Carbon Fiber-reinforced Polymer Matrix Composites Research Articles

Stabilization of polyacrylonitrile-based fiber with a quasi-traveling microwave applicator

This study introduces a novel microwave applicator and optimized processing conditions to enhance the stability of Polyacrylonitrile (PAN)-based precursor fiber (PF). The innovative microwave applicator facilitates the propagation of the electromagnetic (EM) field akin to a quasi-traveling wave, thus circumventing standing wave nodes. This ensures a uniform thermal distribution and broadens the heating zone. Utilizing this applicator, the PF undergoes thermal stabilization in a streamlined two-step process, completing in just 13 min, a significant improvement over the conventional 90-min process. This not only saves manufacturing time, promoting energy efficient manufacturing but also aligns with the global trend towards green energy and lightweight carbon fiber-reinforced polymer matrix composites, potentially catalyzing rapid economic growth. Fiber characterization through Raman spectroscopy (RS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and complex permittivity measurements reveals that the microwave-processed fiber meets the standard of commercial stabilization fiber (SF).

Comparison of Mechanical Property Simulations with Results of Limited Flexural Tests of Different Multi-Layer Carbon Fiber-Reinforced Polymer Composites.

This article is focused on the experimental study of flexural properties in different multi-layer carbon fiber-reinforced polymer (CFRP) composites and correlations with the results of finite element method (FEM) simulations of mechanical properties. The comparison of the results shows the possibility of reducing the number of experimental specimens for testing. The experimental study of flexural properties for four types of carbon fiber-reinforced polymer matrix composites with twill weaves (2 × 2) was carried out. As input materials, pre-impregnated carbon laminate GG 204 T and GG 630 T (prepreg) and two types of carbon fiber fabrics (GG 285 T and GG 300 T (fabric)) were used. Multi-layer samples were manufactured from two types of prepregs and two types of fabrics, which were hand-impregnated during sample preparation. The layers were stacked using same orientation. All specimens for flexural test were cut with the longer side in the weft direction. Pre-impregnated carbon laminates were further impregnated with resin DT 121H. Carbon fabrics were hand-impregnated with epoxy matrix LG 120 and hardener HG 700. To fulfill the aim of this research, finite element method (FEM)-based simulations of mechanical properties were performed. The FEM simulations and analysis were conducted in Hexagon's MSC Marc Mentat 2022.3 and Digimat 2022.4 software. This paper presents the results of actual experimental bending tests and the results of simulations of bending tests for different composite materials (mentioned previously). We created material models for simulations based on two methods-MF (Mean Field) and FE (Finite Element), and the comparative results show better agreement with the MF model. The composites (GG 285 T and GG 300 T) showed better flexural results than composites made from pre-impregnated carbon laminates (GG 204 T and GG 630 T). The difference in results for the hand-impregnated laminates was about 15% higher than for prepregs, but this is still within an acceptable tolerance as per the reported literature. The highest percentage difference of 14.25% between the simulation and the real experiment was found for the software tool Digimat FE 2022.4-GG 630 T composite. The lowest difference of 0.5% was found for the software tool Digimat MF 2022.4-GG 204 T composite. By comparing the results of the software tools with the results of the experimental measurements, it was found that the Digimat MF 2022.4 tool is closer to the results of the experimental measurements than the Digimat FE 2022.4 tool.

Friction and wear properties of polyacrylonitrile‐ and pitch‐based carbon fiber‐reinforced polymer matrix composites containing silicon carbide nanoparticles

AbstractThis study investigated the friction and wear properties of high‐strength polyacrylonitrile (HS‐PAN)‐, high‐modulus polyacrylonitrile (HM‐PAN)‐, and high‐modulus pitch (HM‐pitch)‐based carbon fiber‐reinforced polymer matrix composites (CFRPs) with silicon carbide (SiC) nanoparticles. The fiber orientation of the CFRP specimens was set as 8‐ply unidirectional laminates, denoted as [0]8. Ball‐on‐disk (cyclic) wear testing was accompanied by laser, digital, and scanning electron microscope observations. For the HS‐PAN CFRP, the coefficient of friction and wear resistance were the highest among the HM‐PAN and HM‐pitch CFRPs and wear resistance was improved by the addition of SiC‐nanoparticles. Furthermore, there was no significant enhancement of wear resistance for the SiC‐nanoparticle‐filled and nanoparticle‐unfilled HM‐PAN CFRP. The wear resistance was improved by adding SiC‐nanoparticles for the HM‐pitch CFRP.

