Aquiles Sepúlveda

Damping characteristics of a CuZnAlNi shape memory alloy

Abstract In this paper the mechanical behavior of copper-based SMA bars and their potential use for dissipating seismic energy is examined. In particular, results obtained from cyclic tests under tension–compression loading of martensitic CuZnAlNi shape memory alloy bars, with two different processing histories (hot rolled or extrusion), are reported as a function of strain amplitude, grain size and frequency. The stress–strain loops were asymmetric with respect to the central point and rather stable, especially for strains up to 2%. The effective module of elasticity diminishes significantly with strain. From hysteretic cycles, an equivalent damping of 12% was obtained for the larger strains imposed. The maximum tensile strain attained before fracture was 3.5%. Frequency, in the range of interest for seismic applications, had only a small influence on the damping values. For the conditions studied in this research, the bars produced from both processing histories presented similar mechanical properties. Fracture, when it occurred, was due to tensile action and it was rather brittle and intergranular.

On the determination of grain-size distributions from intercept distributions

A matrix formulation for determining the spatial grain-size distribution of tetrakaidecahedral grains from linear-intercept data is developed. The truncation effect (which stems from the fact that the intersection of single-size grains by a test line gives rise to intercepts of different length) and the sampling effect (which results as a consequence of bigger grains being intersected more frequently than smaller ones) are separately taken into account. The derivation procedure of this formulation is applicable to any other convex shape, provided the linear intercept distribution for single-size grains of the corresponding shape is known. The percentage spatial grain-size distributions obtained by the formulation derived here are similar to those estimated by the Spektors chord method for spherical grains.

An analysis of grain-growth data in duplex materials on static annealing and during superplastic deformation

An analysis of data on grain growth in several duplex materials subjected to superplastic deformation (SPD) and to static annealing (SA) is presented. Also, for comparison, data of a single-phase alloy and a particle-strengthened one were considered. The theoretical equationDn -Don =Kt was employed, whereD andDo are the instantaneous and initial average grain size, respectively,t is the deformation or annealing time,n is the kinetic exponent andK is the rate constant. When analysing the selectedD-t data sets, the fact thatDo values were generally not small enough to be negligible in the above equation was taken into account. It is concluded: (i) for a given duplex allay, the coarsening mechanisms acting during SA and SPD are different, the kinetics being enhanced by concurrent deformation, and (ii) whatever the alloy, the value forn for grain growth under SPD is near to 2, (as for normal grain growth in single-phase materials).

Coarsening of particles connected by dislocations

This work theoretically reinvestigates the coarsening of particles controlled by diffusion through a dislocation network, when the dislocation spacing of the network is larger than the maximum particle size and the volume fraction of the particles vanishes. Both a three-dimensional network and a plane one, such as that which might be encountered at a low-angle grain boundary, are treated. It is considered here that the number of dislocation pipes for diffusion increases with distance away from the particle. Under certain reasonable assumptions, at1/4 steady-state coarsening kinetics is found. The origin of the differences between the present kinetic results (t1/4) and those predictions previously reported in the specialized literature (t1/5), for similar dislocation geometries, are discussed. The effect of the dislocation annihilation phenomenon on the growth kinetics is also examined qualitatively.

Conservation of the secondary structure of protein during evolution and the role of the genetic code

In this communication we demonstrate, in a group of modern proteins, following an algorithm described by Argyle (1980), that the ordination of the amino acids in terms of the most frequent substitutions agrees with the conservation of theα-helix,β-sheet, andβ-turn formation tendencies during evolution. The same correspondence has been demonstrated for the conservation of the physico-chemical properties in the amino acid substitutions. Both parameters are similar in showing higher correlation with the most frequent amino acid substitutions than with the feasibility of changes at the level of the respective codons.Some kind of restrictions for the expression of the genomic changes, due to the conservation of the secondary structure of proteins and/or the physicochemical properties of the substituted amino acids, could account for the differences found between the distribution of the amino acid substitutions and the most probable codon changes.