Study On Fatigue Behavior And Fatigue Life Prediction Of Metal Honeycomb Sandwich Panels

The metal honeycomb sandwich panel is a new type of ultra-light porous material. It is widely used in aviation, aerospace, automobile, construction and many other engineering fields due to its excellent performance such as the high strength and stiffness, acoustic and heat insulation, fireproofing and insulation etc. Since the advent of this new material, considerable researches involving static equivalent model and dynamic behavior have been done by domestic and foreign scholars, but researches in cumulative fatigue damage properties are still rare. Therefore, in this study, steel honeycomb sandwich panels were taken as research object, and with four-point bending fatigue test, cumulative fatigue damage properties of the metal honeycomb sandwich panel were deeply investigated. What's more, generalized bending stiffness was selected as characteristic parameters and a cumulative fatigue damage evolution model was established. On this basis, investigation in fatigue life prediction was carried out and good results were obtained. Details are as follows:1. Take the hexagonal honeycomb sandwich panel for example. Based on the pretheories, the preparation technology of honeycomb sandwich panel was introduced. The equivalent parameters represent its physical and mechanical properties were derived, and meanwhile, the bending and vibration theories for honeycomb sandwich were summarized in this paper.2. Static and fatigue four-point bending experiments were carried out in this paper. Firstly, the bending load-deflection relationship and failure modes of the test specimen were obtained by the four-point bending test. Secondly, according to the results of the static test, four-point bending fatigue tests of the specimens in L-direction (along the honeycomb core strip) and W-direction (perpendicular to the core strip) under 90%,80% and 70% load level were carried out respectively, and fatigue lives of specimens were obtained. The results of fatigue tests indicated that the loading point deflection of the test specimen in fatigue process experienced three increasing stages. They are rapid growth stage, smooth growth stage and a second rapid growth stage until failure.3. Based on Lemaitre classic formula, the generalized residual bending stiffness was selected as a damage parameter and the law of the bending stiffness degradation of honeycomb sandwich panel during the four-point bending fatigue test was analyzed according to the experimental results. Analysis results showed that the stiffness degradation was only related to the directivity of specimens but the load levels. Based on this finding, the three-stage damage (damage coring, damage stably expanding and instability damage growth failure stage) evolution formulas describing the damage progress of the L-direction and W-direction specimen respectively was established. A good agreement was found between the formulas and the experimental results.4. By employing the methods of S-N curves flitting and cumulative damage methods, the four-point bending fatigue lives of the honeycomb sandwich specimens under 65% and 60% load levels were predicted, and corresponding experiments were made for certification. The results indicated that fatigue lives predicted by cumulative damage method were more accurate within 10% relative error compared with the experimental results, which proved that the proposed cumulative damage fatigue life prediction method based on the stiffness degradation is feasible in engineering.