Carbon Fibre-epoxy Laminates Research Articles

Empirical model for the effect of air humidity during manufacturing on the glass transition temperature of carbon fiber reinforced hot curing epoxy – a case study

Carbon fiber-epoxy laminates are used in aerospace manufacturing, e.g. as substrates for solar cells of satellites. Commonly, fibers or fibermats are impregnated with epoxy resin and placed in the required orientation. During subsequent curing, the resin molecules are crosslinked. Cured parts are characterized by their glass transition temperature (Tg). It has been observed that Tg of epoxy matrix resin vary with recorded absolute air humidity during wet fiber placement manufacturing. Based on the production data of a series production of 203 carbon fiber laminates for space application, an empirical linear relationship between the absolute air humidity at the beginning of each production day and the observed glass transition temperature of the fully cured laminate is formulated and validated. The empirical equation describes a linear decrease of achievable glass transition temperature with increasing absolute air humidity. The quantitative nature of the results encourages straightforward practical application to determine the maximum achievable Tg for given production conditions.

Discontinuous interleaving strategies for toughening, damage sensing and repair in multifunctional carbon fibre/epoxy composites

Thermoplastic interleaving is a well-established approach to toughen carbon fibre thermoset laminates, studied over the past five decades. Recently, it has been revisited to create functional smart composites with damage sensing and repair capabilities with a renewed focus on the sustainability and longevity of components. However, the introduction of thermoplastic films within the interlaminar region often lowers fibre volume fraction and performance at elevated temperature, while the addition of impermeable continuous films during manufacture may also limit compatible fabrication methods. Moreover, the incorporation of dielectric thermoplastic films inevitably reduces through-thickness electrical conductivity and prevents accurate damage sensing of delamination in carbon fibre laminates. In this study, strategies of using discontinuous interleaving to improve both fracture toughness and thermomechanical properties of carbon fibre epoxy laminates, with the ability to monitor delamination damage and restore mechanical properties after a short healing step have been explored. Both the interleaving design and the physical properties of the thermoplastic were assessed, which has not been addressed previously. Interleaving high molecular weight thermoplastic with decreasing interleaf width and distance between interleaf zones results in increased fracture toughness (+347 %), by creating a superior toughened interlaminar zone, forcing a migration of delamination into the intralaminar region. A repair efficiency of 77 % was achieved when using a lower molecular weight of thermoplastic; however, the lack of thermoplastic over the entire fracture surface area affects the repairing performance universally. Damage sensing and thermomechanical properties were significantly improved compared to continuous interleaving, demonstrating that discontinuous thermoplastic interleaving strategies offer a favourable combination of toughening, thermal performance and accurate damage sensing for multifunctional high-performance composites.

Pressure resistance characterisation of vascular networks embedded in carbon composites for high energy physics applications

In state-of-the-art tracking detectors, lightweight carbon composite structures support the pixel or micro-strip sensors and provide the main thermal path between the silicon and a network of metallic or plastic pipes containing a cooling fluid. Despite the good results obtained with this design approach, the challenges associated with future high energy physics (HEP) experiments demand even lighter and more efficient technologies. In this regard, replacing the existing piping with a network of channels directly embedded in the composite laminates represents a promising solution to improve the thermal coupling, which also offers additional gains in terms of mass and thermo-elastic stability.While some research has been devoted to assessing the mechanical and thermal performance of such laminates, limited information about their resistance to internal pressure is currently available in the literature. This lack of data constitutes an important obstacle for the use of vascular networks in future HEP applications, which the present paper aims to address.Experimental methods were used to investigate the pressure resistance of channels embedded in carbon composite laminates. Modified poly (lactic) acid (PLA) preforms were embedded in carbon-fibre epoxy laminates. A post-cure vaporization technique removed the PLA, thus producing plates with longitudinal channels. Destructive tests were conducted to determine the burst pressure of the plates depending on the lay-up and the cross-section geometry of the channels. Both circular and oblong channels were evaluated, and various reinforcement techniques were explored to enhance the pressure resistance of the laminates. Micro-graphic examinations and X-ray micro-computed tomography were employed to gain a better understanding of the microstructure and the failure mechanisms of the plates.Plates with circular channels measuring 1.75 mm in diameter, embedded in [0/90/0]S laminates and reinforced with 2 mm lay length fuzzy carbon fibre over-braids, achieved burst pressures exceeding 45 MPa. This result, which is approximately an order of magnitude greater than that obtained for the equivalent non-reinforced laminates, demonstrates the enormous potential of this technology for future particle detectors.

