46 research outputs found
Nanoscale anisotropic structural correlations in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5 MnO3
We report x-ray scattering studies of short-range structural correlations and
diffuse scattering in Nd0.5Sr0.5MnO3. On cooling, this material undergoes a
series of transitions, first from a paramagnetic insulating (PI) to a
ferromagnetic metallic (FM) phase, and then to a charge-ordered (CO) insulating
state. Highly anisotropic structural correlations were found in both the PI and
FM states. The correlations increase with decreasing temperature, reaching a
maximum at the CO transition temperature. Below this temperature, they abruptly
collapsed. Single-polaron diffuse scattering was also observed in both the PI
and FM states suggesting that substantial local lattice distortions are present
in these phases. We argue that our measurements indicate that nanoscale regions
exhibiting layered orbital order exist in the paramagnetic and ferromagnetic
phases of Nd0.5Sr0.5MnO3.Comment: 5 pages, 4 embedded figure
Magnetic-field-induced collapse of charge-ordered nanoclusters and the Colossal Magnetoresistance effect in Nd(0.3)Sr(0.3)MnO(3)
We report synchrotron x-ray scattering studies of charge/orbitally ordered
(COO) nanoclusters in NdSrMnO. We find that the COO
nanoclusters are strongly suppressed in an applied magnetic field, and that
their decreasing concentration follows the field-induced decrease of the sample
electrical resistivity. The COO nanoclusters, however, do not completely
disappear in the conducting state, suggesting that this state is inhomogeneous
and contains an admixture of an insulating phase. Similar results were also
obtained for the zero-field insulator-metal transition that occurs as
temperature is reduced. These observations suggest that these correlated
lattice distortions play a key role in the Colossal Magnetoresistance effect in
this prototypical manganite.Comment: 5 pages, 3 embedded eps figures; to appear in PRB Rapid
Commumication
Microstructural developments of poly (p-phenylene terephthalamide) fibers during heat treatment process: a review
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Failure of a fiber composite lamina under three-dimensional stresses
The efficient use of thick-section fiber composites requires a proven three-dimensional failure model. Numerous failure criteria have been proposed, but the lack of critical experimental results makes it difficult to assess the accuracy of these models. It is shown that the various predictions for failure of a lamina due to the simple state of uniaxial stress plus superposed hydrostatic pressure are disparate. These differences are sufficient to allow evaluation of failure criteria using data that has the normal scatter found for composite materials. A high-pressure test system for fiber composites is described and results for the effects of pressure on the transverse and longitudinal compression strengths of a carbon fiber/epoxy lamina are discussed. Results are compared with a few representative failure models
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Effect of angle-ply orientation on compression strength of composite laminates
An experimental program was initiated to investigate the effect of angle-ply orientations on the compressive strength (X{sub 1C}) of 0{degree} plies in fiber reinforced composite laminates. Graphite fiber-reinforced epoxy test coupons with the generic architecture [0{sub 2}/{+-}{theta}] (where {theta} varied between 0{degree} and 90{degree}) and for the quasi-isotropic architecture were evaluated. The effective compressive strength of the 0{degree} plies varied considerably. The results were related to the Poisson's ratios of the laminates with high Poisson's ratios leading to high transverse tensile strains in the test coupons and lower than expected strengths. Specimens with the [O{sub 2}/{+-}30] architecture had both the highest Poisson's ratio and the lowest calculated ply-level compression strength for the 0{degree} plies. This work has implications in the selection of composite failure criterion for compression performance, design of test coupons for acceptance testing, and the selection of laminate architectures for optimum combinations of compressive and shear behavior. Two commonly used composite failure criteria, the maximum stress and the Tsai-Wu, predict significantly different laminate strengths depending on the Poisson's ratio of the laminate. This implies that the biaxial stress state in the laminate needs to be carefully considered before backing out unidirectional properties
Fatigue and Fracture of Fiber Composites Under Combined Interlaminar Stresses Fatigue and Fracture of Fiber Composites Under Combined Interlaminar Stresses*
