Coalescence Of Micro-damage On Dynamic Tensile Fracture Of Ductile Metal

Dynamic tensile fracture(spallation) of ductile metals is one of the important dynamic behavior and is an essential problem closely related with the physical process of arms. Spall is resulted by tensile stress which is produced due to the rarefaction wave interaction under dynamic loading. Tensile stress activates the non-uniform mesoscopic structure of material,leads to internal damage nucleation, growth, coalescence and final fracture. Currently, nucleation and growth of micro-damage is often studied and well understood, but, for the most important stage of damage evolution, the coalescence is still lack of sufficient knowledge and understanding. The reason is that, on the one hand, with respect to the nucleation and growth of micro-damage, coalescence is a process of rapid development and it is difficult to directly observed by experiments; on the other hand, the coalescence behavior of micro-damage is multi-scale dynamics of strong coupling and the mesoscopic feature is difficult to be associated with the macroscopic fracture behavior.In this thesis, one-stage light gas gun has been used to conduct shock load experiments, the free surface velocity curves and "soft recovery" samples are obtained, with which kinetics of damage evolution has been analyzed. On the basis of the experimental test, high-precision micro-damage quantitative methods are established and damage evolution behavior of mesoscopic scale are outlined. A new bond percolation model of micro-damage coalescence has been proposed, and the applicability of the new model is verified through the numerical simulation. The main study and innovations are as follows:(1) Based on the coalescence of micro-damage being strong temporal correlation characteristics, time regulation dynamic tensile experiments have been disigned and conducted. The design is to decopule the loading stress effect by controlling the impact velocity, and the dynamic tensile duration is controled by varying the thickness of the flyer, with a constant flyer-sample thickness ratio. Based on the theoretical caculation, the design of time regulation is in consistent with the tensile strain rate measured by the free surface velocity. Dynamic experiments are conducted by using one-stage light gas gun and macroscopic response of high-purity copper are obtained: the spall strength of high-purity copper monotonically increases with strain rate increasing, and the bounce of free surface after pullback point exhibits a critical characteristics, with a critical point about 60000s-1.(2) In order to establish the relationship between different scales, a method of damage coalescence measurement has been established. The surface countour instrument is used to achieve large-scale, continuous and high-precision measurement of the sample damage. A quasi-three dimensional(3D) damage identification method is proposed with ?m resolution. The damage calculation for high-purity copper indicated that during damage evolution of vious stages, the spatial discontinuity of micro-damage is strongly coupled with the time discontinuity. Coalescence of micro-damage affects damage distribution characteristics, which shows two critial coalescence size, i.e. the initial coalescence size and the final coalescence size.(3) The bond percolation model for micro-damage coalescence, containing the shape evolution of damage, is established. Through analysis of micro-damage coalescence behavior, it is found that the coalescence process is dominated by the external field. By assuming a single void cell with non-spherical damage, the bond percolation model indicates that the model reflects the initial coalescence and final rupture critical characteristic. Compared to the previous models, the present model shows lower initial coalescence critical damage, lower final fracture damage and faster stress relaxation rate. For high-purity copper, the relationship between macroscopic characteristics and damage evolution, and the model parameters, have been determined.(4) Numerical simulation has been conducted by considering the effect of the coalescence of micro-damage, and results indicate that the model gives well description of the main characteristics of the macroscopic free surface velocity. For high-purity copper with different final damage states, the more micro-damage coalescence proceeded, the more wide rang of free surface velocity affected. The model well reproduces the free surface velocity curves of experiments under different loading conditions. Furthermore, the model simulation demonstrates that, with the increase of strain rate, the rate of damage evolution exhibits criticality transition, confirming the same critical behavior observed in the experiments for the free surface velocity bounce after pullback point.