Key Engineering Materials Vols. 340-341

Abstract: In this paper, via experimental results of ultrasonic wave velocity changes and FEPM simulations of slip system under the pure shear plastic deformation of a pure Aluminum single crystal, the authors verify that the ultrasonic longitudinal wave velocity depends upon the development of point defects caused by intersected cross slip among dislocations and also examine the correlation of the point defect dependence of the longitudinal wave velocity with the elastic stiffness degradation due to plastic deformation damage which is closely related to the deformation instability mechanism.

Abstract: Representative length scale of ULSI (Ultra Large Scale Integration) cells is going to be at a nano-meter order, and the atomic level defects, such as uneven oxide films or dislocation accumulation are becoming more and more important. Among these defects, dislocation accumulation is known to be caused by thermo-plastic deformation in silicon during the processes of device fabrication. In this study, we analyse such thermal stress, plastic slip deformation and accumulation of dislocations in STI (Shallow Trench Isolation) type ULSI devices when the temperature drops from the initial at 1000 °C to room temperature. For the analysis, we use a crystal plasticity analysis code CLP, assuming that lattice friction stress for the movement of dislocations is proportional to the hardness of silicon, which is known to have strong dependency on temperature. The results show that dislocations are generated between the temperature range from 880 to 800 °C, and its maximum density is highly dependent on the lattice friction stress in the temperature range above 800 °C. For example, the difference of 16 MPa in the lattice friction stress at 1000 °C caused increase in dislocation density more than ten times. It is concluded that control of lattice friction stress at high temperatures is one of the most promising way for the suppression of dislocation accumulation.

Abstract: A multiscale model on dislocation patterning of cell structure and subgrain for polycrystal is newly developed on the basis of reaction-diffusion theory. A FD simulation for dislocation patterning and a FE one for crystal deformation are simultaneously carried out for a FCC polycrystal at large strain. Reflecting stress value on stress-effect coefficients, it is numerically predicted that the evolution of dislocation pattern in a polycrystal is different in response to the stress condition of each grain.

Abstract: In this study, effects of typical texture components observed in rolled aluminum alloy sheets (i.e. Copper, Brass, S, Cube and Goss texture components) on plastic flow localization are studied. The material response is described by a generalized Taylor-type polycrystal model, in which each grain is characterized in terms of an elastic-viscoplastic continuum slip constitutive relation. First, forming limits of thin sheet set by sheet necking are predicted using a Marciniak–Kuczynski (M–K-) type approach. It is shown that only the Cube texture component yields forming limits higher than that for a random texture in the biaxial stretch range. Next, three-dimensional shear band analyses are performed, using a three-dimensional version of M–K-type model, but the overall deformation mode is restricted to a plane strain state. From this simple model analysis, two important quantities regarding shear band formation are obtained: i.e. the critical strain at the onset of shear banding and the corresponding orientation of shear band. It is concluded that the Cube texture component is said to be a shear band free texture, while some texture components exhibit significantly low resistance to shear band formation. Finally, shear band developments in plane strain pure bending of sheet specimens with the typical textures are studied.

Abstract: Physical interpretations of the incompatibility tensor are extensively discussed and applied to model several practically-important dislocation substructures in metallic materials. This paper firstly performs a tangible decomposition of the incompatibility tensor into the two types of defects by introducing Nye’s contortion tensor, and also clarifies the interrelationship with expressions given based on differential geometry. The effects on the evolutions of intra-granular substructures like cells and geometrically-necessary type bands are examined based on finite element simulations on multi-grain models under tension and simple shear with several representative orientations.

Abstract: The structural behavior of an old six-span reinforced concrete arch bridge, which has been in service for about 40 years, is investigated. Field monitoring (inclusive of test of material property, static and dynamic test of the bridge) was conducted, static and dynamic responses of the bridge are obtained. Based on the primitive bridge, a scaled one-span bridge model was fabricated by organic-glasses. Both the static and dynamic tests were executed on the bridge model in the laboratory. Since the arch rib is the crucial member for the arch bridge, 7 notches were cut on both arch ribs of the bridge model to simulate different damages of the arch rib. Mechanical responses of the bridge with different damages on the arch ribs were achieved. FEM analyses were preformed on the bridge as well. Numerical results show good agreement with the experimental results.

Abstract: Damage development due to creep under uniaxial tension at elevated temperatures is assessed using destructive and non-destructive methods in steels, commonly used in power plants or chemical industry, and in aluminium alloy used in aircrafts for responsible elements. The results obtained using two different destructive methods for assessments of damage development are critically discussed. In the first method the specimens of steel after different amounts of creep prestraining were stretched up to failure and variations of the selected tension parameters were taken into account for damage identification. In the second one, a damage degree was evaluated by studying variations of an initial yield locus position in the stress space and by determination of the yield loci dimensions. The ultrasonic investigations were selected as the non-destructive method for damage development evaluation.

Abstract: A methodology for evaluating and predicting component lives in creep-fatigue interaction region was investigated for Waspaloy. A unified viscoplasticity constitutive equation including multi-back stresses was used to describe cyclic material behaviors. Also, a continuum damage model coupling with the creep-fatigue damage rules was established based on the analysis of creep and low cycle fatigue behavior. Multi-axial fatigue and creep equivalent stress concepts were employed to predict three dimensional component lives. Notched cyclic tests under various stress conditions in the creep-fatigue interaction region were carried out to validate the life prediction methodology with FEM simulation based on the continuum damage model. The comparison of experimental data and prediction results indicates that the continuum damage model is a powerful approach for the prediction of component lives.

Abstract: An ultralight magnesium alloy AM50A has been investigated for its potential to be used in aerospace and automotive industry. The dynamic stress strain relation of aluminum 6061 T6 and the magnesium alloy AM50A have been obtained by using the Hopkinson bar apparatus. The strain rates range between 600 s-1 and 1300 s-1. The Al 6061 T6 results tally well with those in literature. The magnesium alloy AM50A displays about 50% higher tensile stress at the strain rate of about 1300 s-1 than at static.