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Damage detection and model refinement using elemental stiffness perturbations with constrained connectivity

Description: A new optimal update method for the correlation of dynamic structural finite element models with modal data is presented. The method computes a minimum-rank solution for the perturbations of the elemental stiffness parameters while constraining the connectivity of the global stiffness matrix. The resulting model contains a more accurate representation of the dynamics of the test structure. The changes between the original model and the updated model can be interpreted as modeling errors or as changes in the structure resulting from damage. The motivation for the method is presented in the context of existing optimal matrix update procedures. The method is demonstrated numerically on a spring-mass system and is also applied to experimental data from the NASA Langley 8-bay truss damage detection experiment. The results demonstrate that the proposed procedure may be useful for updating elemental stiffness parameters in the context of damage detection and model refinement.
Date: April 1, 1996
Creator: Doebling, S.W.
Partner: UNT Libraries Government Documents Department

Computation of structural flexibility for bridge health monitoring using ambient modal data

Description: Issues surrounding the use of ambient vibration modes for the location of structural damage via dynamically measured flexibility are examined. Several methods for obtaining the required mass- normalized dynamic mode shapes from ambient modal data are implemented and compared. The method are applied to data from a series of ambient modal tests on an actual highway bridge. Results indicate that for the damage case examined, the flexibility from the ambient mode shapes gave a better indication of damage than the flexibility from the forced-vibration mode shapes. This improved performance is attributed to the higher excitation load levels that occur during the ambient test.
Date: May 1, 1996
Creator: Doebling, S.W. & Farrar, C.R.
Partner: UNT Libraries Government Documents Department

MODEL VALIDATION AND UNCERTAINTY QUANTIFICATION.

Description: This session offers an open forum to discuss issues and directions of research in the areas of model updating, predictive quality of computer simulations, model validation and uncertainty quantification. Technical presentations review the state-of-the-art in nonlinear dynamics and model validation for structural dynamics. A panel discussion introduces the discussion on technology needs, future trends and challenges ahead with an emphasis placed on soliciting participation of the audience, One of the goals is to show, through invited contributions, how other scientific communities are approaching and solving difficulties similar to those encountered in structural dynamics. The session also serves the purpose of presenting the on-going organization of technical meetings sponsored by the U.S. Department of Energy and dedicated to health monitoring, damage prognosis, model validation and uncertainty quantification in engineering applications. The session is part of the SD-2000 Forum, a forum to identify research trends, funding opportunities and to discuss the future of structural dynamics.
Date: October 1, 2000
Creator: Hemez, F.M. & Doebling, S.W.
Partner: UNT Libraries Government Documents Department

From shock response spectrum to temporal moments and vice-versa

Description: Temporal momerils have been used in engineering mechanics to condense the information contained in the shock response spectrum into a few scalar quantities. This paper presents an application of temporal moments to the propagation of an explosive-driven shock wave through an assembly of metallic parts. For this particular application, it is shown that temporal moments characterize the response of the system better than other features traditionally used in the analysis of nonlinear, transient events, such as the peak response or 10% duration of event. The inverse problem is also illustrated: the original, time-domain signals and their shock response spectra can be reconstructed from the temporal moments. This property makes temporal moments features of choice for the analysis of experimental data or the development of numerical models because they are low-dimensional quantities; they capture transient dynamics well; and they can be used to re-generate the original time signals.
Date: January 1, 2002
Creator: Hemez, F. M. (Fran├žois M.) & Doebling, S. W. (Scott W.)
Partner: UNT Libraries Government Documents Department

Damage identification and health monitoring of structural and mechanical systems from changes in their vibration characteristics: A literature review

Description: This report contains a review of the technical literature concerning the detection, location, and characterization of structural damage via techniques that examine changes in measured structural vibration response. The report first categorizes the methods according to required measured data and analysis technique. The analysis categories include changes in modal frequencies, changes in measured mode shapes (and their derivatives), and changes in measured flexibility coefficients. Methods that use property (stiffness, mass, damping) matrix updating, detection of nonlinear response, and damage detection via neural networks are also summarized. The applications of the various methods to different types of engineering problems are categorized by type of structure and are summarized. The types of structures include beams, trusses, plates, shells, bridges, offshore platforms, other large civil structures, aerospace structures, and composite structures. The report describes the development of the damage-identification methods and applications and summarizes the current state-of-the-art of the technology. The critical issues for future research in the area of damage identification are also discussed.
Date: May 1, 1996
Creator: Doebling, S.W.; Farrar, C.R.; Prime, M.B. & Shevitz, D.W.
Partner: UNT Libraries Government Documents Department

