PSI - Issue 52

Ruben I. Erives et al. / Procedia Structural Integrity 52 (2024) 600–610 Ruben Erives et al./ Structural Integrity Procedia 00 (2019) 000–000

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δ ℓ end-opening of linear extrapolated function at crack onset φ phase angle of opening M i moment acting one side of the DCB specimen a 0 initial crack length (slip foil) L AE distance between the AE sensors B width of the DCB specimen 2H thickness of the DCB specimen

1. Introduction Many modern structural components used in aerospace, marine and wind energy industries are made up of fibre reinforced polymer matrix composites. These materials are arranged in layers, where often there exist weak planes such as interfaces between the individual layers. Delamination is one of the commonly observed damage mechanisms along these weak interfaces. The delamination crack growth can occur in different mode mixities and depends on the external loads acting on the laminate. Characterization of delamination fracture resistance in terms of J integral is done using fracture mechanics tests such as the double cantilever beam (DCB) specimens (Sørensen and Jacobsen 2009). A primary challenge in delamination related fracture mechanics tests is the accurate determination of the onset of crack initiation (Zhang et al. 2015). Visual inspection is the simplest method to observe the crack initiation and propagation during the testing and requires no additional equipment. However, the main disadvantage of this method is that the crack becomes visible only when it is grown to edges of the test specimen. Over the years, several non-destructive techniques have been developed, briefly, these can be categorized into active and passive techniques. Active techniques involve exciting the composite laminate separately by an external source and based on the response signal received damage can be detected. On the other hand, passive techniques do not require any external source and the damage can be detected while the composite laminate is being tested. Acoustic emission (AE) is one of the most widely used passive damage monitoring techniques. When cracking occurs within a test specimen, the release of accumulated strain energy generates elastic waves which can be recorded by AE sensors mounted on the specimen surface (Morscher and Godin 2015). One or multiple or a combination of AE features such as number of counts, number of hits, peak amplitude, and peak energy are usually used to characterize the damage initiation in composites. In some studies, the first identified AE event is considered as the onset of delamination (Marec et al. 2008, Oskouei and Ahmadi 2010, Fotouhi et al. 2015, Kharrat et al. 2018). Alternatively, one may use cumulative events, cumulative energy, cumulative counts as features and with the first notable increase in the gradient of these curves can be considered as the onset of damage initiation (Nikbakht et al. 2017, Saeedifar et al. 2018). Davijani et al. 2011 and Barile et al. 2019 used sentry function, defined as logarithmic of the ratio of mechanical energy to AE energy, to uncover the delamination in composite DCB specimens. Based on the trend of the sentry function, delamination initiation and propagation in the test specimen was correlated. For the increasing trend: it indicated that no damage has occurred, for a sudden drop: damage has occurred, for a constant trend: damage growth is impeded by mechanisms like fibre bridging and for a decreasing trend: damage growth is unstable with the loss of load-carrying capability. In view of this, the first significant drop in the sentry function curve is deemed as the damage onset. However, this curve behaviour-based damage prediction is a qualitative criterion that can be interpreted in many ways. Similar approaches have been used for cyclic tests, and different test configurations such as unidirectional tests (e.g., Kalteremidou et al. 2022) Despite the different approaches that exist in the literature, there are still some open challenges in AE-based detection of damage. When the first AE event is considered as the damage initiation, it could lead to conservative