A method to determine resin flow during curing of composite laminates

An experimental method is presented to determine the amount of resin flow within a composite laminate during cure. The method is analogous to the use of radioactive tracers in other applications. Heavier elements such as chlorine and bromine, which may be naturally present in small amounts in epoxy resins are used to follow resin flow and mixing. The presence and the quantity of these “tags” is determined using wavelength dispersive X-ray analysis in a scanning electron microscope. With the resins in this study, it is shown that it is possible to measure volume fractions of resin with accuracies ranging from ±0.5 to ±3 volume %. By using brominated resin in only one layer of a laminate, the degree of flow and mixing can be followed accurately. The results suggest that there is considerable resin mixing as well as flow.     

A model for resin flow during composite processing part 2: Numerical analysis for unidirectional graphite/epoxy laminates

In this paper numerical results are presented for resin flow during processing of unidirectional graphite/epoxy laminates. Resin pressure and velocity profiles, as well as resin loss, specific permeability, and resulting thickness changes, were computed to examine the effects of one and two-dimensional flow, initial laminate thickness, and various cure cycles. Input data to the model are also discussed in detail. Analysis of the input data on the stress-strain behavior of graphite fiber beds showed that the bed consolidation behavior can be divided into three regions. “Free-bleeding” (O psig and negligible bleeder resistance to flow at the boundaries) during two-dimensional resin flow leads to rapid decay in resin pressure. Comparison of the predicted results for the resin mass loss and the average final thickness per ply with experimentally determined values shows good agreement.     

Formation of voids in composite laminates: Coupled effect of moisture content and processing pressure

Void formation as a result of prepreg moisture content and processing pressure during cure was experimentally investigated in thermosetting composite laminates. This was achieved by determining the void contents of eight‐ply laminates fabricated from TenCate^® BT250/7781 E‐glass/epoxy prepreg at processing pressures of 1.7, 3.0, 4.4, and 5.8 atm. At each processing pressure, three types of laminates were fabricated using: (i) unconditioned prepregs (direct from the storage bag); (ii) prepregs conditioned at 25% relative humidity; and (iii) 99% relative humidity. Dynamics of prepreg moisture uptake during conditioning was measured using a moisture analyzer and was shown to exhibit Fickian diffusion behavior. The void contents of the cured laminates were found to vary from 1.6% to 5.0% depending on humidity environment the prepregs were exposed and the pressure applied during fabrication. The void contents of all laminates were observed to approach an asymptotic value of ∼1.6% as pressure was increased. The experimental results indicated the processing pressure applied during fabrication was increasingly carried by the fiber bed, reducing resin pressure during cure. Therefore, an enhanced void formation model was proposed through the addition of a pressure reduction factor and an asymptotic void content term. The proposed model was found to accurately predict the void content of laminates made of prepregs exposed to constant/varying humidity environments and fabricated at a wide range of processing pressures. POLYM. COMPOS., 36:376–384, 2015. © 2014 Society of Plastics Engineers     

Dynamic-mechancial response of graphita/epoxy composite laminates and neat resin

The dynamic mechanical properties of carbon-fiber-reinforced, epoxy-matric-composite laminates subjected to loading perpendicular to the plane of lamination and of neat epoxy resin are reported. The dynamic mechanical measurements were performed in the frequency range from 0.1 to 40 Hz and at temperatures between 20° and 200°C at deformation levels within the linear viscoelastic region by the use of a Dynastat apparatus. It is found that thein-phase and out-of phase stiffnesses superpose to form master curves covering a frequency range of 12 decades. By a suitable scaling procedure of the master curves, it is found that thein-phase stiffiness has tbe same shape and the out-of-phase stiffness has the same dispersion for all laminates irrespective of the stacking sequence and are nearly identical to those for neat epoxy resin, An empirical function for the relaxation modulus is developed that, when converted to dynamic modulus, gives a good overall agreement for both components of the dynamic stiffness as a function of frequency. Absorbed moisture is found to cause a reduction in thhe elevated temperature mechanical properties of a laminate due to a reduction in the glass-transition temperature of the resin. It is also found thatthemositure absorption is a reversible process, in the sense that the initially dry properties of the laminate are recovered afterredrying the wet sample.     