Past, present and future prospective of global carbon fibre composite developments and applications

Carbon fibre and carbon fibre reinforced polymer matrix composites (CFRPs) are important lightweight materials for aerospace, automotive, rail transport, infrastructure, and renewable energy applications. This paper provides a comprehensive review on the history of carbon fibres and carbon fibre composites, the current global CFRPs consumption, and trends for future developments in the aerospace, wind turbine, automotive, pressure vessels, sports and leisure, and construction sectors. The history of carbon fibres and CFRPs is discussed over four representative periods including their early development (1950–60's), growth of carbon fibre composites industry (1970–80's), major adoption of carbon fibre composites (the first wave, 1990–2000's), and expanded use of carbon fibre composites (the second wave, 2010's and beyond). Despite a 37% decline of carbon fibre consumption in the aerospace industry in 2021 caused by COVID-19, the global CFRP demand was around 181 kt which more than doubled its value in 2014. There is tangible projected increase over the next five years and the demand for CFRPs is expected to reach 285 kt in 2025, mainly attributed from the fast expansion of non-aerospace industries such as the wind energy sector. Lower cost carbon fibres (e.g., large tow) and associated manufacturing technologies are continually evolving. Finally, the implications of emerging materials and manufacturing methods in conjunction with recycling and reuse for carbon fibre composites are discussed.

Constructing a Superior Interfacial Microstructure on Carbon Fiber for High Interfacial and Mechanical Properties of Epoxy Composites.

The interface quality is crucial for the properties of carbon fiber-reinforced polymer- matrix composites (CFRPs). In order to improve the interfacial and mechanical properties of CFRPs, a superior gradient modulus interfacial microstructure is constructed on the carbon fiber (CF) surface by chemically grafting a self-assembly carboxyl-terminated hyperbranched polymer (HP-COOH). A monofilament debonding test, a short beam shear test, an impact test and a dynamic mechanical thermal analysis (DMTA) were conducted to investigate the properties of the modified composite. Prominent improvements of 79.6% for the interfacial shear strength, 51.5% for the interlaminar shear strength, and 49.2% for the impact strength, as well as superior heat-resistance properties are achieved for composites with the gradient modulus interface over those of the untreated CF composites. The mechanism for performance improvement is mainly attributed to the enhanced CF surface energy, mechanical interlocking, and chemical bonding interactions. In particular, an atomic force microscopy (AFM) test proved that the gradient modulus interfacial microstructure formed by HP–COOH could widen the interface layer thickness and buffer the sharp variations in the modulus from CF to resin, thereby transmitting an external force and reducing the stress concentration. This work provides a facile and efficient strategy for constructing a superior and versatile interface for high- performance composites.

Tensile properties and fracture behavior of carbon nanotube-sheets/carbon fibers epoxy-impregnated bundle composites

An interesting technique for modifying carbon fiber-reinforced polymer matrix composites is through hybridization with carbon nanotubes (CNTs). Carbon nanotubes sheets/carbon fibers offer potential benefits of nanoscale reinforcement to the well-established fibrous composites by creating multiscale hybrid micro-nano composites. In this study, the tensile properties of high tensile strength polyacrylonitrile (PAN)- and high modulus pitch-based carbon fiber-reinforced polymer matrix composites incorporating CNT sheets (CNT-sh/CFs/Ep-H: CNT sheets/carbon fibers/epoxy hybrid composites) were investigated. To fabricate CNT sheets, CNT was vertically grown on a quartz glass plate by chemical vapor deposition. A solid-state drawing and winding technique was applied to transform the vertically aligned CNT array into horizontally aligned CNT sheets. The tensile modulus of the CNT-sh/CFs/Ep-H was higher than that of the composite in the as-received state (CFs/Ep: carbon fibers/epoxy bundle composite). The tensile strength of the CNT-sh/PAN-based CF/Ep-H was lower than that of the PAN-based CF/Ep, whereas the tensile strength of the CNT-sh/pitch-based CF/Ep-H was higher than that of the pitch-based CF/Ep.

