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