Technique measures structural deformation of materials subjected to wide range of temperatures and other environmental conditions. Effects of convection currents are eliminated by operating a pulsed laser in double pulse mode that exposes hologram twice in quick succession

Aprahamian, R.

Evensen, D. A.

English

NASA Technical Reports Server

March 1973	 B73-10075 
NASA TECH BRIEF 
Langley Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
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Vibration Measurement by Pulse Differential Holographic Interferometry 
A pulsed differential holography technique has been 
devised to measure structural deformations of materials 
that are subjected to a wide range of temperatures and 
other environmental conditions. 
Continuous wave, time-average laser holography has 
been established as a method for mapping the displace-
ments of a vibrating structure at ambient temperatures. 
At elevated temperatures, however, this method is 
severely limited by distortions induced by convection 
currents in the atmosphere adjacent to the test structure. 
The effects of convection currents are eliminated 
by operating a pulsed laser in a double pulse mode which 
exposes the hologram two times in quick succession. 
During the exposure separation, the structure will de-
flect, but the turbulence in the medium will not change 
sufficiently to appreciably alter the optical path of the 
laser light and its reflection. 
While this technique requires additions to existing 
pulsed holography setups, double pulsing should prove 
to be useful for measuring flutter, aerolastic, and other 
vibration characteristics of metal plates, beams, shells, 
and other structures. It isnow possible to accomplish 
large area holography and to measure vibrations with 
large amplitudes. It is also no longer necessary to 
isolate the test structure in a vacuum chamber in order 
to eliminate or reduce atmospheric interferences. 
Pulse differential holography is a variation on double-
exposure holography, in which an interferogram is 
recorded. In this technique, a hologram is made of the 
test object. The object is then subjected to loads which 
cause it to deform, and a second hologram is made on 
the same piece of photographic film. Two images are
produced when the developed film is illuminated by a 
laser light, one image from the undeformed object 
and the other when it is deformed. The light waves 
which form the two images interact with each other to 
create interference patterns. Analysis of the patterns 
reveals the surface deformations of the object caused 
by the load. 
The timing of the two exposures, in pulsed differ-
ential holography, is carefully controlled to be syn-
chronized with the deflection of the object being 
measured. The optimum time interval between exposures 
is dependent on several factors such as the number of 
fringes desired, vibration amplitude and frequency, and 
temperature of the structure. Too long a period between 
pulses results in fringes that are perturbated by the 
convection currents surrounding the object, while the 
shorter time intervals permit operation at higher temper-
atures and higher vibration amplitudes or frequencies. 
The magnitude of the structural deformation is deter-
mined by counting the number of interference fringes 
that appear on the hologram. The first dark fringe 
corresponds to a deflection of one-quarter wavelength 
of the laser light being used, the second corresponds to 
three-quarters of a wavelength, and so on. The larger the 
slope in the surface of the object, caused by bending or 
deflection from an impact or other vibration-producing 
mechanism, the closer the spacing between fringes. 
Using this technique, vibration amplitudes have been 
successfully measured on stainless steel panels heated to 
temperatures from 394 K (2500 F) to 1394 K (20500 
F). For these experiments the pulse duration was 50 x 
10 second, while the interval between pulses was 50 
microseconds. 
0 (continued overleaf) 
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Reference: NASA CR-2028 (N72-29543) 
Pulsed Differential Holographic Measurements 
of Vibration Modes of High-Temperature Panels 
Patent status: 
NASA has decided not to apply for a patent. 
Source: D. A. Evensen and R. Aprahamian of

TRW, Inc.

under contract to

Langley Research Center

(LAR-1 1092)
B73-10075	 Category 03, 06
	
S


Vibration measurement by pulse differential holographic interferometry

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