Figure 3 - uploaded by Janet L. Gbur
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Various tensile grips. A) Roller grips, B) Rubber-lined grips, C-D) Serrated wedge grips, E-F) Aluminum-capped grips.
Source publication
Title Mechanical characterization of 316 LVM wires: A comparative study of flex bending fatigue and rotating bending fatigue and its utility in fatigue testing for biomedical applications Abstract Mechanical characterization of 316 LVM stainless steel was investigated across a range of fatigue life cycles in rotating-bending fatigue and flex bendin...
Contexts in source publication
Context 1
... testing and subsequent analyses followed ASTM E8 in addition to work performed by Benini (2010) and Lavvafi (2013) as the wire diameter was less than 4 mm. Testing was conducted on an Instru-Met uniaxial testing machine Figure 3A, and rubber-lined grips, Figure 3B and induced stress risers were apparent with the serrated wedge grips, Figures 3C-D, even if the wire was protected inside a paper/tape sheath within the grips. The best result for securing the wire resulted from an aluminum cap placed over serrated block grips as show in diameter. ...
Context 2
... testing and subsequent analyses followed ASTM E8 in addition to work performed by Benini (2010) and Lavvafi (2013) as the wire diameter was less than 4 mm. Testing was conducted on an Instru-Met uniaxial testing machine Figure 3A, and rubber-lined grips, Figure 3B and induced stress risers were apparent with the serrated wedge grips, Figures 3C-D, even if the wire was protected inside a paper/tape sheath within the grips. The best result for securing the wire resulted from an aluminum cap placed over serrated block grips as show in diameter. ...
Context 3
... testing and subsequent analyses followed ASTM E8 in addition to work performed by Benini (2010) and Lavvafi (2013) as the wire diameter was less than 4 mm. Testing was conducted on an Instru-Met uniaxial testing machine Figure 3A, and rubber-lined grips, Figure 3B and induced stress risers were apparent with the serrated wedge grips, Figures 3C-D, even if the wire was protected inside a paper/tape sheath within the grips. The best result for securing the wire resulted from an aluminum cap placed over serrated block grips as show in diameter. ...
Citations
Miniaturization of Nitinol-based medical devices requires increased performance from less material, and therefore necessitates investigation of the connection between material processing and performance. Moreover, it is imperative to understand how impurities that result from processing affect its lifetime performance. Research on the microcleanliness (i.e. presence of nonmetallic inclusions, intermetallic particles, etc.) of fine Nitinol wires and its effect on fatigue performance are not frequently found in literature.
This study compared a manufacturer-defined standard (SP) and high (HP) purity, Nitinol superelastic (SE) fine (< 140 μm) wire. Nonmetallic inclusion (NMI) chemistry was determined to be TixNiyOz with varied morphology, occurring with/without pores. Combined NMI/pore area percentages ranged from 0.08% ± 0.04% (HP) to 1.44% ± 0.26 (SP) when measured with scanning electron microscopy (SEM). Plasma focused ion beam (PFIB) serial sectioning was also used characterize the combined NMI/pore volume percentages yielding 0.09% (HP) and 0.47% (SP).
Differential scanning calorimetry was used to identify SE transformation temperatures. Microindentation hardness measurements showed gradients with the center of the wire measuring 359 ± 6.4 HK (HP) and 329 ± 7.2 HK (SP) while mid-radius values were similar for both at 366 ± 5.1 HK and 361 ± 6.0 HK, respectively. Higher tensile and upper plateau strengths were observed in SP (1573 MPa and 515 MPa, respectively) than HP (1420 MPa and 392 MPa, respectively) with greater reduction in area for HP (92.2%) vs. SP (80.2%). Fatigue enhancement was observed in HP wires in flex bending fatigue at strain amplitudes (εa) from 0.67-11% with a run-out at 10^6 cycles and rotating bending fatigue (εa 1.2-1.5%) though run-out at 10^8 cycles. Fractography showed crack initiation attributed to NMIs/pores at the surface or sub-surface with feature areas ranging from submicron to 20.4 μm^2.
Characterization techniques showed that HP contained less large-dimension NMIs/pores than SP leading to less potential stress concentrations and points for crack initiation. These differences in relative purity were reflected in the fatigue results where the HP exhibited enhanced fatigue resistance in both low cycle fatigue and high cycle fatigue regimes.
Fine wires and cables play a critical role in the design of medical devices and subsequent treatment of a large array of medical diagnoses. Devices such as guide wires, catheters, pacemakers, stents, staples, functional electrical stimulation systems, eyeglass frames and orthodontic braces can be comprised of wires with diameters ranging from 10s to 100s of micrometres. Reliability is paramount as part of either internal or external treatment modalities. While the incidence of verified fractures in many of these devices is quite low, the criticality of these components requires a strong understanding of the factors controlling the fracture and fatigue behaviour. Additionally, optimisation of the performance and reliability of these devices necessitates characterisation of the fatigue and fracture properties of its constituent wires. A review of cable architecture and stress states experienced during testing is followed by an overview of the effects of changes in material composition, microstructure, processing and test conditions on fracture and fatigue behaviour of wire and cable systems used in biomedical applications. The review concludes with recommendations for future work.
