Key Engineering Materials Vol. 449

Abstract: The electrochemical behaviour of two carbon steels exposed to the acidic solutions and their resistance to hydrogen induced cracking (HIC) is evaluated in order to determine the effect of hydrogen damage on the failure process of the steels used in line pipe and casing at a geothermal plant. MethodologyApproach: Samples of two different steels, ASTM A-53 grade B (line pipe) and API L-80 (casing) were immersed during 96 hours in the electrolyte proposed by NACE to evaluate the susceptibility to HIC. Samples of the two steels embedded in non-conducting bakelite were exposed to potentiodynamic polarisation at room temperature using brines obtained from different wells at Cerro Prieto geothermal plant as electrolyte. Hardness tests were performed before and after the HIC test on samples in order to determine hardness changes induced by hydrogen penetration, as field results indicated embrittlement of the steels after 4 months of service. Findings: The steels, ASTM A-53 grade B and API L-80 did not present cracking sensitivity, as no cracks were observed in the tests specimens, although they showed an increase in hardness. The steels presented high corrosion rate in the brine media at room temperature (3.3 mm/year), which is expected to increase at high temperatures. Originality: This work revealed that carbon steels used at line pipes and casing at geothermal plants present high resistance to hydrogen induced cracking; however they corrode at high rates and present embrittlement. It is suggested that due to the high operation temperature the damage induced by hydrogen results in hardness increase but was not sufficient to crack development.

Abstract: Fatigue limit for surface crack initiation on shot peened Aluminium Alloys resulted to be determined by surface residual stress and stress concentration, according to the magnitude of the applied stress. The effect of surface roughness considered as micro-notches (dents) has been analysed by using the Vallellano and Navarro’s formulation. It was also determined that the stress gradient associated to those dents dominates over the closure stress effect.

Abstract: A study of the mechanical behavior of three composite materials under compressive, flexural, hardness and impact stress is presented in this research. The experimental work consisted on manufacturing and characterizing the composites. Firstly, the volcanic ashes, clay, water and other adherent ingredients were mixed. Then, this mixture was winterized and vitrified to obtain the ceramic matrix composite. On the other hand, the ashes were mixed with epoxy resin and polyester resin, individually, to origin two polymer matrix composites. Next, physical and mechanical tests were done to determine their physical and mechanical properties. The hardness, compressive, flexural, and impact stresses were determined and analyzed to know the mechanical behavior of the materials and to study their failure. Due to the fact that the mechanical properties of these composites are strongly affected by characteristics of their microstructure, their physical properties such as density, porosity and water absorption were evaluated. Also, an analysis of the materials’ microstructures in the scanning electron microscope and X-ray diffractometer was made. Finally, it could be observed that finer grain size helped reduce the volumes of open pores and stresses that develop at grain boundaries due to anisotropic expansion and contraction in the microstructures. Ceramic composites failed because of their low fracture toughness. However, this mechanism of failure in the polymer matrix composites was different, as the materials were subject to stresses. Considerable heating occurred near some pores and the inter-relationships between fatigue and creep. The results indicate that these products can be used like an added value in the industrial sector.

Abstract: In this paper is presented a study of the analysis of the stress state of micro specimens exposed to a load inside the column of a High Voltage Electron Microscope (HVTEM), in particular for a Mo single crystal. The experimental tangential stresses distribution images were obtained and compared with theoretical calculations using Naiber approach, obtaining an excellent match.

Abstract: It is well known that the formation of a highly hardening state of the standard steels is related with the improvement of its mechanical characteristics, which means an increment in the ductility of the fracture. Commonly, the ductility of the fracture is measured by means of mechanical methods involving large size and geometrical special shape of the sample. Satisfy these requirements in the initial steps of the development of a new material, or during the treatment for improving the hardness, is a dif ficult task. The present work shows the theoretical determination of the ductility of the fracture when the researchers have a small piece of the sample. Also, from the analysis of microscopy images, by the first time, it is demonstrated that an ultra-hardening state of the steel corresponds to a nano-fragmented dislocational structure in accordance to the theory.

Abstract: This work deals with very high cycle fatigue tests on high strength steel subject to the corrosive action of salt solution. Tests were carried out at constant load ratio R = -1 on corrosion treated and non-treated specimens and sea water flow specimens of R5 steel, which according to an International Classification Society of offshore systems is steel graded with the highest ultimate tensile strength to manufacture mooring chains and accessories intended to position mooring applications such as mooring of mobile offshore units, mooring of floating production units and mooring of offshore loading systems. Fatigue endurance on pre-corroded specimens was quite dispersed but with a mean value of 360 MPa, whereas for the non corroded specimens the fatigue endurance was located between 420 MPa at 105 - 106 cycles, and 380 MPa at 109 cycles. Concerning the sea water flow specimens, important increase in fatigue life is observed in reducing the applied load. Dispersion on fatigue endurance for the pre-corroded and sea water flow specimens was related to the complex process of corrosion on the specimen surface; nevertheless, the resulting pitting holes were associated to fracture origin in these specimens. Furthermore, fatigue failure origin was systematically localized at a circular, elliptical or irregular surface pitting hole formed during the corrosion process; special attention was focused on the pitting holes related to fatigue failure. Finite element results were obtained for the hemispherical voids placed on the specimen surface in order to determine the corresponding stress concentration; these results were associated with the real fracture origin pitting holes observed on specimens with pre-corrosion and sea water flow corrosion. Finally, some conclusions were obtained concerning pitting holes, stress concentration and fatigue life for the pre-corroded and sea water flow corroded specimens.