- Email: [email protected]
COST ACTION F3 ‘STRUCTURAL DYNAMICS’ (1997–2001)—AN EUROPEAN CO-OPERATION IN THE FIELD OF SCIENCE AND TECHNOLOGY
Mechanical Systems and Signal Processing (2003) 17(1), 3–7 doi:10.1006/mssp.2002.1533, available online at http://www.idealibrary.com on
COSTACTION F3 ‘STRUCTURAL DYNAMICS’ (1997^2001)}AN EUROPEAN CO-OPERATION IN THE FIELD OF SCIENCE AND TECHNOLOGY J.-C. Golinval Department of Aerospace, Mechanics and Materials, Universite! de Lie"ge, 1, Chemin des Chevreuils (B52/3), LTAS}Vibrations et Identification des Structures, B-4000 Lie"ge 1, Belgium. E-mail: [email protected]
and M. Link
Lightweight Structures and Structural Mechanics Laboratory, Universita.t Kassel, Mo.nchebergstr. 7, 34109 Kassel, Germany (Received 1 October 2002, accepted 1 October 2002) This section is concerned with presenting the objective, the scientific programme and the organisation of COST Action F3 in Structural Dynamics. # 2003 Elsevier Science Ltd. All rights reserved.
1. INTRODUCTION
The key idea of Co-operation in the field of Scientific and Technical Research (COST). Actions is to allow European research institutions working on similar problems in parallel to exchange information with others and let them be aware of similar research programmes. The COST framework is an efficient and simple way of gathering a database and diffusing information among many European partners. The COST Action ‘F3’ [1] in Structural Dynamics was initiated by Prof. Jean-Claude Golinval from the University of Li"ege in Belgium. It was supported for a period of 4 years by the European Community according to the Memorandum of Understanding (MoU) for the implementation of a European Concerted Research Action signed in Brussels on the 25th of June 1997. The purpose of this action was to develop European collaboration, to intensify and to co-ordinate research on the topics of model updating, damage detection, health monitoring and identification of non-linearities in structural dynamics. COST Action F3 grouped 13 European countries (Austria, Belgium, Denmark, Finland, France, Germany, Greece, Italy, Netherlands, Portugal, Spain, Switzerland, United Kingdom), the Joint Research Centre in Ispra and the European Commission.
2. OBJECTIVE OF THE ACTION
Regarding the increased demand for construction or equipment performance in terms of mechanical reliability, lightness, load-carrying capacity, speed, safety, etc., modern design methods become more and more sophisticated. New design objectives need powerful computers, precise description of material behaviour and efficient modelling methods. Furthermore, the complexity of today’s mechanical systems and their increased level of 0888–3270/03/+$35.00/0
# 2003 Elsevier Science Ltd. All rights reserved.
4
J.-C. GOLINVAL AND M. LINK
performance make it necessary to model effects that were not predominantly important in previous analyses. Failure to do so could result in inadequate models and could lead to large discrepancies between the estimation of the structure’s dynamics and its real behaviour. These effects include non-linear dynamics, mechanisms of damping and dissipation of the energy and phenomena of energy localisation. As model updating procedures are used to build representative models of the structural dynamic behaviour, they can also be used to detect damage on structures and monitor the health conditions in general. For example, if the parameters related to a crack geometry are chosen as model updating variables, the updating procedures can be used to predict the crack propagation during the life of the structure. Once the structure is in operation, structural health monitoring (predictive maintenance, default diagnosis, characterisation of the mechanical signature, etc.) becomes the main challenge. The general objective pursued in COST Action F3 was to increase the knowledge required for improving the structural design, the mechanical reliability and the safety of structures in the field of linear and non-linear dynamics.
3. SCIENTIFIC PROGRAMME OF COST ACTION F3
The general features described in the Technical Annex of the MoU are briefly recalled in this section. COST Action F3 focused on three specific research themes through working groups: finite element model updating (WG1), structural damage detection and health monitoring (WG2), and identification of non-linear systems (WG3). 3.1. FINITE ELEMENT MODEL UPDATING METHODS (WORKING GROUP 1) Updating methods are used to adjust analytical models to test results. The field of application of these methods includes: *
*
*
The correction of approximation errors: this type of error is related to assumptions regarding the physics of the model as, for example, the linear behaviour of the structure, the physical behaviour laws (model of elasticity, etc.), in modelling connections and boundary conditions, the limitations of the mathematical formulations used for deriving particular finite elements and the representation of non-accessible structural data (dissipative phenomena); The correction of discretisation errors related to errors arising from a model that is too coarse for capturing some of the significant dynamics of the system: this type of error is related to the optimisation and automation of finite element meshing for dynamic computations; The correction of parametric errors caused by the differences in measured material properties (elesticity modulus, density, section and thickness, etc.), especially for complex new material systems.