Simulation and experimental testing of a bolted carbon-aluminium hybrid wingbox assembly under thermal loads

On the last decades, the aeronatical industry has increased the use of Carbon Fibre Reinforced Polymer matrix composites (CFRP), and at the same time, has kept the use of conventional metals. This has led to the presence of hybrid assemblies in the wingbox, fuselage, etc. Due to their high stiffness and strength, as well as the ease to dissasemble and repair, these assemblies are usually bolted. During its usage, an aircraft is subjected to large thermal jumps (the temperature difference between a landed aircraft and a flying one can reach 140 ºC). Because of the presence of hybrid bolted joints, the materials of the joint undergo different thermal expansion/contraction, which cause significant thermal stresses altering the mechanical behaviour of the joint. Framed in the aeronautical Clean Sky 2 project ‘INNOHYBOX’, this work studies the behaviour of hybrid bolted joints subjected to thermal loads, both numerically and experimentally, following a pyramidal approach. At the coupon level, a hybrid carbon-aluminium single-lap shear specimen with one bolt was used to comprehend and characterise its behaviour under thermal loads. At the substructural level, a subcomponent representative of a wingbox was tested under thermal conditions. Finally, a full wingbox was tested. Furthermore, a computationally efficient finite element model was developed to simulate the complex behaviour of these type of joints without comprimsing the computational cost. The numerical predictions correlate favourably against the experimental data.

Enhancing cryogenic mechanical properties of epoxy resins toughened by biscitraconimide resin

The demand for lightweight carbon fibre-reinforced polymer matrix composites (CFRPs), which typically have an epoxy resin (EP) matrix, have been increasing in cryogenic fields. The mechanical properties of CFRPs depend largely on the mechanical properties of their EP matrix, particularly in cryogenic environments. To date, however, no commercially available EP can comprehensively fulfil the demands. In this study, biscitraconimide resin (BCI) was introduced to enhance the mechanical properties of EP. Results indicated that cryogenic tensile strength, bending strength, impact strength and fracture toughness (KIC) were enhanced for the modified epoxy system at appropriate BCI contents. The tensile strength (144.57 MPa) of EP increased by 67.6%, and its bending strength (229.53 MPa) increased by 94.9% at 77 K. Impact strength (41.16 kJ·m−2) was improved by 195.7% and KIC (2.72 MPa·m1/2) increased by 65.9% at 77 K compared with that of pure EP. The results were properly explained via scanning electron microscopy observation, and the mechanism of the improved mechanical properties was analysed in detail.

Damage by Improvised Incendiary Devices on Carbon Fiber-Reinforced Polymer Matrix Composites

This study focuses on short-term thermal degradation of polymer matrix composites by one-sided impact of improvised incendiary devices (IID). Specimens of two commercial composites HexPly® 8552/IM7 and M18-1/G939 with various thicknesses (1–8 mm) are systematically investigated as well as sandwich structures thereof, applying various amounts of fire accelerant predominantly in laboratory scale fire tests. Results of preceding large-scale fire tests with IIDs justify the chosen conditions for the laboratory-scale fire tests. The aim is to correlate the amount of fire accelerant with heat damage and residual mechanical strength. Thermal damage is characterized visually and by ultrasonic testing, infrared spectroscopy, and residual interlaminar shear strength. Matrix degradation and combustion only contribute to the overall amount of released heat by the fire accelerant for thin and especially vertically aligned panels as tested by a cone calorimeter (without electrical heating), but not for horizontally orientated and thicker panels. Degradation processes are discussed in detail. Protective effects are observed for typical coatings, a copper mesh applied for protection against lightning strike, combinations thereof as well as an intumescent coating. Especially sandwich structures are prone to severe damage by assaults with IID, such as Molotov cocktails.

An integrated methodology for fatigue life prediction of carbon fiber-reinforced polymer matrix composites under constant and variable R-ratio loadings

The progress in the anisomorphic constant fatigue life (CFL) diagram approach for efficiently identifying the fatigue of carbon fiber-reinforced composites under constant waveform loading over the whole range of mean stress is reviewed. The asset of this approach is the concept of critical stress ratio. The anisomorphic formulation of CFL diagram is flexible enough not only to increase the number of segments to accommodate the transient change in sensitivity to mean stress, but also to take a generalized form that considers the effect of temperature as well as the effect of variability in fatigue life. The anisomorphic approach is shown to succeed in describing the asymmetric and nonlinear CFL diagrams for carbon fiber composites of different kinds. It is suggested that the classic procedure for prediction of spectrum loading fatigue life of composites using the Miner rule with the rain flow cycle counting method is powered by integrating with the anisomorphic CFL diagram approach.

