Identification of viscoplastic parameters and characterization of Lüders behaviour using digital image correlation and the virtual fields method

In this study, tensile loading experiments are performed on notched steel bars at an average applied strain rate of 1s-1. Displacement fields are measured across the specimen by coupling digital image correlation (DIC) with imaging using high speed CCD cameras (4796 fps). Results from the experiments indicate the presence of local strain rates ranging from 0.1 to 10s-1 in the notched specimens. The heterogeneity of the strain rate fields provides suitable conditions for determining simultaneously all the elasto-visco-plastic constitutive parameters governing the material behavior. For that, the whole stress fields are reconstructed in the specimen using the full-field deformation measurements. This reconstruction is repeated with different constitutive parameters until the average stress in the specimen matches the one measured with the load cell response. Perzyna’s model is firstly considered for the reconstruction of stresses but it is shown to be unsuited for providing the drop in the average stress that is systematically detected at the onset of plasticity by the load cell. This drop is attributed to the sudden occurrence of plasticity in the material due to Lüders effect. A modified model for elasto-visco-plasticity taking account of Lüders behavior in the material is considered afterwards. It yields a better agreement between the reconstructed stresses and the load cell response, and a more accurate identification of the parameters driving the visco-plastic model. Eventually, it is shown how to use DIC measurements for replacing the load cell measurements when the transient effects in the test reach the resonance frequency of the load cel

Halk Kütüphanelerinin Amacını Yeniden Tanımlamak: Dış Teorik Çerçeveden Esinlenen Eleştirel Bir Yaklaşım

There is a wide variety of views on the public library’s future. In addition, a large number of books and articles have appeared, which address the future existence ofpublic libraries in the age of Google, social media and electronic books. To identify major themes and viewpoints a review was conducted of selected published sources addressing the legitimacy of public libraries along with the efforts to rethink libraries ’ future roles and chief tasks. The analysis of selected publications shows that there is a paucity of critical thinking about public libraries, their mission, priorities and external environment. A major focus of the study is the relevance and feasibility of “importing” a specific body of theory from outside the field of Library and Information Science to enrich theoretical discussions within public librarianship. Hence, the paper considers ideas generated from reading German philosopher Axel Honneth’s seminal work entitled “Das Recht der Freiheit”. An initial discussion is conducted of Honneth’s theoretical perspec­tives and how they can be related to theory building in public librarianship. In considering Honneth’s conceptualisations ofpersonal freedom, attention is particularly given to the public library’s awareness of the growing democratic deficit in the neo-liberal society.Kütüphanecilik mesleğinde, bilgi yönetimi akademik çevrelerinde ve medyada halk kütüphanelerinin geleceğine dair birçok görüş dile getirilmektedir. Bazı profesyoneller, kütüphane müdürleri ve bilgi yönetimi bölümü akademisyenleri halk kütüphanelerinin geleceğini parlak görürken, bazı analist ve yorumcular önümüzdeki 20-30 senelik süreçte halk kütüphanelerine yönelik ihtiyaca daha kötümser bir bakış açısıyla yaklaşmaktadır. Her halükarda, halk kütüphanelerinin amaçlarını ve hedeflerini eleştirel bir bakış açısıyla gözden geçirmeleri ve hizmetlerini yeniden tanımlamayı düşünmeleri gerekmektedir.Google, sosyal medya ve elektronik kitaplar çağında halk kütüphanelerinin yaşamlarım sürdürmeleri­nin yolları tartışılırken, kütüphanelerin rolünü güçlendirip genişletecek bazı görüşler ortaya atılmakta­dır. Halk kütüphanelerini bir buluşma yeri, bir kamusal söylem alanı, bir demokrasi meydanı veya bir öğrenme yeri olarak inceleme görüşü öne çıkmaktadır. Bu çalışmada, küresel kriz ve belirsizlik zaman­larında halk kütüphanelerinin rolü ve faaliyetlerine yönelik bir çalışmanın kavramsal temeline katkıda bulunulurken, Alman filozof Axel Honneth'in, Hegel'in Haklar Felsefesini derinlemesine inceleyen Das Recht der Freiheit isimli çalışmasından bazı düşünceler ele alınmıştır. Honneht'in analizleri adalet te­orisini anlamaya çalışmak ve özgürlük kavramının tüm yönlerini ve çıkarımları ele almak üzerinedir. Honneth'in çalışmasında modern toplumda demokrasi kavramının yanı sıra eşitsizlik, güç ve hakimiyet kavramları çerçevesinde görüşler ve bakış açılarına ağırlık verilmiştir. Halk kütüphanelerinin amacı ve öncelikleri tartışmasında Honneth'in teorik çalışmasının etkileri ilk kez bu çalışmada ele alınmıştır. Honneth'in kişisel özgürlüğü kavramsallaştırması Batılı bir bakış açısı, sosyal düzen ve demokratik yaşam biçiminde düşünülürse, halk kütüphanelerini neo-liberal toplumdaki büyüyen demokrasi açığı konusunda ve toplumları küresel açıdan ilgilendiren çelişkiler, krizler ve zorluklar hususunda farkında- lık getiren bir kuruma dönüştürmek konusu da ayrıca öne çıkmaktadır