Updating methods can be classified in two groups: global and local. Global methods are based on the correction of the global stiffness and/or mass matrices of the finite element model. These methods use optimisation tools for deriving an updated model that is not always physically meaningful even if it reproduces the test data with accuracy. Local methods are usually preferred to global methods because they offer a better potential. They are based on the calculation of modal parameters (frequencies, damping ratios and mode shapes) or frequency response sensitivities with respect to the physical variables of the model (elasticity modulus, density, section and thickness, etc.). The advantages of local methods are their ability to locate erroneous regions of the analytical model and to select the relevant physical or structural parameters for model updating.
COST ACTION F3
5
Whether the methods are global or local in nature, several ‘families’ emerge depending on the mathematical formulations adopted for formulating the updating problem. An important research task of COST Action F3 was therefore to address practical difficulties of model updating (incomplete measurement sets, selection of the optimum data sets for the updating, capability to control the numerical difficulties, assessment of the model’s capability to predict the behaviour of other structural configurations than those used for adjusting the model, etc.). 3.2. STRUCTURAL DAMAGE DETECTION AND HEALTH MONITORING (WORKING GROUP 2) In some aspects, structural damage detection may be considered very similar mathematically to model updating even though objectives of the two problems are different. Model updating seeks the correlation of a mathematical model to experimental data collected from the instumentation of a structure whereas health monitoring is mostly concerned with the variations in time of a structural system. Very often, the system to monitor is represented via mathematical models such as a finite element models, which provides a clear link with updating methods. For example, assuming that data are collected periodically, successive updating of the mathematical models may be performed and comparison of the correlated models to the baseline (undamaged) model provides a practical tool for damage detection. The long-term goal is the ability to realise on-line diagnosis systems which would perform modal tests of the structure throughout its lifetime, identify an experimental model, correlate this model to the analytical one (if it exists) and assess the probability of damage from the comparison between various correlated models in time. For example, bridges can be tested using the natural source of excitation of wind and traffic, and cracks should be located before leading to safety concerns. In structural health monitoring, the structure must be considered under realistic environmental conditions, loading conditions and measurement conditions. Thus, addressing the practical problems of developing long-term stable highly sensitive transducers, systems for easy data transfer and data manipulation, and problems of filtering out changes in environmental and loading conditions are essential. Highly accurate (especially bias-free) system-identification procedures must be implemented in a module-oriented software system, and experience must be gathered using these techniques on large civil engineering structures using only a few sensors. Probabilistic measures must be applied and developed for indication of damage and location and type of damage, and ways of taking a priori information into account must be developed. Further, ways of using the information gathered from analysis of the measured response in updating the reliability of the structure, must be developed to have an appropriate tool for decision making concerning maintenance and visual inspection. 3.3. IDENTIFICATION OF NON-LINEAR SYSTEMS (WORKING GROUP 3) Whenever non-linearities (e.g. when stiffness and damping depend on the load level) are suspected, traditional modal analysis techniques collapse because their underlying mathematics are restricted essentially to the linear domain. The problem of test-analysis reconciliation (model updating, health monitoring, etc.) exhibits several aspects depending upon the type of structure and the type of structural modification involved. In various situations, a local identification of the dynamics of a compnent may be extracted from the modal test of the structure. Therefore, the problem becomes that of ‘modal subtracting’ the behaviour of the known components from that of the whole system. This problem has numerous industrial applications, for example, the inspection of bolted joints in metallic structures or the control of joints in pipe networks.
