Analysis of padloaded honeycomb panels

Before this project took place, there was some issues with certain fatigue test of honeycomb panels at Saab. During these test the panels where loaded with cylindrical pads which were not designed for fatigue testing, this lead to stress concentrations that in turn caused unexpected failure in the panel. The goal of this project was to develop an improved method for designing these pads for testing of honeycomb panels. These pads should be designed for both static and fatigue tests, and the resulting method should focus on simplicity based on general guidelines. To accomplish this an FE model was set up, and analyses were performed on a few parameters in the sandwich panel and pad, and their impact on the stress distribution in the structure. These parameters included the diameter of the pad, the thickness of the face sheet and rubber padding, the density in the structure and finally some variations of the test setup. This in turn lead to a further analysis of the dominant failure modes, as a result of the varied parameters. From the parameter study and analysis of dominant failure modes, certain guidelines were acquired to limit stress concentrations from the pad and reduce the risk of unexpected failure. These guidelines are summarised in the points below. A larger pad diameter resulted in larger shear stress, while a smaller pad diameter lead to larger compressive stress. The density in the structure affected the stress magnitude, but not the stress distribution. A thicker face sheet and rubber padding reduced the maximum stresses in the model, though after a combined thickness of 3 mm the stress distribution did not change much. The impact of some variations on the test was also analysed, this includes using one pad on either side of the panel rather than just a single pad, and bolting the pad(s) to the honeycomb panel rather than just gluing them together. Finally, through theoretical calculations and results from simulations the dominant failure mode was identifed to be fatigue in most cases, with buckling being equally dominant in lower densities. This, together with the guidelines presented above, provide a method in designing the pads for static and fatigue testing