PSI - Issue 37

Carl Fällgren et al. / Procedia Structural Integrity 37 (2022) 948–955 Carl Fällgren / Structural Integrity Procedia 00 (2019) 000 – 000

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approaches of different complexity in order to identify possible crack arrest or crack propagation. Calculated results were compared to experimental test data from component-like specimens. The comparison to the test results showed an overestimation of the predicted fatigue lives. The modelled material behaviour and consequently the residual stress distribution from the simulation models was identified as the decisive factor for the deviation. Still, the comparison showed that the fracture mechanics based approaches are capable of describing the crack arrest and propagation behaviour reliably. Further investigation regarding the modelling of the material behaviour with focus on the Autofrettage process is still required. © 2022 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of Pedro Miguel Guimaraes Pires Moreira Keywords: ultra-high-strength steel; Autofrettage; residual stress 1. Introduction To further raise the pressure in high-pressure components like common-rails, the usage of higher-strength steels has to be examined. For this purpose, the ultra-high-strength steel W360 (brand name by Böhler Edelstahl GmbH & Co. KG) was used to elaborate the possibility of using the material in autofrettaged components. The steel was chosen as it shows a high ductility which is required as very large strains occur at the highly stressed areas of the components during the autofrettage process. The introduction of the compressive residual stresses by autofrettage causes multiple effects with influence on the components' fatigue lives: crack initiation lives are prolonged due to the residual stress at the crack initiation location, crack propagation lives are prolonged as the cracks grow into a compressive residual stress field. Also the crack-arrest phenomenon can occur. These effects have been examined in previous works with the quenched and tempered commonly used steel 42CrMo4 for component-like specimens with intersecting holes subjected with and without foregoing autofrettage treatment, see Beier et al. (2017), Vormwald et al. (2018) and Thumser (2009). To obtain comparable results, in the research project, the same specimen geometry was used as in the previous works. 2. Material testing and modelling The material used was the ultra-high-strength stainless steel W360 (X50CrMoV5-3-1). For material testing, axisymmetric hourglass specimens with a diameter of 5 mm of the net cross section were manufactured out of round bar-steel and heat-treated to obtain an ultimate tensile strength of around 2100 MPa. Tensile Testing showed a high ductility of the material and an elongation at break of about 10 %. Further material testing was performed to evaluate the material's cyclic behaviour. For this purpose, strain controlled tests were performed with the hourglass specimens described with strain amplitudes between 0.33 % and 4.65 %. The strain ratio was set to = −1 . As cracking criterion, a 5 % load-drop was used. The results obtained were used to calculate the parameters for the Ramberg-Osgood equation shown in Eq. (1), cf. Ramberg and Osgood (1943). = + ( ′ ) ( 1 ′ ) (1) To fit the parameters to the experimental data, linear regression was applied to the logarithmic values from stress and plastic strain amplitudes at half crack initiation life as described in Fiedler and Vormwald (2018). The obtained cyclically-stabilised stress-strain curve is shown in Fig. 1. The data from experimental results are shown with blue dots and the derived Ramberg-Osgood curve is shown in fully-drawn and blue. Additionally, the curve from quasi static tensile testing is shown in black and dashed for comparison. From the figure it can clearly be seen, that the material is subject to cyclic hardening. 02 r u i This i a c c unde e lice e bar-steel and heat-treated to obtain an ultimate tensile strength of around 2100 MPa. Tensile Testing showed a high © 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of Pedro Miguel Guimaraes Pires Moreira