6
J.-C. GOLINVAL AND M. LINK
In other cases, for instance, when structural changes originate from localised damage, the problem must be investigated from a non-linear point of view. For example, untightening of bolted joints may determine particular vibrational patterns of the type ‘vibration with contact’, due to joint free play. The analysis of these characteristics has been attempted by authors in the general framework of detection of non-linearities. With respect to identification of non-linear systems for health monitoring, artificialintelligence-based techniques should also be considered. During the recent years, many interesting attempts in this field have been proposed, but the subject still needs a more systematic investigation. Also, the durability of recently proposed genetic algorithms for non-linear identification should be considered. These new techniques are not only interesting for health monitoring, but also for control purposes, i.e. active as well as nonactive control of the dynamic response of structures.
4. ORGANISATION OF THE ACTION
4.1. MANAGEMENT COMMITTEE The work undertaken in COST Action F3 was administered by a Management Committee (MC) composed of the chairman, the co-chairman and two national delegates from each signatory country. MC meetings were held once or twice per year to supervise and to ensure co-ordination of the Action. The scientific secretariat of the Action was provided by the University of Li"ege (Belgium). 4.2. WORKING GROUPS This research Action was divided into three Working Groups (WGs) directed by coordinators. Working Group 1 which was co-ordinated by Prof. M. Link and Prof. M. Friswell concerned itself with a detailed examination of finite element model updating methods. The activities of Working Group 2 which was co-ordinated by Prof. K. Worden concerned with the subject of structural health monitoring. Working Group 3 which was co-ordinated by Dr P. Argoul and Dr F. Thouverez was tasked to investigate non-linear systems. The main task of the WG was to define and to organise benchmarking exercises. Throughout the Action, a total of 19 WG meetings were held in different countries. 4.3. SHORT-TERM SCIENTIFIC MISSIONS The aim of Short-Term Scientific Missions (STSMs) is to contribute to the realisation of the scientific objectives of the Action. STSM strengthened the created network by allowing scientists to go to a laboratory in another country to learn a new technique or to make measurements using instruments and/or methods not available in their own laboratory. Throughout the Action, a total of 20 scientific missions were made covering the subjects dealt within the WG. 4.4. INTERNATIONAL WORKSHOPS/CONFERENCES Dissemination of the results of the Action was performed through the organisation of COST F3 international conferences: 1. International Seminar on Modal Analysis (ISMA 23), Katholieke Universiteit leuven, Belgium, 10–12 September 1998. 2. Identification in Engineering Systems, University of Swansea Wales, U.K. 29–31 March,1999 (www.swan.ac.uk/mecheng/ies99). 3. European COST F3 Conference on System Identification & Structural Health Monitoring, Universidad Politecnica de Madrid, Spain, 6–9 June 2000 (www.dmpa.upm.es/SHM).
7
COST ACTION F3
Table 1 Participation to COST F3 conference Year and place of the conference 1998, 1999, 2000, 2000, 2001,
Leuven, Belgium Swansea, U.K. Madrid, Spain Leuven, Belgium Kassel, Germany
Total number of papers
Number of papers related to COST F3
204 73 85 221 55
55 52 33 38 20
4. International Seminar on Modal Analysis (ISMA 25), Katholieke Universiteit Leuven, Belgium, 13–15 September 2000 (www.mech.kuleuven.ac.be/pma/events/isma/isma25conf/isma25conf.html). 5. International Conference on Structural System Identification, University of Kassel, Germany, 5–7 September 2001. Statistics concerning the participation to these conferences are given in Table 1.
5. CONCLUSION
On the basis of the sum of the efforts related to the work presented in this special issue, the economic dimension of COST Action F3 may be estimated roughly to a 100 manyears. However, the overall benefit of the Action is incommensurable since it contributed to the improvement of the communication between many research groups among the 13 European countries that actively participated in the implementation of the Action. It provided a unique opportunity to co-ordinate research efforts between research groups with complementary domains of expertise in aerospace, mechanical and civil engineering.
ACKNOWLEDGEMENTS
The European Commission is gratefully acknowledged for its financial support to cover the co-ordination costs, the scientific secretariat and the allowance for the scientific missions. The chairman and the co-chairman of COST Action F3 would like also to acknowledge the national delegates who welcomed MC and WG meetings in their institutions. Finally, Prof. J. C. Golinval would like also to acknowledge the General Directorate of Technology, Research and Energy (DGTRE) of the Ministry of the Walloon Region in Belgium for its financial support regarding to the organisation of COST F3 meetings at the University of Li"ege.
