An aircraft stringer lay-up assembly is provided comprising a contoured curing block and a first mandrel element positioned thereon. The first mandrel assembly includes a first bar assembly having a plurality of rigidity reducing first slots formed along a first mandrel length. The plurality of rigidity reducing first slots protruding partially through a first mandrel depth of the first mandrel element to allow the first bar assembly to conform to the contoured curing block. A composite ply assembly is laid up onto the first mandrel element and cured while conformed to said contoured curing block such that a contoured composite stringer element is generated.