Interfacial mechanics in bi-materials and structures

Abstract

Bi-materials and structures are broadly adopted in nature and engineering due to their ability to take advantage of the merits of individual constituents and to minimize their weaknesses. However, there are some interfacial problems impairing their mechanical performance. For example, the interface between the two materials is often much weaker than the bulk constituents, making interfacial crack and subsequent interfacial delamination easy to happen. This problem would become even worse when stress concentration is present on the interface due to the strain misfit between the two distinct materials. This thesis would focus on solving these interface-related problems for an enhanced or controllable mechanical performance in bi-materials and structures. In Chapter 4, a micro-screw dislocation (μ-SD) found in biological laminated composites was systematically studied. Mechanical tests indicate that μ-SDs can greatly enhance the resistance to scratching. Finite element analysis was performed to shed light on the underlying reinforcing mechanism. Results show that the failure of a μ-SD under tension involves the delamination of the prolonged spiral interface, thus giving rise to more energy consumption and higher toughness compared with the planar counterparts. The operation of such reinforcing mechanism requires proclivity of cracking along the spiral interface. Fracture mechanics-based modelling indicates that if the fracture toughness of interface is less than 60% of that of the lamina material, crack would always propagate along the interface. These findings not only reveal the reinforcing mechanism of spiral interface but also imply a great promise of applying μ-SDs in laminated composites for higher fracture toughness. Interfaces in bi-materials such as film-substrate systems are often subjected to shear stress due to distinct deformation responses of two bonded materials to the external stimuli. The distribution of such shear stress over the interface normally exhibits high concentration. To enhance the interface's resistance to delamination, a gradient thickness strategy was proposed to homogenize interfacial shear stress caused by strain misfit in film-substrate bi-material systems in Chapter 5. The solutions to the gradient thickness in the films were obtained based on two typical bi-material systems: continuous film on disk-like substrate and island film on half-space substrate. The effectiveness of these theoretical solutions were well demonstrated via finite element simulation and experimental test. This strategy is believed to be of great value to the enhancement of resistance to interface delamination in diverse film-substrate systems. The strain misfit between the two bonded materials can be exploited instead to achieve controllable morphing behaviors for a broad range of applications. In Chapter 6, the deformation of stacked assembly of graphene (SAG)/polyethylene (PE) bilayer under thermal loadings was systematically studied via a combination of theoretical modelling and finite element simulation. As SAG layer has asymmetric elastoplastic properties, i.e., high plasticity under tension and high elasticity under compression, the strain misfit between the two bonded materials can originate from either thermal mismatch or plastic strain. Through theoretical analysis, the morphing behaviors of SAG/PE bilayer under different thermal loadings were well predicted. These results would provide valuable guidelines when applying such bi-materials in the field of sensors, actuators and soft robotics, etc. Through our investigation, several strategies and guidelines were proposed to resolve the interface-related problems in bi-materials and structures. These results would be of significant value for the design of bi-materials and structures with enhanced or controllable mechanical behaviors.