Nonlinear constitutive model of CFRP composites under constant direct current: Combined effects of thermal damage and dielectric degradation

Prolonged exposure to low-density direct current (DC) can lead to mechanical degradation of carbon fiber-reinforced polymer (CFRP) composites, posing significant risks to material safety and reliability. This paper presents a nonlinear constitutive model to elucidate the mechanical degradation of CFRP composites when subjected to DC influences. Grounded in the principles of non-equilibrium thermodynamics, this model introduces two internal variables to account for the impact of thermal damage and dielectric degradation on the Helmholtz free energy. Furthermore, specialized dissipation functions are employed to derive the evolution equations governing these internal variables. Utilizing the model, we analyze the damage progression in carbon fiber–epoxy laminates subjected to constant DC loading. The theoretically projected resistivity and elastic modulus align closely with available experimental data in literature, thus confirming the rationality and accuracy of the proposed model. This model holds the potential to forecast the long-term evolution of mechanical properties in unidirectionally reinforced composite materials with varying carbon fiber contents under the influence of DC, thereby furnishing a theoretical foundation for enhancing the reliability design of CFRP composites in electrically-charged environments.

Effect of graphene nano-platelets coating on carbon fibers on the hygrothermal ageing driven degradation of carbon-fiber epoxy laminates

While spray coating of graphene nano-platelets (GNPs) on-to long carbon fibers (CF) is a viable strategy to enhance the strength of carbon fiber reinforced plastics (CFRP), the degradation of these laminates under hot and humid conditions remains unexamined. Here, Pristine and 0.4 % GNP coated CF (0.4GNP) laminates are manufactured using vacuum assisted resin transfer molding (VARTM) technique and subjected to hygrothermal ageing in de-ionized water at 75 °C for 30 days. Mechanical and structural characterization are performed after 15 and 30 days of hygrothermal ageing once the specimens were dried long enough to attain a saturation in mass. 0.4GNP laminate offered higher water resistance than Pristine laminate as 0.4 wt% non-functionalized GNP coated CFs were more hydrophobic than Pristine CF as quantified through contact angle-based measurements. Additionally, a thick carbonaceous hydrophobic interphase is formed between the CF and epoxy for 0.4GNP laminate which hinders water transport in composite. The thicker interphase also enhances the tensile strength and interlaminar shear-strength (ILSS) of 0.4GNP laminate in the unaged condition through enhanced CF—epoxy mechanical locking and epoxy strengthening in the vicinity of CF. However, both tensile strength and ILSS decreased with hygrothermal ageing time and the enhancement in strength achieved through GNP addition is completely lost after 15 days of hygrothermal ageing. Under tensile loading, the stress-strain plots showed a step-wise decrement in stress due to a ply-by-ply failure triggered by failure of the weakened inter-ply epoxy regions. In case of ILSS specimens the number of delaminations triggered increased considerably with hygrothermal ageing.

Enhanced Mechanical and Self-Healing Properties of Carbon Fiber-Reinforced Epoxy Laminates Using In Situ-Grown ZnO Nanorods and Thermo-Reversible Bonds.

