A Comprehensive Review of Sandwich Structures with an Emphasis on the Underexplored Potential of Long Fiber Thermoplastic (LFT) Skins

Sandwich structures are extensively used in lightweight structural applications due to their efficient load carrying capability and low mass. While metallic and thermoset composite skins have been widely investigated, long fiber thermoplastics (LFTs) as sandwich face sheets still lack a comprehensive and structure oriented critical review. This paper provides a targeted critical review of sandwich structures with a specific focus on LFT skins, addressing their mechanical behavior under quasi-static and impact loading conditions. Dominant damage and failure mechanisms including core crushing, face-core debonding, delamination, and face-sheet fracture are examined, and the role of key LFT design parameters, such as fiber length, fiber orientation, matrix system, manufacturing route, and joining method, is critically assessed in terms of stiffness, energy absorption, and residual structural performance. The review indicates that LFT skins offer a balanced combination of mechanical performance, damage tolerance, manufacturability, and sustainability, supporting their potential as face-sheet materials for selected sandwich applications. Remaining challenges and research gaps, particularly related to temperature-dependent behavior, interfacial durability, and cross-study comparison metrics, are identified, and future research directions are proposed.