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Delamination is the most common failure mode in composite materials. It takes place in different modes, i.e. mode I, mode II or the combination of these modes. The present study is concerned with an investigation of mechanical and acoustic emission behavior of delamination. In this work, various lay-ups of glass/epoxy composite laminates have been used to study the delamination behavior when subjected to mode I, mode II and the mixed-mode I/II tests. First, the characterization of load-displacement curves of the specimens is done based on the AE parameters and mechanical responses and the curves were divided into three parts. The crack growth in the mode I was stable state and in the mixed-mode and mode II was unstable. In the next, interlaminar fracture toughness of the specimens, Gc, were measured using standard methodologies and acoustomechanical methodologies which is based on the mechanical behavior and AE information. It was found that the acoustomechanical method presents the lower limit of the interlaminar fracture toughness and agrees with the results that obtained from standard. The images were captured with Scanning electron microscope (SEM) from damage surfaces verifies the results that obtained from Acoustic emission.

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Despite the fact that fiber reinforced plastic composites have excellent mechanical properties, various failure mechanisms can be occurred in these materials. Delamination is the most common failure mode in laminated composites that can be occurred under quasi-static and fatigue loading conditions. The present study is concerned with the investigation of mechanical and Acoustic Emission (AE) behavior of delamination in glass/epoxy composites under mode I quasi-static and fatigue loading conditions. First, the unidirectional and woven specimens were subjected to mode I quasi-static loading. The behavior of the delamination in the specimens was investigated and interlaminar fracture toughness of the specimens was calculated. Then, according to the information that obtained from quasi-static loading, the similar specimens were subjected to the fatigue loading. The mechanical and AE behavior of the delamination under fatigue loading was investigated. A linear relationship was established between cumulative AE energy and fatigue crack growth and fatigue crack growth curve was predicted using the AE method. Then, energy release rate variations curve and fatigue crack growth rate diagram were predicted using AE method. The predicted results by AE have a good compatibility with the visually based data that recommended by standard. The results indicate that, the AE method has good applicability for health monitoring of composite structures that subjected to quasi-static and fatigue loading conditions.

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In this paper, acoustic emission monitoring of repaired aluminum 2024-T3 sheet with FML patch is studied. For the experimental investigation, 12 samples were made and classified into 4 categories according to the crack angle (zero and 45 degrees), and repaired or unrepaired state. To reduce manufacturing errors, composite prepreg is used for producing patches, aluminum surfaces is anodized and curing is done in an autoclave. In fatigue crack initiation process by using Acoustic Emission data acquisition, crack initiation moment is detected. By using Acoustic Emission signal cumulative energy parameter onset of delamination, growth of delamination and critical delamination growth is identified. SEM image and investigation of failure surface are used for detecting of failure mechanism. By introducing one frequency analysis method tried to classify frequency range of failure mechanism signals. Because of frequency range intersection of matrix cracking, fiber/matrix separation and delamination of patch from aluminum sheet, force-displacement curve is divided to 3 zone and frequency analysis is done in each zone that occurrence possibility of certain failure mechanism is higher than the others. Signal frequency range of aluminum plasticity and crack growth is in the range of 440-480 kHz, and signal frequency range of delamination is in the range of 100-150 kHz and 200-220 kHz.

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Structures during their lifetime experience plenty of static and dynamic loads. These loads cause failure or undermine the structures. So, reinforcement or repairing failed parts is one way to repair out of service structures. Composite materials have been used to reinforce structures. These materials enjoy advantages such as the proportion of their strength to their weight. As these structures get exposed to some load a number of failures get introduced. This research investigates the failure mechanisms of a notched 2024-T3 aluminum plate repaired with a composite patch using visual and acoustic emission methods. After constructing the specimens, tensile test has been conducted, and acoustic emission sensors have been stocked on the surface of the plate, so that they can record acoustic data. At the first stage, mechanical data obtained from the specimens in different states based on the number of layering have been analyzed. At the second stage, acoustic data, obtained from recording of acoustic emission signals, have been compared with the mechanical data. Also the images obtained from SEM were used to investigation of damages. According to this research, it is identified that a reasonable correspondence between the results obtained from mechanical and acoustic data and the desired functionality of the acoustic emission method in determining failure mechanism in those specimens that are repaired with composite patches.