PSI - Issue 7

M. Wicke et al. / Procedia Structural Integrity 7 (2017) 235–241

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M. Wicke et Al./ Structural Integrity Procedia 00 (2017) 000–000

1. Introduction Very low stress amplitudes occur under very high cycle fatigue (VHCF) loading or if the crack extension of long cracks in the near-threshold regime is studied. Both scenarios come together, if interest is focused on the components containing pre-existing flaws which are subjected to vibrations of very small amplitude. In that case, similar effects as observed for short cracks may occur with long cracks resulting in unexpected crack extension mechanisms. Thus, it may be worthwhile to study the effect of very small cyclic loads on the crack-extension behavior of long cracks. The determination of the long crack threshold has been standardized by ASTM E-647 and relies on the so-called load-shedding procedure, in which the range of the stress intensity factor is decreased in steps until the crack propagation rate falls below a value of typically 10 -10 m/cycle. However, crack closure effects have been reported to shift the threshold to higher values leading to numerous discussions concerning the meaning of the threshold value determined by this procedure in the past years (e.g. [Forth et al. (2003), Newman et al. (2005), Newman and Yamada (2010)]). Non-conservative estimates of the threshold value can be circumvented by an alternative approach developed by, among others, Pippan et al. [Pippan (1987), Pippan et al. (1994)]. After compression pre-cracking of specimens containing a sharp notch, the load amplitude is increased in steps until the crack propagates in a stable manner. The threshold is thus approached from below solving the problems encountered with the load-shedding procedure mentioned above. In a previous study [Stein et al. (2017)] analyzing the near-threshold behavior of long cracks in a commercial aluminum alloy we have shown that the combination of crack generation by compression pre-cracking with a subsequent crack propagation at a constant amplitude is an ideal procedure for investigating the crack extension at very low stress amplitudes. Results pointed out that there are two major mechanisms keeping the crack from continuous extension at a stress ratio of R = -1. First the crack front was pinned by primary precipitates with the amount of pinning found to be dependent on the spatial distribution of the primary precipitates. It was shown that local pinning of the crack front led to significant kinking of the fracture surface with white lines, representing ridges of ductile fracture which bridged the crack faces, extending into the crack surface. The second mechanism was shear controlled crack extension of very long cracks with plastic zones ahead of the crack tip very similar to stage-I small cracks. Misorientation fields derived from EBSD data revealed that such cracks extending in a shear-dominated mode can be deterred by primary precipitates, but remain in the shear-dominated mode afterwards. Shear-dominated crack extension as observed in [Stein et al. (2017)] can occur at a stress ratio of R = -1. This effect is well-known but rarely reported for higher R-ratios. Consequently, it may be worthwhile to study the near-threshold crack extension behavior of long cracks at R ≠ -1. To this end, fatigue crack growth tests at a stress ratio of R = 0.1 were performed in this study using the aluminum alloy investigated in [Stein et al. (2017)] in two heat treatment states with the aim of analyzing the influence of the microstructure and precipitates on crack growth. The paper is organized as follows: In section 2 the material and specimens are described, whereas the experimental methods are presented in section 3. An overview of the fracture mechanic tests is given in section 4. Crack growth of long cracks in the near-threshold regime and crack advance of small cracks are related in section 5, which is followed

by some concluding remarks. 2. Material and Specimens

The material analyzed in this study is the aluminum alloy EN-AW 6082 in peak-aged (6PA) and overaged conditions (6OA). R p0.2 and R m for the material 6PA were determined in a tensile test as 336 MPa and 344 MPa, and 262 MPa and 294 MPa for material 6OA, respectively. A rolled sheet with a thickness of 20 mm and the microstructure shown in Fig. 1a taken in rolling direction (RD) served as base material. The grains are elongated with a maximum length of up to 2000 – 3000 µm in rolling direction and 100 – 200 µm in transverse direction (TD). Two types of primary precipitates can be detected in the microstructure: Mg-based ones (dark spots in Fig. 1a) and Fe based ones (bright spots in Fig. 1a), which are arranged in a line-like fashion in rolling direction.