Génie mécanique
Correlation between defect depth and defect length through a reliability index when evaluating of the remaining life of steel pipeline under corrosion and crack defects.
Despite the various engineering models existing for lifetime investigation of hydrocarbon steel pipelines under localized corrosion and crack defects there is still a lack of information about the correlation between the two main parameters characterizing the geometrical defect model, depth and length. So the aim of the present work is a contribution in evaluating the remaining life of the pipeline using reliability analysis in order to correlate the two parameters through one common parameter, the reliability index. As a first step, the investigation is carried out on four standard and well established engineering models; Irwin, SINTAP, ASME B31G and Modified ASME B31G methods, aiming to coordinate results given by each of the four models. The potential defect depth and length correlation through a reliability index can be used as a decision-making tool to give realistic answers for replacing and/or repairing a tube subjected to internal pressure and on which flaws of different depths and lengths have been detected from inspection of a pipeline after few years of service. Voir les détails
Mots clés : corrosion, Defect, Reliability, Correlation, Sensitivity
Reliability estimation of pressurized API 5L X70 pipeline steel under longitudinal elliptical corrosion defect.
The probabilistic approach is the best way to give realistic answers for design and maintenance and it is considered as a powerful decision-making tool. In the present paper, assessment of the failure probability of corroded pipeline subjected to internal pressure is estimated by using the first order reliability method (FORM). Measurements of defect dimensions have been achieved changed into collected by using ultra sound inspection probe through a 75-km pipeline portion on one of the main Algerian high pressure gas transportation that has been in service for 30 years. The sensitivity analyses have been carried out on random variables to identify the importance of the parameters within the reliability mechanical model. The modified B31G code has been coupled as a mechanical model, with Rackwitz optimization algorithm by using PHIMECA Software. The limit state function resumes the difference between the pipeline burst pressure and the pipeline operating pressure. The aim of the present work is to make in evidence the competitive importance of pressure service, wall thickness and the defect depth with regards to defect length and to determine the influence of the coefficients of variation on the failure probability and the remaining life of the pipeline. A diagram based on the reliability index results is proposed to predict the pipeline degradation and it can be used as a decision tool for maintenance program. Voir les détails
Mots clés : corrosion, Failure probability, Coefficient of variation, Modified ASME B31G
Engineering modeling of wear profiles in tooth flank of polyamidespur gears
The present paper is a contribution in developing an engineering model expressing the evolution of tooth flank wear in polyamide spur gears as a function of number of cycles. The method is based on first observing the behavior of wear on tooth flanks in real test conditions. Then progressive flank wear profiles are plotted in order to sort out the corresponding fitting curves together with the associated mathematical equations. The general engineering model controlling the evolution of wear profile is found of the form of quadratic equation where the variable is the circular thickness of a tooth at a given depth from the top land of the tooth. The respective parameters of the engineering wear equation are determined as a function of number of revolutions of the spur gear. A wear correction parameter kw is added in order to catch up the measuring errors when plotting the wear profile curves using the proposed engineering wear model for spur gears. The data are in good agreement with that obtained from experimental measurements. Voir les détails
Mots clés : polyamide, Spur gear, wear, Tooth flank, lubrication
EFFECT OF ELECTRICAL CURRENT ON FRICTION AND WEAR BEHAVIOR OF COPPER AGAINST GRAPHITE FOR LOW SLIDING SPEEDS
Copper-graphite is an important tribological material used in the applications of electrical sliding contact like generators and electrical brushes. A series of experimental tests were conducted on a pin-disc tribometer in air and dry sliding condition. The pair of material was subjected to electric current ranging from 0 to 10A, normal loads of 5 to 30N and sliding speed of 0.5m/s. The duration of each test was 30 minutes. Experimental results indicated that the friction coefficient decreases and wear rate increases with increasing load with and without applied electric current. The changes in surface chemistry and topography of the tribo-surfaces were characterized using Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). This later technique was used to analyze the transfer of pin materials to the counterface, and also to understand how copper and graphite influence the tribological properties. Results indicated that, electric current and normal load have more or less significant influence on the tribological behavior of the pair of materials and the effect of oxide layer created at interface of the pairs in contact. Voir les détails
Mots clés : friction, wear, Contact temperature, load, Electric current.
