Epoxy Prepreg Research Articles

Derivation and experimental validation of Lamb wave equations for an n-layered anisotropic composite laminate

Lamb waves are ultrasonic guided waves that propagate between two parallel free surfaces and their use for damage detection has been widely explored and demonstrated. Damage in materials/structures can be detected by analyzing the difference between the phase/group velocity and the loss of amplitude of Lamb waves on damaged and un-damaged specimens. The propagation characteristics of Lamb waves are described in the form of dispersion curves, which are plots of phase/group velocities versus the product of frequency-thickness generated by solving the Lamb wave equations. Lamb waves’ dispersion behaviors for isotropic materials are well established in the literature; however, such is not the case for the laminated composites. The most common methods for solving the Lamb wave equations in composites consist of using laminated plate theory or 3D linear elasticity by assuming an orthotropic and/or higher symmetry. This assumption may not be true, if the actuators and sensors in an orthotropic or transversely isotropic laminates are installed in a non-principle direction or the layup is symmetric but not balanced.This paper presents a full derivation of Lamb wave equations for n-layered monoclinic composite laminates based on linear 3D elasticity by considering the displacement fields in all three directions using the partial wave technique in combination with the Global Matrix (GM) approach. In the partial wave technique, the principle of superposition of three upward and three downward travelling plane waves are assumed in order to satisfy the associated boundary conditions. The bounded upper and lower surfaces reflect the waves and the combination of these reflections going towards the upper or lower interfaces results in the propagating guided waves. The GM approach is used to assemble all the equations from each layer to form a global, unified matrix that describes the displacement and stress fields along the entire laminate associated with the wave propagation. A robust method for numerically solving the Lamb wave equations is also presented.The presented method was verified experimentally by analyzing the propagation of Lamb waves in two different composite panels constructed out of unidirectional carbon-fiber epoxy prepreg and fiber-metal laminate (GLARE 3-3/4). The panels were instrumented with lead zirconate titanate (PZT) piezoelectric sensors, which were excited at different frequencies ranging from 20kHz to 500kHz to generate and acquire the waves. The waves were excited and gathered at three different propagation angles of 0°, 45°, and 90° for the carbon-fiber epoxy laminate panel and at six different angles of 0°, 20°, 45°, 70° and 90° for the fiber-metal laminates (GLARE). The phase and group velocities of the fundamental symmetric (So) and anti-symmetric (Ao) Lamb waves were extracted by tracking the peaks of each individual wave phase and the wave envelope respectively using an in-house code developed in MATLAB. It was found that the presented 3D linear elasticity model followed the experimental data closely for both symmetric and anti-symmetric Lamb modes. The analytical method presented in this paper was able to predict the Lamb wave dispersion for both the carbon-fiber epoxy laminate and the hybrid fiber-metal laminate proving the robustness and versatility of the solution method.

Graded conventional-auxetic Kirigami sandwich structures: Flatwise compression and edgewise loading

The work describes the manufacturing and testing of graded conventional/auxetic honeycomb cores. The graded honeycombs are manufactured using Kevlar woven fabric/914 epoxy prepreg using Kirigami techniques, which consist in a combination of Origami and ply-cut processes. The cores are used to manufacture sandwich panels for flatwise compression and edgewise loading. The compressive modulus and compressive strength of stabilized (sandwich) honeycombs are found to be higher than those of bare honeycombs, and with density-averaged properties enhanced compared to other sandwich panels offered in the market place. The modulus and strength of graded sandwich panel under quasi-static edgewise loading vary with different failure mode mechanisms, and offer also improvements towards available panels from open literature. Edgewise impact loading shows a strong directionality of the mechanical response. When the indenter impacts the auxetic portion of the graded core, the strong localization of the damage due to the negative Poisson’s ratio effect contains significantly the maximum dynamic displacement of the sandwich panel.

