Pre-impregnated Materials Research Articles

Smart Carbon Fiber-Reinforced Polymer Composites for Damage Sensing and On-Line Structural Health Monitoring Applications.

High electrical conductivity, along with high piezoresistive sensitivity and stretchability, are crucial for designing and developing nanocomposite strain sensors for damage sensing and on-line structural health monitoring of smart carbon fiber-reinforced polymer (CFRP) composites. In this study, the influence of the geometric features and loadings of carbon-based nanomaterials, including reduced graphene oxide (rGO) or carbon nanofibers (CNFs), on the tunable strain-sensing capabilities of epoxy-based nanocomposites was investigated. This work revealed distinct strain-sensing behavior and sensitivities (gauge factor, GF) depending on both factors. The highest GF values were attained with 0.13 wt.% of rGO at various strains. The stability and reproducibility of the most promising self-sensing nanocomposites were also evaluated through ten stretching/relaxing cycles, and a distinct behavior was observed. While the deformation of the conductive network formed by rGO proved to be predominantly elastic and reversible, nanocomposite sensors containing 0.714 wt.% of CNFs showed that new conductive pathways were established between neighboring CNFs. Based on the best results, formulations were selected for the manufacturing of pre-impregnated materials and related smart CFRP composites. Digital image correlation was synchronized with electrical resistance variation to study the strain-sensing capabilities of modified CFRP composites (at 90° orientation). Promising results were achieved through the incorporation of CNFs since they are able to form new conductive pathways and penetrate between micrometer-sized fibers.

Comparison of the physico-chemical behavior of two prepregs used for body armor

The development of ultra-high molecular weight polyethylene fibers has resulted in continued improvement in ballistic protection, along with reduced weight in body armors. Prepregs from newly developed fibers are constantly available, although prepregs from older brands still remain on the market. In this work, the physicochemical properties of two successive generations of pre-impregnated materials are characterized to compare the two materials in the as-received condition and also after be consolidated in a ballistic plate. The results showed that fibers from the newer brand have a smaller diameter and greater tensile strength. This is an important result because the tensile strength of fibers is one of the variables that influence the ballistic performance of body armors. The smaller diameter of these fibers was associated with a greater degree of stretching during their manufacture, as revealed by the presence of two melting peaks in the DSC analysis.

Orthotropic viscoelastic characterization of thin woven composites by a combination of experimental and numerical methods

Electric mobility is the driving force for the development of power electronic devices embedded in printed circuit boards (PCBs). Fast charge of batteries generates large increase of temperature in PCBs. Therefore, the characterization of the thermo-viscoelastic behavior of laminates is a necessity. Tests have been conducted at elevated temperature, with a specific attention to minimize the oxidation of the material. To model precisely the viscoelastic response of the laminate, since pure resin samples are usually not available, an inverse method based on the comparison of experimental results and finite elements simulations is proposed. Periodic boundary conditions allowing to apply uniaxial tension with an arbitrary orientation are developed. In order to validate this method, resin samples are extracted from pre-impregnated material and tested under the same temperature conditions. In a last step, the cyclic response of a buried hole in a PCB is simulated by finite element calculations, using the identified material data. For large maximum temperature, substantial differences are highlighted between thermo-elastic and thermo-viscoelastic approaches in terms of accumulated plastic strain in the copper barrel. The present work opens new possibilities to model the lifetime of electronic devices facing temperature excursions ranging over the glass transition temperature.

Thickness Control of Autoclave-Molded Composite Laminates

Abstract Composite materials and especially those made from pre-impregnated (prepreg) material are widely used in the aerospace industry. To achieve the tight assembly dimensional tolerances required, manufacturers rely on additional manufacturing steps like shimming or machining, which generate extra waste, which are time-consuming and expensive. Prepreg sheets come naturally with fiber and resin volume content variability that leads manufacturers to guarantee cured ply thicknesses within a typical +/−5% margin of their nominal values. For thick laminates, this can equate to a thickness variability of as much as a few millimeter. To solve the issue, it is proposed to twin in situ laser measurements of the uncured prepreg thickness with numerical simulations of the laminate autoclave consolidation and cure process and to adjust the number of additional sacrificial plies in the laminate based on the model predictions. This reduces the need for expensive and time-consuming trial and error approaches, extra machining operations, and results in the production of a part with high accuracy dimensions. Data for IM7/8552 and IM7/977-3 are presented to demonstrate the potential of the method to reach an almost exact target thickness for flat panels.

