Determination of material parameters of a viscoplastic model by genetic algorithm

WOS: 000274203200026Macroscopic (continuum) and microscopic models, used for simulation of material behaviors under different loading conditions, contain a large number of material parameters and determination of these parameters is an important and difficult issue in the modeling. The aim of this work essentially deals with parameter determination procedure of any viscoplasticity model. In this study, genetic algorithm (GA) parameter optimization procedure has been proposed to determine material parameters of viscoplastic models. Parameter determination capability of the GA optimization method was tested by using VBO model which one of the viscoplasticity theory with no yield surface and loading-unloading conditions. Fourteen material. parameters of VBO model are determined using uniaxial loading-unloading stress strain curves of high density polyethylene (HDPE). Using these material parameters, creep and relaxation behaviors of HDPE are simulated. A good match with experimental results is obtained. Apart from many existing studies in the literature, GA optimization procedure is applied to determine material parameters instead of trial and error procedure. This method can also be used to determine materials parameters of other viscoplasticity theories for all kinds of materials. (C) 2009 Elsevier Ltd. All rights reserved.Scientific and Technological Research Council of Turkey (TUBITAK) [104M215]The support of the Scientific and Technological Research Council of Turkey (TUBITAK) is gratefully acknowledged. The project number: 104M215

Efficacy of nasopharyngeal culture in identification of pathogens in middle ear fluid in chronic otitis media with effusion

Purpose: Chronic otitis media with effusion (OME) is the leading cause of hearing loss during childhood. In bacterial etiology of OME, the most frequent pathogens responsible are Haemophilus influenzae followed by Streptococcus pneumoniae and Moraxella catarrhalis . This study aimed at evaluating the accuracy of nasopharyngeal (NP) specimens in the identification of pathogens in the middle ear fluid (MEF) in patients with OME. Materials and Methods: In this cross sectional, case-control study, 95 MEFs and 53 NP secretion specimens were obtained from 53 children. As a control group, 102 NP specimens were taken from children having an operation other than an otological disease. Conventional culture methods and multiplex-PCR method have been used to determine the etiology of OME; NP carriage between cases and control groups were compared using conventional culture methods. Pearson Chi-Square and Fishers Exact tests were used in statistical analysis. Results : Bacteria were isolated by culture in 37.9 of MEF specimens, 14.7 of which belonged to the group H. influenzae , S. pneumoniae and M. catarrhalis. PCR was positive in 30.5 specimens targeting the same pathogens. There was a two-fold increase in carriage rate of S. pneumoniae and H. influenzae in patients than controls for each pathogen. Conclusion: PCR is a more reliable method to detect middle ear pathogens in MEF in comparison with the conventional culture methods. The NP colonization wasnt found to be an indicator of the pathogen in MEF although middle ear pathogens colonize more in nasopharynx of diseased children