Interfacial synergy and mixed-mode failure in hybrid closed-cell aluminum-polyurethane-Kevlar sandwich structures under flexural and compressive loading

Combining materials that individually excel in different mechanical roles has long been a strategy in structural engineering, yet the specific pairing of closed-cell metallic and polymeric foams within a single layered architecture remains poorly explored. In this work, a five-layer sandwich panel was developed by alternating closed-cell Al6061 foam (CCAF) as the outer and central layers with in-situ foamed polyurethane (PF) in the intermediate positions, taking advantage of the complementary stiffness and damping characteristics of the two foam types. A Kevlar fiber (KF)/epoxy composite face sheet was subsequently bonded to this hybrid core to form the APKC configuration. Mechanical performance was assessed by subjecting four structural variants - bare CCAF, bare PF, the hybrid CCAF-PF panel (APC), and APKC - to uniaxial compression in both in-plane and out-of-plane orientations, as well as three-point bending. A notably remarkable result was that the APC structure demonstrated a compressive plateau stress of around 1.76 MPa, nearly thrice the estimation provided by the rule-of-mixtures (∼0.60 MPa), which considers the proportions of the constituent materials. This improvement primarily stems from interfacial synergy linked to robust Al–PU bonding, localized cell refinement at the interface, and enhanced stress transmission between the metallic and polymeric foam layers, rather than solely from geometric or density influences. With the Kevlar face sheet incorporated, the APKC panel reached compressive and flexural strengths of 2.74 MPa and 7.41 MPa respectively, while its energy absorption capacity was approximately twice that of APC and more than five times greater than either constituent foam alone. Flexural fracture in all structures proceeded through a mixed failure mode, with the relative contributions of indentation, face yielding, and core shear varying systematically with panel configuration. A modified theoretical framework, incorporating mode-specific weightage factors, was developed and found to closely reproduce the measured flexural strengths. Taken together, these results make a quantitative case for multilayer Al-PU-Kevlar sandwich panels as practical, weight-efficient solutions for energy-absorbing structural components in automotive, aerospace, and protective engineering contexts.