Investigation On Fatigue Behavior Of Metallic Honeycomb Sandwich Structures

Due to advantages of light weight, high stiffness and strength ration, design flexibility, etc, honeycomb sandwich structures are attractive structural components and are therefore widely used in aerospace, shipbuilding, automobile, civil engineering and other areas. In practical engineering applications, generally honeycomb sandwich structures are designed to carry cyclic loading. Moreover, mechanical properties can be influenced by loading and service conditions. In order to explore the mechanical behaviors of brazed metal honeycomb sandwich structures, especially to investigate fatigue behavior and reveal different failure modes, also to evaluate the macro structural performance, some research works were carried out as following:Mechanical properties of two kinds of metal honeycomb sandwich structures, such as lateral tension, flatwise compression, three-point bending, four-point bending, SHPB impact compression were experimental investigated in detail. The results show mechanical properties are sensitively influenced by geometries of specimen, different cell orientation, experiment temperatures and so on. With digital cameras and optical stereo microscope, different failure modes were analyzed and discussed in detail.The fatigue behavior of honeycomb sandwich beams was experimentally investigated through four-point bending tests. Two kinds of specimens, initially undamaged and damaged by interface debonding, have been tested. The fatigue tests results were presented in standard SN diagrams and the fatigue limit was obtained. Meanwhile the influence of different debonding size on fatigue life was discussed. Fatigue behaviors of three-point bending and four-point bending at high temperature were investigated in detail. During the three-point bending fatigue experiments, by changing the service temperature (ranges from 200 to 400℃), high temperature, coupled thermal and mechanics limit conditions of actual engineering were simulated. Curves between number of cycles and load level were acquired. Also based on the above results, the effects of cell orientations on the fatigue strength were studied. In the four-point bending fatigue tests, the experimental temperature was focused on 300℃. Different failure modes of the above loading conditions were carried out. Several fatigue life prediction methods were presented in this thesis. Namely:direct SN curve method, stiffness degradation method and strength reduction method. The results between experiments and established method based on stiffness degradation were contrasted. Meanwhile, appropriate corrected work on the stiffness model was finished in order that it can fit for life prediction of honeycomb structure under high temperature. Two non-linear cumulative damage models derived from the chosen stiffness degradation equations, were examined in context with the linear Miner's damage criterion and compared with available experimental results. For the strength reduction method, the effectiveness formula used by Burman was studied for undamaged specimens and damaged by interface debonding specimens. A corrected model was proposed to get a reasonable prediction of bending fatigue life for honeycomb sandwich specimens at room temperature.Finally in this thesis, three non-destructive testing methods were employed to evaluate the macro performance of honeycomb sandwich structures. Surface and inner defects of laminated wood panel were detected by ESSPI. The influence of shearography size, defect geometry, depth and other factors on the detection results were analyzed. Debonding between face layer and honeycomb core and defect information were successfully accomplished by pixel and displacement calibration technique. Program with mean, median, wavelet domain spatial filtering functions was compiled and used to optimize the speckle patterns. Introduced a method based on digital image correlation to obtain the in-plane elastic modulus of honeycomb sandwich structure. Special calculation programs were compiled. Results acquired from different search template size and subpixel algorithms were contrasted and studied. Meanwhile compared results with the traditional machine measurement method proved the validity of the proposed method. Using acoustic emission technique, with the three-point bending test, the damage analysis of honeycomb sandwich with the initial defects was monitored. Variation rules of AE parameters with the load time, different location under three different load conditions were developed. These AE characteristic parameters indicated the damage and fracture process of the steel sandwich beam specimens. A good agreement was found between the experimental and analytical results.