Efforts are under way to develop a technique of noncontact acousto-ultrasonic testing in which (1) a pulsed laser beam excites ultrasonic waves in a plate specimen and (2) the ultrasonic waves are detected by use of one or more focusing air-coupled ultrasonic transducer(s) placed at a short distance away from the specimen and aimed at the spot(s) of interest on the specimen. This technique is intended to be an alternative to an older technique of contact acousto-ultrasonic testing; when fully developed, this noncontact technique could be used to characterize materials and monitor integrity of structures in locations that are inaccessible to contact ultrasonic probes or in situations in which contact ultrasonic probes cannot or should not be used.
The acousto-ultrasonic method has been shown to be useful for assessing mechanical properties of composite-material structures. Plate-wave analysis has been used to quantify moduli of elasticity of composite materials. Rates of decay of ultrasonic signals can be used to monitor residual strengths or crack densities. The present development is expected to extend these capabilities from the contact to the noncontact regime. It has been envisioned that a noncontact acousto-ultrasonic testing technique would be especially useful for monitoring changes in the properties of ceramic- and metal-matrix composite materials, and of aircraft-engine structural components made of these materials, during thermomechanical testing and during engine operation.
The use of a laser as a remote (and thus also noncontact) ultrasound-input source and as part of an ultrasound detector has been under investigation for a number of years. The use of a noncontact piezoelectric ultrasonic transducer coupled through an airgap has also been under study. Laboratory experience has led to the conclusion that a laser is more useful as an ultrasound-input device than as part of a detector, while an air-coupled piezoelectric transducer is more useful as a detector than as an ultrasound-input device. Taking advantage of this lesson of experience, the present laser-in-coupling/air-out-coupling technique combines the two named means of coupling in such a way as to obtain a signal-to-noise ratio greater than can be achieved in other noncontact ultrasonic techniques that involve laser or air coupling.
The figure schematically depicts two acousto-ultrasonic testing apparatuses; one that implements an older contact technique and one that implements the present noncontact technique. In the noncontact apparatus, the beam from a pulsed neodymium:yttrium aluminum garnet (Nd:YAG) laser is aimed at a spot on the specimen; this is the same spot where, in the contact apparatus, the sending transducer would be coupled to the specimen via a dry (silicone rubber) couplant pad. The air-coupled transducer in the noncontact apparatus is positioned and oriented to be sensitive to the spot on the specimen where, in the contact apparatus, the receiving transducer would be coupled to the specimen via another silicone rubber pad. A nonfocusing micromachined capacitance transducer can be used as an alternative to the air-coupled piezoelectric transducer (and is characterized by a broader frequency response), provided that it can be placed close enough to the specimen.
In experiments, rates of decay of ultrasonic energy in SiC/SiC ceramic-matrix and SiC/Ti metal-matrix composite-material specimens were measured by the contact technique and by the present noncontact technique. For each noncontact measurement, the laser pulse energy was ≈ 13 mJ and the air-coupled acoustic transducer was one with a broad frequency response nominally centered at 0.25, 0.5, 1.0, or 2.0 MHz. The rates of decay of ultrasonic energy were found to be higher for the contact measurements; the difference has been attributed to loss of energy via the contacts, and this attribution, in turn, seems to imply that the noncontact rate of decay is a more nearly pure measure of attenuation of ultrasound within the specimen. Still, contact measurements have been successful in revealing mechanical fatigue in the specimen materials; it is important that this be so, inasmuch as in projected uses for monitoring the integrity of aircraft components, it will often be necessary to take measurements in the presence of support structures that cause loss of ultrasonic energy. In both the contact and noncontact techniques, rates of decay of ultrasonic signals have been observed to increase with frequency.
Several concerns must be addressed in further development efforts. One is potential destructiveness of the laser pulse. Another is attenuation of the ultrasonic signal in output coupling via air; this attenuation imposes a practical upper limit on the usable frequency range.
This work was done by Harold E. Kautz of Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
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Commercial Technology Office
Attn: Steve Fedor
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Refer to LEW-16916.