Fault detection in metallic structures requires a detailed and discriminative feature pool creation mechanism to develop an effective condition monitoring system. Traditional fault detection methods incorporate handcrafted features either from the time, frequency or time-frequency domains. To explore the salient information provided by the acoustic emission (AE) signals, a hybrid of feature pool creation and an optimal features subset selection mechanism is proposed for crack detection in a spherical tank. The optimal hybrid feature pool creation process is composed of two major parts:(1) extraction of statistical features from time and frequency domains, as well as extraction of traditional features associated with the AE signals; and (2) genetic algorithm (GA)-based optimal features subset selection. The optimal features subset is then provided to the k-nearest neighbor (k-NN) classifier to distinguish between normal (NC) and crack conditions (CC). Experimental results show that the proposed approach yields an average 99.8% accuracy for heath state classification. To validate the effectiveness of the proposed approach, it is compared to conventional non-linear dimensionality reduction techniques, as well as those without feature selection schemes. Experimental results show that the proposed approach outperforms conventional non-linear dimensionality reduction techniques, achieving at least 2.55% higher classification accuracy.Energies 2019, 12, 991 2 of 14 acoustic emission signals. Detection of cracks in their early stages enables necessary measures to be undertaken in a timely fashion, thereby reducing accident occurrence. Acoustic emission (AE) signals are a promising nondestructive technology, capable of providing the information required for crack classification in the incipient stages. Compared to other nondestructive methods, AE is an economical and efficient alternative for recording the data associated with the health state of an object [6]. Additionally, the low energy signals found in AE signals can provide underlying information for substantial data-driven fault identification approaches [7,8]. Due to these benefits, AE signals are used to record data and develop a data-driven crack classification model for spherical tanks.Traditional data-driven fault identification methodologies rely on two important procedures: handcrafted feature extraction utilizing domain expertise; and detection of the health types using the extracted features. Signal-based health state diagnosis approaches rely primarily on the spectral analysis of the signals [9]. The choice of a signal analysis technique to extract discriminant information from the signals also has an impact in the performance of the fault classification process [10]. Therefore, in this study, the AE signals are analyzed in different domains to explore the detailed intrinsic information contained in the signals. The advantage of analyzing the signals in different domains is the acquisition of multi-domain knowledge of the signals, which would otherwise not...