PSI - Issue 39

Georg Schnalzger et al. / Procedia Structural Integrity 39 (2022) 313–326 Author name / Structural Integrity Procedia 00 (2019) 000–000

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2.2. High pressure torsion The current work uses a modified anvil design as shown in Fig. 4, which can be deformed by HPT to produce severely deformed FCG specimens. A DC bar is machined from undeformed R260. The DC specimen is placed between the HPT tool shown in Fig. 4 (a) and twisted under the pressure of several gigapascals. The DC bar exhibits a shoulder in the middle part with reduced diameter to concentrate the shear deformation. A copper ring surrounds this shoulder during the HPT process to sustain the hydrostatic pressure and consequently raise the achievable deformation degree. For the present study, the HPT machine uses a pressure of 3.5 GPa and deforms the specimen for a half rotation. The corresponding imposed shear strain γ calculates from Eq. (1) taken from Leitner et al. (2019): γ = 2∙ π ∙r h ∙ n (1) with r being the radius in the shoulder, h the height of the shoulder and n the number of rotations. The corresponding von Mises strain ε vM calculates from Eq. (2) and provides a better comparison to longitudinal strains: ε vM = √ γ 3 (2) For the present experiments, r and h amount approximately 8 and 9 mm, respectively. Consequently, the shear strain and von Mises strain amount to approximately 3.5 and 2 after a half rotation, respectively. From the deformed DC specimen the FCG specimen is machined as outlined in Fig. 4 (b). The machined notch is directly situated in the highly deformed region to study the effect of pre-deformation on the FCG behavior.

Fig. 4. HPT setup to produce deformed FCG specimens: (a) sketch of HPT tool and double cone sample and (b) manufacturing of FCG specimen from DC specimen with notch.

2.3. Finite element analysis The work at hand uses the 2D plane stress FEA illustrated in Fig. 5, which is implemented in the commercial Finite Element (FE) software package Abaqus (Dassault Systèmes (2019)). The FEA is a complementary tool to estimate machine loads prior to the tests, compute stress intensity factors and investigate mode-mixity ratios. The model depicts the cylinder surface in the middle section of the FCG specimen including the notch and idealized cracks. The crack is parametrized using its length a 0 and inclination angle β . The current work assumes these geometrical parameters from experimental observations. The mesh consists of approximately 15,000 four-node CPS4 and three-node CPS3 plane