Showing papers on "Structural health monitoring published in 1999"

Impedance-Based Structural Health Monitoring for Temperature Varying Applications

Abstract: A new structural health monitoring technique capable of in-service, on-line incipient damage detection has been proposed by the Center for Intelligent Material Systems and Structures, Physical changes in a structure cause changes in the mechanical impedance. Due to the electromechanical coupling in piezoelectric materials, this change causes a change in the electrical impedance of the piezoelectric sensor. Hence, by monitoring the electrical impedance and comparing this to a baseline measurement, we can determine when structural damage has either occurred or is imminent. However, in almost all practical health monitoring applications, the structure being monitored is constantly undergoing change due to the effect of external boundary conditions. One of the important factors that leads to this change is the temperature variations. In this paper, temperature effects on the electrical impedance of piezoelectric materials and the structures have been investigated. A computer algorithm was developed which incorporates temperature compensation into our health monitoring applications. Three experimental investigations were performed successfully under the temperature varying condition, in the range of 25 to 75°C, including a bolted pipe structure, composite reinforced aluminum and precision part such as gears. It was found that, by this compensation procedure, the impedance based health monitoring tehcnique is able to detect damage in the incipient stage, even with the presence of significant temperature variation.

Optical fiber sensors for spacecraft applications

Abstract: Optical fiber sensors offer a number of advantages for spacecraft applications. A principal application is strain sensing for structural health monitoring, shape determination, and spacecraft qualification testing. This paper will review the results of recent work at the Naval Research Laboratory where optical fiber strain sensors have been used on spacecraft structures and ground test hardware. The sensors have been both surface mounted to the structure and embedded in fiber-reinforced polymer composites. The issue of potential strength reduction of high-performance composites due to embedded optical fiber sensors and leads has been studied, low-cost fabrication of tubular struts with embedded sensors has been demonstrated, and a novel technique for fiber ingress-egress from composite parts has been developed. Applications of fiber sensors discussed in this paper include distributed dynamic strain monitoring of a honeycomb composite plate and a lightweight reflector during acoustic qualification tests, ultrahigh-sensitivity static strain and temperature measurements for precision structures, and on-line system identification of a lightweight laboratory truss.

Impedance-based health monitoring technique for massive structures and high-temperature structures

Abstract: This paper presents the recent research on impedance-based structural health monitoring technique at Center for Intelligent Material Systems and Structures. The basic principle behind this technique is to use high frequency structural excitation (typically greater than 30 kHz) through the surface-bonded piezoelectric sensor/actuator to detect changes in structural point impedance due to the presence of damage. Two examples are presented in this paper to explore its effectiveness to the practical field applications. First, the possibility of implementing the impedance-based health monitoring technique to detect damage on massive, dense structures was investigated. The test structure considered is a massive, circular, three-inch thick steel steam header pipe. Practical issues such as effects of external boundary condition changes and the extent of damage that could be detected were the issues to be identified. By the consistent repetition of tests, it has been determined that this impedance-based technique is able to detect a very small size of hole (4 X 20 mm), which can be considered the mass loss of 0.002% of entire structure. The second example includes the implementation of this technique in the high temperature applications. With high temperature piezoceramic materials, which have a Curie temperature higher than 2000 degrees F, experiments were performed to detect damage on the bolted joint structure in the temperature range of 900 - 1100 degrees F. Through the experimental investigations, the applicability of this impedance-based health monitoring technique to monitor such an extreme application was verified, with some practical issues need to be resolved. Data collected from the tests proved beyond a doubt the capability of this technology to detect both existing and imminent damage.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A Bayesian probabilistic approach to structural health monitoring

Abstract: Some general issues associated with online structural health monitoring are discussed. In order to address the problem of determining the existence and location of damage in the presence of uncertainties, a global model-based structural health monitoring method which utilizes Bayesian probabilistic inference is developed. The results of tests using simulated data are described.

Damage Detection and Evaluation II

Abstract: In the most general terms damage can be defined as changes introduced into a system that adversely effect the current or future performance of that system. Implicit in this definition is the concept that damage is not meaningful without a comparison between two different states of the system, one of which is assumed to represent the initial, and often undamaged, state. This discussion is focused on the study of damage identification in structural and mechanical systems. Therefore, the definition of damage will be limited to changes to the material and/or geometric properties of these systems, including changes to the boundary conditions and system connectivity, which adversely effect the current or future performance of that system.

Dynamic monitoring of structural health in cable-supported bridges

Abstract: A sophisticated instrumentation system, called Wind and Structural Health Monitoring System (WASHMS), is devised by the Hong Kong Highways Department to monitor the structural health and performance of three long-span cable-supported bridges in Hong Kong. This system consists of about 900 sensors including accelerometers. Strain gauges, displacement transducers, level sensing stations, anemometers, temperature sensors and wight-in-motion sensors. The Hong Kong Polytechnic University is commissioned to investigate the feasibility of using measured dynamic characteristics for structural damage detection in these bridges. Explored in this paper are some key issues related to developing a viable vibration-based damage assessment strategy for these bridges: (a) Evaluation of possible damage likely to occur in the three bridges - the detectibility of damage to structural components by using global or local dynamic characteristics is discussed; (b) Concept and rules of damage detection oriented modeling for large-scale structures - a super-element formulation for complicated two-tier bridge deck is developed within this framework; (c) System identification methodology for heavily redundant structures - a neural network based hierarchical identification strategy is pose for successively detecting the occurrence, type, location and extent of the damage.

