Note: Descriptions are shown in the official language in which they were submitted.
1 3 1 ~ ~ 1 2 86-1/9~43 ll
SYSTEM FOR STABILIZING DIMENSIONAL PROPERTIES
OF CURED COMPOST~E STRUCTURES
BACKGROUND OF THE INVENTION
1. Field of Inventicn
This invention relates to stabilizing the
properties of advanced composite materials such as
epoxy resin organic carbon fiber composites. In
particular, the invention relates to a method and
apparatus for inducing potential dimensional stability
in cured composite structures by usinq passive stress
wave acoustic emission analysis to determine the
appropriate level and extent of coincident thermal
conditioning.
Advanced composite materials, primarily
graphite and aramid fiber reinforced epoxy resin
materials, are attractive to a number of applications
due to their high specific stiffness, high strength,
and low coefficients of thermal expansion (CTE). Among
the applications are antennas and microwave components
for satellites and other spacecraft. These
applications impose stringent demands on dimensional
stability over a broad range of operating temperatures
and stress conditions. However, such composite
materials are heterogeneous as well as anisotropic.
Consequently, thermal treatment, including thermal
loading during and subsequent to the formation of the
; solid state of the material, induces internal residual
stresses. These residual stresses are subject to
relaxation with time, which results in dimensional
- modificationt the extent of which may be unacceptable
in certain critical applications, such as spacecraft
antenna systems. Therefore, great interest exists in
techniques for inducing controlled stress relaxation in
composite structures and for verifying consequent
dimensional stability.
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2. Description of the Prior Art
The following patents were uncovered in a
search of the public records of the U. S. Patent and
Trademark Office respecting the sub~ect invention:
INVENTOR U.S. PATENT NO.
Bristoll et al. 3,934,451
Keledy et al. 3,713,127
Vahaviolos 3,924,456
Scott et al. ` 4,089,224
Bartoli et al. 4,126,033
Rosencwaig 4,255,971
Fukuda et al. 4,353,255
Imam et al 4,380,172
~uate 4,430,897
Rosencwaig 4,484,820
Keledy et al, Vahaviolos, Scott et al, and
Fukuda et al disclose acoustic emission monitoring
systems which are principally used to monitor crack
formation and growth in a structure. There is no
disclosure in these patents of conditioning the
structure to relieve residual stress.
Bristoll et al. discloses a method of
detecting imperfections in a lining of foamed material
by first cooling the surface of the lining and then
observing the lining for imperfections in the cooled
state. There is no disclosure of acoustic emission
monitoring or of inducing stress relief to stabilize
dimensional properties.
; Imam et al. discloses methods for detecting
incipient cracks in a turbine rotor in which a
signature analysis is used for comparison of vibration
patterns after perturbation to normal vibration
patterns. There is no suggestion of any mechanism for
; controlling stress-related-characteristics.
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Bartoli discloses a method for determining
the thermal conductances of binding layers of detectors
in infrared detector arrays.
Rosencwaig '820, Quate and Rosencwaig, '971
relate to acoustic and thermoacoustic microscopy.
Additionally, U.S. Patent No. 4,494,408 was
issued January 22, 1985 to the inventor of the present
invention and assigned to the same assignee. That
patent discloses method and apparatus for monitoring
and controlling potential residual stress relief
mechanisms in composite materials by acoustic emission
signature analysis during the cure cycle. The
invention described in the present application is
distinguished in that it provides extended teachings
concerning the use of post-cure constant temperature
conditioning to stabilize dimensional properties of
composite materials.