A study of localisation in dual-phase high-strength steels under dynamic loading using digital image correlation and FE analysis

Tensile tests were conducted on dual-phase high-strength steel in a Split-Hopkinson Tension Bar at a strain-rate in the range of 150-600/s and in a servo-hydraulic testing machine at a strain-rate between 10-3 and 100/s. A novel specimen design was utilized for the Hopkinson bar tests of this sheet material. Digital image correlation was used together with high-speed photography to study strain localisation in the tensile specimens at high rates of strain. By using digital image correlation, it is possible to obtain in-plane displacement and strain fields during non-uniform deformation of the gauge section, and accordingly the strains associated with diffuse and localised necking may be determined. The full-field measurements in high strain-rate tests reveal that strain localisation started even before the maximum load was attained in the specimen. An elasto-viscoplastic constitutive model is used to predict the observed stress-strain behaviour and strain localisation for the dual-phase steel. Numerical simulations of dynamic tensile tests were performed using the non-linear explicit FE code LS-DYNA. Simulations were done with shell (plane stress) and brick elements. Good correlation between experiments and numerical predictions was achieved, in terms of engineering stress-strain behaviour, deformed geometry and strain fields. However, mesh density plays a role in the localisation of deformation in numerical simulations, particularly for the shell element analysis

On the use of simulated experiments in designing tests for material characterization from full-field measurements

The present paper deals with the use of simulated experiments to improve the design of an actual mechanical test. The analysis focused on the identification of the orthotropic properties of composites using the unnotched Iosipescu test and a full-field optical technique, the grid method. The experimental test was reproduced numerically by finite element analysis and the recording of deformed grey level images by a CCD camera was simulated trying to take into account the most significant parameters that can play a role during an actual test, e.g. the noise, the failure of the specimen, the size of the grid printed on the surface, etc. The grid method then was applied to the generated synthetic images in order to extract the displacement and strain fields and the Virtual Fields Method was finally used to identify the material properties and a cost function was devised to evaluate the error in the identification. The developed procedure was used to study different features of the test such as the aspect ratio and the fibre orientation of the specimen, the use of smoothing functions in the strain reconstruction from noisy data, the influence of missing data on the identification. Four different composite materials were considered and, for each of them, a set of optimized design variables was found by minimization of the cost function

The unseen and unacceptable face of digital libraries

The social and organizational aspects of digital libraries are often overlooked but this paper reviews how they can affect users' awareness and acceptance of digital libraries. An analysis of research conducted within two contrasting domains (Clinical and Academic) is presented which highlights issues of user interactions, work practices and the organizational social structures. The combined study comprises an analysis of 98 in-depth interviews and focus groups with lecturers, librarians and hospital clinicians. The importance of current and past roles of the library, and how users interacted with it, are revealed. Web-based digital libraries, while alleviating most library resource and interaction problems, require a change in librarians' and DL designers' roles and interaction patterns if they are to be implemented acceptably and effectively. Without this role change, users will at best be unaware of these digital resources and at worst feel threatened by them. The findings of this paper highlight the importance on DL design and implementation of the social context and supporting user communication (i.e. collaboration and consultation) in their information search and usage activities

Identification of plastic constitutive parameters at large deformations from three dimensional displacement fields

The aim of this paper is to provide a general procedure to extract the constitutive parameters of a plasticity model starting from displacement measurements and using the Virtual Fields Method. This is a classical inverse problem which has been already investigated in the literature, however several new features are developed here. First of all the procedure applies to a general three-dimensional displacement field which leads to large plastic deformations, no assumptions are made such as plane stress or plane strain although only pressure-independent plasticity is considered. Moreover the equilibrium equation is written in terms of the deviatoric stress tensor that can be directly computed from the strain field without iterations. Thanks to this, the identification routine is much faster compared to other inverse methods such as finite element updating. The proposed method can be a valid tool to study complex phenomena which involve severe plastic deformation and where the state of stress is completely triaxial, e.g. strain localization or necking occurrence. The procedure has been validated using a three dimensional displacement field obtained from a simulated experiment. The main potentialities as well as a first sensitivity study on the influence of measurement errors are illustrated

Information (and library) science at City University London; 50 years of educational development

The development of education for information and library science at City University London over a 50-year period is described in this article. The development of the Masters course in Information Science, and the later equivalent courses in Library Science and in Information Management in the Cultural Sector are described in detail, together with shorter-lived Masters courses in pharmaceutical and health information. The rationale for changes to the courses, and the influence of the professional and educational contexts, are analysed. Issues emerging from this analysis are discussed in seven themes: the nature of the discipline; the library/information spectrum; the student group; the academic/professional balance; curriculum design; local and global issues; and teaching methods. The discussions of the courses are set in the wider context of changes in library/information education over the period in the UK and worldwide