Surface Ejection And Melted Fragmentation Of Shocked Metal Sample

Surface ejection and melted fragmentation(micro-spalling)of main body for shocked metal sample keep close relationship with weapon physics,which does not only alter dynamic response of metal flyer,but also has a significant influence on interface instability and mixing at the late stage of implosive compression.These two phenomena are one of critical problems urgently solved by the country owning nuclear weapon and unraveling them possesses an important practical value.Seen from the perspective of basic study,ejecta and fragmentation for shocked metal belong to a complex dynamic issue coupling shock/release wave with material nature,referring to numerous aspects,such as phase transition,melting,fracture and internal defect.As a typical interdisciplinary and preceding physical problem,it is of current focus on the field of basic science.However,limited capability of diagnostics and complicated formation leads to an insufficient knowledge of experimental measurement and physical mechanism about these two phenomena up to date.Therefore,this thesis emphasizes on developing new diagnostic technique used for accurately measuring characteristic information about ejecta production and micro-spalling of melted(or partial melted)metal sample upon shock or release,physical mechanism of which is explored.Further,aiming at high-density spalled layer close to free surface of melted metal sample,we choose solid-solid phase transition as a replaced investigation and the finding indicates that melting relaxation is a major origin for this phenomenon.The primary content and conclusion in this thesis are displayed as follows:(1)The present situation about ejecta production from shocked metal surface is systematically analyzed and summarized.In terms of the current existing drawbacks,diagnostic tool named as Asay-F window is built,which is appropriate for ejecta measurement on the large mass and at the airtight and narrow space environment.We put emphasis on comparison of ejecta formation for melted or solid metal sample and the effect of melted metal surface property on ejecta production.It is found that there exists a distinct difference about total mass and velocity distribution of ejected particles between melting and solid state,and for melted metal surface roughness remains a leading factor determining total ejected mass,velocity and spatial distribution of ejecta.These findings effectively promote the deep understanding about ejecta character,key factor and physical mechanism.(2)Considering the micro-spalling of main body for shocked metal,three stages are proposed to partition this process:a)propagation of loading shockwave before reflecting on the free surface;b)cavitation formation,growth and covalence,triggered by coupling response between release tensile wave and melting metal;c)expansion and dispersion of micro-spalling.According to the characteristics at different stages,the corresponding experimental and numerical approaches are put forward.Experimentally,by incorporating intermediate-energy X-ray radiography,Asay-ML window and DPS velocimetry,important physical information about micro-spalled fragments of Sn sample is obtained under the different explosive conditions,which provides an important data for deeply understanding micro-spalling and breaks through analyzing the data only adopted from abroad.Using molecular dynamics simulations,a great amount of physical processes about micro-spalling are gained,largely improving the understanding.Moreover,physical model describing micro-spalling is constructed for initially performing numerical simulation relying on Lagrange hydrodynamics code CHAP and simulation results are in good agreement with the experiment.(3)For the high-density spalled layer nearby the free surface,we propose an idea that solid-solid phase transition of pure iron material replaces solid-melting process subjected to the shock loading.Combination with dynamic measurement of loaded Fe,microscopic examination of the recovered samples and phase-field modeling,width of spalled layer,morphologic distribution and corresponding dynamic behavior in the region without phase transition are accurately determined and the detailed contribution of phase relaxation and shock front on the formation of this region are explicitly illustrated.Further investigation confirms the main mechanism for the high-density layer induced by melting relaxation,which lays the foundation on the future work about this problem.