Energy absorption of sandwich structures with a kirigami-inspired pyramid foldcore under quasi-static compression and shear

Sandwich structures have found a wide range of applications in engineering fields where lightweight and stiffness are of great interest, like the aerospace industry. The conventional design of sandwich structures comes in many forms and shapes. Honeycomb made of aluminum has been widely used owing to its excellent energy absorption and specific strength. Honeycomb cores are, however, susceptible to accumulated moisture due to the trapped water in the cells. If not addressed in time, the accumulated moisture could deteriorate its mechanical performance over time, leading to structural failure. Thus, effective approaches to diversify the design parameters and address the problem of moisture accumulation are highly desirable. Recently, foldcores, made by folding 2D sheets into 3D structures, have gained research attention as a promising alternative, particularly in the aviation industry.

Unlike honeycomb structures, foldcores comprise open channels that are critical in preventing moisture accumulation. Additionally, their design parameters can be easily diversified to meet the desirable engineering requirements. As per the available literature, the design of foldcore structures is normally based on origami or kirigami patterns. Although sometimes weaker in stress and compression than honeycomb cores, origami and kirigami-based foldcores generally exhibit better performance under bending loads. Moreover, they can be geometrically modified to attain higher energy absorption through indentation and curved creases. The main difference between origami and kirigami foldcores is that the former can be folded from single and continuous sheets while the latter requires cutting or stamping processes before the sheet materials can be folded. The property analysis of various types of foldcores such as cube, miura-ori and eggbox foldcores have been extensively conducted. And the available results provide more insights for improving the performance and applications of origami-kirigami inspires sandwich structures through further research.

Inspired by the previous results, Professor Jiayao Ma, Mr. Huaping Dai, Mr. Sibo Chai and Professor Yan Chen from Tianjin University proposed a kirigami-inspired pyramid foldcore based on a twist unit. The energy absorption and the quasi-static comprehension performance of the resulting sandwich structure under both comprehension and shear were investigated analytically, numerically and experimentally. The deformation mechanism for all the loading scenarios and the effects of geometric parameters were respectively examined via numerical models and parametric analysis. Finally, the researchers established a theoretical model to predict the average shear stress of the resulting foldcores. Their work is currently published in the research journal, Materials & Design.

Results showed that the energy absorption was mainly through the axial crushing of the foldcore and the folding of the stationary plastic contributed to about 8% of the total energy. A decrease in the sector angle increased the average comprehensive stress because the smaller the angle, the larger the angle formed between the horizontal plane and pyramid walls. The number of layers had negligible effects on the energy absorption. Compared with the Miura-ori, honeycomb and eggbox sandwiches, the new sandwich exhibited superior performance with a respective increase in the average compressive stress by 342%, 73% and 130%. Consequently, the average shear stress of the new sandwich was comparable to the honeycomb core with a difference of 11%. However, it was greater than the shear stress of Miura-ori foldcores by 34%.

In summary, the design of a new kirigami-inspired pyramid foldcore and subsequent numerical and experimental analysis of the comprehension and shear properties of the resulting sandwich structures was reported. Under comprehension and shear loading, the newly designed sandwich structure exhibited superior performance than the conventional and well-known sandwiches. In a statement to Advances in Engineering, the authors stated the new foldcore is a promising alternative for designing high-performance sandwich structures for various engineering applications.

Professor Ma and the Motion Structure Laboratory in Tianjin University have been devoted to innovative design of origami/kirigami metamaterials and theoretical characterization of the mechanical properties by combining advanced kinematics and structural mechanics, and produced inter-disciplinary research outcomes with global impacts.