194,745 research outputs found
Cure modelling and monitoring of epoxy/amine resin systems
No full textThermoanalytical techniques and dielectric analysis were used in this study to describe
and characterise the cure processes occurring during the isothermal and dynamic cures
of four epoxy/amine resin systems.
The complexity of the cure reactions was illustrated by results from DSC and FTIR
experiments and was attributed to the variety of chemical reactions between the epoxy
and the amine groups. Several phenomenological and mechanistic cure kinetics models
were constructed, based on the cure reaction mechanisms, in order to simulate the
degree of conversion during the cure. A one-to-one relationship was established
between the degree of cure and the glass transition temperature of the curing resin,
which was finther used in the construction of chemoviscosity models and in a
simulation of the viscosity advancement during the cure.
A number of mathematical techniques were utilised to evaluate the parameters
involved in all the models, varying from simple linear regression methods to complex
non-linear least squared estimation procedures.
An in-situ dielectric monitoring technique was used in combination with the above
mentioned chemorheological models, to investigate the feasibility of a quantitative
correlation between the changes in the dielectric signal, the cure advancement and the
major physical transformations, namely gelation and vitrification. The imaginary
impedance response of the curing resin, as measured by the dielectric technique,
showed good agreement with the degree of conversion, depicting all the crucial
characteristics of the curing mechanism, such as autocatalysis and diflusion. The
endset of the cure reaction was also identified from the endset of the conductivity
changes and correlated to the vitrification time.
The analytical chemorheological models developed in this study to describe the cure
processes for some epoxy/amine resin systems, along with the dielectric monitoring
technique used, suggest that a real-time link between the above mentioned models and
the cure monitoring technique can be achieved. This would greatly enhance the
predictive capability of the technique and form the basis of a future feedback-loop
control system
Monitoring and Heat Transfer Modelling of the Cure of Thermoset Composites Processed by Resin Transfer Moulding.
No full textExperimental studies have demonstrated the existence of significant thermal
gradients during the cure stage of Resin Transfer Moulding (RTM). Presence of
such thermal gradients can affect the final degree-of-cure distribution and
cause the development of residual stresses, leading to a deterioration of the
final composite component properties. Therefore the incorporation of heat
transfer modelling in the general context of RTM modelling and monitoring of the
process is necessary. The present work focuses on the application of combined
monitoring and heat transfer modelling to the process. A finite element heat
transfer model incorporating resin cure kinetics has been developed and tested.
An inverse solution of the heat diffusion model has been implemented in order to
extend the local measurements given by in-situ monitoring to global information
about the temperature distribution and the degree-ofcure distribution during the
cure
Effect of carbon nanoparticle addition on epoxy cure
No full textThe thesis reports studies of cure kinetics and the glass transition
temperature advancements of three commercial epoxy resin systems: MY 750
/ HY 5922 (Vantico), MTM 44 -1 (ACG) and 8552 (Hexcel Composites). This
investigation was conducted with the utilisation of Differential Scanning
Calorimetry (DSC) and Temperature Modulated DSC (TMDSC). Appropriate
phenomenological cure kinetics models were built to predict the degree of
cure as a function of temperature/time profile. The validity of superposition of
dynamic and isothermal experimental data was established. Rheological
measurements were performed in order to determine the gelation region
under given cure conditions. The cure modelling methodology was validated
against an international Round-Robin exercise led by the University of British
Columbia (Canada).
The effects of carbon nanoparticle incorporation on the cure kinetics and
the glass transition temperature advancement of two of the epoxy systems
were also studied. Cure kinetics models were developed for the
nanocomposites containing commercial multiwalled carbon nanotubes and a
direct comparison was made with the models of the neat resin systems. The
glass transition temperature advancement is shown to be affected in the early
stages of the cure.
The state of the dispersion of the nanoparticles was studied in order to
correlate it with the observed effects upon the cure and on the morphology of
the cured samples. The presence of carbon nanotube clusters is shown to
have an influence on the phase separation in the MTM 44-1 resin system.
As a potential industrial application of this study, optical fibre
refractometers were utilised as an on-line cure monitoring technique. A good
correlation was established between the measured refractive index changes
during the cure and the degree of cure predicted by the above mentioned
models, for the neat resin systems and their nanocomposites
Stochastic simulation of the cure of advanced composites
No full textThis study focuses on the development of a stochastic simulation methodology to
study the effects of cure kinetics uncertainty, in plane fibre misalignment and
boundary conditions uncertainty on the cure process of composite materials.