An overview of modal-based damage identification methods

Description: This paper provides an overview of methods that examine changes in measured vibration response to detect, locate, and characterize damage in structural and mechanical systems. The basic idea behind this technology is that modal parameters (notably frequencies, mode shapes, and modal damping) are functions of the physical properties of the structure (mass, damping, and stiffness). Therefore, changes in the physical properties will cause detectable changes in the modal properties. The motivation for the development of this technology is first provided. The methods are then categorized according to various criteria such as the level of damage detection provided, model-based vs. non-model-based methods and linear vs. nonlinear methods. This overview is limited to methods that can be adapted to a wide range of structures (i.e., are not dependent on a particular assumed model form for the system such as beam-bending behavior and methods and that are not based on updating finite element models). Next, the methods are described in general terms including difficulties associated with their implementation and their fidelity. Past, current and future-planned applications of this technology to actual engineering systems are summarized. The paper concludes with a discussion of critical issues for future research in the area of modal-based damage identification.
Date: September 1, 1997
Creator: Farrar, C.R. & Doebling, S.W.
Partner: UNT Libraries Government Documents Department

Statistical damage identification techniques applied to the I-40 bridge over the Rio Grande River

Description: The statistical significance of vibration-based damage identification parameters is studied via application to the data from the tests performed on the Interstate 40 highway bridge in Albuquerque, New Mexico. A test of statistical significance is applied to the mean and confidence interval estimates of the modal properties and the corresponding damage indicators. The damage indicator used in this study is the change in the measured flexibility matrix. Previously presented deterministic results indicate that damage is detectable in all of the damage cases from these data sets. The results of this study indicate that the changes in both the modal properties and the damage indicators are statistically significant for all of the damage cases. However, these changes are distributed spatially for the first three damage cases and do not localize the damage until the fourth and final damage case.
Date: March 1, 1998
Creator: Doebling, S.W. & Farrar, C.R.
Partner: UNT Libraries Government Documents Department

Lessons learned from applications of vibration-based damage identification methods to a large bridge structure

Description: Over the past 30 years detecting damage in a structure from changes in dynamic parameters has received considerable attention from the aerospace, civil, and mechanical engineering communities. The general idea is that changes in the structure`s physical properties (i.e., stiffness, mass, and/or damping) will, in turn, alter the dynamic characteristics (i.e., resonant frequencies, modal damping, and mode shapes) of the structure. Properties such as the flexibility matrix, stiffness matrix, and mode shape curvature, which are obtained from modal parameters, have shown promise for locating structural damage. However, the application of these techniques to large civil engineering structures is limited because of the inability to find structures that the owners will allow to be damaged. Also, the cost associated with testing these structures can be prohibitive. In this paper, the authors` experiences with performing modal tests on a large highway bridge, in its undamaged and damaged state, for the purpose of damage identification will be summarized. Particular emphasis will be made on the lessons learned from this experience and the lessons learned from recent tests on another bridge structure.
Date: October 1, 1997
Creator: Farrar, C.R. & Doebling, S.W.
Partner: UNT Libraries Government Documents Department

ESTIMATION OF UNCERTAINTY BOUNDS ON UNMEASURED VARIABLES VIA NONLINEAR FINITE ELEMENT MODEL UPDATING

Description: Finite element model validation is a topic of current interest to many researchers in the field of linear and nonlinear structural dynamics. Model validation refers to ''substantiation that a model, within its domain of applicability, possesses a satisfactory range of accuracy consistent with the intended application of the model. [1]. Validation is accomplished primarily by comparison of simulation results to experimental results to confirm the accuracy of the mechanics models in the simulation and the values of the parameters employed in the simulation, and to explore how the simulation might be improved. The assessment of uncertainties in the simulation mechanics models and their associated parameters plays a critical role in the credible validation of nonlinear structural dynamics models. The study of the effects that these uncertainties produce is termed uncertainty quantification (UQ). A major issue in UQ is the determination of how the distributions of the model parameters (which essentially form a set of inputs to the simulation) should be represented in order to accurately reflect the real-world response of the structure. In the case of repeated experiments, it is sometimes adequate to monitor the values of the input variables (e.g. forces, temperatures, velocities, etc.) and estimate a distribution from these observations. However, in many structural dynamics experiments, there can be significant input variables that are either unmeasurable (such as the actual orientation of parts during an impact event) or unmeasured (such as the level of torque applied to an interface during assembly). In these cases, it is necessary to estimate the distributions of the key input variables by indirect means. In this paper, a previously developed model updating technique for nonlinear structural dynamics models is applied to data from repeated experimental trials to estimate the distributions of four key input parameters for a transient impact event. The model updating ...
Date: April 1, 2001
Creator: DOEBLING, S. W.; SCHULTZE, J. F. & HEMEZ, F. M.
Partner: UNT Libraries Government Documents Department

