Fatigue life of Drawn Filled NiTi-Ag Wires for Medical Applications

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1 Fatigue life of Drawn Filled NiTi-Ag Wires for Medical Applications, ŠŤEPÁN MAJOR, 3 ŠŤEPÁN HUBÁLOVSKÝ, 3 JOSEF ŠEDIVÝ, JOSEF HANUŠ The Institute of Theoretical and Applied Mechanics AS CR Prosecká 809/76, Praha 9 Institute of Medical Biophysics Charles University of Prague Šimkova 870, Hradec Králové 3 Department of Informatics University of Hradec Králové Rokitanskeho 6, Hradec Králové CZECH REPUBLIC s.major@seznam.cz 3 stepan.hubalovsky@uhk.cz, 3 josef.sedivy@uhk.cz Abstract: - Nickel titanium alloys (Nitinol) which possess the unique thermal shape memory and superelasticity effects are attracting more attention in medical devices. Currently it is mainly about the construction of stents or other types of implants. Researchers are looking for new ways to improve the mechanical and physical properties of materials for the manufacture of stents. New types of wires are tested. This new wires are prepared by drawn filled tubing technology. Drawn filled wires enable the unique ability to match dissimilar materials to provide a variety of properties in a single wire system. This technology can be utilized by the engineer to resolve technical issues. This article is devoted to fractographical analysis of fatigue fracture process in drawn filled wires made of nitinol with silver core and drawn filled wires made of silver with nitinol core. Key-Words: NiTiNol, Drawn Filled Wires, Fatigue Crack Growth, Fatigue Crack Initiation. Introduction Nickel titanium, also known as nitinol, is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages. The term nitinol is derived from its composition and its place of discovery: (Nickel Titanium Naval Ordnance Laboratory). William J. Buehler and F. Wang [,], discovered its properties during research at the Naval Ordnance Laboratory in 96. Nitinol alloys exhibit two closely related and unique properties: shape memory and superelasticity (also called pseudoelasticity). Shape memory alloys have vast use in all areas of engineering, especially in the mechatronics or medical engineering [3-5]. As a biomedical material, Nitinol has to meet several requirements: high corrosion resistance in chloriderich medium and biocompatibility combined with suitable mechanical properties [6]. Nitinol is used for orthodontic treatments, and in cardiovascular surgery for stents, guide wires, filters, etc., in orthopaedic surgery for various staples and rods, and in maxillofacial and reconstructive surgery [6]. Probably the most common medical use of nitinol is construction of stent-grafts. Another example of the use of these materials in medicine are dental applications or use in prosthesis [7, 8]. The role of material science, particularly metallurgy, in biomedical implants has grown considerably in recent years. Perhaps the most common medical applications of shape memory materials are stents, whether coronary stents or stents used in the esophagus or gastrointestinal tract (other applications made from NiTi are guidewires and embolic filters). The main reason for the use of materials with shape memory effect in the design of stents is the ability to self-expand. Selfexpandable metallic stents are typically inserted at the time of endoscopy, usually with assistance with fluoroscopy or x-ray images taken to guide placement [9]. Perhaps the most important properties of stents is their fatigue life, fatigue life of stent must be more than ten years. Most stent failures can cause in-stent restenosis, internal bleeding and often death of the patient. Composite are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or ISBN:

