Evaluation of Acoustic Emission Signals to Detect Stress Corrosion Cracking Mechanisms of AA7075-T6 Aluminum Pipe by Fuzzy Clustering Method (FCM)

High strength aluminum alloy pipes (AA7075-T6) has been widely used in various industries due to their favorable properties, Such as high strength to weight ratio, excellent machinability and proper forming. On the other hand, according to the susceptible to phenomenon of stress corrosion cracking, the evaluation and monitoring of mechanisms related to this issue is of considerable importance in the industry. In this research, Fuzzy clustering method (FCM) has been used to detect acoustic emission signals related to stress corrosion cracking mechanisms of aluminum pipe. Using this method, while revealing the clusters in an unsupervised method, leads to more appropriate classification and separation of data. In this regard, a laboratory system including waveguide, corrosive solution chamber, and arc-shaped aluminum samples according to ASTM-E399 standard along with acoustic emission equipment was designed and prepared. Then, using the slow strain rate test (SSRT) and receiving AE signals simultaneously, the AA7075-T6 aluminum sample was placed in two corrosive environments (HCL9% and HCL33%) to identify the two main mechanisms of stress corrosion cracking, including anodic dissolution and hydrogen embrittlement (cathodic). Five descriptive acoustic parameters including rise time, count, energy, amplitude and duration were used to analyze the obtained signals. In order to select the most effective acoustic characteristics and reduce the amount of information, principal component analysis was used. Subsequently, with the fuzzy method (FCM clustering) based on the optimized data from the analysis of the main components, it was possible to distinguish and separate these two types of corrosion mechanisms. According to the obtained results, it was found that the dominant mechanism in the phenomenon of stress corrosion cracking in HCL corrosive solution is the anodic dissolution mechanism, which in addition to increasing the corrosion current density, increases the dependence of stress corrosion on this mechanism by increasing the concentration of H+ and C- ions.