Multilayer and composition gradient structures with improved damping properties

An uncured structural composite material comprising

• a plurality of layers of structural fibers;
• a thermoset resin component;
• and
• an interlayer positioned between adjacent layers of structural fibers, the interlayer comprising at least two different types of nonwoven fibrous material, each nonwoven fibrous material comprising polymeric fibers,
• wherein
• the layers of structural fibers are impregnated with the thermoset resin component,
• said at least two different types of nonwoven fibrous material comprises a first nonwoven fibrous material located in a core portion of the interlayer and a second nonwoven fibrous material located in outer portions of the interlayer that are on either side of the core portion,
• the polymeric fibers of the first and second nonwoven fibrous materials are formed from a polymeric material comprising:
• (a) at least one thermoplastic elastomer, and (b) at least one thermoplastic polymer or copolymer thereof,
• the interlayer comprises a compositional gradient in which the first nonwoven fibrous material in the core portion differs in composition from the second nonwoven fibrous material in the outer portions, and
• the second nonwoven fibrous material is more permeable to the thermoset resin component than the first nonwoven fibrous material such that, upon curing, the interlayer comprises a resin interpenetration gradient in which the resin penetration in the outer portions of the interlayer is higher than that in the core portion.