Skin

A composite construction in which two thin, stiff skins are bonded to a thick, lightweight core such as foam, honeycomb or balsa. Separating the skins raises bending stiffness dramatically for very little added weight.

The lightweight central layer of a sandwich construction, bonded between two thin, stiff face sheets to raise a panel's bending stiffness with minimal added weight. Common structural cores include polymer foams, honeycombs, end-grain balsa and syntactic materials, each specified by density, thickness and shear properties.

A separation at a bonded interface, such as within an adhesive joint or between the skin and core of a sandwich panel. Disbonds may originate in manufacture or grow in service, and are detected by tap testing or ultrasonic inspection.

The main unidirectional load-bearing element running along the length of a wind turbine blade or wing spar, carrying the principal bending loads. Spar caps are typically thick stacks of unidirectional glass or carbon laminate, or pre-cured pultruded planks bonded into the structure.