Analysis of a pull-out test with real specimen geometry. Part II: the effect of meniscus

This paper continues our study on the platelet model of the pull-out specimen, in which the matrix droplet shape is approximated by a set of thin parallel disks with the diameters varying along the embedded fiber. Using this model, the fiber tensile stress and the interfacial shear stress profiles were calculated for real-shaped matrix droplets, including menisci (wetting cones) on the fibers, taking into account residual thermal stresses and interfacial friction. Then, these profiles were used to numerically simulate the processes of crack initiation and propagation in the pull-out test and to obtain theoretical force-displacement curves for specimens with different embedded lengths and wetting cone angles. Our simulations showed that the interfacial crack in real-shaped droplets initiated at very small (practically zero) force applied to the fiber, in contrast to the popular ‘equivalent cylinder’ approximation. As a result, the equivalent cylinder approach underestimated the interfacial shear strength (IFSS) value determined from the pull-out test and at the same time overestimated the interfacial frictional stress; the smaller was the wetting cone angle, the greater the difference. We also investigated the effects of the embedded fiber length and interfacial frictional stress in debonded areas on the calculated IFSS. The simulated force–displacement curves for the real-shaped droplets showed better agreement with experimental curves than those plotted using the equivalent cylinder approach.     

Effect of Halloysite Nanotube Incorporation in Epoxy Resin and Carbon Fiber Ethylene/Ammonia Plasma Treatment on Their Interfacial Property

Abstract

Effects of halloysite nanotube (HNT) loading of up to 2% in epoxy resin on its mechanical properties were characterized. The interfacial property of the resin with carbon fiber nanocomposite was also studied. Single fiber composite (SFC) technique was used to characterize the carbon fiber/epoxy resin interfacial shear stress. Carbon fibers were also coated with ammonia/ethylene plasma polymer to obtain a thin coating of the polymer with amine groups that could react with the epoxy and thus improve the interfacial property. The results indicated that the Young’s modulus of HNT containing nanocomposites increased slightly up to a loading of 0.25% after which it started to decrease. The tensile strength, however, steadily decreased with increasing of HNT loading although the fracture strain did not change significantly. This might be related to the nanotube shape, size and clustering. The interfacial shear strength (IFSS) was also increased slightly with HNT loading. The ethylene/ammonia plasma polymer coated fibers exhibited significantly higher IFSS by over 150%, independent of the HNT loading. The highest IFSS obtained was almost 79 MPa for plasma treated fibers. The results suggest that the carbon fiber/epoxy interface is not affected by the incorporation of up to 1.5% of HNT. Furthermore, the fiber surface modification through plasma polymerization is an effective method to improve and control the IFSS.     

The interfacial strength and fracture characteristics of ethanol and polymer modified carbon nanotube fibers in their epoxy composites

Carbon nanotube (CNT) fibers spun from CNT arrays were used as the reinforcement for epoxy composites, and the interfacial shear strength (IFSS) and fracture behavior were investigated by a single fiber fragmentation test. The IFSS between the CNT fiber and matrix strongly depended on the types of liquid introduced within the fiber. The IFSS of ethanol infiltrated CNT fiber/epoxy varied from 8.32 to 26.64 MPa among different spinning conditions. When long-molecule chain or cross-linked polymers were introduced, besides the increased fiber strength, the adhesion between the polymer modified fiber and the epoxy matrix was also significantly improved. Above all, the IFSS can be up to 120.32 MPa for a polyimide modified CNT fiber, one order of magnitude higher than that of ethanol infiltrated CNT fiber composites, and higher than those of typical carbon fiber/epoxy composites (e.g. 60–90 MPa). Moreover, the composite IFSS is proportional to the tensile strength and modulus of the CNT fiber, and decreases with increasing fiber diameter. The results demonstrate that the interfacial strength of the CNT fiber/epoxy can be significantly tuned by controlling the fiber structure and introducing polymer to optimize the tube–tube interactions within the fiber.     

