Researchers investigated the vibration patterns resulting from laser-induced shock waves to identify defects in concrete structures.
Experimental setup used to perform the non-destructive telemetry test. Scientists from Shibaura Institute of Technology (SIT)developed a new non-destructive method to investigate cracks in concrete structures. They used shockwaves generated close to the surface of the structure by focusing the beam from a high-powered laser, and defects were detected by analyzing the vibrations induced in the structure. Image Credit: Professor Naoki Hosoya from SIT, Japan.
Regular testing is required for big concrete structures to identify defects that might compromise their stability. Generally, while acoustic tests executed by certified inspectors are the inspection technique of choice, these tests take a long time to perform as the structure’s size tends to increase.
In a new study performed, researchers have come up with a new technique to determine defects with the help of shock waves produced by the laser-induced plasma. The new method is comparably quicker, entirely non-destructive and enables the timely evaluation of concrete infrastructure.
Nothing has been set truly in concrete, and this is particularly true for structures that are made of concrete. When structures made of concrete including buildings, bridges and tunnels are loaded repeatedly over a long period of time, they tend to form cracks that might progress and lead to structural failure. Hence, regular inspections are required to detect cracks before they turn out to be a cause for worry.
Traditionally, defects present in concrete structures are detected with the help of the acoustic test known as the “hammering method” performed by certified building inspectors. But these tests take time to complete, and as with most skill-based techniques, the effectiveness of the test is based on the expertise of the inspector.
Furthermore, as the number of aging infrastructures is on the rise, a technique of inspection that is fast and dependable seems to be paramount for guaranteeing safe operation and the structure’s long-term use.
An alternative inspection technique for testing includes generating shock waves next to the surface of the concrete structure. The shock waves tend to induce vibrations on the structure which can be examined to detect defects.
But in such tests, it is essential to produce shock waves that do not cause any harm to the structure. Laser-induced plasma (LIP) shock wave excitation has shown great potential for this. The method has been utilized to detect defects in a range of structures, varying from pipes to fruit surfaces. In this method, the shock waves are produced by colliding laser-generated plasma with air.
In a new study, scientists from the Shibaura Institute of Technology and the National Institutes for Quantum Science and Technology, Japan, tested the efficiency of this technique at detecting cracks in concrete structures.
We used LIP shock waves as a non-contact, non-destructive impulse excitation. This allows for remote and completely non-destructive detection of defects in concrete structures.
Naoki Hosoya, Study Corresponding Author and Professor, Department of Engineering Science and Mechanics, Shibaura Institute of Technology
The study findings have been reported in the International Journal of Mechanical Sciences.
To assess the new technique, the scientists exposed a concrete block that consisted of an artificially made defect to a shock wave produced by a high-power pulsed laser. The vibrations were examined at multiple points on the concrete surface inside and outside the defect region.
The analysis disclosed the existence of Rayleigh waves at the site of the defect. These are surface waves that tend to move at a faster velocity compared to other shock waves. The scientists were successful in determining the defect areas by detecting the points where there was a reflection of these Rayleigh waves.
Defects in the concrete specimen can be detected and the location of the approximate boundary can be determined using the propagation of Rayleigh waves.
Naoki Hosoya, Study Corresponding Author and Professor, Department of Engineering Science and Mechanics, Shibaura Institute of Technology
By imagining Rayleigh waves, defects in a structure could be detected much quicker compared with other telemetric methods which examine vibrations. This makes it a hopeful method for the non-destructive testing of concrete structures.
The advantage of using Rayleigh waves to detect defects is that fewer measurement points are necessary compared to measuring the natural mode. Additionally, the time required for defect detection can be shortened. Visualizing Rayleigh waves propagation has potential for practical detection of the configurations and defects in concrete.
Naoki Hosoya, Study Corresponding Author and Professor, Department of Engineering Science and Mechanics, Shibaura Institute of Technology
The use of LIP shock waves to evaluate cracks in concrete structures is a secure and quick technique that can be utilized to retain infrastructure and avoid structural failure.
Journal Reference:
Wakata, S., et al. (2022) Defect detection of concrete in infrastructure based on Rayleigh wave propagation generated by laser-induced plasma shock waves. International Journal of Mechanical Sciences. doi.org/10.1016/j.ijmecsci.2021.107039.