RP035 -
Correlation of Dielectric Cure Index to Degree of Cure for
3501-6 Graphite Epoxy; Micromet Instruments, Inc., Dr. David
R. Day and David D. Shepard
ABSTRACT
Dielectric sensing
is currently the most promising technique for monitoring and
controlling composite curing. Several sensors have been developed
to withstand the rugged and abusive environments found in production
presses and autoclaves. Much work has been done interpreting
the dielectric signals and using them to control various processes
in real time.
Measurements
are typically taken at several frequencies (0.1Hz to 100KHz)
over several orders of magnitude and the ionic conductivity
is extracted from the response. During the course of the cure
the ionic conductivity is a strong function of cure state and
temperature. In this work we have measured the temperature
dependence of the uncured and cured resin. The temperature
dependence of the dielectric response during cure is then removed
from the data. Finally, the dielectric cure index is compared
to degree of cure data from both the Springer
cure model and the Landuyt
cure model. In all cases the cure index was very sensitive
to small changes in cure state near the end of cure.
Return
to Top
RP039 -
On the Use of Cure Modeling in Honeycomb Processing; Hexcel
Corporaton., L. Chiao and P.W.Borris
ABSTRACT
A general cure
model for honeycomb core based on physical principles has been
applied to both aramid paper and a fiberglass fabric honeycomb
coated with phenolic resin. The cure model was used to design
cure cycles which are much shorter than those currently used
in industry, but advance the resin to the same degree of cure,
as verified by microdielectrometer experiments. Plant trials
show that these modified cure cycles result in core with equivalent
mechanical properties to those which go through the standard
cure cycles.
The model was
also used to predict cure reaction runaway (exotherm) behavior.
This informaton is useful in analyzing regions of the honeycomb
block where the airflow is poor and also situations of sudden
airflow loss.
Return
to Top
RP041 -
Dielectric Cure Monitoring During Composite Lamination; IBM
Corporation, Jeffery Gotro
ABSTRACT
Oscillatory
parallel plate rheometry is commonly used to measure the viscosity
of neat resins during curing. The objective of this work was
to use a dielectric sensor embedded in an epoxy/glass cloth
prepreg layup to infer the viscosity profile during lamination.
This method allowed qualitative characterization of the viscosity
profile during lamination at different heating rates.
Correlations
between the dielectric loss factor and the complex viscosity
were established using simultaneous viscosity and dielectric
measurements. Viscosity measurements during composite lamination
would be virtually impossible. The dielectric method is simple
and relatively inexpensive compared to the cost of most rheometers.
Return
to Top
RP044 -
PMR-15: Aging Detection Using Dielectric Sensors; Micromet
Instruments, Inc., David. D. Shepard
ABSTRACT
The aging of
PMR-15 graphite reinforced prepregs prior to processing can
affect the performance of the resin system. PMR-15 consists
of three monomers blended together in anhydrous methanol which
is prepregged and stored in the monomer stage. Out time can
result in aging which will increase the viscosity and reduce
the flow of the resin. This in turn may affect the thermal
and mechanical properties of the cured part. Dielectric measurements
can monitor the entire cure cycle of PMR-15 graphite prepregs
in both research and production environments.
The progressive
room temperature aging of the prepreg over a 31 day period
was observed using a rugged, reusable dielectric sensor. During
a typical cure cycle, major differences in the Ion Viscosity
(resistivity) were observed prior to imidization in samples
aged to various degrees. These differences were attributed
primarily to solvent volatilization and monomer reaction. The
more aged samples appeared to complete their final cross-linking
reaction at an earlier time than the fresher samples.
Return
to Top
RP051 -
A New Dielectric Instrument for In-Mold Statistical Process
Control of RIM, SMC, and RTM; Micromet Instruments, Inc.,
Dr. David R. Day, Huan L. Lee, David D. Shepard, and Norman
F. Sheppard
ABSTRACT
The usefulness
and versatility of dielectric measurements in monitoring the
curing behavior of thermosetting resins and composites has
been widely demonstrated. The ability to make dielectric measurements
in the process environment allows the technique to be used
for research, process development, and production applications.
The ICAM-1000
is a new instrument designed for process monitoring and Statistical
Process Control of short cycle time processes such as RIM,
SMC, and RTM. The system utilizes high-speed dielectric, pressure,
and temperature measurements to monitor the intrinsic and extrinsic
properties of the resin during processing. Measurements are
made through reusable sensors flush mounted in the mold. Software
provides the ability for window analysis and provides output
for SPC use. Examples of the application of the instrumentation
in RIM, SMC, and RTM processes will be discussed.