Influence of low-velocity impact-induced delamination on electrical resistance in carbon fiber-reinforced composite laminates

In this paper, experiments have been performed and finite element models have been developed for studying the influence of low-velocity impact damage on the four-probe electrical resistance of carbon fiber-reinforced polymer matrix laminates. Sixteen-ply and 32-ply AS4/3501-6 laminates with quasi-isotropic layup were analyzed. Electrical resistance was evaluated using a four-step procedure. First, finite element models were created in Abaqus Finite Element Analysis (FEA) for simulating low-velocity impact using a quasi-static loading approach. Second, matrix rupture in the inside plies was evaluated, and delamination analysis was performed at the corresponding interfaces to determine delamination patterns. Third, four-probe electrical finite element models were developed in Abaqus FEA for specimens before and after impact using the concept of effective conducting thickness and the delamination patterns obtained from the delamination analysis. Effects of the low-velocity impact delamination on four-probe top and oblique electrical resistance were studied. Electrical resistance predictions were compared to the experimental data. Both top and oblique resistance planes were sensitive to presence of delamination with the oblique resistance measurement being more sensitive as compared to the top resistance measurement. In addition, the resistance of the 16-ply specimens was more greatly affected by the delamination compared to the 32-ply specimens. The proposed analysis can be utilized for design of carbon fiber-reinforced polymer matrix composites with optimized damage sensing capabilities.

The Attenuation of Galvanic Corrosion between Mechanically-Coupled Aluminum and Carbon-Fiber Reinforced Polymer Matrix Composites in Outdoor Environments

The effect of hydrophobic coatings on insulative skirts to attenuate galvanic corrosion between mechanically-fastened 6061-T6 aluminum (Al) alloy and carbon-fiber reinforced polymer-matrix composites (CFR PMC) was studied in outdoor environments. The utilization of hydrophobic coatings on the insulative skirt can help to attenuate galvanic corrosion by disrupting the formation of a continuous electrolyte film. Our recent work* (Srinivasan et.al. 2016) showed a novel approach of utilizing hydrophobic coatings on insulative skirts (ranging from 0 inch to 8 inch in length) to mitigate galvanic corrosion between Al alloy and CFR PMC couples in laboratory humidity chamber corrosion tests (90% relative humidity (RH) and 30°C). Shorter skirt lengths (i.e., 0.25 inch) with a hydrophobic Siloxel TM coating was shown to be most effective in attenuating galvanic corrosion based on the capillary effect that wicked away large water droplets, generated from a mechanical sprayer. During outdoor exposures, airborne salt particles are on the order of a few microns, and hence, may settled on the skirts as a relatively uniform distribution. To study the effectiveness (due to capillary wicking) of shorter skirt lengths during outdoor exposure, Siloxel™-coated G10 fiberglass insulating skirts of 0.25 inch and 0.5 inch were compared at rain forest (Manoa) and severe marine (MCBH) environments for 72 days. The natural salt particles deposited on the Siloxel™ coated skirts, and galvanic corrosion rates were higher for the shorter skirt length contrary to the laboratory humidity chamber results, indicating that the capillary wicking mechanism was not dominant in the field. In outdoor field environments, since natural salt particles (on the order of a few microns) are deposited on the substrate and not sprayed in solution as in the laboratory experiments, the capillary-wicking mechanism does not have a chance to operate in the absence of substantial rain. To study the effectiveness of different coatings as a function of coating contact angle, CFR PMC/Al galvanic couples with 0.50 inch insulating skirts coated with either the Siloxel™ , an epoxy coating, or no coating were also exposed at the MCBH test site. The CFR PMC/Al galvanic specimens at the severe marine site were exposed in the sheltered and unsheltered conditions. The higher contact angle SiloXelTM-coated G10 induced greater attenuating of galvanic corrosion in the sheltered condition; whereas, the epoxy-coated G10 induced greater attenuation in the unsheltered condition. In the unsheltered condition, the epoxy matrix of the G10 fiber glass substrate degraded due to exposure to sunlight causing the Siloxel™ coating (only several microns thick) to debond and crack resulting in a loss of hydrophobicity; whereas, the much thicker epoxy coating retained its integrity for the duration of the test and hence induced better attenuation. *Raghu Srinivasan and Lloyd Hihara, “Utilization of Hydrophobic Coatings on Insulative Skirts to Attenuate Galvanic Corrosion between Mechanically-Fastened Aluminum Alloy and Carbon-Fiber Reinforced Polymer-Matrix Composites”, Electrochemistry Communications, Volume 72 (2016).

