Thermosetting Epoxy Matrix Research Articles

Investigation of Fiber-Matrix Interface Strength via Single-Fiber Pull-Out Test in 3D-Printed Thermoset Composites: A Simplified Methodology.

The emergence of additive manufacturing technologies for fiber-reinforced thermoset composites has greatly bolstered their utilization, particularly within the aerospace industry. However, the ability to precisely measure the interface strength between the fiber and thermoset matrix in additively manufactured composites has been constrained by the cumbersome nature of single-fiber pull-out experiments and the need for costly instrumentation. This study aims to introduce a novel methodology for conducting single-fiber pull-out tests aimed at quantifying interface shear strength in additively manufactured thermoset composites. Our findings substantiate the viability of this approach, showcasing successful fiber embedding within composite test specimens and precise characterization of fiber pull-out strength using a conventional mechanical testing system. The test outcome revealed an average interfacial strength value of 2.4 MPa between carbon fiber and the thermoset epoxy matrix, aligning with similar studies in the existing literature. The outcome of this study offers an affordable and versatile test methodology to revolutionize composite material fabrication for superior mechanical performance.

Comparative Life Cycle Assessment of molding process and 3D printing of High-Performance Long-fiber Reinforced Composites

Composite materials are widely employed to produce high-performance components. However, their usage is associated with significant environmental impacts, deriving from the constituent materials (e.g., carbon fiber and epoxy resin) and the manufacturing process. Traditional methods of composite manufacturing such as bag molding or compression molding were subjected to discussion due to their high energy requirements, related to the required heat and pressure, the use of molds and the production of composite scraps. In contrast, 3D printing presents a promising alternative, that could reduce material waste and energy consumption.This study investigates an innovative 3D printing process for high-performance composites, in particular continuous carbon fiber and thermosetting epoxy matrix, with the aim of investigating its environmental impact.A comprehensive Life Cycle Assessment (LCA) was conducted to compare the environmental footprints of traditional molding and 3D printing processes of continuous fiber composites. A wide range of factors such as energy consumption, resource utilization, greenhouse gas emissions, and waste generation was considered.The outcomes of this comparative LCA reveal a significant reduction in environmental impacts associated with the innovative 3D printing system with respect to traditional methods. These findings offer crucial insights into the sustainability of 3D printing for high-performance composites, providing a valuable decision-making tool for industries seeking eco-friendly manufacturing solutions.

Effect of Fiber Sizing Levels on the Mechanical Properties of Carbon Fiber-Reinforced Thermoset Composites.

Fiber sizing is one of the most important components in manufacturing composites by affecting mechanical properties, including strength and stiffness. The sizing of manmade fibers offers many advantages, such as improving fiber/matrix adhesion and bonding properties, protecting fiber surfaces from damage during the processing and weaving stages, and enhancing the surface wettability of polymer matrices. In this work, the influence of fiber sizing levels on carbon fibers' (CFs) mechanical properties is reported at room temperature using single fiber tensile testing (Favimat+), single fiber pullout testing (SFPO), and interfacial elemental analysis by X-ray photoelectron spectroscopy (XPS). Standard modulus CFs (7 ± 0.2 μm in diameter) were sized using two commercially available Michelman sizing formulations. The average solid content for each sizing formulation was 26.3 ± 0.2% and 34.1 ± 0.2%, respectively. HEXION RIMR 135 with curing agent RIMH 137 was used as a model thermoset epoxy matrix during SFPO measurements. A predictive engineering fiber sizing methodology was also developed. Sizing amounts of 0.5, 1, and 2 wt.% on the fiber surface were achieved for both sizing formulations. For each fiber size level, 50 single-fiber tensile testing experiments and 20 single-fiber pull-out tests were conducted. The ultimate tensile strength (σult) of the carbon fibers and the interfacial shear strength (τapp) of the single fiber composite were analyzed. The sizing levels' effect on interfacial shear stress and the O/C (Oxygen/Carbon) surface composition ratio was investigated. Based on our experimental findings, an increase of 6% in fiber performance was recorded for ultimate tensile and interfacial shear strengths. As a result, generalized fiber sizing and characterization methods were established. These developed methods can be used to characterize the strength and interfacial shear strength of manmade fibers with different sizing formulations and solid contents irrespective of the matrix, i.e., thermoset or thermoplastic.

