 |
Description  |
|
|
FIELD OF THE INVENTION
This invention relates to an apparatus for applying thermal shock to a
sample and a method thereof. More particularly, this invention relates to
an apparatus for applying a convective thermal shock to a sample and a
method thereof.
BACKGROUND OF THE INVENTION
Many applications of materials such as ceramics, glasses, composites, and
polymers involve thermal shock conditions. Stresses may develop in the
materials as a result of heating or cooling during applications. If the
stresses are sufficiently high and the volume of the material under
stresses is sufficiently large, the material will fracture and fail. This
is called thermal shock fracture. For example, ceramic materials that are
transparent in the infrared range (3-5 micrometer wavelength) are
typically used as domes for guidance of missiles. During service, these
materials heat up rapidly due to aerodynamic heating, and are subject to
thermal shock fracture if the heating rate is too high or the material's
resistance to thermal shock is too low. The present invention addresses
the need of a simple thermal shock testing method for such infrared dome
applications. The results of the invention, the apparatus and method are,
however, not limited to the infrared dome application, and may be useful
for any thermal shock environments that involve heating in which
convective heat transfer is the dominant heat transfer mode.
Hasselman developed a series of equations for calculating thermal shock
resistance figures of merit (R parameters) which took into account both
mechanical add thermal properties of the materials: D.P.H. Hasselman,
"Thermal Stress Resistance Parameters for Brittle Refractory Ceramics, A
Compendium," Bulletin of the American Ceramic Society 49, 1033-1037
(1970).
These equation can be applied to a wide variety of thermal shock
conditions. These theoretical figures of merit are useful in comparing the
theoretical thermal shock behavior of different materials. However, they
must be substantiated by the results of thermal shock testing. Therefore,
simple thermal shock resistance tests for ranking materials are strongly
desirable. This is especially true for the case of infrared transmitting
materials for dome applications.
Traditional methods of thermal shock testings include the following: (1)
quenching bars into water or oil and determining the strength reduction
caused by the quenching (2) laser thermal shock testing involving rapidly
heating the central portions of disk samples irradiated with high power
laser, (3) wind tunnel testing of actual domes, (4) impinging cold air jet
on hot disks, (5) exposing the central portions of disk samples to
radiation heat flux from a high-temperature furnace, (6) rapidly heating
disk samples in a lamp furnace equipped with focussing mirrors to allow
very fast heating, (7) quenching bars into fluidized beds for convective
heat transfer, (8) exposing samples to heating in a gas flame of a burner,
and (9) fast heating using an array of solar mirrors. These testing
methods were discussed in the following references: D. Lewis, "Thermal
Shock Testing of Optical Ceramic," SPIE Vol. 297, 120-124 (1981); J. R.
Brockenbrough, L. E. Forsythe, and R. L. Rolf, "Reliability of Brittle
Materials in Thermal Shock,"
Journal of the American Ceramic Society 69[8]634-637 (1986); K. T. Faber,
M. D. Huang, and A. G. Evans, "Quantitative Studies of Thermal Shock in
Ceramics Based on a Novel Test Technique," Journal of the American Ceramic
Society 64[5]296-301 (1981); and F. A. Strobel "Thermostructural
Evaluation of Spinel Infrared Domes", SPIE Vol. 297 pp. 125-136 (1981).
The disadvantages of the above testing methods are:
(1) Quench testing--Results strongly depend on sample size, heat transfer
mode complicated by water boiling, heat transfer coefficient is limited if
oil is used as the quenching medium, stress corrosion effect unavoidable,
and edge or corner effects are undefined.
(2) Laser thermal shock test--Large capital investment for the stable,
flat-top type of laser required in the laser thermal shock test. Heat
transfer mode involves radiation to the coated layer absorbing the laser
radiation and conduction within the sample.
(3) Wind tunnel test--The test involves very sophisticated equipment and
instruments, and the calibration and reproducibility of the test
conditions are difficult.
(4) Cold-air-jet on hot disk--This test suffers from the fact that thermal
conductivity of ceramics typically decreases with increasing temperature.
