(Article) Fracture of rigid solids: a discrete approach based on damaging interface modelling

International audienceWe describe the progressive and delayed fracture of rigid solids by a discrete modelling. Each rigid solid is considered as an assembly of particles with initial cohesive bonds, the latter decreasing progressively during the loading. A damaging interface model is proposed to describe this progressive phenomenon. The model has been implemented in a discrete element code. The first illustrative example, which is actually a parametric study, deals with the progressive damage and sudden fracture of a single interface submitted to an uniaxial tension. The second example is related to the crushing of an assembly of rigid solids--i.e. a granular medium--submitted to an oedometric compression

Discrete element models for grain breakage

International audienc

A discrete model for fracture of rigid solids based on a damaging interface

We describe the progressive and delayed fracture of rigid solids by a discrete modelling. Each rigid solid is considered as an assembly of particles with initial cohesive bonds, the latter decreasing progressively during the loading. A damaging interface model is proposed to describe this progressive pehenomenon. This model has been implemented in a numerical code based on a discrete element method. The illustrative example is related to the crushing of an assembly of rigid solids - i.e. a granular medium - due to an œdometric compression

Global economic burden of unmet surgical need for appendicitis

Background: There is a substantial gap in provision of adequate surgical care in many low-and middle-income countries. This study aimed to identify the economic burden of unmet surgical need for the common condition of appendicitis. Methods: Data on the incidence of appendicitis from 170 countries and two different approaches were used to estimate numbers of patients who do not receive surgery: as a fixed proportion of the total unmet surgical need per country (approach 1); and based on country income status (approach 2). Indirect costs with current levels of access and local quality, and those if quality were at the standards of high-income countries, were estimated. A human capital approach was applied, focusing on the economic burden resulting from premature death and absenteeism. Results: Excess mortality was 4185 per 100 000 cases of appendicitis using approach 1 and 3448 per 100 000 using approach 2. The economic burden of continuing current levels of access and local quality was US $92 492 million using approach 1 and$73 141 million using approach 2. The economic burden of not providing surgical care to the standards of high-income countries was $95 004 million using approach 1 and$75 666 million using approach 2. The largest share of these costs resulted from premature death (97.7 per cent) and lack of access (97.0 per cent) in contrast to lack of quality. Conclusion: For a comparatively non-complex emergency condition such as appendicitis, increasing access to care should be prioritized. Although improving quality of care should not be neglected, increasing provision of care at current standards could reduce societal costs substantially

Pooled analysis of WHO Surgical Safety Checklist use and mortality after emergency laparotomy

Background The World Health Organization (WHO) Surgical Safety Checklist has fostered safe practice for 10 years, yet its place in emergency surgery has not been assessed on a global scale. The aim of this study was to evaluate reported checklist use in emergency settings and examine the relationship with perioperative mortality in patients who had emergency laparotomy. Methods In two multinational cohort studies, adults undergoing emergency laparotomy were compared with those having elective gastrointestinal surgery. Relationships between reported checklist use and mortality were determined using multivariable logistic regression and bootstrapped simulation. Results Of 12 296 patients included from 76 countries, 4843 underwent emergency laparotomy. After adjusting for patient and disease factors, checklist use before emergency laparotomy was more common in countries with a high Human Development Index (HDI) (2455 of 2741, 89.6 per cent) compared with that in countries with a middle (753 of 1242, 60.6 per cent; odds ratio (OR) 0.17, 95 per cent c.i. 0.14 to 0.21, P <0001) or low (363 of 860, 422 per cent; OR 008, 007 to 010, P <0.001) HDI. Checklist use was less common in elective surgery than for emergency laparotomy in high-HDI countries (risk difference -94 (95 per cent c.i. -11.9 to -6.9) per cent; P <0001), but the relationship was reversed in low-HDI countries (+121 (+7.0 to +173) per cent; P <0001). In multivariable models, checklist use was associated with a lower 30-day perioperative mortality (OR 0.60, 0.50 to 073; P <0.001). The greatest absolute benefit was seen for emergency surgery in low- and middle-HDI countries. Conclusion Checklist use in emergency laparotomy was associated with a significantly lower perioperative mortality rate. Checklist use in low-HDI countries was half that in high-HDI countries.Peer reviewe

Global variation in anastomosis and end colostomy formation following left-sided colorectal resection

