Opening and closing of folds in superposed deformations

During superposed deformation, an early fold (F1) may either tighten or open out. Opening out is possible if, during the second deformation, there is a bulk extension across the F1 axial plane. The theoretical model indicates that the rate of opening out is largely controlled by the initial tightness of the fold. The results of the numerical model are in agreement with analog experiments of superposed buckling and show that if there is a relatively small range of variation of initial tightness of F1 folds, the range of fold tightness becomes much larger after the opening out of the folds in the second deformation. Consequently, the refolded structure may show a close association of different modes of superposed buckling. In a type-2 fold interference, the continued opening out of F1 invariably causes a replacement of the initial F1 hinge line by a new and more sinuous hinge line. When the shortening direction (P2) of the second deformation was at an acute angle to the F1-axis, depending upon their local orientation with respect to P2, the F1 folds were tightened in some places and were opened out elsewhere. When the enveloping surface is inclined in the direction of maximum stretching of second deformation, segments of F1 and F2 hinge lines may both rotate to lie in the extension field and both F1 and F2 may open out with progressive deformation. If F1 has a low pitch, and P2 is parallel to the F1-axis, the two sets of hinge lines may become subparallel at an advanced stage of deformation. The resulting structure is morphologically similar to non-planar sheath folds.