###### Abstract  
Understanding the deterioration and predominant damage mechanisms of structures is highly relevant, especially for safety-critical components. Non-Destructive Testing (NDT) plays a crucial role in assessing and monitoring their integrity by evaluating damage evolution. However, when it comes to complex structures, the existing NDT methods face challenges in their application, as is the case with sandwich structures. This study employs two NDT methods to analyze the initiation of damage in such structures during a fatigue test. The Acoustic Emission (AE) technique utilizes membrane-free microphones with a broad bandwidth to capture acoustic events from difficult-to-access areas. A machine learning algorithm is used to classify these events to determine their source and associated damage mechanisms. Additionally, Digital Image Correlation (DIC) is employed to measure strain evolution without contact during the test. This method is particularly relevant due to the complex and thin geometry of sandwich structures, where other methods are not applicable. Strain redistributions are considered relevant damage indicators. The results indicate that Acoustic Emission serves as an early indicator of damage, with the cumulative number of events and peak frequency correlating well with the severity of the damage. In contrast, DIC revealed clear indications of damage or deterioration, albeit at a later stage compared to AE. acoustic emission; digital image correlation; sandwich structures; damage mechanism +
Footnote †: journal: Medical Imaging Sciences  
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**Acoustic Emission and Digital Image Correlation-Based Study for Early Damage Identification in Sandwich Structures**  
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**Acoustic Emission and Digital Image Correlation-Based Study for Early Damage Identification in Sandwich Structures**  
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**Acoustic Emission and Digital Image Correlation-Based Study for Early Damage Identification in Sandwich Structures**  
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in their production is the brazing of plates and fins. Due to the presence of a large number of fin-plate joints in a conventional heat exchanger, ensuring the strength and reliability of these joints is essential [5; 6; 7; 8]. Considerable efforts have been directed towards detecting and identifying defects in these brazed joints, as they play a critical role in ensuring the proper performance of heat exchangers during operation [9; 10]. Considering the main requirements for the inspection is crucial when selecting the most suitable technologies. In this context, damage initiation is defined as a permanent loss of integrity that reduces the structure's capacity to withstand loads, ultimately leading to failure after several cycles. Early identification of such damage is essential, providing a wider margin, in terms of the number of cycles to failure, before the component becomes compromised. Therefore, experimental techniques capable of early identification of damage are crucial. The target defects in this work include fins breakage or fins debonding. Another requirement is the real-time identification of defects to ensure early detection. Additionally, the system should withstand high temperature operations, limiting the use of monitoring systems to non-contact methods or methods capable of enduring high temperatures for extended periods. With respect to the literature, non-destructive testing has been practiced for a long time. Techniques such as Eddy Current have been used in heat exchangers in industrial pressure vessels for periodic inspections. Some authors [11] used it to identify circumferential cracks during maintenance inspection with good results. Remote visual inspection by a videoscope is another common practice, where a small chip at the tip of the videoscope can be introduced into difficult to access areas. However, when it comes to sandwich structures with alternating layers, traditional methods face challenges due to the tight spacing among the fins (a few millimeters).