The durability and safety of wood constructions and wood products are susceptible to micro-cracks in the wood. Acoustic emission technique can be used to evaluate the crack growth and propagation in woods. Acoustic emission characteristics signals for wood are affected by numerous factors, including moisture content, wood species, loading type and density. In addition to how these factors influence the characteristic acoustic emission signals, their relationship with the mechanical properties of wood has been the focus of most studies.
Generally, softwoods have more acoustic emission counts than hardwoods, suggesting that softwoods are more ductile and facilitate the creation of a process zone consisting of more micro cracks. Under fatigue and static torsional loading, however, the failure process of hardwoods produces more acoustic emission counts than softwoods. As such, the testing and loading methods considerably influence the acoustic emission counts. In addition, the sensor location has also been found to influence the acoustic emission counts. It is noteworthy that analysis based on the correlation between acoustic emission signals and other factors is useful not only in assessing the damage but also distinguishing the possible causes of such damage processes.
Being a non-destructive method, acoustic emission can be combined with digital correlation imaging technology. The combination is deemed effective for understanding a wide range of problems, such as the underlying damage and fracture mechanism, biological decay effects and moisture effects. However, the extreme variability of wood properties presents a big challenge for accurate result interpretation. Therefore, further studies are still needed to facilitate the effective application of acoustic emission in wood and wood objects.
To this note, Professor Wengang Hu from Nanjing Forestry University and Dr. Jilei Zhang from Mississippi State University investigated the failure process of southern yellow pine (SYP) blocks under compact tension load in the radial longitudinal crack system. They searched acoustic emission signals to quantify or differentiate the cracking behaviors with crack tips located at earlywood, early-latewood interface and latewood. The effects of growth rings on the acoustic emission characteristic signals and fracture toughness of the SYP were also investigated. Finally, the failure modes of SYP were analyzed to illustrate the relationship between the wood microstructure and acoustic emission characteristic signals. The work is currently published in the journal, Construction and Building Materials.
The authors showed that the crack tip locations significantly affected the counts and energy of the acoustic emission characteristic signals but not the acoustic emission amplitude. It was possible to characterize all the investigated acoustic emission characteristic signals versus time curves as three distinct stages: initiation, acceleration and falling. Cracking at earlywood and at the early-latewood interface generated more total acoustic emission cumulative counts than at latewood. However, the difference in the total acoustic emission cumulative at the earlywood and at early-latewood was not significant.
In the initiation stage, the cumulative acoustic emission counts at earlywood were remarkably greater, followed from a distance by that at early-latewood and lastly that at latewood. In the acceleration stage, the acoustic emission count rates at earlywood were greater, followed by that at latewood and early-latewood interface. However, the cumulative acoustic emission counts at earlywood and early-latewood were significantly greater than at latewood. Furthermore, it was observed that the fracture toughness of the SYP blocks had a strong negative linear correlation with their total cumulative energy and maximum acoustic emission counts of the SYP blocks.
In summary, the effects of growth rings on the acoustic emission characteristic signals of SYP wood were investigated in terms of the crack tip location. The different patterns in the acoustic emission characteristic signals were useful for the non-destructive detection of crack generation in woods. In a statement to Advances in Engineering, Professor Wengang Hu stated that their findings provided valuable insights that would contribute to enhancing the durability, safety and performance of wood products.
Hu, W., & Zhang, J. (2022). Effect of growth rings on acoustic emission characteristic signals of southern yellow pine wood cracked in mode Ⅰ. Construction And Building Materials, 329, 127092.