SUMMARY OF THE INVENTION
According to the invention, a method and
apparatus are provided for stabilizing dimensional
properties of advanced composite materials, and
particularly for predicting dimensional stability of
cured composite structures using acoustic emission
analysis while actively stabilizing dimensional
properties by controlled thermal conditioning. The
invention is based on a discovery that thermal
conditioning at a constant temperature may bias the
mechanisms of stress relief to induce dimensional
stability in composite structures. Furthermore, by
analyzing selected interdependent parameters manifest
as features of a time-dependent function (herein
interdependent waveform features) of the acoustic
emissions, it is possible to detect the presence of
specific stress relaxation mechanisms in the composite
and to thereby generate a signature of stress
relaxation events with respect to time. (Events
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correspond to acoustic reports associated with
mechanical stress waves generated upon the breaking of
bonds in the subject material. Events are generally
identified by the detection of a transient sound having
an amplitude above a predetermined threshold and
substantially discriminated from sources of external
and known noise.) This signature may then be compared
against selected criteria to identify a state of
stress/strain equilibrium, and corresponding
dimensional stability, of the composite.
According to a specific embodiment of the
invention, sensor means, such as a passive transducer,
sensitive to acoustic signals generated by relaxation
of stress mechanisms within the subject composite
structure, are coupled to the composite and a
controlled thermal load is placed on the subject
structure, thereby ramping the temperature at a
constant rate to a low limit. Analyzer means receive
the signals from the sensor means after conditioning
(filtering and thresholding) and digitizing of the
signal. The analyzer means analyze the interdependent
waveform features, such as one or more ratios of
parameters, and compare them to empirically derived
data to recognize specific stress relaxation events.
When the occurrence rate of such events drops below a
level characteristic of dimensional stability, and
remains below that level for a specific duration,
controller means terminate the thermal conditioning
period.
One of the purposes of the invention is to
provide noninvasive, in situ means for inducing the
relaxation of internal residual stresses (herein stress
relaxation response) in advanced composite structures.
Another purpose of the invention is to
provide noninvasive, in situ means for recognizing
acoustic signals indicative of stress relaxation events
in advanced composites.
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Another purpose of the invention is to
provide constant temperature conditioning in lieu of
conventional cyclic temperature conditioning to
dissipate induced residual stresses naturally, i.e. via
redistribution networking, without degrading mechanical
or thermal properties.
Another purpose of the invention is to use
the distributed properties response (viscoelastic
signature response) of the composite to qualify like
properties for quality assurance.
Another purpose of the invention is to
provide means for confirming dimensional stability of a
composite subject to post-cure thermal condition.
These and other purposes of the invention
will be apparent by reference to the following detailed
description taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram of an apparatus for
monitoring and controlling the post-cure thermal
conditioning of advanced composite structures.
Fig. lA is a schematic diagram showing a
temperature control means of Fig. 1.
Fig. 2 is a flow diagram of a method for
analyzing acoustic emissions from a composite structure
to recognize acoustic signals indicative of stress
relaxation events.
Fig. 3 is a representative signature of
stress relaxation events over time for a composite
structure undergoing thermal conditioning.
Fig. 4 s representative of an actual data
plot of a stable composite element showing stress
relaxation events per unit time.
Fig. 5 is a representation of an actual data
plot of a "quasi"-stable composite element showing
stress relaxation events per unit time.
DESCRIPTION OF PREFERRED EM~ODIMENT
Fig. 1 illustrates a specific embodiment of a
system 10 for stabilizina the dimensional properties of
cured composite structures. The structures, as, .or
example, a specimen 30, are typically formed of
graphite fiber reinforced epoxy resin materials,
hereinafter referred to as composites, which may be in
the form of a tubular element, laminate sheet or other
shaped form. The thermal conditioning system 10
comprises a digital controller 12, timing means 14,
terminal means 16, printer means 18, recorder means 20,
analog-to-digital signal converter means 22, analyzer
means 24, temperature control means 26, and a
conditioning chamber 28. Specimen 30 of the composite
is placed in the conditioning chamber 28 and sensor
means 32 are coupled to the specimen 30. Noninvasive
coupling of sensor means 32 may be accomplished using a
medium having a high viscosity index, such as
anti-seizing compound, which will retain its
consistency during the temperature excursion applied to
the specimen 30. Low thermal expansion tape, such as
Kapton tape, may be used to attach the sensor means 32
to the specimen 30. The sensor means 32 should be
thermally conditioned prior to attachment to the
specimen 30.