Testing a hot disk does not stimulate the same thermal conditions in a
cold disk subjected to aerodynamic heating.
(5) Furnace radiation--Usually insufficient heat flux to initiate thermal
shock fracture in ceramics with reasonable strength. The heat transfer
mode is primarily radiation.
(6) Lamp furnace heating--This test uses radiation heat transfer.
(7) Quenching in fluidized bed--This test utilizes a low heat transfer
coefficient.
(8) Gas burner--The flame temperature is nonuniform; therefore,
reproducibility is difficult. (9) Solar mirrors--The test involves very
sophisticated equipment and instruments.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a new and improved
thermal shock apparatus comprises a hot gas stream impinging means, a hot
gas stream impinging control means, a positioning means, and a sample
holding means. The hot gas stream impinging means is adapted to provide
impingement of a hot gas stream upon a predetermined impingement area of a
sample at a predetermined instant. The hot gas stream impinging control
means is adapted to deflect the hot gas stream away from the sample when
the hot gas stream impinging control means is positioned between the hot
gas stream impinging means and the sample. The hot gas stream impinging
control means is adapted to provide an opening for positioning the hot gas
stream impinging means close to the ample to provide impingement of the
hot gas stream upon the predetermined hot gas stream impingement area of
the sample. The positioning means is adapted to position the hot gas
stream impinging means, and the sample holding means is adapted to the hot
gas stream impinging means and the positioning means to hold the sample in
a predetermined position.
In accordance with another aspect of the present invention, a new and
improved method for thermal shocking a sample comprises the following
steps:
Step 1--A sample is placed in the sample holding means of the thermal shock
apparatus of claim 1.
Step 2--The predetermined hot gas stream impingement area of the sample is
impinged with the hot gas stream from the hot gas stream impinging means
by moving the hot gas stream impinging means past the hot gas stream
control means towards the predetermined hot gas stream impingement area of
the sample.
Step 3--The impinging hot gas stream is maintained upon the predetermined
hot gas stream impingement area of the sample for a predetermined time
imparting a thermal shock to the sample.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
FIG. 1 is a side view of a thermal shock apparatus in accordance with the
present invention.
FIG. 2 is a side view of the thermal shock apparatus of FIG. 1 depicting
the positioning of the hot gas stream impinging means above the sample in
accordance with the present invention.
FIG. 3 is a perspective view of the heat deflection means of the thermal
shock apparatus of FIG. 1 in accordance with the present invention.
FIG. 4 is a plan view of the sample holder of the thermal shock apparatus
of FIG. 1 in accordance with the present invention.
FIG. 5 is a curve of the probability of failure of samples as a function of
the critical temperature difference in accordance with the present
invention.
For a better understanding of the present invention, together with other
and further objects, advantages and capabilities thereof, reference is
made to the following disclosure and appended claims in connection with
the above-described drawing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method and apparatus of the present invention alleviate the
disadvantages associated with the prior-art tests discussed above. The
heat transfer mode of the testing method of the present invention is
convective. This is particularly important for infrared dome applications
in which convection is the dominant mode of heat transfer. The apparatus
of the present invention imparts a thermal shock to a given material such
as a ceramic, so that the thermal shock can cause mechanical failure of
the material by stresses induced by a rapid temperature change experienced
by the sample. The thermal stresses developed in the material are
controlled by the temperature distribution and thermomechanical properties
of the material. The temperature distribution is determined by the initial
condition and boundary conditions which, in turn, depend on the dominant
heat transfer mode. For radiation heat transfer, the boundary condition is
Q=h(T.sup.4 -To.sup.4) (1)
where is the flux across the boundary, T the surface temperature, To the
temperature of the surrounding medium, h is a constant. For forced
convection, which is the case of infrared dome heating during launch of
projectile, the boundary condition is
Q=h(T-T0). (2)
In the case of conduction heat transfer,
k.sub.1 /Cp.sub.1 .rho..sub.1 (.differential.T/.differential.n).sub.1
=k.sub.2 /Cp.sub.2 (.differential.T/.differential.n).sub.2(3)
where k is the thermal conductivity, Cp the heat capacity, .rho. the
density, .differential.T/.differential.n the temperature gradient along
the normal to the surface of separation, 1 and 2 denote the two media.
