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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1. Introduction Initiation of fatigue cracks in polycrystalline materials represents an important period in the fatigue life of a specimen or structural component. In material containing macroscopic defects the stress and strain concentration determines usually the crack initiation location. If defect size and shape are below a certain limit fatigue cracks initiate in several locations on the surface. It was clearly documented (Antolovich et al. (2014 ), Polák et al. (2017)) that at room temperature the deciding role in fatigue crack initiation plays the cyclic strain localization. Contrary to the unidirectional straining the strain in each half-cycle is small but is applied repeatedly. The plastic component of the strain is realized mostly by the slip of dislocations. Though cyclic slip is initially homogeneous soon it is localized in persistent slip bands (PSBs). Repeated cyclic straining in PSBs with high local plastic strain amplitude results in the formation of persistent slip markings (PSMs) on the surface of the material in the form of extrusions and intrusions. PSMs, preferentially intrusions, represent the sites for fatigue crack initiation in single crystals, in the grains of polycrystals (Man et al. (2015)) or at the twin boundaries (Heinz and Neumann (1991)). Experimental findings have stimulated models of PSM formation and fatigue crack initiation. The first physically founded model by Essmann et al (1981) was based on the measurement of the production of vacancies during cyclic straining (Polák (1969)) and emphasized the role of extrusions. It was shown later (Polák (1987)) that both extrusions and intrusions are produced in localized cyclic straining and crack-like intrusions lead to fatigue crack initiation in the single crystal or the grain of a polycrystal. Quantitative description of PSM formation ( Polák and Man (2014) ) allowed to evaluate the shape of extrusions and intrusions. Fatigue cracks in polycrystalline materials arise not only within grains or at twin boundaries but also in high angle grain boundaries (HAGBs) (Boettner et al (1964)). Kim and Laird (1978) using interferometric observations of polycrystalline copper concluded that cyclic loading produces steps at grain boundaries and the stress concentration developed at this step is the source of grain boundary fatigue crack initiation. Figueroa and Laird (1983) also cycled polycrystalline copper having large grain size (~ 400  m) and monitored the development of the surface using a plastic replica. In the low amplitude region, the propagating cracks were initiated by the PSBs by the impingement mechanism. Liang and Laird (1989) found that intergranular initiation is preferred in coarse-grained material while for fine-grained material fatigue cracks initiated mostly intragranularly from PSMs. Models and mechanisms of intergranular fatigue crack initiation are until now based on the initial idea of Tanaka and Mura (1981) who supposed the origin of the high local stress at the grain boundary and brittle fracture appearance of incipient cracks. Tanaka and Mura considered the stress comes from the stacked pile-ups of vacancy dipoles oriented normal to the grain boundary. Mughrabi et al. (1982) considered the model of PSBs of Essmann et al (1981) and proposed that static extrusions in the grain produce superdislocations in PSB/matrix interface and these pile-up against the grain boundary while resulting stress causes the initiation of “brittle - type” PSB -GB cracks. Christ (1989) evaluated the critical stress that can induce the grain boundary cracking due to pile-up of the interface dislocations related to the saturated concentration of static vacancies in the PSBs. The idea of dislocation pile-ups and brittle fracture appearance of incipient cracks due to high local stress appeared in many theoretical and experimental papers (see e.g. Liu et al. (2020)). In the study of the fatigue crack initiation and early growth of fatigue cracks in superaustenitic Sanicro 25 steel Mazánová et al. (2022) analyzed also fatigue cracks generated at the grain boundaries. It was found that the system of extrusions and intrusions in adjoining grains heading under high angle to the grain boundary has damaging effect on the grain boundary bonding. Chlupová et al. (2022) studied intergranular cracks in polycrystalline copper cycled in strain control with low and high strain amplitude. The purpose of the present paper is to report recent experimental data and the analysis showing the mechanism of grain boundary fatigue crack initiation in polycrystalline materials. 2. Experimental study of intergranular fatigue crack initiation The majority of recent experiments were performed on 99.97% pure polycrystalline copper rods which, after drawing, were annealed for one hour in a vacuum at 600 °C. Cylindrical specimens with flat surfaces parallel to the stress axis were cycled in strain-control with constant strain rate of 5x10 -3 s -1 in a symmetrical cycle and various strain amplitudes. During cycling the stress amplitude  a , the plastic strain amplitude  ap , and the loop shape parameter V H were recorded. Fig. 1 shows the plot of the stress amplitude and the loop shape parameter during cycling.