Effect of 60Co gamma radiation on the mechanical properties of epoxy blends and epoxy-graphite fiber interface

The effect of ⁶⁰Co gamma radiation of up to 100 Mrads on an IM6-G graphite fiber-epoxy interface was studied using the single-fiber-composite (SFC) technique. Flexible epoxy blends were formulated using diglycidylether of bisphenol-A (DGEBA) based and polyglycol diepoxide epoxies which were cured with aliphatic and aromatic curing agents. Bulk epoxy specimens and graphite fibers were tension tested to obtain their tensile properties. The fragment length distribution from SFC tests, single fiber strength data, and a Monte Carlo simulation of Poisson/Weibull model for fiber strength and flaws were used to obtain the effective interfacial shear strength values. The results indicate that while graphite fiber strength is not affected by radiation, the tensile properties of the epoxies used are adversely affected by the radiation. The interfacial shear strength, however, increases significantly with the radiation dose. The study also supports the earlier results of many workers that the interfacial shear strength for flexible epoxies is much higher than the shear yield strength of the epoxies.     

Study on the interfacial properties of two‐dimensionally arranged glass fiber/epoxy resin model composites

The effect of interfiber distance on the interfacial properties in two dimensional multi‐E‐glass fiber/epoxy resin composites has been investigated using fragmentation test. In addition, the effect of the fiber surface treatment on the interfacial properties has been studied. We found that the interfacial shear strength decreased with the decreasing interfiber distance at the range of <50 μm and the extent of the decreasing was more serious as the increasing of the number of adjacent fiber. This is probably that the interface between the fiber and the resin was damaged by the breaking of adjacent fibers and the damage increased with minimizing the interfiber spacing and the number of adjacent fibers. We can guess that interfacial shear strength in real composites is much smaller than that of multifiber fragmentation sample with touched fiber. When the interfiber distance was >50 μm, the interfacial shear strengths were saturated regardless of fiber surface treatment and were in close agreement with those of the single fiber fragmentation test. Finally, the interfacial shear strength evaluated using two dimensional fragmentation tests are shown as real values in‐site regardless of fiber surface treatment, interfiber distance, and existing matrix cracks. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1541–1551, 2006     

Study of interfaces of high-performance glass fibers and DGEBA-based epoxy resins using single-fiber-composite test

The single-fiber-composite (SFC) technique was used to study the interfacial behavior between two flexible blends of diglycidylether of bisphenol A (DGEBA)-based epoxy and polyglycol epoxide and three glass fibers. Dog-bone-shaped SFC specimens were made and strained to obtain a distribution of fragment lengths. The fibers were tension-tested at two different gauge lengths. The fragment length distributions, the fiber strength data, and a Monte Carlo simulation of a Poisson/Weibull model for fiber strength and flaws were used to obtain the effective interfacial shear strength values. The results show that the interface does not fail. Instead, penny-shaped transverse cracks appear at every fiber break and grow as the specimen is strained. The interfacial shear strength values are many times higher than the yield shear strength values of bulk epoxy obtained from the tension test.     

Multiple fiber technique for the single fiber fragmentation test

The single fiber fragmentation test has been modified by embedding multiple fibers into matrix resin. During testing, we examined the interfacial shear strengths between the fibers and the matrix. In addition, the time-dependent nature of the fragmentation process was considered. In the fragmentation test, we examined the failure process of two fibers placed far from each other, and we found that the failure profile of the two fibers were similar to the failure profiles from tests done on single fibers. When we examined three fibers, we found that the measured interfacial shear strength values were much greater than the shear strength values from either the single or two fiber tests. However, when we used three fibers, we found it difficult to control the interfiber spacing. Consequently, whenever the interfiber spacing was too small, breaks in one fiber caused breaks in the adjacent fiber. In conclusion, using multiple fibers in a fragmentation test has many merits, such as saving time in testing, ease of comparing the effects of fiber surface treatment, and testing different fibers in the same matrix exposed to the same processing conditions. © 1998 John Wiley & Sons, Inc. J Appl Polm Sci 67:1701–1709, 1998     

Interfacial improvement of carbon fiber/epoxy composites using a simple process for depositing commercially functionalized carbon nanotubes on the fibers

An aqueous suspension deposition method was used to coat the sized carbon fibers T700SC and T300B with commercially carboxylic acid-functionalized and hydroxyl-functionalized carbon nanotubes (CNTs). The CNTs on the fiber surfaces were expected to improve the interfacial strength between the fibers and the epoxy. The factors affecting the deposition, especially the fiber sizing, were studied. According to single fiber-composite fragmentation tests, the deposition process results in improved fiber/matrix interfacial adhesion. Using carboxylic acid-functionalized CNTs, the interfacial shear strength was increased 43% for the T700SC composite and 12% for the T300B composite. The relationship between surface functional groups of the CNTs and the interfacial improvement was discussed. The interfacial reinforcing mechanism was explored by analyzing the surface morphology of the carbon fibers, the wettability between the carbon fibers and the epoxy resin, the chemical bonding between the fiber sizing and the CNTs, and fractographic observation of cross-sections of the composites. Results indicate that interfacial friction, chemical bonding and resin toughening are responsible for the interfacial improvement of nanostructured carbon fiber/epoxy composites. The mechanical properties of the CNT-deposited composite laminate were further measured to confirm the effectiveness of this strategy.     