Carbon-fibre epoxy prepreg (CFC) curing in an autoclave analogue process controlled by Dynamic Mechanical Analysis (DMA)

Carbon fibre prepregs have found widespread application in lightweight constructions. They are based on a carbon-fibre fabric impregnated with reactive epoxy resin. DMA measurements under temperature conditions similar to an autoclave programme were carried out using commercially available prepreg material with a high glass transition temperature. The characteristic of the temperature programme was a dynamic heating segment at 1.5 K/min followed by a longer isothermal segment at 180 °C. The courses of the storage modulus E′, loss modulus E″ and tanδ were recorded. The measuring frequency was varied between 1 Hz and 33.3 Hz. Gelation and vitrification are assigned. The influence of the measuring frequency on the time to vitrification and the correlation with DSC are discussed. The reaction does not end even after 10 h curing at 180 °C, which is interpreted as the slow cessation of the reaction caused by vitrification.

Effects of Stacking Sequence on the Dynamic Characteristics of Hybrid Sandwich Composites Plates

In this paper, the vibration performance of hybrid sandwich composites plates with different stacking was investigated by modal test. The results showed that the reduction in layer numbers of the unidirectional carbon fiber epoxy prepreg in 0odirection will result the decrease in the natural frequency values for bending modal, the increment of layer numbers of 2D woven epoxy prepreg with plain weave will result the enhancement the of torsional modal. The reinforcements of unidirectional carbon fiber epoxy prepreg in ±45odirection has a preferential effect on the torsional modal natural frequency of the plate than that of 2D woven epoxy carbon prepreg with plain weave in 0odirection.

Cure state detection for pre-cured carbon-fibre epoxy prepreg (CFC) using Temperature-Modulated Differential Scanning Calorimetry (TMDSC)

Carbon-fibre prepregs have found widespread use in lightweight applications. They are based on a carbon-fibre fabric impregnated with reactive epoxy resin. Prepreg materials are generally pre-cured so that they have a higher molecular weight than typical resins in order to reduce resin flow, which facilitates storage and later processing properties.The measurements were carried out using commercially available materials and follow the published DMA investigations of the same material [1]. TMDSC was used to find the correlation between curing conditions, the degree of cure and glass transition temperature. TMDSC has the advantage over standard DSC that it enables better determination of the glass transition temperature, which is often accompanied by an exothermic curing reaction, and thus overshadowed. The influence of the amplitude of temperature modulation was tested in preliminary experiments. For non-cured material a glass transition temperature of approximately 0 °C was determined; whereas for the totally cured material it was approximately 230 °C. The changes in degree of cure, temperature of actual glass transition and post-reaction are given as a function of curing time at 180 °C. The correlation between actual glass transition temperature and degree of cure is derived.

The prediction capability of the kinetic models extracted from isothermal data in non-isothermal conditions for an epoxy prepreg

In this article, the kinetic behavior of a glass/epoxy prepreg, HexPly® 1454, has been investigated using differential scanning calorimetry data. The kinetic parameters of three different kinetic models have been calculated using non-linear curve fitting of isothermal differential scanning calorimetry data. Curve fitting was done based on genetic algorithm to allow to obtain a unique value for each kinetic parameter. The non-isothermal differential scanning calorimetry experiments were predicted using the obtained models. Prediction results demonstrated that only model with the minimum number of kinetic parameters and unique value for each of them can correctly predict non-isothermal experiments at all heating rates.

Advanced integral isoconversional analysis for evaluating and predicting the kinetic parameters of the curing reaction of epoxy prepreg

The kinetic parameters of the curing reaction of epoxy/glass prepreg have been evaluated and predicted by three different methods using the non-isothermal differential scanning calorimetry (DSC) data. In the first method, the dependence of activation energy, pre-exponential factor and reaction model on the extent of conversion have been calculated in the tabular form using advanced integral isoconversional method and then converted to the equation form. In the second method, the kinetic parameters of the Kamal and diffusion models have been evaluated using the activation energy dependency on the extent of conversion to reduce the number of kinetic parameters in minimization procedure. In the third method, a conventional non-isothermal model-fitting method has been used to evaluate the kinetic parameters of Sourour and Kamal's model. The predictions made by the methods based on the advanced isoconversional analysis show good agreement to both isothermal and non-isothermal experiments outside the experimental temperature regions.