Vibration-based health monitoring and model refinement of civil engineering structures

Description: Damage or fault detection, as determined by changes in the dynamic properties of structures, is a subject that has received considerable attention in the technical literature beginning approximately 30 years ago. The basic idea is that changes in the structure`s properties, primarily stiffness, will alter the dynamic properties of the structure such as resonant frequencies and mode shapes, and properties derived from these quantities such as modal-based flexibility. Recently, this technology has been investigated for applications to health monitoring of large civil engineering structures. This presentation will discuss such a study undertaken by engineers from New Mexico Sate University, Sandia National Laboratory and Los Alamos National Laboratory. Experimental modal analyses were performed in an undamaged interstate highway bridge and immediately after four successively more severe damage cases were inflicted in the main girder of the structure. Results of these tests provide insight into the abilities of modal-based damage ID methods to identify damage and the current limitations of this technology. Closely related topics that will be discussed are the use of modal properties to validate computer models of the structure, the use of these computer models in the damage detection process, and the general lack of experimental investigation of large civil engineering structures.
Date: October 1, 1997
Creator: Farrar, C.R. & Doebling, S.W.
Partner: UNT Libraries Government Documents Department

A Validation of Bayesian Finite Element Model Updating for Linear Dynamics

Description: This work addresses the issue of statistical model updating and correlation. The updating procedure is formulated to improve the predictive quality of a structural model by minimizing out-of-balance modal forces. It is shown how measurement and modeling uncertainties can be taken into account to provide not only the correlated model but also the associated confidence levels. Hence, a Bayesian parameter estimation technique is derived and its numerical implementation is discussed. Two demonstration examples that involve test-analysis correlation with real test data are presented. First, the validation of an engine cradle model used in the automotive industry shows how the design's uncertainties can be reduced via model updating. The second example consists of employing test-analysis correlation for identifying the degree of nonlinearity of the LANL 8-DOF testbed.
Date: February 8, 1999
Creator: Hemez, F.M. & Doebling, S.W.
Partner: UNT Libraries Government Documents Department

Test-Analysis Correlation and Finite Element Model Updating for Nonlinear Transient Dynamics

Description: This research aims at formulating criteria for measuring the correlation between test data and finite element results for nonlinear, transient dynamics. After reviewing the linear case and illustrating the limitations of modal-based updating when it is applied to nonlinear experimental data, simple time-domain, test-analysis correlation metrics are proposed. Two implementations are compared: the conventional least-squares technique and the Principal Component Decomposition that correlates subspaces rather than individual time-domain responses. Illustrations and discussions are provided using the LANL 8-DOF system, an experimental testbed for validating nonlinear data correlation and model updating techniques.
Date: February 8, 1999
Creator: Hemez, F. M. & Doebling, S. W.
Partner: UNT Libraries Government Documents Department

Damage detection in aircraft structures using dynamically measured static flexibility matrices

Description: Two methods for detecting the location of structural damage in an aircraft fuselage using modal test data are presented. Both methods use the dynamically measured static flexibility matrix, which is assembled from a combination of measured modal vectors, frequencies, and driving point residual flexibilities. As a consequence, neither method requires a mode-to-mode correlation, and both avoid tedious modal discrimination and selection. The first method detects damage as a softening in the point flexibility components, which are the diagonal entries in the flexibility matrix. The second method detects damage from the disassembled elemental stiffnesses as determined using a presumed connectivity. Vibration data from a laser vibrometer is used to measure the modal mechanics of a DC9 aircraft fuselage before and after induced weakening in a longitudinal stringer. Both methods are shown to detect the location of the damage, primarily because the normal stiffness of the reinforced shell of the fuselage is localized to a few square centimeters.
Date: February 1, 1996
Creator: Robinson, N.A.; Peterson, L.D.; James, G.H. & Doebling, S.W.
Partner: UNT Libraries Government Documents Department

