Mechanical Properties Of Fatigue And Impact Of Metallic Honeycomb Sandwich Panel

Metal thermal protection system has the outstanding advantages, such as large-size, reusability, low life-cycle cost, and it is the first choice of widespread passive thermal protection system of reusable launch vehicle in sub-high-temperature area. Metal thermal protection system is mainly composed of three parts: metal honeycomb panel, multilayer reflection insulation structure and connecting structure. As the main component of metal thermal protection system, the reliability of performance and structure of metal honeycomb panel structure is related with lifecycle of metal thermal protection system and safety of reusable launch vehicle.Metal honeycomb panel is widely used in the rockets, missiles, aircraft, satellites and other aerospace area for its light weight, high specific stiffness, high specific strength, and excellent heat-shielding performance. Honeycomb sandwich panel is the main surface load-bearing component of reusable launch vehicle, it has to bear the aerodynamic force, aerodynamic heat, alternating fatigue load, impact load inevitably. As the honeycomb sandwich panel is a composite structure, and there is a big difference between different forecasting methods, in order to obtain accurate material parameters, it is necessary to operate the mechanical properties test. Moreover, in use process metal honeycomb panel will be attacked in various extents and the mechanical property of materials under fatigue loads and dynamic loads is not clear, therefore, it is necessary to study the fatigue property and the impact property at different temperatures. The debonding defects between panels and honeycomb core are one of the common and most serious defects of honeycomb panel. The defects are in the internal area, so it is hard to observe and detect, there are no sign before the damage or fracture. The suddenness of its destruction often cause lethal threat to the structure and form a security risk. Thus, it is a great help to understand damage mechanism of metal honeycomb panel with the accurate positioning of internal defects and forecast the intensity of metal honeycomb panel with debonding interface, as well as the analysis of crack propagation of debonding interface.As the study of metal thermal protection system is still in its developing period, thus the failure mechanism of honeycomb sandwich panel is not clear enough, and there are relative limited characterization and evaluation methods of these failure processes. This paper illustrated the theory, experiment and numerical simulation of mechanical properties in honeycomb sandwich panel's using process. The problems of mechanical properties, fatigue properties, impact properties, interface debonding strength prediction and crack propagation of debonding interface were illustrated in this paper as well. It provided some theoretical basis for honeycomb sandwich panel design and development, at the same time it was significant to engineering application.In chapter two, experimental method was used to test the mechanical properties of honeycomb sandwich panel. Using digital speckle correlation technology and time-series speckle detection technology, flatwise tension of honeycomb sandwich panel was studied and the tensile modulus of in-plane has been got. Compared with the honeycomb sandwich panel efficient elastic modulus calculated with the equivalent stiffness method, the efficiency and practicability of digital speckle correlation technology were verified. Using flatwise compression experiments, three-point bending experiments; it generalized the mechanical performance parameters of honeycomb sandwich panel under different temperature. With increasing temperature, elasticity modulus of flatwise compression, platform stress and three-point bending stress intensity were reduced in various degree; with the increasing span of three-point bending, yield load decreased.Aiming at debonding defects, non-destructive testing experiment based on electronic speckle shearing pattern interferometry technology was conducted. In the experiment, phase-shifting technique, gray-scale extraction and binarization processing technology were added, and more satisfactory results were achieved. In view of three-point bending mechanical properties of honeycomb sandwich panel, the finite element model of honeycomb sandwich panel with defects was established. Based on the bilinear cohesive model and B-K criterion, mechanical property of honeycomb sandwich panel with debonding defects was simulated. Through calculations, it is showed that the interface debonding will lead to stress concentration, which is the main reason of significant drop of mechanical properties in the honeycomb sandwich panel test. And this coincides with the experimental results, it verify the validity of the model. In the fatigue experiments, it was found that the fatigue life of honeycomb sandwich panel was different due to the different direction of honeycomb core, and the fatigue life in high temperature was higher than in room temperature. When the bearing load was near to the limit load, fatigue failure of materials was the main control factors for the failure of honeycomb sandwich. The crack became nucleus under the repeat function of loads below material yield stress, and caused subcritical expansion. When the length of crack reached critical value, the crack propagated unstably which led to the overall damage. When the bearing load was smaller than the limit load, the material fatigue damage was difficult to occur because the stress was lower or near to the threshold of crack nucleation. While there were inevitable micro-defects (weld, bonding locations) in the preparation process of honeycomb sandwich panel, these defects would become new source of crack under the fatigue load. The crack propagation would lead some damage in the connecting area, result in stress concentration, and lead to an overall failure.Using dynamic compression experimental device Split Hopkinson Pressure Bar and the Instron testing machine to study the compression mechanical properties of honeycomb sandwich panel, and compare the mechanical properties under the effect of the dynamic load and quasi-static load. As the honeycomb sandwich panel with a more pronounced heterogeneity of the stress, using pulse shaper technology and smoothing the incident wave, by comparing the experimental results, selected the size of theΦ10mm×1mm soft materials as pulse shaper, and realize deformation of constant strain rate of the specimen during the deformation process. Using Split Hopkinson Pressure Bar experiment, the stress strain curve of honeycomb sandwich panel within the 2500/s-3850/s strain rate was obtained. The results showed that the honeycomb sandwich panel is rate related material, and strain rate had a significant effect on dynamic maximum stress of honeycomb sandwich panel. Dynamic maximum stress increased with the strain rate. The test was operated under 20℃, 200℃, 400℃and 800℃and examined the stress strain curve, it was found that at 800℃and the same condition of strain rate, the temperature did not affect the dynamic maximum stress.Interface debonding is one of the reasons of honeycomb sandwich panel the mechanical properties reduction. This paper studied the crack propagation process of honeycomb sandwich panel interface, and established the crack dynamic model to crack propagation in the process of debonding. Based on complex analysis, using self-similar function, the problem is transformed into Riemann—Hilbert problem, the general expression of the analytical solution of the crack tip stress, displacement and dynamic stress intensity factor of typeⅢasymmetric dynamic interface crack propagation under the effect of movement varying load Pt n /x n,Px n +1 /tn was got.