The sensor means 32 may comprise a
piezo-electric transducer either having a flat
(broadband) response or having a suitable resonant
response, that is, responsive to acoustic stress waves
generated within the specimen between approximately 150
~ kHz and 2 ~Hz freauency. The electrical output of the
; sensor means 32 is coupled to an amplifier 34, the
output of which is coupled to a band-pass filter 36.
The output of the filter 36 is coupled into signal
converter means 22. Signal converter means 22 receives
analog input signals at a sample-and-hold circuit 38,
the output of which is coupled into an
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analog-to-digital converter 40. The sample-and-hold
circuit 38 and the analog-to-digital converter 40 are
clocked by timing means 14 via clock~ line 15. The
outputs of the signal converter means 22 are coupled
into analyzer means 24 via lines 23. Analyzer means 24
may be a programmable 8 bit microprocessor with memory
means, or may be other suitable means for analyzing the
incoming digital signal. Optional thermocouples 42 mav
be thermally attached to the specimen 30 and coupled
via signal lines 43 into analyzer means 24 so that the
analyzer means 24 may optionally receive both acoustic
and thermal input signals direct~y from the specimen
30. Additionally, the analyzer means 24 may receiv~
command input and timing input signals from, for
example, the digital controller 12.
In general, temperature of the specimen 30
need not be closely correlated with the acoustic
signals of specimen 30 when it is subjected to constant
temperature loading, as described below for this
embodiment. For simplicity, temperature of the ambient
environment rather than actual surface temperature of
the specimen may be monitored, if constant temperature
loading is employed. There are various means for
conveying ambient temperature information from the
conditioning chamber 28. In the example shown in Fig.
1, ambient temperature signals are conveyed by a line
44 to external parameter input means 45, digitized and
conveyed to digital controller 12 via digital input
signal bus 47 as a part of the so-called external
parameter inputs.
Accepted acoustic signals from analyzer means
24 are coupled into the digital controller 12. The
digital controller 12 also receives inputs from timing
means 14 via line 15' and from external parameter input
means 45 via bus 47, which provides such external
parameters as the ambient temperature inside the
conditioning chamber, as discussed above.
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Referring to Fig. lA, the digital controller
12 cooperates with the temperature control means 26 via
lines 49 and 49'. The digital controller 12 may convey
simple on-off signals to temperature control means 26
to alternately regulate heat input and withdrawal from
the conditioning chamber 28. The temperature control
means 26 may take the form of a first relay 50 and a
second relay 52 for switching a power source 55 to a
heater (not shown) in the conditioning chamber 28 via
line 51 or to a cooler (not shown) in the conditionina
chamber 28 via line 53. Line 49 may control relay 50
and line 49' may control relay 52.
Returning to Fig. 1, the digital controller
12 may be bidirectionally coupled to both analyzer
means 24 and terminal means 16. The terminal means 16
is for operator interface and for communication to the
printing means 18 and recording means 20. In one
embodiment the digital controller 12 may therefore be a
simple data acquisition and signal processing device to
provide controlled temperature regulation, analogous to
a thermostat. Alternatively, the digital controller 12
and temperature control means 26 may be in combination
a commercially available controller such~as the Sym-Com
No. 3 temperature controller and Sym-Com 5 digital rate
programmer by Sym-Tek company of Santa Clara,
California. One of the features of these commercial
apparatuses is the ability to positively control
temperature ramp rate during the initial application of
a thermal load.
The invention operates as follows: following
attachment of the sensor means 32 to the cured
composite specimen 30, the temperature inside the
conditioning chamber 28 is first raised, under control
of temperature control 26 via line 51, to a level
sufficient to drive off volatiles such as moisture
which may be absorbed or otherwise entxapped by the
specimen 30. The temperature in the conditioning
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chamber 28 is then lowered using suitable coolinq
means, such as, for example a dry nitrogen gas
injection system (not shown) under control of
temperature control 26 via line 53. The temperature
normally is lowered according to a bounded ramp rate
under control of digital controller 12. The ramp rate
should be sufficiently slow to inhibit stress damage
which might otherwise result if a temperature ramp rate
is applied which is extreme in comparison to the
maximum contemplated temperature ramp rate for the
contemplated operating environment. The temperature is
dropped to a level approximating the low temperature
limit to which the specimen will be exposed in actual
application and held at that level. For most
applications of thermal conditioning of composite
structures for use in spacecraft, the temperature ramp
rate generally should not exceed about -10C/minute,
and is pre~erably between approximately -1C/minute and
-5C/minute; the low temperature limit will generally
be in the range -40C to -100C, with a typical extreme
temperature for a component which is protected in its
contemplated operating environment at about -50C.