Performance of polylactide biopolymer as matrix material for woven fiber composite laminates

The first purpose of this study was, as the first time in the literature, to investigate usability of polylactide (PLA) biopolymer as the matrix of high-performance composite laminated structures having woven forms of glass and carbon fibers. For this purpose, 2-14 layers of 2 × 2 twill Carbon Fiber (CF) and 1 × 1 plain weave Glass Fiber (GF) forms were stacked by PLA powders followed by consolidation of these layers by compression molding technique. After conducting various tests and analysis it was observed that significant improvements in the mechanical and thermal properties could be obtained by laminating PLA with woven CF and GF layers. For instance, mechanical properties obtained for PLA/CF laminates with 14 layers were flexural strength of 641 MPa, flexural modulus of 34 GPa and interlaminar shear strength of 38 MPa. Another purpose of this study was to compare mechanical performance of the PLA matrix laminates with Epoxy matrix laminates having exactly the same type and number of GF and CF layers. It was revealed that mechanical properties of the PLA/CF laminates were approaching to the values of the Epoxy/CF laminates (e.g. flexural strength of 607 and 673 MPa, respectively); while due to poor adhesion between the PLA matrix and Glass Fiber surfaces, the properties were lower in the PLA/GF laminates (e.g. flexural strength of 302 and 553 MPa, respectively). Therefore, it could be generally concluded that, in terms of mechanical performance; traditional thermoset Epoxy matrix could be replaced with the renewable biopolymer PLA matrix in the Woven Carbon Fiber laminates. For the Woven Glass Fiber laminates, a proper sizing treatment with a PLA compatible silane coupling agent would be necessary.

Friction and thermo mechanical characterization of nano basalt reinforced epoxy composites

In the present investigation nano basalt epoxy composites have been examined for its possibility in employing new wear resistant material with improved friction and thermo mechanical characteristics. The nanocomposites were fabricated by dispersing (1–5 wt.%) nano basalt particles in epoxy using sonication process. The density, mechanical properties (tensile strength and micro hardness), thermal stability along with friction and wear behavior have been investigated as per ASTM standard. The result shows that all the properties found to be increased with nano basalt content in the thermosetting epoxy matrix. The FESEM studies reveal the homogeneous dispersion of nano basalt (≤3 wt.%) in the epoxy matrix. This uniform dispersion improves the thermomechanical and friction characteristics. The specific wear rate of the nano epoxy composites reinforced with nano basalt particulates exhibits the superior wear-resistance properties. The results reveal that 3 wt.% nano basalt in the epoxy matrix yields maximum improvement in tensile strength, micro hardness, thermal stability, wear resistance and coefficient of friction when compared to other composites subjected to investigation. The excessive addition (>3 wt.%) forms basalt clusters as agglomeration which pull down the frictional and thermo mechanical performance.

The influence of temperature gradient thermal shock cycles on the interlaminar shear strength of fibre metal laminate composite determined by the short beam test

The paper presents research on the interlayer adhesion of fibre metal laminate composites in a 3/2 configuration made of 0.4 mm thick 2024-T3 ALCLAD aluminium sheets. A polymer-fibrous layer is used to make a prepreg based on glass fibres with a thermosetting epoxy matrix. Experimental research presented in this paper was aimed at determining the impact of thermal shocks on the mechanism of composite destruction in the three-point-bending test. The samples were subjected to 100 °C temperature gradient thermal shocks in the form of cyclic temperature changes in the range of −40 °C and +60 °C. The number of cycles was 500 and 1000, according to the specific variant. The damage characteristics were observed by Scanning Electron Microscopy to assess failure modes. Samples subjected to thermal shocks are characterised by lower stiffness in relation to samples not exposed to repeated thermal shocks. It was found that both the failure mode and interlaminar shear strength depend on the number of thermal shock cycles. Under the influence of cyclic temperature changes in the test samples, metallic layers undergo cyclic expansion and shrinkage while the layer of fibre-reinforced composite with relatively low thermal expansion does not undergo large deformations.

