Influence of cold expansion and aggressive environment on crack growth in aluminum alloy
PI Alberto Mello
This research aims to establish the effect of hole cold expansion on fatigue life of pre-cracked material under aggressive environment.
This research investigates the relationship between crack propagation and secondary crack initiation in aluminum alloys with cold worked holes subjected to cyclic loads to determine the impact on fatigue life of joints in presence of aggressive environment. We work with experiments and analysis of fatigue life of bolted joints with coldworked holes in presence of galvanic corrosion. This investigation is examining the effect of local plastic deformation and localized galvanic corrosion on small cracks and fatigue life of bolted joints. The benefits of cold work are well known and its application is widely used in new and repaired structures, even in crack arrester holes. However, coldworked holes are usually fastened to dissimilar materials, what may induce localized galvanic corrosion. When applied in the field, damaged material removal in a cold work procedure may be limited to a maximum allowable diameter for reaming and finishing, what may leave micro/small cracks on the strained region. To completely understand the effect of initial cracks as a function of initial plastic deformation level in a coldworked hole it is necessary to fully evaluate strain distribution during and post cold work with microscopic detail. In a first approach, we have analyzed (FEM and classic analysis) and measured strain distribution during the process using digital image correlation (DIC). In the next step, we have tested specimens under fatigue. Pre cold work induced micro cracks was monitored in-situ via digital optical microscopy. In sequence, the coldworked holes were filled with a dissimilar material fastener in saline environment and the impact of galvanic corrosion on crack growth rate was determined for AA 2024-T3.
In a next step, we will further investigate the formation of critical secondary cracks. The probable cause could be a local corrosion around cathodic precipitates, but a detailed study is necessary to confirm this hypothesis. The tested samples must be prepared for use in scanning electron microscope (SEM) to identify the local pit formation at the plate edge, find the point of crack initiation, and determine the propagation path. Using striation counting technique, we may be able to estimate the number of cycles to failure and, consequently, the time necessary for the crack initiation under aggressive environment. Additionally, we will plan to use and analyze a special ceramic coating to mitigate galvanic corrosion effect on loaded components.