Background End colostomy rates following colorectal resection vary across institutions in high-income settings, being influenced by patient, disease, surgeon and system factors. This study aimed to assess global variation in end colostomy rates after left-sided colorectal resection. Methods This study comprised an analysis of GlobalSurg-1 and -2 international, prospective, observational cohort studies (2014, 2016), including consecutive adult patients undergoing elective or emergency left-sided colorectal resection within discrete 2-week windows. Countries were grouped into high-, middle- and low-income tertiles according to the United Nations Human Development Index (HDI). Factors associated with colostomy formation versus primary anastomosis were explored using a multilevel, multivariable logistic regression model. Results In total, 1635 patients from 242 hospitals in 57 countries undergoing left-sided colorectal resection were included: 113 (6·9 per cent) from low-HDI, 254 (15·5 per cent) from middle-HDI and 1268 (77·6 per cent) from high-HDI countries. There was a higher proportion of patients with perforated disease (57·5, 40·9 and 35·4 per cent; P < 0·001) and subsequent use of end colostomy (52·2, 24·8 and 18·9 per cent; P < 0·001) in low- compared with middle- and high-HDI settings. The association with colostomy use in low-HDI settings persisted (odds ratio (OR) 3·20, 95 per cent c.i. 1·35 to 7·57; P = 0·008) after risk adjustment for malignant disease (OR 2·34, 1·65 to 3·32; P < 0·001), emergency surgery (OR 4·08, 2·73 to 6·10; P < 0·001), time to operation at least 48 h (OR 1·99, 1·28 to 3·09; P = 0·002) and disease perforation (OR 4·00, 2·81 to 5·69; P < 0·001). Conclusion Global differences existed in the proportion of patients receiving end stomas after left-sided colorectal resection based on income, which went beyond case mix alone

Une modélisation discrète du comportement mécanique des enrochements

Rockfill are made by coarse materials, blocks, whose size can reach one meter. Rockfill dams, composed by these blocks, show some relatively important deformations during time and during their filling or during an accidental flood. These deformations seems to be related to blocks ruptures in the inner structure, but they are unfortunately unknown after the dam construction.The settlements' prevision is then particularly important to ensure the structural integrity of the dam in a long time.The granular media behaviour is strongly related to the discrete nature of the medium. A discrete model is proposed to take into account rockfill particularities with local parameters that have clear physical sense. Ruptures are rockfill major features. The major idea in this work, consisted in developing a numerical discrete model to account for progressive and delayed blocks failure. Each block is considered as an assembly of particles with initial cohesive bonds which can decrease progressively during the loading. A damaging interface model is proposed to describe this progressive phenomenon. The model is composed by two yield surfaces (damage and fracture yield surfaces). A characteristic time is related to the damage evolution. The water influence is introduced by couplage with the parameters of the cohesion model, by decrease of the wedge and of the local friction. The modelling adopted is based on the “Non Smooth Contact Dynamics Method”, where grains and particles are rigid. The model is implemented on the discrete element code, LMGC90.Numerical simulations are realised at contacts, block and rockfill scales. First simulations consisted in modelling an assembly of unbreakable blocks disposed in a rockfill column and progressively filled by water. Simulations reveal that the buoyancy forces and the diminution of the local friction produce only low displacements in the medium. These simulations verify that blocks fracture is the major phenomenon responsible of the settlements of rockfill dams.Simulations of compression of breakable blocks and oedometric compression of an assembly of these blocks are realised. Results show that bonds ruptures and deformations are well correlated. Numerical simulations of oedometric compression tests composed by rockfill reveal some creep and relaxation responses. These tests reproduce the basic phenomena with only few parameters.Les enrochements sont des matériaux granulaires constitués de blocs rocheux dont la taille peut atteindre plusieurs dizaines de centimètres. Les barrages en enrochements, constitués par ces matériaux grossiers, présentent des déformations relativement importantes au cours du temps et peuvent également tasser au moment de leur remplissage ou lors d'une crue accidentelle. Ces déformations semblent liées à des ruptures de blocs à l'intérieur de l'ouvrage, mais ne sont malheureusement pas connues après la construction. La prévision de ces tassements est donc particulièrement importante pour s'assurer de l'intégrité structurelle de l'ouvrage à long terme.Le comportement des matériaux granulaires étant fortement lié à la nature discrète du milieu, un modèle discret est proposé afin de prendre en compte les particularités des enrochements avec des paramètres locaux au sens physique clairs. Les phénomènes de rupture sont les principales caractéristiques des enrochements. La démarche retenue dans ce travail s'est attachée à développer un modèle numérique discret capable de prendre en compte la fissuration progressive et différée des blocs rocheux. Chaque bloc est représenté par un assemblage de particules, initialement liées par une cohésion qui peut diminuer progressivement au cours du chargement. Un modèle d'endommagement interfacial est proposé pour décrire cette décroissance progressive au cours du temps. Le modèle comporte deux surfaces seuil (endommagement et de rupture). Un temps caractéristique d'endommagement règle la vitesse d'endommagement. L'effet de l'eau est introduit par couplage avec les paramètres du modèle de cohésion, par la diminution des forces de pesanteur et du coefficient de frottement local. La modélisation adoptée est de type « Non Smooth Contact Dynamics », où les grains et particules sont supposés rigides. Le modèle est implémenté sur la plateforme logicielle LMGC90. Des simulations numériques à l'échelle des contacts, à l'échelle du bloc et à l'échelle de l'enrochement sont effectuées. Des premières simulations consistent à simuler un ensemble de blocs rocheux incassables placés dans une colonne d'enrochements progressivement remplie en eau. Les simulations révèlent que la prise en compte du déjaugeage et d'une diminution réaliste du coefficient de frottement local ne génèrent que des déplacements faibles dans le milieu. Ces simulations permettent de vérifier que le phénomène essentiel à l'origine des tassements dans un barrage en enrochement est lié à la rupture des blocs rocheux.Des simulations de compression de blocs cassables (constitués de particules) et de compression œdométrique d'un ensemble de ces blocs sont réalisées. Les essais montrent que le temps caractéristique d'endommagement permet de mettre à l'échelle le comportement du matériau. Les résultats indiquent que l'histoire des évènements de rupture et des déformations sont bien corrélées. Les simulations numériques d'essais de compression œdométrique d'enrochements modèles de barrage mettent en évidence des réponses assimilables au fluage ou à la relaxation. Ces essais reproduisent les phénomènes de base avec très peu de paramètres

