PSI - Issue 13

F. Bülbül et al. / Procedia Structural Integrity 13 (2018) 590–595

591

Fatih Bülbül / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

The propagation behaviour of long fatigue cracks in aluminium alloys under Very High Cycle Fatigue (VHCF) loading is not yet understood in detail. Due to the very small dimension of the cyclic plastic zone in front of the crack tip, a strong interaction with microstructural features must be expected leading to deviations in the conventional long crack regime. Crack initiation in the VHCF regime often was detected in the interior underneath the material surface [Höppel et al. (2010), Schwerdt et al. (2011)] leading to crack propagation without contact to the surrounding atmosphere. Thus, this crack propagation takes place in vacuum. Studies by Stein et al. [Stein et al. (2017)] and Wicke et al. [Wicke et al. (2017)] in laboratory air in the Δ K region of the long fatigue crack threshold value have shown that shear-dominated long crack propagation can occur, if the applied load is small enough. Moreover, primary precipitates can pin the crack front leading to a local retardation of the crack propagation, whereby the amount of the pinning effect significantly depends on the spatial distribution of the primary precipitates. Studies by Bülbül et al. [Bülbül et al. (2018)] have ascertained that in vacuum very pronounced single slip takes place leading to shear-stress-controlled VHCF long crack propagation in EN-AW 6082 in the peak aged (pa) condition. In this study, the interaction between the VHCF long crack propagation and the microstructure is investigated in detail for the aluminium alloy EN-AW 6082 (pa). Fatigue experiments with constant and increasing stress amplitude were carried out in order to identify the change in the crack propagation mechanism. Possible effects of the microstructure on the long crack propagation will be discussed in this paper. The fatigue experiments in vacuum were performed on the aluminium alloy EN-AW 6082 in the peak aged (pa) condition. The aluminium alloy was delivered as sheet material exhibiting a microstructure with a rolling texture. Thus, the microstructure shows elongated grains in rolling direction (Fig. 1a). The heat treatment of the material was performed at the Technische Universität Dresden. The hardenable aluminium alloy was homogenized at 540 °C for 1 h, followed by water quenching and four-hour artificial aging at 200 °C. Figure 1b, which was taken at the Technische Universität Dresden, shows needlelike secondary precipitates generated by the heat treatment. 2. Experimental

Fig. 1. (a) Microstructure showing elongated grains; (b) semi-coherent needlelike secondary precipitates showing a preferred orientation; (c) in situ setup of the ultrasonic fatigue testing system with small vacuum chamber.

For the secondary precipitates an average length of 67 nm with a standard deviation of 20 nm and an average width of 5nm with a standard deviation of 1 nm were experimentally determined. The secondary precipitates are semi-coherent obeying preferred orientation relationships. Figure 1c shows the in-situ installation of the ultrasonic fatigue testing system (USFT). The fatigue experiments were carried out in vacuum under fully reversed tension-compression loading (R = -1) at a resonance frequency of about 19.2 kHz. A strict compliance with the temperature limit (30 °C) for the ultrasonic specimens was achieved by adjusting the pulse and pause sequences in the USFT. The ultrasonic specimens contain a single shallow-notch in order to monitor the long fatigue crack propagation by means of the long distance microscope. Prior to the fatigue