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

168

3

platform, thin-walled tubular specimens were built with an orientation of both 90° (also named vertical specimens in the following) and 45° (also named diagonal specimens). After manufacturing, AM specimens were machined in order to obtain an average surface roughness value within the specifications of ASTM Standard E2207 (that is, lower than 0.200 μm). Then, such specimens were subjected to a post-processing heat treatment, named Hot Isostatic Pressing (HIP), which allows closing the internal defects and relaxing the tensile residual stresses (Tammas-Williams et al. (2016)). Such a treatment together with the above scanning strategy consisting in a hatch rotation of 67° are able to almost completely remove the inhomogeneous and anisotropic microstructures of the present material (Yadollahi et al. (2020), Robinson et al. (2019)). The testing machine used was a closed-loop servo-hydraulic axial-torsion Instron machine (equipped with an Epsilon axial-torsion extensometer); fatigue tests were performed under both strain-controlled and load/torque controlled modes on vertical and diagonal specimens. The details of the experimental loading conditions examined are reported in Molaei et al. (2018): they consisted in tensile, torsional, and combined tensile/torsional (with a phase shift,  , equal to 0° or 90°) loading with fatigue ratio R equal to -1. Based on experimental results, only negligible differences in fatigue behaviour between vertical and diagonal AM Ti-6Al-4V specimens were observed. From uniaxial data, the parameters of the tensile and torsional Manson-Coffin curves are computed (Table 1). As far as static mechanical properties are concerned, the elastic modulus E and the shear modulus G are taken equal to 118.9 GPa and 45.7 GPa, respectively, for both vertical and diagonal specimens. Consequently, the elastic Poisson ratio e  turns to be equal to 0.3.

Tab. 1 Fatigue properties of AM titanium alloy Ti-6Al-4V.

0 b

0 c

' f 

' f 

' f 

' f 

b

c

Specimen Vertical Diagonal

1240 MPa 1573 MPa

-0.0547 1.04 -0.0776 2.61

-0.735 -0.879

683 MPa -0.0417 558 MPa -0.0282

0.40 0.41

-0.481 -0.453

3. Critical plane-based criterion applied to AM metals The fatigue tests presented in Section 2 are hereafter simulated through the critical plane-based fatigue criterion proposed by Carpinteri et al. (2015). Although such a criterion has been originally formulated for conventional metallic materials (homogeneous and isotropic), it can be also applied to the above experimental data since the anisotropic and inhomogeneous microstructures of the present AM metal were removed by adopting, respectively, a specific scanning strategy and a post-manufacturing heat treatment. Note that the presence of both tensile residual stresses and high surface roughness (which are peculiar features of AM metals) are not taken into account in the formulation of the present criterion since: - tensile residual stresses were completely relaxed by means of the HIP treatment; - high surface roughness was removed by machining and polishing the AM specimens. According to the Carpinteri et al. criterion, the fatigue life assessment is carried out at a verification point (point P ) located on the specimen surface. Once the strain state at point P is obtained, the averaged directions ( ˆ ˆ 1, 2 and ˆ 3 ) of the principal strain axes are determined on the basis of their instantaneous directions by means of the averaged values of the principal Euler angles. The orientation of the critical plane is assumed to be linked to the above averaged directions through the off-angle  formed by the normal w to the critical plane and the averaged direction ˆ 1 . The empirical expression of  is as follows:

   

   

2

  

   

3 2

1



1

45

a









(1)

 2 1  



a 



eff