Advanced hierarchical carbon fiber epoxy laminates with an engineered interface using in situ-grown ZnO nanorods on carbon fiber resulted in strong mechanical interlocking with the matrix. To further strengthen the interface, "site-specific" modification was realized by modifying the ZnO nanorods with bismaleimide (BMI), which facilitates "thermo-reversible" bonds with graphene oxide (GO) present in the matrix. The resulting laminates exhibited an improvement in flexural strength by 20% and in interlaminar shear strength (ILSS) by 28%. In order to gain a mechanistic insight, few laminates were prepared by "nonselectively" modifying the ZnO-grown carbon fiber (CF) with BMI. The "nonselectively" modified laminates showed flexural strength and ILSS improvement by 43 and 39%, respectively. The "nonselective" modification resulted in a strong improvement in mechanical properties; however, the "site-specific" modification yielded a higher self-healing efficiency (81%). Raman spectroscopy, scanning electron microscopy (SEM) micrographs, atomic force microscope (AFM) analysis, and contact angle analysis indicated a strong interaction of the modified CFs with the resin. Enhanced surface area and energy, along with a decrease in segmental molecular mobility observed from dynamic mechanical analysis, confirmed the mechanism for a better performance. Microscopic images revealed an improved interfacial behavior of the fractured samples, indicating a higher interfacial adhesion in the modified laminates. Besides mechanical properties, these laminates also showed excellent electromagnetic interference (EMI) shielding performance. The laminates with only ZnO-modified CF showed a high shielding effectiveness of -47 dB.

Experimental Investigation of Aerospace Grade Composites Porosity Laminates Under Hygro-Thermal Environment Condition

The effect of porosity has been studied by developing different levels of porosity laminates through a new autoclave based manufacturing technique known as Diverse Cure System (DCS). Further assessment of porosity levels by mechanical characterization (Inter-laminar Shear Strength), microscopic studies and acid digestion method have been achieved in Carbon Fiber Epoxy laminates. Correlations are obtained on the different levels for porosity laminates of two different thicknesses. ILSS test results are compared with the literature results. Assessment of porosity effect on ILSS is performed at room temperature and environmentally conditioned specimens. Strength degradation of 20 to 30% has been noticed on porosity laminates. Hygro-thermal specimens are tested at room temperature, elevated temperature and hot wet condition. A large reduction in the range of 45 to 53% in ILSS has been noticed on the moisture conditioned specimens tested at elevated temperature of 150°C.

Production and characterization of cellulose nanocrystals of different allomorphs from oil palm empty fruit bunches for enhancing composite interlaminar fracture toughness

The effect of different alkaline treatments during the extraction of α-cellulose from oil palm empty fruit bunches on the properties of subsequently extracted cellulose nanocrystals are reported. Sodium hydroxide in an alkaline process step results in a change of native cellulose from type I to II. Treatment with potassium hydroxide maintains the native allomorph of cellulose I. Compared to commercial cellulose nanocrystals (CNCs), oil palm empty fruit bunch (OPEFB) CNCs type I materials have higher aspect ratios, higher crystallinity index, but lower charge, and thermal stability compared to commercial CNCs. OPEFB CNCs type II have a higher aspect ratio, but lower charge than both types of CNC. Type I CNCs exhibit intermediate charge, but higher thermal stability which might make them more suitable for processing with thermoplastic polymers. A full map of physical properties, comparing the oil palm based CNCs to both commercial and lab-based materials is presented to better understand how they might be exploited in composite applications. It is demonstrated that aligned electrospun cellulose acetate butyrate nanofibrous networks containing these CNCs can enhance the mode-II interlaminar fracture toughness of a carbon fibre-epoxy laminate.

