Charca-Mamani, Samuel

Profile Picture

Filters

2005 - 20092
Reset filters

Settings

Citations

Publication Search Results

Now showing 1 - 2 of 2
  • Thumbnail Image
    Publication
    Study of hydrogen permeation and diffusion in steels
    (2005) Charca-Mamani, Samuel; Uwakweh, Oswald; College of Engineering; Basir Shafiq, A.; Just, Frederick; Perez, Nestor; Department of Mechanical Engineering; Marcelo Suarez, Oscar
    The high mobility of hydrogen is due to the relative small size compared to the atomic size of transition metals. However, the presence of defect (dislocations, voids, grain boundaries), can hinder hydrogen transport as they are potential trapping sites. The main objective is to study the hydrogen diffusivity and permeability in steels; it is possible to perform an electrochemical hydrogen permeation process in a thin film (foil or sheet) as a function of certain variables such as prior cold work, thickness, polarization charging current, grain size, electrolyte medium, type of promoter, and concentration. Devanathan and Stachurski (DS) cell is adapted to the study of hydrogen cathodic charging and permeation behavior. Cold work in Armco-Fe sample (cold rolled condition) increases the dislocation density with these sites acting as irreversible trapping sites as reflected in the reduction of diffusivity and permeability parameters. Same behavior was observed for grain boundary. In both cases the trap binding energy obtained was 20.81KJ/mol and 15.29KJ/mol for Armco-Fe respectively. Promoters added into the acid solution (charging electrolyte medium) accelerate hydrogen entry in to the material. The promoter that is best in acid solution (pH=1.2) is compose of arsenic, in a concentration range of 0.25 to 1.00 g/l g/l Na2HAsO4 7H2O. Furthermore, the charging surface roughnesses have a significant effect in hydrogen permeation due to the reduction of rate of hydrogen ingress into the material. Based on desorption test conducted on prior hydrogen charged materials at room temperature, it appears that the solubility of hydrogen in AF1410 steel is about three times approximately higher than in Armco-Fe. As is well known hydrogen leads to a reduction in plasticity due to its embrittling properties, and consequently the effect in the reduction of fatigue life. The fatigue test performed was based on load decrement, showed a fatigue life reduction of approximately 45% than in specimen tested in air. Additionally, SEM image showed a brittle fracture surface (intergranular combined with transgranular), in the areas with presence of high hydrogen concentration.
  • Thumbnail Image
    Publication
    Experimental study of wave slamming of sandwich composites
    (2009) Charca-Mamani, Samuel; Shafiq, Basir; College of Engineering; Godoy, Luis A.; López, Ricardo R.; Suárez, Luis E.; Just, Frederick; Department of Civil Engineering; Otero, Ernesto
    Slamming of ship hull structures was simulated using sandwich composite panels that were repeatedly slammed on to a body of calm water with the main objective of understanding the damage accumulation mechanism and corresponding lifetime. Literature is abundant on ship hull slamming; however, it is limited to single slamming; while damage accumulation and progression under repeated slamming is largely absent. Therefore, an extensive experimental program was carried out to understand damage accumulation and failure in sandwich composites under repeated slamming as a function of deadrise angle and slamming energy. The two model material systems used consisted of polyester foam filled honeycomb sandwich composites and polyurethane foam core sandwich composites. Honeycomb core sandwich composites indicted a significant damage accumulation as a function of increasing slamming energy. Similarly, foam core sandwich composites revealed a gradual but substantial damage accumulation as a function of increasing slamming energy or decreasing deadrise angle. The modes of failure corresponded primarily to local facesheet yielding with evidence of core crushing for the honeycomb core sandwich composites. While, the modes of failure indicated mainly interface tearing, core shear and facesheet buckling in the case of foam core sandwich composites. Interestingly, the peak pressures and strains were observed to occur near the keel while the maximum damage was obtained near the chine at deadrise angles between 15o and 20o ; as ship hull design is primarily based on peak pressures, this result is quite significant.