Production and processing of pre-impregnated thermoplastic tapes by pultrusion and compression moulding

Although there is no doubt that composite materials are the future of lightweight structures and components, most of composites currently produced are made from thermoset polymers, which are not able to be recycled or reprocessed. In contrast, thermoplastic polymers offer the possibility to recycle and reprocess and when combined with a fibrous reinforcement, provide interesting mechanical properties. This work reviews the production of two thermoplastic pre-impregnated materials in a tape form, one of which is produced on new prototype equipment developed in our laboratories. The method for the production of tape is described, and the prepregs presented here were subjected to two processing techniques. The first processing method, pultrusion, is an efficient and autonomous method to produce composite profiles, marking itself as a continuous and cost-effective way to produce these materials. Pultrusion bars were then subjected to heated compression moulding, a process that allows to obtain more complex-shaped parts. The second method, heated compression moulding, is a relatively simple process which was used to obtain composite laminates. The pultrusion bars and composite laminates were then subjected to mechanical testing to evaluate the levels of consolidation of the final material. A microscope testing was also carried out to analyse the dispersion of fibres and polymer, as well as the amount of voids present in the composite.

A Facile Molding Method of Continuous Fiber-Reinforced Thermoplastic Composites and Its Mechanical Property.

The mechanical properties of continuous fiber-reinforced thermoplastic (C-FRTP) composites are commonly lower than those of continuous fiber-reinforced thermosetting plastic (C-FRP) composites. We have developed a new molding method for C-FRTP. In this study, pre-impregnated materials were successfully prepared by polymer solution impregnation method and, finally, C-FRTP was fabricated. The viscosity of the thermoplastic matrix was decreased to approximately 3dPa×s, the same level of epoxy, and the fiber volume fraction was increased from approximately 45 to 60%. The cross-section of specimens were polished by an ion milling system and impregnation condition was investigated by scanning electron microscopy (SEM). The micrographs suggested that thermoplastic polymer was impregnated to every corner of the fiber, and no void was found on the cross-section. It revealed that void-free composites with perfect mechanical properties can be manufactured with this new molding method. All specimens were submitted to a mechanical measuring equipment, and the mechanical properties of the composite specimens were investigated. Mechanical analysis revealed that tensile property and flexural property of C-FRTP were enhanced up to the same level with C-FRP.

Manufacturing tests of the lower skin of a wing in dry fiber by laying in FP and subsequent forming and infusion

In recent years many of the developments in the field of composite materials have focused on processes of dry carbon fiber, and efforts have been directed to achieve with these materials the same levels of industrialization as with pre-impregnated materials, since it is a technology of low cost and with a high productivity. The objective of this project has been to develop dry carbon fiber manufacturing methods to obtain an automated, robust and efficient process for reaching manufacture the lower skin of a wing with stringers and integrated spars by automatic laying in Fiber Placement, posterior Hot forming and finally a process of resin infusion by "Vacuum Assisted Process". To achieve this objective, the Fiber Placement machine has been set up to optimize the laying parameters for the processing of the material to be used (½” Hexcel Hi-Tape fiber). On the other hand, several tests of hot forming of this laminates have been carried out by means of the previous process, modifying process variables, tooling details and forming bag configuration, in order to obtain the preforms of stringers and spars. Finally, the integration of spars, stringers and skin was performed by a resin infusion process using the VAP technique "Vacuum Assisted Process".

A Contribution to the Study of the Forming of Dry Unidirectional HiTape® Reinforcements for Primary Aircraft Structures

In the context of developing competitive liquid composites molding processes for primary aircraft structures, modeling the forming stage of automatically-placed initially flat stacks of dry reinforcements is of great interest. In the case of HiTape®, a dry unidirectional carbon fiber reinforcement designed to achieve performances comparable to state-of-the-art pre-impregnated materials, the presence of a thermoplastic veil on each side of the material for both processing and mechanical purposes should also be considered when modeling forming in hot conditions. As a dry unidirectional reinforcement, HiTape® is expected to exhibit a transversely isotropic behavior. Computation cost and strong characterization challenges led us to model its behavior at the forming process temperature (above the thermoplastic veil melting temperature) through a homogeneous equivalent continuous medium exhibiting four ‘classical’ deformation modes and a specific structural mode, namely out-of-plane bending. The response of both single plies and stacks of HiTape® to this latter structural mode was characterized at the forming process temperature using a modifiedPeirce flexometer. Results on single plies showed a non-linear softening moment-curvature behavior and a corresponding flexural stiffness much lower than what can be inferred from continuum mechanics. Moreover, testing stacks revealed that the veil acts as a thin load transfer layer between the plies undergoing relative in-plane displacement,i.e.inter-ply sliding. This inter-ply response was then characterized separately at the forming process temperature thanks to a specific method relying on apull-throughtest. Experiments performed at pressures and speeds representative of the forming stage revealed that a hydrodynamic lubricated friction regime predominates,i.e.a linearly increasing relationship between the friction coefficient and the modified Hersey number. From an industrial point of view, high forming pressures and low speeds are therefore recommended to promote inter-ply slip to limit the occurrence of defects such as wrinkles.

