Sandwich composites stand out especially in the aerospace industry owing to their high strength-to-weight ratio, one of the most prominent factors for material selection. Polymeric foams as core material in sandwich composites are likely to prevent delamination between face sheets and core by augmenting the contact surface area, resulting from their closed-cell structure. Polymeric foam properties can be enhanced by adding nanomaterials such as carbon nanotubes (CNTs), however increased CNT content or the type of CNTs might arise critical problems such as agglomeration and irregular distribution of nanomaterials. Cellulose nanocrystals (CNCs) are claimed to be good candidates to prevent nanomaterial reinforcing related issues and further, their inclusion enables reinforcing of polymeric foams using an optimum CNT/CNC concentration. In this work, CNT/CNC reinforced polyurethane (PU) foam-cored sandwich composites were manufactured and characterized for the influence of nanomaterial addition on the mechanical properties with an aim to find the optimum nanomaterial content. 0.1 wt.% CNT, CNC, CNT/CNC (1:1), and CNT/CNC (1:2) reinforced PU foam-cored sandwich composites were subjected to simultaneous three-point bending tests and acoustic emission tests, one of the promising non-destructive testing methods enabling in-situ monitoring of the damage mechanisms to understand how damage evolves. The effects of these nanomaterial additives on damage mechanisms and the mechanical properties were examined thoroughly via both mechanical and morphological characterizations. The test results were found to be promising in terms of revealing how these reinforcements affect the retardation and/or elimination of damage mechanisms including core damage, face sheet-core debonding, matrix cracking, and fiber breakage in the sandwich composite structures. The results suggested that with the addition of 0.1 wt.% CNT, the mechanical properties of PU foam were increased; therefore, the ratio of AE signals related to fiber breakage and core damage were decreased because of the strengthened core material.@en