Thermal analysis of epoxy resin matrix and carbon fiber epoxy laminate cured by imidazole

Thermal analysis of epoxy resin matrix and carbon fiber epoxy laminate cured by imidazole

Degradation mechanisms in carbon fiber–epoxy laminates subjected to constant low-density direct current

This paper reports on an experimental investigation on the mechanical degradation of carbon fiber reinforced composites (CFRP) subjected to long-term exposure of low-density electric field by passing direct current (DC) through cross-ply laminates. In this study, an in-house fabricated experimental setup is used to study the effects of long-duration electrical conduction through cross-ply CFRP laminates. The experiments are conducted under constant current conditions. The change in laminate resistance is measured by monitoring the voltage across the specimen. Meanwhile, the effects of Joule heating are measured by monitoring the surface temperature across the length of the composite sample. Both voltage and temperature measurements are made continuously through the duration of the experiment. Experiments are conducted for continuous three (3) and twenty-four (24) hour duration and for cyclic three-hour repeated duration. Combined loading-compression (CLC) mechanical testing, ultrasonic pulse-echo test and dynamic mechanical analysis (DMA) is performed after electrical degradation to quantify mechanical and thermo-physical changes in the material. It is found that the electrical degradation leads to a 13%–15% reduction in compressive strength , 6%–7% reduction in Young’s modulus in thickness direction, 3%–4% decrease in glass transition temperature , and 41%–45% increase in tan δ . Post-degradation electrical resistivity change in through-thickness and in-plane directions is also correlated to damage. Comparison of change in resistivities of electrically degraded samples to thermally loaded samples indicates that the primary degradation is in the form of delamination caused by Joule heating and localized dielectric breakdown of epoxy near the carbon fiber–epoxy interface. Additionally, X-ray computed tomography (CT) images confirm that damage accumulated in electrically degraded samples is a combination of thermal damage due to Joule heating and dielectric degradation between plies due to passage of electric current.

Observing progressive damage in carbon fiber epoxy laminate composites via 3D in-situ X-ray tomography

As the use of fiber-reinforced polymer composites grows in aerospace structures, there is an emerging need to implement damage tolerant approaches. The use of in-situ synchrotron X-ray tomography enables direct observations of progressive damage relative to the microstructural features, which are studied in a T650/5320 laminate composite with two layups via monotonic tension. Specifically, the interactions of micromechanical damage mechanisms at the notch tip were analyzed through 3D image processing as the crack grew. The analysis showed intralaminar cracking was dominant during crack initiation, delamination became prevalent during the later stages of crack progression, and fiber breakage was, in general, largely related to intralaminar cracking.

Toughening mechanisms in cost-effective carbon-epoxy laminates with thermoplastic veils: Mode-I and in-situ SEM fracture characterisation

This study investigates the influence of thermoplastic non-woven veils on the interlaminar fracture energy and toughening mechanisms under mode-I dominant loading conditions in out-of-autoclave resin-infused carbon fibre-epoxy laminates. Two different non-woven micro-fibre veils, Polyetherimide (PEI) and Polyphenylene Sulfide (PPS), of low areal weight (∼10 g/m2) are introduced between carbon fibre non-crimp fabric (NCF) laminae prior to vacuum assisted resin infusion (with a low viscous two-part epoxy resin, i.e. Araldite 564/Aradur 2954). Double Cantilever Beam (DCB) specimens are tested for characterising mode-I fracture energies, and a novel miniature specimen geometry with an embedded fibre-discontinuity to trigger delamination under mode-I dominant loading is employed for in-situ SEM characterisation of micro-damage evolution and toughening mechanisms. The results obtained from DCB specimens showed that the micro-fibre thermoplastic veils considerably improved the mode-I fracture energy when compared with the untoughened fracture energy (i.e. ∼13% increase with PEI and ∼60% with the PPS veils, at the onset of crack propagation). Optical and scanning electron microscopy revealed that the veil parameters such as fibre dispersion, fibre diameter and specific surface area play an important role in enhancing fracture energy. Fracture surface micrographs indicated that progressive fibre-matrix debonding followed by fibre pull-out and fibre bridging resulted in higher fracture energy within the PPS veils. Interleaving a relatively low-cost NCF composite with thermoplastic veils could be as resistant to delamination as aerospace-grade carbon fibre-epoxy systems.