Differential Scanning Calorimetry was used to characterise cure kinetics variability of
a commercial epoxy resin used in aerospace applications. It was found that cure
kinetics uncertainty is associated with variations in the initial degree of cure,
activation energy and reaction order. Image analysis was employed to characterise
in plane fibre misalignment in a carbon fibre ±45º non-crimp fabric. The experimental
results showed that variability in tow orientation was significant with a standard
deviation of about 1.2º. A set of experiments using an infusion set-up was carried
out to quantify boundary conditions uncertainty related to tool temperature, ambient
temperature and surface heat transfer coefficient using thermocouples (tool/ambient
temperature) and heat flux sensors (surface heat transfer coefficient). It was
concluded that boundary conditions uncertainty can show considerable short term
and long term variability. Conventional Monte Carlo and Probabilistic Collocation
Method were integrated with a thermo-mechanical cure simulation model in order to
investigate the effect of cure kinetics, fibre misalignment and boundary conditions
variability on process outcome. The cure model was developed and implemented
using a finite element model incorporating appropriate material sub-models of cure
kinetics, specific heat capacity, thermal conductivity, moduli, thermal expansion and
cure shrinkage. The effect of cure kinetics uncertainty on the temperature overshoot
of a thick carbon fibre epoxy flat panel was investigated using the two stochastic
simulation schemes. The stochastic simulation results showed that variability in cure
kinetics can introduce a significant scatter in temperature overshoot, presenting a
coefficient of variation of about 30%. Furthermore, it was shown that the collocation
method can offer an efficient solution with significantly lower computational cost
compared to Monte Carlo at comparable accuracy. Stochastic simulation of the cure
of an angle shaped carbon fibre-epoxy component within the Monte Carlo scheme
showed that fibre misalignment can cause considerable variability in the process
outcome. The coefficient of variation of maximum residual stress can reach up to
approximately 2% (standard deviation of 1 MPa) whilst qualitative and quantitative
variations in final distortion of the cured part occur with the standard deviation in twist
and corner angle reaching values of 0.4 º and 0.05º respectively. Simulation of the
cure of a thin carbon fibre-epoxy panel within the Monte Carlo scheme indicated that
surface heat transfer and tool temperature variability dominate variability in cure time,
resulting in a coefficient of variation of about 22%. In addition to Monte Carlo, the
effect of surface heat transfer coefficient and tool temperature variations on cure
time was addressed using the collocation method. It was found that probabilistic
collocation is capable of capturing variability propagation with good accuracy while
offering tremendous benefits in terms of computational costs
Spatially-resolved volume monitoring of adhesive cure using correlated-image optical coherence tomography
No full textCross-correlation of optical coherence tomography (OCT) images has been applied to internal imaging of PVA and two-part epoxy adhesives during cure, providing information on relative viscosity at different positions within the sample volume. Spatial resolution of a few micrometres is obtained in the original OCT images, and a few tens of micrometres in the correlation images, enabling the progress of cure to be mapped in fine detail within a small cure volume. Evidence of phase separation is seen in the OCT and correlation images of a partially-cured PVA emulsion. Mixing structure and regions of poor cure can be observed in a poorly-mixed two-part epoxy
Parameter estimation in equivalent circuit analysis of dielectric cure monitoring signals using genetic algorithms.
No full textThis communication concerns the treatment of dielectric data obtained from
experiments following the chemical hardening process (cure) in thermosetting
resins. The aim is to follow, in real time, the evolution of the individual
parameters of an equivalent electrical circuit that expresses the electrical
behavior of a curing thermoset. The article presents a methodology for the
sequential inversion of impedance spectra obtained in cure monitoring
experiments. A new parameter estimation technique based on genetic algorithms is
developed and tested using different objective functions. The influence of the
objective functions on the modelling performance is investigated. The new
technique models successfully spectra contaminated with high noise levels. The
introduction of regularization in the optimization function rationalizes the
effects of outliers usually detected in cure monitoring dielectric spectra. The
technique was successfully applied to the analysis of a series of spectra
obtained during the cure of an epoxy thermosetting resin
Cost-effectiveness of sofosbuvir plus ribavirin with or without pegylated interferon for the treatment of chronic hepatitis C in Italy
No full textObjective:Across Italy up to 7.3% of the population is infected with hepatitis C virus (HCV), with long-term complications resulting in high medical costs and significant morbidity and mortality. Current treatment options have limitations due to side effects, interferon intolerability and ineligibility, long treatment durations and low sustained virological response (SVR) rates, especially for the most severe patients). Sofosbuvir is the first nucleotide polymerase inhibitor with pan-genotypic activity. Sofosbuvir, administered with ribavirin (RBV) and with or without pegylated interferon (PEG-INF), resulted in >90% SVR across treatment-naïve (TN) genotype (GT) 1-6 patients. It is also the first treatment option for patients that are unsuitable for interferon (UI). This analysis evaluates the cost-effectiveness of sofosbuvir for GTs 1-6 in Italy.Research design and methods:A Markov model followed a cohort of 10,000 patients until they reached 80 years old. Approximately 20% of naïve and 30% of experienced patients initiated treatment at the cirrhosis stage. Comparators included PEG-INF + RBV for all GTs and plus telaprevir or boceprevir for GT1, or no treatment. Costs and outcomes were discounted at 3% and the cost perspective was that of the National Health Service in Italy.Results:Sofosbuvir was cost-effective with incremental cost-effectiveness ratios (ICERs) below 40,000/QALY in all patient populations, particularly in cirrhotic patients. The exception was for a mixed cohort of GT2 TN patients where the ICER was 68,500/QALY and for a cirrhotic cohort of GT4/5/6 where the ICER was 68,434/QALY. Nevertheless, the prevalence of HCV in this patient population is expected to be low. Results were robust to sensitivity analysis.Conclusions:Sofosbuvir-based regimens are cost-effective in Italy, particular for the most severe patients. The interferon-free regimens are a real treatment option for UI patients. The high cure rates of this breakthrough treatment are expected to substantially reduce the burden of HCV in Italy