Surface modification of ultra high modulus polyethylene fibers by an atmospheric pressure plasma jet

To improve their adhesion properties, ultra high modulus polyethylene (UHMPE) fibers were treated by an atmospheric pressure helium plasma jet (APPJ), which was operated at radio frequency (13.56 MHz). The surface properties of the fibers were investigated by X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and contact angle measurement. The surface dyeability improvement after plasma treatments was investigated using laser scanning confocal microscopy (LSCM). The adhesion strengths of the fibers with epoxy were evaluated by microbond tests. In addition, the influence of operational parameters of the plasma treatment including power input and treatment temperature was studied. XPS analysis showed a significant increase in the surface oxygen content. LSCM results showed that the plasma treatments greatly increased fluorescence dye concentrations on the surface and higher diffusion rate to the fiber center. The tensile strength of UHMPE fiber either remained unchanged or decreased by 10–13.6% after plasma treatment. The contact angle exhibited a characteristic increase in wettability, due to the polar groups introduced by plasma treatment. The microbond test showed that the interfacial shear strengths (IFSS) increase significantly (57–139%) after plasma treatment for all groups and the optimum activation is obtained at 100°C and 5 W power input. SEM analysis showed roughened surfaces after the plasma treatments. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Effect of electrochemical treatment on pull‐out properties of 73/27 HBA/HNA copolyester fibers in thermosetting matrix

The surface of high‐strength 73/27 HBA/HNA fibers was electrochemically modified in aqueous sodium hydroxide solution (10 wt %) with the change of the applied current to improve the interfacial shear strength (IFSS) of the fiber in the thermosetting matrix. A sodium component was identified on the surface of the treated fibers by scanning electron microscopy (SEM). The detection of sodium is best interpreted as the existence of a negative charged functional group ( COO⁻Na⁺) on the surface of the treated fibers. A pull‐out test was used to measure the IFSS of the HBA/HNA fibers and epoxy/amine cure system. According to the calculating value from Greszczuk's geometrical model, the IFSS increased with the applied current in the range of 50–450 mA. However, no further increase in the IFSS was observed for strong treatment (1500 mA). There was an optimum by applied current to obtain maximum IFSS. The electrochemical treatment of the 73/27 HBA/HNA copolyester fibers was effective in altering their surface chemistry and improving the interfacial adhesion at the moderate treating conditions. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 15–21, 1999     

Modulus-Graded Interphase Modifiers in E-Glass Fiber/Thermoplastic Composites

A process for coating E-glass fibers with polystyrene–polyethyleneimine (PEi) core–shell particles was developed, and uniform monolayers of particles of 143 and 327 nm diameter were covalently bonded to the glass surface. The effect of the particle coatings on the mechanical properties of fiber-reinforced composites of poly(vinyl butyral) (PVB) was investigated. The interfacial shear strength (IFSS) was measured for specimens containing one to 20 fibers each using the tensile fiber fragmentation test, and significant enhancements were found, in particular for samples containing larger numbers of fibers. The smaller-particle (143 nm) coatings in the 20-fiber specimens produced approximately a 100% enhancement in IFSS over equivalent specimens with bare or aminosilane-treated fibers, while the 327 nm particle coatings produced only approximately a 25% enhancement. The greater effectiveness of the smaller particles was attributed, at least in part, to the larger effective interfacial area they provide and their relatively greater shell-to-core ratio, providing greater interphase stiffness. The greater enhancements achieved for the multi-fiber vs single-fiber specimens suggest that the coatings produce a more uniform fiber–fiber spacing and, therefore, a more thorough wetting of the fibers by the resin in the multi-fiber samples. Composites formed using fiber tows of 3200 fibers each showed more than a 100% increase in composite toughness and 35% increase in ultimate tensile strength as compared to samples with bare fibers due to the presence of the 143 nm particle coatings, and somewhat more modest increases for the 327 nm particle coatings.     