Analysis of flexible composites for coiled tubing applications

The present paper investigates thick-walled composite pipes subjected to simultaneous multi-axial loads common to those experienced in coiled tubing applications. The pipes were assumed to be filament wound carbon fiber-epoxy laminates with multiple layers and variable fiber orientations. MATLAB-based software was used to calculate stresses and carry out failure analyses through the thickness of the pipes when subjected to pressure, axial and bending (spooling) loads. Analyses were performed to determine if the composite tubes could achieve comparable strengths and spooling diameters as equivalent steel tube geometries.

The key role of thread and needle selection towards ‘through-thickness reinforcement’ in tufted carbon fiber-epoxy laminates

Tufting of dry preforms is one of the means of accomplishing through-thickness reinforcement (TTR) in a liquid composite molding process. Herein, once a preform is laid up, an automated robotic tufting setup is used to introduce the TTR. The selection of thread and needle for tufting process go hand-in-hand as tufting operation involves the act of penetrating preform by the needle as well as the trauma thread has to endure during this act. This paper explores the methodology of thread and needle selection through studies at different levels right from tufting of preforms to testing of tufted laminates. Glass, carbon and Kevlar threads in combination with a tufting needle and two different sewing needles are explored in this study. The effect of tufting speed on the quality of tuft is analyzed in terms of damage of fabric yarn, thread, and needle breakage. Layer-wise analysis of damage due to needle penetration in fabric yarns is carried out. Key mechanical properties of tufted composite samples are evaluated to determine the effect of tufting on the in-plane and out-of-plane properties.

Characterizing and modelling delamination of carbon-fiber epoxy laminates during abrasive waterjet cutting

Delamination is a common defect when abrasive waterjet (AWJ) cutting composite laminates. This paper presents experimental and numerical results used to characterize delamination when AWJ cutting a carbon-fiber/epoxy laminate. A fluid-structure interaction model was used to simulate the AWJ cutting. The structural domain used cohesive zone modeling to predict delamination along the ply interfaces. The numerical results showed that cutting delamination of the carbon-fiber/epoxy was primarily dependent on the normal interlaminar stress, with relatively large damage zones occurring ahead of the cutting front. This trend was also observed in X-ray micro-tomographs of an AWJ cut. To distinguish between side-wall and cutting-front forces, the loading generated by the jet was measured using a strain gage. This showed that the loads applied to the cutting front were larger than those generated on the side walls, and thus the likelihood of delamination was greater on the cutting front. The amount of delamination during AWJ cutting was measured using a moisture uptake methodology that was implemented with a six-factor (pressure, stand-off distance, abrasive flow rate, traverse speed, mixing-tube size, and fiber orientation) Taguchi experimental design. The trends evident in these data were consistent with the extent of delamination predicted by the numerical models, and show that traverse speed, abrasive flow rate, and mixing tube size had the most significant effect on delamination.

Novel carbon nanotube interlaminar film sensors for carbon fiber composites under uniaxial fatigue loading

This paper discusses the real-time sensing behavior of carbon nanotube film sensors interleaved between cross-ply carbon fiber-epoxy laminate under uniaxial fatigue loading conditions. The carbon nanotube film integrated cross-ply specimens were subjected to tension-tension fatigue load (R = 0.5) up to 10,000 cycles, and the electrical-mechanical response of the sensors was monitored in situ. The damage progression of the composite laminate was monitored during fatigue loading through non-destructive acoustic emission and microscopic edge replication techniques. The damage state of the laminate was also characterized using ultrasonic c-scan and X-ray computed tomography. The carbon nanotube film sensors are highly capable of strain sensing under fatigue loading. As damage accumulates in the fiber composite through the formation of matrix cracks and delamination the shift in internal stresses can be utilized to monitor damage progression in situ. In particular, the quasi-static piezoresistive response of the laminate at low strains becomes increasingly negative and can be utilized as an indicator of damage.

