Impact of machine stiffness and heat treatments on crack propagation instabilities in an Al-Mg-Si alloy
Résumé
This study investigates rapid but limited crack propagation associated with a sudden decrease in load some-times observed during fracture toughness tests. This phenomenon is often referred to as “pop-in”. Tensile and compact tension specimens made of 6061 Al-alloy were machined from blocks which had been subjected to different isothermal ageing (4 or 8 or 12 or 16 h at 175 °C) heat treatments to obtain different mechanical properties and keep the same microstructure except the nanometric hardening precipitates. Ten-sile and fracture toughness tests have been performed. The observed mechanical behavior was correlated to metallurgical characterizations in two dimensions (scanning electron microscopy, EBSD, profilometer) and three dimensions (tomography, synchrotron laminography, atom-probe tomography). An influence of ageing time on both tensile and fracture toughness properties was revealed. Increasing the ageing time reinforced the yield and tensile strengths. For 4 and 8 hours of ageing time the material investi-gated did not show pop-in occurrence whereas pop-in is observed on 50% of specimens after 12 hours and on 100% after 16 hours. 3D scans were made via synchrotron laminography (ESRF, France) on flat specimens extracted from a CT specimen, containing a stopped crack. Reconstructions show very rough crack surface and the heterogeneous presence of large clusters of porosity at the crack tip. However, no difference between the differently heat treated materials and associated tearing moduli was found in terms of fracture mechanism. An innovative assembly has been designed in order to vary machine compliance during a toughness test. It is shown that the appearance of pop-in is related to the competition between crack growth toughness, i.e. tear-ing modulus and the double system stiffness (contributions of machine and specimen). An instability criteri-on has been investigated accounting for machine and sample stiffness and displacement considerations. It showed an excellent agreement with the experimentally found pop-ins.
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