Epoxy Prepreg Research Articles

Experimental Verification of Numerical Calculations with the Use of Digital Image Correlation

Abstract The article presents the results of research work performed under the TEBUK project, aiming primarily to develop a reference methodology for assessing the impact of damage on the strength of structures made of carbon epoxy prepregs. The tests described in the paper were concerned with a fragment of the structure (FS) of the TEBUK project demonstrator, made of carbon epoxy composite, with an artificial circular delamination measuring 40 mm in diameter. Numerical and experimental test of FS have been performed under quasi-static compression load. The buckling of the skin observed in the delamination area, as well as the propagation of the latter were investigated. The numerical calculations have been performed with the use of the commercially available MSC Marc/Mentat calculation suite based on the Finite Elements Methods. Results of the numerical calculations have been compared with experimental measurements made with the use of the Digital Image Correlation (DIC) method. The tests performed aimed to provide a preliminary verification of the numerical model. The results obtained have shown a very good correlation between the numerical and experimental results concerned with critical load levels at which stability of the layers separated by delamination is lost (buckling). The lack of convergence of the numerical model’s results after exceeding the critical load values has rendered it impossible to unequivocally compare the results concerned with propagation of the delamination area.

Effects of plies orientations and initial geometric imperfections on buckling strength of a composite stiffened panel

The use of composite stiffened panels is common in several activities such as aerospace, marine and civil engineering. The biggest advantage of the composite materials is their high specific strength and stiffness ratios, coupled with weight reduction compared to conventional materials. However, any structural system may reach its limit and buckle under extreme circumstances by a progressive local failure of components. Moreover, stiffened panels are usually assembled from elementary parts. This affects the geometric as well as the material properties resulting in a considerable sensitivity to buckling phenomenon. In this work, the buckling behavior of a composite stiffened panel made from carbon Epoxy Prepregs is studied by using the finite element analysis under Abaqus software package. Different plies orientations sets were considered. The initial distributed geometric imperfections were modeled by means of the first Euler buckling mode. The nonlinear Riks method of analysis provided by Abaqus was applied. This method enables to predict more consistently unstable geometrically nonlinear induced collapse of a structure by detecting potential limit points during the loading history. It was found that plies orientations of the composite and the presence of geometric imperfections have huge influence on the strength resistance.

Multi-step cure kinetic model of ultra-thin glass fiber epoxy prepreg exhibiting both autocatalytic and diffusion-controlled regimes under isothermal and dynamic-heating conditions

As packaging technologies are demanded that reduce the assembly area of substrate, thin composite laminate substrates require the utmost high performance in such material properties as the coefficient of thermal expansion (CTE), and stiffness. Accordingly, thermosetting resin systems, which consist of multiple fillers, monomers and/or catalysts in thermoset-based glass fiber prepregs, are extremely complicated and closely associated with rheological properties, which depend on the temperature cycles for cure. For the process control of these complex systems, it is usually required to obtain a reliable kinetic model that could be used for the complex thermal cycles, which usually includes both the isothermal and dynamic-heating segments. In this study, an ultra-thin prepreg with highly loaded silica beads and glass fibers in the epoxy/amine resin system was investigated as a model system by isothermal/dynamic heating experiments. The maximum degree of cure was obtained as a function of temperature. The curing kinetics of the model prepreg system exhibited a multi-step reaction and a limited conversion as a function of isothermal curing temperatures, which are often observed in epoxy cure system because of the rate-determining diffusion of polymer chain growth. The modified kinetic equation accurately described the isothermal behavior and the beginning of the dynamic-heating behavior by integrating the obtained maximum degree of cure into the kinetic model development.

Nonlinear and complex cure kinetics of ultra-thin glass fiber epoxy prepreg with highly-loaded silica bead under isothermal and dynamic-heating conditions

In the advent of the miniaturized mobile devices, the packaging technology is required utmost high performance of the thin composite laminates in such materials properties as the coefficient of thermal expansion (CTE) and stiffness. Accordingly, the composition of a thermosetting resin becomes extremely complicated often including multiple fillers, monomers and/or catalysts in thermoset-based glass fiber prepregs. The prepreg systems is so complicated that it is usually difficult to obtain a reliable kinetic description and methodology that could be used for the complex thermal cycles including both isothermal and dynamic-heating segments in a facile manner. In this investigation, we propose an isoconversional kinetic using an ultra-thin glass fiber epoxy prepreg with highly loaded silica filler (the ultra-thin glass fiber/silica bead epoxy prepreg) as a model system. The activation energy was determined as a function of the conversion of curing reactions, which was fitted to linear models. The kinetic prediction using the linear models showed an excellent agreement to isothermal experiments. The master curve of a conversion-dependent function which derived from activation energy dependency used to investigate the complex reactions of the ultra-thin glass fiber/silica bead epoxy prepreg.

