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One of the intricate issues of using acoustic emissions (AEs) for mechanical diagnostics in composite materials is establishing a relation between acoustic features and fracture behaviors. Therefore Hilbert Transform (HT) which is a new method to investigate fracture and damage evolution were used to analyze AE features of cross ply polymer composite material subjected to ENF test which simulate mode II delamination. Two different mid-plane interface layups are used. HT is used to extract frequency range of each damages occurring in different stages of loading process for each layup. Phase angle of HT is used as a novel feature to distinguish frequency ranges of AE waveforms and the resulting ranges are correlated to the damage mechanisms of the material under investigation. Three important stages in mode II delamination, i.e. initiation, maximum load nearby and the region where crack reaches to the middle of the specimens, are investigated using the introduced method and the details of damages in those regions are characterized. The results of Hilbert transform was in good agreement with scanning electron microscope (SEM) pictures. As a result, it is shown that AE method accompanied with the presented method is a new appropriate approach to examine damage mechanisms in composite materials.

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