Experimental analysis of toughness in 6156 Al-alloy sheet for aerospace application

Analysis of toughness in 6156 Al-Mg-Si-Cu sheet has been performed using enhanced Kahn tear tests on samples quenched at different rates, whilst microstructures of the samples have been assessed using differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. Crack initiation energies were unaffected by changing water quench temperature from 20°C to 60°C, however a significant reduction was evident on air cooling. Crack propagation resistance was reduced for both 60°C water quenched and air cooled materials. The failure morphology of the air cooled material appears consistent with classical intergranular ductile failure. Coarse voiding and shear decohesion was prevalent in the 20°C water quenched material, whilst the 60°C water quenched material showed a mixture of transgranular and intergranular fracture modes. Changes in microstructure and precipitation behaviour resulting from reduced quenching rate were identified and related to the observed fracture behaviour, particularly in terms of precipitate free zone formation and the simultaneous presence of coarse particles at grain boundaries

Quench sensitivity of toughness in an Al alloy: direct observation and analysis of failure initiation at the precipitate free zone

Analysis of toughness in 6156 Al-Mg-Si-Cu sheet has been performed using enhanced Kahn tear tests on samples quenched at different rates. Crack initiation energies were hardly affected by changing water quench temperature from 20°C to 60°C; however a significant reduction was evident on air cooling. Crack propagation energy was reduced for both 60°C water quenched and air cooled materials. Observation of failure initiation through synchrotron radiation computed tomography (SRCT), for the 60°C water quenched material revealed failure ahead of the crack tip of grain boundaries oriented at 45° to the main loading axis and crack “tongues” extending into the material ahead of the main crack. Failure was predominantly intergranular. Fractographic assessment revealed predominantly voiding and shear decohesion in the 20°C water quenched material. With the aid of the new findings past models on the influence of precipitate free zone parameters on toughness have been revise

Direct observation and analysis of failure initiation and crack progression in tear toughness of two Al sheet alloys

The failure initiation and crack progression in tear toughness tests on 2 sheet alloys have been analysed through synchrotron radiation computed tomography (SRCT), supported by TEM, SEM and DSC studies. For the quench sensitive 6156 Al-Mg-Si-Cu alloy propagation energy was reduced by changing water quench temperature from 20°C to 60°C. Observation of failure initiation for a 60°C water quenched sample revealed failure ahead of the crack tip of grain boundaries oriented at 45° to the main loading axis and crack “tongues” extending into the material ahead of the main crack. The fracture toughness of AA2139 (Al-Cu-Mg-Ag) is anisotropic. SRCT reveals that three-dimensional distribution and morphology of pores and defects in the as-received state are anisotropic, with chains of voids and void elongation in the L (longitudinal) direction. For toughness testing in L-T orientation (T is transverse), voids ahead of the crack grow and link in the L direction. In T-L tests, voids ahead of the crack tip also grow in the loading direction. New models for crack propagation and toughness have been derived on the basis of the quantitative microstructural data

Experimental and numerical analysis of toughness anisotropy in AA2139 Al-alloy sheet

Toughness anisotropy of AA2139 (Al–Cu–Mg) in T351 and T8 conditions has been studied 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 were investigated via scanning electron microscopy and synchrotron radiation computed tomography. Contributions to failure anisotropy are identified as: (i) anisotropic initial void shape and growth; (ii) plastic behaviour including isotropic/kinematic hardening and plastic anisotropy; and (iii) nucleation at a second population of second-phase particles leading to coalescence via narrow crack regions. 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. Its transferability has been shown by simulating tests of large M(T) samples

Numerical investigation of dynamic strain ageing and slant ductile fracture in a notched specimen and comparison with synchrotron tomography 3D-DVC

AbstractIn ductile tearing experiments, a flat crack is often observed to develop normal to the loading direction that subsequently turns into a slant crack. The underlying physical mechanisms are poorly understood. The numerical strategy for reproducing such kind of slant fracture remains a challenge. Recently, the strain field of a 2198 aluminium alloy CT-like specimen has been measured by lamino(tomo-)graphy combined with digital volume correlation (DVC) (Morgeneyer et al. (2014)). Multiple localisation bands were observed at notch area at early loading stages. The final fracture occurred within an inclined band. Experiments showed evidences of Portevin-Le Chatelier (PLC) effect in this alloy at room temperature. Previous simulations with von Mises, anisotropic plasticity or Gurson-Tvergaard-Needleman (GTN) models were not able to simulate these observations. As the PLC effect produces instabilities and multiple inclined localisation bands, it is considered to be a candidate for the underlying mechanism related to slant fracture. A fully coupled model (Rousselier and Quilici (2015)) combining polycrystalline, PLC, porous plasticity and Coulomb fracture implemented in FE code is used for simulating these phenomena. The PLC model gives intermittent and moving oscillations of the macroscopic plastic strain rate bands. The multiple strain localisation bands obtained by FE simulation for a thin sheet CT-like specimen are similar to the ones observed in laminography with 3D DVC. Crack propagation occurs during strain rate surges. A flat to slant fracture surface observed in laminography is reproduced successfully by the current FE simulation