A unified theory for brittle and ductile shear mode fracture

A unified theory captures both brittle and ductile fracture. The fracture toughness is proportional to the applied stress squared and the length of the crack. For purely brittle solids, this criterion is equivalent to Griffith's theory. In other cases, it provides a theoretical basis for the Irwin-Orowan formula. For purely ductile solids, the theory makes direct contact with the Bilby-Cottrell-Swinden model. The toughness is highest in ductile materials because the shielding dislocations in the plastic zone provide additional resistance to crack growth. This resistance is the force opposing dislocation motion, and the Peach-Koehler force overcomes it. A dislocation-free zone separates the plastic zone from and the tip of the crack. The dislocation-free zone is finite because molecular forces responsible for the cohesion of the surfaces near the crack tip are not negligible. At the point of crack growth, the length of the dislocation-free zone is constant and the shielding dislocations advance in concert. As in Griffith's theory, the crack is in unstable equilibrium. The theory shows that a dimensionless variable controls the elastoplastic behaviour. A relationship for the size of the dislocation-free zone is derived in terms of the macroscopic and microscopic parameters that govern the fracture.     

The deformation and fracture of balloons

Inflation of balloons provides a straightforward way of achieving large biaxial deformations. Previous studies have shown that when a balloon bursts, crack propagation occurs at very high speed – much higher than would be expected from the low strain modulus and elastic wave velocity of the rubber. The present paper is concerned with studies of the deformation and fracture of cylindrical balloons. On inflation, the deformations of such a balloon pass through an unstable region but subsequently increase monotonically with pressure. In this relatively high pressure region, the ratio of the longitudinal and circumferential extension ratios is broadly in accord with expectations from high-strain elasticity theory when the ratio of the corresponding stresses is taken into account. On bursting, crack speeds up to around 300 m/s in this region. It is shown that these speeds are in accord with large increase in incremental moduli for the highly-strained rubber. Marked changes in crack tip profile observed at very high crack speeds are consistent with control of the rate of growth by inertia rather than by the viscoelastic properties of the rubber (as is believed to be the case at lower speeds). Consistent with this, various elastomers having different glass transition temperatures show similar crack growth behaviour in the very high speed region.     

Characterization of strongly interacted multiple cracks in an infinite plate

Based on the Kachanov method and the alternating iteration technique, a new method is proposed to deal with the problem of the strongly interacted multiple cracks in an infinite plate. Unlike the Kachanov method which neglects the interaction of the tractions of the non-uniform components, the tractions of the non-uniform components on the surfaces of cracks are considered through the alternating technique. The accuracy and efficiency of present method are validated by comparing the results of two collinear and two parallel overlapped open the cracks obtained by the present method with those of the exact solutions, the results of the Kachanov method and the alternating iteration technique. Applications of present method in solving sliding close crack problems and evaluating the plastic zones demonstrate the versatility of present method.     

Morphology Engineering of Fullerene (C60) Microstructures Featuring Surface Cracks with Enhanced Photoluminescence and Microscopic Recognition Properties

Surface cracks could improve the optical and photoelectronic properties of crystalline materials as they increase specific surface area, but the controlled self-assembly of fullerene (C₆₀) molecules into micro-/nanostructures with surface cracks is still challenging. Herein, we report the morphology engineering of novel C₆₀ microstructures bearing surface cracks for the first time, selecting phenetole and propan-1-ol (NPA) as good and poor solvents, respectively. Our systematic investigations reveal that phenetole molecules initially participate in the formation of the ends of the C₆₀ microstructures, and then NPA molecules are involved in the gradual growth of the sidewalls of the microstructures. Therefore, the surface cracks of C₆₀ microstructures can be finely regulated by adjusting the addition of NPA and the crystallization time. Interestingly, the cracked C₆₀ microstructures show superior photoluminescence properties relative to the smooth microstructures due to the increased specific surface area. In addition, C₆₀ microstructures with wide cracks show preferential recognition of silica particles over C₆₀ particles owing to electrostatic interactions between the negatively charged C₆₀ microstructures and the positively charged silica microparticles. These C₆₀ crystals with surface cracks have potential applications from optoelectronics to biology.     