Tailoring the performance of bamboo filler reinforced epoxy composite: insights into fracture properties and fracture mechanism

Due to the graded micro-structure and high specific strength-stiffness, bamboo micro fillers are systematically utilized in reinforcing different thermoset and thermoplastic polymers as replacement of conventional glass and carbon fillers. In this work, micro-size bamboo particle fillers are reinforced in ‘specific grade’ thermoset epoxy matrix and its fracture properties has been evaluated by following linear elastic fracture mechanics. To enhance its compatibility with the polymer matrix and to reduce the hydrophilicity, the bamboo micro fillers are surface modified through alkaline treatment. The extent of surface modification and removal of lower weight polymers from filler surface are examined and established by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction analysis and thermogravimetric analysis. The fracture properties of bamboo-epoxy composite material are observed to be increasing with the addition of bamboo fillers and the maximum value of fracture toughness is 0.678 MPa.m0.5 which is 32% higher than the same for neat epoxy samples. In addition, the mechanisms of notch initiated fracture propagation have also been explained for the understanding of stress singularity present at the preexisted crack tip.

Mechanical and thermal properties of Moringa oleifera cellulose-based epoxy nanocomposites

Cellulose nanowhiskers were prepared by acid hydrolysis of drumstick fruit fibers from Moringa oleifera. It was incorporated into thermosetting epoxy matrix at various weight fractions (0.06, 0.12 or 0.18 wt%) to investigate their effect on the mechanical and thermal properties of resulting epoxy resin composite. Drumstick fruit nanofibers slightly increased the thermal stability of the prepared composites. The tensile modulus (31.4%) and bending modulus (38.2%) of the epoxy composites filled with 0.18 wt% cellulose nanowhiskers significantly increased in comparison with that of the neat cured epoxy resin. However, the tensile strength (48.7%) and bending strength (41.6%) of the composites significantly decreased as the 0.18 wt% nanowhiskers were added into the epoxy matrix. The nanowhiskers (0.18 wt%) can be efficiently used in the epoxy matrix to improve considerably the tensile and bending modulus of the composites as well as a slight increase in the thermal stability.

Atomistic Modeling of the Formation of a Thermoset/Thermoplastic Interphase during Co-Curing

Co-curing of a thermoset epoxy matrix in contact with thermoplastic foils is an essential step in damage-free joining of polymers or polymer-based composites. We present results of all-atom molecular dynamics simulations that shed light onto the resulting hybrid interface. Using poly(vinylidene difluoride) (PVDF) and a multicomponent epoxy resin as model systems, we have developed a computational co-curing protocol that ensures both adequate structural representation and mobility of the PVDF chains and a realistic cross-linking conversion and topology of the epoxy resin. As a result, we reveal that mutually entangled loops of thermoplastic chains and resin strands form across the interface within the extended interphase region separating the two polymers. In tensile stress simulations we find that these loops contribute to a surprisingly large interfacial strength. In the absence of extrinsic defects, failures nucleate at the PVDF side of the interphase and propagate via a chain-pullout mechanism characteristic of semi-interpenetrating polymer networks involving thermoplastic materials.

