PSI - Issue 26

Sabrina Vantadori et al. / Procedia Structural Integrity 26 (2020) 106–112 Vantadori et al. / Structural Integrity Procedia 00 (2019) 000 – 000

108

3

whereas the definition of the shear stress amplitude a C is not unique. In the following, a C is computed through the Maximum Rectangular Hull method by Araujo et al. (Ref. by Araujo et al., 2011). For finite life, the number of loading cycles to failure, N cal , can be determined by iteratively solving the following equation:

2

m

2 * m

m

2

   

     



0 cal N N N N      

  

  

  

N

(

)

2

af

, 1 −

2

N

C

+

=



cal

0

(3)

, eq a

a

af

, 1 −

N





af

cal

, 1 −

0

This Equation is obtained from Eq.(1) by replacing the fatigue strengths 6 2(10) loading cycles) with the fatigue strengths at fatigue finite life N cal , exploiting the Basquin expressions for both fully reversed normal stress and fully reversed shear stress. Further, m and m  are the slopes of the SN curve for fully reversed normal stress and fully reversed shear stress. Hollow cylindrical dog bone-shape specimens made of 316 were tested (Refs by Morishita et al., 2018, and Cruces et al., 2019). The chemical composition of such a material (in wt%) is listed in Table 1, and the mechanical and fatigue properties are listed in Table 2 (Refs by Itoh et al., 2011, and by Cruces et al., 2019). , 1 af  − and , 1 af  − (generally referred to 0 N =

Table 1. Chemical composition of 316 stainless steel (in wt %).

Material SS 316

C

Si

Mn

P

S

Ni

Cr

Mo

0.06

0.46

1.33

0.32

0.28

10.15

16.97

2.03

Table 2. Mechanical and fatigue properties of 316 stainless steel.

N 0 cycles

ν

σ y [MPa]

σ u [MPa]

σ α f,-1 [MPa]

τ α f,-1 [MPa]

Material

E [GPa]

m

m*

SS 316

193

0.3

240

575

168

11.07

97

11.07

2(10) 6

The tests herein examined were performed by means of an electric servo-controlled multiaxial fatigue testing machine (Figure 1), designed by some of the present authors (Ref. by Morishita et al., 2016), which can combine push pull, reversed torsion, and cyclic inner pressure. Such a machine was equipped with three actuators that allowed to perform tests under a wide range of multiaxial stress ratio. The maximum pressure on the inner surface was 200MPa, the maximum axial loading for push-pull was ± 50kN, and the maximum torque moment for reversed torsion was 250Nm. An extensometer to measure the axial displacement was attached directly to the specimen, and its gauge length was 7mm. Another extensometer to measure the hoop displacement was attached to the specimen with clamps held together by rubber bands. The experimental tests were carried out under cyclic axial loading ( 0 R = ), cyclic inner pressure ( 1 R = − ), push pull ( 1 R = − ), and a combination of cyclic axial loading and inner pressure (Refs by Morishita et al., 2018, and Cruces et al., 2019). The load-controlled fatigue tests under constant amplitude loadings were carried out at room temperature, with the specimens being subjected to a quenching heat treatment (1050°, WQ) before testing. The computed radial, r  , hoop,   , and axial stress, z  , related to the inner surface of each specimen, are listed in Table 3, where  is the phase shift between the radial and the hoop stress and the fatigue life N exp for each specimen is reported in the last column.