The invention is a process for forming a preform for use in a structure having at least one curved portion of a specific length, including the steps of: 1) providing a preform having cable of expanding in length threads in rows parallel to the direction of curvature over a length equal to the length requiring curvature, such that the gaps between each thread row are spaced from the gaps in the adjacent tread rows; and 2) stretching the portions of the preform requiring curvature in a sine-wave pattern.