高铁锻钢制动盘热疲劳裂纹扩展仿真及寿命评价

巨大的制动热能导致高铁制动盘面出现热斑或热点,形成沿径向扩展的热疲劳裂纹,严重危及行车安全.通过仿真获得热疲劳裂纹扩展规律,据此制定出合适、经济的检测周期,具有重要意义.基于断裂力学理论,采用扩展有限单元法和自主研发的裂尖网格加密技术,得到热点形成规律,植入半椭圆形裂纹进行热疲劳断裂仿真,得到制动速度为400 km/h...     

Effective elastic moduli of an inhomogeneous medium with cracks

In the present paper, the effective elastic moduli of an inhomogeneous medium with cracks are derived and obtained by taking into account its microstructural properties which involve the shape, size and distribution of cracks and the interaction between cracks. Numerical results for the periodic microstructure of different dimensions are presented. From the results obtained, it can be found that the distribution of cracks has a significant effect on the effective elastic moduli of the material. The project supported by the National Education Committee for Doctor     

INVERSE OPALINE STRUCTURE TITANIA PHOTONIC CRYSTALS BY FUNCTIONALLY MODIFIED COLLOIDAL CRYSTALS TEMPLATING

Ordered macroporous titania photonic crystals （PCs） and photonic balls were fabricated by functional modified polymer colloidal crystals. The TiO2 PCs and balls formed through this method exhibit no cracks and lacunae in large areas on their surface and their inner structures.     

Self-healing slip pulses and the friction of gelatin gels

We present an extensive experimental study and scaling analysis of friction of gelatin gels on glass. At low driving velocities, sliding occurs via propagation of periodic self-healing slip pulses whose velocity is limited by collective diffusion of the gel network. Healing can be attributed to a frictional instability occurring at the slip velocity V = V _c. For V > V _c, sliding is homogeneous and friction is ruled by the shear-thinning rheology of an interfacial layer of thickness of order the (nanometric) mesh size, containing a solution of polymer chain ends hanging from the network. In spite of its high degree of confinement, the rheology of this system does not differ qualitatively from known bulk ones. The observed ageing of the static friction threshold reveals the slow increase of adhesive bonding between chain ends and glass. Such structural ageing is compatible with the existence of a velocity-weakening regime at velocities smaller than V _c, hence with the existence of the healing instability. Received: 7 March 2003 / Accepted: 2 May 2003 / Published online: 11 June 2003 RID="b" ID="b"e-mail: ronsin@gps.jussieu.fr     

Earth surface effects on active faults: An eigenvalue asymptotic analysis

We study in this paper an eigenvalue problem (of Steklov type), modeling slow slip events (such as silent earthquakes, or earthquake nucleation phases) occurring on geological faults. We focus here on a half space formulation with traction free boundary condition: this simulates the earth surface where displacements take place and can be picked up by GPS measurements. We construct an appropriate functional framework attached to a formulation suitable for the half space setting. We perform an asymptotic analysis of the solution with respect to the depth of the fault. Starting from an integral representation for the displacement field, we prove that the differences between the eigenvalues and eigenfunctions attached to the half space problem and those attached to the free space problem, is of the order of d^-2, where d is a depth parameter: intuitively, this was expected as this is also the order of decay of the derivative of the Green's function for our problem. We actually prove faster decay in case of symmetric faults. For all faults, we rigorously obtain a very useful asymptotic formula for the surface displacement, whose dominant part involves a so called seismic moment. We also provide results pertaining to the analysis of the multiplicity of the first eigenvalue in the line segment fault case. Finally we explain how we derived our numerical method for solving for dislocations on faults in the half plane. It involves integral equations combining regular and Hadamard's hypersingular integration kernels.     

Damage in fiber bundle models

We introduce a continuous damage fiber bundle model and compare its behavior with that of dry fiber bundles. Several interesting constitutive behaviors, such as plasticity, are found in this model depending on the value of the damage parameter and on the form of the disorder distribution. We compare the constitutive behavior of global load transfer models, obtained analytically, with local load transfer models numerical simulations. The evolution of the damage is studied analyzing the cluster statistics for dry and continous damage fiber bundles. Finally, it is shown that quenched random thresholds enhance damage localization. Received 27 March 2000