Une modélisation discrète du comportement mécanique des enrochements

Les enrochements sont des matériaux granulaires constitués de blocs rocheux dont la taille peut atteindre plusieurs dizaines de centimètres. Les barrages en enrochements, constitués par ces matériaux grossiers, présentent des déformations relativement importantes au cours du temps et peuvent également tasser au moment de leur remplissage ou lors d une crue accidentelle. Ces déformations semblent liées à des ruptures de blocs à l intérieur de l ouvrage, mais ne sont malheureusement pas connues après la construction. Le comportement des matériaux granulaires étant fortement lié à la nature discrète du milieu, un modèle discret est proposé afin de prendre en compte les particularités des enrochements avec des paramètres locaux au sens physique clair. Ce travail a donc consisté à développer un modèle numérique discret capable de prendre en compte la fissuration progressive et différée des blocs rocheux. La modélisation adoptée est de type "Non Smooth Contact Dynamics"Rockfill are made by coarse materials, blocks, whose size can reach one meter. Rockfill dams, composed by these blocks, show some relatively important deformations during time, during their filling or during an accidental flood. These deformations seems to be related to blocks ruptures in the inner structure, but they are unfortunately unknown after the dam construction. The settlements prediction is then particularly important to ensure the structural integrity of the dams all along their life. The granular media behaviour is strongly related to the discrete nature of the medium. A discrete model is proposed to take into account rockfill particularities with local parameters which have a clear physical sense. Ruptures are rockfill major features. The major idea in this work, consisted in developing a numerical discrete model to account for progressive and delayed blocks failure. Each block is considered as an assembly of particles with initial cohesive bonds which can decrease progressively during the loading. A damaging interface model is proposed to describe this progressive phenomenon. The model is composed by two yield surfaces (damage and fracture yield surfaces). A characteristic time is related to the damage evolution. The water influence is introduced by coupling the parameters of the cohesion model, by introducing the buoyancy forces and by decreasing the local friction. The modelling adopted is based on the "Non Smooth Contact Dynamics Method", where grains and particles are rigid. The model is implemented on the discrete element code, LMGC90. Numerical simulations are realised at contacts, block and rockfill scales. First simulations consisted in modelling an assembly of unbreakable blocks disposed in a rockfill column and progressively filled by water. These simulations verify that blocks fracture is the major phenomenon responsible of the settlements of rockfill dams. Simulations of compression of breakable blocks and oedometric compression of an assembly of these blocks are realised. Numerical simulations of oedometric compression tests composed by rockfill reveal some creep and relaxation responses. These tests reproduce the basic phenomena with only few parameters.AIX-MARSEILLE1-BU Sci.St Charles (130552104) / SudocSudocFranceF

Residual strength of underground structures in service

International audienceÉtant donné que les tunnels anciens se détériorent au cours du temps, il est nécessaire d’évaluer leur résistance résiduelle, afin de pouvoir planifier correctement leur réparation et leur consolidation. L’état de contraintes des structures souterraines évolue au fil du temps, d’abord en raison de la progression du front de chantier durant les travaux de construction, ensuite à cause des modifications graduelles des propriétés du sol et des propriétés mécaniques des matériaux qui composent les infrastructures, telles que la diminution de la cohésion, la perte de résistance ou de rigidité, etc. Ces modifications sont provoquées par des phénomènes combinés, tels que la reptation ou l’altération météorique, et par l’apparition de fissures, lesquelles entraînent une redistribution des contraintes et donc des déformations. Dans la présente étude, il est question des tunnels et des galeries situés dans le sous-sol de Paris. Une macromodélisation basée sur la méthode des éléments finis permet d’étudier différents scénarios de détérioration de revêtements de tunnel, en particulier au niveau de l’extrados des galeries ou des tunnels. Pour comprendre les variations d’état de contrainte, de même que l’apparition de détériorations et les redistributions (des contraintes ou des déformations) associées, il est nécessaire de modéliser les déformations retardées. Pour modéliser le comportement des infrastructures, on a choisi d’utiliser une méthode dépendante du temps. Cette méthode permet de quantifier les détériorations et de prédire la résistance résiduelle d’infrastructures de type galerie ou tunnel. On se sert d’un modèle constitutif élastique et viscoplastique avec atténuation des déformations pour reproduire l’aspect des zones dégradées et leur comportement. Les résultats obtenus avec la méthode numérique permettent de reproduire le comportement des tunnels du métro parisien, de confirmer les résultats d’auscultation, de clarifier leur interprétation et de mieux planifier les réparations des infrastructures