Acoustic and Emission Analysis of the Defect Nucleation Process in Carbon Fiber Reinforced Plastic Samples

The results of acoustic emission (AE) control of samples consisting of nine monolayers of [±45/90/03/90 /±45] pre-impregnated material Torayca T800 with geometric dimensions of 600×100×0.9 mm are considered. A hole with a diameter of 12 mm was made in the center of each sample. Two samples were exposed to a positive temperature T = 100 °C, three samples were tested at a temperature T = 20 °C. The location of AE signals during loading is shown. The analysis of the main informative parameters of AE signals recorded in the working area of the samples is carried out. Using cluster analysis of the digital form of AE signals, three types of clusters are determined. The first type of clusters was characterized by AE signal frequency exceeding 175 kHz, and a relatively large level of the MARSE energy parameter. AE signals with lower MARSE values and a median frequency not exceeding 170 kHz when the sample was heated and not exceeding 140 kHz when loading without heating were observed in the second type of clusters. The third type of clusters was formed from AE signals whose frequency exceeded 400 kHz, and the MARSE energy parameter was 20 mV∙μs. Such clusters were characteristic only for samples tested at a temperature of T = 20 °C.

Influence of introducing organic solvents of different nature on the gelation time of a binder based on methylphenyl organosilicon oligomer during curing

The influence of the type of organic solvent, the acidity index, and the content of MF-12 methylphenyl organosilicon oligomer on the gelation time during polycondensation for the manufacture of heat-resistant electrical insulating materials is considered. The choice of the type of solvent that provides the best viscosity characteristics during impregnation and the greatest stability of the properties of the binder intended for use in combination with selective latent polycondensation catalysts, which exclude the effects of undesirable processes in the production and storage of pre-impregnated materials, has been justified.

Is the Viscoelastic Sheet for Slamming Impact Ready to Be Used on Glass Fiber Reinforced Plastic Planning Hull?

Planing hull vessel built with polymer matrix laminates and fiberglass reinforcements (GFRP) suffer structural damage due to the phenomenon of slamming during navigation, due to the impact of the boat hull on the free surface of the water at high speed. A modification in the manufacture of the laminates for these fast boats is proposed, consisting of the insertion of an additional layer of a hybrid material, formed by elastomer encapsulated in an ABS polymer cell. Using GFRP specimens made from pre-impregnated material and reproducing the characteristic impacts of slamming, it is possible to compare the modified material with the introduction of the viscoelastic layers with the response under the same conditions as the unmodified laminates. Additionally, the panels have been tested using impacts due to weight drop at different energies, which allow determining the material damage threshold as a function of the energy absorbed, and to establish a comparison with the GFRP panels modified by observation in fluorescent light. It is verified that the proposal to reduce the effect of these impacts on the generation of damage to the material and its progression throughout the service life of the vessel is effective.

Creep behavior of polyetherimide semipreg and epoxy prepreg composites: Structure vs. property relationship

In the present study, the creep behavior of polyetherimide semipreg and epoxy prepreg composites was studied using dynamic mechanical analyzer and focused on structure vs. property relationships in glassy, glass transition, and elastomeric regions. The main contribution to the field is to study pre-impregnated materials concerning creep behavior, mainly based on different analytical models, and microstructure. Two different reinforcements were used (carbon fiber and glass fiber) for each matrix. Findley, Burger, and Weibull analytical models were applied with an excellent fit for the most of them. The impregnation quality, verified by C-scan and the void content by acid digestion, shows different impregnation behaviors, mainly for epoxy/CF, which also influenced molecular motion behavior. The creep behavior was mainly influenced by matrix type than reinforcement architecture and void content. In addition, the creep was higher for epoxy in the glassy region; however, in the glass transition region, higher deformation was found for polyetherimide composites. Previous behavior is mainly attributed to higher energy storage in the glassy region which plays a significant role in the dissipation (glass transition energy), resulting in the energy loss or the drop of storage modulus in a narrow temperature range – more abrupt. This behavior was corroborated by time-temperature superposition curves in which the low deformation obtained for polyetherimide composites at low temperatures is maintained only until the glass transition temperature. Epoxy composites showed a higher initial creep deformation, but the values were almost constant with temperature, even when the temperature passes by the glass transition temperature.

