The NASA Lewis Research Center has developed and implemented a unique process for forming flawless three-dimensional, compound-curvature laser windows to extreme accuracies. These windows represent an integral component of specialized nonintrusive laser data acquisition systems that are used in a variety of compressor and turbine research testing facilities. These windows are molded to the flow surface profile of turbine and compressor casings and are required to withstand extremely high pressures and temperatures. This method of glass formation could also be used to form compound-curvature mirrors that would require little polishing and for a variety of industrial applications, including research view ports for testing devices and view ports for factory machines with compound-curvature casings. Currently, sodium-alumino-silicate glass is recommended for three-dimensional laser windows because of its high strength due to chemical strengthening and its optical clarity. This paper discusses the main aspects of three-dimensional laser window formation. It focuses on the unique methodology and the peculiarities that are associated with the formation of these windows

Kowalski, David

Verhoff, Vincent G.

English

NASA Technical Reports Server

-THREE-DIMENSIONAL LASER WINDOW FORMATION
FOR INDUSTRIAL APPLICATION
Vincent G. Verhoff and David Kowalski
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
The NASA Lewis Research Center has developed and implemented a unique process for forming flawless
three-dimensional, compound-curvature laser windows to extreme accuracies. These windows represent an
integral component of specialized nonintrusive laser data acquisition systems that are used in a variety of com-
pressor and turbine research testing facilities. These windows are molded to the flow surface profile of turbine
and compressor casings and are required to withstand extremely high pressures and temperatures. This method
of glass formation could also be used to form compound-curvature mirrors that would require little polishing and
for a variety of industrial applications, including research view ports for testing devices and view ports for
factory machines with compound-curvature casings. Currently, sodium-alumino-silicate glass is recommended
for three-dimensional laser windows because of its high strength due to chemical strengthening and its optical
clarity. This paper discusses the main aspects of three-dimensional laser window formation. It focuses on the
unique methodology and the peculiarities that are associated with the formation of these windows.
INTRODUCTION
An increased interest in fundamental research in turbines and compressors has created a need for nonintru-
sive optical flow measurement systems. The state-of-the-art systems used in obtaining detailed velocity data are
called nonintrusive laser data acquisition systems. These systems seed the airflow with small particles that flow
through a fringe pattern which is created by intersecting laser beams. Data are obtained by measuring the
pulsating light that is reflected as the seed particles pass through the fringe pattern.
Optically clear laser windows are used so that the laser beams and reflected light can pass through. Nor-
mally the laser window glass that is used in wind tunnels is approximately 1.0-in.-thick flat quartz. For turbine
and compressor testing facilities, however, the windows are approximately 0.100 in. thick and have three
dimensions. The difference between the two windows is shown in figure 1.
Two types of errors can be introduced to the system by the laser window: spatial error of the measurement
volume and reduced signal amplitude. Spatial error of the measurement volume is caused when the laser beams
pass through the window at incident angles. The actual focal point is then skewed from the desired focal point,
resulting in an error [1], as depicted in figure 2. Errors associated with reduced signal amplitude are caused by
reflection of the laser beam as it passes through the window. Window size, curvature, thickness, surface quality,
and contour tolerance are the major factors that control the magnitude of error.
Because laser windows are molded to the flow surface profile of the turbine and compressor casings, their
size and curvature cannot be altered. Window thickness, on the other hand, can vary. Thinner windows are
desirable because they minimize error, but the glass must maintain high strength with the reduced thickness. In
addition, these thinner curved windows must be able to withstand high pressure and temperature differentials
while preserving surface quality. This paper narratively addresses three-dimensional laser window formation and
the process that maximizes the surface quality and the contour accuracy. A more detailed explanation of three-
dimensional laser window formation is given in [2]. In this paper three-dimensional laser windows will be
referred to as "windows."
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BACKGROUND
Laser systems are commonly used at NASA Lewis. Most of these systems are located in the engine com-
ponent test facilites that test compressor and turbine rotors and components. The windows are usually located
over rotating hardware where typical instrumentation cannot be used.
Overall safety is of primary concern. Therefore, the windows are hydrostatically pressure tested to 1.5
times the maximum operating pressures of the facilities. Operating pressures for various facilities range between
