A new adaptive response surface method for reliability analysis

Response surface method is a convenient tool to assess reliability for a wide range of structural mechanical problems. More specifically, adaptive schemes which consist in iteratively refine the experimental design close to the limit state have received much attention. However, it is generally difficult to take into account a lot of variables and to well handle approximation error. The method, proposed in this paper, addresses these points using sparse response surface and a relevant criterion for results accuracy. For this purpose, a response surface is built from an initial Latin Hypercube Sampling (LHS) where the most significant terms are chosen from statistical criteria and cross-validation method. At each step, LHS is refined in a region of interest defined with respect to an importance level on probability density in the design point. Two convergence criteria are used in the procedure: The first one concerns localization of the region and the second one the response surface quality. Finally, a bootstrap method is used to determine the influence of the response error on the estimated probability of failure. This method is applied to several examples and results are discussed

Reliability approach in spacecraft structures

This paper presents an application of the probabilistic approach with reliability assessment on a spacecraft structure. The adopted strategy uses meta-modeling with first and second order polynomial functions. This method aims at minimizing computational time while giving relevant results. The first part focuses on computational tools employed in the strategy development. The second part presents a spacecraft application. The purpose is to highlight benefits of the probabilistic approach compared with the current deterministic one. From examples of reliability assessment we show some advantages which could be found in industrial applications

Analyse de la fiabilité d’une structure spatiale

L’analyse de la fiabilité des grandes structures est difficile car le nombre de variables aléatoires d’entrée et le nombre de réponses à traiter sont généralement grands. Lorsque le comportement mécanique de la structure n’est pas fortement non-linéaire, les surfaces de réponse présentent des outils intéressants pour évaluer la fiabilité. Ce papier expose une stratégie de résolution appliquée à une structure spatiale. Elle consiste à simplifier le problème initial dans le but de conserver l’information essentielle. Pour cela, la réduction s’appuie sur la sélection des variables les plus pertinentes. Un indicateur d’erreur est utilisé pour valider la précision des résultats. Des tirages de Monte Carlo viennent corroborer certains résultats

Approche fiabiliste pour l’optimisation locale d’un problème couplé fluide-structure

Le projet OSYCAF (Optimisation d’un Système Couplé fluide/structure représentant une Aile Flexible) a pour objectif de proposer une méthodologie d’optimisation multidisciplinaire dans un contexte aéronautique. Plus précisément, il s’agit d’optimiser une aile d’avion en tenant compte des interactions fluide-structure. Les modèles de mécanique des fluides et des structures sont des disciplines devant communiquer entre elles et avec l’optimiseur global. L’optimisation est réalisée sur deux niveaux : par rapport aux paramètres globaux, communs aux deux disciplines, et par rapport aux paramètres locaux, propres à chacune. Le travail présente l’optimisation de la structure par rapport aux paramètres locaux. Dans ce cadre, il est proposé d’introduire des incertitudes probabilistes permettant de tenir compte de contraintes de fiabilité

Application des surfaces de réponse pour l’analyse fiabiliste d’une structure spatiale

Cette communication présente une application des surfaces de réponse pour l’analyse de la fiabilité d’une structure satellite. Les méta-modèles sont construits par régression itérative où seul les termes significatifs sont sélectionnés parmi une liste de régresseurs potentiels préalablement déterminée par une analyse de sensibilité. Les méta-modèles sont ensuite vérifiés par une méthode de bootstrap où les variations observées sur les prédictions sont prises en compte dans le calcul des probabilités de défaillance afin de valider le résultat

Changes of Sagittal Spinopelvic Parameters in Normal Koreans with Age over 50

Study Design A prospective comparative study. Purpose To describe the changes in the spinopelvic parameters on normal Koreans more than 50 years of age. Overview of Literature There are differing opinions regarding the changes in the thoracic kyphosis, lumbar lordosis, C7 plumb with age in the elderly population. Methods Sagittal standing radiographs of the whole spine including the pelvis in 132 Korean adult male volunteers more than 50 years of age were evaluated prospectively. Volunteers with a history of spine operation, spinal disease, pain in their back or legs, scoliosis, spondylolisthesis, monosegment disc space narrowing, or compression fracture in radiographs were excluded. The following parameters were included: thoracic kyphosis (T5 upper end plate [UEP]-T12 lower end plate [LEP]), thoracolumbar kyphosis (T10 UEP-L2 LEP), lumbar lordosis (T12 LEP-S1 UEP), lower lumbar lordosis (L4 UEP-S1 UEP), sacral slope, pelvic incidence, and the distances from the C7 plumb to the posterosuperior endplate of S1. These parameters in the 6th, 7th and 8th decade groups were compared and the changes in these parameters according to age were examined. Results The thoracic kyphosis demonstrated significant differences in the in the three age groups (p = 0.019), and increased with age (r = 0.239, p < 0.006). The other parameters did not show any significant difference or correlation. Conclusions Similar global sagittal balances and spinopelvic parameters may be observed in Korean males older than 50 years, with a trend towards increasing thoracic kyphosis with age

Characterizing the shape of the lumbar spine using an active shape model: reliability and precision of the method

Copyright © 2008 Lippincott, Williams & WilkinsThis is a non-final version of an article published in final form in Spine Vol. 33 (7), pp. 807-813 (2008)Study Design. Analysis of positional magnetic resonance images of normal volunteers. Objective. To compare the reliability and precision of an active shape model to that of conventional lordosis measurements. Summary of Background Data. Characterization of lumbar lordosis commonly relies on measurement of angles; these have been found to have errors of around 10[degrees]. Methods. T2 weighted sagittal images of the lumbar spines of 24 male volunteers in the standing posture were acquired using a positional magnetic resonance scanner. An active shape model of the vertebral bodies from S1 to L1 was created. Lumbar lordosis was also determined by measuring the angles of the superior endplates. All measurements were performed twice by one observer and once by a second observer. Results. The shape model identified 2 modes of variation to describe the shape of the lumbar spine (mode 1 described curvature and mode 2 described evenness of curvature). Significant correlations were found between mode 1 and total lordosis (R = 0.97, P < 0.001) and between mode 2 and mean absolute deviation of segmental lordosis (R = 0.80, P < 0.001). Intra- and interobserver reliability was higher for the shape model (intraclass correlation coefficients, 0.98-1.00) than for the lordosis angle measurements (intraclass correlation coefficients, 0.68-0.99). The relative error of the shape model (mode 1 = 4%; mode 2 = 9%) was lower than the conventional measurements (total lordosis = 10%). Conclusion. The shape of the lumbar spine in the sagittal plane can be comprehensively characterized using a shape model. The results are more reliable and precise than measurements of lordosis calculated from endplate angles

Global spinal deformity from the upper cervical perspective. What is Abnormal in the upper cervical spine?

Hypothesis: Reciprocal changes in the upper cervical spine correlate with adult TL deformity modifiers. Design: This was a retrospective review. Introduction: The upper cervical spine has remarkable adaptability to wide ranges of thoracolumbar (TL) deformity. Methods: Patients \u3e18 years with adult spinal deformity (ASD) and complete radiographic data at baseline (BL) and 1 year were identified. Patients were grouped into component types of the Roussouly classification system (Type 1: Pelvic incidence [PI] Results: A total of 343 ASD patients were analyzed. When grouped by BL Schwab and Roussouly, Group Conclusions: Our study suggests that upper cervical alignment remains relatively stable through most broad variations of adult TL deformity. Changes in SVA correlated most with upper cervical changes