Relation of interfacial adhesion in Kevlar®/epoxy systems to surface characterization and composite performance

A central problem in composite materials is the poorly understood relation between the nature of the surfaces at the fiber/matrix interface, the actual interfacial bond strength, and interface-sensitive composite properties, in this study on the Kevlar®/epoxy composite system, the interface was varied chemically by fiber sizings. The sized and unsized fiber surfaces and the cured matrix surface were characterized by contact angle measurements. The interfacial shear strength was directly measured by single-filament pull-out tests of sized and unsized fibers in epoxy matrix. The shear strengths of the composites made with sized and unsized fibers were measured. The results from surface analysis, interfacial shear tests, and composite shear tests were consitent. This suggests that surface-contact-angle analysis and single-filament pull-out tests may be helpful in screening strength of the composite.     

Study on the interfacial properties of three‐dimensionally arranged glass fiber/epoxy resin model composites

The effect of interfiber distance on the interfacial properties in three‐dimensional multi‐E‐glass fiber/epoxy resin composites has been investigated using fragmentation test. In additions, the effect of the fiber surface treatment on the interfacial properties has been studied. The interfacial shear strength decreased with the decreasing the interfiber distance at the range of under 50 μm and the extent of the decreasing was more serious as the increasing of the number of adjacent fiber. This is probably due to the fact that the interface between the fiber and the resin was damaged by the adjacent fiber breaks and the damage increased with closing the interfiber spacing and the number of adjacent fiber. It was found that the interfacial shear strengths saturated when the interfiber distance was over 50 μm, the ones were saturated regardless of fiber surface treatment and the ones were in close agreement with those of the single fiber fragmentation test. Finally, the interfacial shear strength evaluated using three‐dimensional fragmentation tests are shown as real values in‐site regardless of fiber surface treatment, interfiber distance and existing of matrix cracks. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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.     

Interfacial studies on the surface modified aramid fiber reinforced epoxy composites

In this work, solutions of rare earth modifier (RES) and epoxy chloropropane (ECP) grafting modification method were used for the surface treatment of aramid fiber. The effect of chemical treatment on aramid fiber has been studied in a composite system. The surface characteristics of aramid fibers were characterized by Fourier transform infrared spectroscopy (FTIR). The interfacial properties of aramid/epoxy composites were investigated by means of the single fiber pull‐out tests. The mechanical properties of the aramid/epoxy composites were studied by interlaminar shear strength (ILSS). As a result, it was found that RES surface treatment is superior to ECP grafting treatment in promoting the interfacial adhesion between aramid fiber and epoxy matrix, resulting in the improved mechanical properties of the composites. Meanwhile, the tensile strengths of single fibers were almost not affected by RES treatment. This was probably due to the presence of reactive functional groups on the aramid fiber surface, leading to an increment of interfacial binding force between fibers and matrix in a composite system. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:4165–4170, 2006     

Mechanical Behavior at 20° and 1200°C of Nicalon-Silicon-Carbide-Fiber-Reinforced Alumina-Matrix Composites

Tensile and fracture tests were conducted at 20° and 1200°C on a ceramic-matrix composite that was composed of an alumina (Al₂O₃) matrix that was bidirectionally reinforced with 37 vol% silicon carbide (SiC) Nicalon fibers. The composite presented nonlinear behavior at both temperatures; however, the strength and toughness were significantly reduced at 1200°C. In accordance with this behavior, matrix cracks were usually stopped or deflected at the fiber/matrix interface, and fiber pullout was observed on the fracture surfaces at 20° and 1200°C. The interfacial sliding resistance at ambient and elevated temperatures was estimated from quantitative microscopy analyses of the saturation crack spacing in the matrix. The in situ fiber strength was determined both from the defect morphology on the fibers and from the size of the mirror region on the fiber fracture surfaces. It was shown that composite degradation at elevated temperature was due to the growth of defects on the fiber surface during high-temperature exposure.     

Foreword

The effect of surface treatments and fiber sizings on the stress transfer characteristics and composite properties of AS-4 carbon/epoxy materials has been determined. Fiber surface chemistry was systematically varied from acidic to basic with RF glow discharge plasmas of CO₂ and NH₃ and characterized with ESCA techniques. Sizings applied to some of the treated fibers consisted of diglycidyl ether of bisphenol-A(DGEBA). Single fiber tension tests were used to measure the interfacial shear strength of samples made with DGEBA/metaphenylene diamine resin. Short beam shear and transverse flexure tests were used to examine the composite properties of modified materials.

Results showed that the plasma treatments were effective in altering the surface chemistry of the fiber but that changes in surface chemistry had surprisingly little effect on the critical stress transfer length. Sizing had a more significant effect on the transfer length. The interlaminar shear strength of the composites were unaffected by the treatments. Transverse flexure tests were more sensitive to the changes in surface characteristics. The work indicates that the interface properties of AS-4 fibers are close to optimal but that improvements in composite performance are possible through interphase formation.     

