Non-contact ultrasonic techniques

doi:10.1016/j.ultras.2004.01.101

Ultrasonics

Volume 42, Issues 1–9, April 2004, Pages 9-16

https://doi.org/10.1016/j.ultras.2004.01.101 Get rights and content

Abstract

Non-contact generation and detection of acoustic and ultrasound waveforms is of practical importance, since it permits making acoustic and ultrasonic measurements at elevated temperatures, in corrosive and other hostile environments, in geometrically difficult to reach locations, in outer space and doing this at relatively large distances from the test structure. Non-contact acoustical and ultrasonic techniques currently available are laser generation, optical interferometric detection, electromagnetic acoustic transducers (EMATs), air(gas)-coupled systems and hybrid combinations of the above. The present paper will describe how several such systems have been used in unique materials characterization applications.

Introduction

A problem which can arise with conventional ultrasonic techniques used for measurements in solids is the requirement that piezoelectric transducers be acoustically coupled to the test structure with an acoustical impedance matching coupling medium such as oil or grease or immersing the entire material structure to be tested in a tank of water or using a water squirter system. For velocity measurements the coupling medium can cause transit time errors, while partial transmission and partial reflection of the ultrasonic energy in the couplant layer may cause a change in the shape of the waveform leading to errors in attenuation measurements. Other techniques must be developed if we are to be able to meet future needs. One of the most unique of these is non-contact ultrasound.

Section snippets

Non-contact methods

Non-contact techniques permit generation and detection of acoustic waves with less modification of the detected waveform or frequency spectrum, although all systems are bandwidth limited, some more than others. Non-contact techniques can make measurements in hot and cold materials and in other hostile environments, in geometrically difficult to reach locations and at relatively large distances from the test structure. Several non-contact acoustical techniques are presently available using

Acoustic emission

In the 1970s, a US government agency calibrated piezoelectric transducers for acoustic emission applications [1]. This was done by using a large solid cylindrical “transfer block” which was polished to a high degree of flatness. A test specimen was coupled to the transfer block through a thin layer of liquid or grease. When the test specimen was deformed by a threaded indenter, acoustic emission signals were picked up by a piezoelectric transducer. After observing this system, the present

Holographic imaging

Optical holographic interferometry permits full-field imaging of ultrasonic wave propagation in materials. In one application [6], [7], a full-field laser pulse was used to record a holographic image of the front surface of a composite panel. Then, a point source laser pulse, possessing sufficient energy to cause ultrasonic wave generation in the material by thermoelastic heating, was directed to the center of the panel back surface. After sufficient time for the thermoelastically generated

Types of ultrasonic scanning systems

There are various types of ultrasonic scanning systems, Fig. 3 and some of them may be used in a non-contact manner, Fig. 4.

Electromagnetic acoustic transducers (EMATs)

Also since the late 1970s, EMATs have been successfully used for inspection of metal bars, tubes, pipes and plates [8], [9], [10], [11]. One major problem with EMAT's is that their efficiency rapidly decreases with lift-off distance between the EMAT's face and the surface of the test object. The footprint of a generating EMAT transducer is much larger than a detecting one. They can only be used for electrically conducting materials and are much better detectors than generators of ultrasound.

Laser generation and interferometric detection

In 1963 White reported the generation of elastic waves in solid materials by transient surface heating [12] and research has continued in this field up to the present time. Pulsed lasers are able to simultaneously produce shear and longitudinal bulk waves as well as surface modes. Laser beam ultrasound generation and detection permits non-contact ultrasonic measurements in both electrically conducting and non-conducting materials, in materials at elevated temperatures, in corrosive and other

Air-coupled generation and detection

Air-coupled generation and detection has been used to detect splits, checks, delamination and voids in various materials in a non-contact manner. Highly anisotropic and inhomogeneous materials such as wood and wood products, with surface layers of gesso, gesso and linen, paper and wood veneer have been inspected. Two paintings were tested; one was an oak-cradled panel painting, Fig. 5, and the other was illustration board mounted on hardboard. In all cases, the air-coupled ultrasound technique

Laser generation/air-coupled detection

Fiber tow-placement is used in aerospace applications for large scale non-autoclave processing of high-temperature thermoplastic composites. To improve the overall quality and performance of the final part, real-time process control is necessary. Ultrasonic surface waves are well-suited for this application. One non-contact system used for this purpose consisted of laser generation of narrowband surface waves and an air-coupled transducer [31], Fig. 6, Fig. 7. An optical fiber delivered Nd:YAG

Inspection of railroad rails and wheels

Non-destructive inspection of railroad rails and wheels is one of the greatest problems facing the transportation industry today. Ranging from visual inspection, to magnetic induction and water coupled ultrasonics, the techniques currently used are far from ideal. Conventional water-coupled contact ultrasonics using piezoelectric transducers in rolling rubber wheels filled with water or oil can detect both surface and internal horizontal cracks (parallel to the rail top surface), but cannot

Monitoring radiation embrittlement of metals

As nuclear reactors age the mechanical properties of their steel containment vessels change due to neutron irradiation embrittlement. Charpy impact specimens identical to the steel in the vessel are placed in surveillance capsules and exposed to similar conditions of radiation and temperature as the inner surface of the vessel. At periodic intervals the capsules are removed and the specimens are subjected to Charpy impact tests to measure the radiation-induced upward shift in the ductile to

Conclusions

Over the past 40 years non-contact acoustic and ultrasonic systems have been developed and proven to be optimal for a number of applications including scans of wooden panel paintings, composite panels, graphite/epoxy tape placement, railroad rails and wheels and potentially for applications including remote, non-contact detection of radiation embrittlement of metals and for non-destructive measurements in other hazardous environments. Because of the vast amount of activity in this field, the

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Citation Excerpt :
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View full text

Non-contact ultrasonic techniques

Abstract

Introduction

Section snippets

Non-contact methods

Acoustic emission

Holographic imaging

Types of ultrasonic scanning systems

Electromagnetic acoustic transducers (EMATs)

Laser generation and interferometric detection

Air-coupled generation and detection

Laser generation/air-coupled detection

Inspection of railroad rails and wheels

Monitoring radiation embrittlement of metals

Conclusions

Ultrasonics

Ultrasonics

Ultrasonics

Ultrasonics

Acoustic emission: some applications of Lamb's problem

J. Acoust. Soc. Am.

Optical detection of acoustic emission waves

Appl. Opt.

Optical probing of acoustic emission waves

Acoustic emission waveforms from cracking steel: experiment and theory

J. Appl. Phys.

Laser interferometric probe for detection of acoustic emission

Mater. Evaluat.

Interferometric and holographic testing of composite materials

Rotation sensing through electromagnetic–surface–acoustic–wave transduction

J. Appl. Phys.

A laser-ultrasound/EMAT imaging system for near surface examination of defects

Generation of elastic waves by transient surface heating

J. Appl. Phys.

Quantitative studies of thermally generated elastic waves in laser-irradiated metals

J. Appl. Phys.

Laser-ultrasonic determination of elastic constants at ambient and elevated temperatures

Ultrasonic generation by pulsed lasers

Phys. Acoust.

Laser Ultrasonics: Techniques and Applications