PSI - Issue 31

M. Mlikota et al. / Procedia Structural Integrity 31 (2021) 3–7 Marijo Mlikota et al. / Structural Integrity Procedia 00 (2019) 000–000

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al. (2011), Božić et al. (2018), Baragetti et al. (2019)), which can lead to catastrophic failure due to fracture which may occur at a critical load (Babić et al. (2018), Babić et al. (2019), Babić et al. (2020), Božić et al. (2018), Vučković et al. (2018), Vukelić et al. (2020), Baragetti et al. (2020). It has been long known that the local residual stresses underneath the surface of a material can contribute to its durability (compressive) or, in contrast, can harm the structure (tensile), depending on their nature. A common treatment technique, named as shot-peening (SP), where a surface of a structure is shot by small hard particles is often used to introduce compressive residual stresses at the surface and, to a certain degree, in the depth of the material of the structure. These compressive stresses have well-known beneficial effect on the overall fatigue life, where the crack initiation phase amounts often to more than 90% in the case of high-cycle fatigue (HCF). In the present study, compressive residual stresses are considered in combination with a fatigue crack initiation modelling approach with the aim to test their influence on the fatigue initiation life ( S - N ini ) curve of AISI 1141 steel.

Nomenclature CRSS critical resolved shear stress d s slip band segment length FEM finite element method G shear modulus HCF high cycle fatigue N ini W c crack initiation energy Poisson’s ratio TM Tanaka-Mura Δ � N s S applied loading stress level SP shot pinning

number of cycles to initiate the short crack

number of cycles to nucleate a crack segment in a single grain

average shear stress range along the slip band segment

2. FE crack initiation analysis A micromechanical damage model for predicting material failure in the initiation stage due to high-cycle fatigue (HCF) is the physically-based model initially proposed by Tanaka and Mura (1981, 1982), which relies on dislocation pileup. In combination with a finite element method (FEM) based analysis, the model is capable of determining when a grain, subjected to an outer cyclic loading, will develop a slip band and subsequently a crack. The number of cycles N s needed for micro-crack nucleation within a single grain can be derived by means of the Tanaka-Mura (TM) equation: � = ��� � ������� � ��� � ������� � (1) According to TM, micro-cracks form along slip band segments, depending on segmental length d s , the average shear stress range on the segment Δ τ s (derived from FEM simulation), the shear modulus G , the crack initiation energy W c , the Poisson’s ratio ν , and the critical resolved shear stress CRSS. The number of cycles for the initiation phase ( N ini ) is estimated by summing the cycles required for all segmental cracks ( N s , Eq. 1) that nucleated during the initiation phase. The FEM-based crack initiation analysis has been performed with the microstructural model exposed to compressive residual stresses that have been superimposed to the outer loading (Fig. 1b), and has been compared with the results where solely outer loading (224 MPa) has been considered (Fig. 1a). Boundary conditions have been transferred to the microstructural model from the global model (see Fatemi et al. (2004) for details on the considered