Periodic Void Formation in Chevron Folds

An energy-based model is developed to describe the periodic formation of voids/saddle reefs in hinge zones of chevron folds. Such patterns have been observed in a series of experiments on layers of paper, as well as in the field. A simplified hinge region in a stack of elastic layers, with straight limbs connected by convex segments, is constructed so that a void forms every m layers and repeats periodically. Energy contributions include strain energy of bending and work done both against a confining overburden pressure and an axial compressive load. The resulting total potential energy functional for the system is minimised subject to the constraint of non-interpenetration of layers, leading to representation as a nonlinear second-order free boundary problem. Numerical solutions demonstrate that there can exist a minimum-energy m-periodic solution with m≠1. The model shows good agreement when compared with experiments on layers of paper.