Surface Characterization of Carbon Fiber Polymer Composites and Aluminum Alloys After Laser Interference Structuring

The increasing use of carbon fiber-reinforced polymer matrix composites (CFPC) and aluminum alloys as lightweight materials in the automotive and aerospace industries demands enhanced surface preparation and control of surface morphology prior to joining. In this study, surfaces of both composite and aluminum were prepared for joining using an Nd:YAG laser in a two-beam interference setup, enabling the (1) structuring of the AL 5182 surface, (2) removal of the resin layer on top of carbon fibers, and (3) structuring of the carbon fibers. CFPC specimens of T700S carbon fiber, Prepreg—T83 epoxy, 5 ply thick, 0°/90° plaques were used. The effects of laser fluence, scanning speed, and number of shots-per-spot were investigated on the removal rate of the resin without an excessive damage of the fibers. Optical micrographs, 3D imaging, and scanning electron microscope imaging were used to study the effect of the laser processing on the surface morphology. It was found that an effective resin ablation and a low density of broken fibers for CFPC specimens was attained using laser fluences of 1–2 J/cm2 and number of 2–4 pulses per spot. A relatively large area of periodic line structures due to energy interference were formed on the aluminum surface at laser fluences of 12 J/cm2 and number of 4–6 pulses per spot.

The Utilization of Hydrophobic Coatings on Insulative Skirts to Galvanically Decouple Mechanically-Fastened Aluminum Alloy and Carbon-Fiber Reinforced Polymer-Matrix Composites

In a quest to reduce weight and improve performance, aluminum (Al) alloys have been used in conjunction with lightweight materials such as carbon-fiber reinforced (CFR) polymer-matrix composites (PMCs). The mechanical contact of CFR PMCs (which serves as cathode) with Al alloys (which serves as anode) can result in galvanic corrosion of the Al alloys in a corrosive environment. The major objective of this research was on devising corrosion control strategies for couples of 6061-T6 Al that were mechanically fastened to CFR PMCs using Ti-6Al-4V bolts. A typical method used to mitigate galvanic corrosion when bolting Al to CFR PMCs is to insert an insulating layer (G-10 fiberglass or E-glass–reinforced PMC) between the members. Although the direct contact between the aluminum and CFR PMC is eliminated, the aluminum and the CFR PMC are still galvanically coupled through the Ti fastener, and galvanic corrosion on Al will occur if the electrolyte bridges over the insulating layer. Galvanic corrosion can be mitigated by increasing the IR loss across the skirt, which can be achieved by lengthening the skirt or breaking up the continuity of the salt bridge that forms on the skirt. The continuity of the salt bridge can be disrupted by applying hydrophobic coatings on the insulative skirt. In this study, galvanic corrosion rates were monitored for 6061-T6 Al—CFR PMC couples (with insulating skirts) subjected to the following conditions: 1) Insulting skirts having coatings (silane-based coating, epoxy, polyurethane, or latex) of various levels of hydrophobicity 2) Salt solutions of various concentrations (0, 10, 100, 1000, 10000, and 20000 ppm of chlorides) 3) Various insulating skirt lengths (0.25 inch or 0.5 inch), 4) Various exposure conditions (controlled humidity laboratory exposure, outdoor exposure with sheltered and unsheltered conditions). The effect on galvanic corrosion by the different coatings was studied as a function of hydrophobicity, as determined by the contact angle for wetting. The contact angle was measured on the coating surface before and after exposure to the outdoor test sites to study the effects of environmental degradation on hydrophobicity. The degradation of the coatings was also studied using other techniques such as Raman spectroscopy. The ingress of environmental constituents (i.e., chlorides, sodium ions, etc.,) towards the anode and cathode surfaces was analyzed using scanning electron microscopy (SEM) and energy dispersive-X-ray analysis (EDXA).