Estimated life time in an HDPE pipe with a semi-elliptical defect under internal pressure
High density polyethylene has been widely used in marine and gas transportation, storm sewers, culverts and city drainage system. Compared with other conventional pipes, it has its own advantages, such as, chemical and corrosion resistance, toughness, flexibility, easy splicing and consequent easy handling and installation. In this paper, and in order to predict the remaining lifetime of a pipe having a defect and subjected to internal pressure, it is necessary to estimate KI stress intensity factor during the propagation of the crack. The finite element method is used to assess the values of the stress intensity factor of an axially oriented semi-elliptical crack located at the inner surface of an HDPE pipe. The crack configuration is described by the relative wall thickness (t/R), the relative crack depth (a/t) and the crack aspect ratio (a/c). The lifetimes presented are discussed according to the size of the crack and the stress intensity factor in each case. Voir les détails
Mots clés : HDPE pipe, Crack, rupture, Finite Element Method FEM, stress intensity factor, Lifetime
Estimated life time in an HDPE pipe with a semi-elliptical defect under internal pressure.
High density polyethylene has been widely used in marine and gas transportation, storm sewers, culverts and city drainage system. Compared with other conventional pipes, it has its own advantages, such as, chemical and corrosion resistance, toughness, flexibility, easy splicing and consequent easy handling and installation. In this paper, and in order to predict the remaining lifetime of a pipe having a defect and subjected to internal pressure, it is necessary to estimate KI stress intensity factor during the propagation of the crack. The finite element method is used to assess the values of the stress intensity factor of an axially oriented semi-elliptical crack located at the inner surface of an HDPE pipe. The crack configuration is described by the relative wall thickness (t/R), the relative crack depth (a/t) and the crack aspect ratio (a/c). The lifetimes presented are discussed according to the size of the crack and the stress intensity factor in each case. Voir les détails
Mots clés : HDPE pipe, Crack, rupture, Finite Element Method FEM, stress intensity factor, Lifetime.
A method for mechanical property assessment across butt fusion weldedpolyethylene pipes
The use of high-density polyethylene pipes in gas and water distribution networks is steadily growing worldwide. If the resistanceof plain pipes is at present time well established using appropriately designed standards, welding issues continue to be globallyapproached equally in terms of structure and mechanical properties. Consequently, further practical investigations should beaimed at studying mechanical properties in the weld region which includes the melt zone and its heat-affected zones. This workpresents a method based on removing layers in order to assess localized variances in mechanical properties throughout the weldseam in both radial and circumferential directions. An experimental plan based on specific machining operations allowed testing39 standard specimens representing the weld volume matter in three concentric layers for given pipe dimensions and theircounterpart standard unwelded ones. The typical stress–strain behavior of semi-crystalline materials is preserved in weldedand unwelded specimens but with different characteristic limits. At the weld inner layers, properties such as elastic modulus,yield, and failure stresses displayed lower values, whereas in welded outer layers, the tendency is inversed. The cold drawingextend remained approximately steady for unwelded and welded cases across the pipe wall. This property is less affected by thepresence of the weld as it described a constant material flow which is mostly a function of available material quantity for yielding.The approach developed in this study gives consistent indications on welding quality around the pipe weld and across thethickness. Accordingly, outermost and innermost welded layers may exhibit lower or even bad-quality welds as imperfectionscan concentrate stresses at the joint interface because of cold weld problems. Such method enabled detecting 23% of failures atthe weld seam from outer and inner layers while the middle layer did not reveal any failure at the weld. The causes of thisbehavior are approached using crystallinity evolution in welded and unwelded pipes. Voir les détails
Mots clés : HDPE pipe . Butt fusion welding . Mechanical properties . Radial direction . Circumferential direction . St ru ct ur al variances
ETUDE ET SIMULATION DE L’ENDOMMAGEMENT DES PIECES MECANIQUES DE REVOLUTION
