What are the “dispersive shear bands” on the surfaces of layered heterostructured materials?

Hetero-deformation-induced (HDI) strengthening mechanism has been well investigated in heterostructured materials including layered/gradient materials prepared by surface attrition or other processing techniques. While the roles played by the geometrically necessary dislocations (GNDs) and the forward/back stresses on the grain scale have been a focus in nearly all these studies, some latest works reveal the peculiar formation of “dispersive shear bands” or strain localizations on the surfaces of layered/gradient materials. Feature sizes of these “shear bands” are commensurate with the macroscopic sample size, but not on the microstructural length scales, thus excluding the HDI strengthening as the primary mechanism. In this work, using a sandwich structure as an illustrative example, we demonstrate that the origin of these shear bands be localized necking with intermediate wavelengths and inclined orientations, which are critically dictated by the hardening behavior of the constituent layers, the geometric parameters, and the initial morphological perturbations. The layered structure under tension may not neck with an infinite wavelength (i.e., the Considère mode), but neck at intermediate wavelengths which actually correspond to a much larger uniform ductility than the commonly observed Considère necking mode. The arrangements of these shear bands can be further classified as I, X, and W types. Findings in this work not only resolve the origin of recent unusual experimental observations, but also suggest an alternative way of understanding and improving the ductility in heterostructured materials.