From Equations (1) to (3), it is obvious that the boundary conditions
representing radiation, conduction, and convection types of heat transfer
are quite different. Thus, it is essential to devise a test apparatus and
procedure that involve a dominant heat transfer mode identical or close to
that expected in service.
In addition to the boundary condition, the temperature distributions during
transients are affected by the initial condition. It is desirable to have
the same initial conditions in the test as in the actual application. For
example, infrared domes typically heat up from room temperature during
service. Therefore, the simulated test should start with a dome material
at room temperature. This is important from thermal property standpoints.
Thermal conductivity in ceramics typically decrease with increasing
temperature. Thermal conductivity values at high temperatures can be less
than 1/2 or 1/4 of those near the ambient temperature. Thus a test
involving quenching bars from high temperatures to water bath does not
simulate the transient conditions of infrared domes, since the actual
application involves heating from room temperature at which the material's
thermal conductivity is somewhat higher than the elevated temperatures
employed in quench tests.
The apparatus and method of the present invention solved the problems of
heat transfer mode, initial conditions, edge or corner effects and stress
corrosion effect in the prior-art techniques. It also has a major
advantage in terms of simplicity. The apparatus and testing procedure are
relatively simple. The geometry of the samples required for the testing of
the present invention is relatively simple.
In FIG. 1 is shown a thermal shock apparatus 10 comprising a hot gas stream
impinging apparatus 20, the hot gas stream impinging control 30, a
positioning mechanism 40, and a sample holder 50. The hot gas stream
impinging apparatus 20 comprises a heat gun 60 having a heater coil 70
whose heat output is controlled by power supply 71 with a volt meter 72
and amperage meter 73 to indicate the power being supplied to heater coil
70 and a source of gas controlled by gas regulator 85, pressure gauge 86,
and gas flow indicator 87. The quartz envelope 80 has an inside diameter
of about .4 inches. The quartz envelope 80 is tapered at one end to reduce
the area of the opening to form an orifice 81. The taper has a radius of
curvature 82 of approximately 0.16 inch. The heater coil 70 is positioned
along the center line of quartz envelope 80 and having its exit end 83 of
heater coil 70 positioned approximately 0.5 inches from orifice 81. The
temperature and flow rate of hot gas stream 84 is controlled by the gas
regulator 85, the pressure gauge 86 and the gas flow indicator 87 in
combination with the amount of current applied to heater coil 70. The
uniformity of the temperature of the hot gas stream 84 at the orifice 81
region is not only dependent upon the flow rate of the incoming gas in
combination with the uniformity of the heat dissipated from the heater
coil to the flowing gas along the axis of the coil, it is also dependent
upon the geometry of the quartz envelope 80, the position of exit end 83
of heater coil 70, the radius of curvature 82 at the taper of the quartz
envelope 80 as well as the diameter of orifice 81.
The hot gas stream impinging control 30 comprises two thin foils 88 and 89
which are made from a metal such as stainless steel to act as a hot gas
stream deflection shield to prevent the hot gas stream 84 from reaching
sample 90 in the position as indicated at FIG. 1. The thin foils 88 and 89
are attached to holders 91 and 92 which are adapted to a fixed position
relative to sample holder 50. FIG. 3 shows hot gas stream impinging
control 30 in a perspective view of thermal shock apparatus 10 as shown in
FIG. 1. As shown in FIG. 3, thin foil 88 has a notch 93 which coacts with
thin foil 89 to maintain the position of thin foil 88 and thin foil 89 as
shown in FIG. 1 and FIG. 3.
Sample holder 50 as shown in FIG. 1 and FIG. 4 comprises a stage 94 made
from a heat resistant material such as quartz and clamps 95, 96 and 97 for
holding sample 90 in a predetermined position relative to hot gas stream
impinging apparatus 20, hot gas stream impinging control 30 and
positioning mechanism 40. Clamp 97 is provided with a spring 98 adapted
with a clamp holder 99. The clamps are made from a heat resistant material
similar to stage 94 material, and are attached to stage 94.
