Multi - Field Coupling Interference Of Dislocations In Different Heterogeneous Materials And Typical Defects At Different Scales

Various defects will inevitably produced in the course of manufacturing and employing of crystal materials, such as micro-cracks, inclusions or dislocations, and interactions among these defects have crucial effect on macroscopic mechanical properties of materials, especially the dislocation emission from crack tips is the key transformation between brittle and ductile. In addition, the influence of micro structures near crack tips, such as grain size and phase particles, on the crack propagation is also great. Therefore, research on the interaction between dislocations and various cracks and inclusions contributes to understand the balance stability of dislocation and the evolving trends of micro structures in the vicinity of the crack tip due to the movement of dislocation, as well as its effects on materials fracture toughness, and also can provide scientific basis for the micro structure of the composite design and damage fracture.In this paper, with the elastic materials, piezoelectric/magnetoelectroelastic materials and nanocrystalline materials containing typical defects as the object of study, and using the sectionally holomorphic function theory, Cauchy singular integral, singularity analysis of complex functions and theory of Riemann boundary problem, the interactions between the generalized dislocations, including wedge disclination dipole and quadrupole, screw dislocations and dislocation dipole, and inclusions containing cracks or the surface/interface crack are systematically studied. The main research results are as follows:First, the interactions between screw dislocation or dislocation dipole and enhanced phase containing cracks in elastic materials, including the elliptical inclusion with a confocal crack, strained reinforcement with a lip-shaped crack are studied. The closed solutions of complex potentials are obtained corresponding problems, and the analytical expressions of interference stress strain field, the image torque and image force, the stress intensity factor of the crack tip are obtained in the series form. The calculation results show that the internal crack in inclusions obviously enhance the rejection of the hard matrix on dislocation and weaken the attraction of soft matrix on dislocation, even make the attraction into rejection. The image force, image torque and stress intensity factor periodically change with dip angle of screw dislocation dipole. shielding or anti shielding effect of dislocation dipole on crack enhances with increase of the relative shear modulus of inclusion and matrix. The influence of buried force on dislocation force component of the same direction is small, while on the vertical direction is bigger, that is, it can make the dislocation force magnitude increase greatly, even change the direction of the image force, making the attraction into or the rejection into attraction.Second, the electro-mechanical coupling interference between screw dislocation or dislocation dipole and typic defects, including elliptic crack and circle inhomogeneity are studied. The closed solutions of complex potentials are obtained corresponding problems, and the explicit series solution of stress and strain field, image force, generalized stress intensity factors, and the energy release rate and the stress energy density of crack tip are obtained. The calculation results show that the shielding effect of dislocation on crack in piezoelectric materials is different from elastic material, and the electric displacement intensify factor has the same distribution with the stress intensity factor. The strain energy density is constant positive while the energy release rate can be positive or negative under a given load; negative electric field always suppress crack propagation, while the positive electric field can promote or inhibit the crack propagation; the strain energy density of a point under pure electric field is far less than under pure mechanical field, and increasing the external force field and electric field can increase the strain energy density. Properly increasing the inclusion layer or setting soft inclusion in the crack position can inhibit crack propagation; under the same condition, energy release rate, which can inhibit the crack propagation; dislocations can decrease the strain energy density under positive electric field, while the negative electric field are opposite; the interfacial effect can weaken the attraction of soft and enhance the repulsion of hard inclusion on dislocation.Third, the electro-magnetic-mechanical coupling interference between screw dislocation and elliptic inhomogeneity of a confocal crack in piezoelectric/piezomagnetic composites is studied, and the closed-form solutions are obtained for the dislocation locating both in matrix and inhomogeneity. The expressions of the generalized stress/strain field, image force, the generalized stress intensity factor and energy release rate of crack tip, and strain energy density are derived explicitly. The calculation results show that the image force in magnetoelectroelastic materials has different distribution law from that in elastic materials.The stress intensity factor has the same distributing law as the electric displacement intensity factor, but that of magnetic induction intensity factor is different. The stress intensity factor and electric displacement intensity factor is negative, which shows that positive screw dislocation has shielding effect on crack expansion. The strain energy density remains positive while energy release rate can be negative or positive in magnetoelectroelastic materials. The increasing of applied mechanical load and electric load is not always making energy release rate increased, and it depends on the combined action of applied fields. Moreover, the influence of applied magnetic field on energy release rate is small, and the contributions of applied electric field and magnetic load to strain energy density are far less than that of applied mechanical load.Four, the mechanism of dislocation emission and crack propagation of both blunt crack tip in nanocrystalline solid and interface collinear crack tip in nanocrystalline bi-material interface under plane loadings is studied, including the influence law of nano-crystal rotation and cooperative grain boundary sliding and migration on dislocation emission of blunt crack tip in nanocrystalline solid. The calculation results show that the nano-crystal rotation can inhibit the edge dislocation emitting form blunt crack tip, so as to reduce the toughening of materials caused by dislocation emission. The in-plane shear loads more easily make the dislocation emission from the blunt crack tip than tensile loads, and the cooperative grain boundary sliding and migration can promote dislocation emitting from crack tip to toughen materials.