Effects of Microwave Processing on Fiber-matrix Adhesion in Composites

Experiments have been done using a single mode (TE₁₁₁, 2.45 GHz) cylindrial microwave cavity with single fiber composite specimens. After obtaining a cure cycle with microwaves to match that achieved with a conventional thermal cure cycle as measured by tensile tests, dynamic mechanical analysis and differential scanning calorimetry, quantitative measurements of interfacial shear strength and physical properties have been carried out and compared with the results from conventional thermally-cured systems. Under the conditions studied for single fiber specimens, the fiber-matrix interfacial shear strength decreases slightly in both glass-epoxy and aramid-epoxy cases as comapared with thermally-cured specimens. Graphite fiber-epoxy adhesion, on the other hand, increases significantly in these single fiber studies in microwave processed specimens as indicated by an increase in the interfacial shear strength. The failure mode changes from interfacial (thermal curing) to matrix failure.     

Application of the microbond technique: Characterization of carbon fiber-epoxy interfaces

Microbonding has been applied to measure the interfacial shear strength, τ, between single carbon fiber and microdroplets of epoxy resins. The effect of thermoset cure and resin modification on this initial parameter for composite performance have been studied. The interfacial shear strength for the host fiber/epoxy system (T-300/Epos 828) increased 3 fold from a B-stage to a fully cured material. The addition of a toughening agent called “Fortifier P” to the host resin system increased T by 40%. Residual thermal stresses were calculated and their contribution to mechanical adhesion were related to friction components.     

Avoidance of fabricational thermal residual stresses in co-cure bonded metal-composite hybrid structures

In this work, a smart cure cycle with cooling, polymerization and reheating was devised to nearly completely eliminate thermal residual stresses in the bonding layer of the co-cure bonded hybrid structure. In situ dielectrometry cure monitoring, DSC experiments and rheometric measurements were performed to investigate the physical state and the cure kinetics of the neat epoxy resin in the carbon fiber/epoxy composite materials. From the experimental results, an optimal cooling point in the cure cycle was obtained. Also, process parameters such as cooling rate, polymerization temperature and polymerization time in the curing process were investigated. Then, the thermal residual stresses were estimated by measuring the curvatures of co-cure bonded steel/composite strips and their effects on the static lap-shear strengths of co-cure bonded steel/composite lap joints were measured. Also, the effects of thermal residual stresses on the tensile strength, the interlaminar shear strength and the interlaminar fracture toughness of the composite material itself were measured using tensile, short beam shear and double cantilever beam tests. From these results, it was found that the smart cure cycle with cooling, polymerization and reheating eliminated the thermal residual stresses completely and improved the interfacial strength of the co-cure bonded hybrid structures, as well as the tensile strength of the composite structures.     

Interfacial adhesion and failure modes in single filament thermoplastic composites

The single filament composite (SFC) test has been uses to investigate the adhesion of carbon and glass fibers to thermoplastic matrices. A new modification of the test is proposed, consisting of SFC specimens stretched until the neck formation is complete. This makes the measuring of fiber fragment lengths much easier and allows a wider class of matrices to be investigated. The adhesion between fiber and thermoplastic matrix is strongly dependent on the contact forming conditions. With increased time of thermal treatment, interfacial bonding is improved and the failuere mode is changed. In the case of por fiber-mitrix adhesion, interfacial failure occurs. With an increase in interfacial shear strength, the martix cracks perpendicular to the fiber at the fiber breaks.     

Interfacial properties of kevlar-49 fiber-reinforced thermoplastics

A single-filament pull-out test was used to study adhesion of Kevlar-49 fibers to thermoplastic polymers. The test involved pulling a partially embedded fiber out of a polymer film. Kevlar-49 fibers with three different surface treatments were used with five thermoplastic materials. The test resulted in the measurement of two properties, an interfacial bond strength and a frictional shear strength. The interfacial bond strength is an essential factor in determining the critical aspect ratio of discontinuous fibers in a composite. The frictional shear strength was found to correlate with the tensile strength of discontinuous fiber composites which fail by fiber pull-out. Scanning electron microscopy was used to examine the fiber pull-out specimens after testing. Observations of the fiber showed that the failure mode at the fiber–matrix interface was complex. The predominant failure mode was fracture at the interface (or in some weak boundary layer). In some cases, cohesive failure of the fiber surface was observed, with the result that strips of material were torn from the fiber surface.     