Impact behavior of carbon fiber/ultra-high modulus polyethylene fiber hybrid composites

Carbon fiber (CF)/ultra-high modulus polyethylene (UHMPE) fiber hybrid composites were fabricated using vinyl ester resin as a matrix. Interfacial adhesion of carbon fiber/vinyl ester composites and UHMPE fiber/vinyl ester composites as model composites was optimized using low temperature plasma treatment. Interlaminar shear strengths of carbon fiber/vinyl ester and UHMPE fiber/vinyl ester homocomposite were greatly increased by plasma and silane coupling agent treatment. From the result of the impact test, total absorbed energy of carbon fiber/UHMPE fiber hybrid composites was correlated with laminating sequences at optimized interfacial adhesion between the reinforcing fiber and matrix resin. UHMPE fiber layers of hybrid composites played an important role in absorbing energy. Elastic and plastic deformation of UHMPE fiber layers also played a key role in improving the impact properties of carbon fiber/UHMPE fiber hybrid composites.     

Influence of temperature and relative humidity on aging of atmospheric plasma jet treatment effect on ultrahigh-modulus polyethylene fibers

The aging effects of atmospheric pressure plasma treated fiber surfaces are important for storage and processing of the fibers. One of the high-performance fibers, ultrahigh modulus polyethylene (UHMPE) fiber, was chosen as a model system to investigate the aging process of atmospheric pressure plasma jet (APPJ) treated fibers surfaces 0, 7, 15 and 30 days after initial plasma treatment. The fiber was first plasma-treated and then stored at temperatures varying from ?80 to 80°C on the same relative humidity (RH, 0%) and on RH of 0%, 65% and 100% at the same temperature of 20°C. Immediately after the plasma treatment, scanning electron microscope (SEM) showed the roughened fiber surface. X-ray photoelectron spectroscopy analysis showed changed surface chemical compositions. Contact-angle measurement showed increased surface wettability and microbond test showed an increase in IFSS. With increasing relative humidity or decreasing temperature, the IFSS value decreased and the contact angle increased more slowly. However, after 30 days, the IFSS values and contact angles reached a similar level for all groups. Moisture showed no effect on the single fiber tensile strengths during aging. The reasons for the observed aging behavior could be that decreasing temperature or increasing relative humidity hindered the surface rearrangement of polymer chains after plasma treatment.     

Modification of pitch-based carbon fibers using a nickel-catalyzed oxidation treatment: Effect of treatment on fiber-matrix interfacial shear strength

Pitch-based carbon fiber surfaces were modified using a nickel-catalyzed, dry oxygen etch (NCDO) and a dry oxygen etch (DO), both of which were applied using a continuous process. The treatments differed in that DCDO produced preferential pitting on fiber surfaces at nickel particle sites. Tensile strengths of all etched fibers were slightly lower than values for untreated, or “as received” (AR), fibers. Reductions in tensile strength were independent of the type of treatment administered, indicating that most degradation resulted from damage produced by the mechanics of the continuous process, not the etching treatments. Fiber-matrix interfacial shear strengths (IFSS) of AR and etched fibers were evaluated using the Microbond test. At first glance, Microbond test data indicated that NCDO produced large increases in IFSS. However, inconsistencies in the data raised questions regarding the validity and usefulness of the Microbond test when applied to this system. Only after detailed analysis of the data for this series of fibers was it concluded that NCDO did indeed produce an increase in IFSS over AR fibers and DO fibers.     

Analysis of micro-failure behaviors in hybrid fiber model composites

Micro-failure modes and statistical fragment lengths in the hybrid fiber and non-hybrid reference composites in the uniaxial tension were investigated. Similar to the reference experiments, fibers in hybrid strong interface/medium interface fiber composites display a decrease in aspect ratio and an increase in interfacial shear stress (IFSS) with the increase of inter-fiber spacing. While for the fibers with weak interfaces in the hybrid strong interface/weak interface fiber composites, the aspect ratio increases and IFSS decreases with enlargement of inter-fiber spacing, which is contrary to other systems. Finite element numerical analysis was used to interpret the special phenomena.     

