Fracture mechanics of creeping solids applied to pre-cracked NOL ring specimens to predict residual lifetime of high density polyethylene pipes

International audienceHigh Density Polyethylene (HDPE) is widely used for the distribution of drinking water and are exposed to an internal pressure due to water flow. Furthermore, when they are in contact with disinfectants, oxidation of HDPE occurs at the immediate surface of the inner wall. This leads to a decrease in the HDPE molar mass and consequently, to a hardening as well as an embrittlement of the material. The oxidised layer thickness seems to stabilize at 200 μm whatever the initial pipe thickness due to the diffusion of reactive species in the inner wall. Inspections with a scanning electron microscope (SEM) of the inner wall of pipes collected on site, after several years of service, showed a network of cracks. The most noxious (deepest) longitudinal crack propagates under the steady internal pressure until the complete failure of the pipe. The experimental investigations consisted of creep crack growth tests, carried out on an original geometry for this type of test using NOL ring. These specimens were cut from the pipes and an internal longitudinal crack implanted. Creep crack growth tests were performed at various net stresses and at various crack depth ratios. At the end of each test, the time to failure was recorded. Before applying the theory of fracture mechanics of creeping solids, 3D finite element simulations were carried out to assess the suitability of assuming plane strain conditions. To this end, a porous viscoplastic model was implemented into an in-house finite element code. A fracture criterion based on critical porosity allowed for the simulation of creep crack growth. The localization of the maximum damage at mid-thickness and the thickness reduction were well captured. In these conditions, the crack front curvature is located near the surface so that a 2D calculation with plane strain conditions can then be justified to determine the C* load parameter. The residual lifetime of a real pipe containing a longitudinal defect, under in-service loading was estimated by using the correlation between the time to failure and the C*-integral

Experimental study of the crack depth ratio threshold to analyze the slow crack growth by creep of high density polyethylene pipes

International audienceTo assess the durability of drinking water connection pipes subjected to oxidation and slow crack growth, a comprehensive database was constructed on a novel specimen geometry: the pre-cracked NOL ring.135 tests were carried out consisting of initial crack depth ratio ranging from 0.08 to 0.6; single or double longitudinal cracks: tensile with steady strain rate and creep loading. A threshold value of the crack depth ratio of 0.2, induced by the oxidation was determined by analyzing several mechanical parameters. This threshold value was shown to be independent on the strain rate effects, single or double crack configuration and the kind of loading: tensile or creep. Creep test results with crack depth ratio larger than 0.2 were then utilized to establish a failure assessment diagram. A methodology allowing the prediction of residual lifetime of in-service pipes was proposed, using this diagram

Interactions chimiques des tubes en polyéthylène avec les désinfectants en eau potable

Distribuer une eau potable de qualité constante et conforme aux impératifs sanitaires constitue une des préoccupations majeures de VEOLIA EAU. Le respect de cette exigence implique à la fois une maîtrise des facteurs qui influencent la qualité de la ressource et une connaissance approfondie de l’état structurel des réseaux. Dans le cadre du consortium VEOLIA/Suez/Saur/ENSAM, il est clairement démontré depuis 2006 que le dioxyde de chlore présent dans l’eau distribuée consomme les antioxydants contenus dans le polyéthylène. Cette altération est ciblée en paroi interne sur le premier millimètre de tuyau en contact avec l’eau. Elle s’accompagne d’une oxydation du polyéthylène qui induit une fragilisation structurelle du branchement et explique les ruptures prématurées observées sur sites depuis 2003. Ce phénomène est accentué par la température, la pression et leurs variations dans le réseau. Cet article porte sur l’interaction chimique du polyéthylène avec le dioxyde de chlore et le chlore, et leurs conséquences respectives sur la tenue mécanique résiduelle du branchement. À partir d’essais à l’échelle pilote en conditions de vieillissement accéléré au dioxyde de chlore (70 mg.L-1, 40 °C) et de prélèvements «terrain» sur des sites exploités en conditions moyennes d’utilisation (~0,5 mg.L-1 ClO2, 20 °C, 3 bar), un mécanisme de rupture mécanique d’un branchement PEHD en service a été établi et sert de base pour les futures expertises en routine. L’oxydation du polyéthylène par le chlore a été démontrée à l’échelle du laboratoire en conditions extrêmement accélérées (400 et 4000 mg.L-1 de chlore). L’examen de prélèvements «terrain» exploité en conditions particulières (0,8–1 mg.L-1 de chlore résiduel, 8 bar, Teau = 25−27 °C) met en évidence des altérations semblables (couche oxydée et réseau de microfissures en paroi interne) à celles observées en présence de dioxyde de chlore en conditions moyennes d’exploitation. Le tube multicouche Excel Plus®, élaboré en partenariat avec ALIAXIS R&D, ARKEMA et VEOLIA Environnement, offre une réponse concrète à l’action de ces désinfectants et se place comme un candidat pertinent pour le remplacement des branchements PEHD

The "Woundosome" Concept and Its Impact on Procedural Outcomes in Patients With Chronic Limb-Threatening Ischemia

This editorial assembles endovascular specialists from diverse clinical backgrounds and nationalities with a global call to address key challenges to enhance revascularization in chronic limb-threatening ischemia (CLTI) patients.- Dedicated below-the-ankle (BTA) angiography and revascularization is underutilized in ischemic foot treatment. Existing guidelines do not address comprehensive BTA vessel analysis. CLTI trials also often lack data on in-line arterial flow to the ischemic lesion and BTA vessel evaluation, hindering outcome assessment.- Dedicated multi-planar angiographic evaluation of the distal microcirculation is key: Direct arterial flow or good-quality collaterals are crucial in influencing wound healing and need to be assessed diligently to the level of the distal ischemic wound territory, termed “woundosome.”- An important primary emphasis of future trials should be on validating technologies and strategies for assessing tissue perfusion before, during, and after revascularization undertaken to heal tissue loss in CLTI patients. This will allow determination of a potentially significant delta in tissue perfusion prior to and following intervention at the “woundosome” level. Once changes in arterial perfusion have been identified as positively correlated to wound healing, these could serve as a much-needed novel primary technical outcome measure for patients with tissue loss undergoing surgical, hybrid, or endovascular revascularization