REFERENCES 1. COST F3 web site: http://www.ulg.ac.be/ltas-vis/costf3/costf3.html.
COSTACTION F3 ‘STRUCTURAL DYNAMICS’ (1997^2001)}AN EUROPEAN CO-OPERATION IN THE FIELD OF SCIENCE AND TECHNOLOGY J.-C. Golinval Department of Aerospace, Mechanics and Materials, Universite! de Lie"ge, 1, Chemin des Chevreuils (B52/3), LTAS}Vibrations et Identification des Structures, B-4000 Lie"ge 1, Belgium. E-mail: [email protected]
and M. Link
Lightweight Structures and Structural Mechanics Laboratory, Universita.t Kassel, Mo.nchebergstr. 7, 34109 Kassel, Germany (Received 1 October 2002, accepted 1 October 2002) This section is concerned with presenting the objective, the scientific programme and the organisation of COST Action F3 in Structural Dynamics. # 2003 Elsevier Science Ltd. All rights reserved.
1. INTRODUCTION
The key idea of Co-operation in the field of Scientific and Technical Research (COST). Actions is to allow European research institutions working on similar problems in parallel to exchange information with others and let them be aware of similar research programmes. The COST framework is an efficient and simple way of gathering a database and diffusing information among many European partners. The COST Action ‘F3’ [1] in Structural Dynamics was initiated by Prof. Jean-Claude Golinval from the University of Li"ege in Belgium. It was supported for a period of 4 years by the European Community according to the Memorandum of Understanding (MoU) for the implementation of a European Concerted Research Action signed in Brussels on the 25th of June 1997. The purpose of this action was to develop European collaboration, to intensify and to co-ordinate research on the topics of model updating, damage detection, health monitoring and identification of non-linearities in structural dynamics. COST Action F3 grouped 13 European countries (Austria, Belgium, Denmark, Finland, France, Germany, Greece, Italy, Netherlands, Portugal, Spain, Switzerland, United Kingdom), the Joint Research Centre in Ispra and the European Commission.
2. OBJECTIVE OF THE ACTION
Regarding the increased demand for construction or equipment performance in terms of mechanical reliability, lightness, load-carrying capacity, speed, safety, etc., modern design methods become more and more sophisticated. New design objectives need powerful computers, precise description of material behaviour and efficient modelling methods. Furthermore, the complexity of today’s mechanical systems and their increased level of 0888–3270/03/+$35.00/0
# 2003 Elsevier Science Ltd. All rights reserved.
4
J.-C. GOLINVAL AND M. LINK
performance make it necessary to model effects that were not predominantly important in previous analyses. Failure to do so could result in inadequate models and could lead to large discrepancies between the estimation of the structure’s dynamics and its real behaviour. These effects include non-linear dynamics, mechanisms of damping and dissipation of the energy and phenomena of energy localisation. As model updating procedures are used to build representative models of the structural dynamic behaviour, they can also be used to detect damage on structures and monitor the health conditions in general. For example, if the parameters related to a crack geometry are chosen as model updating variables, the updating procedures can be used to predict the crack propagation during the life of the structure. Once the structure is in operation, structural health monitoring (predictive maintenance, default diagnosis, characterisation of the mechanical signature, etc.) becomes the main challenge. The general objective pursued in COST Action F3 was to increase the knowledge required for improving the structural design, the mechanical reliability and the safety of structures in the field of linear and non-linear dynamics.
3. SCIENTIFIC PROGRAMME OF COST ACTION F3
The general features described in the Technical Annex of the MoU are briefly recalled in this section. COST Action F3 focused on three specific research themes through working groups: finite element model updating (WG1), structural damage detection and health monitoring (WG2), and identification of non-linear systems (WG3). 3.1. FINITE ELEMENT MODEL UPDATING METHODS (WORKING GROUP 1) Updating methods are used to adjust analytical models to test results. The field of application of these methods includes: *
*
*
The correction of approximation errors: this type of error is related to assumptions regarding the physics of the model as, for example, the linear behaviour of the structure, the physical behaviour laws (model of elasticity, etc.), in modelling connections and boundary conditions, the limitations of the mathematical formulations used for deriving particular finite elements and the representation of non-accessible structural data (dissipative phenomena); The correction of discretisation errors related to errors arising from a model that is too coarse for capturing some of the significant dynamics of the system: this type of error is related to the optimisation and automation of finite element meshing for dynamic computations; The correction of parametric errors caused by the differences in measured material properties (elesticity modulus, density, section and thickness, etc.), especially for complex new material systems.