2 chemical properties which remain separate and distinct within the finished structure. Currently, some researchers are testing the possibility of using such materials for the manufacture of stents and other implants. into Martens tic phase (Assuming that the temperature is higher than active austenitic finish temperature A F. A F is the temperature at which the unloaded material has completely transformed to Austenite. Alloys have completed its shape memory transformation above this temperature). This produces an almost constant stress level over a relatively large range of strain, up to about 0%. The unloading plateau stress is the stress level at which the Martens tic phase will return to the Martensite. The loading plateau is defined as K [7,8] : CLP ( R ) σ R = () SLP D UTC D σ σ + σ CLP is composite loading plateau strength, σ SLP is sheath loading plateau strength, σ UTC is ultimate tensile strength of core. The definition of unloading plateau is in principle the same K [7.8]where, σ CUP ( R ) σ R = σ, () SUP D UCC D Fig. Cross section of composite wires Drawn Filled Wires Composites now have great importance in all fields of technology. Drawn filled wires are composites developed to combine the required physical and mechanical properties of two alloys into a single system [0,]. As a result of extreme compressive forces imparted during the processing of the dissimilar materials, the mechanical bond formed between surfaces has been found to be metallurgically sound. This feature has given rise to a number of novel applications of drawn filled wire. The composite typically uses the outer sheath to impart strength, while the core material is designed to provide elasticity and for biomedical applications very important properties, such as radiopacity and enhancement to MRI.. Physical characteristics Properties of NiTi-Ag composite wire are described by several physical parameters. The important characteristic of composite is R D ratio defined as R D = D C / D W, where D C is core diameter and D W is the wires diameter. Two critical characteristics specific to combination of an alloy with shape memory effect with another alloy are loading and unloading plateau. Both critical characteristics are associated with austenitic phase of alloy. The loading plateau stress is the stress level at which is material forced from Austenitic phase where σ CUP is composite unloading plateau strength and σ SUP is sheath upper plateau strength. The σ UCC is ultimate compressive strength of core. The ultimate tensile strength of the composite K [7.8]: UC ( R ) σ R σ = σ + (3) SU D UCC D where, σ CU is composite tensile ultimate strength and σ SU is sheath tensile ultimate strength. 3. Experimental procedure In this study were used four different wires: solid Nitinol wire, drawn filled wire NiTi-DFT-5%Ag (the silver core represents 5% of wires crosssection), drawn filled wire NiTi-DFT-40%Ag (30% of wires cross-section), and pure Ag wire, see Fig.. The diameter of tested wire was 0,035, then 0,889 mm. The Nitinol specimens used in this study was made of alloy composed 50.6% of Nickel and Titanium 48.9%. The alloys were further identified the following elements: Oxygen 0.06%, Carbon 0.09%, Hydrogen 0.008%. The core was made from silver with puritty 99.95% commercially and its ultimate tensile strength was 70 MPa, Young modulus. Physical properties of analyzed Nitinol alloy (every physical property must be determined separately for austenitic and martensitic phase): density 6.45 g/cm 3 (austenitic phase), 6.44 g/cm 3 ISBN:

3 (martensitic phase); modulus of elasticity: 75GPa (austenitic phase), 4GPa (martensitic phase); coefficient of thermal expansion: x0-6 C - (austenitic phase), 6.65x0-6 C - (martensitic phase). The platinium core was processing according to the thermal and reduction schedule used to produce the Nitinol. Specimens used in this study were heat straightened at a temperature of 578 C in atmosphere composed of technicaly pure Argon (99.98%) at pressure about 0.MPa and the process was halted for dwell time of 4.5 minutes. The specimens were subjected to pull-off loading and another experiments done in this study are rotary-beam tests. All specimens were loaded to final fracture. All fatigue fracture surfaces were examined with scanning electron microscopy (SEM). Mechanical properties of used wires are expressed in Table. performed at temperature of C. Usual separation of fatigue tests on low and high-cycle region (with a boundary around 0 5 ) is not suitable for the shape memory alloys []. The rotary-beam fatigue tests were realised at three different stress levels. At high strain levels ten samples are typically chosen, due to the expected shortness of the fatigue life; in contrast, six samples are tested at low strain levels. Low strain experiments represents the time to failure higher then 0 7 cycles (N f >0 7 ). High strain levels samples have the time to failure about 0 3 cycles (N f ~0 3 ). Ten samples were loaded to the fracture in region from 0 5 to 0 7 cycles. For the pull-pull test were used only six samples for every strain level. Table. Ultimeta strength of wires Wire Nitinol NiTi-Ag5% NiTi-Ag40% σ U Fig.3 Fractographical observation: a) represent fatigue surface of pure NiTi wires; b) incluson; c) region grain boundary as the suspect stress raiser; d) the fatigue crack initiated on defects closed near the interface between core material and outer material. Fig.. Fatigue life diagram The cyclic frequency of 60 Hz was used for rotarybeam testing in a system equipped with wire fracture detection. The rotary-beam tests were carried out on the Rotary Beam U-Bend Wire Spin Fatigue Tester. The wire, with a known length, is mounted into the drive chuck system while the other end is inserted into the free bushing. To prevent vibration, two support guides are positioned on the radius of the specimen, but outside of the apex, such that the guides do not affect the region of maximum strain. Pull-pull test were caried on tensile fatigue machine. Working frequency of tensile fatigue machine was 60 Hz. All experiments were 4. Results of Fatigue Experiments Nitinol fatigue test data are presented in the form of an ε ~ N f strain-life diagram as shown in Fig.. This figure shows that pure nitinol have higher strain ε in the (N f ~0 3 ) and (N f >0 7 ) region, in the region (0 5 < N f < 0 7 ) strain portion is higher for composite wires. The fracture surfaces may be analyzed using SEM. Fig. 3 (a) represent fatigue surface of pure NiTi wires and Fig. 3(b) shows inclusion. The nonmetallic inclusion is located at the wire surface of the initiation site.the fatigue fracture area comprised about 50 % of the wire cross section. The inclusion had an angular shape and was of approximately 6 µm across. High cycle ISBN:

