An experimental and numerical investigation of the biaxial tensile behaviour of biomedical alloys, nitinol and SS304

The tensile test is one of the most commonly used techniques to establish the mechanical properties of a material. The test is accomplished by extending the specimen with known cross sectional area in the direction perpendicular to its cross-sectional area axis. The loading of the specimen is increased until fracture occurs. This simple and accurate method of defining material properties has been the subject of recent research investigations when used to characterise the tensile behaviour of sheet metal. Forming of sheet metal leads to largely anisotropic behaviour of the material under variable loading conditions. It has varying mechanical strength in different directions due to rolling (type of manufacturing process used). The resulting data obtained by the standard tensile test may not be applicable to multi directional forming processes used in biomedical alloys. Biaxial testing has become an increasingly used common technique to determine mechanical properties of sheet materials. It provides closer approximation of the behaviour of sheet metals (anisotropic materials) during deformation processes used in sheet metal forming. Mechanical loading is applied to a cruciform specimen in two directions simultaneously. One of the aims of this research was to design and build a biaxial planar testing system to study the properties of biomedical materials. A biomedical grade Stainless Steel and Nitinol, near equi-atomic alloy of Nickel and Titanium, were investigated and their tensile properties were established under biaxial loading conditions as well as the mechanisms of their fracture. The goals of this research are outlined in Chapter 1. These goals are followed by hypotheses used in this study. A comprehensive literature review of four areas: biomedical materials, sheet metal formability, biaxial planar testing and cruciform specimen design was conducted and is presented in Chapter 2. Chapter 3 accounts for the activities necessary to design and manufacture biaxial testing system and cruciform specimens. The biaxial planar testing system was designed, manufactured and assembled at the University of Limerick. Chapter 4 presents the integrations of all the controls and data acquisition components (hardware and software) required for successful testing. Most of the components required for the system were available from the previous research activities in the Department of Design and Manufacturing Technology. Additional items were purchased to satisfy the requirements of the controls and data acquisition system. The biaxial testing system underwent a series of steps to prove its validity. These steps are explained in Chapter 5. The Finite Element Analysis models of Stainless Steel SS304 and Nitinol were developed in ABAQUS software. Stainless Steel SS304 and Nitinol specimens were tested biaxially and also underwent microscopic analysis. Chapter 6 contains the results of biaxial testing of Stainless Steel SS304 and Nitinol, their FEA models and microscopic analysis of the materials. Chapter 7 summarises the experience gained from the project. Conclusions drawn from the analysis are presented in Chapter 8. Chapter 9 presents a reference list and is followed by Appendices.