José María Santín Amo

Laser welding of Al55-Zn coated steel sheet

A study has been made of the procedures used for the lap and butt welding of 0.62 and 1.16 mm thickness steel sheets coated with Al55-Zn alloy, using laser-beam technology. The main problems arise in lap welding as a result of zinc evaporation from the coating. The vapour pressures of this element lead to cratering and blistering of the welds which are visible from the external surface and imply severe weld defects. An efficient solution to this problem consists in leaving a gap between the internal faces, thus allowing the zinc vapours to escape. Excessive gaps, however, lead to the molten metal collapse and to lack of penetration. In the present work, acceptable results were obtained with a maximum interface separation of 0.1 mm. This allows excellent continuity of the joints, without cracks, pores or non-metallic inclusions and with moderate hardnesses, thus demonstrating the good laser weldability of the coated material. A difficulty arises, however, in relation to the coating in zones close to the weld. In these high-temperature zones the coating is lost, and in the immediately adjacent areas the coating microstructure and phases are altered, as was shown by scanning electron microscope analyses.

Confident methods for the evaluation of the hydrogen content in nanoporous carbon microfibers

Nanoporous carbon microfibers were grown by chemical vapor deposition in the vapor-liquid solid mode using different fluid hydrocarbons as precursors in different proportions. The as-grown samples were further treated in argon and hydrogen atmospheres at different pressure conditions and annealed at several temperatures in order to deduce the best conditions for the incorporation and re-incorporation of hydrogen into the microfibers through the nanopores. Since there are some discrepancies in the results on the hydrogen content obtained under vacuum conditions, in this work, we have measured the hydrogen content in the microfibers using several analytical methods in ambient conditions: surface tension, mass density, and Raman measurements. A discussion on the validity of the results obtained through the correlation between them is the purpose of the present work.

Study of elastic-plastic fracture toughness determination

The elastic-plastic fracture toughness viaJ-integral and crack tip opening displacement (CTOD) has been obtained in two structural steels using several fitting equations representing the resistance curve of the material. The toughness is determined as the values corresponding to the critical stretched zone width (SZW) on theR-curves and with respect to 0.2 mm crack growth. The SZW measurements were performed by scanning electron microscopy. The various toughness values have been compared and the importance of using appropriateR-curves based on physical considerations has been pointed out. TheJ-CTOD relationship during the blunting process has been experimentally investigated from load-displacement records of the fracture test.

Changes in the thermoelectric response of vitreous carbon due to the irradiation by γ-rays

In order to study variations in the thermoelectric properties, some commercial glassy carbon samples were subjected to a sequence of steps consisting of a combination of irradiation with γ-rays produced by radioisotopes 60Co, and hydrogen adsorption when the samples were put in an over pressured atmosphere of this gas. With this procedure it was possible to observe that the irradiation decreases the electrical conductivity of glassy carbon samples and the hydrogenation changes the sign of Seebeck coefficient. The material initially is an n-type semiconductor, but with hydrogenation changes to p-type semiconductor. X-ray diffraction analysis showed that the hydrogenated vitreous carbon is more amorphous than the pristine material and the γ-rays irradiation produces changes in the crystallite size and shape.

Effect of mechanical heterogeneity on the behavior of a crack in the weld heat affected zone: A numerical analysis

Abstract Both mechanical heterogeneity and crack position effects on the stress and strain fields at the crack tip of overmatching and undermatching weld metal joints are investigated. Analyses under sustained plane strain yielding conditions of a notch “near to” and “at” the interface between two elastic-plastic materials of a double edge notch tensile specimen have been carried out by finite element method. The results are used to study the implications of the difference in plastic properties on the behavior of a crack in the weld heated affected zone (HAZ), as well as to discuss its more critical position, regarding the fusion line, for the evaluation of HAZ weld fracture toughness. The contribution to the notch tip opening displacement of each material in heterogeneous cases, and the notch tip sliding displacement, have been related to remote load for the different cases considered.

An experimental investigation of blunting and stable crack growth processes of a crack in the weld HAZ

Abstract This work describes the crack tip blunting, initiation and stable crack growth processes of a crack localized in the heat affected zone of a weld. In the first stages of the crack tip blunting process, the same behaviour as that for a crack in a homogeneous medium under mode II was observed. It is also shown that, in the central zone of the thickness, two cracks grow in a stable manner from the blunted crack tip: one in tearing mode and another one in shearing mode. The experimental results were then analysed to discuss the implications for the evaluation of fracture toughness of the weld heat affected zone.

Spot welding of metal laminated composites

Composite materials of steel sheets joined by interlayers of zinc or lead- tin, show very good impact and corrosion resistance properties. Resistance spot-weld characteristics of these composite materials made of steel sheets and non-ferrous metals have been tested. Spot welds of composites with both zinc and lead- tin interlayers present good behaviour in peeling tests. Shear tests of the welds also show very high strength, probably as a consequence of simultaneous brazing because of the alloyed layer of non-ferrous material around the weld spot. This good welding behaviour enhances the possibilities of application of this composite.

Influence of the measurement errors of crack growth on fracture toughness evaluation. A monte carlo simulation

Abstract The effect of random error, which inevitably occurs in measurement of stable crack growth, on estimation of fracture toughness obtained by means of the multispecimen technique, is analyzed. Using the Monte Carlo simulation method, statistical distributions of the toughness parameters are generated, as well as the statistical distribution of the lower bounds as estimated by the model of simple regression. Two examples are analyzed, which are representative of the majority of cases which occur in practice, pointing out the dispersions which occur in toughness values as a consequence of errors in measurement of stable crack growth.

Stress and strain fields ahead of a crack in ductile materials. Application to approximate evaluation of fracture ductility and fracture toughness

Abstract In this paper a model is presented to obtain non-singular stress and strain distributions ahead of a crack in a ductile material. The model is based on both crack tip curvature effects from external loads and appearance of internal damage as a consequence of strong strain concentration. Both effects are coupling and affect the stress and strain fields. The results are utilized to do both plane strain ductility and fracture toughness evaluation. Those results are compared with experimental data from the bibliography.