Modified Hartman-Schijve fitting of mode I delamination fatigue data and the resulting variation in threshold values Gthr

With a view toward applications in structural composite design, mode I (tensile opening) fatigue delamination test data obtained for carbon fiber-reinforced polymer-matrix (CFRP) composites have recently been (re-)analyzed with different fitting approaches for the determination of threshold values Gthr. One approach that looks promising as an alternative to the conventional Paris-law type presentation of the data (a double logarithmic plot of delamination length increment per cycle da/dN versus GImax or ΔGI) is a modified Hartman-Schijve fit (also a double logarithmic plot of da/dN, but versus a square-root function of G) requiring four fitting parameters (labelled A, β, D and Gthr). The present paper details a procedure for selecting the fitting parameters A, β, and D in order to determine Gthr in a consistent way from the experimental data. For some design approaches, it is important to also have an estimate of the possible scatter in experimentally derived values of Gthr. The procedure chosen by the authors yields Gthr values and a measure of the scatter due to experimental variation of the other fitting parameters. Selected mode I delamination fatigue data for IM7/8552 CFRP composites from literature are used to compare Gthr values and their scatter from Paris-law type fitting with those from the modified Hartman-Schijve approach.

Cyclic fatigue delamination of carbon fiber-reinforced polymer-matrix composites: Data analysis and design considerations

Activities toward standardization of fracture mechanics tests on carbon fiber-reinforced polymer-matrix (CFRP) composites have recently focused on cyclic fatigue under mode I (tensile opening), mode II (in-plane shear) and mixed-mode I/II loading. Data from recent round robins performed by Technical Committee 4 (TC4) of the European Structural Integrity Society (ESIS) and from preliminary testing of additional CFRP epoxy laminates at the authors’ laboratories are analyzed with different approaches in attempts to reduce scatter and to identify parameters for CFRP structural design. Selected test data comparing load and displacement control for the cyclic fatigue tests are also discussed. Specifically, threshold values from Paris-law data fitting are compared with values from fitting with a modified Hartman–Schijve approach. Independent of the approach used for the analysis, mode I threshold values of selected CFRP seem to be in the range between about 30 and 100J/m2, i.e., roughly around the range of critical mode I energy release rate values (denoted by GIC) obtained from fracture testing of neat commercial epoxy resins, but clearly below quasi-static initiation GIC-values for unidirectional CFRP composites. Implications for CFRP structural design based on mode I fatigue fracture mechanics test data are briefly discussed.

The Effects of Hydrophobic Coatings on an Insulative Skirt Layer to Decouple Galvanic Corrosion Between Mechanically-Coupled Aluminum Alloy and Carbon-Fiber Reinforced Polymer-Matrix Composites

Lightweight metals such as aluminum (Al) are being frequently coupled to polymer-matrix composites (PMCs) to reduce weight and improve performance in a multitude of applications in the defense, aerospace, automotive and civil-infrastructure industries. The intimate contact of carbon-fiber reinforced (CFR) PMCs with Al alloys can result in galvanic corrosion of the Al alloys. The electrically conductive carbon fibers in the CFR PMCs serve as cathodes and induce galvanic corrosion of the Al. The galvanic corrosion between 6061-T6 Al and CFR PMC can be effectively attenuated by decoupling Al from CFR PMC using an insulating skirt layer. The deposition of salts on the insulative skirt, however, can create a continuous ionically-conductive electrolyte film that can bridge the CFR PMC and Al, thus reducing the effectiveness of the skirt. The utilization of hydrophobic coatings on the skirt can help to attenuate galvanic corrosion by breaking up the continuous electrolyte film. In this study, the effect of hydrophobic coatings on insulative skirt materials was investigated to attenuate galvanic corrosion between mechanically coupled 6061-T6 Al and CFR PMC. Experiments were conducted in accelerated laboratory and outdoor conditions. The galvanic corrosion rates were monitored for coatings of different degrees of hydrophobicity, various skirt lengths, and exposure conditions and. Initially, the galvanic corrosion rates for couples with different insulating-skirt lengths coated with a silane-based ceramer (i.e., Siloxel) were monitored in a humidity chamber at 30o C and 90% RH over a period of up to 3 days using the zero resistance ammeter (ZRA) technique. The couples were sprayed with salt solutions of different concentrations. Results from this accelerated humidity ZRA experiment showed that a skirt length of only 0.5 inch was very effective in attenuating galvanic corrosion. To study the effectiveness of different coatings, specimens were fabricated with a 0.5-inch long skirt and coated with either Siloxel, epoxy, polyurethane or latex. The resulting galvanic corrosion rates were then compared. The two best coatings in attenuating galvanic corrosion (i.e., Siloxel and polyurethane) were selected for outdoor exposure. CFR PMC/Al galvanic couples with 0.5 inch insulating skirts (with either the Siloxel or polyurethane coatings, or no coatings) were exposed at rain forest (Manoa) and severe marine (MCBH) environments for 2 months. The CFR PMC/Al galvanic specimens at the severe marine site were exposed in two conditions: sheltered and unsheltered. The sheltered specimens were subjected to significant salt build up; whereas, the unsheltered specimens were subjected to cleansing effects of rain. The effect the salt deposition characteristics on galvanic corrosion between the sheltered and the unsheltered conditions was studied.

