Method and apparatus for producing ultrasonic waves in light absorbing surfaces of workpieces

In order to stabilize the acoustic wave amplitude in a workpiece, where such acoustic wave is generated by a laser beam pulse transmitted upon the workpiece surface, laser pulses preceding the laser pulse used for acoustic evaluation are utilized to clean the workpiece surface from contamination. Cleaning of the workpiece surface to the bare metal provides a normalized surface condition in respect to absorbed pulsed laser beam energy and, hence, produces acoustic waves of substantially constant amplitude. Either the same laser producing the acoustic wave or a separate laser is used for vaporizing contamination at the workpiece surface where an acoustic wave is subsequently to be produced.

BRIEF SUMMARY OF THE INVENTION 
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.

DETAILED DESCRIPTION OF THE INVENTION 
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.