Estimating the error in simulation prediction over the design space

Description: This study addresses the assessrnent of accuracy of simulation predictions. A procedure is developed to validate a simple non-linear model defined to capture the hardening behavior of a foam material subjected to a short-duration transient impact. Validation means that the predictive accuracy of the model must be established, not just in the vicinity of a single testing condition, but for all settings or configurations of the system. The notion of validation domain is introduced to designate the design region where the model's predictive accuracy is appropriate for the application of interest. Techniques brought to bear to assess the model's predictive accuracy include test-analysis coi-relation, calibration, bootstrapping and sampling for uncertainty propagation and metamodeling. The model's predictive accuracy is established by training a metalnodel of prediction error. The prediction error is not assumed to be systcmatic. Instead, it depends on which configuration of the system is analyzed. Finally, the prediction error's confidence bounds are estimated by propagating the uncertainty associated with specific modeling assumptions.
Date: January 1, 2003
Creator: Shinn, R. (Rachel); Hemez, F. M. (Fran├žois M.) & Doebling, S. W. (Scott W.)
Partner: UNT Libraries Government Documents Department

A statistical comparison of impact and ambient testing results from the Alamosa Canyon Bridge

Description: In this paper, the modal properties of the Alamosa Canyon Bridge obtained using ambient data are compared to those obtained from impact hammer vibration tests. Using ambient sources of excitation to determine the modal characteristics of large civil engineering structures is desirable for several reasons. The forced vibration testing of such structures generally requires a large amount of specialized equipment and trained personnel making the tests quite expensive. Also, an automated health monitoring system for a large civil structure will most likely use ambient excitation. A modal identification procedure based on a statistical Monte Carlo analysis using the Eigensystem Realization Algorithm is used to compute the modal parameters and their statistics. The results show that for most of the measured modes, the differences between the modal frequencies of the ambient and hammer data sets are statistically significant. However, the differences between the corresponding damping ratio results are not statistically significant. Also, one of the modes identified from the hammer test data was not identifiable from the ambient data set.
Date: December 31, 1996
Creator: Doebling, S.W.; Farrar, C.R. & Cornwell, P.
Partner: UNT Libraries Government Documents Department

DIAMOND: A graphical interface toolbox for comparative modal analysis and damage identification

Description: This paper introduces a new toolbox of graphical-interface software algorithms for the numerical simulation of vibration tests, analysis of modal data, finite element model correlation, and the comparison of both linear and nonlinear damage identification techniques. This toolbox is unique because it contains several different vibration-based damage identification algorithms, categorized as those which use only measured response and sensor location information (non-model-based techniques) and those which use finite element model correlation (model-based techniques). Another unique feature of this toolbox is the wide range of algorithms for experimental modal analysis. The toolbox also contains a unique capability that utilizes the measured coherence functions and Monte Carlo analysis to perform statistical uncertainty analysis on the modal parameters and damage identification results. This paper contains a detailed description of the various modal analysis, damage identification, and model correlation capabilities of toolbox, and also shows a sample application which uses the toolbox to analyze the statistical uncertainties on the results of a series of modal tests performed on a highway bridge.
Date: June 1, 1997
Creator: Doebling, S.W.; Farrar, C.R. & Cornwell, P.J.
Partner: UNT Libraries Government Documents Department

Effects of measurement statistics on the detection of damage in the Alamosa Canyon Bridge

Description: This paper presents a comparison of the statistics on the measured model parameters of a bridge structure to the expected changes in those parameters caused by damage. It is then determined if the changes resulting from damage are statistically significant. This paper considers the most commonly used modal parameters for indication of damage: modal frequency, mode shape, and mode shape curvature. The approach is divided into two steps. First, the relative uncertainties (arising from random error sources) of the measured modal frequencies, mode shapes, and mode shape curvatures are determined by Monte Carlo analysis of the measured data. Based on these uncertainties, 95% statistical confidence bounds are computed for these parameters. The second step is the determination of the measured change in these parameters resulting from structural damage. Changes which are outside the 95% bounds are considered to be statistically significant. It is proposed that this statistical significance can be used to selectively filter which modes are used for damage identification. The primary conclusion of the paper is that the selection of the appropriate parameters to use in the damage identification algorithm must take into account not only the sensitivity of the damage indicator to the structural deterioration, but also the uncertainty inherent in the measurement of the parameters used to compute the indicator.
Date: December 31, 1996
Creator: Doebling, S.W.; Farrar, C.R. & Goodman, R.S.
Partner: UNT Libraries Government Documents Department