The acoustic signals from the sensor means 32
are amplified by the amplifier 34 and conditioned by
band-pass filter 36 to remove spurious signals, such
as, for example, signals from mechanical noise having a
frequency below about 100 kHz. The resulting signals
are digitized by signal converter means 22 and sent on
to the analyzer 24 for analysis. Acoustic signals
desired to be monitored from the specimen 30, called
accountable signals, i.e., those signals representative
of stress relief mechanisms occurring in the specimen
30 in response to the controlled thermal load, are
recognized by the analyzer 24 and sent to the
controller 12. A signature of the rate of stress
relaxation events per unit time versus elapsed
conditioning time is extracted from the accountable
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signals, where stress relaxation events are defined as
acoustic reports of the breaking of atomic and/or
molecular konds. The signature of the structure
comprises at least indicia at selected times of the
rate stress relaxation events per unit time. The
extracted signature is compared with selected criteria
from a predefined signature template (and optionally
preprogrammed limits) to determine whether the indicia
satisfy the selected criteria and thus the course and
need for continued conditioning. The predefined
signature template is derived from an observed response
of an eauivalent composite structure when exposed to an
analogous thermal load. Analyzer means 24 and
controller means 12 act together to continuously
analyze the signals from sensor 32 to determine the
point in time where stress/strain equilibrium is
reached. Thereafter the thermal load may be removed.
In general, temperature control means 26 is
controlled by the digital controller 12 for applying
heating and cooling for sustained conditioning of the
specimen 30, or for terminating the conditioning period
in response to the analyzed acoustic signal. Terminal
16 provides means for memory access and programming of
analyzer 24 to facilitate on-line diagnostics to study
various composite forms, and to initiate changes, such
as sampling interval, signal conditioning, and display
options. An operator may manually override the
controller means 12 to alter the conditioning
parameters (via temperature control means 26) according
to the analyzed response of the sensor means 32, as
observed through printer means 18 and recorder means
20. Timing means 14 provide a clock for initiation and
coordination of the interdependent functions for data
sampling, analysis, and routing of signals.
The recognition of accountable acoustic
signals from the sensor 32 will be understood with
reference to Fig. 2. The signal from the sensor 32 is
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conditioned (60~ by the band-pass filter 36, and those
components which do not reach a preselected minimum
voltage threshold are rejected (62). Specific
interdependent parameters, such as event duration,
ringdown count, and/or peak amplitude are then measured
(65,66~. ~wo or more of these interdependent
parameters are then interrelated (68) and compared (70)
to empirically derived data correlating to accountable
stress relaxation mechanisms. In the preferred
embodiment, the interdependent waveform features
derived from the parameters are frequency dependent and
consist of a quotient derivation equal to the number of
threshold crossings (ringdown count) of the signal,
divided by event duration (68~. Event duration is
determined by the elapsed period from the time of the
first signal threshold crossing to the last signal
threshold crossing, corresponding to the energy of the
mechanical wave exciting the attached piezo-electric
transducer and its dependent signal decay. Other
interdependent waveform features, such as the peak
amplitude of the signal divided by the ringdown count,
or the energy of the signal divided by the ringdown
count, may be substituted within the scope of the
invention.
The interdependent waveform features may be
used to identify specific stress relaxation mechanisms
while discounting the signal from fiber fretting and
extraneous electrical and mechanical noise sources.
The resulting signature data may be used in conjunction
with selected benchmarks from exemplary signature
templates to monitor and control thermal conditioning.
