Metallic foams have been recently investigated as a potential material for medical implants. Novel processing methodologies to create porous microstructures of nickel-titanium (NiTi) have been developed at Northwestern University by the Dunand group. Due to their pseudoelastic properties, NiTi foams allow local residual strains to be reduced in the microstructure while preserving low modulus and light-weight features. We have used the finite element code ABAQUS and a user-developed SMA constitutive subroutine in order to investigate the mechanical behaviour of porous SMA under cyclic loading conditions. The average material response has been compared to that of more traditional Ti foams. As an example, Figure 1 shows a 2D porous microstructure with a contour of the residual martensite volume fraction after five loading cycles. Figure 2a illustrates the average macroscopic stress-strain response of the porous SMA for the first and fifth loading cycle. The material exhibits an accumulation of residual strain with cycling which tends to an asymptotic value. In Figure 2b the stress-strain response of a porous SMA is compared to that of a Ti foam with the same microstructure for the same value of applied maximum strain. We can notice a much smaller amount of average residual strain for SMA.
To reveal the interactions of pores and to illustrate the mechanism in the SMA foams, a series of SMA 2D plates with structured arrays of holes are modeled and simulated (Fig 3). A newly developed elastic-transformation-plastic constitutive model is utilized in this study. The conclusions drawn from this work help provide a deeper insight into the underlying transformation mechanisms of porous SMA structures, and provide a tool to aid in the design and optimization of pore architectures in SMAs. It is observed that clustered pores can lead to more broadly distributed stress states, while dispersed pores can lead to locally higher plasticity effects.
P. Zhu, A.P. Stebner, L.C. Brinson, Plastic and Transformation Interactions of Pores in Shape Memory Alloy Plates, Smart Materials and Structures, 2014; 23(10).
P. Zhu, A.P. Stebner, L.C. Brinson, A Numerical Study of the Coupling of Elastic and Transformation Fields in Pore Arrays in Shape Memory Alloy Plates to Advance Porous Structure Design and Optimization, Smart Materials and Structures, 2013; 22(9).