Return
to Top
RP050 -
Moisture Diffusion Monitoring of Polyetheretherketone (PEEK)
with Microdielectric Sensors; Micromet Instruments, Inc.,
Dr. David R. Day, David D. Shepard, and Kelly J. Craven
ABSTRACT
Classical diffusion
analysis involves weight uptake measurements as a function
of time on thin polymeric samples. The use of Microdielectrometry
to monitor the diffusion of moisture through polymers is a
relatively new technique. Sensors have been developed which
are sensitive to the small changes in permittivity (dielectric
constant) of polymeric materials as water concentration (and
thereby, mobile dipole concentration) increases. In addition,
sensors exist which can withstand the high temperatures (greater
than 350C) required for the consolidation of high temperature
thermoplastics. The diffusion coefficient can be estimated
through a simple Fickian diffusion model of the dynamic dielectric
response. The Microdielectric sensors measure a very localized
area of approximately 0.01cm into the material. This allows
monitoring diffusion properties of thin films, or of many simultaneous
and independent locations in a bulk material.
Previous work
has studied the diffusion characteristics of epoxies and polyimides
by this technique. This study examines the diffuson of water
through a thin film of polyetheretherketone (PEEK) and compares
it with published data obtained by weight uptake studies. The
diffusion rate of moisture through amorphous PEEK is shown
to be slightly faster than diffusion through more crystalline
PEEK.
Return
to Top
RP056 -
Automotive/Aerospace Synergism in Computer-Aided Composites
Processing; Advanced Composites Engineering, GM Delco Products
Division, Johnny Gentry
ABSTRACT
In-mold dielectric
cure monitoring techniques pioneered by the aerospace industry
were applied to the manufacturing process of LITEFLEX® springs.
Real-time data was collected in-situ and used to interpret
the progression of cure in an epoxy/fiberglass composite. This
information was used to trigger mold pressurization in an attempt
to lower void content and improve part appearance.
A microdielectric
sensor placed on the mold surface collected temperature and
conductivity data that was converted to a cure percentage scale
(cure index). The optimum pressurization point for the epoxy
system was at approximately 40% conversion. Although results
varied somewhat depending on spring geometry, void contents
were reduced to less than 1%, composite shear strength increased
10-15% and surface appearance was also significantly improved.
Return
to Top
RP070 -
Computer-Aided Curing of Composites; McDonnell Aircraft Company
ABSTRACT
The objective
of this project is to demonstrate an artificial intelligence
based control system using hardened sensor systems and an advanced
knowledge base in an advanced composite facility. To achieve
this objective, the program was divided into three tasks.
I. Sensor development:
we are developing sensor technologies to interrogate composite
materials during processing to allow intelligent processing
decisions to be made based on actual material states. The developed
sensors will be production-hardened for real-time composite
processing control in a manufacturing environment.
II. Control
Rules and Strategies: we investigate three critical composite
properties (void volume, thickness, and resin state). A thorough
understanding of these properties, and how to control each
one, is crucial to this program. They are the goals which must
be met for each part produced by the intelligent controller.
We add on-line modeling capabilities to the QPA control system.
Finally, we develop and demonstrate new control rules and strategies.
III. Advanced
Processing Demonstration: we demonstrate the intelligent control
system on a stiffened wing box configuration that represents
one of the leading design candidates for a future fighter aircraft
wing. We study innovative processes to identify the best unit
process for QPA applications and assess the effect of the QPA
approach to the candidate unit process.
Return
to Top
RP083 -
Cure Characterization of Thick Composite Parts Using Dielectric
and Finite Difference Analysis, Micromet Instruments, Inc.,
David R. Day
ABSTRACT
Curing characteristics
of thick composite parts have long been known to be a function
of thickness and location within the parts. While cure state
as a function of thickness has often been modelled, actual
experimental verification has been difficult. In this work,
disposable and permanently mounted dielectric sensors were
used to characterize the cure in a thick composite part a various
locations through the thickness in a simulated molding environment.
Using established techniques, the dielectric and temperature
information were combined to yield local cure state information
for each sensor. The cure state was found to be significantly
nonuniform through the thickness of the part. These observed
cure state data are compared to model predictions.