Updating methods can be classified in two groups: global and local. Global methods are based on the correction of the global stiffness and/or mass matrices of the finite element model. These methods use optimisation tools for deriving an updated model that is not always physically meaningful even if it reproduces the test data with accuracy. Local methods are usually preferred to global methods because they offer a better potential. They are based on the calculation of modal parameters (frequencies, damping ratios and mode shapes) or frequency response sensitivities with respect to the physical variables of the model (elasticity modulus, density, section and thickness, etc.). The advantages of local methods are their ability to locate erroneous regions of the analytical model and to select the relevant physical or structural parameters for model updating.
COST ACTION F3
5
Whether the methods are global or local in nature, several ‘families’ emerge depending on the mathematical formulations adopted for formulating the updating problem. An important research task of COST Action F3 was therefore to address practical difficulties of model updating (incomplete measurement sets, selection of the optimum data sets for the updating, capability to control the numerical difficulties, assessment of the model’s capability to predict the behaviour of other structural configurations than those used for adjusting the model, etc.). 3.2. STRUCTURAL DAMAGE DETECTION AND HEALTH MONITORING (WORKING GROUP 2) In some aspects, structural damage detection may be considered very similar mathematically to model updating even though objectives of the two problems are different. Model updating seeks the correlation of a mathematical model to experimental data collected from the instumentation of a structure whereas health monitoring is mostly concerned with the variations in time of a structural system. Very often, the system to monitor is represented via mathematical models such as a finite element models, which provides a clear link with updating methods. For example, assuming that data are collected periodically, successive updating of the mathematical models may be performed and comparison of the correlated models to the baseline (undamaged) model provides a practical tool for damage detection. The long-term goal is the ability to realise on-line diagnosis systems which would perform modal tests of the structure throughout its lifetime, identify an experimental model, correlate this model to the analytical one (if it exists) and assess the probability of damage from the comparison between various correlated models in time. For example, bridges can be tested using the natural source of excitation of wind and traffic, and cracks should be located before leading to safety concerns. In structural health monitoring, the structure must be considered under realistic environmental conditions, loading conditions and measurement conditions. Thus, addressing the practical problems of developing long-term stable highly sensitive transducers, systems for easy data transfer and data manipulation, and problems of filtering out changes in environmental and loading conditions are essential. Highly accurate (especially bias-free) system-identification procedures must be implemented in a module-oriented software system, and experience must be gathered using these techniques on large civil engineering structures using only a few sensors. Probabilistic measures must be applied and developed for indication of damage and location and type of damage, and ways of taking a priori information into account must be developed. Further, ways of using the information gathered from analysis of the measured response in updating the reliability of the structure, must be developed to have an appropriate tool for decision making concerning maintenance and visual inspection. 3.3. IDENTIFICATION OF NON-LINEAR SYSTEMS (WORKING GROUP 3) Whenever non-linearities (e.g. when stiffness and damping depend on the load level) are suspected, traditional modal analysis techniques collapse because their underlying mathematics are restricted essentially to the linear domain. The problem of test-analysis reconciliation (model updating, health monitoring, etc.) exhibits several aspects depending upon the type of structure and the type of structural modification involved. In various situations, a local identification of the dynamics of a compnent may be extracted from the modal test of the structure. Therefore, the problem becomes that of ‘modal subtracting’ the behaviour of the known components from that of the whole system. This problem has numerous industrial applications, for example, the inspection of bolted joints in metallic structures or the control of joints in pipe networks.
6
J.-C. GOLINVAL AND M. LINK
In other cases, for instance, when structural changes originate from localised damage, the problem must be investigated from a non-linear point of view. For example, untightening of bolted joints may determine particular vibrational patterns of the type ‘vibration with contact’, due to joint free play. The analysis of these characteristics has been attempted by authors in the general framework of detection of non-linearities. With respect to identification of non-linear systems for health monitoring, artificialintelligence-based techniques should also be considered. During the recent years, many interesting attempts in this field have been proposed, but the subject still needs a more systematic investigation. Also, the durability of recently proposed genetic algorithms for non-linear identification should be considered. These new techniques are not only interesting for health monitoring, but also for control purposes, i.e. active as well as nonactive control of the dynamic response of structures.
