PSI - Issue 43

Jaroslav Polák et al. / Procedia Structural Integrity 43 (2023) 197–202 Jaroslav Polák, Alice Chlupová / Structural Integrity Procedia 00 (2022) 000 – 000

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Cycling was interrupted at 5%, 10%, and 50% of the fatigue life to make SEM, EBSD observations and selected FIB cutting. Material cyclically hardens and reaches saturation. Temporary changes of the stress amplitude and V H parameter after interruption are mostly due to changes of the applied strain amplitude. The minimum of the V H parameter indicates the start of the cyclic strain localization and reaching the maximum indicates that cyclic strain is localized in a majority of grains. We can see that all the interruptions were in the domain of cyclic strain localization. llllll

(a) (b) Fig. 1. Mechanical behavior of specimens cycled with two strain amplitudes (a) stress amplitude vs. number of cycles, (b), loop shape parameter vs. number of cycles.

Secondary grain boundary crack in a specimen cycled to fracture ( N f = 5600) with strain amplitude of 5x10 -3 is shown in Fig. 2. Both adjoining grains have well-developed system of distinctive PSMs consisting of alternating extrusions and intrusions on the surface. The insets in the picture show the detail of the crack and partially also the cracked surface. It is apparent that the extrusions and intrusions emanate not only on the surface but also on the grain boundary. An extrusion on the surface corresponds to an intrusion in the grain boundary and similarly, an intrusion on the surface corresponds to an extrusion in the grain boundary. The early evolution of the grain boundary crack in cycling with high strain amplitude is shown in Fig. 3. The insets show details of the PSMs heading from both adjoining grains to the grain boundary. Here the surface intrusion corresponds to the grain boundary intrusion and the surface extrusion corresponds to the grain boundary extrusion. PSMs from both adjoining grains contribute to the grain boundary damage in the form of tiny void-like defects produced by grain boundary intrusions. Grain boundary extrusions locally push the neighbor grain apart.

Fig. 2. Surface of the specimen cycled with strain amplitude 5x10 -3 to fracture ( N f = 5600 cycles); details of a cracked grain boundary reveal PSMs from neighbour grains impinging the grain boundary, eventually leading to crack initiation.