Return
to Top
RP046 -
Production Implementation of Fully Automated, Closed Loop
Cure Control for Advanced Composite Structures; General Dynamics,
Sean Johnson and Nancy Roberts
ABSTRACT
Economics of
advanced composite part production requires development and
use of the most aggressive cure cycles possible without sacrificing
quality. As cure cycles are shortened and heating rates increase,
tolerance windows for process parameters become increasingly
narrow. These factors are intensified by condensation curing
systems which generate large amounts of volatiles.
Management of
the situation requires fully automated, closed loop process
control and a fundamental understanding of the material system
used for the application. No turnkey system for this application
is currently available. General Dynamics Pomona Division (GD/PD)
has developed an integrated closed loop control system which
is now being proofed in production. Realization of this system
will enable cure time reductions of nearly 50 percent, while
increasing yield and maintaining quality.
Return
to Top
RP110 -
In-Process Cure Monitoring of Fiber Reinforced Polymer Composites
Using Ultrasound, United Technologies Research Center, William
Veronesi
ABSTRACT
Compression
molding of fiber reinforced thermosetting polymer composite
parts typically involves controlling the temperature and pressure
of a molding system. Changes in temperature cause changes in
the viscosity of the plastic system during the consolidation
phase of processing, and the final modulus of the plastic system
is largely determined by the length of the final hold at high
temperature. Pressure is typically adjusted based on an estimate
of what these two key parameters, viscosity and modulus, are
at any point in a cure cycle and what they will be in the near
future. Using ultrasonic methods, these parameters can be measured
in real time during the cure cycle. Real-time access to these
parameters allows for the potential control and optimization
of the cure cycle. A non-intrusive sensor capable of operating
in the high temperature environment of polymer composite processing
has been developed. A complete ultrasonic cure monitoring system
has been developed to complement the sensor. The complete system
is described, and experimental data, obtained in-process with
this system, is compared with off-line laboratory measures
of viscosity and in-process dielectric sensor measurements.
Return
to Top
RP096 -
A Microdielectric Analysis of Moisture Diffusion in Thin
Epoxy/Amine films of Varying Cure State and Mix Ratio, Micromet
Instruments, Inc. David Day, David Shepard, and Kelly Craven
ABSTRACT
This work shows
that dielectric sensors can be used to monitor moisture diffusion
in polymer materials ranging from composites to thin films.
When the technique is used with thin films, diffusion coefficients
and degree of moisture uptake can be determined very quickly.
This paper describes preliminary moisture diffusion analysis
carried out in thin epoxy films as a function of ultimate cure
state and mix ratio of epoxide to amine components.
Return
to Top
RP114 -
Application of Dielectric and Thermal Analysis to the Curing
of Epoxy Resins, Toshiba Corporation, T.Nakano, S.Makishima,
Y.Inoue, K.Goto
ABSTRACT
Upon studying
the application of dielectric analysis (DEA) to techniques
for non-destructive on-line measuring of the curing process
of epoxy casting resin in molds, it was found that the ionic
conduction portion derived from dielectric loss factor was
closely correlated to glass transition temperature and resin
viscosity. A new study was undertaken using DEA, differential
scanning calorimetry (DSC), dynamic mechanical analysis (DMA)
and curing volume shrinkage measurement to investigate in more
detail the relation between the electrical conductivity of
resins and the degree of cure during the curing process. A
relation was quantitatively determined between the equivalent
resistivity derived from dielectric loss factor and the degree
of cure, viscoelasticity and volume shrinkage.
Return
to Top
RP118 -
- Effect of Advancement of Epoxy Prepreg Processing - A Dielectric
Analysis, Micromet Instruments, Inc., David R. Day and David
D. Shepard
ABSTRACT
With the relatively
recent trend toward adaptive process control during processing
of structural composites, there is need for an understanding
of the nature of variations during cure that are encountered.
Part-to-part variations can arise because of differences in
thermal exposure arising from part placement or part thickness,
or from chemical differences due to formulation or advancement.
Microdielectric sensors are particularly well suited for monitoring
thermoset cures from initial point of flow, through the liquid
region, and on into the solid state. In this study, microdielectric
sensors are used to systematically study the influence of resin
advancement on cure behavior. Relationships between advancement
and critical cure phenomena such as flow temperature, point
of viscosity minimum, and reaction end point are discussed.