Investigation of the curing behaviour of carbon fibre epoxy prepreg by Dynamic Mechanical Analysis DMA

Carbon fibre prepregs have found widespread application in lightweight constructions. They are based on a carbon fibre fabric impregnated with reactive epoxy resin. Measurements were carried out using commercially available prepreg material. For Dynamic Mechanical Analysis (DMA), a single cantilever measuring device was applied. The DMA results were refined by additional DSC measurements. The measurements were carried out with dynamic heating in the temperature range −90 to 280 °C. The heating rates were 1 and 2 K/min, respectively. A glass transition of the uncured material (Tg0) near 1 °C, and crosslinking-induced vitrification and devitrification at the maximal glass transition temperature of the cured material (Tgmax) in the temperature range 220 to 230 °C were found. The activation energies for the glass transitions were determined using an Arrhenius plot. By detailed consideration of the influence of the frequency on the DMA data, indications for gelation were deduced.

Effect of room-temperature out-time on tow impregnation in an out-of-autoclave prepreg

Tow impregnation as a function of material out-time was investigated for an out-of-autoclave carbon fiber–epoxy prepreg. Prepreg was aged at ambient temperature for 56days. Every 7days, laminates were laid up and cured using vacuum bag only processing. Void content was calculated through image analysis of polished sections. Experimental results were used to validate an analytical model for tow impregnation. Model predictions were based on flow kinetics during processing conditions, taking into account increasing degree of cure and evolution of resin viscosity as a function of ambient aging time. The study found that no significant tow porosity occurred within the material’s stated out-life, that tow porosity increased once this out-life was exceeded and eventually stabilized due to the room-temperature vitrification of the resin. The model’s predicted trends were consistent with experimental results, suggesting that an increase in resin viscosity is indeed the main cause of out-time induced tow porosity and providing a means of predicting laminate quality as a function of room temperature aging time.

PROCESS OPTIMIZATION OF FRP SANDWICH COMPOSITE STRUCTURE FOR ENGINEERING APPLICATION

“Composites are the combination of a reinforcement fibre in a thermoset polymer resin matrix, where the reinforcement has an aspect ratio that enables the transfer of loads between fibres, and the fibres are chemically bonded to the resin matrix.” FRP (fibre reinforced plastic) Sandwich structure are defined as a three layer type of construction a thick layer known as core and two thin polymer layer known as face . In automobile and aerospace industries FRP composites are used due to numerous advantage like mechanical, chemical, thermal properties and light weight. The mechanical, thermal chemical properties can be optimize by stabilizing the core[1][2]. For this study various raw material characterization done .Aluminium honeycomb used as a Core material (25mm thick) and Epoxy prepreg (layer used as a face sheet material) & PU foam provide housing for core [5]. The experiment conducted according to raw material characterization test report . by autoclave curing At a particular temperature, pressure ,vacuum with respect to time process optimized ,sandwich cured and stabilized. Finally the test conducted According to ASTMD & DIN standard as a result it shows that the stabilized FRP sandwich having excellent mechanical properties compare to core crushing FRP sandwich.

The effect of thermal cycles on the mechanical properties of fiber–metal laminates

The effect of thermal-shock cycles on the mechanical properties of fiber–metal laminates (FMLs) has been evaluated. FML plates were composed by two AA2024 Al sheets (1.6mm thick) and one composite ply formed by two layers of unidirectional glass fiber epoxy prepreg and two layers of epoxy adhesive tape of glass fiber reinforced epoxy adhesive. The set was manufactured by hand layup and typical vacuum bag technique. The curing cycle was in autoclave at 125±5°C for 90min and an autoclave pressure of 400kPa. FML coupons taken from the manufactured plate were submitted to temperature variations between −50 and +80°C, with a fast transition between these temperatures. Tensile and interlaminar shear strength were evaluated on samples after 1000 and 2000 cycles, and compared to nonexposed samples. 2000 Cycles corresponds to typical C Check interval for commercial aircraft maintenance programs. It was observed that the thermal-shock cycles did not result in significant microstructural changes on the FML, particularly on the composite ply. Similarly, no appreciable effect on the mechanical properties of FML was observed by the thermal-shock cycles.