4 fatigue failure in wires widouth core is initiated by either surface defects or internal non-metallic inclusions closed to the surface. In the (N f >0 7 ) region grain boundary as the suspect stress raiser and initiation place was found, see Fig.3(c). These facts are consistent with the observation In the case of wires with core made of different metal is proces more complicated and fatigue crack often initiated on defects closed near the interface between core material and outer material, see Fig. 3 (d). Metallographic analysis has provided evidence of a bond that is theoreticaly able eliminate interface slippage. This bond is a result of the significant amount of co-processing that is involved in the production of wires. This includes the extreme compressive forces associated with typical wire drawing, and the thermal processes needed to impart ductility to the highly cold-worked materials prior to further deformation. In the case of (N f ~0 3 ) region partial tensile rupture of high strength core is typical. On the other hand, in and (N f >0 7 ) region, partial tensile rupture of outer is occur frequently. Only short cracks were observed in nitinol and in platinium core of NiTi-DFT-5%Ag wires. On other hand, both the short and long crack growth regime was found in platinium core of NiTi-DFT-40%Ag wires. The stress intensity factors K were calculated for pull-pull loaded specimens. Fatigue crack growth data for ductile materials are usually presented in terms of the crack growth rate, da/dn, and the stress intensity factor range, K [3,4] Fig.4. Diagram versus (da/dn) K da dn m = C K (4) The Fig.4 shows the speed (da/dn) versus K in the case of NiTi-DFT-40%Ag, pure platinium wire with the same diameter as the core. The speed (da/dn) is in the case of pure platinium wire higher then in the case of DFT wire. 4 Conclusion A fractography study revealed that different fatigue crack initiation mechanisms are active. Based on these different fatigue crack initiation mechanisms, the fatigue data can be divided into three groups. All three groups are significantly different in fatigue process, meaning that the different fatigue crack initiation mechanisms lead to different fatigue lives. Although the ultimate strength of the wire NiTi- DFT-40%Ag appears to be higher then NiTi-DFT- 5%Ag and NitiNol wires, its fatigue performance is lower. Fig.6 MOdelof frce 3D- SEM photo of stengraft Acknowledgement The author acknowledged the financial support provided by the Czech Science Foundation in the frame of the Project No. SVV References: [] W.J. Buehler, J.W. Gilfrich, R.C. Wiley, Effects of low-temperature phase changes on the mechanical properties of alloys near composition TiNi, Journal of Applied Physics, No.34, 963, pp. 475 ISBN:

5 [] F.E. Wang, W.J. Buehler & S.J. Pickart, "Crystal structure and a unique martensitic transition of TiNi, Journal of Applied Physics, No. 36 (965) pp [3] J. Van Humbeeck: Non-medicals applications of shape memory alloys, Materials Science and Engineering A, Volumes 73-75, 5 December 999, pp [4] J. Van Humbeeck, M. Chandrasekaran, L. Delaey: Shape memory alloys, Materials in action, Volume 5, Issue 4, pp [5] J.D. Chute, D.E. Hodgson. Shape Memory Alloys, In: T.W. Duerig, K.N. Melton, D. Stöckel and C.M. Wayman Editors, Engineering Aspects of Shape Memory Alloys Butterworth-Heineman, Guildford, UK, 990, pp. 40. [6] S.A. Shabalovskaya, Surface, Corrosion and Biocompatibility Aspects of Nitinol as an Iimplant Material Bio-Med. Mater. Eng., No., 00, pp [7] T.W. Duerig, K.N. Melton, D. Stöckel, and C.W. Wayman, Engineering Aspects of Shape Memory Alloys, Butterworth-Heinemann Ltd., London, 990, pp [8] D.J. Wever, A.G. Veldhuizen, J. de Vries, H.J. Busscher, D.R.A., J.R. van Horn, Biomaterials, No.9,998, pp [9] G. Kauffmann, T. Roeren, P. Friedl, H. Bramb, G. Richter. Interventional radiological treatment of malignant biliary obstruction. Eur J Surg Oncol, 990, No.6 (4), pp [0] J. E. Schaffer, General characteristics of DFT composite wire, Fort Wayne, Indiana, USA, 00. [] J.E. Schaffer, R.Gordon, Engineering characteristics of drawn filled nitinol tube, 003. [] Characterizing Fatigue Response of Nickel- Titanium Alloys by Rotary Beam Testing, Journal of ASTM International, Vol. 4, No. 6 [3] P.C. Paris. A rational analytic theory of fatigue, The Trend in Engineering at the University of Washington, No.3(), 96, pp [4] American Society of Testing Materials. International, Subcommittee E08.06, standard test method for measurement of the plane strain fracture toughness in metallic materials, ASTM Book of Standards, v.3.0, Standard E 399, 005. ISBN: