a method of continuously forming a composite structure comprised of a three - dimensional fabric including first and second sheets of fabric which face each other and which are connected by intermediate yarn , the method including providing a continuous supply of three - dimensional fabric ; impregnating the three - dimensional fabric with resin which is thermoreactive ; mating the three - dimensional fabric with a first outer strip which contacts the first sheet of fabric , and a second outer strip which contacts the second sheet of fabric to form a stratified structure containing the impregnated three - dimensional fabric ; feeding the stratified structure along a path in a feed direction x which is straight ; heating the stratified structure to at least partially polymerize the resin between the first outer strip and the first sheet of fabric and between the second outer strip and the second sheet of fabric , while the resin impregnating the intermediate yarn of the three - dimensional fabric remains substantially in the plastic state , so that the first and second outer strips adhere respectively to the first and second sheets of fabric ; parting the first and second outer strips as the stratified structure is fed in feed direction x to at least partially distend the intermediate yarn of the three - dimensional fabric ; and fully solidifying the resin impregnating the three - dimensional fabric .