HOLOGRAPHIC ANALYSIS OF AIRFOILS
Ryszard J Pryputniewicz
and
Wallace W Bowley
The University of Connecticut
ABSTRACT
Turbine airfoil deflections in three-coordi-
nate directions were measured using the meth-
od of hologram interferometry. Holographie
interferograms, depicting the displacemerts of
turbine blades loaded in a specially designed
test stand, have been recorded using double-
exposure hologram interferometry. The param-
eters obtained during multiple observations
of virtual images reconstrueted from the holo-
grams were analyzed, using computer, to obtain
the displacement patterns for the tested air-
foil. This paper describes the test facili-
ties and experimental procedures, and pre-
sents experimental results. Some of the holo-
graphically determined deflections were com-
pared with the theoretical results obtained
from finite element analysis of the tested
airfoils. The agreement was very good.
INTRODUCTION
Study of turbine airfoil deflections is a con-
tinuous problem in the testing and development
of today's sophistieated jet engines. Pre-
sently, such analysis is usually carried out
on a scaled up model of a blade using dial
indicators or strain gauges. This approach,
however, is very troublesome because of the
inherent limitations of these devices.
To circumvent the limitations of mechanical
devices, recent developments in the field of
hologram interferometry were utilized, in the
present study, to analyze turbine blade de-
fleetions resulting from static loads. ^A spe-
cial test stand was designed and constructed.
This test stand was capable of delivering
tensile, bending, and torsion loads one at a
time, independent of each other, and any
combination of the above mentioned loads.
Double-exposure holograms were used to record
deflections of the blade corresponding to the
applied load. These holograms, upon recon-
struction, provided rapid, full surface in-
22
spection capability which led to quick iden-
tification of areas of large deformations and
hence high stress concentrations.
Every hologram for each loading condition was
analyzed using the least-squares theory of
holographic displacement analysis [1-4] and
computer program [3] developed for solution
of the governing equations of this theory.
In this study, two airfoils with different
aspect ratios (airfoil aspect ratio = length
of the airfoil/average cord) were studied.
These airfoils were: (i) an airfoil with high
aspect ratio and (+4) an airfoil with low as-
pect ratio, from now on referred to as air-
foil No. 1 and airfoil No. 2, respectively.
EXPERIMENTAL METHOD
Double-Exposure Holography
A method of double-exposure hologram inter-
ferometry was used to determine deflections
of the blade. In this method, two consecutive
positions of the blade were pecorded on the
same photographic plate. The surface of the
blade was deformed between the two exposures.
Upon reconstruction of the hologram, two
three-dimensional images of the blade were
formed. Since both reconstructed images ap-
peared in coherent light and existed in ap-
proximately the same loeations in space, they
interfered with each other and produced a set
of bright and dark interference fringes over-
lying the reconstrueted image. ALT of the
information about the blade's motion was
stored in this fringe pattern. Obviously, a
unique deformation of a blade, for a given
illumination and observation, did produce a
unique three-dimensional interferogram, with-
in a hologram reconstruction, from which the
deflections of the blade were determined.
Experimental Apparatus and Procedure
Experiments were carried out using a continu-
ous-wave laser holographie system, shown in
Fig. 1. Components of the experimental Setup
were supported on a 1.8 m x 2.5 m flat opti-
cal table with air suspension!, and the il-
lumination for both recording and reconstruc-
tion of holograms was provided by a He-Ne
laser?. All holograms were recorded using
high resolution holographic plates?®.
Turbine blade was placed in a special test
stand, see Figs 1 and 2, which was designed
to deliver three loading modes: (i) tension,
(11) bending, and (111) torsion; the blade
could be loaded in any of the modes individ-
ually or.in eombination, e. g., applying
tension and torsion simultaneously. Both,
tension and bending were applied by pistons,
motion of which was controlled by carefully
adjusting flow of gas from a high pressure
reservoir. Finally, torsion was produced by
applying equal and opposite forces at the ex-
Manufactured by Newport Research Corp.,
Fountain Valley, California.
Manufactured by American Optical Corp.,
Keene, New Hampshire.
3 Type 10E75. 102 mm x 127 mm, Agfa-Gevaert,
Antwerp, Belgium.