On the analysis of force during secondary processing of natural fiber‐reinforced composite laminates

The rising concern regarding global warming has ignited a quest in the research fraternity towards development of sustainable materials that can reduce the carbon footprint. Natural fiber reinforced composites have made an excellent impression in the area of sustainable development because of their environmental and ecological aspects. These materials are widely used for manufacturing of engineering products. But, the use of natural fiber composites for engineering applications necessitates making of holes in order to ascertain assembly of several components by means of mechanical fastening. In this research endeavor, the drilling behavior of nettle/polypropylene (PP) composites has been experimentally investigated. The relative significance of the input process variables has also been studied with the help of statistical technique called analysis of variance (ANOVA). The present study established few significant facts in context of drilling of natural fiber reinforced composites. It has been observed that the facts established for drilling of synthetic fiber reinforced thermoset composites cannot be directly applied for natural fiber reinforced composites. POLYM. COMPOS., 38:164–174, 2017. © 2015 Society of Plastics Engineers     

A study of composite foams for diving suits subjected to high hydrostatic pressure

In order to obtain foams possessing flexibility and at the same time heat insulation under high hydrostatic pressure, composite foams with spherical rigid foams filled in flexible rubber foam at certain intervals were prepared and their thermal conductivity and flexural rigidity were studied. The following points were found: (1) With a unit model having a spherical rigid foam in the middle, the thermal conduction of a composite foam was analyzed under the conditions of steady one-dimensional heat flow. Theoretical equations giving overall coefficients of heat transmission under atmospheric and hydrostatic pressures were obtained, and the adequacy of these theoretical equations was confirmed by the measurement of overall coefficients of heat transmission of composite foams in an apparatus so constructed as to allow heat conduction experiments under pressures ranging from atmospheric to the hydrostatic pressure corresponding to 100-m depth in water. (2) The effect of the filled spherical rigid foams on heat insulation is notable under hydrostatic pressures corresponding to a 20-m depth or more in water. Under the hydrostatic pressure corresponding to a 100-m depth in water, the coefficient of heat insulation of the most closely filled composite foam used in the experiment was approximately 35% larger than that of the unfilled foam, while the theoretical most closely filled composite foam gives an approximately 110% increase. (3) Under the hydrostatic pressure corresponding to a 100-m depth in water, the flexural rigidity of the most closely filled composite foam used in the experiment was approximately one half that of an unfilled foam of the same heat insulating property.     

Effects of resin storage aging on rheological property and consolidation of composite laminates

The ability to predict the viscosity of thermoset resin is important to understand the manufacturing process of composites and optimize the processing parameters. During resin or prepreg storage course, the cure reaction may happen and the degree of cure increases gradually. The storage aging effect reduces the fluidity of resin, and hence alters the processability of resin. In this article, the rheological properties of an epoxy resin and a bismaleimide resin used in composite autoclave process were measured and a viscosity model was established, which can predict the viscosity progression during cure for different aging degree of resin. Moreover, a computer simulation method was used to study the effects of aging degree on the composite consolidation and the processing operations. It is found that the viscosity model of aged resin can be obtained by modified dual Arrhenius model of fresh resin with the dynamic rheological measurement. The resin aging strongly alters the flowability, so influences composite consolidation. According to the simulated results, the processing parameters need to be adjusted to achieve cured composites with appropriate fiber content. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Technical justification of repairs to composite laminates

Composite repair in the commercial aviation environment is largely evaluated and applied using rules of thumb and excessively conservative repair configurations. This is primarily due to the perception that the material is inherently too complex for reliable analysis and the data needed for such an analysis would be too difficult for a commercial operator to acquire or would not be accurate due to variables in the application process. Most composite design engineers have been exposed to the classical laminate theories, with their labor-intensive matrix analysis techniques, which require computerized calculation to make them feasible. Although computer advancement has made this approach more acceptable, the deep levels of understanding and large numbers of mechanical properties needed for accuracy make it impracticable for a commercial operator to design repairs. Some analysis methods, originally created to optimize design of graphite laminates in manufacturing prior to classical analysis, can be utilized with a fraction of the effort and still provide accurate results. These techniques reduce the complexity of composites to a level of difficulty that is manageable in a repair design environment. The equations to accomplish this have been found to be valid for other common substrate materials and their hybrid combinations. The majority of mechanical properties necessary for the accomplishment of such an analysis can be determined by directly measuring the capability of the material to be used in the repair process. This not only lessens the number of mechanical property variables needed to perform an accurate calculation but also takes into account any variability that exists between the processes used in repair. In addition, when comparing the results of an original structure with that of a repair analyzed using the same method, any bias inherent in the analysis technique will be nullified in their comparison. This type of analysis makes it possible for a commercial operator to provide accurate and feasible technical justification for repairs to composite laminates.     

