Investigation On Fracture Behaviors Of Micro/Nano-scale Structrues With Interfaces

Micro/nano-scale structures have been widely applied in the areas of aerospace,national defense,computer,high-speed rail and automobile related to microelectronic technology,e.g.,ultra-large-scale integrated circuits,which make great contribution to the national economy.However,complicated fracture behaviors,i.e.,interface failure or cracking caused by inner defects(such as tiny crack or void),are the major factors preventing their further development and miniaturization.As the material size shrinks down to micro/nano-scale,the dimension of the controlling region for fracture(e.g.,singular stress field in the vicinity of a crack tip)is also limited to few nanometers.In this case,the classical fracture mechanics based on continuum assumption maybe no longer describe the fracture behavior controlled by few discrete atoms.Therefore,it is of central importance to directly investigate fracture behaviors at the nanoscale experimentally and theoritically.The main contents of this thsis include:Crack initiation from the interface edge in micro/nano-scale structures is investigated at first.A series of micro/nano-cantilever specimens with different dimensions are fabricated by using Focused Ion Beam(FIB)technique,and in situ mechanical bending experiments are conducted by means of Transmission Electron Microscopy(TEM).The results show that large-scale plastic deformation governs the region near the interface edge where the crack initiates.Based on the continuum interface-edge singular field theory,the stress distribution near the interface edge under the large-scale yielding(LSY)condition is investigated.The size of the singular stress field and the critical stress intensity parameter are yielded.The results show that the critical stress intensity parameter is independent of the size of the singular stress field.This indicates that the singular stress field on the order of 10 nm still governs the crack initiation from the interface edge,indicating the validity of the classical interface-edge singular field theory at such a nanometer scale.The unstable cracking behavior in nanoscale single crystal silicon is then investigated.By fabricating micro/nano-scale trapezoidal-double-cantilever-beam specimens,the in situ unstable cracking experiment is conducted in TEM.The entire cracking process,including initiation,propagation and arrest,along the(011)cleavage plane is in situ observed.The fracture toughness,the arrest toughness and the specific surface energy of(011)cleavage plane are obtained experimentally,and then a criterion for unstable cracking in micro/nano-scale structures is proposed.Based on in situ fracture experiments with pre-cracked micro/nano-cantilever specimens,the competitive fracture behavior in micro/nano-scale structures is investigated.A new competitive fracture analysis model is proposed by combining the interaction integral method and the cohesive zone model.Based on the experimental results,key fracture parameters for different fracture patterns are determined,i.e.,fracture toughness of the Si N layer and the cohesive zone parameters of the Si N/Cu interface,i.e.,the cohesive strength and the interfacial characteristic length.With experimental verification,the proposed analysis model for competitive fracture behavior is able to predict the fracture pattern in the pre-cracked micro/nanocantilever specimens.This study will provide reliable guidance for the fracture strength test and the design of micro/nano-scale structures.Considering that it is currently hard to conduct dynamic fracture experiments at the micro/nano-scale so far,the theoretical model for an interface crack problem under dynamic loading is investigated.A new domain-independent interaction integral(DII-integral),which can evaluate the dynamic stress intensity factors(DSIFs)of an interface crack in nonhomogeneous materials under dynamic loading conditions,is derived.Since no material property derivative is involved in the DII-integral formulation,it can be applied to the interface crack problems with both differentiable and non-differentiable material properties.The DII-integral is then proved to be domain-independent of arbitrary interfaces emerged in the integral domain.By using the DII-integral combined with the extended finite element method(XFEM),several benchmark problems are investigated to examine the validity of the proposed DIIintegral and the influence of material nonhomogeneity and the inclusion close to the crack tip on the DSIFs.The results show that the present DII-integral is effective to evaluate the DSIFs of an interface crack in nonhomogeneous materials.This study provides a feasible tool for the future dynamic fracture experiments of micro/nanoscale structures.