Surface modification of PEEK by UV irradiation for direct co-curing with carbon fibre reinforced epoxy prepregs

Adding a layer of thermoplastic resin on the surface of thermoset composites enables welding as a possible joining method for thermoset composites. Adhesion at the thermoset/thermoplastic interface was achieved by direct co-curing of an UV irradiation treated PEEK film with aerospace grade carbon/epoxy prepregs. The effectiveness of UV irradiation for surface modification of PEEK was characterized using Fourier-Transformed InfraRed (FTIR) spectroscopy and contact angle measurement. The adhesion quality at the thermoset/thermoplastic interface was evaluated using a double cantilever beam test and Atomic Force Microscopy (AFM). UV treatment was found to effectively modify the chemical structure of PEEK surface and improve the wettability, which enables the development of a thermoset/thermoplastic interface by direct co-curing.

Low velocity impact response of basalt-aluminium fibre metal laminates

This work reports on a preliminary study to assess the response of fibre metal laminates (FMLs) made of stacked layers of basalt fabric epoxy prepreg and aluminium sheets to low velocity impact and, through comparison with published results for Glare materials, benchmark their impact performance. Comparison is also made with results from plates of monolithic aluminium with comparable thickness. The effect of impactor size is also investigated whilst the impact energies were selected with the aim of inducing first crack (FC) and through the thickness crack (TTT) on the FML samples. Optical microscopy and profilometry were used to elucidate fracture mechanisms and the dent depth. The response of basalt fibre metal laminates was found to be both significantly influenced by the impactor size and fairly positive, proving to be competitive with monolithic aluminium plates although some issues about the aluminium/basalt interface have been raised.

Improvement in mechanical properties of recycled GF prepreg with CNT reinforced composites using a spray coating method

Dispersion and shape of nanoparticles, as well as interfacial conditions, add significantly to difficulties in composite manufacture. In the work reported here, an innovative method of recycling composites using out-of-date prepreg was investigated in which the carbon nanotube (CNT) on the prepreg was optimally coated. Nanocomposites utilizing the out-of-date prepreg were coated with CNT and fabricated by a sheet molding method. CNT nanofillers were observed to be uniformly dispersed on epoxy prepreg by spray coating. The mechanical and interfacial properties of these CNT coated nanocomposites were improved over those of more conventionally manufactured carbon fiber/epoxy composites. The CNT nanofillers were embedded at the epoxy and fiber interface, as a result of etching of the epoxy prepreg surface by a CNT dispersion solution which enhanced interfacial reactivity.

Damage tolerance of carbon/flax hybrid composites subjected to low velocity impact

Abstract Hybrid laminates produced using carbon (C) and flax (F) fibre epoxy prepregs were fabricated with two different stacking sequences based on the presence of flax fibre laminates as outer layers and carbon as inner layers (FCF) or vice versa (CFC). Pure flax and pure carbon fibre reinforced laminates were also fabricated as a reference. Experimental tests were performed, which included four-point bending, falling weight impact tests at energies ranging from 5 to 30 J with determination of the barely visible impact damage (BVID) and post-impact flexural tests. As a whole, CFC proved slightly superior to FCF as for flexural performance, although the presence of flax laminates on the outside guaranteed a higher impact damage tolerance, acting as hindrance to crack propagation in the laminate. Specimens impacted at 10 J were also subjected to tensile tests monitored by Digital Image Correlation (DIC) that allowed preliminary identification of peculiar failure modes of the hybrid laminates.