FUNCTIONAL DEPENDENCE OF LASER POWER AND LAYUP SPEED FOR AUTOMATIC FIBRE PLACEMENT TEMPERATURE CONTROL

Automated Fiber Placement (AFP) is a relatively new technology that has revolutionized the production of composite structures in the aerospace and space industries for more than two decades and is nowadays increasingly used in new industries such as the wind energy. Generally, the AFP machine consists of an automatic manipulator (robot) on which a layup head is fixed for laying multiple individual composite strips at once (certainly not excluding the possibility of laying a single wider tape when it comes to Automatic Tape Placement). The layup process is performed on a mandrel or tool with a certain geometric shape. The laying head should at least include a feeder, a cutting mechanism, a compaction mechanism (usually roller) and a certain type of heater (depending on the material type). Conventionally, three types of composite materials are used in combination with AFP technology: continuous fibers reinforced with thermoset resin (usually epoxy resin), same continuous fibers reinforced with thermoplastic resin as well as bonded continuous carbon fibers. Depending on the type of material this technology uses various types of heat sources in order to achieve a good adhesion to the individual fibers that are deposited in the laying process and the pre-laid composite layers. Thermoplastic pre-impregnated material requires high temperature to reach degree of melting of the resin used to achieve complete 'welding' with the previous layers. The melting temperature varies for different materials and ranges from 130°C to 200°C for low melting thermoplastics (such as Polyamide PA and Polypropylene PP), 280°C to 350°C for Polypropylene Sulfide (PPS) up to 400°C - 450oC for Polyether Ether Ketone (PEEK). For more than two decades, hot gas torches have been used for thermoplastic layup - not a very expensive system but very difficult to control. One of the newer sources of heat close to infrared radiation (λ = 0.9-1.1 μm) is diode laser heating. This research presents a simple thermal model of the process which correlates the heater power and the layup speed with the temperature of the heating area. The deposition temperature was measured over a range of heater powers and layup speeds. The experimental data is used to define and validate a thermal model for thermoplastic material used in conjunction with a diode laser: carbon fibre reinforced thermoplastics PEEK. This enables open-loop, speed dependent heater power control, based on defining and programming the speed dependent heater power function in the machine controls. Obtained functional dependency was implemented in the AFP machine control system and tested for production of several plates with different layup angles. The achieved temperature during layup process is monitored on the thermal camera and through several pyrometers.

Challenges and opportunities on nano-enabled multifunctional composites for aerostructures

The incorporation of carbon-based nanomaterials in the polymeric matrix of carbon fibre reinforced polymer composites has recently received worldwide attention, aiming to enhance their performance and multifunctionality. In this work, different loadings of nanoparticles from the graphene family, including reduced graphene oxide (rGO) and graphene nanoplatelets (GNPs), were produced from graphite exfoliation. The mixing conditions for the production of epoxy-based suspensions were optimized using a three-roll mill, by changing the residence time and hydrodynamic shear stresses. The rheological behaviour, electrical conductivity and optical assessment were performed to study the influence of these nanoreinforcements on the resin properties. Afterwards, pristine and modified suspensions containing 0.089 wt. % of rGO or 2.14 wt. % of GNPs were used for manufacturing pre-impregnated materials with carbon fibre volume fractions of approximately 59 %. The nano-enabled CFRPs presented improved transverse electrical conductivity between 48 and 64 % when compared to the reference material. Significant enhancement of interlaminar fracture toughness (98.4 %) was found with GNPs.

Analysis of Mechanical and Impact Properties of Prepreg Composites under Elevated Temperature

This paper presents an experimental investigation of mechanical and impact properties of carbon and Kevlar-glass composites prepared from pre-impregnated materials. Namely, flexural performance in three-point bending at different temperatures is evaluated. Moreover, Charpy impact test and low-velocity impact test are also conducted for classification of impact properties and character of rupture. These all properties are important for material design of sport bike rims and many sport and other applications.