Positioning mechanism 40, as shown in FIG. 1, has a movable portion 100
adapted to traverse vertically when control wheel 101 is activated.
Movable portion 100 is attached to heat gun 60 as depicted in FIG. 1. As
shown in FIG. 2, hot gas stream impinging apparatus 20 is relocated
directly above sample 90 within less than one second when the movable
portion 100 of the positioning mechanism 40 is activated. Stop 102 of
movable portion 100 coacts with the stationary portion of the positioning
mechanism 40 as depicted in FIG. 2 to locate the orifice 81 of heat gun 60
directly above sample 90 to effect the impingement of hot gas stream 84
upon the predetermined hot gas stream impingement area 103 of sample 90 as
shown in FIG. 4. The hot gas stream 84 impinges sample 90 normal to the
surface of sample 90 or at another angle of impingement. Thermocouples
104, 105 are located directly under sample 90 as indicated in FIG. 1 and
FIG. 4 to facilitate temperature measurement of sample 90 before and
during the hot gas stream impingement upon the predetermined hot gas
stream impingement area 103 of sample 90.
As shown in FIG. 2, the exit end of quartz envelope 80 containing orifice
81 is rapidly moved from its position as shown in FIG. 1 to a position
directly above sample 90 as depicted in FIG. 2 by the operation of control
wheel 101 of positioning mechanism 40. The exit end of heat gun 6 is moved
vertically past foil 88 and 89 which move from their position as depicted
in FIG. 1 to the new position as depicted in FIG. 2 exposing the
predetermined hot gas stream impingement area 103 of sample 00 to the hot
gas stream 84 of heat gun 60 thereby imparting a thermal shock to sample
90.
The heat source is capable of heating the upper surface of sample 90 to
.perspectiveto.750.degree. C. when the orifice 81 of the heat gun 60 is
located 5/32"directly above the area 10 of the top surface of the sample
90. Other types of hot gas heaters employing a coil heater, a quartz
envelope, and argon gas can produce a hot gas stream of argon at
temperatures up to 1000.degree. C. If a ceramic, such as alumina,
zirconia, or yttria, is used as an envelope, and a gas, such as argon
helium, hydrogen, or nitrogen hydrogen mixture, is used as the hot gas,
temperatures of the hot gas stream up to 1900-2000.degree. C. are
expected. The heat gun 60 is raised and lowered by the upward and downward
motion of positioning mechanism 40 such as a press which has been fitted
with a control wheel 101 as opposed to a lever. A stop 102 on the movable
portion 100 ensures a constant distance between the orifice 81 of the heat
gun 60 and the upper surface of the sample 90 for each test. The orifice
81 of the heat gun 60 is about 0.146 inches in diameter and the orifice
end of the hot air gun 60 is rounded to provide a more uniform
distribution of the hot gas stream from the orifice.
The thin foils 88 and 89 acting as heat deflection shutters assist in
cooling the sample 90 to room temperature after each test. These are
purposely made very flexible so that the heat gun 60 can move up and down
without encountering undue obstruction.
Two thermocouples 104, 105 are in contact with the lower surface of the
sample 90 during the test to record temperatures at the center and at an
area adjacent to the periphery of sample 90. A chart recorder is used for
recording these temperatures. A chart of the hot gas stream 84 temperature
vs applied voltage for the heat gun 60 was made prior to the use of
thermal shock apparatus 10 for the thermal shock tests. This calibration
was checked periodically during sample testing.
The test sample was positioned in a sample holder 50, which consists of a
three-point, lightly spring-loaded clamping fixture made of quartz. This
holder is located below and directly in line with the heat gun 60, and
when the heat gun 60 is in the raised position, the thin foils 88, 89
isolate the sample from the hot gas stream 84 of the raised heat gun 60.
This is necessary since the hot gas stream 84 temperature must be
maintained between tests in order for temperature stabilization to take
place before the succeeding step in the test can be carried out.