Dynamics of model polycyclic aromatic hydrocarbon compound-epoxy composites: A dielectric study

A model polycyclic aromatic hydrocarbon (PAH) compound, anthracene, was incorporated into a thermosetting epoxy matrix as a reinforcing moiety via physical dispersion and/or chemical modification. In order to understand the “additive effect” of glass transition temperature (Tg) observed with the variation of free anthracene (AN) and bonded 2-aminoanthracene (2-AM) loading, the relaxation dynamics were investigated by broadband dielectric spectroscopy. Within the measurement range of 0.01 Hz–1 MHz and −60 to 130 °C, three relaxation processes, namely normal mode (n-mode) relaxation, α relaxation, and β relaxation, were observed for all epoxy composites with bonded and/or unbound anthracene. After eliminating the strong effect of ionic conduction by using the logarithmic derivative approximation ε''∝∂ε'∂lnω, derived from the Kramers-Kronig relations, the n-mode relaxation occurring at low frequencies above Tg for the rigid epoxy system is revealed. The Arrhenius diagram showing the temperature dependence of each relaxation process for the PAH-epoxy composites was obtained after parametric fitting using the Havriliak-Negami (HN) function in the frequency domain. The segmental α relaxation was more strongly impacted than the long-range n-mode relaxation by the different reinforcing approaches. The correlation of Tg -scaled fragility to molecular structures reveals the different mechanisms for the retardation effects on cooperative segmental and chain relaxation time. The localized β relaxation below Tg was not seemingly affected by the incorporation of bound and/or unbound anthracenes as indicated by the characteristic relaxation time and the activation energy.

Local repair procedure for carbon‐fiber‐reinforced plastics by refilling with a thermoset matrix

ABSTRACTTextile‐reinforced composites have given rise to an increasingly important key technology for lightweight construction in aerospace, automotive, civil engineering, and many other industries. Because there exists no suitable repair procedure for carbon‐fiber‐reinforced plastics (CFRPs), damaged parts have to be replaced completely; this is extremely disadvantageous both ecologically and economically. With fiber‐reinforced composites used being more and more often, fast and efficient methods for the local repair of damaged CFRPs are essential. In this article, a novel repair procedure for CFRP is presented. The thermal activation by IR radiation of oxide semiconductors was used to locally degrade the thermoset matrix of the damaged CFRP through the maintenance of its structural stability and properties. The matrix‐free textile structure was then refilled with a thermoset epoxy matrix. Carbon fibers from the treated area were characterized with scanning electron microscopy, IR spectroscopy, thermogravimetric analysis, and subsequently, tensile strength for single fibers to verify the effectiveness of the procedure. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42964.

Thermal and ablative properties of binary carbon nanotube and nanodiamond reinforced carbon fibre epoxy matrix composites

The thermal and ablative properties of carbon nanotube (CNT) and nanodiamond (ND) reinforced carbon fibre epoxy matrix composites were investigated by simulating shear forces and high temperatures using oxyacetylene torch apparatus. Three types of composite specimens—(i) carbon fibre epoxy matrix composite (CF/Epoxy), (ii) carbon fibre epoxy matrix composite containing 0.1 wt-% CNTs and 0.1 wt-% NDs, and (iii) carbon fibre epoxy matrix composite containing 0.2 wt-% CNTs and 0.2 wt-% NDs—were explored. The ablative response of composites was studied through pre- and post-burnt SEM analysis and further related with thermogravimetric analysis, weight loss profile and thermal conductivity measurements. The novel nanofiller composites showed marked improvement in their thermal and ablative properties. A 22% and 30% increase in thermal conductivity was observed for composites containing 0.1 wt-% CNTs/0.1 wt-% NDs and 0.2 wt-% CNTs/0.2 wt-% NDs respectively. These nanofillers also improved the thermal stability of thermosetting epoxy matrix, and an increase of 13% and 20% was recorded in the erosion rate of composites containing 0.1 wt-% CNTs/0.1 wt-% NDs and 0.2 wt-% CNTs/0.2 wt-% NDs respectively. This improvement is due to the increased char yield produced by the increase in the loading of nanofillers, i.e. CNTs and NDs. Insulation index and insulation to density performance have also been improved due to increased thermal conductivity and char yield.