Return
to Top
RP120 -
Ultrasonic Cure Monitoring of Advanced Composites, Micromet
Instruments, Inc., David D. Shepard, Kim R. Smith
ABSTRACT
Measurements
of the ultrasonic sound speed of thermosetting resins and composites
can be used as an in-process cure monitoring technique. Ultrasonic
measurements have an advantage over other in-process techniques
in that ultrasonic sensors do not make contact with the part
(thus leaving no imbedded sensor or witness mark) and can make
true bulk measurements of the part. A new, commercially available
ultrasonic cure monitoring system for the cure monitoring of
thermosetting resins and composites has been developed. Advancements
in ultrasonic sensor technology enable the self-contained ultrasonic
sensor to be easily installed in a mold and maintain good coupling
to the part during thermal cycling to 260°C. Data is presented
showing the change in ultrasonic sound speed during compression
molding of graphite-epoxy prepreg. The data shows a good relationship
to the electrical resistivity data collected via dielectric
cure monitoring. The ultrasonic technique is also applicable
to phenolic based materials.
Return
to Top
RP071 -The
Curing of Epoxy/Amine Systems Viewed Through Microdielectrometry;
University of Lyon, C.Mathieu, G.Boiteux, G.Seytre, Laboratory
of Macromolecular Materials, INSA, M.Feve, J.P.Pascault,
Technical University of Lodz, Poland, J.Ulanski, and Aerospatiale,
P.Dublineau
ABSTRACT
Aim of this
article is to show how microdielectrometry can improve the
curing of epoxy resins by detecting in real-time critical events
like gelation or vitrification. The isothermal curing of DGEBA/3DCM
in stoichiometric proportions is followed with: electrical
techniques of microdielectrometry and DC measurements, analysis
of insolubles, dynamic mechanical analysis and viscosimetry.
Electrical techniques have evidenced their own efficiency to
detect chemical phenomena with parameters sensitive to the
modification of the system during the network formation.
Return
to Top
RP112 -
Processing and Properties of IM7/LARC-IAX2 Polyimide Composites,
Lockheed Engineering & Sciences Company, T.H.Hou and
A.C.Chang, NASA Langley Research Center, N.L. Johnston and
T.L.St.Clair
ABSTRACT
LARC-IAX2 (Langley
Research Center-Improved Adhesive eXperimental resin 2) aromatic
polyimide, based on oxydiphthalic anhydride (ODPA), benzophenone
tetracarboxylic acid dianhydride (BRDA), and 3,4'-oxydianiline
(3,4'-ODA), was evaluated as a matrix for high performance
composites. This polymer is a modified version of the baseline
LARCIA polyimide made from ODPA and 3,4'-ODA. Two poly(amide
acid) solutions end-capped with phthalic anhydride were synthesized
in N-methypyrrolidone and N,N-dimethylacetiamide (DMAc) with
ODPA to BTDA ratios of 4:1a and 3:1, respectively. Fully imidized
films exhibited improved solvent resistance in acetone, meghylethylketone,
toluene, DMAc, and chloroform compared to the baseline LARCIA
film. Unidirectional prepregs were fabricated from both solutions
using the Langley multi-purpose prepreg machine. A separate
molding cycle (350C/250psi) was developed for each prepreg
based upon solvent/volatile depletion characteristics. These
cycles consistently yielded void-free consolidated laminates
but required more severe processing conditions than those for
the baseline material. Short beam shear strength and longitudinal
flexural properties were measured at room temperature, 93,
150, and 177C. Notatbly, the flexural moduli were about 5%
higher than those reported for the baseline composite. In addition,
engineering properties such as fracture toughness, unnotched
tensile strength, notched and unnotched compressive strengths
and moduli, compression strength after impact and open hole
compression strength were also measured.
The enhanced
solvent resistance, more difficult processability, and higher
flexural moduli compared to those for the baseline composites
were attributed to an increase in polymer backbone stiffness
due to the presence of BTDA. No differences in the physical
and mechanical properties were observed between composites
made in NMP and DMAc. The results have helped define a chemical
approach for improving solvent resistance in thermoplastic
polyimide materials.
Return
to Top
RP085 -
Chemorheological and Dielectric Behavior of the Epoxy Matrix
in a Carbon Fiber Prepreg, Dept. of Materials and Production
Engineering, University of Naples, Italy, J.M.Kenny and A.Trivisano
ABSTRACT
A scientific
approach to the problem of autoclave curing of thermoset composites
includes modelling of the cure kinetics and viscosity of the
matrix. The objective of the present study was to develop and
test a chemorheological model for the commercial carbon fiber/epoxy
prepreg Fibredux HTA/6376 with a toughened epoxy matrix based
on TGMDA/DDS. The model for the cure kinetics takes into account
the effect of vitrification prior to full cure at cure temperatures
below the glass transition temperature of the fully cured epoxy.