In-situ cure monitoring of epoxy resin systems
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis describes the work carried out at Brunel University to develop novel optical fibre sensors capable of monitoring the cure state of an epoxy/amine resin system. The sensors were of simple construction, consisting of an optical fibre from which the silicone cladding layer had been removed over a short length. This stripped length was embedded into the curing resin system. The sensor was successfully used in two ways:
i) as an evanescent absorption sensor to monitor specific absorption bands of the resin system. The absorption of energy from the evanescent wave of the optical fibre by absorbing media allows evanescent absorption spectra to be obtained. Absorption spectra were obtained from sensors embedded in a model curing resin system over narrow wavelength ranges. These wavelength ranges corresponded to positions of known absorptions in the spectra of active components in epoxy/amine systems. By monitoring the change in these absorptions it was possible to obtain information about concentration of the amine hardener functional group throughout cure;
ii) as a refractive index sensor capable of monitoring the changes in the refractive index of the resin system during cure. A laser diode was used to launch light into the sensor and the intensity of light emerging from the other end of the fibre was monitored. Changes in the resin system refractive index caused changes in the guiding properties of this the sensor. This resulted in a significant change in the intensity of light recorded by the detector and allowed the cure process to be followed. This sensor was also embedded into a unidirectional pre-preg system and was able to follow the cure of the system.
The results from the two sensing methods have been compared with data obtained using FTIR spectroscopy and Abbe refractometry during the resin system cure. A theoretical model of sensor response has been developed and compared with the experimental data obtained. The sensor response has also been compared to predictions made by several models of evanescent sensor systems obtained from the literature. These models have been modified so that they can be applied to a sensor embedded into a curing resin system. An analysis of the correspondence between theory and experiment is presented
Acquisition and interpretation of dielectric data for thermoset cure monitoring
No full textThe interpretation and modelling of the dielectric response of thermosetting materials
during cure was the main focus of this study. The equivalence of complex permittivity
and complex impedance in terms of information content was outlined in a series of
case studies covering the separate effects of dipolar movements and charge migration
as well as the combined effect of the two polarisation mechanisms. Equivalent
electrical circuits were used in order to model the evolution of the complex impedance
during cure. A numerical method that can model consecutive spectra throughout the
cure was developed. The method is based on Genetic Algorithms and requires only
input from the modelling of the initial spectra.
Complex impedance spectra were collected during the cure of a commercial epoxy
resin formulation under isothermal and dynamic heating conditions. The spectra were
analysed and modelled. The modelling was successful over the whole frequency range
of the measurements (1 Hz – 1 MHz). The analysis of the estimated model parameters
showed that charge migration dominates the dielectric response in a wide frequency
range. In addition, the modelling algorithm also distinguished between the effects of
electrode polarisation and dipolar movements in the signal. A new equivalent circuit
was used in order to map the frequency regions where the each one of the three
phenomena that together comprise the dielectric signal can be monitored most
effectively.
A chemical cure kinetics model was developed for the studied system. A correlation
between the maximum point of the imaginary impedance spectrum and the reaction
conversion was established. A mathematical model, based on a simple linear
dependence of the dielectric signal on conversion and temperature, was built. The
model predictions agreed well with the experimental data.
The aim of simplifying the interpretation of the dielectric signals led to the
development of a new experimental technique. Temperature Modulated Dielectric
Analysis employs temperature modulations superimposed on an underlying thermal
profile in order to separate the influence on the signal of the temperature alone from
that of the cure reaction. The early study carried out here shows that such
measurements are feasible and reveals important issues for its further development.EPSR
Inverse heat transfer for optimization and on-line thermal properties estimation in composites curing.
No full textAbstract This article presents the development and application of a heat
transfer inversion procedure to the cure of thermoset-based composites based on
genetic algorithms. The procedure is utilized for process optimization applied
to the curing of carbon fiber reinforced composites. The optimization objective
is the selection of an appropriate cure schedule so that the duration of the
curing is minimized subject to constraints related to the thermal gradients
developed during the cure. An alternative use of inversion concerns the
integration of monitoring signals with modeling. Inversion is utilized to alter
on-line the thermal properties used in the direct model so that monitoring
results coincide with simulation predictions. This procedure is applied to the
curing of a carbon fiber reinforced thermoset-based composite, using thermal
conductivity as the variable thermal property
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