Pultrusion of fibre reinforced thermoplastic pre-impregnated materials

Fibre reinforced thermoplastic pre-impregnated materials produced continuously by diverse methods and processing conditions were used to produce composites using pultrusion. The processing windows used to produce these materials and composites profiles were optimized by using the Taguchi/DOE (Design of Experiments) methods. Those composites were then submitted to mechanical testing and microscopy analysis. The obtained results were compared with the expected theoretical ones predicted from the Rule Of Mixtures (ROM) and with those of similar engineering conventional available materials. The results obtained shown that produced composites have adequate properties for applications in common and structural engineering markets.

Effects of thickness and fiber volume fraction variations on strain field inhomogeneity

In this study, variations in thickness and fiber volume fraction are investigated as causes of elastic strain inhomogeneity in composite laminates under an applied transverse load. Standard carbon/epoxy tensile specimens were fabricated from unidirectional pre-impregnated material using two different manufacturing techniques that produced two different levels of surface roughness. Fiber volume fraction variation was computed by analyzing optical micrographs of the samples. During loading and unloading of the samples two-dimensional surface strain fields were measured on the specimen using digital image correlation. It was shown that in both cases the strain in the specimen is not uniform, as is generally assumed. Using finite element simulations the effects of fiber volume fraction variation and thickness variation were modeled individually and in combination. The simulations agree well with the experimental results and suggest that thickness variations are the dominant mechanisms involved in this elastic strain inhomogeneity.

Experimental Validation on Time Base Analysis of Various Aircraft CFRP Panel Conditions for Structural Health Monitoring

This paper evaluates the feasibility and effectiveness, within controlled conditions of an active pitch catch sensing PZT sensors on two panels and an aircraft structure made from carbon fiber reinforced plastic (CFRP) pre-impregnated materials. Once cured, the exhibits were subjected to partial and full penetration damages. Two PZT sensors each acting as an actuator and receiver were placed across the investigated region at 100mm apart. Three conditions were set on each panel for each undamaged, damaged and repaired area. Fifty readings were carried out on each panel for each condition. Feature extraction of the wavelet propagations were applied for the post processing from the captured Lamb wave data. The aircraft structure was also used to acquire data with the same above condition. A promising result shows that the interrogation of the actuating PZT sensors can differentiate the studied condition for both panels and structure. In addition, distinguished wavelet patterns were contributed by the surface irregularities due to damage and additional repair plies for both panels and aircraft structure.

Production of thermoplastics matrix preimpregnated materials to manufacture composite pultruded profiles

The aim of this work is to optimize the production of new continuous carbon fibers reinforced polypropylene matrix pre-impregnated materials (towpregs) continuously processed by dry deposition of polypropylene (PP) powder by our own developed dry coating line. The processing of the produced towpregs by pultrusion, using a prototype equipment, was also optimized. The optimization of both processes was made by studying the influence of the most relevant processing parameters in the final properties of the produced towpregs and composites. The method of Taguchi/DOE (Design of Experiments) was used to allow the determination of the optimal processing windows. The possibility of using maleic anhydride as compatibilizer for polypropylene reinforced carbon fiber composites (CF/PP) was also analyzed. A pre-consolidate tape was produced by the melting cross-head extrusion process and its proprieties compared with those of towpregs. Finally, towpregs were characterized by scanning electron microscopy (SEM) and visual analysis. Their polymer mass contents were determined and the final pultruded composite profiles were also submitted to tensile, interlaminar, flexural, calcination and optical microscopy tests.

Validation of Macroscopic Forming Simulations of a Unidirectional Pre-Impregnated Material through Optical Measurements

Presenting interesting aspects such as a high strength-to-weight ratio, Carbon Fibre Reinforced Plastic components are frequently used in the aerospace industry. The forming step, which conforms the reinforcement to a specific geometry, is a sensitive phase of the manufacturing process. In order to detect the occurrence of defects prior to any trial, forming methods are often simulated via finite element software. The presented work will detail the simulation validation of a double curved helicopter frame forming out of a unidirectional carbon fibre pre-impregnated material (M21E, Hexcel®). The finite element model was based on an explicit approach at a macroscopic level and developed via the commercially available software Visual-Crash PAM (ESI®) [1]. The validation was carried out on six different preforms. Measurements of the top layers were performed by an enhanced version of a 4D measuring system, originally developed for non-woven fabric [2], able to make reproducible photographic and height measurements (Fig. 1). Experimental results were then compared to simulated ones. Due to material specificities, the photo quality reached for non-crimp fabrics could not be achieved [2]. After hardware and software modifications, measurements and analyses were eventually successfully completed. The validation of the simulation reached an accuracy of 1° to 3° depending on the geometrical features of the preform (Fig. 2).