Mixed-mode fracture of composites using Iosipescu shear test

Finite element analysis is used to determine the influence of elastic properties on the stress distribution in the Iosipescu shear test specimen. Two different boundary conditions are used which assume either the application of force couples or specified displacements. The effect of orthotropy ratio is investigated with E ₁₁/E ₂₂ranging from 1 to 14.2. The analysis is extended to partially cracked specimens and used to calculate the basic fracture parameters in aligned composite materials. In particular, the failure of specimens with two axial splits, nucleated at the roots of the notches, and extended in the fibre direction is analysed. It is shown that the stress distribution is strongly dependent on both the elastic properties and boundary conditions. The mixed mode stress intensity factors K _Iand K _IIat the crack tips tend to increase with orthotropy ratio. The analysis is discussed with respect to the limited experimental data available for this test geometry. From the numerical and experimental results the mixed mode toughness is estimated in terms of the critical stress intensity factors and the critical energy release rate.Résumé On recourt à une analyse par éléments finis pour déterminer l'influence des propriétés élastiques du matériau sur la distribution des contraintes dans l'éprouvette de l'essai de cisaillement de Iosipescu. Deux conditions aux limites sont considérées: l'application de couples de forces ou de déplacements imposés. L'effet du rapport d'orthotropie est étudié, pour des valeurs de E ₁₁/E ₂₂comprises entre 1 et 14,2. L'analyse est étendue au cas d'éprouvettes partiellement fissurées, et est utilisée pour le calcul des paramètres fondamentaux de rupture dans des matériaux composites à fibres alignées. On analyse en particulier la rupture d'éprouvette comportant deux séparations axiales, prenant naissance aux racines ds entailles et s'étendant dans la direction de la fibrosité.On montre que la distribution des containtes dépend fortement des propriétés élastiques du matériaux et des conditions aux limites. Les facteurs d'intensité de contrainte correspondant au mode mixte K _Iet K _IIde rupture aux extrémités de la fissure tendant à croître avec le rapport d'orthotropie.Une discussion sur cette analyse tient compte du nombre limité de données expérimentales disponibles pour cette géométrie d'éprouvettes. On peut estimer la ténacité sous mode mixte à partir des résultats numériques et expérimentaux, et l'exprimer par les facteurs critiques d'intensité de contraintes et la vitesse critique de relaxation de l'énergie.     

Hybrid composite rods subjected to excessive bending loads

Glass fiber/carbon fiber/epoxy hybrid composite rods were investigated in this research for their resistance to excessive bending. The rods are presently being used as the load bearing component of the Aluminum Conductor Composite Core/Trapezoidal Wire (ACCC/TW™) design. The ACCC/TW™ design is one of the most serious candidates to replace the existing conductor designs based on steel and aluminum wires. The effects of mandrel size and thickness of the insulating glass fiber composite sheath on the axial compressive stress state during bending of the ACCC rod were numerically investigated by performing non-linear finite element analyses of the conductor wrapping process. In addition, two sets of compression experiments were performed on composite specimens in order to determine the ultimate compressive strength of the ACCC rod and of the carbon fiber composite alone. During the compression tests, acoustic emissions were monitored from the specimens to determine if a different failure process exists for the hybrid composite as opposed to a traditional uni-directional long fiber composite. Proof tests, and subsequent Scanning Electron Microscope (SEM) work of each type of composite were also performed to better understand the failure process. It was clearly demonstrated in this research that ACCC rods will be mechanically damaged by excessive bending over small diameter mandrels used for transportation and installation purposes. This work should be of great help to the manufacturers and potential users of the ACCC conductors around the world.     

Determination of shear strength of unidirectional composite materials with the Iosipescu and 10° off-axis shear tests

The purpose of this research was to determine the shear strength of a unidirectional carbon-fibre/epoxy composite by means of the 10° off-axis and 0° Iosipescu specimens subjected to shear. Detailed non-linear finite-element computations of these two tests were conducted, taking into account the actual non-linear material behavior of the composite. The tests were compared in terms of stresses and strains at failure. It was found that the shear strength of the composite can be very accurately determined by using the two independent testing techniques only if fully non-linear finite element computations of the tests are performed. The stresses and strains at failure in the 10° off-axis specimen closely match the stresses and strains at the onset of intralaminar damage near the roots of the notches in Iosipescu specimens. Owing to the difficulties associated with the measurement of the shear strength of the composite using the Iosipescu test, and in particular, with the interpretation of the experimental data, this test was found to be almost impractical for the determination of shear strength. The test can only be used if fully non-linear finite element computations of uncracked and axially cracked Iosipescu specimens are conducted in conjunction with the continuous monitoring of intralaminar damage near the roots of the notches during testing. In addition, the shear strength results obtained from the Iosipescu specimen should be independently verified by using another method, such as the 10° off-axis test.     