The rheology of the matrix was modelled applying a similar
viscosity model as was previously verified for commercial epoxy
based prepregs containing the TGMDA/DDS system. The cure kinetic
model was verified by calorimetric measurements simulating
different processing conditions. Predictions of the viscosity
qualitatively agreed with dielectric measurements during prepreg
cure of the inverse of the ionic conductivity in the epoxy
matrix. In particular, predicted and measured times at which
minimum viscosity occured showed excellent agreement, even
for complicated cure cycles. Experimental measurements revealed
that the investigated epoxy matrix shows incomplete cure, approximately
90%, after the cure cycle suggested by the prepreg manufacturer.
Return
to Top
RP047 -
Monitoring Rheology During Cure of Prepreg in Autoclaves,
Tonen Co. Ltd. Carbon Fiber Project, O.Watanabe, S.Murakami,
H.Inoue
ABSTRACT
Several techniques
have been used to establish the monitoring and control of prepreg
curing in autoclaves. Dielectric analysis techniques were used
in this paper. Prepregs were made using Tonen pitch-based Ultra
High Modulus carbon fiber and three kinds of epoxy resins.
These prepregs were cured in an autoclave and their dielectric
properties monitored. The dielectric data were compared with
DSC reaction conversion data. These data show a very high correlation
and the results suggest that the optimum processing conditions
for prepreg can be accurately determined by DEA.
Return
to Top
RP121 -
End of Cure Sensing using Ultrasonics for Autoclave Fabrication
of Composites, Johns Hopkins University Applied Physics Laboratory,
Paul J. Biermann and Joan H. Cranmer; Edison Welding Institute,
Carol A. Lebowitz; Carderock Division Naval Surface Warfare
Center, Lawrence M. Brown
ABSTRACT
The objective
of this work was to demonstrate the use of ultrasonics to determine
the end-of-cure for autoclave cured, graphite/epoxy composite
laminates. The fundamental benefit of this work will be understanding
when to complete the temperature hold and cool down the autoclave
and, therefore, consistently produce composite laminates with
the desired material properties. An additional benefit is the
ability to follow the changing viscosity of the resin during
the initial part of the cure.
The general
approach to this program involved using pulse-echo ultrasonics
to measure the transit time for longitudinal ultrasonic waves
to pass through a graphite/epoxy composite laminate during
cure. Sixteen, 32 and 64 ply [0/90]s graphite/Fiberite 934
epoxy panels were fabricated and cured to various end-of-cure
conditions. Additionally, panels with various starting conditions
were run. Sound speed was calculated using the panel thickness
(number of plies multiplied by the manufacturer specified thichness
of each ply) divided by the measured transit time.
Mechanical (maximum
fiber stress and maximum strain) and glass transition temperature
(Tg) tests are being performed on all the fabricated panels.
Determining the relationships between the velocity profiles,
mechanical properties, and Tg data is in progress. Ultimately,
the goal is to understand the relationship between the ultrasonic
data and state-of-cure, and to incorporate the ultrasonic sensor
into an operational expert system for sensor feedback control
of the autoclave process.
Return
to Top
RP127 -
Dielectric and Thermal Cure Characterization of Resins Used
in Pultrusion, University of Mississippi, Reshma Shanku,
James G. Vaughan, Jeffrey A. Roux
ABSTRACT
The ability
to conduct on-line measurements of a resin's degree of cure
is important to a pultruder for setting optimum pultrusion
processing parameters. In this study, in-situ dielectric measurements
were conducted to determine the degree of cure as a function
of the pultrusion die axial position. Dielectric analysis utilizes
a sensor which is inserted at the center of the impregnated
fibers and travels along with the fibers through the heated
die. The conductivity due to ionic movement in the resin is
measured and a cure index is calculated from this data. The
purpose of this study was to determine degrees of cure for
pultruded epoxy/graphite, epoxy/glass and polyester/glass composites
using dielectric measurements and to compare the on-line measured
values with values obtained using differential scanning calorimetry
(DSC) and with values predicted using a numerical heat transfer/chemical
kinetic pultrusion model. A 2.54cm x 0.32cm profile was produced
at a pull speed of 30.5cm/min for the epoxy composites and
at 61cm/min for the polyester composite. DSC samples were taken
from the composite close to the location of the embedded sensor
to verify the values of degrees of cure obtained from the dielectric
measurements.
Return
to Top
|