Fiber/Matrix load transfer in cyanate resin carbon fiber systems

Comparative single fiber fragmentation test measurements are used to charcterize cynate and epoxy resin interface load transfer with high modulus (HMS4) and high strength (AS4) carbon fibers. The HMS4 fiber forms a weak interface with a fiber controlled failure mode, and the AS4 fiber forms a strong interface with the resin properties, apparently determining the level of load transfer. The resin properties examined are critical surface energy for wetting, cure shrinkage, thermal shrinkage, and mechanical modulus and strength. The cynate and epoxy resins display no significant difference in critical surface energy. Cure shrinkage has a negligible effect on load transfer. The compressive force from thermal shrinkage is significant, but the larger Tg to testing temperature range of the cyanate resin is offset by the larger thermal expansion coefficient of the epoxy, resulting in a near equal compressive force for the two resins. There is little difference in modulus between the two resins but a significant difference in shear strength. This difference is reflected in the larger load transfer measurement for the cyanate resin. A comparison of simple one-dimensional elastic and plastic models for the fiber fragmentation experiment resulted in better conformity with the plastic model. This would indicate that interfacial failure occurs by plastic deformation of the resin for the systems of this study.     

Influence of processing rate and formulation on the interface strength of vinyl ester/E‐glass composites

The single fiber fragmentation test was used to investigate the effect of gelation time on interfacial shear properties of fast reacting resin systems. We developed a processing system capable of producing single fiber fragmentation samples with gelation times that ranged from 2 min to 45 min. The interfacial properties of E‐glass fibers in vinyl ester resin were measured with single fiber fragmentation tests using a manual and an automated testing machine. We found that vinyl ester resins catalyzed with methyl ethyl ketone peroxide and promoted with cobalt naphthenate and dimethylaniline gelled in less than two minutes and had an estimated interfacial shear strength of 105 MPa. Specimens cured without the promoter gelled in 45 min and had an interfacial shear strength of 72 MPa. Further curing of the unpromoted specimens resulted in an increase in shear strength to 96 MPa. We have demonstrated the ability to make and test rapidly cured specimens, thus expanding the range of materials that can be tested using the single fiber fragmentation testing technique.     

Interfacial behavior of high performance organic fibers

The surface and interfacial properties of different high performance fibers of current interest have been analyzed. The pyridobisimidazole fiber M5 shows a markedly higher polar contribution to its surface free energy than the rest of the organic fibers under study. Interfacial shear strength (IFSS) values measured by means of the microdroplet test indicate that M5 fiber has an IFSS that doubles that of the Kevlar fibers, in agreement with the observed results from surface free energy tests. Armos fiber, a para-aramid material that incorporates imidazole functional groups, shows an average IFSS 30-35% higher than the Kevlar fibers. SEM micrographs of failed microdroplet specimens show different failure mechanisms for the Kevlar KM2, Armos and M5 fibers. The KM2 specimens fail due to complete detachment of surface fibrils from the bulk of the fiber, while Armos specimens fail by the combined effect of microfibrillation on the fiber surface coupled with adhesive failure. In contrast, M5 microdroplet specimens exhibit failure surfaces consisting of partial matrix yielding during droplet debonding, indicative of the high level of interfacial bonding to the surface and higher levels of hydrogen bonding within the fiber that suppress microfibrillation. The higher polar character of the M5 surface can lead to the presence of an interphase region with different mechanical properties from the bulk matrix.     

Interfacial shear strength studies using the single-filament-composite test. I: Experiments on graphite fibers in epoxy

The failure of the interface in a carbon fiber-epoxy system was studied for six different epoxy blends using the single-filament-composite technique. The blends were formulated to yield a wide range of stiffnesses, and their effect on interfacial failure was examined. Specimens were made from Hercules IM6-G carbon fiber and the different blends of epoxy, and then strained to obtain a distribution of fiber fragment lengths. Birefringence patterns near the fiber breaks were observed and recorded. Some of the specimens were strained until they failed and the resulting fracture surfaces were observed under a scanning electron microscope to determine fracture patterns and the existence of debonding. The fragment length distributions were interpreted using a Monte-Carlo simulation of a Poisson/Weibull model for fiber strength and flaw occurrence. The results were used to calculate an effective interfacial shear strength. From this analysis we conclude that one cannot accurately predict the interfacial properties of a composite based solely upon conventional single fiber and bulk matrix properties. Local matrix properties and fiber/matrix interactions, on a microscale, play a key role in composite strength.     

The effects of surface treatments of fibers on the interfacial properties in single-fiber composites

The interfacial properties between fibers and the matrix contribute to the overall properties in high performance composites. Plasma treatments (Ar, O₂, CF₄/O₂, N₂/H₂) have been performed on carbon fibers to improve the fiber-matrix interaction. The treatment efficiency was checked by the single-fiber technique, while the surface chemistry and morphology were characterized by X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectroscopy (SSIMS), and scanning electron microscopy (SEM). The O₂- and N₂/H₂-plasma treatments proved most effective both for introducing oxygen-containing functionalities at the fiber surface and for improving the interfacial shear strength of carbon fiber/epoxy composites. A relationship between the oxygen concentration at the fiber surface and the interfacial shear strength is demonstrated.     

