Alexandros A. Skordos

A novel strain sensor based on the campaniform sensillum of insects

The functional design of the campaniform sensillum was modelled as a hole in a plate using two– and three–dimensional finite–element modelling. Different shapes of opening in a fibrous composite plate amplify differently the global strains imposed on the plate, and different configurations of reinforcement also have an effect. In this paper, the main objective is to study the strain and displacement fields associated with circular or elliptical openings in laminated plates in order to investigate their potential for integrated strain sensors. Since we are therefore primarily interested with the detection of displacement, the detailed stress concentration levels associated with these openings are not of primary concern. However, strain energy density levels associated with different hole and fibre configurations have been used to assess the relative likely strength reduction effect of the openings. To compare the relative strain amplification effect of drilled and formed holes of the same size in loaded plates, we have used the relative change in length of diameters (circular) or semi–axes (elliptical) in directions parallel and normal to the load. Various techniques which could sense this deformation were investigated, in particular, the coupling mechanism of a campaniform sensillum of Calliphora vicina. This mechanism was resolved into discrete components: a cap surrounded by a collar, a joint membrane and an annulus–shaped socket septum with a spongy compliant zone. The coupling mechanism is a mechanical linkage which transforms the stimulus into two deformations in different directions: monoaxial transverse compression of the dendritic tip and vertical displacement of the cap. The mechanism is insensitive to change of the material properties of the socket septum, the cuticular cap and the spongy cuticle. The joint membrane may serve as a gap filler. The material properties of the collar have a substantial influence on the coupling mechanisms output. A 30% change of stiffness of the collar causes 45% change in the output of the coupling mechanism. The collar may be able to tune the sensitivity of the sensillum by changing its elastic properties.

Investigation of cure induced shrinkage in unreinforced epoxy resin

Abstract Changes in volume and thermal expansion coefficient have been investigated during the cure of a high temperature curing epoxy resin containing a thermoplastic modifier. The measurements were carried out using a combination of standard and novel thermoanalytical techniques. It is shown that the chemical shrinkage of the curing resin is a linear function of the degree of cure, whereas the coefficient of thermal expansion depends on the temperature and on the degree of cure. This experimental information is translated to an incremental model that simulates the volumetric changes occurring as the resin follows a programmed thermal profile. Such a model can serve as a density submodel in simulating heat transfer or residual stress development in composites during the manufacturing process.

A dielectric sensor for measuring flow in resin transfer moulding

The development, analysis and experimental validation of a novel flow and cure sensor for use in the resin transfer moulding of composites are presented. A linear relationship is established between the flow front position in the mould and electrical admittance measurements gathered using the sensor setup, allowing accurate flow front location. The sensor performance as an indicator of flow front position is evaluated using visual verification. Its efficiency for monitoring of the curing stage is assessed by comparison of the measurements with data obtained from more conventional microdielectrometry. Experimental results demonstrate that the sensor can locate the flow front accurately. The measurement output is in the form of a complex number; this suggests a potential qualitative self-assessment method. The monitoring of the cure process using the new sensor shows performance similar to that of the established microdielectrometric techniques.

Strain development in curing epoxy resin and glass fibre/epoxy composites monitored by fibre Bragg grating sensors in birefringent optical fibre

Fibre Bragg gratings (FBGs) fabricated in linearly birefringent fibres were embedded in glass fibre/epoxy composites and in the corresponding unreinforced resin to monitor the effective transverse strain development during the cure process. The optical fibres containing the FBG sensors were aligned either normal or parallel to the reinforcement fibres in unidirectional glass fibre/epoxy prepregs. The chemical cure kinetics of the epoxy resin system used were studied using differential scanning calorimetry, in order to investigate the correlation between the strain monitoring results and the evolution of the curing reaction. A non-parametric cure kinetics model was developed and validated for this purpose. The effective transverse strain measured by the FBGs demonstrated high sensitivity to the degree of cure as a result of the densification of the resin caused by the curing reaction. The effective compressive transverse strain developed during the reaction, and thus the corresponding sensitivity to chemical changes, was higher in the case of the sensing fibre aligned normal to the reinforcement fibres than in the case of the sensor fibre parallel to the reinforcement fibres. Small but measurable sensitivity to cure induced changes was observed in the case of the unreinforced resin.