Mode I delamination fatigue crack growth in unidirectional fiber reinforced composites: Results from ESIS TC4 round-robins

Two round robins on mode I fatigue delamination propagation organized by Technical Committee 4 of the European Structural Integrity Society compared three unidirectional carbon fiber reinforced composites, one with thermoplastic (poly-ether-ether-ketone) and two with thermoset (epoxy) matrix tested at five laboratories. Different approaches for data evaluation and their effect on the in- and inter-laboratory scatter are discussed and compared. Calculated delamination rates da/dN and applied GImax from displacement controlled tests are sensitive to small scatter in the load signal, and, therefore, a new route to evaluate the crack growth rate from pairs of load and displacement data is presented.

Structure and electric heating performance of graphene/epoxy composite films

We have prepared a series of graphene/epoxy composite films by thermal curing of diglycidyl ether of bisphenol-A with an amine-functionalized agent casted on a polyimide film, and investigated their structures and electrical properties as a function of graphene content of 0.0–10.0wt%. X-ray diffraction patterns and TEM images show that graphene nanoplatelets are well dispersed in the epoxy resin matrix. The electrical resistance of the composite films varies dramatically from ∼1013 to ∼103Ω with increasing the graphene content, especially at a certain graphene content between 1.0 and 2.0wt%. Accordingly, electric heating behavior of the composite films with 2.0–10.0wt% graphene is strongly dependent on graphene content as well as applied voltage. For the composite film with 10.0wt% graphene, a maximum temperature of ∼126°C is stably maintained over a cyclic voltage variation of 30V. The excellent electric heating performance such as rapid temperature response, high electric power efficiency, and operational stability at applied voltages is believed to be owing to the presence of highly conductive graphene sheets interconnected in the thermosetting epoxy matrix.

Simple Approach for Preparation of Epoxy Hybrid Nanocomposites Based on Carbon Nanotubes and a Model Clay

We report a simple, yet effective solution method for dispersion of carbon nanotubes and inorganic nanoplatelets into a thermosetting epoxy matrix. With an aid of exfoliated inorganic nanoplatelets, carbon nanotubes can be easily deroped in aqueous solution. Subsequently, the aqueous dispersion of carbon nanotubes and nanoplatelet hybrid can be easily transferred to organic solvents by a drying-redispersion process, and then to the epoxy matrix. Transmission and scanning electron microscopy images show that carbon nanotubes have been individually dispersed and nanoplatelets have been fully exfoliated in the epoxy matrix. X-ray diffraction is also used to investigate the dispersion mechanism. The resulting epoxy nanocomposites, containing both individually dispersed carbon nanotubes and exfoliated nanoplatelets, exhibit greatly improved modulus (up to 41%) and strength (up to 55%) at low nanotube loadings (up to 0.4 wt.%) without compromising the strain at failure. This approach is promising for large-scal...

Influence of Fiber Fineness, Fiber Maturity, and Nep Content on the Properties of Natural Fiber Reinforced Cotton-Epoxy Composites

ABSTRACT This article reports a systematic study of the effects of two different kinds of cotton fibers on properties of composites. Both cotton fibers exhibit similar mechanical properties. However, they vary in fineness and maturity. The intention of this study is to investigate the influence of these fiber properties on the characteristics of composites. In the present work composites with a thermosetting epoxy matrix with fiber mass proportions of 0 to 60% were produced by compression molding and tested for stiffness and impact strength. Contrary to expectations, the best mechanical values could be obtained from the composites with coarser fibers. This phenomenon was ascribed to the lower maturity and higher feasibility of nep formation of the finer fibers.

