Evaluation of local stress and strain state at notch root by means of a new method valid for multiaxial random loadings

International audienceNeuber type methods are widely used to predict the local stress-strain behaviour at notch root in specimens or industrial components. Some limitations of these methods are pointed out in the present paper, especially when the global loading is multiaxial and/or random. A fully new approach is then presented: it introduces a phenomenological model describing the development of residual stresses that can be calibrated by reference to Eshelby's type approaches. A tensorial variable is used for this purpose. Its evolution rule allows us to represent the stress redistribution at the surface of the component. Isotropic and anisotropic constitutive equations are accepted for the description of the material behaviour. It has been successfully used for several complex situations (cyclic, multiaxial, random loadings)

A strategy for the estimation of the fatigue life of notched components under random multiaxial fatigue

International audienceThis paper describes three new models that can be used (1) to determine the stress-strain response in stress concentration zones for components submitted to complex multiaxial loading paths, (2) to extract relevant cycle sequences from a general threedimensional loadin path, (3) to find the hypersphere enclosing any type of cyclic load history. These three models can be combined to post-process an elastic Finite Element Analysis, and provide a fast estimation of the fatigue life of the components

Une méthode de correction élastoplastique pour le calcul en fatigue des zones de concentration de contraintes sous chargement cyclique multiaxial non proportionnel

The first step of a low cycle fatigue life calculation consists in determining the stress and strain histories at critical points. To avoid time-consuming complete nite element computations, local elasto-plastic corrections may be used to evaluate those histories in a very short time. Existing approaches are not accurate enough, thus a new method inspired from homogenisation models is proposed. It was implemented in the ZeBuLoN FE code, and validated through multiaxial non proportional random loadings, which is not the case for any existing method. The obtained histories are used as an input for fatigue life calculation methods. A new cycle counting algorithm was also implemented to extract cycles from a multiaxial random loading sequence. It is a rainow technique, which takes into account all components of the loading, and denes a \cycle" in the deviatoric stresses space. Fatigue life calculations have been performed to evaluate the relevance of this calculation chain. Finally, we apply the method to an industrial component (crane frame arm).La première étape d'un calcul de durée de vie en fatigue oligocyclique consiste à obtenir l'historique des contraintes et des déformations sur des points jugés critiques. Pour éviter des calculs complets par éléments finis (force brute) particulièrement longs, les corrections élasto-plastiques locales évaluent ces historiques en un temps très court. Les approches existantes manquant de précision, une nouvelle méthode de calcul qui s'inspire des modèles d'homogénéisation a été proposée. Elle a été implémentée dans le code de calcul ZéBuLoN et validée sur des cas de charge quelconques en multiaxial non proportionnel aléatoire, ce qui n'est le cas d'aucune autre méthode de la littérature. Ces historiques servent d'entrée aux méthodes de prévision de durée de vie. Un nouvel algorithme de comptage de cycles a été utilisé pour extraire une série de cycles d'un chargement multiaxial aléatoire. Il s'agit d'une technique de rainow qui permet de conserver toutes les composantes du chargement, et de définir un "cycle" dans l'espace des contraintes déviatoriques. Des calculs de durée de vie ont été réalisés pour évaluer la pertinence de l'ensemble de la chaîne de calcul ainsi constituée. On montre pour finir des applications à une pièce industrielle (bras de châssis de grue)

Epidural analgesia in ICU chest trauma patients with fractured ribs: retrospective study of pain control and intubation requirements

International audienceBackground Nonintubated chest trauma patients with fractured ribs admitted to the intensive care unit (ICU) are at risk for complications and may require invasive ventilation at some point. Effective pain control is essential. We assessed whether epidural analgesia (EA) in patients with fractured ribs who were not intubated at ICU admission decreased the need for invasive mechanical ventilation (IMV). We also looked for risk factors for IMV. Study design and methods This retrospective, observational, multicenter study conducted in 40 ICUs in France included consecutive patients with three or more fractured ribs who were not intubated at admission between July 2013 and July 2015. Results Of the 974 study patients, 788 were included in the analysis of intubation predictors. EA was used in 130 (16.5%) patients, and 65 (8.2%) patients required IMV. Factors independently associated with IMV were chronic respiratory disease (P = 0.008), worse SAPS II (P < 0.0001), flail chest (P = 0.02), worse Injury Severity Score (P = 0.0003), higher respiratory rate at admission (P = 0.02), alcohol withdrawal syndrome (P < 0.001), and noninvasive ventilation (P = 0.04). EA was not associated with decreases in IMV requirements, median numerical rating scale pain score, or intravenous morphine requirements from day 1 to day 7. Conclusions EA was not associated with a lower risk of IMV in chest trauma patients with at least 3 fractured ribs, moderate pain, and no intubation on admission. Further studies are needed to clarify the optimal pain control strategy in chest trauma patients admitted to the ICU, notably those with severe pain or high opioid requirements