Morphology of an Interface between Polyetherimide and Epoxy Prepreg

The morphology of the interface between a commercial epoxy prepreg resin (HexPly M18/1) and Polyetherimide (PEI) is studied. Different cure cycles prescribed by the supplier were investigated to determine the influence of temperature and pressure on interface formation. Atomic Force Microscopy (AFM) was used as the main means of investigation. A phase-separated interphase spanning 9-10μm was observed for the PEI/epoxy prepreg interface. It was found that the temperature profile of the cure cycle strongly influences the formation of this interphase. Rapid formation of these relatively large interphase areas suggests that mechanisms other than diffusion are responsible for this phenomenon.

Motorcycle swingarm redesigned in carbon composite

Nigel O'Dea explains how the redesign of a motorbike swingarm in carbon fibre epoxy prepreg was made possible by using the Composites Modeler ply modelling and fibre simulation software developed by Simulayt Ltd.

Through-thickness compression response of uncured prepreg during manufacture by automated layup

A better understanding of the mechanical properties of prepreg in the uncured state is required to establish process models for automated layup of thermoset prepregs. Such models would enable control of the bulk factor and defects, such as fibre waviness and voidage, during layup. Pressure application time and temperature during automated layup are typically low while the through-thickness strains can be high due to a reduction of gaps between the plies. A viscoelastic analysis for the through-thickness compression response of a toughened carbon/epoxy prepreg is compared against an elastic–plastic material behaviour with respect to automated layup for three representative temperatures. It is shown that a simple plastic material law with strain hardening can be used to accurately model the material response during automated layup of toughened epoxy prepreg. A finite-element model shows good agreement with experimental results.

Thermal, rheological, and mechanical properties of a polymer composite cured at different isothermal cure temperatures

Thermal, rheological, and mechanical properties of a commercial carbon fiber epoxy prepreg, Cycom 977-2 UD, were obtained for isothermal cure temperatures ranging from 149°C to 182°C. For each cure profile, an encapsulated-sample rheometer (ESR) was used to measure the storage modulus. Each ESR cure profile was followed by the glass transition temperature ( Tg) test. The degree of cure ( α) during cure and the heat of reaction of the prepreg were obtained using a differential scanning calorimeter (DSC). Combined loading compression (CLC) and short-beam shear (SBS) tests were performed to obtain compressive properties and SBS strength, respectively. It was observed that the compressive properties did not vary significantly for the studied isothermal cure temperatures; likewise, the compressive failure mode was the same for all the CLC specimens. However, the SBS strength for the specimens cured at 149°C was approximately 10% less than that of those cured at isothermal cure temperatures ranging from 160°C to 182°C. Further, the failure mode of the SBS specimens cured at 149°C was also different from other specimens. The storage modulus of the ESR sample cured at 149°C also showed a 10% decrease compared to other ESR samples. The SBS strength exhibited a good correlation with the storage modulus and a weak correlation with Tg and α.

