With the increasing seriousness of electromagnetic pollution in civil applications and national defense, current radar absorbing structures (RASs) with narrow absorption performance and high density are inadequate to meet the demands for excellent electromagnetic absorption performance. Therefore, achieving broadband absorption capabilities in RASs across the frequency range of 2 to 40 GHz is a pressing issue and a topic of significant interest. This review article summarizes the multi-dimensional design of broadband RASs by integrating materials, structures, and manufacturing processes, promoting the application of novel materials in three-dimensional structures through advanced manufacturing processes in the future. Meanwhile, the multi-scale absorption mechanism, including the micro-scale absorption attenuation mechanism and macro-scale absorption resonance, has been discussed. Finally, the major challenge of current RASs and their relatively new frontier has been discussed, highlighting their potential for diverse applications across multiple fields.

Sandwich structures are vulnerable to multi-point impacts, and such impacts can result in a reduction in residual strength even catastrophic accident. Therefore, the multi-point impact behaviors of PMI foam sandwich structure are investigated and studied using experimental and numerical coupled methods. Three impact energy levels and five Distances Between Impact Positions (DBIP) are considered in details, and representative impact characteristics are compared to reveal the association between Compression After Impact (CAI) strength and DBIP. Results indicate that the interference between the multi-point impact events has a dominant effect on CAI strength when DBIP is small, and the variation in bending stiffness induced by the boundary effect is the dominant factor affecting CAI strength when DBIP ranges from 20 mm to 60 mm. In addition, matrix damage represents the primary damage mode in multi-point impact, and the calculated ratio of energy absorbed by the top face sheet and honeycomb core, in relation to the total absorbed energy, serves as a clear indicator of the damage severity experienced by both components. This work is enlightening for the structural design of impact-resistant composites.