PSI - Issue 13

Marijana Milković et al. / Procedia Structural Integrity 13 (2018) 1861 – 1866 Marijana Milkovi ć / Structural Integrity Procedia 00 (2018) 000–000

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Surface roughness influences fatigue life, so McMillan et al. (2018) demonstrated in the finite element analysis of the stresses that surface roughness can give rise to localized regions with significantly higher stresses state than it would be predicted assuming a perfectly smooth surface. According to the guide of Fitzpatrick et.al. (2005), the X-ray measurement is dependent on surface preparation (mechanical methods introduce additional surface residual stresses into the measured sample), grain size, texture, low net intensity (it can lead to scattering), number of phases in the sample etc... In this paper will be monitored residual stresses on the surface obtained after manufacturing and on the electropolished surface. Withers and Bhadeshia (2001) assessed the capability of a range of techniques for residual stress measurement. According to them, spatial resolution of X-ray diffraction is 1 mm laterally and 20  m in depth. Damjanović et.al. (2017) compared results of residual stresses measured by the incremental hole drilling method, X ray diffraction and the splitting method. It was concluded that X-ray diffraction method is more sensitive in the region of the surface than incremental hole drilling method. J. Keckes et al. (2018) investigated residual stress distribution in iron disks subjected to high pressure torsion by using high energy X-ray diffraction to access regions deep under the surface. In this paper is used low energy X-ray diffraction to get the actual surface condition. 2. Material and method For the experiments was used the tensile flat bar, cut from sheet plate of Al7075-T6 alloy. The plate was manufactured with cold rolling. Plate Al7075-T6 was stretched up to 10% plastic strain after cold rolling to relieve residual stresses and to receive minor straightening after rolling, as was reported by Trško et.al. (2017). Corresponding chemical composition of Al7075-T6 alloy in weight % is Al 89.406%, Zn 6.0426%, Mg 2.235%, Cu 1.744%, Cr 0.19%, Fe 0.099%, Si 0.0908%, Ti 0.0303, Mn 0.016%, Ga 0.0115, Zr 0.0043, Pb 0.0032%, Bi 0.0012, As 0.0011%, Ca 0.001%, P 0.0009%, Sn 0.0006%, In 0.0004%, Hg 0.0002% and Tl 0.0002%. Mechanical properties of the utilized Al7075-T6 alloy are following: yield strength is around 460 MPa, ultimate tensile strength is 527 MPa, Young’s modulus is around 64 GPa and Poisson’s ratio is 0.33. Average crystal grain size of Al7075-T6 is 31.3  m in longitudinal direction and 29.6  m in transversal direction. Figure 1.a) shows the microstructure of Al7075-T6 under magnification 50x. Samples were extracted from both untested Al7075-T6 plate and fatigue tested specimen (Milković et.al. 2017). Both samples were polished with the diamond paste. On the polished surface of the material were arbitrarily marked 21 points for measurement of the residual stresses. Residual stresses were measured with Pulstec µ-X360 in longitudinal direction with Aluminium (311) crystallographic structure.

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b) Fig. 1 a) Microstructure of Al7075-T6 (50x) b) Dimensions of the Al7075-T6 specimen with thickness t=3.14 mm,