Mandrel with differential in thermal expansion to eliminate wrinkles in ply

A method of laying up an elongated contoured aircraft stringer comprising:

• manufacturing a first mandrel element from a first mandrel material having a first mandrel coefficient of thermal expansion, said first mandrel element having a first mandrel vertical upper surface, a first mandrel vertical lower surface, two first mandrel side surfaces, a first mandrel right end surface and a first mandrel left end surface, said first mandrel right end surface and said first mandrel left end surface having a smaller surface area than said side surfaces, said vertical upper surface and said vertical lower surface of said first mandrel, said mandrel vertical upper surface and said vertical lower surface each including an elongated curved contoured bend line,
• laying up a composite ply assembly onto said first mandrel element to generate a primary lay-up surface, said composite ply assembly being longer than said first mandrel element, said composite ply assembly comprising a first primary fold-over portion and a second primary fold over portion extending from said primary lay-up surface;
• folding over said first primary fold-over portion onto said first mandrel right end surface to partially engage the first mandrel right end surface;
• folding over said second primary fold over portion onto said first mandrel left end surface to partially engage the first mandrel left end surface to help maintain tension, said composite ply assembly being dimensioned so as to conform to the shape of the first mandrel element following heating of said first mandrel element;
• heating said first mandrel element through the use of a heating element positioned within said first mandrel element so that said first mandrel moves between a first mandrel normal condition to a first mandrel heated condition, said first mandrel coefficient of thermal expansion moving said first mandrel right end surface away from said first mandrel left end surface as said mandrel enters said first mandrel heated condition, said first primary fold-over portion moving away from said secondary primary fold-over portion in response to relative movement of said first mandrel right end surface and said first mandrel left end surface, whereby the difference in thermal expansion between the first mandrel and said ply assembly increases the tension in the composite ply assembly in the heating step and any wrinkles are pulled out of said primary lay-up surface;
• and
• curing said composite ply assembly to form a contoured composite stringer element corresponding along essentially its entire length to the elongated curved contoured bend line and therewith to the shape of said mandrel.