and periodic boundary condition are used. Predictions for the stress field and driving force of multiple surface cracks in the film/substrate system are presented. It is seen that the interface roughness has significant effects on the strain energy release rate, the interfacial stress distribution, and the crack propagation patterns. One can see the completely different distributions of stress and strain energy release rate in the regions of convex and concave asperities of the substrate. Variation of the interface asperity is responsible for the oscillatory characteristics of strain energy release rate, which can cause the local arrest of surface cracks. It is concluded that artificially created rough interface can enhance the durability of film/substrate system with multiple cracks.     

Photoelastic investigation of interfacial fracture between orthotropic and isotropic materials

Static and dynamic fracture of interfaces between orthotropic and isotropic materials were studied using photoelasticity. In this study, a bi-material specimen made of PSM-1^® and Scotchply^® 1002, a unidirectional glass fiber reinforced epoxy composite, was used. Two fiber orientations, fibers parallel to the interface (α=0°) and fibers perpendicular to the interface (α=90°) were considered. Center crack bi-material specimens having different crack lengths were loaded quasi-statically and the full-field isochromatics were recorded using a digital camera. The complex stress intensity factor corresponding to each crack length was calculated from the isochromatics and the values were compared to that obtained from boundary collocation method. Dynamic interfacial fracture was studied with an edge crack bi-material geometry for the two different fiber orientations. The isochromatics around the propagating crack were recorded using a digital high-speed camera. The fracture parameters such as crack speed, complex stress intensity factor and energy release rate were extracted from the isochromatics using the asymptotic stress field equations. The complex stress intensity factor obtained from the static experiments was in close agreement with that calculated using the boundary collocation method. The results also indicated that the fiber orientation with respect to the interface influences the fracture parameters for stationary and propagating cracks.     

Finite strain stress fields near the tip of an interface crack between a soft incompressible elastic material and a rigid substrate

We present a numerical study of finite strain stress fields near the tip of an interface crack between a rigid substrate and an incompressible hyperelastic solid using the finite element method (FEM). The finite element (FE) simulations make use of a remeshing scheme to overcome mesh distortion. Analyses are carried out by assuming that the crack tip is either pinned, i.e., the elastic material is perfectly bonded (no slip) to the rigid substrate, or the crack lies on a frictionless interface. We focus on a material which hardens exponentially. To explore the effect of geometric constraint on the near tip stress fields, simulations are carried out under plane stress and plane strain conditions. For both the frictionless interface and the pinned crack under plane stress deformation, we found that the true stress field directly ahead of the crack tip is dominated by the normal opening stress and the crack face opens up smoothly. This is also true for an interface crack along a frictionless boundary in plane strain deformation. However, for a pinned interface crack under plane strain deformation, the true opening normal stress is found to be lower than the shear stress and the transverse normal stress. Also, the crack opening profile for a pinned crack under plane strain deformation is completely different from those seen in plane stress and in plane strain (frictionless interface). The crack face flips over and the tip angle is almost tangential to the interface. Our results suggest that interface friction can play a very important role in interfacial fracture of soft materials on hard substrates.     

Co元素对硬质合金基底金刚石涂层膜基界面结合强度的影响

采用基于密度泛函理论的第一性原理平面波赝势方法, 研究了硬质合金刀具基底黏结相Co元素对金刚石涂层膜基界面结合强度的影响机理. 借助Materials Studio软件建立了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型, 采用CASTEP仿真软件计算了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型的最优稳定结构. 通过仿真计算, 获得了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型的界面结合能、电荷密度图及Mulliken重叠布居数. 经对比分析后发现, 硬质合金基底中磁性元素Co的存在能转移金刚石涂层膜基界面处W元素及C元素的电荷, 从而使膜基界面处的原子因失电荷而相斥, 这直接导致了金刚石涂层膜基界面间距变大, 使得金刚石涂层膜基界面结合能降低.     

Study of electronic states for V thin films deposited on 6H-SiC substrates by soft X-ray emission spectroscopy

Silicon carbide (SiC) is a candidate material for electronic devices to operate upon crucial environment. Electronic states of silicides and/or carbide/graphite formed in metal/SiC contact system is fundamentally important from the view point of device performance.We study interface electronic structure of vanadium (V) thin-film deposited on 6H-SiC(0 0 0 1) Si-face by using a soft X-ray emission spectroscopy (SXES). For specimens of V(38 nm)/6H-SiC (substrate) contact systems annealed at 850 °C, the Si L_2,3 emission spectra indicate different shapes and peak energies from the substrate. The product of materials such as silicides and/or ternary materials is suggested. Similarly, the C Kα emission spectra show the shape and peak energy characteristic of vanadium carbide including substrate 6H-SiC signal.     

