PSI - Issue 23

Petra Ohnišťová et al. / Procedia Structural Integrity 23 (2019) 469 –474 Petra Ohnišťová et al. / Structural Integrity Procedia 00 (2019) 000 – 000

474

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The initial local cracks were observed in some intermetallic particles when number of cycles 1,000 cycles was reached. The cracks were initiated mainly inside of the intermetallic particles. When the number of cycles was increased, the cracks propagated to the boundary between the particle and matrix. The cyclic testing was stopped when number of cycles 6,000 was reached. At the moment, the cracks had remained inside of the intermetallic particle and did not propagate into the material matrix. DIC analysis was afterwards used to display engineering strain evolution in the selected intermetallic particles and the specimen.

4. Conclusions

The studied aluminum alloy 7475-T7351 consisted of a heavily deformed structure and strong anisotropy that had been made by rolling. Static and dynamic mechanical tests were performed in order to observe crack nucleations and their propagations. The results from in-situ tensile testing can be summarized as following:  three types of the secondary phases in matrix were observed (intermetallic particles, dispersoid precipitates and fine metastable precipitates) occurring in different morphologies (as large particles, elongated particles or clusters of the intermetallic particles).  there was crack initiation in intermetallic particles observed when the yield strength was reached (415-419 MPa)  the crack propagation from a secondary phase to the material matrix at the angle of 45° (in the direction of the maximum shear stresses) was observed before reaching the tensile strength limit (484 MPa). The results from in-situ fatigue cyclic loading can be summarized:  the surface topography after machining was not the key factor influencing the fatigue crack initiation. Crack were preferably initiated at the intermetallic particles larger than 10 µm ;  two basic mechanisms of the fatigue crack initiations were observed (inside of the intermetallic particles or at the boundary of a particle and a matrix);  moreover, the cracks were initiated at the intermetallic particles located in the vicinity of machined surface also. On contrary, the machining process and plastic deformation of the surface layer can eventually suppress the initiation mechanism and crack propagation (due to the strain hardening and compressive residual stresses).

Acknowledgements

This research work was supported by The TESCAN Orsay holding a.s. in Brno and by the Brno University of Technology, Faculty of Mechanical Engineering, Specific research 2018, in the frame of grant "Research of perspective production technologies", FSI-S-19-6014, and also by the project CEITEC 2020, LQ1601, Ministry of Education, Youth and Sports of the Czech Republic.

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