A further aspect of the invention is based on
the generation and identification of a state of induced
stress/strain equilibrium, and corresponding
dimensional stability, within the intended operational
temperature limits of any given composite form.
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Referring to Fig. 3, a representative
signature 80 is shown, plotting stress relaxation
events per unit time vs. elapsed conditioning time.
The point of onset 82 of accountable stress relief is
seen where the slope of the curve abruptly rises
towards a peak value which is reached at point 84. The
onset stress temperature is dependent upon the
constituents of the composite and its previous thermal
history. In general, the onset stress temperature may
be presumed to be based on the previous low-temperature
history of the specimen. In the example shown in Fig.
3, the onset stress temperature for significant stress
relaxation, corresponding to point 82, is reached at
approximately -5C during the downward temperature
ramping. The measured stress relaxation response is
maximum at a point 84 at a time after the lower limit
of the imposed temperature conditioning is reached, at
approximately -50C. In this example, this low
temperature is maintained for the remainder of the
conditioning period.
In the typical case illustrated by Fig. 3,
localized stresses continue to develop in the composite
(due to redistribution phenomena) for some time after
the point of maximum stress 84. It is believed that
the viscoelastic response of the composite is
responsible for renewed stress relaxation activity, as
evidenced by the second peak 88 in the signature. A
state of "quasi-stability" is reached at point 90,
wherein the rate of stress relaxation events has fallen
below a benchmark of approximately ten percent of the
maximum sustained acoustic emission event rate response
indicated by point 84. The material is considered
suitable for its contemplated use when operational
stability is reached. Operational stability is defined
in terms of a multiple of the elapsed time required to
generate a quasi-stable state, as defined above. In
the example illustrated by Fig. 3, thermal conditioning
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continues for an additional time period of at least two
times (2T) the duration of the dwell period (T)
required to achieve quasi-stability. The dwell period
T commences at the onset (point 82~ of accountable
stress relief. In the example shown, the rate of
acoustic emission events continues to remain below the
ten percent threshold 92, reaching a point of
operational stability as defined above after a period
3T at point 94. If the event rate increases above the
ten percent threshold 92, prior to the time represented
by point 94, conditioning continues for at least an
additional time period equal to two times the duration
from the point of onset 82 of accountable stress relief
to the last point of quasi-stability (For example,
after point l00 of Figure 5).
Fig. 4 is an example of an actual data plot
95 of a stable composite structure. Once a condition
of quasi-stability is reached, it is maintained until
operational stability is reached. Fig. 5 is an example
of an actual data plot 96 of a sample which would
require further conditioning after first reaching
quasi-stability as described above. At point 98, the
event rate rises before operational stability is
reached, thus reauiring further conditioning.
Conditioning should continue after point l00 for a
period of at least twice the dwell period (T') needed
to reach quasi-stability at point l00. In general, the
period required for conditioning varies with each
sample specimen and may range from two to more than
thirty hours, depending on the nature of and
configuration of the specimen.
By imposing a controlled thermal load on the
specimen, while detecting the initiation and arrestment
of microcracks or other stress relief mechanisms, it is
possible to bias the nature of stress relaxation and to
induce and identify a state of operational stability.
For example, in the case of graphite-epoxy composites
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such as represented by the signature of Fig. 3, the
incremental strain energy developed as a crack is being
formed becomes greater than the stored energy beyond a
few fiber diameters of the crack sites, and is
independent of any existing crack site. Accordingly,
if the induced stress is sufficiently high, it is more
probable that new cracks will form than that old cracks
will propagate in order to stabilize the composite.
The viscoelastic behavior exhibited by the composite at
low temperatures is utilized to redistribute the
induced thermal stresses around neighboring good bonds
in a manner which minimizes the residual stresses and
the possibility of further change in the material.
The invention has now been explained with
reference to a specific embodiment. Other embodiments
and applications of the invention will be apparent to
those of ordinary skill in the art upon reference to
this application. It is therefore not intended that
this invention be limited except as indicated by the
appended claims.
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