Abstract
To gain a better understanding of the mechanisms operating during the first stage of fatigue-crack propagation in aluminium alloys, large slip-band facets of an age-hardened Al-Zn-Mg alloy, produced by fatiguing notched single-crystal specimens, were studied in detail using optical and electron fractographic methods. The features observed on the fracture surfaces were found to be a function of (i) crack propagation velocity, (ii) orientation of the crystallographic deformation modes with reference to macroscopic crack-propagation direction, and (iii) moisture level in the test environment. The observations have led to the following conclusions: (1) crack propagation in crystallographically favoured slip directions is energetically favoured; (2) nucleation and propagation of secondary cracks ahead of the main crack front plays an important role during slipband cracking, propagation of secondary cracks occurring parallel to the slip band preferentially in crystallographically favoured slip directions; (3) moisture in the test environment introduces a cleavage component into the fracture process and suppresses secondary cracking.
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