This thermal shock test apparatus 10 utilizing a heat gun 60 such as a
serpentine heater Sylvania No. 010226 is capable of thermal shocking
ceramic samples such as lanthana-doped yttria to the point of fracture.
One example is described below. Standard La.sub.2 O.sub.3 -doped Y.sub.2
O.sub.3 (containing 9 mole percent La.sub.2 O.sub.3) sample disks (3/4
inch diameter by 2 mm thick) typically fractured during the test when the
hot gas stream temperature was 350-530.degree. C. Fracture initiated from
the central portion of the sample, eliminating edge or corner effects
encountered in other tests such as quenching tests. The time to fracture
ranged from about 10 sec to about 90 sec, depending on the test sample.
The sample holder 50 was stationary and the heat gun 60 was mounted on
movable portion 100 which was injected toward the sample 90. This
configuration produced a good repeatibility of the test conditions. The
test method consisted of (1) injecting the hot gas gun 60 toward the
sample 90 (2) maintaining the hot gas gun 60 at a position (.about.5/32")
above the sample 90 for 2 min, (3) if the sample 90 did not fracture in
the 2 min exposure period, raising the hot gas gun 60 to let the sample
cool to ambient, and then restarting steps (1) to (2) again at hot gas
temperatures incrementally higher until the sample 90 fractured, and (4)
if the sample 90 fractured (this was typically observed by visual
examination, or the acoustic emission of fracture typically produced a
sound that was audible to the observer), raising the hot gas gun 60 and
removing the sample 90 when it cooled to ambient. The test results are
expressed in terms of critical temperature difference (.DELTA.Tc) which is
defined as the critical hot gas temperature (the hot gas stream
temperature at which the sample fractures) minus the initial temperature
(room temperature). The results of thermal shock testing on two types of
La.sub.2 O.sub.3 -doped Y.sub.2 O.sub.3, YL466 and YL489, are listed in
Table 1. The probability of thermal shock fracture (probability of
failure) can be calculated by the standard procedure of (1) ranking the
.DELTA.Tc, and (2) calculating the failure probability by R/(1+N) where R
is the order in the ranking, and N the total number of test samples. One
example of the probability of failure vs critical temperature difference
(.DELTA.Tc) is shown in FIG. 5.
TABLE 1
______________________________________
RESULTS OF HOT-GAS JET THERMAL SHOCK TESTING
Critical Temperature Difference (.degree.C.)
.DELTA.Tc, (critical hot gas stream temperature -
room temperature)
Sample Type Sample Type
YL489 YL466
.DELTA.Tc .DELTA.Tc
______________________________________
508 508
483 508
464 483
464 464
438 464
438 438
438 438
415 438
415 438
374 438
324 415
303 389
______________________________________
An alternative procedure consists of testing with a high .DELTA.Tc. For
example, .DELTA.Tc=508.degree. (hot gas stream temperature =530.degree.
C.) is used for all samples, and the time to fracture is measured. The
absolute value of an average time of fracture provides a measure of the
resistance to fracture.
The structure of the apparatus can be easily be modified for automation.
For example, the press can be replaced with a solenoid device that injects
the heat gun upon activating the solenoid. The hot gas stream impinging
control 30 can be replaced with a diaphragm that opens up automatically
when the heat gun 60 is injected toward sample 90. Heaters of higher
temperature capabilities such as tungsten-wire heater or platinum-wire
heater with quartz, alumina, zirconia, or yttria envelopes can be used for
producing hot air (platinum-wire heater) or hot gas (e.g. Ar, He, or
N.sub.2 with tungsten heater) at high temperatures (>1000.degree. C.)
required in thermal shock testings of materials such as SiC or Si.sub.3
N.sub.4 that are more resistant to thermal shock than yttria. The thermal
shock apparatus 10 and the methods described are not limited to ceramics,
other materials such as glass, composites, and polymers can also be
tested.
While there has been shown and described what is at present considered the
referred embodiment of the invention, it will be obvious to those skilled
in the art that various changes and modifications may be made therein
without departing from the scope of the invention as defined by the
appended claims.
* * * * *
|
|
 |
|
 |
Description |
|
