PSI - Issue 75

Luca Vecchiato et al. / Procedia Structural Integrity 75 (2025) 602–608 Luca Vecchiato et al. / Structural Integrity Procedia (2025)

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were repeatedly applied until failure in a descending-descending sequence. In all cases, loads were applied using a nominal load ratio R σ = R τ = -1. All tests were performed in laboratory conditions and the recorded number of cycles to failure corresponded to a through-the-thickness crack. The latter was real-time monitored by measuring the pressure inside the tube, which were inflated with a small amount of compressed air (~50 mbar) before running the test. Fatigue failure always occurred at the weld toe, the latter being characterized by a tip radius ρ ranging from 13 to 20 mm, i.e. more than 100 times larger than the control radius R 0 = 0.12 mm.

0.20

0.18

0.16

0.14

0.12

0.10

n i / L s [-]

0.08

0.06

0.04

0.02

0.00

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Δ σ /Δ σ max or Δ τ /Δ τ max [-]

Fig. 2. Stress spectrum used for VA tests.

Given the relatively large notch tip radius, it was necessary to consider the presence of the weld toe radius to properly apply the PSM and evaluate the mode I and mode III equivalent peak stresses according to Eq (3). The smallest fillet radius, i.e. 13 mm, was selected for modelling the weld geometry, both as a conservative choice and because this is where fatigue cracks were most likely to occur during experimental testing. Consequently, it was essential to accurately evaluate the elastic peak stress values under both bending and torsion, to be used in Eq. (3). To this end, 2D axisymmetric linear elastic FE analyses (Figure 3) were performed using Ansys ® Mechanical APDL. A free mesh pattern of 2D 4-node quadrilateral harmonic elements (PLANE 25) with a global element size d = 2.5 mm was used to discretize the FE model. A refined element size d local = 0.8 mm was adopted at the weld toe to reach mesh convergence and evaluate the exact values of the maximum elastic stresses. All displacements of nodes located along the X-axis were constrained to simulate the flange connection to the rigid frame (Figure 3). Then, a nominal bending (Δ σ = 1 MPa) or torsional stress range (Δ τ = 1 MPa) was applied remotely on the tube side to replicate the experimental loading conditions (Figure 3). The maximum elastic stresses at the notch tip (Δ σ ρ,max and Δ τ ρ,max ) evaluated from FE simulations allowed to to convert experimental data, originally expressed in terms of nominal stress range, into the corresponding equivalent peak stress range Δ σ eq,peak . Specifically, they were scaled by the maximum nominal stress range and introduced into Eq. (3) along with the relevant coefficients f s1 and f s3 (for both CA and VA loads) as well as f w1 and f w3 to determine the mode I Δ σ eq,peak,I and mode III Δ σ eq,peak,III equivalent peak stresses. Then, Δ σ eq,peak was calculated using Eq. (1).