Investigation On The Multi-field Coupling Interaction Of Dislocations With Interfacial Secondary Crack Or Nanovoid

Various defects,such as micro-cracks,void or dislocations,will inevitably produced in the manufacturing and employing of crystal materials,and they tend to nucleate or accumulate at the interface of the material.It is remarkable that the effect of the interaction between dislocations and those other defects on macroscopic mechanical properties of material.The interaction of dislocation and secondary cracks will be conducive to the understanding the evolution trend of crack tip caused by dislocation behavior at the secondary crack tip and its influence on the fracture properties of materials.In addition,nanovoid nucleation and growth is an important part of material strengthening-toughening and fracture mechanism.Dislocation emission from the nanovoid have contributed to explaining the nanostructured materials physical and mechanical properties.Considering crystalline composite materials with dislocations and complicated micro-structure,the ineraction between generalized dislocations(screw dislocation,edge dislocation and extended dislocation)and interfacial secondary cracks are systematically studied.The corresponding model is constructed,and a series of closed-form solutions is derived for complex potentials.The influence of dislocation location,material constant mismatch and crack morphology on the crack propagation and dislocation emission of the secondary crack tip are discussed.Dislocation emission from the nanovoid with surface effects is investigated,in order to reveal nanovoid growth mechanism by dislocation emission.The research results is contribute to the enrichment and development of dislocation theory.In addition,the effect of thermal activation energy on dislocation emission from an elliptically blunted crack tip is researched at high temperature.The research contents are as follows:1.The interaction between screw dislocations and two asymmetrical interfacial cracks emanating from an elliptically interfacial blunted crack under loads at infinity is studied.The closed-form solution is derived for complex potentials.The stress intensity factor and the critical applied stress for the dislocation emission are also calculated.In the limiting cases,well-known results can be obtained from the present solutions.Moreover,new exact solutions for a screw dislocation interacting with some complicated cracks are derived.The results show that positive screw dislocations can reduce the stress intensity factor,and the magnitude of the stress intensity factor increases with the increase in the length of the other cracks and the minor semi axis,but decreases with the increase of dislocation azimuth.The repulsion acting on the dislocation from the other phase or the other crack extending in the horizontal direction makes the dislocation emission at the crack tip take place more easily,but the minor semi axis of the elliptical hole extending in the vertical direction makes it more difficult.2.The interaction between piezoelectric/piezomagnetic screw dislocations and two asymmetrical interfacial cracks emanating from an elliptically interfacial blunted crack under generalized load at infinity is dealt with.The closed-form solutions are derived for complex potentials and generalized stress fields.In the limiting cases,some well-known results can be obtained from the present solutions.Moreover,some new exact solutions are shown.The energy release rate at the right tip due to a piezoelectric/piezomagnetic screw dislocation near the right interfacial crack are also calculated.The results show that the elastic repulsion.stronger piezoelectric effect or weak electric conducting effect from the other half plane acting on the dislocation demotes crack propagation for piezoelectric materials,and the elastic repulsion?weaker magnetic conducting effect or weaker electric conducting effect from the other half plane acting on the dislocation demotes crack propagation for piezomagnetic materials.And the energy release rate on crack expanding decreases with the increase in dislocation azimuth angle and the distance between the dislocation and the crack tip.The increasing of the length of the other crack promotes crack growth,but the increasing of the minor semi-axis demotes it.3.The interaction between edge dislocations and two asymmetrical interfacial cracks emanating from an elliptically interfacial blunted crack is discussed.The closed-form solution is derived for complex potentials.In the limiting cases,well-known results can be obtained from the present solutions.Moreover,new exact solutions for a screw dislocation interacting with some complicated blunted cracks are derived.The results show with the decrement of the length of the elliptically crack or the passivation rate of the elliptically crack tip,the threshold value of dislocation emission from the right tip of the secondary crack at the right side and the most probable dislocation emission angle increase.When dislocation is emitted along a slip plane,the higher length and passivation rate of the elliptically blunted crack,the more difficultily dislocation emission at the secondary crack tip take place.And the influences of the crack length and radius of curvature on dislocation emission increase,while emission angle is farther away from the most probable emission angle.The other secondary crack extending in the horizontal direction makes the dislocation emission at the crack tip take place more easily.4.Dislocation emission from the nanovoid with surface effects under combined loading is investigated.Using the complex variable method,the explicit solutions of the stress fields and the critical stress for dislocation emission are obtained.The influence of the surface properties and the size of the nanohole on the critical stress are discussed.The results show that dislocations are more difficultly/easily emitted from void due to the local hardening/softening on the surface.And the smaller values of intrinsic lengths make the dislocation emission take place more easily.At the same time,the smaller radius of the hole,the greater effect of the surface stress.5.The effect of thermal activation energy on dislocation emission from an elliptically blunted crack tip is researched.Critical stress intensity factors are calculated for an edge dislocation emission from an elliptically blunted crack under mode I and mode II loading conditions at high temperature.The results show that the impact of thermal activation processes is remarkable,the value of the critical stress intensity factor for dislocation emission decreases at high temperature or high strain rate,which means the applied loads for dislocation emission will decrease with increment of temperature or strain rate.