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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Fig. 4 shows another area of the surface of the specimen cycled with low strain amplitude to only 4% of the fatigue life. A grain boundary, oriented on the surface perpendicular to the stress axis is already cracked. Several PSBs originated in the left grain produce both surface and grain boundary extrusions and intrusions. The extrusions exert local compression pressure on the neighbor grain. The local internal compression stress tries to separate both grains. Small intrusions, parallel with extrusions, produce void-like defects at the grain boundary. These microvoids help in the separation of both grains and the formation of an intergranular crack. FIB cuts (see the insets in Fig. 4), produced at several locations of the grain boundary crack, illustrate the length and the direction of the grain boundary crack. The crack follows the grain boundary under angle close to 45°. The de pth of the crack varies along the grain boundary and the highest crack depth is around 3  m. Similar grain boundary cracks were observed in fatigued austenitic Sanicro 25 stainless steel cycled surface of the specimen and on the grain boundary. The inset in Fig. 5b shows that grain boundary extrusions are high and during cyclic loading were bent and even pushed into neighbor intrusion (see white arrow in Fig. 5b). The arrangement of extrusions and intrusions at the grain boundary in stainless steel is similar to that in polycrystalline copper (compare Fig. 2 and Fig. 3). 3. Discussion The effect of extrusions and intrusions on the development of intergranular cracks has been neglected until recently. The role of extrusions and intrusions has been recognized in the initiation of intragranular cracks (Polák et al. (2017)) . Here the intrusions serve as a crack-like defect and fatigue crack initiate in their tips and follow the primary slip plane along the PSB or the matrix/PSB interface. The initiation of intergranular cracks has been attributed to the very high stress concentration produced by the dislocations in PSBs piling-up on the grain boundary (Christ (1989)). The sudden brittle-fracture-like fatigue crack initiation due to

Fig. 3. Surface of the specimen cycled 5x10 -3 for 560 cycles (10% N f ); the insets with higher magnification show extrusions and intrusions (white arrows) both on the surface and at the grain boundary.

in the low cycle fatigue domain (Mazánová et al . (2022)). Fig. 5 shows the heavily damaged area of the surface of the specimen cycled up to fracture. A grain boundary crack is perpendicular to the loading direction. The system of PSBs impinge on the grain boundary from grains No. 2 and No.3. Extrusions and parallel intrusions appear both on the

Fig. 4. Surface of the specimen cycled with 1x10 -3 for 4000 cycles (4% N f )). A cracked grain boundary is oriented perpendicular to the stress axis. The insets show the details of the impinging PSMs and also the FIB cuts illustrating the local grain boundary crack depth and direction.