Mechanisms and Implications of Kink-Banding in Layered Materials

Abstract

An abstruse mode of deformation, widely referred to as kink-banding, has been observed over a wide range of length-scales, loading conditions, and in a wide variety of materials across the physical and engineering sciences. Kink-banding leads to inelastic deformation in thin bands wherein the material is microstructurally misoriented. A key ingredient for kink-banding is plastic anisotropy which is readily present in materials with layered structures. Despite the ubiquitous observations of kink-banding, it is only well studied in fiber reinforced composites where kink-banding is an unstable failure mode. However, in many other classes of materials kink-banding has been hypothesized to enhance their mechanical performance. Following this, the overarching goal of this dissertation is to understand the onset and propagation, and the effects of kink-banding on the mechanical performance of a few selected classes of layered materials. In the first part of this dissertation, the influence of structural and material parameters on kink-banding in bimetallic layered composites is analyzed through extensive finite element calculations. The results show that depending on the properties of the constituent phases, kink-banding can either occur abruptly or progressively by nucleation and growth of stable inclined wedge-shaped kink-bands similar to recent experimental observations. In the second part, the focus is confined to kink-banding in crystalline materials with layered crystal structures such as Micas and MAX phases using novel in-situ SEM experiments. The in-situ experiments are carried out using an in-house designed and build fixture that enables deformation of single-crystal specimens with a rigid punch along predetermined directions, with and without lateral deformation constraint. Although mechanical behavior of Micas has been well studied, the effect of isomorphic substitution on kink-banding in this class of material is not well understood. Herein, it is shown that isomorphic substitution in Micas that leads to a difference in T-O-T structure charge and, hence, inter and intra layer properties, significantly affects the onset and morphology of kink-bands as well as the extent of deformation or layer rotation within the kink-bands. Next, through these in-situ SEM experiments, an intriguing possibility of intrinsic and autonomous crack-healing (even at room temperature) in MAX phases is demonstrated. MAX phase crystals readily fracture along weakly bonded crystallographic planes, but the onset and propagation of kink-banding in these materials induces large crystallographic rotations and plastic deformation that physically heal the cracks. Furthermore, the qualitative effect of local stress state on the ease of formation and morphology of kink-bands in MAX phase single crystals is analyzed by in-situ SEM deformation using cylindrical and flat punch as well as finite element analysis. The results show that more inclined and thinner kink-bands readily formed under cylindrical punch compared to flat punch.