Effect of thermoplastic toughening agent on glass transition temperature and cure kinetics of an epoxy prepreg

The isothermal time–temperature-transformation (TTT) cure diagram is developed in this article to investigate the effect of thermoplastic toughening agent on glass transition temperature (Tg) and cure kinetics of an epoxy carbon fiber prepreg, Cycom 977-2 unidirectional (UD) tape. The glass transition temperature was measured using differential scanning calorimetry (DSC) over a wide range of isothermal cure temperatures from 140 to 200 °C. Times to gelation and vitrification were measured using shear rheometry. The glass transition temperature master curve was obtained from the experimental data and the corresponding shift factors were used to calculate the activation energy. The kinetic rate model was utilized to construct iso-Tg contours using the calculated activation energy. It was observed that the iso-Tg contours did not follow the behavior of the neat epoxy resin, since they deviated from the gel time curve. This deviation was believed to be the effect of the thermoplastic toughening agent. The behavior of the neat epoxy resin in 977-2 was shown by constructing the iso-Tg contours using the activation energy obtained from gel time modeling.

New method for estimating the extent of curing of thermosetting prepregs

AbstractHere, we propose a new method for estimating the extent of curing of thermosetting prepregs. In the proposed method, the extent of curing is estimated with the curing index (Ci), defined as the ratio of the glass‐transition temperature (Tg) to the ultimate glass‐transition temperature of the material. The advantages of this new method over the conventional degree of conversion (α) for estimating the extent of curing of thermosetting prepregs are discussed in detail. Ci and α of a toughened epoxy prepreg (977‐2 unidirectional) were obtained for a wide range of isothermal curing temperatures with a differential scanning calorimeter. The ultimate heat of reaction varied inconsistently with decreasing curing temperature; this resulted in erratic behavior of α. However, Ci provided a more consistent estimate of the extent of curing because Tg, unlike α, did not need to be modified on the basis of the curing history of the material and was measured directly with the heat‐flow data from differential scanning calorimetry. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

The Effects of Seawater Immersion on the Bearing Strength of Woven-Glass–Epoxy Prepreg Pin-Loaded Joints

This study considers the bearing strengths and failure modes of woven-glass–epoxy prepreg composite mechanically fastened joints. For the study, the specimens were immersed in natural seawater in the laboratory for over 1 year. The edge distance-to-hole diameter ( E/ D) ratio and the specimen width-to-hole-diameter ratio ( W/ D) were systematically altered during experiments. Failure modes were determined by observing the failure regions on the specimens. Damage progression in pin-loaded specimens prepared from woven-glass–epoxy prepreg was also examined using a scanning electron microscopy for three different percentages of ultimate failure load. According to the experimental investigation results, it was observed that the bearing load values obtained from the specimens immersed in seawater decreased considerably in comparison to unimmersed specimens.

Hybrid processing of thick skins for honeycomb sandwich structures

Carbon–epoxy prepregs are generally used to form the skins of honeycomb sandwich structures used in aerospace or racing yachts. For some applications, it is desirable to increase the thickness of the skins. In order to achieve an ideal core pressure level during cure for maximal skin–core bonding, the issue of air extraction from the honeycomb cells through the skin during processing thus becomes critical, in particular if vacuum only processing is used. In the present work, partially impregnated prepregs, called semipregs, having high initial transverse permeability to air, are combined with traditional prepregs to form a hybrid skin. Results are presented on the pressure change inside the honeycomb cells and the skin permeability to air during cure, as well as on skin–core adhesion. The final sandwich quality is assessed and found to be comparable to that obtained with prepreg skins.

Mechanical behavior of bio-inspired laminated composites

To investigate if the relative high strength and stiffness of biological composites can be translated into man-made materials, innovative bio-inspired laminated composites with commercial materials (glass epoxy prepreg) were designed and manufactured by incorporating the distinctive helicoidal morphology observed in the exoskeletons of crustaceans. The helicoidal structure is characterized by a stacking sequence consisting of a gradual rotation of each lamina in the multi-layered laminated composites. Variations of the helicoidal structure were designed and produced to address some important issues encountered in the practical composites manufacturing process, including mid-plane symmetry. In addition, composite structures with a general quasi-isotropic configuration were made as a baseline structure. Selected material properties (flexural properties, shear strength) were evaluated through appropriate ASTM tests and laminated composites theory. Structure post-damage behavior was also investigated. Significant improvement in the mechanical performance of the bio-inspired structure was observed over the baseline structure.