Among the failure modes of composite materials, delamination is the most striking one. It may result in the reduction of stiffness and long-term performance of composite materials. The initiation stage is very crucial in delamination. This is because initiation stage requires higher amount of force, and after this stage, there is a stable propagation of delamination that requires little force and causes the failure of the structure. Initiation of delamination consists of two main stages including minor initiation and major initiation. These two stages are vital stages in the delamination process. Understanding the delamination behavior and their internal micro events in these stages may lead to better design and result in enhanced strength against crack initiation. This article proposes a combined method to discriminate various fracture mechanisms in the initiation of delamination. The combination of sentry function analysis and frequency analysis of acoustic emission (AE) waveforms are used to analyze crack initiation and to discriminate its internal micro failures during mode I delamination. Discrimination process is derived from the power spectrum density of AE waveform in distinct frequency intervals. Scanning electron microscopy (SEM) observation was used to determine different fracture mechanisms and associate these observed fracture mechanisms and their equivalent AE signal frequency range. It is shown that this combined method is a powerful device to study the behavior of the initiation of delamination.

In this study, two types of specimens, woven [0, 90]_s and unidirectional [0]_s glass/epoxy composites, were studied. Acoustic emission (AE) monitoring with wavelet-based signal processing technique and Fuzzy C-Means (FCM) clustering method was used to detect the different fracture mechanisms during delamination in quasi-static three-point bending test. The three-point bending test simulates thrust force, the most effective factor in delamination, throughout the process of drilling without backup plate. The obtained AE signals were decomposed into various wavelet levels and classified using FCM. Scanning Electron Microscopy (SEM) observation was used to determine the different fracture mechanisms. The results show that the energy of AE signals has been concentrated in two significant components for both types of specimens, and the dependency percentage of damages in the two specimens is different.

In this article, acoustic emission monitoring was used to identify different types of damage mechanisms which occur during delamination of sandwich structures. Features of the obtained acoustic emission signals and wavelet-based signal processing technique were utilized to discriminate failure mechanisms during the delamination test. Dominant damage mechanisms were classified based on their power spectral density in distinct frequency ranges and the energy distribution criterion in each component. The scanning electron microscopy was also employed to verify the results which were obtained from the acoustic emission investigation. It was concluded that frequency domain analysis and wavelet analysis are efficient tools for identification and discrimination of different damage mechanisms in sandwich structures.

In this study acoustic emission monitoring was used for detection and classification of damage mechanisms during mode I delamination test of glass/polyester composites. For this purpose, the integration of harmony search and k-means algorithms was applied to cluster acoustic signals generated during delamination test. The clustering analysis represented three clusters, each one related to a distinct damage mechanism. Considering the relationship between acoustic emission parameters and damage mechanisms, the acoustic emission signals of each cluster were assigned to a distinct damage mode. Furthermore, the dominance of various damage types was investigated based on the distribution of signals in different clusters. Finally, scanning electron microscopic observation was employed to verify the obtained results. The results indicate good performance of proposed algorithms in damage classification procedure.

Delamination is the most common failure mode in laminated composites and it leads to loss of structural strength and stiffness. In this paper, acoustic emission monitoring was applied on the carbon/epoxy laminated composites when subjected to mode I, mode II, and mixed-mode I and II loading conditions. The main objective is to investigate delamination behavior and to predict propagation curve of the delamination in different G_II/G_T modal ratio values by the acoustic emission. First, a combination of acoustic emission and mechanical data (sentry function) is used to characterize the propagation stage of the delamination. Next, the crack tip location during propagation of the delamination in the specimens is identified using two methods. In the first method, by determining the velocity of the acoustic emission waves in the specimens, the position of the crack tip can be estimated throughout the tests. In the second method, the cumulative energy of the acoustic emission signals is utilized for localization of the crack tip. Agreement between the predicted crack length and the actual crack length verifies the presented procedures. It can be concluded from the results that the acoustic emission method is a powerful approach to investigate the delamination behavior and to estimate the crack tip position in the composites.