Method and Apparatus for Producing Ultrasonic Waves in Light Absorbing Surfaces of Workpieces

This invention concerns a method and apparatus for producing ultrasonic waves resulting from thermal excitation in the light absorbing surface of a workpiece. More particularly, the invention concerns the production of an ultrasonic wave in the surface of a workpiece caused by the absorption of a laser beam pulse, and such ultrasonic wave being adapted for nondestructively testing such workpiece for internal defects.

The generation of acoustic waves responsive to sudden heating of a surface portion of a workpiece is well known, see "Werkstoffprufung mit Ultraschall" (book) J. & H. Krautkramer, 3rd edition, pages 148 to 149, Springer Verlag, Berlin/Heidelberg/New York (1975) and U.S. Pat. No. 3,978,713, dated Sept. 7, 1976 to C. M. Penney entitled "Laser Generation of Ultrasonic Waves for Nondestructive Testing".

The amplitude of the ultrasonic wave produced by thermal excitation is dependent upon the absorbed energy from the pulsed laser beam. The frequency spectrum of the acoustic wave is determined by the shape of the laser beam pulse. When performing nondestructive testing of workpieces with ultrasound, the thermal method of producing an acoustic wave is used when the acoustic energy cannot be imparted to the workpiece by a conventional coupling medium. For locating defects in the workpiece, the workpiece must be scanned by a finite acoustic beam propagated from the workpiece surface, the beam having generally a cross sectional area not exceeding a few square centimeters. When utilizing the contact-free generation of ultrasonic waves resulting from the thermal effect produced by a pulsed laser beam, different absorption characteristics prevailing along the surface of the workpiece lead to local variations of the sonic wave amplitude.

In practice, workpieces are contaminated unevenly along their surface. Cleaning of the surface, particularly when workpieces to be tested by the contact-free method are involved, is difficult if not impossible on account of the shortcomings inherent in such workpieces. Varying amplitudes of the acoustic wave due to different absorption of the laser energy complicate the quantitative evaluation of the test result since constant acoustic pressure generation from location to location is a prerequisite for such evaluation.

An object of this invention is the provision to assure the condition of constant laser pulse energy absorption from location to location when producing ultrasonic waves with laser beam energy and, thereby, providing constant acoustic wave amplitude along the entire workpiece surface.

In accordance with the invention disclosed hereafter, the problem pointed out heretofore is solved by providing for each laser energy responsive acoustic wave generation process, two or more coherent light pulses upon the workpiece surface portion at which the acoustic wave is to be generated. Only the acoustic wave caused by the last transmitted laser pulse is used for the ultrasonic test whereas the preceding transmitted laser beam pulses serve for eliminating surface contamination. In accordance with the invention, the known phenomenon that laser beam radiation can be utilized to vaporize a contaminant is employed.

It is apparent that clean metallic surfaces reflect more energy and, hence, absorb less energy than contaminated surfaces. The light energy absorbed by an oxidized or contaminated surface causes a localized heating and ultimately produces vaporization of the contaminant. It is advantageous that the contaminant layer generally has a lower thermal conductivity and a lower specific heat capacity than that of the base material. However, if in contrast clean metallic material is irradiated with pulsed laser beams of the same energy, no or only insignificant vaporization occurs since, on account of the higher reflectivity, a smaller amount of energy is absorbed. Moreover, by virtue of the higher thermal conductivity and higher specific heat capacity of the base material, a significantly lower degree of heating is obtained. It is not detrimental if the laser pulse energy is selected for removal of the most severe contamination, or layer of foreign substance, since the material erosion becomes self-limiting as soon as the clean surface presents itself.

The present method and several embodiments thereof will be more clearly apparent from the following description when taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic block diagram of a typical electrical control circuit of the invention, and

FIG. 2 is a schematic block diagram showing an alternative embodiment.

The method forming the present invention can be understood most clearly from the following example. A workpiece having a partially oxidized surface is to be tested by ultrasonic energy without establishing physical contact. The generation of a sonic energy pulse is accomplished by means of a laser beam pulse and reception of sonic energy is to be made, for instance, with a known transit time interferometer arrangement not forming a part of the present invention. The sonic waves are to be produced by means of a laser beam pulse of about 30 nanoseconds duration. The variations in sound pressure amplitude arising from the thermal sound generation in the oxidized region and in the clean metallic region of the workpiece surface have an adverse effect on the test result. As an example, the sound pressure amplitude in the oxidized region is 20 db greater than that in the clean workpiece surface region.

Using the region of heaviest oxide layer, tests are made to reveal the quantity of laser beam pulses which must precede the laser beam pulse producing the same sound wave amplitude as had been obtained at the clean surface region of the workpiece. The adjustment so derived empirically, that is the determination of the quantity of laser pulses needed for cleaning the workpiece, can be maintained constant for a particular workpiece. Of course, for each laser beam pulse, including those used for cleaning, there is produced a respective acoustic wave. Only the sonic wave produced responsive to the last-transmitted laser beam pulse is used for test purposes. Such condition can be met in several ways. The pulse repetition rate of the laser source is known and, hence, the time delay from the initial laser pulse to the first laser beam pulse useable for acoustic exploration of the workpiece is programmable. By providing a time delay gate in the electrical output of the interferometer as seen in FIG. 1, the initial outputs arising from the laser pulses and used for cleaning are suppressed. Only after the passage of a delay, the time interval between the start signal and the end of a preset adjustable time delay, are the output signals permitted to pass to an evaluation unit.

In another embodiment, instead of setting the delay for a predetermined constant quantity of laser beam pulses used for cleaning the surface of the workpiece, photoelectric means are used for determining the surface condition of the workpiece. The cleaning process performed by means of laser beam pulses is terminated when the output signal from the photoelectric means corresponds to a value corresponding to that obtained from a clean metal surface, see FIG. 2. In another embodiment, a circuit can be provided for sensing at the photoelectric means the condition when the reflected light responsive to two consecutive transmitted laser beam pulses remains constant, and thereafter a release signal is sent to the receiver for providing the acoustic wave responsive signal to the evaluation circuit.

It will be apparent that the invention is not limited to the use of a single laser source. For example, several laser sources can be used in such a manner that one laser source serves for cleaning the surface of the workpiece and another source for producing the acoustic wave. In the latter case, a sequencing circuit first operates the cleaning laser and subsequently activates the second laser used for producing the acoustic wave. Concurrently with rendering the second laser source operative the sequencing circuit also opens a gate circuit associated with the receiver means for permitting the acoustic wave responsive output signal derived from the deflection of the workpiece surface to pass to an evaluation circuit.

Moreover as used heretofore, the term "laser source" or equivalent expression shall be interpreted as including also a combination of laser beam sources.