The effects of rate sensitivity and plastic potential surface curvature on plastic flow localization in porous solids

Plastic flow localization in porous elastic-viscoplastic solids is analyzed with an emphasis on the effects of material rate sensitivity and plastic potential surface curvature. The effect of rate sensitivity is included in a material model that accounts for a change of yield surface curvature in a rate-insensitive porous ductile solid. Shear band formation under plane strain and axisymmetric tension, and localized necking in biaxially stretched sheets are analyzed by using the present material model. The results illustrate the interactions of the effects of void nucleation and growth, material rate sensitivity and plastic potential surface curvature on plastic flow localization. The effects of nonproportional straining paths on localized necking in thin sheets are also demonstrated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42772/1/10704_2004_Article_BF00015862.pd

Mechanical properties of S-65C grade beryllium at elevated temperatures

Tensile property measurements and fractographic analysis of S-65C beryllium are reviewed. Tests were performed on specimens oriented in the longitudinal and transverse directions with respect to the direction of vacuum hot pressing. Specimens were tested in air at RT, 100 C, 200 C, 300 C, 415 C and 500 C at an initial strain rate of 1.1 {times} 10(sup {minus}4) per second. Ductility of the material was strongly affected by the test temperature, exhibiting a peak ductility at 300 C. The material displayed a yield point phenomenon which was most pronounced at this same temperature. Scanning electron microscopy was performed on the resulting fracture surfaces and observations are reported

Mechanical properties of a structural polyurethane foam and the effect of particulate loading

The room temperature mechanical properties of a closed-cell, polyurethane encapsulant foam have been measured as a function of foam density. Tests were performed on both unfilled and filler reinforced specimens. Over the range of densities examined, the modulus of the unloaded foam could be described by a power-law relationship with respect to density. This power-law relationship could be explained in terms of the elastic compliance of the cellular structure of the foam using a simple geometric model found in the literature. The collapse stress of the foam was also found to exhibit a power-law relationship with respect to density. Additions of an aluminum powder filler increased the modulus relative to the unfilled foam

Direct Measurement of Extension and Force in Conductive Polymer Gel Actuators

The synthesis of a polythiophene-based conductive polymer gel is described. Preliminary measurements of the electrochemically driven extension and force response of this gel are reported when driven under the action of an applied square-wave potential. Over each square wave interval (i.e., one oxidation pulse followed by one reduction pulse), the axial change in dimension was found to be approximately 2%. Some hysteresis was noted in that the cylindrical specimens did not return to their original axial dimension. The axial pressure generated by the expansion of the gel against a fixed surface was also measured and found to be on the order of 15 kPa

Helium induced degradation in the weldability of an austenitic stainless steel

Autogenous gas tungsten arc welding was performed on He-doped type 316 stainless steel. Helium was uniformly implanted in the material using the ''tritium trick'' to levels of 27 and 105 appm. Severe intergranular cracking occurred in both fusion and heat-affected zones. Microstructural observations of fusion zone indicated that the pore size, degree of porosity, and tendency to form cracks increased with increasing helium concentration. Scanning electron microscopy showed that cracking in He-doped materials was due to the precipitation of helium bubbles on grain boundaries and dentrite interfaces. Results of the present study demonstrate that the use of conventional welding techniques to repair materials degraded by exposure to radiation may be difficult if the irradiation results in the generation of even rather small amounts of helium. 23 refs., 9 figs., 2 tabs