Comparison of Quasi-static and Cyclic Fatigue Delamination Resistance of Carbon Fiber Reinforced Polymer-matrix Laminates under Different Mode Loading

Delamination resistance data from different carbon-fiber reinforced polymer-matrix (CFRP) composites are compared for different loading modes, i.e., quasi-static and cyclic fatigue, opening tensile mode I, in-plane shear mode II, and fixed-ratio mixed-mode I/II. For this, data from round robin tests conducted at the authors laboratories will be complemented by selected results from literature. Questions related to delamination resistance of CFRP composites with implications for composite structural design and testing include, e.g., the determination of threshold values in cyclic fatigue, the question of conservative mode (mode I versus mode II), approaches for data analysis, and possible analogies in short crack cyclic fatigue between fracture behavior of structural metal alloys and CFRP. The scatter in Paris-type law data analysis of cyclic fatigue tests and the resulting apparent threshold behavior that has implications for composite structural design will be presented. Load measurement resolution yields the major contribution to scatter in displacement controlled fatigue tests. The analogous displacement resolution for load controlled tests is discussed and limitations in test control and of power law displacement data fitting for analysis are pointed out.

Fracture toughness of fiber metal laminates: Carbon nanotube modified Ti–polymer–matrix composite interface

Multifunctional hybrid composites are proposed as novel solutions to meet the demands in various industrial applications ranging from aerospace to biomedicine. The combination of carbon fiber and/or fabric, metal foil, and carbon nanotubes is utilized to develop such composites. This study focuses on processing and fracture toughness characterization of the carbon fiber-reinforced polymer–matrix composites and the carbon nanotube modified interface between the polymer–matrix composite and titanium foil. Vacuum Assisted Resin Transfer Molding (VARTM) process is used to fabricate the laminate. Double cantilever beam tests at both room temperature and high temperature are conducted to assess the mode I interlaminar fracture toughness. The experimental and characterization efforts suggest that carbon nanotubes improve bonding at the hybrid interface. Simple computational models are developed to assist the interpretation of experimental results and further investigate the damage modes. The numerical results agree well with the limited experiments at crack initiation and furthermore support the absence of mode mixity.

Interlaminar shear fatigue behavior of glass/epoxy and carbon/epoxy composites

Abstract This work addresses a strong need in accurate fatigue properties of glass- and carbon-fiber reinforced polymer–matrix composites. In particular, interlaminar shear fatigue properties of glass/epoxy and carbon/epoxy composite tape material systems used in aircraft fatigue-critical applications are needed. The interlaminar shear fatigue material properties, essential for the development of the analysis methods able to capture fatigue delamination failure onset in composite structures, are presented in this work. S–N curves are generated based on custom short-beam shear (SBS) fatigue tests. The custom SBS test configurations ensure a consistent interlaminar shear failure mode. This work extends the recently developed methodology, published in Composites Science and Technology to characterize nonlinear shear properties of composite materials using digital image correlation (DIC) and finite element analysis, to fatigue loading. Test data sets used to develop the fatigue properties, include approximately 20 glass/epoxy and 30 carbon/epoxy SBS coupons. All tests were run in load control at 0.1 load ratio. To better understand material behavior under cyclic loading, surface shear strain was monitored using the DIC technique. Accurate shear stress approximation resulted in similar S – N curve shapes for the glass-fiber and the carbon-fiber composites.