Embedded Die Packages and Modules for Power Electronics Applications

Packages and modules with embedded semiconductor dies are of interest for various application fields and power classes. First packages in the lower power range are available in volume production since almost six years. Recent developments focus on medium and higher power applications raging over 500W into the kW range. Different approaches are available to realize such packages and modules. This paper will give an overview and detailed description of the latest approaches for such embedded die structures. In common of all of these approaches, is the use of laminate based die embedding, which uses standard PCB manufacturing technologies. Main differences are the used base substrate, which can still be a ceramic (DBC), Cu leadframe or high current substrate. Examples for the different methods will be given. As the main part, this paper will describe concepts, which enable significant smaller form-factor of power electronics modules, thereby allowing for lower price, high reliability, capability of direct mounting on e.g. a motor so as to form one unit with the motor housing, wide switching frequency range (for large application field) and high power efficiency. The innovative character of this packaging concept is the idea to embed the power drive components (IGBTs, MOSFETs, diode) as thinned chips into epoxy-resin layer built-up and to realize large-area interconnections on both sides by direct copper plating the dies to form a conductor structure with lowest possible electrical impedance and to achieve an optimum heat removal. In this way a thin core is formed on a large panel format which is called Embedded Power Core. The paper will specifically highlight the first results on manufacturing an embedded power discrete package as an example of an embedded power core containing a thin rectifier diode. For module realization, the power cores are interconnected to insulated metal substrates (IMS) by the use of Ag sintering interconnection technologies for the final manufacturing of Power modules. The paper will elaborate on the sintering process for Power Core/IMS interconnections, the microscopically features of the sintered interfaces, and the lateral filling of the sintering gap with epoxy prepregs. Firstly, 500W power modules were manufactured using this approach. Reliability testing results, solder reflow testing, temperature cycling test and active power cycling, will be discussed in detail.

The Influence of Features of Aluminium Alloys 2024, 6061 and 7075 on the Properties of Glare-Type Composites / Wpływ Właściwości Stopów Aluminium 2024, 6061, 7075 Na Cechy Kompozytu Typu Glare

The above paper presents the assumptions and results of the research whose aim was to determine the influence of 2024, 6061 and 7075 aluminum alloys on the final properties of GLARE-type composites. GLARE 3 2/1 type composites, made of two layers of the epoxy prepreg, reinforced with unidirectional glass fibers, arranged in the direction of 0°/90°, and two sheets of aluminum with a thickness of 0.4 mm, were investigated. Composites of various stacking configurations of alloy layers, made of one type of aluminum alloy (so-called ‘homogeneous composites’), and two different alloys (mixed composites), were analyzed. The properties of the composites were evaluated with the use of the mixing rule and compared with the test results. The influence of the used aluminum alloys on mechanical properties of GLARE-type composites has been determined. GLARE-type composite made of 7075 alloy sheets had the most favorable mechanical properties in comparison to properties of composites with 2024 and 6061 sheets. It has been shown how the properties of GLARE-type composites depend on the type of the aluminum alloy. It has been also proved that the properties of GLARE-type composites can be evaluated with the use of the mixing rule.

Influence of cure kinetic, rheological and thermo-mechanical behavior on micro-level curing strain of an epoxy prepreg

The coupled nature of chemical kinetics, mechanical and rheological behavior of a curing prepreg dictates the dimensional accuracy of a composite laminate. In this work, an integrated study was carried out to understand the influence of thermo-chemo-mechanical and thermo-chemo-viscous effect on the curing strain of an epoxy laminate. The chemical kinetics of the resin was obtained from differential scanning calorimeter. The “instantaneous elastic modulus” was measured by using dynamic mechanical analyzer. An empirical model as a function of temperature and degree of cure (thermo-chemo-mechanical) was proposed to fit the elastic modulus data of the curing resin. This model correlates well with the experimental results offering improved accuracy than existing linear models and simpler than viscoelastic models. Rheology of the resin was investigated to obtain gelation, vitrification and viscosity information. The temperature-degree of cure-dependent viscosity was modeled empirically from the rheological data. The empirical models obtained for degree of cure, modulus and viscosity were used to predict the micro-level cure-induced strain of a curing composite and compared with experimental results.