An investigation of moisture and leakage currents in GRP composite hollow cylinders

The applicability of using flat composite plates and hollow core composite cylinders for moisture absorption testing of unidirectional glass/polymer composites used in high voltage composite (non-ceramic) insulators was examined. Two main issues were addressed in this work. First, the effect of specimen geometry (cylinders vs. plates) on moisture absorption by the composites was investigated both numerically and experimentally. Both classical Fickian and non-Fickian diffusions were considered. Subsequently, hollow core cylinders made up of ECR (low seed)-glass fibers and epoxy resin were tested for their high voltage properties under controlled moisture diffusion conditions. The specimens were exposed to warm, moist air and their high voltage properties were ascertained using a modified version of the ANSI test (standard C29.11 Section 7.4.2) for water diffusion electrical testing. It was found that the behavior of the hollow core cylinder and flat plate composite specimens subjected to moisture compared reasonably well experimentally and very well numerically. From the high voltage tests, a direct correlation was found between the amount of moisture that had been absorbed by the specimens and the amount of leakage current that was detected. It was shown that using the thin walled composite cylinders leakage currents could be predicted based on the amount of absorbed moisture in the insulator composites. The predictions can be made based on relatively short term moisture data even if the diffusion process in the composites is anomalous in nature with long times required for full saturation. After additional verifications, considering other composite systems, the hollow core cylinder testing under controlled moisture and high voltage conditions could become a screening test for selecting suitable glass/polymer composites for insulator applications.     

Finite element analysis of substation composite insulators

Composite insulators are rapidly replacing their porcelain counterparts in electrical substation applications. These insulators consist of a glass-reinforced polymer (GRP) rod, with two metal end fittings radially crimped onto the ends of the rod during assembly. In this paper, axisymmetric finite element models are developed to evaluate the mechanical performance of composite insulators under externally applied axial compression. The analyses are performed by assuming both a perfectly bonded interface between the composite rod and the end fittings, and an imperfect interface which permits large relative sliding with Coulomb friction. Results indicate that the perfect interface model is unrealistic since it predicts singular stresses at the interface comer and an overall linear structural response. On the other hand, the imperfect interface model is found to simulate accurately the structural non-linearity caused by relative sliding of the GRP rod within the end fittings. The imperfect interface model has therefore been used to evaluate the effects of interface friction, and the extent of crimping, on the maximum load-bearing capacity of substation composite insulators.     

Hydroxyl Group Separation Distances in Anti-Freeze Compounds and Their Effects on Ice Nucleation

Since the discovery of biological antifreeze glycoproteins (AFGPs), which can inhibit ice nucleation, there has been considerable interest in understanding their mechanisms and mimicking them in synthetic polymers. In this study, we used molecular dynamics simulations of modified polyvinyl alcohol (PVA) compounds to show that the hydroxyl (OH) group distance is a key factor in whether certain compounds promote or inhibit ice nucleation. A hydroxyl distance smaller than ~2.8 Å but greater than ~7.1 Å in modified PVA (MPVA) compounds was associated with the promotion of ice nucleation, while a hydroxyl group separation distance of approximately ~5.0 Å was correlated with a delay in ice nucleation, owing to changes in the energy of the system. Thus, these results may help explain some of the mechanisms of current known anti-freeze compounds and may have implications for designing new anti-freeze compounds in the future.     