淀粉纳米晶对玻璃纤维的表面改性研究

采用自制的淀粉纳米晶(SNC)对玻璃纤维进行表面处理,增加其与环氧树脂基体的界面剪切强度(IFSS)。研究了处理方式、处理时间、SNC乙醇分散液浓度、热处理温度等工艺参数对SNC在玻璃纤维表面沉积情况的影响,以及对改性玻璃纤维与环氧树脂的界面性能的影响规律。采用扫描电子显微镜、单纤维强力仪对处理前后玻璃纤维进行表征,并采用微脱粘法测试玻璃纤维与环氧树脂的界面粘结情况。结果表明,当重力静置处理时间24 h,SNC乙醇分散液浓度为1 g/100 m L时,SNC在玻璃纤维表面均匀沉积,且能显著提高玻璃纤维与环氧树脂的IFSS,为27.29 MPa,较未处理的纤维增加29.3%。150℃热处理4 h后,X射线光电子能谱结果显示SNC与玻璃纤维形成化学键合,进一步增加纤维与环氧树脂的界面粘结,IFSS值达到32.30 MPa,较未处理的纤维增加53%,且纤维的拉伸强度得到较好的维持。     

Catalytic grafting: A new technique for polymer/fiber composites. II. Plasma treated UHMPE fibers/polyethylene composites

In this paper, the catalytic grafting technique for preparation of polymer/fiber composites is extended to plasma treated ultra-high modulus polyethylene (UHMPE) fiber/high density polyethylene (HDPE) system. The OH groups introduced on the UHMPE fiber surface by oxygen plasma treatment were used to chemically anchor Ziegler-Natta catalyst which then was followed by ethylene polymerization on the fiber surface. The morphology and interfacial behavior, as well as the mechanical properties, of the HDPE composites reinforced by catalytic grafted or ungrafted UHMPE fibers were investigated by SEM, DSC, polarized light optical microscopy, and tensile testing. The experimental results show that the polyethylene grafted on the fibers acted as a transition layer between the reinforcing UHMPE fibers and a commercial HDPE matrix. The interfacial adhesion was also significantly improved. Compared with the composite reinforced by ungrafted UHMPE fibers, the composite reinforced by catalytic grafted UHMPE fibers exhibits much better mechanical properties.     

Injection molded noil hemp fiber composites: Interfacial shear strength,fiber strength,and aspect ratio

Noil hemp fiber‐reinforced polypropylene composites were fabricated using intermixer and injection molding machines. X‐ray microtomography and Weibull statistical methods were employed to characterize the aspect ratio distributions of noil hemp fibers in the polypropylene matrices. The influence of fiber content (0–40 wt%) and compatibilizer addition (5 wt%) on IFSS (interfacial shear strengths) was evaluated by means of the modified Bowyer and Bader model. The evaluated IFSSs decreased from 9.7 to 7.2 MPa as the fiber content increased from 10 to 40 wt%. Also, the outcomes indicated increases to IFSSs for the maleic anhydride grafted polypropylene (MAPP)‐coupled composites than uncoupled ones. They were used to predict theoretical tensile strength of the composites. A good agreement has been found between the theoretical and the experimental tensile strengths of composites indicating that the developed model has excellent capability to predict the tensile strength of noil hemp fiber reinforced polypropylene composites. Ultimately, the influences of interfacial shear strength; fiber strength and fiber aspect ratio were investigated using the developed model to predict composite tensile strengths. POLYM. COMPOS., 213–220, 2016. © 2014 Society of Plastics Engineers     

Analysis of a pull-out test with real specimen geometry. Part I: matrix droplet in the shape of a spherical segment