Drawdown prepreg coating method using epoxy terminated butadiene nitrile rubber to improve fracture toughness of glass epoxy composites

Laminates of fibre-reinforced prepreg have excellent in-plane mechanical properties, but have inadequate performance in the through thickness direction. Here, we address this issue by application of epoxy-terminated butadiene nitrile (ETBN) liquid rubber between the prepreg laminae using an automatic draw bar coating technique. Test results reveal that by adding ETBN in small quantities in the range of 9.33–61.33 g/m2, the interlaminar critical energy release rates (GIc and GIIc) are improved by up to 122% in mode-I and 49% in mode-II. Moreover, this finding is further supported by the dynamic mechanical analysis thermograms that clearly indicate that coating has not altered the Tg of ETBN-coated samples. Scanning electron microscopic analysis of fracture surfaces showed that rubber particles formed micro cavitations in the epoxy, causing localised rubber rich regions. These resin-rich regions require more energy to fracture, resulting in increased toughness of the glass epoxy prepreg systems.

Analysis of notched woven composites and fiber metal laminates with previous fatigue damage

Fiber metal laminates are hybrid materials consisting of alternating layers of thin metallic sheets and fiber reinforced epoxy prepregs. In this work, the fatigue damage effect on the residual strength, stiffness, and strain field in woven carbon-reinforced aluminum laminates with a circular open hole is studied. The strain fields were obtained by means of a noncontact optical deformation measuring system and then used to determine the normal stress distribution around the open hole. A squared pattern is observed in the strain mapping of the specimens with previous fatigue damage that disturbed the stress profile. The experimental stress profiles were compared with finite element modeling and an analytic solution; clear agreement is observed on this comparison for fatigue damage free specimens, except near the hole edge due to stress redistribution. Previous fatigue damage reduces residual strength and stiffness of notched woven composite samples and fiber metal laminates; it was also observed that notched fiber laminates are more fatigue tolerant than the base composite material. It is important to note that the fatigue loading caused not only damage such as matrix cracking and delamination, but also a weave style strain/stress redistribution along the warp direction.

Effects of composite lamina properties on fundamental Lamb wave mode dispersion characteristics

Lamb waves are useful for detecting changes in material properties, as their propagation characteristics are sensitive to changes in the medium through which they travel. Defects and damage in materials/structures can be detected by analyzing the difference between the phase and/or group velocity amplitude and mode changes of Lamb waves’ propagation in damaged versus un-damaged specimens. The propagation characteristics of Lamb waves are described in the form of dispersion curves, relating phase and/or group velocities to the product of frequency and thickness, with these curves being generated by solving the Lamb wave equations. In this article, the effect on the Lamb wave propagation/dispersion due to changes in the material properties such as E11, E22, G12, and density were analyzed for three different composite laminate types – unidirectional, cross-ply, and quasi-isotropic, all constructed out of 16-plies of unidirectional carbon-fiber epoxy prepregs. The analysis was performed by obtaining the characteristic Lamb waves’ phase velocity dispersion and slowness curves for each laminate type. This was done by reducing E11, E22, and G12 by 5%, 10%, 15% and 30%, and density by 1%, 2%, 5% and 10% with the intent of representing manufacturing and in-service defects. Since the Lamb waves’ velocity in a composite laminate varies with the propagation direction, the laminates were analyzed at 0°, 20°, 45°, 70°, and 90° propagation angles, with all the aforementioned variations. A program developed in MATLAB was used to generate the dispersion curves for the fundamental anti-symmetric and symmetric Lamb wave. The information provided in this paper can be used to select the proper Lamb wave modes, excitation frequency, which in turn governs sensor selection, data acquisition hardware, and damage detection algorithm for structural health monitoring of composite structures.

Stress and Deflection Development During Die Embedding into Printed Circuit Boards

Thermally induced stresses created during package manufacturing and their roles in mechanical failure are important issues for the microelectronic industry. In the present paper, a numerical analysis of the die embedding process into a printed circuit board by means of the package assembling and lamination is presented. The complex package consisting of a silicon die, an adhesive, a copper foil, an epoxy resin and prepregs (an E-glass woven structure pre-impregnated with an epoxy resin) is investigated in terms of warpage and stress development. Both are mainly introduced due to a mismatch of coefficients of thermal expansion and particularly to shrinkage of the polymer parts (adhesive, epoxy resin). Their impact on possible package failures is discussed. A two-dimensional axisymmetric numerical model is employed for investigation of the embedding process flow. Temperature dependent material properties for all materials are used in the analysis. A special focus is set on the orthotropic properties of the prepreg materials. Those are analytically homogenized based on the lamination theory of plain woven fabric composites and implemented into the numerical model. The numerical results of embedding process steps are validated experimentally by an X-ray diffraction method (Rocking-Curve-Technique) showing a good agreement of the calculated and measured curvature radius values.

