Crack Initiation and Growth in a Notched NiTi Shape Memory Alloy Sheet
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Crack Initiation and Growth in a Notched NiTi Shape Memory Alloy Sheet Wei Tong, Hong Tao, and Nian Zhang Department of Mechanical Engineering, Yale University, New Haven, CT ABSTRACT An experimental investigation was carried out to study the crack initiation and growth in a single-edge notched NiTi shape memory alloy sheet under tension. It is observed that a crack initiated at the tip of a V-shape notch before the peak axial load was reached and it grew steadily across the width of the NiTi sheet until final fracture. In-plane crack-tip deformation fields at various stages of the crack growth were measured based on an image correlation technique and the crack-tip opening displacement (CTOD) and crack-tip opening angle (CTOA) were subsequently determined. The fracture surface of the NiTi sheet was dimpled based on scanning electron microscopy examinations. INTRODUCTION Shape memory materials such as NiTi alloys have found increasing applications in medical devices and MEMS components [1]. Most experimental and modeling efforts on the NiTi shape memory alloys have focused primarily on the thermal-mechanical deformation characteristics and their micromechanisms [2-9]. Fatigue and fracture failure is one of important design considerations in many NiTi shape memory alloy products [1,10,11] but extensive experimental investigations are lacking [11]. As NiTi shape memory alloys used in MEMS and other applications often in the form of wires, thin strips and films, the traditional fracture and fatigue testing methodologies developed for bulk specimens to obtain plane-strain fracture toughness data in conventional structural design analyses may not be feasible. Direct experimental characterization of the fracture and fatigue properties of NiTi shape memory alloy strips and films is thus desirable. We reported in this paper a research effort on measuring the crack-tip plastic deformation field of a single-edge notched NiTi shape memory alloy sheet by in-situ monitoring the crack initiation and growth at the notch. MATERIAL AND EXPERIMENTAL PROCEDURE The material studied in this investigation is a NiTi shape memory alloy (Nitinol SM495) obtained from Nitinol Devices and Components (Fremont, CA). The nominal composition of the NiTi sheet is 54.8wt% Ni and 45.2wt% Ti with only a trace amount of O, H, and C. The transformation temperature (Af) is 60°C and typical mechanical properties of Nitinol SM495 wires at ambient conditions are listed in Table 1. The thin strip form (with a dimension of 50mm long, 7mm wide, and 0.25mm thick) of the Nitinol SM495 in the as-received condition was tested. A rectangular strip of the Nitinol SM495 sheet was clamped down at both ends and stretched quasi-statically under displacement control by a compact desktop tensile tester (100mm-by-125mm-by-50mm in total dimensions, a total crosshead travel of 50mm, and a load cell of 4,400N maximum capacity). A total of four cycles of combined mechanical loading (at a strain rate of 5x10-5 1/s for a total strain of 4-4.5%) and thermal annealing (at t
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