We compared two models of the pull-out specimen – the ‘equivalent cylinder’ and the platelet models in which the matrix droplet is represented as a set of thin parallel disks with the diameters varying along the embedded fiber to approximate the real droplet shape. Analytical expressions for the profiles of the fiber tensile stress and the interfacial shear stress have been derived for the matrix droplet in the shape of a spherical segment, including the effects of residual thermal stresses and interfacial friction. Using these expressions, we analyzed the process of crack initiation and propagation in the platelet model and investigated the effect of the specimen shape on the force–displacement curves. The interfacial stress near the loaded fiber end in the platelet model is higher than in the equivalent cylinder model, which gives rise to earlier crack initiation and smoother shape of the force–displacement curve. As a result, the calculated interfacial shear strength values may be underestimated by 10–20%, if the equivalent cylinder is used instead of the real droplet shape. A method of correction to the equivalent cylinder model in order to avoid this underestimation is proposed.     

A note on polymeric inner-layered single fiber interfacial strength determination

The role of inner-layer (interphase) in fiber-reinforced composites is often assessed by measuring the interfacial shear strength of single fibers embedded in matrix blocks. Interfacial shear strength is calculated using simple shear lag analyses (Kelly and Tyson model). This paper shows that application of Kelly and Tyson's equation may result in gross overprediction of interfacial shear strengths if the thickness of the inner-layer is not negligible in comparison with the fiber diameter.     

Measuring effective interfacial shear strength in carbon fiber bundle polymeric composites

Adhesion in composite materials is often quantified using the single fiber fragmentation (SFF) test. While this method is believed to provide accurate values for the fiber–matrix interfacial shear strength (IFSS), these may not accurately reflect the macroscopic mechanical properties of specimens consisting of tows of thousands of tightly spaced fibers embedded in a resin matrix. In these types of specimens, adhesion may be mitigated by fiber twisting and misalignment, differences in the resin structure in the confined spaces between the fibers and, most importantly, by any incompleteness of the fiber wetting by the resin. The present work implements fiber band fragmentation (FBF) testing to obtain effective interfacial shear strengths, whose values reflect the importance of these factors. The fiber fragmentation in these specimens is tracked through the counting and sorting of acoustic emission (AE) events occurring during the tensile testing of the specimen and yields the average critical fiber fragment length. AE results, in conjunction with stress-strain data, show that fiber breakage events occur at acoustic wavelet amplitudes substantially greater than those generated by fiber/matrix debonding. Kelly–Tyson analysis is applied, using the measured critical fiber fragment length together with known values for the fiber diameter and tensile strength to yield the effective IFSS. FBF tests are performed on carbon fiber/poly(vinyl butyral) (PVB) dog-bone fiber-bundle systems, and effective IFSS values substantially lower than those typically reported for the single fiber fragmentation testing of similar systems are obtained, suggesting the importance of multi-fiber effects and incomplete fiber wetting.     

Improvement of the interfacial shear strength of poly(p‐phenylene benzobisoxazole) fiber/epoxy resin composite via a novel surface coating agent

Poly(p‐phenylene benzobisoxazole) (PBO) fiber with a smooth surface exhibits limited interfacial interaction with resin matrix. One of the effective strategies to improve the adhesion between the fiber and resin matrix is through surface modification of the fiber. In this study, we have proposed a novel surface treatment agent based on phosphoester cross‐linked castor oil (PCCO) for effective surface treatment of PBO fibers. The surface treatment agent was prepared by a simple cross‐linking reaction between hydroxy phosphorylated castor oil (PCO) and epoxy resin, with alcohol as the solvent at 65°C. Once the PBO fiber was treated with this agent, the interfacial adhesion between the PBO fiber and the epoxy resin could then be improved. Systematic analyses suggest that the surface treatment with (PCO + epoxy)/alcohol solution improves the interaction of the PBO fiber with the epoxy resin matrix. The PCCO coated onto the surface of PBO fiber acts as a coupling agent, improving the interfacial shear strength (IFSS) of the PBO fiber/epoxy resin composite. Results indicate a 156% increase in IFSS without compromising the mechanical properties of the fiber. POLYM. COMPOS., 37:1198–1205, 2016. © 2014 Society of Plastics Engineers     

