Showing papers by "Charles R. Farrar published in 1998"

A summary review of vibration-based damage identification methods

Abstract: This paper provides an overview of methods to detect, locate, and characterize damage in structural and mechanical systems by examining changes in measured vibration response. Research in vibration-based damage identification has been rapidly expanding over the last few years. 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 presented. The methods are categorized according to various criteria such as the level of damage detection provided, model-based versus non-model-based methods, and linear versus nonlinear methods. The methods are also 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 vibration-based damage identification.

Comparative study of damage identification algorithms applied to a bridge: I. Experiment

Abstract: Over the past 30 years detecting damage in a structure from changes in global dynamic parameters has received considerable attention from the civil, aerospace and mechanical engineering communities. The basis for this approach to damage detection is that changes in the structure's physical properties (i.e., boundary conditions, stiffness, mass and/or damping) will, in turn, alter the dynamic characteristics (i.e., resonant frequencies, modal damping and mode shapes) of the structure. Changes in properties such as the flexibility or stiffness matrices derived from measured modal properties and changes in mode shape curvature have shown promise for locating structural damage. However, to date there has not been a study reported in the technical literature that directly compares these various methods. The experimental results reported in this paper and the results of a numerical study reported in an accompanying paper attempt to fill this void in the study of damage detection methods. Five methods for damage assessment that have been reported in the technical literature are summarized and compared using experimental modal data from an undamaged and damaged bridge. For the most severe damage case investigated, all methods can accurately locate the damage. The methods show varying levels of success when applied to less severe damage cases. This paper concludes by summarizing some areas of the damage identification process that require further study.

Comparative study of damage identification algorithms applied to a bridge: II. Numerical study

Abstract: This paper extends the study of damage identification algorithms summarized in the accompanying paper `Comparative study of damage identification algorithms: I. Experiment' to numerical examples. A finite element model of a continuous three-span portion of the I-40 bridges, which once crossed the Rio Grande in Albuquerque, NM, was constructed. Dynamic properties (resonant frequencies and mode shapes) of the undamaged and damaged bridge that were predicted by the numerical models were then correlated with experimental modal analysis results. Once correlated with the experimental results, eight new damage scenarios were introduced into the numerical model including a multiple damage case. Also, results from two undamaged cases were used to study the possibility that the damage identification methods would produce false-positive readings. In all cases analytical modal parameters were extracted from time-history analyses using signal processing techniques similar to those used in the experimental investigation. This study provides further comparisons of the relative accuracy of these different damage identification methods when they are applied to a set of standard numerical problems.

Bridge modal properties using simplified finite element analysis

Abstract: A simplified approach to the dynamic finite element modeling of composite girder-slab bridges is presented using the limiting case of a single beam element to represent the girder-slab cross section. Dynamic properties calculated with this simplified model are compared with experimental results obtained from an in situ composite girder bridge and with more detailed shell element model predictions. The simplified beam element model accurately calculates the mode shapes of the structure. This agreement, however, is dependent on accurate modeling of the piers and kinematic constraints between the bridge and piers and the appropriate representation of the bridge's torsional properties. The calculated resonant frequencies associated with these modes show some discrepancy when compared with the experimental results. This discrepancy is attributed to the inability of the single beam element model to simulate the three-dimensional boundary conditions found in the actual structure. The simple models provide approximations to the dynamic properties that are accurate enough to be useful in preliminary seismic scoping studies and in the simplified modeling of long, multispan bridges.

A comprehensive monitoring system for damage identification and location in large structural and mechanical systems

Abstract: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project conducted at the Los Alamos National Laboratory (LANL). This project has focused on developing and experimentally verifying a suite of analytical tools for identifying the onset of damage in structural and mechanical systems from changes in their vibration characteristics. A MATLAB-based computer code referred to as Damage Identification And Modal aNalysis of Data @IAMOND) was developed. The code was then extensively exercised on data obtained from a variety of test structures. The most notable structure was an in situ bridge located ten mile north of Truth or Consequences, New Mexico. The suite of tools contained in DIAMOND is now being applied to the nuclear weapons enhanced surveillance program and an industrial partner has asked to enter into a partnership so that they can implement routines from DIAMOND into their commercial damage assessment hardware for large civil engineering structures. Because of the large volume of requests from around the world for DIAMOND, it can now be downloaded from the web site: http://esaea-www.esa.lanl.gov/damagejd. Background and Research Objectives The interest in the ability to monitor a structure and detect damage at the earliest possible stage is pervasive throughout the civil, mechanical, and aerospace engineering communities. Current damage detection methods are either visual or localized experimental methods such as acoustic or resonant ultrasonic (RUS) methods, magnetic field methods, "Principal Investigator, e-mail: farrar@lanl.gov