Dynamic Yielding Theory And Impact Response Of Multi-layer Unsymmetric Sandwich Beam

Due to the light weight, high strength characteristics, foam sandwich structures are widely used in aerospace, military,marine,automotive, high-speed trains,wind power,packaging and medical equipment. The design of a new type of high performance sandwich structure has always been a frontier subject in the field of material engineering. A typical sandwich structure is usually composed of two thin face sheets and a central core material with a certain thickness. In recent years, with the continuous development of new materials and structures, a variety of novel sandwich structures have been proposed by changing the material and structure of the core materials, which show excellent performance in practical applications. Therefore, a new dynamic plastic constitutive model for sandwich structures is established by considering the deformation characteristics and application requirements, it is of great significance for the dynamic optimization design and application of new sandwich structures. In this dissertation,based on the deformation characteristics of metal foam core under the high-speed impact, the dynamic plastic yield theory of the sandwich beams with time-varying metal foam is established, and the theoretical model of multi-layer sandwich beam with strength gradient is proposed, and the corresponding dynamic yielding theory is established. The main contents are as follows(1) The coupling relationship between the front and back face sheet of sandwich beam model under the mass impact is developed. First, different from the previous pressure cavity model, the core of the sandwich structure simplified as rigid-plastic spring, connecting the front and back facesheet. In order to consider the bending effect of foam core, the mass and bending moment of the foam core are evenly distributed on the front and back facesheet. During deformation, the bending moment of the foam core is considered. The curves of velocity and deflection versus time scale are obtained and the model is applied to the small deformation analysis of sandwich beams. The results show that the prediction model agrees well with finite element simulation and experimental results, the model could effectively predict the final deflection of the beam midpoint.(2) Considering the progressive deformation of sandwich foam core under high velocity impact, a time-varying dynamic yield criterion is proposed for sandwich beams with time inhomogeneity. Under a high-velocity impact, the foam core is local deformation; this is different from quasi-static uniform deformation. So that the cross-section of sandwich structure change from symmetrical shape to asymmetrical shape. The model is applied to sandwich beams under blast loading. Using the membrane force factor method as aforementioned, under blast loading, the dynamic model of large deformation of sandwich beam is established. The velocity and deflection of the midpoint of sandwich structure are analyzed. Considering time inhomogeneity, the dynamic yield criterion can be used to predict the dynamic response of sandwich beam under high velocity impact. The results show that dynamic yield criterion can be well applied to high-speed impact of sandwich structure. The above analysis model can provide theoretical reference for the design optimization of the sandwich structure. It has important theoretical significance for the dynamic performance design of sandwich structure under different conditions.(3) The dynamic yield criterion for the four layers sandwich structures with core strength gradation is derived. The dynamic response of the sandwich structure with same facesheet and two different core materials and thickness are discussed. And the effect of core strength and thikness on the carrying capacity and energy absorption capacity of multilayer beams are analyzed. The results showed that the carrying capacity and energy absorption capacity increase with the increase of the core strength ratio under large deformation. The ratio of the thickness of the core and the facesheet has a similar effect on the carrying capacity and energy absorption capacity of the sandwich structure. The influence of the strength of the front core is more obvious,particularly when the deflection exceeds the depth of the beam.(4) The indentation of sandwich beam with core gradation is investigated. Based on the analytical model of the typical sandwich structure, a large indentation deflection analysis model of sandwich beam with two layers of foam core is presented. The effects of face sheet strength, core strength, and thickness ratio on the initial failure load of sandwich beams with core gradation are discussed. The results show that through variation of the core thickness ratio and core strength ratio, the failure mode of the sandwich beams could be controlled. Meanwhile, the load-carrying and energy absorption capacity of the sandwich beams can also be improved. This work provides theoretical guidance in the sandwich core design.