title: Study on deformation and fracture behaviour of bi-layered aluminium/steel and bronze/steel materials
Creators: name: Jiang, Xingrui
Publisher: name: University of Liverpool Repository
Year: 2027

description: Layered materials are widely used in industry due to their excellent performance. By utilizing laminated structures, layered materials can leverage the advantageous properties of their constituent layers, such as wear resistance, corrosion resistance, thermal and electrical conductivity. However, the complex interaction mechanisms between material layers, particularly the microscopic deformation mechanisms under mechanical loads, are not yet fully understood, which poses significant challenges for assessing mechanical properties and engineering applications. Therefore, understanding the deformation and fracture behaviour of layered materials is crucial for optimizing their performance and expanding their applications. As the simplest and most widely used type of layered material, the study of the mechanical properties of bi-layered structures is of great significance. Existing research often relies on engineering experience, and the interaction mechanisms between layers, especially the impact of different coating organizations, remain unclear.
This thesis explores the mechanical behaviour of bi-layered materials, with a focus on how the layered structure affects deformation and fracture characteristics. Specifically, it investigates two types of bi-layered sheets: 1050 aluminium/low-carbon steel (referred to as aluminium/steel) and lead bronze/low-carbon steel (referred to as bronze/steel). Both materials consist of a thin layer with coarse grains and a relatively thick, non-textured low-carbon steel substrate. The aluminium/steel sheets are produced through cold rolling and annealing, with the aluminium layer having equiaxed grains without texture, while the bronze/steel sheets are fabricated using a solid-liquid continuous casting method, with the bronze layer exhibiting highly heterogeneous microstructures, including columnar copper grains and a continuous Pb phase. These differences result in distinct mechanical properties for the two sets of materials.
The thesis employs advanced characterization techniques, including in-situ methods and digital image correlation (DIC), to study the deformation behaviour of the materials at various deformation stages. It evaluates the impact of residual stresses on the mechanical properties of bi-layered materials and finds that residual stresses affect the yield strength of bronze/steel materials. The thesis focuses on inhomogeneous deformation caused by plastic instability and combines in-situ techniques with finite element modelling to analyse stress distribution and its impact on material behaviour. Finite element simulations combined with surface layer model indicate that the layered structure mitigates stress concentration at the Lüders front, thereby suppressing Lüders band formation. In-situ tensile tests using scanning electron microscopy show that the bi-layered structure restricts the development of single shear strain bands within the bronze layer and promotes the formation of multiple strain bands, which enhances material ductility. The study also explores how the layered structure influences fracture mechanisms that delay early failure, finding that the bi-layered structure effectively suppresses crack propagation. Microcracks and multiple cracks observed in copper layers with different microstructures (HP and SP) are suppressed by the layered structure, enhancing material work hardening and resulting in higher fracture elongation.
Overall, this thesis provides an in-depth understanding of the relationships between microstructures, thickness ratio, residual stress, and the mechanical properties and deformation behaviour of bi-layered materials. It offers theoretical support for optimizing layered structure parameters in engineering to improve material performance and applications.