Key Engineering Materials Vols. 348-349

Abstract: A hierarchical multiscale procedure for softening materials is proposed in this paper. A standard multiscale model has been analysed with respect to macro-level mesh dependence and meso-level cell size dependence. In order to eliminate spurious macro-level mesh dependence and meso-level cell size dependence a coupled-volume approach has been proposed. A discussion on the various interacting length scales in the model is included.

Abstract: This paper concerns about a failure analysis of an electric all aluminum alloy conductor (AAAC) damaged and broken for fretting fatigue phenomena induced by aeolian vibrations. Life of electric conductors is often reduced by various degradation mechanisms such as repeated bending, fluctuating tension, distortion, fatigue, wear and corrosion phenomena. However the main limiting factor of the electrical conductors is related to aeolian vibrations in the high frequency range (between 5 to 50 Hz). Conductor oscillations may lead to fretting fatigue problems (otherwise called fretting wear) caused by wind excitation, mainly in the suspension clamp regions, spacers or other fittings. The induced aluminium wire fracture imply a drastic reduction in the transmission line service. Vibration dampers are considered the most effective method to extend service life of electric conductors, as they are the means to reduce fretting damage of aluminium wires. The aim of the present work is to investigate the failure of an AAAC conductor of a 400kV overhead transmission line (twin conductors) located in Touggourt Biskra (Algeria); the damaged and broken conductors were operated in-service only for six months without spacers or dampers. Three different types of conductors have been taken as experimental samples: the in-service broken conductor, another in-service damaged conductor and a new conductor from warehouse as terms of comparison. Samples have been analysed to identify the root cause of the failure and to verify the conformity of the conductor elements to the international standards. The investigation has outlined the morphology of the fretting damage: in all cases the fractured wires have shown typical static deformation marks and dynamic fretting wear tangential marks associated with intense presence of Al2O3 debris.

Abstract: The fatigue behavior assessment of high strength low alloy steel has main importance for reliable and safe service of welded structure. The mechanisms of fatigue crack initiation and propagation always represent a potential danger of crack propagation during the exploitation of a structure, which can lead to catastrophic failure. If the fatigue’s crack appear at the welded structure it is important to reliable find loading regime which can reduce risk of failure for still operating structure.

Abstract: This paper deals with the application of Cohesive Zone Model (CZM) concepts to study mode I fracture in adhesive bonded joints. In particular, an intrinsic piece-wise linear cohesive surface relation is used in order to model fracture in a pre-cracked bonded Double Cantilever Beam (DCB) specimen. Finite element implementation of the CZM is accomplished by means of the user element (UEL) feature available in the FE commercial code ABAQUS. The sensitivity of the cohesive zone parameters (i.e. fracture strength and critical energy release rate) in predicting the overall mechanical response is first examined; subsequently, cohesive parameters are tuned comparing numerical simulations of the load-displacement curve with experimental results retrieved from literature.

Abstract: The paper presents a simplified version of the Notch Stress Intensity Factor (NSIF) approach useful for fatigue strength assessments of welded joints. The evaluation of the NSIF from a numerical analysis of the local stress field usually needs very refined meshes and then large computational effort. A relationship is proposed here to estimate the Notch Stress Intensity Factor from finite element analyses carried out by using a mesh pattern with a constant element size. The main advantage of the presented relationship is that only the elastic peak stress numerically evaluated at the V-notch tip is necessary to estimate the NSIF instead of the whole stress-distance set of data (that is why the method has been called Peak Stress Method, i.e. PSM). An application of the PSM to fatigue strength assessment of fillet welded joints made of structural steels and aluminium alloys under tensile or bending loads is presented. In those joints, only mode I stress distribution is singular at the weld toe due to the presence of a V-notch angle equal to 135 degrees.

Abstract: This paper reports on the use of the Modified Wöhler Curve Method (MWCM) applied along with the Theory of Critical Distances (TCD) to estimate fatigue lifetime of steel welded joints subjected to both uniaxial and multiaxial cyclic loading. In a recent work [1] we have proved that the above engineering method is highly accurate when calibrated by using standard fatigue curves characterised by a probability of survival equal to 50%. In order to better check its accuracy and reliability, in the present study our approach is systematically applied to a large amount of experimental data by calibrating it using standard fatigue curves having a probability of survival equal to 97.7%. This exercise allowed us to prove that the in-field application of such an engineering procedure results in estimates which fully comply, from a statistical point of view, with Eurocode 3’s recommendations. This result strongly supports the idea that our approach can safely be employed to perform the fatigue assessment of real mechanical assemblies, with the advantage over other existing methods that fatigue lifetime under any kind of fatigue loading can be estimated by simply post-processing linear-elastic Finite Element Models.

Abstract: The microbiological influenced corrosion (MIC) behaviors of the ship plate steel directly exposed in different medias (the sterile seawater, the ferrous bacteria solution and the sulfate-reducing bacteria solution) were investigated with electrochemical impedance spectroscopy (EIS), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Corrosion potential, electrochemical impedance and micrographs of specimens under different experimental conditions were obtained. Results show that the FB and SRB in the marine environment affect the corrosion behavior of the ship plate steel greatly. The corrosion process in FB and SRB environment was controlled by both bacteria and corrosion products. The mechanism of MIC is discussed.

Abstract: The purpose of this study is to establish an analytical formulation for determination of the critical chloride content in concrete. In this study, factors affecting the critical chloride content such as mix proportion, environment, chemical evolution of pore solution with time, carbonation and so on are taken into account. The numerical simulation program of cementitious materials, HYMOSTRUC was utilized for the construction of the formulation. This was expressed as a free chloride content in a mass unit of concrete as a time function.