4. ORGANISATION OF THE ACTION
4.1. MANAGEMENT COMMITTEE The work undertaken in COST Action F3 was administered by a Management Committee (MC) composed of the chairman, the co-chairman and two national delegates from each signatory country. MC meetings were held once or twice per year to supervise and to ensure co-ordination of the Action. The scientific secretariat of the Action was provided by the University of Li"ege (Belgium). 4.2. WORKING GROUPS This research Action was divided into three Working Groups (WGs) directed by coordinators. Working Group 1 which was co-ordinated by Prof. M. Link and Prof. M. Friswell concerned itself with a detailed examination of finite element model updating methods. The activities of Working Group 2 which was co-ordinated by Prof. K. Worden concerned with the subject of structural health monitoring. Working Group 3 which was co-ordinated by Dr P. Argoul and Dr F. Thouverez was tasked to investigate non-linear systems. The main task of the WG was to define and to organise benchmarking exercises. Throughout the Action, a total of 19 WG meetings were held in different countries. 4.3. SHORT-TERM SCIENTIFIC MISSIONS The aim of Short-Term Scientific Missions (STSMs) is to contribute to the realisation of the scientific objectives of the Action. STSM strengthened the created network by allowing scientists to go to a laboratory in another country to learn a new technique or to make measurements using instruments and/or methods not available in their own laboratory. Throughout the Action, a total of 20 scientific missions were made covering the subjects dealt within the WG. 4.4. INTERNATIONAL WORKSHOPS/CONFERENCES Dissemination of the results of the Action was performed through the organisation of COST F3 international conferences: 1. International Seminar on Modal Analysis (ISMA 23), Katholieke Universiteit leuven, Belgium, 10–12 September 1998. 2. Identification in Engineering Systems, University of Swansea Wales, U.K. 29–31 March,1999 (www.swan.ac.uk/mecheng/ies99). 3. European COST F3 Conference on System Identification & Structural Health Monitoring, Universidad Politecnica de Madrid, Spain, 6–9 June 2000 (www.dmpa.upm.es/SHM).
7
COST ACTION F3
Table 1 Participation to COST F3 conference Year and place of the conference 1998, 1999, 2000, 2000, 2001,
Leuven, Belgium Swansea, U.K. Madrid, Spain Leuven, Belgium Kassel, Germany
Total number of papers
Number of papers related to COST F3
204 73 85 221 55
55 52 33 38 20
4. International Seminar on Modal Analysis (ISMA 25), Katholieke Universiteit Leuven, Belgium, 13–15 September 2000 (www.mech.kuleuven.ac.be/pma/events/isma/isma25conf/isma25conf.html). 5. International Conference on Structural System Identification, University of Kassel, Germany, 5–7 September 2001. Statistics concerning the participation to these conferences are given in Table 1.
5. CONCLUSION
On the basis of the sum of the efforts related to the work presented in this special issue, the economic dimension of COST Action F3 may be estimated roughly to a 100 manyears. However, the overall benefit of the Action is incommensurable since it contributed to the improvement of the communication between many research groups among the 13 European countries that actively participated in the implementation of the Action. It provided a unique opportunity to co-ordinate research efforts between research groups with complementary domains of expertise in aerospace, mechanical and civil engineering.
ACKNOWLEDGEMENTS
The European Commission is gratefully acknowledged for its financial support to cover the co-ordination costs, the scientific secretariat and the allowance for the scientific missions. The chairman and the co-chairman of COST Action F3 would like also to acknowledge the national delegates who welcomed MC and WG meetings in their institutions. Finally, Prof. J. C. Golinval would like also to acknowledge the General Directorate of Technology, Research and Energy (DGTRE) of the Ministry of the Walloon Region in Belgium for its financial support regarding to the organisation of COST F3 meetings at the University of Li"ege.
REFERENCES 1. COST F3 web site: http://www.ulg.ac.be/ltas-vis/costf3/costf3.html.