微波固化成型三维中空复合材料结构的力学性能

为了改善三维中空复合材料结构微波固化成型的固化均匀性,提出了添加外部导热附加模具和内部微波吸收剂两种不同的方法。通过试验研究了附加模具的材料、厚度以及微波吸收剂种类和含量对三维中空复合材料结构力学性能的影响。结果表明,结构的平压失效模式包括芯柱失稳和压溃,剪切失效模式为芯材剪切失效和界面脱粘,短梁弯曲的失效模式为面板/芯材界面的脱粘后屈曲破坏。相比于未添加附加模具,AlN和Al_2O_3陶瓷均可以提高结构的力学性能,但AlN的增强效果更显著。AlN模具厚度的增加不利于结构的力学性能,模具厚度从0.5 mm增加到1.5 mm时,结构的平压、剪切和芯材剪切强度均随之降低。微波吸收剂的添加均可提高中空结构的力学性能,其中剪切和芯材剪切强度随着石墨含量的增加而增加,平压强度随着石墨含量的增加先增加后降低,而Fe_3O_4含量变化则对结构力学性能的影响不显著。     

Influence of Matrix Properties on Fragmentation Test

Physical aging was used to vary the mechanical properties of model single fiber composites without changing the chemistry at the interface in order to study how property changes affect the measurement of interfacial adhesion by the fragmentation test. The properties of epoxy matrix/AS4 single fiber composites driven to full cure (T_g = 166°C) are altered by annealing below T_g . Neat resin samples with identical thermal histories are tested. All aged panels show roughly the same embrittlement with aging characterized by an average 30% decrease in tensile failure strain and 7.3% increase in compressive yield relative to quenched samples. Fragmentation results indicated no change between aged and quenched samples. Results are discussed in terms of micromechanics models for the fragmentation test. Strain at fragmentation increased with aging. This was related to the residual stress state in the model composite and the possibility of the zero stress state of the single fiber composites increasing with thermal annealing.     

Interfacial Characteristics of Sisal Fiber and Polymeric Matrices

Sisal-fiber-reinforced composites, as a class of eco-composites, have attracted much attention from materials scientists and engineers in recent years. In this article, the effects of fiber surface treatment on fiber tensile strength and fiber-matrix interface characteristics were determined by using tensile and single fiber pullout tests, respectively. The short beam shear test was also employed to evaluate the interlaminar shear strength of the composite laminates. Vinyl ester, epoxy, and high-density polyethylene (HDPE) were chosen as matrix materials. To enhance the interfacial strength, two kinds of fiber surface-treatment methods, namely, chemical bonding and oxidisation, were used. The results obtained showed that different fiber surface-treatment methods produced different effects on the tensile strength of the sisal fiber and fiber-matrix interfacial bonding characteristics. Hence, valuable information on the interface design of sisal fiber–polymer matrix composites can be obtained from this study.     

A new method for study of the fiber-matrix interface in composites: single fiber pull-out from a microcomposite

—A new method, single fiber pull-out from a microcomposite (SFPOM), was developed to study the fiber/matrix interface in composites. By pulling a fiber out of a seven-fiber microcomposite, the SFPOM test provides the real feeling of a fiber pulled out of an environment similar to that in a real composite. Interfacial shear strength decreased as the fiber volume fraction increased in the fiber-matrix system tested in the experiment. Three factors were suggested to be responsible for the phenomenon: (1) poor bonding between fibers when close to each other; (2) shear stress concentration in the matrix between neighboring fibers; and (3) possible change in matrix properties, thus altering the failure mechanism from interfacial debonding to a mixture of interfacial debonding and matrix fracture.     

Interfacial Characteristics of Sisal Fiber and Polymeric Matrices

Sisal-fiber-reinforced composites, as a class of eco-composites, have attracted much attention from materials scientists and engineers in recent years. In this article, the effects of fiber surface treatment on fiber tensile strength and fiber-matrix interface characteristics were determined by using tensile and single fiber pullout tests, respectively. The short beam shear test was also employed to evaluate the interlaminar shear strength of the composite laminates. Vinyl ester, epoxy, and high-density polyethylene (HDPE) were chosen as matrix materials. To enhance the interfacial strength, two kinds of fiber surface-treatment methods, namely, chemical bonding and oxidisation, were used. The results obtained showed that different fiber surface-treatment methods produced different effects on the tensile strength of the sisal fiber and fiber-matrix interfacial bonding characteristics. Hence, valuable information on the interface design of sisal fiber-polymer matrix composites can be obtained from this study.     

