Abstract: A flexible core element having a large bonding surface area suitable for sandwich type construction comprising a plurality of first hexagonal nodes defining a first surface, a plurality of second hexagonal nodes defining a second surface spaced apart from and parallel to the first surface, and a plurality of rectangular wall members which interconnect said first hexagonal nodes to said second hexagonal nodes and define the depth of the core element. Each hexagonal node corresponds to an open hexagonal cell on its obverse surface and serves as a bonding surface. The core element is fabricated from a continuous sheet of material that has been die-cut with a repeating geometrical design, creased and folded, concertina style, in upon itself to make a double-sided core material that is flexible, able to vent, exhibits good bend and shear strength, and has a large surface bonding area rendering it suitable in the construction of lightweight sandwich panels and offering a wide array of other applications.