Using acoustic emissions (AE) to monitor mode I crack growth in bonded joints

The usual way to evaluate the fracture toughness of bonded joints is via experimental characterization of the critical strain energy release rate. Different test procedures and data reduction methods for mode I fracture characterization can be found in the literature, such as ISO-25217, where crack length measurement is required. However, obtaining an accurate visual determination of crack length is often a challenge due to large fracture process zones (FPZ) and difficulties in reaching the bonded path. To compensate, structural health monitoring (SHM) techniques such as embedded Fiber Bragg Grating (FBG), digital image correlation (DIC), backface strain gauges and ultrasonic inspection are used as crack length monitoring. However, experimental work demonstrates the need for experimentation with non-intrusive methods. In the present work, acoustic emission (AE) testing is proposed to measure mode I crack growth in bonded joints. This is valid both for rigid adhesives with a small FPZ ahead of the crack tip and for flexible adhesives in which a correlation between AE event location and external analytical and numerical models confirm that a large FPZ is behind the crack tip. A correlation between the AE source location, visual location and numerical models determines the nature of AE events during the fracture process.