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Stress relaxation in discontinuously reinforced composites

Description: It has been observed that in discontinuously-reinforced Al{sub 2}0{sub 3}/NiAl composites that as the reinforcement size increases the average density of dislocations generated from the relaxation of the thermal stresses increases, and the corresponding thermal residual stresses slightly decrease. Similar changes result when the reinforcement morphology changes from spheres to short fibers to continuous filaments. The changes of dislocation density and thermal residual stresses with respect to particle size are in contrast to those observed in the SiC/Al counterpart A previously developed simple model used to explain the SiC/Al data, which was based on prismatic dislocation punching, suggested that the density of the misfit dislocations decreases when the reinforcement size increases. In this investigation, a simple model is proposed to explain the anomaly in the development of thermal residual stresses and the generation of misfit dislocations as a function of the particle size and shape in Al{sub 2}0{sub 3}/NiAl composites. As a result of a lack of sufficient independent-slip-systems in low symmetry materials such as NiAl, plastic relaxation of the thermal stresses is severely constrained as compared to fcc Al. As such, plastic relaxation requires collaborative slips in an aggregate of grains. This only occurs when the length scale of the varying misfit thermal stress field is much larger than the average grain size. That is, the mechanism of plastic relaxation becomes operative when the reinforcement size increases.
Date: May 1, 1995
Creator: Shi, N. & Arsenault, R.J.
Partner: UNT Libraries Government Documents Department

Particle shape effects on the fracture of discontinuously-reinforced 6061-A1 matrix composites

Description: Effects on fracture and ductility of a spherical and an angular particulate-reinforced 6061-Al composite containing 20(vol)% Al{sub 2}O{sub 3} were studied using SEM fractography and modeled using finite element method (FEM). The spherical particulate composite exhibited a slightly lower yield strength and work hardening rate but a considerably higher ductility than the angular counterpart. SEM fractography showed that during tensile deformation the spherical composite failed through void nucleation and linking in the matrix near the reinforcement/matrix interface, whereas the angular composite failed through particle fracture and matrix ligament rupture. FEM results indicate that the distinction between the failure modes for these two composites can be attributed to differences in development of internal stresses and strains within the composites due to particle shape.
Date: May 1, 1996
Creator: Shi, N.; Song, S.G.; Gray, G.T., III & Roberts, J.A.
Partner: UNT Libraries Government Documents Department

Influence of thermal residual stresses on the elastic phase-strain

Description: The development of elastic lattice phase strains in a 15 vol. pct TiC particulate reinforced 2219-T6 Al composite was modeled as a function of tensile uniaxial loading by finite element method (FEM). In the relationship of applied stress vs. elastic lattice phase strain, the slopes vary with the applied load even before the macroscopic yielding. The slopes for the phase-strain perpendicular to loading follow nonmonotonic changes with loading, while, in the direction parallel to loading, the slopes change monotonically with the applied load. In this investigation, we have demonstrated via FEM that thermal residual stresses from thermal expansion mismatch between phases affect initiation of matrix plasticity. And the differences in the matrix plasticity initiation influence the internal stress distribution. The changes in the slope are dictated by the internal stress transfer between phases. FEM models with and without thermal history show significant differences in the response of elastic strain component, a mechanics equivalent of the lattice elastic strain. Agreement with experiment can only be obtained by including the thermal history. From a simple elasto-plastic spring model we are able to demonstrate that, with matrix plasticity propagating as predicted by FEM, the elastic strain component responds similarly to the more rigorous numerical predictions, suggesting that the morphology of elastic strain evolution is dictated by the development of matrix plasticity.
Date: April 1, 1996
Creator: Shi, N.; Bourke, M.A.M. & Goldstone, J.A.
Partner: UNT Libraries Government Documents Department

Relaxation of thermal mismatch in discontinuously reinforced composites

Description: We have measured the dislocation density vol % and thermal residual elastic strain in NiAl matrices of 20 vol % Al{sub 2}O{sub 3} discontinuously-reinforced composites. As the size of the reinforcement increases the average dislocation density increases, and the corresponding thermal residual elastic strains decrease. The changes with respect to particle size in the dislocation density and residual strain can neither be explained by continuum theory nor by dislocation mechanics for homogeneous medium. A previously developed model (that satisfactorily describes the SiC/Al system) suggests that the misfit dislocation density decreases with increase in reinforcement size but this disagrees wit the current Al{sub 2}O{sub 3}/NiAl results. A new model is proposed to describe low-symmetry intermetallics, which are constrained in their ability to relax thermal mismatch because of a paucity of independent slip systems. The results are discussed in the context of continuum mechanics using finite element analyses and crystal plasticity.
Date: August 1, 1995
Creator: Shi, N.; Bourke, M.A.M.; Goldstone, J.A. & Arsenault, R.J.
Partner: UNT Libraries Government Documents Department

The Bauschinger effect in a SiC/Al composite

Description: SiC/Al composites have interesting mechanical properties, the tensile yield stress, whereas, the apparent modulus in tension is greater than that in compression. The Bauschinger effect of SiC/Al composites is also asymmetric with regard to loading directions. Quantitative measurements of the asymmetry of composite Bauschinger Effect was made in this research . An investigation was undertaken to determine the origin of the asymmetrical Bauschinger effect. We have successfully reconstructed the observed asymmetry using an internal stress model based on the development of internal stresses, conveniently referred to as the ``black stress``, and work hardening.
Date: September 1, 1995
Creator: Shi, N.; Pillai, U.T.S. & Arsenault, R.J.
Partner: UNT Libraries Government Documents Department