Predicted Effects of Interfacial Roughness on the Behavior of Selected Ceramic Composites

Potential effects of interfacial roughness in ceramic composites were studied using a model that included the progressively increasing contribution of roughness with relative fiber/matrix displacement during debonding of the fiber/matrix interface. A parametric approach was used to study interfacial roughness in conjunction with other parameters such as the strength, radius, and volume fraction of the fiber. The progressive roughness contribution during initial fiber/matrix sliding caused a high effective coefficient of friction, as well as an increased clamping stress, which led to rapidly changing friction with increasing debond length. Calculated effects implied a potentially significant contribution to the behavior of real composite systems and the necessity for explicit consideration in the interpretation of experimental data to understand composite behavior correctly. In a tension test, the Poisson's contraction of the fiber may negate the effects of roughness, allowing an "effective constant shear stress" (tau) approximation. This was evaluated using a piecewise linear approximation to the progressive roughness model in an analysis of composite stress-strain behavior; for the Nicalon/SiC system, the effective tau value was lower than the values that would be obtained from fiber pushout tests and/or matrix crack spacings.     

Fiber Effects on Minicomposite Mechanical Properties for Several Silicon Carbide Fiber-Chemically Vapor-Infiltrated Silicon Carbide Matrix Systems

Several different types of SiC fiber tows were coated with BN and composited using chemically vapor-infiltrated SiC to form single-tow minicomposites. The types of SiC fiber included Nicalon^™, Hi-Nicalon^™, and the new Sylramic^™ polycrystalline SiC fiber. The interfacial shear stresses were determined from unload–reload tensile hysteresis-loop tests. The ultimate stress and strain properties also were determined for the minicomposites. The ultimate strengths of the newer Hi-Nicalon and Sylramic fibers were superior to that of Nicalon minicomposites with similar fiber volume fractions. The Sylramic minicomposites had the lowest strain to failure and highest interfacial shear strength, respectively, because of the high modulus of the fiber and the rough surface of this fiber type. The apparent interfacial shear strength increased as the stress increased for the Sylramic minicomposites, which also was attributed to the surface roughness of this fiber.     

Evaluation of interfacial properties and microfailure mechanisms in single fiber‐reinforced epoxy composites at low temperature

In this preliminary study, micromechanical techniques were used to compare the interfacial properties of both carbon and glass fiber composites with two structurally different epoxy matrices (YD‐114 and YDF‐175) at ambient and relatively low temperatures (25°C and −10°C). Tensile modulus of elasticity for both epoxies was higher at lower temperature. Although both fibers exhibited more bimodality at lower temperature than at ambient temperature, glass fiber composites exhibited a statistically greater improvement in tensile strength. This may be attributed to differences in inherent flaws and rigidity. A decrement in stress was observed for YDF‐175 epoxy composites under cyclic loadings at both temperatures, which was attributed to lower interfacial shear strength (IFSS). In contrast to the IFSS of conventional YD‐114 epoxy composites, the IFSS of both the carbon and glass fibers/YDF‐175 epoxy composites studied was higher at the lower temperature. The microfailure pattern observed in microdroplet pullout tests was consistent with the other IFSS results. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

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 stress transfer in nylon‐6/E‐Glass microcomposites: Effect of temperature and strain rate

In this study, the interfacial properties between E‐glass fibers with different commercial sizings have been investigated on model composites with a nylon‐6 matrix. In particular, the fiber critical length was measured by means of the single‐fiber fragmentation test over a wide range of temperatures (from 25 to 175°C) and strain rates (from 0.0008 to 4 min⁻¹). The general trend observed is that the fiber critical aspect ratio increases as the temperature increases and it decreases as strain rate is increased. The fiber critical aspect ratio for unsized fibers resulted to be reasonably well linearly related to the square root of the fiber to matrix modulus ratio. This results is in accordance with the Cox's shear‐lag theoretical model and the Termonia's numerical simulations. Sized fibers display an higher deviation from the theoretical prevision probably because of the presence of interphases whose properties are different from the bulk matrix. As a consequence, the interfacial shear strength values resulted to be dependent on the fiber sizing. In particular, the fibers coated with an epoxy sizing showed a superior thermal stability of the fiber matrix‐interface with respect to the unsized or nylon compatible sized fibers.     

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.     

The effect of fiber-matrix stress transfer on the strength of fiber-reinforced composite materials

A model for the mechanism of tensile failure in oriented fiber composites based on random fragmentation of the reinforcing fibers biased by stress concentrations at fracture sites has been developed. Single-fiber composites and composite strands of 34 to 36 volume percent fiber were prepared from an epoxy resin reinforced with Hercules AS4, HMS4, and IM6G carbon fibers. Fiber strength distributions and single-fiber composite fragmentation data were used to calculate theoretical composite tensile strengths, which were then compared with experimental values. The fractures in single-fiber composites were observed in situ under cross-polarized light, and the mechanisms of interfacial failure were discussed.     

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.     

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.