End-shaped copper fibers in an epoxy matrix—predicted versus actual fracture toughening

End-shaped copper fibers are placed in a brittle thermoset epoxy matrix at 10 vol% and tested in four-point bending to determine the fracture toughness of the composite. Results from four-point bend tests agree well with the theoretical predictions of the fracture toughness increment ‘Δ G’ of a metal fiber/brittle thermoset matrix composite based on single fiber pullout (SFP) tests. This close agreement demonstrates that SFP testing, along with the theoretical model, can be used as an effective end-shape screening tool for ductile fibers before full scale composite testing. The model predicts that the composite’s fracture toughness will be 46% higher with flat end-impacted fibers and 4% lower with rippled fibers compared to straight fibers at a 0° orientation. Four-point bend results show the actual composite’s fracture toughness is 49% higher with flat end-impacted fibers and 5% lower with rippled fibers compared to straight fibers. Further, four-point bend results show that end-shaped copper fibers improve both the flexural strength and modulus of the composite, demonstrating that end-shaped ductile fibers provide a good stress transfer to the fibers by anchoring the fibers into the matrix. Lastly, experimental validation of the model also indicates that at low fiber volume fractions, fiber–fiber interaction has only a minor influence on the fracture toughness for the tested ductile fiber/brittle matrix composite.

Graphitic Carbon Nanofiber (GCNF)/Polymer Materials. I. GCNF/Epoxy Monoliths Using Hexanediamine Linker Molecules

Processing methods have been optimized for the formation of graphitic carbon nanofiber (GCNF)/epoxy nanocomposites containing GCNFs highly dispersed throughout a thermoset epoxy matrix. GCNFs having a herringbone atomic structure are surface-derivatized with bifunctional hexanediamine linker molecules (GCNF-HDA) capable of covalent binding to an epoxy matrix during thermal curing and are cut to smaller dimension using high-power ultrasonication. GCNF-HDA nanofibers are dispersed in epoxy resin at 0.3 wt.% loading using variable levels of ultrasonication processing prior to thermal curing. Effects of sonication power on the quality of the GCNF-HDA/epoxy material obtained after curing have been determined from flexural property measurements, thermomechanical analysis and SEM/TEM imaging. GCNF-HDA/epoxy material of the highest quality is obtained using low-power sonication, although high-power sonication for short periods gives improved flexural properties without lowering the glass transition temperature. Good dispersion and polymer wetting of the GCNF component is evident on the nanoscale.

Blends of a new thermoplastic in a thermoset epoxy matrix

AbstractProperties of thermoplastic modified epoxy network have been studied. The particularity of this work is the use of new thermoplastic epoxies whose structure is close to the final matrix. Blends of thermoset epoxy (Diglycidyl Ether of Bisphenol A/4‐4′ methylenebis [3‐chloro 2,6‐diethylaniline]) with a thermoplastic content from 5 to 40%w have been realised. Initial miscibility in the thermoset precursors shows an UCST behaviour with a maximal value near 130°C for a thermoplastic content of 10%. Due to the presence of tertiary amine and pendant hydroxyl groups on the thermoplastic backbone, epoxy amine reactions are faster than for the neat system but the thermoplastic seems not to have reacted with the thermoset network. The final blends are transparent but toughening increase is rather low.

A microcrack growth law for multiaxial fatigue : McDowell, D.L. and Bennett, V.P. Fatigue and Fracture of Engineering Materials and Structures (1996) 19, 821–837

Two round robins on mode I fatigue delamination propagation organized by Technical Committee 4 of the European Structural Integrity Society compared three unidirectional carbon fiber reinforced composites, one with thermoplastic (poly-ether-ether-ketone) and two with thermoset (epoxy) matrix tested at five laboratories. Different approaches for data evaluation and their effect on the in- and inter-laboratory scatter are discussed and compared. Calculated delamination rates da/dN and applied GImax from displacement controlled tests are sensitive to small scatter in the load signal, and, therefore, a new route to evaluate the crack growth rate from pairs of load and displacement data is presented.