Improvement of Interfacial Adhesion of Glass Fiber/Epoxy Composite by Using Plasma Polymerized Glass Fibers

This study intends to produce plasma polymer thin films of γ-glycidoxypropyltrimethoxysilane (γ-GPS) on glass fibers in order to improve interfacial adhesion of glass fiber-reinforced epoxy composites. A low frequency (LF) plasma generator was used for the plasma polymerization of γ-GPS on the surface of glass fibers at different plasma powers and exposure times. X-ray photoelectron spectroscopy (XPS) and SEM analyses of plasma polymerized glass fibers were conducted to obtain some information about surface properties of glass fibers. Interlaminar shear strength (ILSS) values and interfacial shear strength (IFSS) of composites reinforced with plasma polymerized glass fiber were evaluated. The ILSS and IFSS values of non-plasma polymerized glass fiber-reinforced epoxy composite were increased 110 and 53%, respectively, after plasma polymerization of γ-GPS at a plasma power of 60 W for 30 min. The improvement of interfacial adhesion was also confirmed by SEM observations of fractured surface of the composites.     

Effects of electrophoretically deposited carbon nanofibers on the interface of single carbon fibers embedded in epoxy matrix

Electrophoretic deposition (EPD) was used to deposit carboxylic acid-functionalized carbon nanofibers (O-CNFs) on the surface of single carbon fibers. Using the single fiber fragmentation technique and Weibull analysis, interfacial shear strength (IFSS) was estimated for different fiber surface treatments. Samples for sized, unsized, O-CNF deposited sized, and O-CNF deposited unsized carbon fibers were tested. Additionally, the effects of EPD were investigated by testing sized and unsized carbon fiber samples exposed to an electric field in water. Removal of the fiber sizing decreased IFSS by approximately 27%, but addition of O-CNFs to the unsized fiber surface led to an increase of 15% compared to the sized base fiber. The O-CNF deposited sized fibers provided IFSS increases of 207.6% and 66.9% for 1 and 5 min deposition durations, respectively. The surface morphology of all samples was characterized, and those containing homogeneous deposition of closely bound O-CNFs provided the highest IFSS values. Exposing sized fibers to the electric field for 1 min led to an IFSS increase of 79%, while unsized fibers undergoing the same treatment provided increases of 7.7% and 46% compared to the base sized fiber and unsized fiber samples, respectively.     

Influence of environmental moisture on atmospheric pressure plasma jet treatment of ultrahigh-modulus polyethylene fibers

One of the main differences between low-pressure and atmospheric-pressure plasma treatments is that there is little moisture involved in the low-pressure plasma treatment, although moisture could exist at the wall of the vacuum chamber or react with the substrate after plasma treatment, while in the atmospheric-pressure plasma treatment moisture exists not only in the environment but also in any hygroscopic substrate. In order to investigate the influence of environmental moisture on the effect of atmospheric pressure plasma treatment, ultra-high-modulus polyethylene (UHMPE) fibers were treated using an atmospheric-pressure plasma jet (APPJ) with 10 l/min helium gas-flow rate, treatment nozzle temperature of 100°C and 5 W output power. The plasma treatments were carried out at three different relative humidity levels, namely 5, 59 and 100%. After the plasma treatments, the surface roughness increased while the water-contact angle decreased with increasing relative humidity. The number of oxygen containing groups increased as the environmental moisture content increased. The interfacial shear strength of the UHMPE fiber/epoxy system was significantly increased after the plasma treatments, but the moisture level in the APPJ environment did not have a significant influence on the adhesion properties. In addition, no significant difference in single fiber tensile strength was observed after the plasma treatments at all moisture levels. Therefore, it was concluded that the environmental moisture did not significantly influence the effect of atmospheric-pressure plasma treatment in improving interfacial bonding between the fiber and epoxy. The improvement of the interfacial shear strength for the plasma-treated samples at all moisture levels was mainly due to the increased surface roughness and increased surface oxygen and nitrogen contents due to the plasma etching and surface modification effect.