Enhanced flexural properties of bionic carbon fiber sandwiches inspired by the coconut shell

Carbon fiber reinforced plastics (CFRP)/metal foam sandwiches with high strength-to-weight ratios are in growing demand for applications in architecture, automobile, and aerospace. However, most of the sandwiches have low mechanical properties due to the weak metal foam cores and the low interfacial bond strength between cores and skins. In this study, inspired by the natural coconut shell, novel sandwich structures containing magnesium syntactic foam (MSF) cores were designed to improve flexural properties. The CFRP skins were selected to mimic the high strength and multilayer structure of the exocarp and mesocarp, and two types of MSF cores (PMSF with low density and SMSF with high density) were used to mimic the hierarchical cellular structure of the endocarp. The cores with different structures will significantly affect the failure mechanisms and flexural properties of the corresponding sandwich structures. Compared with conventional CFRP/MF, the peak load and flexural stiffness of bionic CFRP/PMSF and CFRP/SMSF are significantly improved due to the hierarchical cellular structure of MSF cores. The enhanced skin-core debonding resistance allows bionic CFRP/PMSF to have better structural integrity under bending loads. Therefore, the CFRP/PMSF exhibits the highest specific flexural strength and specific energy absorption capacity, though the PMSF core has the lowest specific flexural properties. This result confirms that the reasonable matching between the PMSF core and the CFRP skin transfers the load more effectively and improves the strength-to-weight ratio of the sandwich structure. The proposed bio-inspired design method is expected to supply an effective way to enhance the mechanical properties of conventional CFRP/MF sandwich structures in a wide range of engineering applications.