Influence of the interfacial resin layer on delamination initiation in broken ply composite laminates

The present study has investigated the influence of a resin layer on the delamination initiation at the interface of broken and continuous plies in the case of GR/E (graphite/epoxy) laminates with broken central plies. A full three-dimensional (3D) finite element (FE) analysis was performed with each layer of the laminate modelled as homogeneous and orthotropic. The interface between the broken and the continuous plies was modelled with a thin resin-rich layer. Eight-noded isoparametric layered elements were used to model the laminate specimen. Also, 3D contact elements were used to prevent inter-penetration of the delaminated faces at the interface. Based on the results of the 3D FE analysis, strain energy release rates were calculated at the delamination front using Irwin's 'crack closure integral'. Using the concepts of linear elastic fracture mechanics (LEFM), the strain energy release rate was used as a parameter for assessing delamination initiation. The effects of various factors such as resin layer stiffness, resin layer thickness, and fibre orientation at the interface on the three components of the strain energy release rates, namely G_I, G_II and G_III, were studied for laminates with various crack sizes of the broken ply, and the influence of the resin layer in the delamination initiation was established. It was observed that delamination initiation is a mixed-mode phenomenon even in the case of uniaxial loading and the dominance of the mode of delamination is governed by the resin layer stiffness, thickness, and lamina orientation at the interface. The present work also concludes that an increase in the resin layer modulus leads to an increase in the probability of mode I delamination while the probability of mode II delamination decreases. A 0/90 interface exhibits a higher chance of delamination in modes I and II, while mode III delamination is maximum for 0/30 and 0/60 fibre orientation interfaces. It was also observed that the larger the crack width, the greater the probability of delamination initiation at the interface.     

A model for resin flow during composite processing: Part 1—general mathematical development

A generalized three-dimensional model for resin flow during composite processing has been developed. The model is based on a theory of consolidation and flow through a porous medium, which considers that the total force acting on a porous medium is countered by the sum of the opposing forces, including the force due to the spring-like effect of the fiber network and the hydrostatic force due to the pressure of the liquid within the porous medium. The flow in the laminate is described in terms of Darcy's Law for flow in a porous medium, which requires a knowledge of the fiber network permeability and the viscosity of the flowing fluid. Unlike previous resin flow models, this model properly considers the flows in different directions to be coupled and provides a unified approach in arriving at the solution. Comparison of numerical solutions with the closed form analytical solution shows good agreement. Resin pressure profiles show that the pressure gradients in the vertical and horizontal directions are not linear, unlike the assumption of linearity made in several previous resin flow models. The effects on the resin pressure of both linear and nonlinear stress-strain behavior of the porous fiber network were considered. The nonlinear behavior simulates a rapidly stiffening spring and the resin pressure decreases much more rapidly after a given initial period compared to the linear stress-strain behavior.     

Expansion characteristics of thermally expandable microcapsules for dismantlable adhesive under hydrostatic pressure or in resin

ABSTRACT

The expansion characteristics of thermally expandable microcapsules (TEMs) under hydrostatic pressure or in resin were experimentally investigated. For the experiments, the expansion of the microcapsules was observed in high-pressure nitrogen at high temperature utilizing optical microscopy with a digital camera installed. The TEMs used for the experiments were expanded by heating under hydrostatic pressure up to 3 MPa, but the expansion degree decreased with increasing pressure. A cured bulk specimen of epoxy resin containing the microcapsules was made, and the expansion of the microcapsules was again observed with the microscope. It was found that the expansion of the microcapsules in the resin was saturated at a certain temperature. The stress distribution in the resin produced by the expansion of the microcapsules was calculated by the finite-element method. It was found that normal stress occurred, but it was mainly compressive. Tensile stress was also generated, although the maximum value was smaller than that of shear stress. It was observed that the expansion of the microcapsules was limited when there were many microcapsules in the vicinity of the interface. In other words, a complicated stress state occurred, inducing interfacial failure along the interface.     

Impact response of composite laminates with a hemispherical indenter

A model for predicting the contact duration, force, indentation, and displacement under impact has been developed for impact of a composite laminated plate with a hemispherical indenter. The governing equations were first proposed and then solved with the initial conditions along with the peak force condition. An expression forr each parameter has been obtained in a simple expression through straightforward derivation. Both static and dynamic tests have been conducted. Ten impact tests were conducted using a weight drop tower tester. Comparison between the, model predictions and the experimental data on the impact duration and impulse are provided, and were found to be within 14% for all tests involved. Comparison of the impact force-history also shows good accord.     

Online monitoring and analysis of resin pressure inside composite laminate during zero‐bleeding autoclave process

Resin pressure is one of the most important parameters in manufacturing composites during autoclave process. It not only greatly influences resin flow behavior, but also has effects on void formation and elimination. Online monitoring resin pressure can provide an important guidance for the optimization of the processing parameters and the control of the quality of composites. In this study, a resin pressure online measuring system for autoclave process was established based on the principle of pressure transfer in liquid, and the size of the measuring probe of the system was optimized to increase the accuracy of measured resin pressure. The results indicate that the accuracy and the dynamic response of the system can meet the requirements of resin pressure measurement during autoclave process. Furthermore, by means of this proposed resin pressure measuring system and the measurements of compaction properties of the fabric stacks, the resin pressures inside carbon fiber fabric/epoxy resin and glass fiber fabric/epoxy resin prepreg stacks during autoclave process were investigated, especially for the zero‐bleeding process which is prevailing for aircraft composite structures. It is demonstrated that during zero‐bleeding process, the resin pressures, which conform to the spring and piston model, uniformly distribute along through‐thickness and in‐plane directions. In addition, the resin pressure profile is significantly influenced by the fiber volume fraction of the prepregs, indicating that fiber content of prepreg should be optimized for achieving free defects and uniform fiber distribution. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

