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A non-destructive thermographic methodology based on the temperature field is utilized to determine the crack tip position during the very high cycle fatigue (VHCF) test of pure iron and deduce the corresponding fatigue crack growth rate (FCGR). To this end, a piezoelectric fatigue machine is employed to test 1 mm thick pure iron samples at 20 kHz in push–pull fatigue loading. Two cameras are placed on each side of the plate sample, an infrared one for measuring the temperature fields on the specimen surface and an optical one for visualizing the crack tip verification. The centre section of the specimen is notched to initiate the crack. The temperature field is converted into intrinsic dissipation to quantify the inelastic strain energy according to energy conservation. The maximum value of intrinsic dissipation in each thermal image is related to the position of the crack tip and thus allows monitoring of the crack evolution during the fatigue test. The obtained results show that one specific specimen broke at 7.25 × 107 cycles in the presence of a very low-stress amplitude (122 MPa). It is observed that the intrinsic dissipation has a low-constant level during the initiation and the short cracking, then sharply grows during the long cracking. This transition is visible on the polished surface of the sample, where the plasticity appears during the long cracking and slightly before. The material parameters in the Paris equation obtained from the intrinsic dissipation in the short crack growth are close to the results available in the literature as well as those obtained by the optical camera. [ABSTRACT FROM AUTHOR]

• A dual-scale modelling approach combining sub-modelling approach with crystal plasticity constitutive model. • Efficient procedure for fatigue life prediction of anotch parts using the Tanaka-Mura-Wu model and Paris law. • The microstructural size effect on the mechanical and fatigue behaviors of Ni-based superalloy GH4169. Fatigue performances of critical structures are strongly affected by the microstructural (e.g. grain, defect, inclusion, etc.) size effect, and it is thus important to quantify their detrimental effect. In this work, a numerical procedure is constructed to quantify the influence of microstructure on the mechanical and fatigue behaviors of Ni-based superalloy GH4169. Specifically, by combining sub-modelling approach with crystal plasticity constitutive model, a dual-scale modelling approach is developed for studying grain-level mechanical behavior of Ni-based superalloy GH4169 notched components. In addition, the dislocation-based Tanaka-Mura-Wu model is applied for fatigue crack initiation life prediction. The Paris law is then utilized for fatigue crack propagation analysis based on the simulated short crack. To study the microstructural size effect on fatigue crack initiation and propagation behaviors, sub-models containing various grain orientation, grain size, defect size/shape are built and analysed. Finally, a series of fatigue tests on notched specimens of Ni-based superalloy GH4169 were carried out for method validation. Results indicate that the established dual-scale modelling approach and fatigue life prediction framework yields good agreement with experimental results. [ABSTRACT FROM AUTHOR]

• The fatigue crack growth tests of 9%Cr martensitic steel are conducted under R = 0.1, 0.3 and 0.5 at 630 ℃. • The FCG path and secondary crack growth behavior under different stress ratios are discussed. • The martensitic lath structures can lead to a more tortuous crack growth path. • The finite element method is introduced to calculate the evolution of the plastic zone and the secondary crack growth behavior. To understand the effect of mean stress and microstructure on the fatigue crack growth (FCG) behavior of 9%Cr martensitic steel under high temperatures, the FCG experiments are conducted on three stress ratios (R = 0.1, 0.3, and 0.5) at 630 ℃. The results indicate that the high-stress ratios have a higher fatigue crack growth rate, and the Paris region can be divided into two regions by the transition point (da / d N = 1 × 10 - 4 mm/Cycle) with different fracture morphology. For da / d N > 1 × 10 - 4 mm/Cycle, due to the influence of high crack driving force and the microstructure, the FCG path presents in a zigzag way. For da / d N ⩽ 1 × 10 - 4 mm/Cycle, the FCG path presents the characteristic of purely horizontal and no obvious crack deflection. Meanwhile, the martensitic lath fracture angles near the FCG path have an obvious statistical relationship with the stress ratio, which is mainly concentrated at 60° to 80° and 100° to 130°. Additionally, according to the EBSD map of secondary cracks, when the secondary cracks encounter the HAGBs, the larger angle of secondary crack deflection occurs. [ABSTRACT FROM AUTHOR]