Influence Of Micro-cracks And Its Propagation Evolution On Fracture Behaviour Of Materials

In real materials,there are a large number of dispersed micro-cracks near the tip of the macro-crack.They will not only occur in the process of material service but also in the production of materials.Their existence and growth is an extremely important factor affecting the mechanical properties of materials.The stress field at the tip of a macro-crack is different from that of a single crack under the same external load due to the interaction of cracks.Naturally,the crack propagation behavior and mechanism will be different.Therefore,the fracture and damage problems of macro-crack and micro-cracks in materials are studied based on the linear elastic fracture mechanics to explore the propagation mechanism of crack and find out the rule of crack propagation.Based on the Muskhelishvili's complex function method and step-by-step recursive method,the theoretical solution of a central macro-crack and multiple arbitrarily oriented micro-cracks in an infinite plane is derived.This method considers the interaction between micro-cracks.It is a theoretical basis method and the solution speed is fast.It is convenient to solve the problem of crack growth and evolution under different loads.Based on the above theoretical methods,this dissertation investigates the influence of some typical micro-crack distribution forms on the stress field at the tip of the macro-crack,such as parallel distribution,fan-shaped distribution,chain distribution,disordered distribution,etc.Moreover,the influence of the distribution parameters of micro-cracks on the stress field of the macro-crack tip and the micro-crack tip is discussed,including the number of micro-cracks,the distance between micro-cracks and the macro-crack tip,the orientation angle of micro-cracks,the inclination angle of micro-cracks,and the length of micro-cracks.The enhancement effect and shielding effect of micro-cracks on the stress field of the macro-crack tip are analyzed.Then,based on the abovementioned theoretical method,the plastic zone at the tip of the I-II mixed mode crack is modified,the influences of crack closure effect and the size of crack on crack growth behaviou are considered.By employing the special properties of physical short crack,the different growth behaviors of macro-crack and micro-cracks under uniaxial tension,uniform shear,and tension-shear mixed load are studied.Besides,the equivalent crack method is used to solve the propagation of the kinked crack.Based on the physical short crack propagation model and linear elastic fracture mechanics,the process of micro-cracks with different distribution forms are coalescing with the macro-crack is studied.The propagation law of the macro-crack and the physical short crack is described,the propagation path of the crack is predicted,and the possible propagation mechanism of the micro-cracks and the macro-crack is revealed.Finally,combined with the abovementioned theoretical method and damage mechanics method,the damage of material is introduced into the analysis of the stress field at the crack tip.The damage parameters of the macro-crack and micro-crack tip under uniaxial tensile conditions are defined.In this dissertation,the effect of micro-crack on the macro-crack is described by damage parameters,and the effect of micro-crack damage evolution on the macro crack tip damage is studied.With the continuous propagation of some micro-cracks,the macro-crack tip area transits from continuous damage to damage localization,and the bearing capacity of the material decreases.According to the growth behavior of micro-cracks,the direction of the damage localization zone is predicted,which provides a new way for the study of plane multiple cracks in fracture mechanics and damage mechanics.Based on the study of the interaction between cracks,the possible propagation mechanism of micro-cracks and macro-crack is analyzed in this dissertation,and the influence of micro-cracks on the failure process of materials is better understood,which has guiding significance for the development of material design theory.