In this thesis the influence of microstructure on fracture toughness is investigated for two different medium/high strength Al-alloys for aerospace application. In weldable AA6156 (Al-Mg-Si-Cu) alloy sheet, the quench sensitivity in toughness is assessed via enhanced Kahn tear tests. Toughness was seen to be reduced for both 60°C water quenched and air cooled materials cf. 20°C water quench material. Fractography via scanning electron microscopy (SEM) and synchrotron radiation computed tomography (SRCT), as well as Differential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM) studies, have clarified the mechanisms of the quench sensitivity with respect to toughness. Both the coverage of grain boundary decoration and precipitate free zone (PFZ) width increase with reduced quench rates. The failure morphology of the air cooled material appears consistent with classical intergranular ductile failure. Coarse voiding and shear decohesion was prevalent in 20°C water quenched material (depending on local triaxiality), whilst the 60°C water quenched material showed a mixture of transgranular and intergranular fracture modes. The experimental toughness trends are compared to models in the literature and a simple new model is suggested. Fracture toughness anisotropy of AA2139 (Al-Cu-Mg), a candidate alloy for age forming, in T351 and T8 conditions has been investigated via mechanical testing of smooth and notched specimens of different geometries, loaded in the rolling direction (L) or in the transverse direction (T). Fracture mechanisms are again investigated via SEM and SRCT. Fracture toughness is seen to be anisotropic for both heat treatment conditions tested, but is substantially reduced for the T8 condition compared to the T351. Contributions to failure behaviour have been identified with: (i) anisotropic initial void shape and growth, (ii) plastic behaviour, including isotropic/kinematic hardening and plastic anisotropy, and (iii) nucleation at a 2nd population of 2nd phase particles leading to coalescence via narrow crack regions. SRCT analysis of arrested cracks revealed alignment of voids in the crack during propagation in the rolling direction, resulting in shorter intervoid ligaments than for crack propagation in the transverse direction. Coalescence through shear decohesion in the crack initiation and propagation region was found indicating the necessity to investigate and account for this mechanism. A model based in part on the Gurson-Tvergaard- Needleman approach is constructed to describe and predict deformation behaviour, crack propagation and, in particular, toughness anisotropy. Model parameters are fitted using microstructural data and data on deformation and crack propagation for a range of small test samples. The model accounts for the material features found in the experimental study and its transferability has been shown by simulating tests of large M(T) samples showing strong fracture toughness anisotropy. A parametric study shows that nucleation of small voids at different strains for different loading directions is crucial for a correct model of toughness anisotropy; the combined effects of kinematic hardening and void growth anisotropy can not fully describe fracture toughness anisotropy.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:548211 |
| Date | January 2008 |
| Creators | Morgeneyer, Thilo F. |
| Contributors | Sinclair, Ian ; Starink, Marco |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://eprints.soton.ac.uk/64860/ |
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