A simple calorimeter to measure specific heat of activated carbon prepared for pressure swing adsorption refrigeration system

A new calorimeter to measure specific heat of indigenously prepared granular activated carbon (AC) for application in a refrigeration system based on gas adsorption–desorption mechanism is presented. This calorimeter allows specific heat measurements in high (305.5–317.8 K) as well as low (254.6–264 K) temperature ranges. The specific heat of prepared AC on measurement using developed calorimeter has been found to be 1.062 J g^?1 K^?1. Preparation, instrumental analyses and influence of various physical properties of AC on its specific heat have also been discussed. Characterisation results of AC demonstrate that it can be efficiently used in such cold producing system down to refrigeration and cryogenic temperature. © 2012 Canadian Society for Chemical Engineering     

Water-tolerant catalysis of a silica composite of a sulfonic acid resin, Aciplex

A silica composite of a perfluorocarbonsulfonic acid resin, Aciplex, has been used as a solid acid catalyst for a variety of reactions concerning water. The Aciplex–SiO₂ composite containing 20 wt% Aciplex has a surface area of 1.3 m² g⁻¹ and possesses an ion-exchanged capacity of 0.46 meq. g⁻¹ after pretreatment at 423 K, which is higher than that of 13 wt% Nafion–SiO₂ (0.12 meq. g⁻¹). The acid strengths estimated from an initial heat of adsorption of NH₃ were similar for these polymer resin composites. It was found that the Aciplex–SiO₂ was more active than typical solid acids such as Cs_2.5H_0.5PW₁₂O₄₀, H-ZSM-5, and SO₄²⁻/ZrO₂ for hydrolysis of ethyl acetate in excess water and esterification of acrylic acid with 1-butanol, while it was less active than Cs_2.5H_0.5PW₁₂O₄₀ for N-alkylation of acrylonitrile with 1-adamantanol and solid–solid hydrolysis of 2-naphthyl acetate. The Aciplex–SiO₂ was superior in activity to Nafion–SiO₂ for all the above reactions and in thermal stability. These results indicate that Aciplex–SiO₂ is a promising solid acid catalyst for reactions involving liquid phase water.     

Simple dispersion of graphene incorporated rubber composite for resistive pressure sensor application

We developed a resistive pressure sensor based on stand-alone film graphene incorporated natural rubber latex (NR/G) composite fabricated in a simple solution casting method. The graphene dispersion had multilayer properties and an average lateral size of ~60 nm successfully incorporated into the NR matrix to form bilayer graphene composite with the I_2D/I_G calculated ratio of 1.301. The pressure sensor shows high sensitivity (~ 0.2 kPa⁻¹), high cycle stability, and selective response to the applied pressure with a linear relationship of ~6.0 kPa. This work shows that graphene incorporated natural rubber latex pressure sensors with applicable potentials for smart wearable devices to allow pressure detection.     

Dielectric spectroscopy during extrusion processing of polymer nanocomposites: a high throughput processing/characterization method to measure layered silicate content and exfoliation

Dielectric spectroscopy was conducted during extrusion processing of polyamide-6 (PA6) and layered silicate/polyamide-6 nanocomposites. Dielectric dispersion parameters were identified that appear sensitive to layered silicate concentration and degree of exfoliation. Specific to measuring layered silicate concentration is that the Maxwell-Wagner strength of dispersion, Δε_mw, increases linearly with the % mass fraction layered silicate content. This relationship is independent of exfoliation resulting in nanomorphology-averaged Δε_mw values that reflect layered silicate concentration; i.e. 12,800±519 indicates 1.29% mass fraction of a layered silicate in PA6. The nanomorphology is primarily reflected in the Maxwell-Wagner characteristic relaxation frequency value, f_mw, where, for example, 80.4±5 Hz indicates a mixed intercalated/exfoliated nanomorphology. However, following the nanomorphology with the f_mw value can in some cases be complicated because different nanomorphologies can yield the same f_mw value. In these cases we have found that there is a significant difference in the conductive resistance and segmental mobility of these polymers, as indicated by the σ_DC and fα values. For example, the intercalated and exfoliated nanocomposites have a f_mw value of about 5.1 Hz, but the exfoliated nanocomposites have σ_DC and fα values that are much larger than determined for the intercalated nanocomposites.