The Effect of Thermooxidative Aging on the Durability of Glass Fiber-Reinforced Epoxy

Thin-skinned organic matrix composites within aeronautical structures are subjected to thermooxidative aging during their service life, leading to reductions in their durability. In this paper, a durability evaluation of fiberglass epoxy prepreg is performed on the original composite thickness before and after 800 h isothermal aging at 82°C. The characterization of both aged and unaged composites comprised tensile tests, DMA, FTIR, weight loss measurements, SEM, and DSC. The tensile strength and modulus of the composites increased after being exposed to pronounced aging conditions, whereas a decrease was observed in the toughness. DMA results revealed that the glass transition temperature and rubbery state modulus increased as a result of the thermooxidative aging. FTIR spectroscopy demonstrated the formation of carbonyl compounds due to oxidation of the chemical structure of the resin. SEM observations indicated the existence of minor superficial cracking and poor fiber-matrix adhesion after aging. In addition, a minor mass change was observed from mass loss monitoring methods. The overall findings suggest that postcuring and physical aging enhanced the brittleness of the resin, leading to a significant decline in the useful structural life of the thin-skinned composite.

Investigation of optimal lamination condition of CFRTP by compression molding method and a comparison of mechanical properties with CFRTS

The variables of die temperature, pressure, time, lamination method, and forming count are applied to fabricate thermosoftening CFRP (CFRTP) with compression molding method. The mechanical properties of CFRTP are compared with those of thermosetting CFRP (CFRTS). The first lamination method is that one hotmelt is inserted into carbon fiber (five carbon fibers and four hotmelts). The optimal lamination conditions are the die temperature of 220°C, pressure of 6 MPa, and pressurization time for 10 min. The tensile strength of CFRTP of this lamination method is 400 MPa. For higher tensile strength value, the second lamination method was applied. CFRTP prepreg was prepared with one hotmelt and one carbon fiber by applying die temperature at optimal lamination conditions. Five CFRTP prepregs were laminated under the same conditions. When the CFRTP sheet was three formings, the tensile strength of 494 MPa could be obtained. CFRTS are prepared by laminating the epoxy prepregs at 140°C with the compressive pressure of 0.5 MPa for 30 min. CFRTP sheet has the lower tensile strength than CFRTS sheet by 223 MPa, but the flexural strength was higher by 61 MPa and by 1.0 J/cm2 for Charpy impact test.

Effects of moisture ingression on polymeric laminate composites and its prevention via highly robust barrier films

Three different fiber-reinforced composite test laminates were laid up using carbon-, glass-, and Kevlar-reinforced epoxy prepregs, and then four different hydrophobic barrier films were placed as the out-of-most ply (last ply) on top of the test laminates. The prepared samples were co-cured through an autoclave per recommended cure cycles. These hydrophobic barrier films included polyether ether ketone or PEEK (12.7- and 25.4-μm thicknesses), polytetrafluoroethylene or Teflon (25.4 μm), and polyvinyl fluoride (PVF) or Tedlar (25.4 μm). Tedlar films have been the only source used for moisture prevention in Aerospace composites, so the purpose of the present study was to determine other alternatives and their moisture ingression prevention characteristics. The tape adhesion tests conducted on the barrier films of the composite panels indicated that PEEK and Tedlar films were well bonded on the composite surfaces, while Teflon films failed the tape adhesion tests. The laminate composites that were co-bonded with barrier films were immersed in water up to 29 days, and then 3-point bend tests were conducted on each sample before and after immersion. Test results show that 25.4-μm thick PEEK and Tedlar films on the carbon, glass, and Kevlar laminate composites provided similar mechanical properties. Also, the laminates incorporated with barrier films exhibited significantly higher mechanical properties when compared to the same laminates without any barrier films. This study indicated that these barrier films considerably reduced moisture ingression into the laminate composite structures, which may be useful for applications in composite aircraft and wind turbines.

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

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.