PSI - Issue 34

Camilla Ronchei et al. / Procedia Structural Integrity 34 (2021) 166–171 C. Ronchei, S. Vantadori, D. Scorza, A. Zanichelli, A. Carpinteri / Structural Integrity Procedia 00 (2021) 000 – 000

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and automotive applications, where a reduced weight contributes to a reduced fuel consumption and increased payload. As is well-known, the mechanical properties of AM metals can be significantly different from those of the same metals produced through conventional techniques, and these differences are more pronounced in presence of fatigue loading (Yadollahi et al. (2018)). In particular, AM metals experience shorter fatigue lives in comparison to their traditional counterparts due to the presence of both internal defects (porosity and lack of fusion), residual stresses and high surface roughness, which are material inherent features deriving from the manufacturing process (Solberg et al. (2021)). Moreover, also the anisotropic microstructure of AM metals, caused by high thermal gradients occurring during the building process, strongly affects the fatigue behaviour (Stern et al. (2019)). Similar to most conventional metallic components in different engineering applications, AM components typically undergo cyclic loadings through their service life; consequently, understanding their fatigue behaviour is one of the most important steps in design process in order to avoid catastrophic failures. In such a context, the present paper is on the fatigue failure of AM metallic specimens, and fatigue life is estimated as is discussed below. The analysed experimental data available in the technical literature (Molaei et al. (2018)) are related to uniaxial and biaxial fatigue tests performed on tubular specimens made of AM titanium alloy Ti-6Al-4V. Such fatigue tests are hereafter simulated through a critical plane-based fatigue criterion proposed by Carpinteri et al. (2015). This criterion, formulated in terms of strains, can be applied to model both stress- and strain-controlled tests, and the fatigue strength is evaluated by means of an equivalent strain amplitude related to the critical plane, together with the tensile Manson – Coffin curve.

Nomenclature b

fatigue strength exponent under normal strain fatigue strength exponent under shear strain fatigue ductility exponent under normal strain fatigue ductility exponent under shear strain

0 b c 0 c E

elastic modulus shear modulus

G

experimental fatigue life theoretical fatigue life

exp N

f N

unit vector normal to the critical plane fatigue ductility coefficient under shear strain off-angle defining the normal to the critical plane fatigue ductility coefficient under normal strain normal displacement vector related to the critical plane tangential displacement vector related to the critical plane equivalent normal strain amplitude

w

' f 



eq ,a 

' f 

N  C 

elastic Poisson ratio effective Poisson ratio

e 

eff  ' f 

fatigue strength coefficient under normal strain fatigue strength coefficient under shear strain

' f 

2. Experimental data The fatigue tests examined are related to an experimental campaign carried out by Molaei et al. (2018) on AM titanium alloy Ti-6Al-4V specimens. Such specimens were produced by using a Laser-Beam (LB) based Powder Bed Fusion (PBF) system with the following parameters: laser power of 220 W, scanning speed of 756 mm/s, layer thickness of 50  m, hatch pitch of 110  m, and hatch rotation of 67°. The material employed in the manufacturing process was a gas-atomized Ti-6Al-4V powder with the particle size range of 15 – 45  m. With respect to the build