1.3 and 72 psia. Windows are also thermally qualified at facility temperatures and pressures if thermal differen-
tials are significant. The windows are safety tested on both concave and convex surfaces.
GLASS SELECTION
Several types of glass have been used for window formation at NASA Lewis. Sodium-lime, borosilicate,
and sodium-alumino-silicate glasses are among these types. On the basis of the Coming glass code 0317 [2],
sodium-alumino-silicate glass has been determined to be the preferred glass for windows. This glass is rec-
ommended because of its ultrahigh strength through chemical strengthening, which is unavailable with other
glass types. This increased strength allows the use of thinner windows, which reduce spatial error and produce
higher quality laser data.
In addition, failure of a chemically strengthened sodium-alumino-silicate laser window will cause the
window to shatter in tiny particles, only millimeters in cross-sectional area. This characteristic is advantageous
because the smaller particles contain less kinetic energy and are less likely to damage blades or rotors. The
panicles from other failed glass windows are relatively large in cross-sectional area. A failed chemically
strengthened sodium-alumino-silicate window is represented in figure 3.
MOLD DESIGN
The mold material that offers the best results for forming windows is machinable ceramic. The molds for
window formation consist of a male and female mold. The male mold is machined to match the internal flow
path surfaces, whereas the female mold is machined to these coordinates plus the glass thickness. The overall
dimensions of the molds are 1 in. greater than the actual final size of the window glass. Both molds have
threaded holes on the perimeter to fit alignment bars.
In order to maintain surface quality, accurate machining and polishing of the molds that are used for win-
dow formation are essential. The molds are machined and polished to a tolerance of 0.005 in. of the desired
contour with a 16 Ra surface.
All slumping components are machined out of the same material m ensure similar coefficients of thermal
expansion. Orientation of the slumping component configuration is shown in figure 4.
GLASS MOLDING
Inert gas furnaces are preferred for slumping because of the heating characteristics of the molds. Other
types of furnaces usually introduce contaminants into the slumping environment that degrade glass surface
quality. Cleanliness of the molds, glass, and furnace is critical to glass quality. These components should be
thoroughly cleaned at the beginning of every slumping operation,
The forming temperature of windows is found by an iterative process. The mean annealing temperature of
about 1100 *F is the theoretical starting temperature for new window formation. Adequate visual inspection of
the window after slumping will indicate whether the temperature should be increased or decreased. The ideal
slumping temperature for three-dimensional window formation is usually within 2 percent of the annealing
temperature.
455
The procedure for window formation is as follows:
tions
(1) Cut the glass to overall mold dimensions (i.e., 1 in. greater than the final window design dimensions).
(2) Thoroughly clean the glass with soap and water. Oil from fingers will develop into surface imperfec-
during the slumping operation.
(3) Thoroughly clean the male and female molds with an alcohol-based cleaner.
(4) Thoroughly clean the inert gas furnace.
(5) Bolt the alignment bars onto the molds.
(6) Position the male mold into the inert gas furnace.
(7) Insert the glass on top of the male mold.
(8) Position the female mold onto the male mold with the glass positioned in between them.
(9) Heat the furnace to slumping temperature.
(10) Soak the glass at slumping temperature for 4 to 6 hr.
(11) Cool the glass down to ambient temperature.
(12) Examine the slumped glass for proper curvature and quality.
(13) After desired curvature and quality are obtained, anneal the glass to relieve residual stresses.
Visual inspection of the slumped window will provide adequate information for altering the slumping
temperature. If the slumping temperature is excessive, surface imperfections will be apparent. These surface
imperfections will appear along the plane of severe three-dimensional contour or at the inflection point. This
problem can be solved by decreasing the slumping temperature by 1 to 2 percent of the annealing temperature.
The combination of insufficient slumping temperature and excessive mold pressure may result in low-
quality windows. These windows will appear wavy in the area where the glass was stretched. The solution to
this problem is to increase both the slumping temperature and duration.
The windows can be molded in several steps by alternating slumping temperatures, modifying slump soak
time, varying molding force, inverting mold positions, or any combination of these methods. When molding
glass in several steps the female mold is always used first and by itself. After the glass is partially formed, the
male mold is added, the entire assembly is inverted, and the slumping process is then repeated. Windows with
extreme curvature or compound curvature may require repeated slumping operations. These complex windows
often require the addition of weight to the molds to adequately form the glass.
Window development can be a lengthy process, but once the parameters are found for a particular window,
reproduction is routine. The last process in the formation of windows is to anneal the glass. Annealing relieves