Theoretical estimation of discontinuity flaw of adhesive contacts between multilayer elements of the liquid metal blanket in a fusion reactor

A technique developed for calculating the discontinuity flaw of an adhesive contact and the adhesion energy for two bodies makes it possible to take into account these effects in designing a multilayer cermet wall (insulating barrier) of the liquid-metal blanket in a fusion-type reactor. The technique is based on a model of a linearly elastic medium, in which strains are proportional to external forces, and on the assumption that any body in equilibrium possesses minimum internal potential energy. Expressions are derived for calculating the area of an adhesive contact between two bodies and their adhesion energy. Calculation data illustrate the dependence of these two parameters on the values of the Young modulus and Poisson’s ratio for various pairs of materials, including the most suitable materials for insulating barriers of liquid-metal blankets.     

Highly adhesive calcium phosphate layer on UHMWPE prepared by IBAD

A study has been made on adhesive strength and chemical states of calcium phosphate layer deposited on the surface of ultra-high molecular weight polyethylene (UHMWPE) by ion beam assisted deposition (IBAD). IBAD is well known to be able to give an atomic intermixed zone at layer/substrate interface during deposition. The X-ray diffraction (XRD) results demonstrated that the main structure is amorphous calcium phosphate (ACp) in the layer. Tape test and scratch test were used to measure the adhesive strength between layer and substrate. Both results show that the adhesive strength between layer and substrate of samples prepared by IBAD is much stronger than that of contrast samples prepared by ion beam sputtering deposition (IBSD). It has been found by XPS that there are much more stable covalent bonds between layer and substrate in IBAD samples than that in IBSD samples. The results above lead us to a conclusion that besides the atomic intermixed zone at the interface, the increment of covalent bonds between layer and substrate also enhanced the adhesive strength between layer and substrate in IBAD samples.     

Geometry- and rate-dependent adhesive failure of micropatterned surfaces

The dynamic nature of adhesive interface failure remains poorly understood, especially when the contact between the two surfaces is localized in microscopic points of adhesion. Here, we explore the dynamic failure of adhesive interfaces composed of a large number of micron-sized pillars against glass. Surprisingly, we find a large influence of the microcontact geometry; ordered arrays of these pillars exhibit significantly stronger adhesive properties than equivalent surfaces in which the pillars are disordered. This can be understood with a simple geometric argument that accounts for the number of adhesive bonds that needs to be broken simultaneously to propagate the crack front. Moreover, the adhesive strength in both cases depends largely on the velocity with which the surfaces are separated. This rate dependence is explained on the basis of a semi-phenomenological model that describes macroscopic failure as a consequence of microscopic bond-rupture events. Our results suggest that the dynamics of adhesive failure, in the limit explored here, is predominantly stress-driven and highly sensitive to local geometry effects.     

表面黏附导致的接触行为转变

应用大规模分子动力学方法, 模拟了具有不同原子级粗糙形貌的两种刚性球形探头与弹性平面基体的黏附接触行为. 研究了载荷与真实接触面积、接触界面排斥力与真实接触面积, 以及黏附力与真实接触面积之间的关系. 分子模拟得到的载荷与真实接触面积的关系, 与连续力学接触理论预测很好地定性一致. 无论是原子级光滑探头还是粗糙探头, 黏附接触下的排斥力与真实接触面积的关系, 都与无黏附接触时的规律相一致, 即黏附力对接触行为的影响作用, 可以等效为附加在真实外载荷基础上的虚拟载荷, 将对黏附接触行为的分析转变为无黏附接触分析. 两种探头的黏附力随真实接触面积都呈幂函数形式的增长, 但是, 原子级光滑探头的幂指数大于1, 而原子级粗糙探头的幂指数小于1. 关键词： 接触行为 表面黏附 分子动力学模拟     

Database Construction for Two-Dimensional Material-Substrate Interfaces

Interfacial structures and interactions of two-dimensional(2D) materials on solid substrates are of fundamental importance for fabrications and applications of 2D materials. However, selection of a suitable solid substrate to grow a 2D material, determination and control of 2D material-substrate interface remain a big challenge due to the large diversity of possible configurations. Here, we propose a computational framework to select an appropriate substrate for epitaxial growth of 2D material and to predict possible 2D material-substrate interface structures and orientations using density functional theory calculations performed for all non-equivalent atomic structures satisfying the symmetry constraints. The approach is validated by the correct prediction of three experimentally reported 2D material-substrate interface systems with only the given information of two parent materials. Several possible interface configurations are also proposed based on this approach. We therefore construct a database that contains these interface systems and has been continuously expanding. This database serves as preliminary guidance for epitaxial growth and stabilization of new materials in experiments.     

Interfacial Debonding Around Rectangular Features in Multilayer Structures

This article describes a numerical approach for predicting debonding in multi-layers comprised of rectangular features of different sizes. A single global meshing strategy for finite element analysis is described that includes focused radial meshes at every material junction to accurately capture crack tip and material junction singularities. The strategy is designed to allow straightforward yet accurate calculations of energy release rates for steady-state interfacial debonding in complex architectures. Guidelines for meshing parameters (such as element density and distribution) are discussed on the basis of convergence studies of stress distributions along interface junctions, crack opening displacements and crack driving forces along interfaces. The utility of the approach is demonstrated via a study of debonding around rectangular inclusions, which illustrates relationships between energy release rates, inclusion aspect ratio and elastic mismatch. The paper concludes with an illustration of how interfacial debonding along finite-sized features can promote cracking in adjacent layers.     