Les roues dentées, principales pièces mécaniques de révolution connaissent beaucoup d’amélioration grâce à l’apport de nouveaux matériaux comme les polymères. Cependant elles sont sujettes aux différents phénomènes de détérioration comme l’usure, la fatigue, le fluage, qui restent peu connus. La compréhension de ces phénomènes permet d’une part de mieux appréhender l’optimisation des paramètres de fonctionnement dans les applications industrielles et d’autre part d’augmenter leur durée de vie. Ainsi le présent travail s’inscrit dans ce cadre et consiste en une contribution à l’étude et simulation de l’endommagement des roues dentées en polyamides particulièrement l’usure des flancs des dents de roue.La première partie de cette étude est dédiée à l’approche expérimentale qui a permis de faire le suivi de l’évolution de l’usure d’une roue dentée en PA66, dans trois conditions de contact : à sec, dans l’eau et dans le mélange eau-détergent et aussi de voir le comportement tribologique des engrenages combinés, polyamide-polyamide. La deuxième partie consiste à analyser les résultats d’usure et de caractérisation du matériau avant et après usure afin de modéliser et de simuler le comportement à l’usure d’un engrenage en polyamide.Les résultats expérimentaux menés dans un cas réel comme le lave-linge qui a fait l’objet du banc d’essai, montrent que l’usure apparaît dans la saillie de la dent et est plus exprimée à la tête de la dent qu’au niveau du diamètre primitif. Par ailleurs, les dents travaillant dans l’eau subissent une usure qui mènent vers l’arrêt de la machine après 3.5 millions de cycles. Elle est deux fois plus rapide que l’usure observée dans une dent travaillant à sec. Les observations et analyses morphologiques du comportement à l'usure des dents par Microscopie Électronique à Balayage (MEB) permettent de constater que la détérioration dans les engrenages en PA66 est générée par une combinaison de deux types d’usures abrasion et adhésion avec un effet partiel de la déformation plastique dans certains endroits d’engrenage. C’est le cas de la dent d’injection où l’usure est plus exprimée.L’analyse profonde des profils de l’usure sur le flanc des dents a permis d’une part de développer un modèle géométrique qui montre l’évolution de l’usure, et d’autre part d’établir un modèle mathématique qui prend en compte le profil du nouveau flanc de la dent, provoqué par l’usure. Une loi polynomiale d’ordre deux est retenue sur la base de cinq mesures longeant le profil de la dent de la tête jusqu’au diamètre primitif. Un coefficient de correction de l’usure kw a été intégré pour corriger les courbes de tendance avec les courbes réelles. Enfin, cette loi prend en compte le comportement à l'usure dans les flancs des dents en fonction de la nature du milieu environnant pour servir d’un modèle de prédiction de la durée de vie des engrenages en polyamide. Voir les détails
Mots clés : tribologie, Engrenage, Polyamide PA66, Environnement, Modélisation.
Study of reliability index for high-density polyethylene based on pipestandard dimension ratio and fracture toughness limits
The reliability of high-density polyethylene (HDPE) pipes remains a central issue for gas transportation and distribution net-works. The objective of this study is to investigate the estimation of reliability index ( β) for a plastic pipe using the critical stressintensity factor ( KIC) as the maximum limit for safe operating conditions. Simulations are performed as a function of operatingpressure, crack length, and standard dimension ratio (SDR) for three fracture toughness levels (low, moderate, and high). Inaddition, the study compares results from three hoop stress calculations methods (thin, thick, and ISO plastic pipe equation).Based on design recommendation for reliability index, it is found that both operating pressure and crack length show comparablebehaviors. However, the thick wall pipe results overestimate (β) for every KIClevel. In all cases, it is found that the higher thecritical stress intensity factor, the better the reliability index. Results obtained with the standard ISO pipe formula are morerealistic, as they are usually around the design recommendation, i.e., SDR basis indicates that it is a true conservative designapproach incorporating both upper and lower thickness limits. The importance of all variables (thickness, diameter, crack length,pressure, and fracture toughness) is also discussed Voir les détails
Mots clés : HDPE pipe . Critical stress intensity factor . Standard dimension ratio (SDR) . Crack length . Reliability index . Importance of variables
Reliability Index of HDPE Pipe Based on Fracture Toughness
his work presents a contribution to evaluate the reliability of a high density polyethylene (HDPE) pipe using the PHIMECA Software. The critical stress intensity factor (KIC) is adopted as a criterion to the maximum limit of a numerically calculated KI. The reliability index β is obtained using failure probability and a mechanical model. It is found that at lower KIC, no safe domain for actual service pressures existed while for moderate and higher values of KIC (above 3.5 MPa.√m); the β design index is reached and even exceeded. In terms of increasing crack length, β decreased systematically for all toughness cases supporting the idea that reliability and fracture toughness designate similar properties for service life or material resistance to cracking. For a KIC=5 MPa.√m, the pipe is considered safe when crack length is below 370μm. Finally, it is shown that SDR basis is a reasonable and conservative design approach for plastic pipes. Voir les détails
Mots clés : HDPE pipe, critical stress intensity factor, crack length, SDR, reliability index, PHIMECA software