residual stresses that build up in the glass during the development process.
GLASS EDGING
Previous methods of edging the windows to size involved scoring the glass to the desired size, then break-
ing the glass along this scored edge. During this process, small fragments of glass are broken away along the
line of scoring, creating voids in the glass. Minute cracks remain along the edge of the glass after it is broken.
These cracks weaken the glass and cause it to fail under load.
Minimizing cracks is important during the edging process. A perfected process for edging glass by using a
numerically controlled water-jet cutting machine has been developed and is recommended for cutting windows.
The machine abrades glass away along the desired cut line, reducing glass fragmentation. The water-jet cut edge
of the glass is less densely populated with cracks, and crack penetration is less severe than with scored glass.
The desired window dimensions are programmed into the numerically controlled water-jet cutting machine.
In order to alleviate window glass damage from the baeksplash of the water jet, a 0.0625-in.-thick aluminum
plate is placed across the ways of the water-jet cutting machine bed. In order to protect the glass from frac-
turing, duct seal is placed over the surface of the aluminum plate to absorb the high cutting frequencies of the
456
waterjet. Thewindowglassis thensecuredintopositionbypressingit intotheductseal.Thisentireassembly
is thensubmersedunderwatertofurtherassistinabsorbingthehighcuttingfrequenciesofthewaterjet. The
maximumdesiredwater-jetcuttingrateforedgingis4.75in./min,withanozzlepressureof 30000psig.
II
After the glass is cut to size, the edges are rounded. Using aluminum oxide sanding belts helps protect the
glass from damage because glass edges are most susceptible to impairment. The minimum radius of the rounded
edges is equivalent to half the glass thickness, as shown in figure 5.
GLASS STRENGTHENING
Chemical strengthening of the glass through ion exchange is a readily available technology. The process
will only be highlighted in this paper; detailed information is given in references 3 to 8.
Windows are chemically strengthened through ion exchange so that they earl endure facility operating pres-
sures and temperatures and to increase their relative impact strength. These windows are chemically strength-
ened to contain a 0.010-in. or greater compression layer. The specially designed chemical composition of
Coming glass code 0317 sodium-alumino-silicate glass enhances this ion exchange to ensure maximized glass
strength. This is the principal reason why this glass is used for window development.
' CONCLUDING REMARKS
High-quality contoured windows are a key part of laser anemometry systems for compressor and turbine
facilities. The window development process that was described herein ensures accurate tolerances and flawless
quality that could only be previously obtained from grinding and polishing. Although they have been tested for
flaws, windows should still be considered and treated as a fragile material. Routine visual inspection and hydro-
static evaluations are considered important to their integrity.
Future experiments will need larger windows to increase the fields of unobstructed view. Future windows
must also endure higher pressures and temperature gradients. With the present development processes these
windows should successfully withstand these future requirements.
REFERENCES
1. Bueche, F.J.: Introduction to Physics for Scientists and Engineers. Third ed. McGraw Hill, 1980, pp. 614-634.
2. Verhoff, V.G.: Three-Dimensional Laser Window Formation, NASA RP-1280, 1992.
3. Coming, Inc.: Coming Glass Code 0317 Specification Sheet. Coming, New York, 1990.
4. MeLellan, I.; and Shgnd, G.W. II: Glass Engineering Handbook. Third ed. McGraw Hill, 1984.
5. Nordberg, M.E., et al.: Strengthening by Ion Exchange. Am. Ceram. Soc. J., vol. 47, no. 5, May 1964,
pp. 215-219.
6. Hagy, H.E.: Design Strength of a Chemically Strengthened Glass. Central Glass Ceram. Res. Inst. Bull.,
vol. 13, no. 1, 1966, pp. 29-31.
7. Blizard, J.R.: Chemically Strengthened Glass. SAE Paper 680485, 1968.
8. Miska, H.A.: Understanding the Basics of Chemically Strengthened Glass. Mater. Eng., vol. 83, no. 6, 1976,
pp. 38-40.
9. Olcott, J.S.: Strengthening of Glass. Science, voi. 140, no. 3572, June 14, 1963, pp. 1189-1193.
457
0"_.],._1N,_L PA. _,.=
BLACK AND WHITE pHOTOCH:_APh
(a) Wind tunnel window.
(b) Turbine or compre_ window.
Figure 1 .--Typical laser windows.
C-91-02489
H
/
Spatial error
/-- Focal point
without window
Displacement
measurement
volume without
window
(a) Focal distance without window.
Actual focal point
_ _ / ' wlthwindow
"_ L Actual displacement
with window
(b) Focal distance with window.
Figure 2._Spatial error associated with laser windows.
458
BLACK A_ID ';','FfiTEPH©TCG"_RAPH
(a) Window before failure.
-' C-92-06406
G
Stainless steel bolt with drilled-
through center hole
"x _ Graphite alignment bar
F Graphite male mold
I __i
g
Glass -,, _ , _. curvature
Glass 1 'x/ ] / _ ,-"_
i i
P j _
_ 1 _- Casing curvature plus
eP_ glass thickness
Figure 4.---Slumping components configuration.
(b) Window after failure.
Figure 3.---Chemically strengthened sodium-alumino-silicate laser
window,
adius is equal to 1/2 glass thicknessl_s thickness
ai radius is equal to 1/2 glass thickness
Typical laser
window glass -_.
j
Figure 5.--Rounded edges of typical laser window.
459


Three-dimensional laser window formation for industrial application

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