Mechanical Properties of Cellular Core Structures
Cellular core structures are the state-of-the-art technology for light weight structures in the aerospace industry. In an aerospace product, sandwich panels with cellular core represent the primary structural component as a given aerospace product may contain a large number of sandwich panels. This reveals the necessity of understanding the mechanical behavior of the cellular core and the impact of that behavior on the overall structural behavior of the sandwich panel, and hence the final aerospace product. As the final aerospace product must go through multiple qualification tests to achieve a final structure that is capable of withstanding all environments possible, analyzing the structure prior to testing is very important to avoid any possible failures and to ensure that the final design is indeed capable of withstanding the loads. To date, due to the lack of full understanding of the mechanical behavior of cellular cores and hence the sandwich panels, there still remains a significant lack of analytical capability to predict the proper behavior of the final product and failures may still occur even with significant effort spent on pre-test analyses. Analyzing cellular core to calculate the equivalent material properties of this type of structure is the only way to properly design the core for sandwich enhanced stiffness to weight ratio of the sandwich panels. A detailed literature review is first conducted to access the current state of development of this research area based on experiment and analysis. Then, one of the recently developed homogenization schemes is chosen to investigate the mechanical behavior of heavy, non-corrugated square cellular core with a potential application in marine structures. The mechanical behavior of the square cellular core is then calculated by applying the displacement approach to a representative unit cell finite element model. The mechanical behavior is then incorporated into sandwich panel finite element model and in an in-house code to test the predicted mechanical properties by comparing the center-of-panel displacement from all analyses to that of a highly detailed model. The research is then expanded to cover three cellular core shapes, hexagonal cores made of corrugated sheets, square cores made of corrugated sheets, and triangular cores. The expansion covers five different cell sizes and twenty one different core densities for each of the core shapes considering light cellular cores for space applications, for a total of 315 detailed studies. The accuracy of the calculated properties for all three core shapes is checked against highly detailed finite element models of sandwich panels. Formulas are then developed to calculate the mechanical properties of the three shapes of cellular cores studied for any core density and any of the five cell sizes. An error analysis is then performed to understand the quality of the predicted equivalent properties considering the panel size to cell size ratio as well as the facesheet thickness to core thickness ratio.
The research finally expanded to understand the effect of buckling of the unit cell on the equivalent mechanical property of the cellular core. This part of the research is meant to address the impact of the local buckling that may occur due to impact of any type during the manufacturing, handling or assembly of the sandwich panels. The variation of the equivalent mechanical properties with the increase in transverse compression load, until the first folding of the unit cell is complete, is calculated for each of the three core shapes under investigation.