Research On Dynamic Response And Failure Mechanism Of PVC Foam Sandwich Structures Under Near-field And Contact Blast Loadings

Sandwich structures,constructed from two metal face sheets separated by compressible porous(cellular)cores,have been widely employed in the aerospace,building,automobile and naval industries for their high strength-to-weight ratios,high stiffness-to-weight ratios,and superior energy absorbing capability.Based on the traditional ungraded sandwich panel with a uniform core layer,the functionally graded sandwich panel is developed by varying the density and strength of core in the thickness direction.Recently,numerous attentions have been devoted to study the mechanical properties of ungraded/graded sandwich structures under quasi-static,impact and mid-and far-field blast loading.However,the studies on the dynamic behaviors of sandwich structures under near-field air blast loading and contact underwater explosion are relatively few.Under such circumstance,it has significant meaning to conduct the experimental,numerical and theoretical investigations on the failure mechanism of sandwich structures subjected to near-field and contact blast loading.The main highlights in the present thesis are as follows:(1)Taking ungraded/graded rectangle sandwich panels with polyvinyl chloride(PVC)foam core as the study object,an experimental investigation was carried out on the failure modes and associated mechanisms under near-field air blast loading.Nine ungraded and three functionally graded specimens were designed and tested,and the microstructural profile of cores was examined by using Dino-Lite Microscope.Various failure modes including the localized deformation of front face,the global deformation of back face,the crushing deformation and cracking failure of core,the collapse failure of core cell and the debonding between faces and core were observed at studied panels.The effect of structure parameters on the deformation/failure of sandwich panels as well as the intrinsic mechanisms underlying these phenomena were investigated in terms of macroscopic and microscopic aspects.Further insight was given into the relationship between optimal core gradation and response types of graded sandwich panels by comparing with other reports of functionally graded sandwich panel.(2)For the problem of multi-physical-field coupling,a coupled Eulerian-Lagrangian approach(CEL)in the FE software Abaqus was employed to implement the simulations on the dynamic responses of ungraded/graded sandwich panel under near-field air blast loading.The numerical model was firstly validated by comparing the deformation/failure mode of sandwich panels and the deformation of face sheets to experimental results.Then,the simulation gives insight into the response process and several typical physical quantities.The effects of structure parameters on the deformation/failure modes,central velocity response and energy absorption were evaluated.(3)By employing an experiment methodology quantifying the blast performance,blast experiments to investigate the failure mechanisms of circular sandwich panels subjected to contact underwater explosion were carried with six ungraded and two graded specimens.The deformation/failure modes of the sandwich panels were identified and classified in terms of face sheets,core layers and witness plates.Experimental results revealed that the front face would suffer severe petalling failure,the PVC foam would experience spiderweb-like crack failure,the back face would fail into petalling mode or plastic deformation without fracture,while the witness plate failed into localized deformation.Particular attention was placed on the effects of the charge weight,face-sheet configuration,core height and core gradation.(4)Considering the solid-liquid-gas interaction,numerical simulations were implemented and validated to study the dynamic responses of PVC foam core sandwich panel and witness plate subjected to contact underwater explosion.The results revealed a series of complex physical phenomenons,including the detonation and propagation process of the charge,the deformation/failure of sandwich panel,the release and impact of face caps,the water entry of petals and face caps as well as the coupling of the fluid to the witness plate.Then,the attention was focused on the characters of the multiple destructive loads working on the witness plates.It was found that the impact and shock wave caused by the water entry of caps were the main resource of the damage of witness plate.Finally,the influences of charge weight and key geometry parameters on the velocity response and energy absorption was investigated.(5)A dynamic yield criterion for graded sandwich structures with three core layers was derived.Based on the proposed yield criterion and member factor method,a simplified analytical model was developed to predict the deformation response of grade sandwich beam subjected to localized central blast.The analytical model was validated by detailed finite element simulations.The effects of key load and structure parameters on the response were discussed.Theoretical results revealed that the sandwich beam with negative gradient core outperformed their counterparts with uniform or positive core under high-intensity impulse loading.The increase of the number of core layers and core gradient would improve the blast resistance of the sandwich beam with a negative gradient core.(6)Taking stiffened sandwich beam as the study object,a dynamic yield criterion was derived.Then,a simplified analytical model was established to predict the dynamic response of a stiffened sandwich beam under localized central blast by combining the member factor method.The validation of the accuracy of analytical results was assessed through comparison with the results from finite element simulations.A comparison of the structure response of stiffened sandwich beam and monolithic counterpart with equal mass was performed.Then,the effect of key structure parameters on the dynamic response was carried out.Theoretical results demonstrated the superior performance of stiffened sandwich beams.The main attention of presented work was focused on the dynamic behavior and failure mechanism of ungraded/graded PVC foam core sandwich structure subjected to near-field air blast loading and contact underwater explosion.It provides a useful reference for the blast resistance design of sandwich structures.