Inverse heat transfer for optimization and on-line thermal properties estimation in composites curing

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.

Dielectric monitoring of carbon nanotube network formation in curing thermosetting nanocomposites

This paper focuses on monitoring of carbon nanotube (CNT) network development during the cure of unsaturated polyester nanocomposites by means of electrical impedance spectroscopy. A phenomenological model of the dielectric response is developed using equivalent circuit analysis. The model comprises two parallel RC elements connected in series, each of them giving rise to a semicircular arc in impedance complex plane plots. An established inverse modelling methodology is utilized for the estimation of the parameters of the corresponding equivalent circuit. This allows a quantification of the evolution of two separate processes corresponding to the two parallel RC elements. The high frequency process, which is attributed to CNT aggregates, shows a monotonic decrease in characteristic time during the cure. In contrast, the low frequency process, which corresponds to inter-aggregate phenomena, shows a more complex behaviour explained by the interplay between conductive network development and the cross-linking of the polymer.

Parameter estimation in equivalent circuit analysis of dielectric cure monitoring signals using genetic algorithms

This 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.

Stochastic heat transfer simulation of the cure of advanced composites

A stochastic cure simulation approach is developed to investigate the variability of the cure process during resin infusion related to thermal effects. Boundary condition uncertainty is quantified experimentally and appropriate stochastic processes are developed to represent the variability in tool/air temperature and surface heat transfer coefficient. The heat transfer coefficient presents a variation across different experiments of 12.3%, whilst the tool/air temperatures present a standard deviation over 1℃. The boundary condition variability is combined with an existing model of cure kinetics uncertainty and the full stochastic problem is addressed by coupling a cure model with Monte Carlo and the Probabilistic Collocation Method and applied to the case of thin carbon epoxy laminates. The overall variability in cure time reaches a coefficient of variation of about 22%, which is dominated by uncertainty in surface heat transfer and tool temperature; with ambient temperature and kinetics contributing variability in the order of 1%.

Cure of a Carbon Nanotube Modified Multiphase Epoxy- Thermoplastic Resin System

This work focuses on the cure of an epoxy-thermoplastic resin system modified with multi-walled carbon nanotubes. The cure kinetics behavior is investigated using differential scanning calorimetry and the advancement of glass transition temperature during the cure is studied in modulated calorimetry experiments. Cure kinetics models are developed for the unfilled and the filled resin system and the morphology of the heterogeneous thermoset/thermoplastic system is examined using scanning electron microscopy. The dielectric response of the filled and the unfilled systems during the cure are compared. The addition of carbon nanoparticles slows down the curing reaction in the thermoset/thermoplastic resin system. Changes in the activation energies and the exponents of cure kinetics models in the range of 5-20% are in agreement with this observation. The transition from a chemically controlled to a diffusion controlled reaction mechanism appears to be broader in the case of the nanofilled material. Reduced reactivity of the filled systems results in lower final glass transition temperature of the resin. The morphology of the cured material is affected by the presence of nanoparticles which act as seeds for nucleation and growth of large thermoset-rich domains. The dielectric cure monitoring response is influenced by the presence of carbon naotubes. The reaction can still be monitored using the intercept of the real impedance spectrum, whilst phenomena such as phase separation and vitrification are hindered by the predominantly resistive behavior of the filled system.

Stochastic Geometric Properties Of Woven Textiles

This paper addresses geometric variability of woven textiles used in composites manufacturing. Variability in tow directions and unit cell size is quantified by applying an image analysis procedure to two representative materials; a pre-impregnated carbon/epoxy satin weave textile and a commingled glass/polypropylene fabric, and the spatial autocorrelation of stochastic variables is characterized. It is found that variability in tow orientations is significant in both the pre-impregnated material and the fabric, whereas variability in the unit cell size is significant only in the commingled fabric. Variability in the weft directions is more significant than in the warp direction. Highly anisotropic spatial autocorrelation of tow orientations is observed in both materials with the major direction of autocorrelation normal to the corresponding set of tows. The correlation structures identified are decomposed using Cholesky factorization and Monte Carlo simulation of a stochastic textile is performed. This enables stochastic simulations of forming to be carried out based on a simplified finite element model of woven material draping. The results of these simulations show that variability in the woven material geometry induces significant variations in the formed geometry.