Ding Xia TECLAB shaw@teclab.cn

abstract：With the widespread application of fiber-reinforced composite components in the production field, especially in the automotive and aerospace industries, new challenges have been posed to non-contact non-destructive testing technology due to the demand for quality control. Ultrasonic testing, as a flexible and reliable non-destructive testing technique, has been widely used for defects such as internal delamination, cracks, and voids in materials. This article introduces a new non-contact non-destructive testing device based on laser excited ultrasound and using a broadband air coupled photoacoustic sensor for reception. The device can achieve non-contact detection of bilateral transmission and one transmitter and one receiver on the same side. It is a relatively economical and practical new non-destructive testing technology.

Keywords:Laser ultrasound, optical microphone, non-contact non-destructive testing, air coupled ultrasound

introduce

Air coupled ultrasonic testing and laser ultrasonic testing are currently popular non-contact ultrasonic non-destructive testing methods. Laser ultrasonic testing generates ablation or thermoelastic effects on the surface of materials by exciting nanosecond level laser pulses to excite ultrasonic waves. The reliability of this method is limited by the optical parameters and surface properties of the tested material surface. Due to the strong scattering on the surface of materials, laser ultrasonic detection requires high-energy lasers and complex and expensive interferometers, making it difficult to miniaturize and reduce the cost of the equipment.

This article introduces a new type of non-contact air coupled ultrasonic signal receiving device, which uses broadband photoacoustic sensors to receive ultrasonic signals generated by laser or air coupled piezoelectric probes. Due to the attenuation of ultrasonic signals in air, the bandwidth of this method is limited to 10 Hz to 1 MHz. Compared to traditional air coupled piezoelectric ultrasonic reception of KHz transmission signals, this method has significant advantages. More importantly, this receiving method is independent of the surface characteristics and optical parameters of the sample. Therefore, it is particularly suitable for fiber-reinforced composite materials, especially sandwich structured materials.

principle

Figure 1: The wavelength of light in a medium depends on its refractive index and is a local density function. The propagation of ultrasound in the air causes a change in the density of the air, resulting in a change in the wavelength of light. The emission intensity of light depends on the wavelength of the medium between the mirrors. A precise Fabry Perot calibrator is used to measure the reflected light intensity through a photodiode for acousto-optic sensing.

The density p and optical refractive index n of the acoustic modulation medium, as well as the wavelength λ and refractive index of light, follow the following relationship formula:

In the formula, f represents frequency, C represents the speed of light, and λ 0 is the wavelength in vacuum.

The reflection intensity of a monochromatic laser beam between two mirrors is determined by the input intensity I₀Obtain with a transfer function TR (q).

Coefficient F=4R/(1-R) in the transfer function²R is the reflectivity of the mirror. The optical path phase shift q, laser wavelength λ (n), and mirror distance d comply with the following disclosure:

Therefore, any change in laser wavelength caused by the sound field will result in a variation in the reflected light intensity within the cavity between the mirrors. The change in light intensity is detected by a photodiode. It can be seen that this method is not affected by electromagnetic interference from sensors and signal lines.

Application Example 1 Spot Welding Transmission C-scan Imaging

Figure 2: Spot welding inspection of 20 cm x 20 cm steel plate, with a thickness of 1 mm; 1: Fiber coupled laser excitation device; 2: Spot welding samples; 3: Optical microphone.

Figure 3: Transmission C-scan imaging results of spot welding. a: A-scan signal and C-scan signal; b: Weld well with C-scan and centerline A-scan; c: Unfused spot welding C-scan imaging and A-scan signal, with a small weld diameter.

Application Example 2 Spot Welding Same Side One Send One Receive Inspection

Figure 4: Spot welding on the same side with one sending and one receiving inspection. The excitation laser is 7cm away from spot welding, and the optical microphone is located on the same side to receive signals. Scan a 5 cm x 5.5 cm area.

Figure 5: Scanning results of spot welding on the same side. a: Typical time-domain signal, guided wave signal within 110 us; B-d: Evolution diagram of broadcasting time; b: Spot welded S₀Wave and A₀weight; c. D: Diffraction of A0 mode near spot welding; e: Maximum amplitude after 20 us, distribution diagram of amplitude at the welding site.