June 25, 1998 This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author. PREPRINT ABSTRACT As part of efforts to develop a three-dimensional failure model for composites, a study of failure and fatigue due to combined interlaminar stresses was conducted. The combined stresses were generated using a hollow cylindrical specimen, which was subjected to normal compression and torsion. For both glass and carbon fiber composites, normal compression resulted in a significant enhancement in the interlaminar shear stress and strain at failure. Under moderate compression levels, the failure mode transitioned from elastic to plastic. The observed failure envelope could not be adequately captured using common plylevel failure models. Alternate modeling approaches were examined and it was found that a pressure-dependent failure criterion was required to reproduce the experimental results. The magnitude of the pressure-dependent terms of this model was found to be material dependent. The interlaminar shear fatigue behavior of a carbon/epoxy system was also studied using the cylindrical specimen. Preliminary results indicate that a single S/N curve which is normalized for interlaminar shear strength may be able to reproduce the effects of both temperature and out-of-plane compression on fatigue life. The results demonstrate that there are significant gains to be made in improving interlaminar strengths of composite structures by applying out-of-plane compression. This effect could be exploited for improved strength and fatigue life of composite joints and other regions in structures where interlaminar stress states are critical
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Failure Plane Orientations for Fiber Composites
Using a recently developed failure theory for transversely isotropic fiber composites, it is shown how the orientation of the failure surface can be determined for transverse tension and compression. Experimental data on failure surface orientations have been obtained for four carbon fiber composite systems based on both thermoplastic and thermosetting matrix materials. Average compression failure planes for the different composite materials were measured to range from 31{sup o} to 38{sup o} from the load axis. Reasonable agreement was obtained between these measured angles and those predicted from application of the new failure theory
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Fatigue and fracture of fiber composites under combined interlaminar stresses
As part of efforts to develop a three-dimensional failure model for composites, a study of failure and fatigue due to combined interlaminar stresses was conducted. The combined stresses were generated using a hollow cylindrical specimen, which was subjected to normal compression and torsion. For both glass and carbon fiber composites, normal compression resulted in a significant enhancement in the interlaminar shear stress and strain at failure. Under moderate compression levels, the failure mode transitioned from elastic to plastic. The observed failure envelope could not be adequately captured using common ply- level failure models. Alternate modeling approaches were examined and it was found that a pressure-dependent failure criterion was required to reproduce the experimental results. The magnitude of the pressure-dependent terms of this model was found to be material dependent. The interlaminar shear fatigue behavior of a carbon/epoxy system was also studied using the cylindrical specimen. Preliminary results indicate that a single S/N curve which is normalized for interlaminar shear strength may be able to reproduce the effects of both temperature and out-of-plane compression on fatigue life. The results demonstrate that there are significant gains to be made in improving interlaminar strengths of composite structures by applying out-of-plane compression. This effect could be exploited for improved strength and fatigue life of composite joints and other regions in structures where interlaminar stress states are critical
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Advanced composites technology
The development of fiber composite components in next-generation munitions, such as sabots for kinetic energy penetrators and lightweight cases for advanced artillery projectiles, relies on design trade-off studies using validated computer code simulations. We are developing capabilities to determine the failure of advanced fiber composites under multiaxial stresses to critically evaluate three-dimensional failure models and develop new ones if necessary. The effects of superimposed hydrostatic pressure on failure of composites are being investigated using a high-pressure testing system that incorporates several unique features. Several improvements were made to the system this year, and we report on the first tests of both isotropic and fiber composite materials. The preliminary results indicate that pressure has little effect on longitudinal compression strength of unidirectional composites, but issues with obtaining reliable failures in these materials still remain to be resolved. The transverse compression strength was found to be significantly enhanced by pressure, and the trends observed for this property and the longitudinal strength are in agreement with recent models for failure of fiber composites