Thermal response of carbon fiber epoxy laminates with metallic and nonmetallic protection layers to simulated lightning currents

Nonlinear finite element (FE) simulations to characterize lightning‐induced thermal damage in AS4/3506 carbon/epoxy composites with metallic and nonmetallic protection layers were performed and then compared with those for unprotected composites. In this study, FE estimates of matrix thermal decomposition in composites subjected to 40 kA peak currents were performed. Two protection layers were considered: (1) a traditional copper mesh (CM) commonly used for aircraft lightning strike protection and (2) a single layer of highly conductive pitch carbon fiber paper (PCFP). Temperature‐dependent material properties of each constituent were used to predict the matrix thermal decomposition induced by simulated lightning current waveforms. The lightning strike FE models suggest that both the CM and the PCFP lightning protection layers successfully mitigated thermal damage development in the underlying composites as a consequence of reduced through‐thickness electrical current flow/heat conduction. The CM provided excellent protection from thermal damage; the predicted matrix decomposition only penetrated the first AS4/3506 ply. Similarly, the PCFP outer layer limited thermal damage to the top three composite plies, while the predicted matrix decomposition of the unprotected composite penetrated the sixth ply. Improved PCFP protection layers appear possible to design by varying the in‐plane and through‐thickness electrical conductivities, as well as the PCFP‐to‐first ply gap conductance. This suggests that PCFP outer layers or similar lightweight carbon‐based layers may serve as efficient lightning protection layers. POLYM. COMPOS., 39:E2149–E2166, 2018. © 2017 Society of Plastics Engineers

Delamination in cross-ply laminates: Identification of traction–separation relations and cohesive zone modeling

Identification of traction separation relations is at the forefront of damage and fracture mechanics of composite laminates. Such relations are of particular interest in fracture of laminates consisting of layers of different fiber orientations. In this study, an iterative method based on internal strain measurements and parametric numerical modeling is employed to identify a traction-separation relation in quasi-static mode I dominated delamination of a cross-ply carbon fiber epoxy laminate. The results demonstrate that crack propagation is accompanied by a large bridging zone which is smaller than the zone in a uniaxial composite specimen of the same materials and linear dimensions. While the initiation value of the energy release rate (ERR) is the same, the ERR at steady state propagation and the maximum stress in the bridging zone are respectively 1.7 and 3.1 times larger than the corresponding values of the unidirectional specimen. The obtained traction-separation relation is employed in a cohesive zone model to predict the loading response and crack growth. The adopted approach is a step towards a better understanding of delamination in cross-ply composites. (C) 2015 Elsevier Ltd. All rights reserved.

Delamination Fatigue Properties of Z-Pinned Composites

This paper presents an experimental investigation into the mode I interlaminar fatigue resistance of carbon fibre-epoxy laminate reinforced in the through-thickness direction with z-pins. The effects of the volume content, diameter and length of z-pins on the interlaminar toughness, fatigue resistance and crack bridging toughening mechanisms are determined. The delamination growth rate also slowed when the volume content or length of the z-pins was increased or the z-pin diameter was reduced.

Mechanical Properties of Self-Healing Carbon Fiber-Epoxy Composite Stitched with Mendable Polymer Fiber

Carbon fiber composites are self-healed by advance embedding of repairing agents in the composites. However, the repairing agent will influence the mechanical properties of the carbon fiber composites. In this study, poly(ethylene-co-methacrylic acid) (EMAA) filaments were stitched into carbon fiber-epoxy laminates to create a three-dimensional (3D) self-healing fiber system. Specimens with unmodified and self-healing laminates were manufactured. The mechanical properties of the carbon fiber-epoxy composite stitched with mendable polymer fiber for self-healing and unmodified laminates were compared experimentally. Results from the double cantilever beam test revealed that the stitched EMAA fibers increased the mode I interlaminar fracture toughness of the laminate by ∼120%. However, short-beam shear (SBS) strength of the composite laminates with the healing agents was slightly degraded, with a 37% reduction in the average SBS strength. The compressive-after-impact assessment showed that the strength was reduced by 6.6%. C-Scan revealed the 3D inter-connected self-healing EMAA network within the composite laminates.