Issue 35

Š. Major et alii, Frattura ed Integrità Strutturale, 35 (2016) 379-388; DOI: 10.3221/IGF-ESIS.35.43 380 prediction of pedicle-screws was used the method proposed by Navarro [6, 7, 8] and comparison with other methods used in multiaxial fatigue [9]. In the work, fatigue resistance of hollow pedicle-screw and solid pedicle-screws were compared. M ATERIALS CHARACTERIZATION crews used in this study were made of commercially titanium alloy 6AL4V ELI also known as Grade 23. It is an α-β phase titanium alloy made of 6% Al and 4% V in weight with a reduced content of interstitial elements such as oxygen and carbon (Extra Low Intersticials) and also iron. Chemical composition of material is shown in Tab. 1. This table compared the maximum value of the chemical elements specified by the manufacturer and the values measured by X-ray Spectrometers. C[%] N[%] O[%] H[%] V[%] Al[%] Fe[%] Manufacturer 0.08 0.03 0.013 0.0125 3.5-4.5 5.5-6.5 0.25 Measurement 0.073 0.025 0.012 0.0113 4.12 6.015 0.17 Table 1 : Chemical composition of titanium alloy Grade 23. In the first row are the maximal values of the elements specified by the manufacturer. In the second row line element values are obtained by measuring. Figure 1 : Crack growth rate of Grade 23 titanium alloy. The crack growth properties were measured on standard test method for measurement of fatigue crack growth rates introduced by ASTME 647. Mechanical properties of material are: the Young modulus of this material is E = 104.5 GPa. Test were performed to determine the ultimate tensile strength, σ u = 860 MPa and yield stress σ y = 820 MPa. Further mechanical properties are elongation at break A = 14%, reduction area S i = 25% and Vickers hardness 350 HV. The crack growth properties were measured on Standard test method for measurement of fatigue crack growth rates introduced by ASTME 647 [9]. This test method covers the determination of fatigue crack growth rates from near- threshold to K max controlled instability. Results are expressed in terms of the crack-tip stress-intensity factor range ( ΔK ), defined by the theory of linear elasticity. The experimental measurement was performed on specimen with diameter D S = 4 mm. The measured properties, obtained constants are C = 5.1 10 -12 and n = 4.2 for the crack growth in m/cycle, stress intesity factor K in MPa.m 0.5. The mechanical properties and biocompability of implant can be improved by surface S