The effects of atmospheric aging on a hybrid polymer matrix composite

The effect of thermal exposure in an atmospheric environment for up to 1 year on the flexural performance, under both static and fatigue loading, of a glass fiber/carbon fiber hybrid polymer matrix composite material was evaluated. It was found that exposure to a temperature near, but below, the glass transition temperature resulted in diminished flexure strength as well as reduced fatigue performance. The magnitude of property reduction was, in general, proportional to the amount of aging time, and was found to be dictated by the dominant aging mechanism. Scanning electron microscopy revealed that the modest reduction in mechanical properties at intermediate aging times was predominantly attributed to thermal oxidation, while for longer aging times thermal aging (dimensional relaxation) was the primary cause for the substantial reduction. Dimensional relaxation of the composite was measured at several isothermal aging temperatures, from which, the activation energy of the aging process was determined. This work provides insight into the evolution of mechanical properties as a function of aging time in an atmospheric environment for a hybrid polymer matrix composite.     

Critical Examination of the Iosipescu Shear Test as Applied to 0degrees Unidirectional Composite Materials

Several issues regarding the application of the shear and biaxial Iosipescu tests for the shear strength characterization of unidirectional composite materials are addressed in this article. First, the nonlinear effects of specimen sliding and geometric nonlinearity on the mechanical response of 0degrees standard unidirectional graphite/polyimide Iosipescu specimens with different loading conditions and loading block geometries have been investigated. Second, an attempt has been made to improve the Iosipescu shear test to eliminate normal compressive stresses in the specimen gauge section and at the same time prevent axial splitting. Finally, several Iosipescu shear and biaxial experiments have been performed to select proper specimen geometry and loading conditions for the shear strength measurements of unidirectional composites. The nonlinear effects are examined with respect to various coefficients of friction, displacements, loading angles, and fixtures (biaxial with short and modified biaxial with long loading blocks) using nonlinear finite-element techniques. It is shown that the effect of nonlinearity is small on the stresses at the center of the standard Iosipescu specimen, but significant for the stresses near the notch root up to 2 mm applied displacements. In some cases, significant differences in the stresses calculated for different coefficients of friction have been observed. All of these results are somewhat consistent for both fixtures, but with the stress components sigma x, sigma y, and sigma xy significantly lower in the standard Iosipescu specimens tested in the fixture with the long blocks. Numerical load/displacement diagrams show that specimen sliding and geometric nonlinearity have a negligible effect on reaction forces in the biaxial fixture, and a significant effect on the reaction forces in the modified biaxial fixture. Since the various combinations of the loading conditions evaluated in this study do not eliminate transverse compressive stresses in the gauge section of the standard Iosipescu specimens, a major improvement to the Iosipescu shear test has been proposed. Using an optimized specimen geometry subjected to biaxial shear/tension loading conditions, a state of almost uniform pure shear stress can be generated in 0degrees unidirectional composite Iosipescu specimens without the possibility of axial splitting along the fibers at the roots of the notches. However, it is shown in the experimental part of this study that for the optimized Iosipescu specimen, crushing at the inner loading blocks can significantly affect the shear intralaminar failure process. Only by reducing the cross-sectional area of the optimized Iosipescu specimen can the effect of crushing on the failure process be reduced without, however, highquality shear stress fields present in the gauge section at failure.     

ADHESIVELY BONDED COMPOSITE IOSIPESCU SPECIMENS WITHOUT SINGULAR STRESS FIELDS

The asymptotic singular stress fields at bi-maSerial wedge corners in adhesively bonded composite losipescu specimens have been studied using two-dimensional finite-element techniques. In particular, the singular powers of the fields have been analyzed using the finite-element iterative method. Different combinations of the elastic properties of the composite adherends have been considered, taking into account various volume fractions of fibers as well as different fiber orientations with respect to the adherend/adhesive interface. It has been shown that there are critical angles of the interfaces that separate the regions with positive and negative singular powers. This creates stress fields that are not singular in nature. It appears that the critical angles are dependent on the elastic properties of the composite adherends. In the second part of this work, the critical angle approach has been applied to the design of adhesively bonded composite losipescu specimens free from singular stress fields at their bi-material wedge corners. The samples will not develop singular stress fields either in shear or under biaxial shear-dominated loading conditions.     

Failure analyses of nonceramic insulators: part II - the brittle fracture model and failure prevention

In this work, an improved version of a brittle fracture model, based on the formation of nitric acid in service through corona discharges, ozone, and moisture, is presented and is used to explain several different modes of brittle fracture. Similar to Part I, we refer throughout this article to the insulators as nonceramic insulators (NCIs). To prevent brittle fracture in-service, its causes must be first established. To prevent brittle fracture in-service, its causes must be first established.