Contact-free ultrasonic transduction for flaw and acoustic discontinuity detection

• US 4,567,769 A
• Filed: 03/08/1984
• Issued: 02/04/1986
• Est. Priority Date: 03/08/1984
• Status: Expired due to Fees

• Current Assignee: Rockwell International Corporation
• Original Assignee: Rockwell International Corporation
• Inventors: Barkhoudarian, Sarkis
• Primary Examiner(s): Birmiel, Howard A.
• Application Number:
  US06/587,713
• Time in Patent Office: 698 Days
• Field of Search: 73/643, 73/655, 374/5, 350/96.24, 350/358
• US Class Current: 73/643
• CPC Class Codes: G01H 9/006 the vibrations causing a variation in the relative position of the end of a fibre and another element
• CPC Class Codes: G01N 29/2418 using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics photoacoustic cells G01N21/1702; measuring characteristics of vibrations by using radiation-sensitive means G01H9/00; acousto-optical conversion techniques for short-range imaging G01S15/8965; sound-producing devices using laser bundle G10K15/046
• CPC Class Codes: G01N 29/26 Arrangements for orientation or scanning by relative movement of the head and the sensor mechanical steering of sound transducers or their beams G10K11/35
• CPC Class Codes: G01N 29/28 providing acoustic coupling , e.g. water impedance matching G10K11/02
• CPC Class Codes: G10K 15/04 Sound-producing devices G10K15/02 takes precedence

Abstract

A system for the examination of a test object for internal flaws (or acoustic discontinuities such as solid-to-gas or solid-to-liquid interfaces) by means of ultrasonic waves. The ultrasonic waves 18 are induced in the object 16 by projecting at least one intense, pulsed laser beam 13 on the object 16 and scanning the beam 18 along the object 16. Another laser beam 23 is projected on the object 16, either opposite the first beam 14 or on the same side adjacent to the first beam 14, so that is reacts with and is frequency-modulated by the ultrasonic wave. The reflected, frequency-modulated beam 24 is fed to an optical heterodyning means 26 which passes through only an optical beam 28 modulated with the ultrasonic frequency to a photodetector 30. A flaw 36 (or acoustic discontinuities such as solid-to-gas or solid-to-liquid interfaces) in the test object 16 results either in a decrease or an increase in the output of the photodetector 30, depending on whether the detection laser beam 23 is on the opposite or same side, respectively, of the test object 16 as the ultrasonic-wave-inducing laser beam 13. The system is characterized by non-contact with the test object by the ultrasonic-wave generation-and-detection means.

Claims

1. Apparatus for inducing an ultrasonic wave in a test object, comprising:

laser pulse means for providing a series of laser pulses; and

,

a plurality of spaced, partially transmissive, partially reflective mirrors arranged to transmit and reflect the output of said laser pulse means,

wherein, the beam reflected from said mirrors are a plurality of spaced parallel laser beams said beams being directed to impinge on said test object, each beam being spaced from its neighboring beam a distance equal to one-half the wavelength of said ultrasonic wave.

2. Apparatus as in claim 1, wherein;

the transmittivity-reflectivity ratios of said mirrors are such that each directs an equal amount of energy upon said test object in order to maintain the amplitude of the acoustic wave at a constant level.

3. Apparatus as in claim 1, wherein:

the number of mirrors is in the range of 5 to 10.

4. Apparatus for inducing an ultrasonic wave in a test object, comprising:

laser-pulse-generating means for providing a series of laser pulses; and

fiber optic means for directing the output of said laser-pulse-generating means upon said test object, said fiber optic means comprising a bundle of optical fibers each spaced from its neighbor a distance equal to one-half of the wavelength of said ultrasonic wave,

wherein, the output from said fiber optic means is a plurality of spaced parallel laser beams, each beam being spaced from its neighboring beam a distance equal to one-half the wavelength of said ultrasonic wave.

5. A system for detecting flaws and acoustic discontinuities such as solid-to-liquid or solid-to-gas interfaces in test objects which is capable of sustaining the generation and propagation of ultrasonic waves comprising:

light means for inducing ultrasonic waves in said test object, said light means being coupling-free of said test object said light means providing a plurality of spaced parallel light beams directed to impinge on said test object, each beam being spaced from its neighboring beam a distance equal to one-half the wavelength of said ultrasonic wave; and

light means for detecting said ultrasonic waves in said test object, said detection means being coupling-free of said test object.

6. A flaw-detection or acoustic-discontinuity-detection system as in claim 5, wherein:

said light beams are laser pulses.

7. A flaw-detection or acoustic-discontinuity-detection system as in claim 6, wherein said light means comprises:

a plurality of spaced partially transmissive, partially reflective mirrors arranged to transmit and reflect said laser pulses.

8. A flaw-detection or acoustic-discontinuity-detection system as in claim 7, wherein:

the transmittivity-reflectivity ratios of said mirrors are such that each directs sufficient energy upon said test object to compensate for the energy loss which occured during the travel time of the preceding wavelength of the ultrasonic wave.

9. A flaw-detection or acoustic-discontinuity-detection system as in claim 7, wherein:

the number of mirrors is in the range of 5 to 10.

10. A flaw-detection or acoustic-discontinuity-detection system as in claim 6, wherein said detection means comprises:

laser-beam-generating means;

second means for propagating and directing the laser beam from said laser-beam-generating means upon a spot on said test object at which the laser beam frequency can be modulated by the frequency of said ultrasonic wave.

11. A flaw-detection or acoustic-discontinuity-detection system as in claim 10, further including:

optical heterodyning means to which the modulated laser beam is fed for separating out the ultrasonic wave component from the modulated laser beam.

12. A flaw-detection or acoustic-discontinuity-detection system as in claim 11, wherein:

the two laser beams which are directed upon said test object are made to scan the test object in synchronism.

13. A flaw-detection or acoustic-discontinuity-detection system as in claim 11, further including:

an optical detector connected to said optical heterodyning means for producing an output signal from the ultrasonic wave component, the amplitude of the output signal varying when flaws in the test object are encountered by the ultrasonic wave.

14. A system for directing flaws and acoustic discontinuities such as solid-to-liquid or solid-to-gas interfaces in test objects which are capable of sustaining the generation and propagation of ultrasonic waves comprising:

fiber optic means for inducing ultrasonic waves in said test object, said induction means being coupling-free of said test object, said fiber optic means comprising a bundle of optical fibers each spaced from its neighbor a distance equal to one-half the wavelength of said ultrasonic wave; and

,

light means for detecting said ultrasonic waves in said test object, said detection means being coupling-free of said test object.