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Quality Control and Characterization
Published in B. T. Åström, Manufacturing of Polymer Composites, 2018
Although not common with composites, it is also possible to locally introduce a sound wave into a component using a laser; both pulse-echo and through-transmission laser ultrasonics have proved feasible. When heated by the laser beam the component surface locally expands to generate a sound wave, which once introduced into the material behaves in the same fashion as discussed above; reflected sound waves are detected by a laser. Laser ultrasonics has advantages in not requiring a coupling medium or a normal angle between laser beam and component, thus facilitating inspection of very complex geometries. Disadvantages include that care must be taken not to damage the component thermally and that, since sound waves are weak, background noise may lead to reduced contrast between flawed and unflawed material.
Non/Loosely Contacting NDE Techniques
Published in J. David, N. Cheeke, Fundamentals and Applications of Ultrasonic Waves, 2017
Laser ultrasonics comprises the generation and/or detection of ultrasonic waves by laser techniques. Historically, practical interest in the use of lasers and optical effects of ultrasound was for visualization of ultrasonic fields. However, as the technology developed, laser ultrasound became a recognized NDE tool in its own right, and it has now become perhaps the most widely used of the noncontact approaches. Some of its principal advantages are as follows: No need to make an acoustic bond to a transducer.The user can control the position and shape of the source.Ability to access hostile environments, such as high temperature.A variety of detection methods are available.Can use in situ.Applies to curved and complex shapes.
A review of NDE techniques for hydrogels
Published in Nondestructive Testing and Evaluation, 2023
Sasidhar Potukuchi, Viswanath Chinthapenta, Gangadharan Raju
Despite such a wide range of applications, UT has some drawbacks. For instance, Scruby [21] reported that acoustic techniques have low sensitivity to slow-varying events. Therefore, it is not very useful in tracking quasi-static variations in the physical (water diffusion) and mechanical behaviours (low strain rate tension or cutting) of gels. The total focusing method used for ultrasonic imaging is one of the most versatile methods due to its use of transducer arrays instead of a single transducer. But this method had limited acoustic power resulting in a lower signal-to-noise ratio for attenuating materials, which is a characteristic of some gels [35]. Also, this method has a limit on the frame rate because it would require the processing of N2 signals for every N transmission. This method is efficient when the number of signals to be processed is low [36]. Laser ultrasonics is a non-contact NDE method that generates and detects ultrasonic waves using lasers. It is used for depth evaluation but is time-consuming and requires good operator skills [37]. Solodov [38] reported that increasing the vibration amplitude of the specimen led to nodal lines in the standing waves that caused a ‘missing’ of the defect.
Comparative study of active infrared thermography, ultrasonic laser vibrometry and laser ultrasonics in application to the inspection of graphite/epoxy composite parts
Published in Quantitative InfraRed Thermography Journal, 2020
V.P. Vavilov, A.A. Karabutov, A.O. Chulkov, D.A. Derusova, A.I. Moskovchenko, E.B. Cherepetskaya, E.A. Mironova
Laser ultrasonics utilizes broad-band ultrasonic waves generated by a laser pulse. The waves scattered by structural inhomogeneities of composites can be analyzed with a high temporal resolution thus allowing depth evaluation more efficiently that in the case of conventional ultrasonic NDT. The probe pulse width is 70 ns to provide the frequency band from 0.2 to 6.6 MHz at half maximum and the in-depth resolution of about 0.2 mm in metals like steel and aluminium. The 65 dB dynamic range allows investigation of materials with high ultrasonic attenuation, such as composites.
Nondestructive evaluation of rebar corrosion in concrete structures using ultrasonics and laser-based sensing
Published in Nondestructive Testing and Evaluation, 2022
Prasanna Kumar Mayakuntla, Debdutta Ghosh, Abhijit Ganguli
The issue of coupling of the transducers to the concrete surface has been partially circumvented through the development of dry contact shear wave transducers [31,32], which have been used to detect deterioration in concrete pavements [33]. To eliminate the effect of coupling, Choi et al. [34] have applied air-coupled ultrasonic transducers and dry contact shear wave transducers for the detection of artificial defects in concrete and for thickness measurements in RC columns. Air-coupled transducers have been applied for detection of cracks in concrete [35] and for determination of the depth of surface breaking cracks in concrete slabs [36]. The limitation of this technique is that the extent of penetration of ultrasonic energy by overcoming the difference in the acoustical impedance between the concrete and the air is not very clear. Further evolutions include fully non-contact laser ultrasonics that employs a high-energy laser pulse-based excitation on the material surface and subsequent reception of the waves, using a laser vibrometer. Recent work in [37,38] reports on the application of this technique for detection of voids in prestressed concrete tendon ducts and RC specimens and automatic crack visualisation in plate structures [39]. The measurement of the diameter of artificially drilled holes in aluminium samples and the investigation of rebar corrosion and debond regions around rebars in nuclear power plants [40,41]. Assessment of rebar debonding in mortar specimens has been carried out using laser vibrometer [42]. However, due to the high energy associated with the laser generation, practical application of this technique has been difficult. Therefore, a hybrid ultrasonic methodology involving the advantages of both contact and non-contact ultrasonics has promise, until fully non-contact techniques that are both safe and efficient are developed.