A Numerical Study Comparing The Effect on Residual Stresses of Two Different Types of Projectiles During Shot Peening

Shot peening is a widely used technique to improve fatigue life in metallic alloys. This processing technique introduces a subsurface compressive residual stress field through a plastic deformation of the surface caused by the impact of a large number of high-speed projectiles. There are a number of parameters that affect the residual stress field depth and magnitude. The effects of the impact angle, shot speed and shot geometry are currently being researched. In particular, substituting spherical cast shots by cylindrical cut wire shots is an attractive option, especially in terms of cost. The effect of shot geometry on residual stresses, however, needs to be further investigated. Because industrial-scale experimentation is costly and cumbersome, mathematical modeling offers a convenient alternative to carry out this type of research. The present work shows a comparison between the residual stresses generated by the impact of spherical and cylindrical projectiles on a steel substrate. This threedimensional model was developed using ABAQUS finite element commercial software (Release 6.12, Dassault Systémes, France). The results show that cylindrical shots generate residual stress fields that are higher in magnitude than those generated by a spherical shot. However, the residual stress field of cylindrical shots impacting the surface at an oblique angle shows an important degree of asymmetry. This effect is not found when spherical shots impact the surface at the same oblique angle.     

The surface modification of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymers to improve the attachment of urothelial cells

Biocompatible and biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(HB-co-HHx)] substrates were modified to improve the attachment of porcine urothelial cell culture. The pristine copolymer exhibits excellent mechanical properties to replace the bladder tissue, but its surface lacks chemical functionalities to interact with cells. Thus, wet chemical treatments based on NaOH and ethylenediamine in aqueous [ED(aq)] and isopropanol [ED(isoOH)] media to functionalize the P(HB-co-HHx) films surfaces were compared. Among these treatments, short ED(aq) treatment was able to decrease the hydrophobicity, rendering a surface with amino groups and without a significant alteration of the mechanical properties. Furthermore, to enhance the interaction with urothelial cells, laminin derived YIGSR sequence was covalently bound to these amino functionalized substrates. The focal attachment was clearly improved with this last treatment, comparing with those results found with the unmodified and first-step functionalized P(HB-co-HHx).     

Structural Ultrafine Grained Steels Obtained by Advanced Controlled Rolling

Steels with ultrafine grains (lower than 5 μm), which usually known as ultrafine ferrite or ultrafine grained materials, are presently the object of intense research, because of the improvement in resistance and fracture toughness they may reach compared to conventional steels (with grain sizes above this value). It is shown that the forenamed steels designated in the Euronorm EN 10149-2, which are manufactured by advanced techniques of controlled rolling and mainly used in automotive industry, have an ultrafine grain size in the range of 2.5 to 3.5 μm, and with elastic yield stresses higher than 400 MPa. Based on the Morrison-Miller criterion, it is shown that values of the strain-hardening coefficient lower than 0.08 would make the industrial application of these steels unfeasible.     

Effect of the High-Temperature Deformation on the M s Temperature in a Low C Martensitic Stainless Steel

The effects of cooling rate and plastic deformation at 1100 °C on the start of the austenite-to-martensite transformation in a low carbon Fe-12Cr stainless steel were investigated by dilatometry and thermal analysis of pre-strained samples. The experimental M _s was found constant (397.2 °C) for cooling rates between 15 and 30 °C/min. Outside this range, the M _s apparently decreases as the cooling rate decreases below 15 °C/min or increases above 30 °C/min. An overall average value of 380.9 ± 11.8 °C was observed within the complete range of cooling rates investigated (8-40 °C/min). This value is 50 °C higher than that predicted from the chemical composition of the steel and Andrews’ equation for M _s. Prior plastic deformation of austenite at 1100 °C increases the M _s temperature and broadens the martensitic domain (M _f-M _s). This effect increases as the magnitude of the applied pre-strain increases.     

Self-feed device behaviour evaluation designed for assessment of operability of coated electrodes

The objective of the present work is to evaluate the effectiveness of a self-feed device for coated electrodes. This device is able to eliminate disturbances from manual operation or the use of automatic control systems during the study of coated electrode behaviour. The conjugation of different methods such as short-circuit time statistical analysis and metallographic analysis (penetration, heat-affected zone, shape and mean distance between successive solidification fronts) together with the analysis of the electric arc and electrode melting parameters allows us to obtain relationships between arc behaviour, mass transfer processes and bead appearance. This permits the extraction of criteria to describe process stability and the behaviour of the feed device employed. An appropriate statistical methodology was obtained to process the ‘duration of the short-circuit’ parameter through an appropriate adjustment of empirical distributions to a log-normal model. The self-feed device for coated electrode welding allows the repeatable and reproducible execution of welding beads in the flat position, keeping the arc length based on physical and chemical characteristics of the electrode during welding trials using 125, 140 and 160 A.