Adhesion control by inflation: implications from biology to artificial attachment device

There is an increasing demand for materials that incorporate advanced adhesion properties, such as an ability to adhere in a reversible and controllable manner. In biological systems, these features are known from adhesive pads of the tree frog, Litoria caerulea, and the bush-cricket, Tettigonia viridissima. These species have convergently developed soft, hemispherically shaped pads that might be able to control their adhesion through active changing the curvature of the pad. Inspired by these biological systems, an artificial model system is developed here. It consists of an inflatable membrane clamped to the metallic cylinder and filled with air. Pull-off force measurements of the membrane surface were conducted in contact with the membrane at five different radii of curvature r _c with (1) a smooth polyvinylsiloxane membrane and (2) mushroom-shaped adhesive microstructured membrane made of the same polymer. The hypothesis that an increased internal pressure, acting on the membrane, reduces the radius of the membrane curvature, resulting in turn in a lower pull-off force, is verified. Such an active control of adhesion, inspired by biological models, will lead to the development of industrial pick-and-drop devices with controllable adhesive properties.     

Increase in the molecular component of the adhesive pressure in polymer composites induced by electric charge

The effect of electric charge, present at a glass fiber–polymer interface during the composite material formation, on the adhesive pressure between the components was studied. Additional electric charge was delivered to the interface by means of the deposition of charged polymer particles onto the fiber surface from a fluidized bed in an electric field. The adhesive pressure was calculated using the results of the variational mechanics analysis by Scheer and Nairn from the data obtained with a single fiber microbond test. Our experiments showed that the adhesive pressure increased by 15–20% in the case of charged polymer matrices. This can be attributed to both intensification of van der Waals forces due to extra interfacial pressure and, for polar polymers, the increase of the surface concentration of local adhesive bonds. The mechanical and kinetic models have been proposed to describe the observed behavior of the adhesive pressure; theoretical curves for the adhesive pressure as a function of the time and temperature of the contact formation, including the effect of electric factors, have been obtained. From the kinetic model, the activation energies for the process of adhesive contact formation were determined. The effect of acceleration of adhesive bonding in the presence of electric charge can be explained in terms of a decrease of the activation energy for local bond formation in an electric field.     

Adhesive contact of rough surfaces: Comparison between numerical calculations and analytical theories

The authors have employed a numerical procedure to analyse the adhesive contact between a soft elastic layer and a rough rigid substrate. The solution to the problem, which belongs to the class of the free boundary problems, is obtained by calculating Green’s function which links the pressure distribution to the normal displacements at the interface. The problem is then formulated in the form of a Fredholm integral equation of the first kind with a logarithmic kernel. The boundaries of the contact area are calculated by requiring the energy of the system to be stationary. This methodology has been employed to study the adhesive contact between an elastic semi-infinite solid and a randomly rough rigid profile with a self-affine fractal geometry. We show that, even in the presence of adhesion, the true contact area still linearly depends on the applied load. The numerical results are then critically compared with the predictions of an extended version of Persson’s contact mechanics theory, which is able to handle anisotropic surfaces, as 1D interfaces. It is shown that, for any given load, Persson’s theory underestimates the contact area by about 50% in comparison with our numerical calculations. We find that this discrepancy is larger than for 2D rough surfaces in the case of adhesionless contact. We argue that this increased difference might be explained, at least partially, by considering that Persson’s theory is a mean-field theory in spirit, so it should work better for 2D rough surfaces rather than for 1D rough surfaces. We also observe that the predicted value of separation is in agreement with our numerical results as well as the exponents of the power spectral density of the contact pressure distribution and of the elastic displacement of the solid. Therefore, we conclude that Persson’s theory captures almost exactly the main qualitative behaviour of the rough contact phenomena.     

Energy release rate and stress field calculation for debonding crack extension at the fibre-matrix interface during single-fibre pull-out

For the micromechanical modelling of the macroscopic failure of fibre-reinforced composites the formulation of a critical parameter for initiation and extension of debonding cracks at the fibre-matrix interface is essential. This point is discussed for the 'fibre pull-out' specimen, a test commonly used to measure the adhesion quality of fibre-matrix systems. Some of the simplifying assumptions fundamental to shear lag theory-based models of the fibre pull-out test are compared with results from a detailed finite element (FE) model to examine their validity. The FE model strongly contradicts assumptions made with the shear lag theory that the axial stress gradient in the matrix can be neglected from the equilibrium equation. A critical interface shear strength is commonly used as a measure of adhesion quality. But for elastic materials the nature of the stress concentrations at the fibre end and interface crack-tip are singular. Therefore a fracture mechanic approach is better suited for a debonding criterion than a simple finite shear strength. The energy release rate shows a minimum for short crack lengths and may stabilize the moving crack.