环境因素对玻璃纤维增强聚丙烯界面的损伤

采用单丝临界长度法测定玻璃纤维增强聚丙昨合体系的界面剪切强度，研究了湿热及交替变化的温度等环境因素对复合体系界面的损伤。结果表明，湿热及高、低温的冷热循环均能引起玻璃纤维与聚丙烯复合体系的界面脱粘；采用偶联剂处理玻璃纤维、在体系中形成较强的界面粘结（如化学键结合），可提高复合体系界面抗湿热损伤的能力，在复合体系中引入容易变形的界面以生层可提高体系界面的冷热循环疲劳性能，界面结合较强的复合体系经冷热     

SiC fiber reinforced geopolymer composites,part 2: Continuous SiC fiber

In this paper, unidirectional SiC fiber (SiC_f) reinforced geopolymer composites (SiC_f/geopolymer) were prepared and effects of fiber contents on the microstructure and mechanical properties of the composites in different directions were investigated. The XRD results showed that addition of SiC_f retarded geopolymerization process of geopolymer matrix by weakening the typical amorphous hump. SiC_f in all the composites were well infiltrated by geopolymer matrix, but microcracks which were perpendicular to the fiber axial direction were noted in the interface area due to the thermal shrinkage of matrix during the curing process. With the increases in fiber contents, although Young's modulus of the composites increased continuously, flexural strength, fracture toughness and work of fracture increased at first, reached their peak values and then decreased. And when fiber content was 20 vol%, the composites showed the highest flexural strength, fracture toughness and work of fracture, which were 14.2, 15.2 and 81.6 times as high as those of pristine geopolymer, respectively, indicating significant strengthening and toughening effects from SiC_f. Meanwhile, SiC_f/geopolymer composites failed in different failure modes in the different directions, i.e., tensile failure mode in the x direction (in-plane and perpendicular to the fiber axial direction) and shear failure mode in the z direction (laminate lay-up direction).     

环氧基体与竹节状有机纤维之间的界面性能研究

本文采用单丝拔出试验和动态力学分析研究了环氧树脂基复合材料中基体与竹节状有机短纤维之间的界面特性.有关的试验结果表明:在弱界面结合的条件下,由于在竹节状有机短纤维中凸节的存在,可以提高纤维与基体之间的界面结合强度,也有利于纤维末端界面剪切应力的传递.     

Influence of Interface Characteristics on the Mechanical Properties of Hi-Nicalon type-S or Tyranno-SA3 Fiber-Reinforced SiC/SiC Minicomposites

The tensile behavior of CVI SiC/SiC composites with Hi-Nicalon type-S (Hi-NicalonS) or Tyranno-SA3 (SA3) fibers was investigated using minicomposite test specimens. Minicomposites contain a single tow. The mechanical behavior was correlated with microstructural features including tow failure strength and interface characteristics. The Hi-NicalonS fiber-reinforced minicomposites exhibited a conventional damage-tolerant response, comparable to that observed on composites reinforced by untreated Nicalon or Hi-Nicalon fibers and possessing weak fiber/matrix interfaces. The SA3 fiber-reinforced minicomposites exhibited larger interfacial shear stresses and erratic behavior depending on the fiber PyC coating thickness. Differences in the mechanical behavior were related to differences in the fiber surface roughness.     

Adhesion characteristics of oxygen plasma-treated rayon fibers

The effect of oxygen plasma treatment of fiber on the adhesion between regenerated cellulose fiber and polyethylene (PE) was investigated using the single-fiber fragmentation test. In addition to allowing the determination of the interfacial shear strength, the fragmentation test provided a great deal of useful information on shear stress transfer and failure mechanisms in the systems. It was found that oxygen plasma treatment considerably enhanced the interfacial adhesion, as established by both the shear strength values that were measured and the birefringence patterns observed. The influence of the duration of treatment on adhesion was studied and found to be a very important parameter. The roles of surface chemistry, surface energetics, and surface topography of fiber in the interaction balance were investigated using electron spectroscopy for chemical analysis (ESCA), contact angle measurements, and scanning electron microscopy (SEM). It was seen that neither the plasma-induced changes in the surface energetics nor those in the surface topography could have exerted a positive effect on adhesion. Instead, the improved adhesion was ascribed to covalent bonds formed between the fiber and the matrix, as hydroperoxides, which were created on the fiber surface by the plasma treatment, decomposed during the fabrication of single-fiber specimens.