PSI - Issue 42

Kamila Kozáková et al. / Procedia Structural Integrity 42 (2022) 270–275 K. Koza´kova´ et al. / Structural Integrity Procedia 00 (2019) 000–000

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The presence of a notch in a specimen (or in a component) significantly a ff ects the behavior of material and can lead to unexpected short lifetime of components. Cracks usually initiate in notches, that act as stress concentrators. The article aims to present the results of the fatigue tests of notched and smooth specimens using the ultrasonic fatigue machine. Also, an e ff ect of notch machining and its impact on the fatigue lifetime is studied.

2. Material

An EN AW 7075 - T6511 aluminum high-strength, curable alloy was employed in this study. The nominal chem ical composition is listed in Tab. 1. A heat treatment such as solution annealing, stress-relieving by stretching, and subsequent artificial aging were applied. Due to the high strength and low weight of the material, this alloy is used for structural components in defense technology and aviation. The test samples were machined from extruded bars.

Table 1: Nominal chemical composition of the experimental material in wt. % Jambor et al. (2020)

Al Other-each Other-total 87.1-91.4 5.1-6.1 2.1-2.9 1.2-2 0.18-0.28 max. 0.5 max 0.3 max. 0.4 max. 0.2 max. 0.25 max. 0.05 max 0.15 Zn Mg Cu Cr Fe Mn Si Ti Ti + Zr

The microstructure of AW7075 alloy consists of the solid solution of Zn in Al with intermetallic phase particles. The characteristic feature is the elongation of grains in the direction of extrusion. Longitudinal and transverse cuts are shown in Fig. 1 and 2. The Al 2 CuFe, Al 2 CuMgZn, Al 7 Cu 2 Fe intermetallic phase particles can be observed at elongated grain boundaries Fintova´ et al. (2020). There are small sub-grains crated during the extrusion in the large grains (Fig. 3). A number of fine strengthening particles in the grains were created during the aging. Grain orientation was analyzed by electron backscatter di ff raction (EBSD) technique. The high angle grain boundaries were identified with the grain misorientation of 10° and the low angle boundaries (characteristic for subgrains) with a grain misorientation of 2°. In Fig. 4, the grain elongation is observed in the direction of extrusion, whereas the individual grains are oriented randomly. A detailed description of the material and its preparation for microstructure analysis is in Fintova´ et al. (2020). An e ff ect of surface treatment of notches in AW 7075 alloy is studied also in Jambor et al. (2020).

Fig. 1: Grains and particles of intermetallic phases on the grain bound aries, longitudinal cut, SEM, BSE

Fig. 2: Grains and particles of intermetallic phases on the grain bound aries, transverse cut, SEM, BSE

Fig. 3: Sub-grains in large primary grains, transversal cut, SEM, BSE Fig. 4: Primary grains, subgrains and their orientation, SEM, EBDS