Internal strain measurement using pulsed neutron diffraction at LANSCE

Description: The presence of residual stress in engineering components can effect their mechanical properties and structural integrity. Neutron diffraction in the only technique that can make nondestructive measurements in the interior of components. By recording the change in crystalline lattice spacings, elastic strains can be measured for individual lattice reflections. Using a pulsed neutron source, all lattice reflections are recorded in each measurement, which allows for easy examination of heterogeneous materials such as metal matrix composites. Measurements made at the Manuel Lujan Jr. Neutron Scattering Center (LANSCE) demonstrate the potential at pulsed sources for in-situ stress measurements at ambient and elevated temperatures.
Date: December 1, 1994
Creator: Goldstone, J. A.; Bourke, M. A. M. & Shi, N.
Partner: UNT Libraries Government Documents Department

A study of internal damage of metal matrix composites by neutron diffraction

Description: Using neutron diffraction, we have measured the elastic phase strains of Al/TiC and Al/SiC composites under uniaxial tensile loading. The phase strains were used to reconstruct the global elastic strain. It has been found that, above macroscopic yield, the global elastic strain response is not linear. A theoretical model shows that the nonlinearity is dictated by changes in the ratio of longitudinal phase stresses. Furthermore, the changes in this ratio resulting from matrix plasticity and reinforcement fracture are different which leads to distinct slope changes in the global elastic strain response that can be used to distinguish the onset of these two processes on the global elastic strain loading curve.
Date: December 31, 1994
Creator: Shi, N.; Bourke, M. A. M. & Goldstone, J. A.
Partner: UNT Libraries Government Documents Department

Development of the average lattice phase-strain and global elastic macro-strain in Al/TiC composites

Description: The development of elastic lattice phase strains and global elastic macro-strain in a 15 vol% TiC particle reinforced 2219-T6 Al composite was modeled by finite element method (FEM) as a function of tensile uniaxial loading. The numerical predictions are in excellent agreement with strain measurements at a spallation neutron source. Results from the measurements and modeling indicate that the lattice phase-strains go through a ``zigzag`` increase with the applied load in the direction perpendicular to the load, while the changes of slope in the parallel direction are monotonic. FEM results further showed that it is essential to consider the effect of thermal residual stresses (TRS) in understanding this anomalous behavior. It was demonstrated that, due to TRS, the site of matrix plastic flow initiation changed. On the other hand, the changes of slope of the elastic global macrostrain is solely determined by the phase-stress partition in the composite. An analytical calculation showed that both experimental and numerical slope changes during elastic global strain response under loading could be accurately reproduced by accounting for the changes of phase-stress ratio between the matrix and the matrix.
Date: February 1, 1994
Creator: Shi, N.; Bourke, M. A. M.; Goldstone, J. A. & Allison, J. E.
Partner: UNT Libraries Government Documents Department

Innovative Composites Through Reinforcement Morphology Design - a Bone-Shaped-Short-Fiber Composite

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The objective of this project is to improve the strength and toughness of conventional short-fiber composites by using innovative bone-shaped-short (BSS) fibers as reinforcement. We fabricated a model polyethylene BSS fiber-reinforced polyester-matrix composite to prove that fiber morphology, instead of interfacial strength, solves the problem. Experimental tensile and fracture toughness test results show that BSS fibers can bridge matrix cracks more effectively, and consume many times more energy when pulled out, than conventional-straight-short (CSS) fibers. This leads to both higher strength and fracture toughness for the BSS-fiber composites. A computational model was developed to simulate crack propagation in both BSS- and CSS-fiber composites, accounting for stress concentrations, interface debonding, and fiber pullout. Model predictions were validated by experimental results and will be useful in optimizing BSS-fiber morphology and other material system parameters.
Date: June 29, 1999
Creator: Zhu, Y.T.; Valdez, J.A.; Beyerlain, I.J.; Stout, M.G.; Zhou, S.; Shi, N. et al.
Partner: UNT Libraries Government Documents Department

The use of pulsed neutron diffraction to measure strain in composites

Description: Neutron diffraction is a technique for measuring strain in crystalline materials. It is non destructive, phase discriminatory and more penetrating than X rays. Pulsed neutron sources (in contrast with steady state reactor sources) are particularly appropriate for examining heterogeneous materials or for recording the polycrystalline response of all lattice reflections. Several different aspects of composite behavior can be characterized and examples are given of residual strain measurements, strain relaxation during heating, applied loading, and determination of the strain distribution function.
Date: March 1, 1994
Creator: Bourke, M. A. M.; Goldstone, J. A.; Shi, N.; Gray, G. T. III; James, M. R. & Todd, R. I.
Partner: UNT Libraries Government Documents Department

Influence of reinforcement morphology on the mechanical properties of short-fiber composites

Description: A major problem of short-fiber composites is that the interfaces between the fiber and matrix become a limiting factor in improving mechanical properties such as strength. For a short fiber, a strong interface is desired to effectively transfer load from matrix to fiber, thus reducing the ineffective fiber length. However, a strong interface will make it difficult to relieve fiber stress concentration in front of an approaching crack. Stress concentrations result in fiber breakage. The authors report in this paper an innovative approach to overcome this problem: reinforcement morphology design. Short-fibers with enlarged ends are processed and used to reinforce a polyester matrix. The initial results show that the bone-shaped short-fibers produce a composite with significantly higher strength than can be attained with conventional short, straight fibers.
Date: December 1, 1997
Creator: Zhu, Y.T.; Valdez, J.A.; Shi, N.; Lovato, M.L.; Stout, M.G.; Zhou, S. et al.
Partner: UNT Libraries Government Documents Department