Structural health monitoring using active sensors and wavelet transforms

Abstract: Health monitoring methods using active sensors (e.g. piezoelectric transducers) and wavelet transforms are being developed in the Department of Mechanical Engineering at the University of South Carolina. In these methods, wave propagation signals are collected using arrays of piezoelectric transducers placed on or embedded in a structure. The collected signals are analyzed using appropriate wavelet transforms. The final interpretation of the sensor signals is based on signal patterns uncovered by the wavelet transforms in correlation with elastic-wave propagation theory. A number of specimens have been instrumented and tested, which include simple steel beams and actual composite aircraft panels. Impact tests simulating low velocity impact by foreign-object have been conducted and will be used to illustrate the wavelet-based methods.

Longitudinal wave propagation measuring technique for structural health monitoring

Abstract: This work presents a practical and quantitative technique for assessment of the location and characterization of damages by longitudinal wave propagation measuring method. It is shown that the damage location can be simply determined by taking the difference of the time response between the healthy condition and the damaged condition, and the damage state would be quantified by the power consumption metric defined in frequency domain. The accuracy and the efficiency were validated by the experiment results on a simple aluminum beam. Further, the technique using piezoelectric materials as actuator and sensor separately and simultaneously is also demonstrated in order to reduce the numbers of the PZT patches embedded in the structures and the cost for wiring.

Long-term health monitoring of an advanced polymer composite bridge

Abstract: A long-term, structural health monitoring system has been designed and installed on the firs polymer composite bridge to be built in Delaware. The system is designed to monitor and record strains and deflections of the bridge, and the temperature and humidity of the surrounding area. Two types of information are gathered, 'monitor' data and 'event' data. The monitor data records very slow gradual changes in the bridge behavior, while the event data captures the bridge response due to truck loads. The system has been on- line since June, 1998. Sample result are presented in the paper of event and monitor data. The event data shows that the transverse strain of the deck is greater than the longitudinal strain, by a factor of about 1.5, and that the absolute deflection of the deck at mid-span is due mostly to the deflection of the edge girder. Monitor data from a one month period is presented that shows the thermal variations in strain due to daily temperature changes, and the gradual, changes due to the average daily temperature. The long-term monitoring system should provide valuable data for assessing the long-term performance and durability of this unique polymer composite bridge.

On-line Damage Assessment Using Operating Deflection Shapes

Abstract: Structural health monitoring using measured vibration data may (or not) be based on a numerical model. If a structural model is not available, the measurements of the nominally healthy structure have to be used as the baseline for comparison. In this way, alterations of the behavior may be tracked. This approach can be considered as a reactive way to handle the problem since the engineer will detect that the characteristics of the structure changed, but he will not know if the modification(s) render(s) the structure unreliable. The exploitation of a numerical model allows the application of a more rich, proactive strategy. In some conditions, the analyst may even be able to diagnose the remaining lifetime of the structure. A common approach to health monitoring is to use identified modal data. In this paper, the direct use of operating deflection shapes (ODS) is considered The main advantage of this approach is that ODS are more sensitive to structure modifications than mode shapes which have to be identified; moreover, the modal analysis efforts (and errors) are avoided. In this paper, a two level approach for damage assessment is presented. In the reactive level, current experimental ODS are compared to the healthy measured ODS. The Frequency Domain Assurance Criterion -FDAC- is used to track a global evolution and the shifted residual ODS technique is used to obtain a first damage localization. If changes in the ODS are significant, the proactive level of damage assessment is activated. It uses a FE model and is based on the theory of Minimization of Errors on Constitutive Equations -MECE- to locate and quantify damage in terms of physical parameters. The proposed methods are applied on the example of an actual civil engineering structure on which the time evolution of damage is known.

Micromachined ultrasonic transducers for damage detection in CFRP composites

Abstract: Fiber-reinforced composite materials are increasingly being used in the construction of advanced aircraft structures. The detection of damage in such materials is critical to ensure safe application. An in-situ structural health monitoring system, either embedded in the composite structure or surface-mounted, would permit the timely detection of damage in such structures. There has been much work reported on the application of, for example, piezoelectric and optical fiber sensing technology for the detection of composite damage. In this paper we outline our approach using silicon microsystem technology.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Microsensors and MEMS for health monitoring of composite and aircraft structures in flight

Abstract: Microsensors and Microelectromechanical Systems (MEMS) are currently being applied to the structural health monitoring of critical aircraft components. The approach integrates acoustic emission, strain gauges, MEMS accelerometers and vibration monitoring devices with signal processing electronics to provide real-time indicators of incipient failure of aircraft components with a known history of catastrophic failure due to fracture.

An overview of vibration‐based damage detection research at Los Alamos National Laboratory

Abstract: The ability to monitor the structural health of our aging infrastructure is becoming increasingly important A wide variety of highly effective local nondestructive evaluation tools are available However, damage identification based upon changes in vibration characteristics is one of the few methods that monitors changes in the structure on a global basis The material presented herein will summarize the structural health monitoring research that has been conducted at Los Alamos National Laboratory over the last 8 years First, the process of vibration‐based damage detection will be described as a problem in statistical pattern recognition This process has three portions: (1) data acquisition and cleansing; (2) feature selection and data compression; and (3) statistical model development Current research regarding feature selection and statistical model development will be emphasized with the application of this technology to large‐scale, in situ bridge structures and to bridge columns that were tested

Details of the electromechanical (E/M) impedance health monitoring of spot-welded structural joints

Abstract: Health monitoring of structural joints is a major concern of the engineering community. Among joining techniques, spotwelding play a major role. Spot welding is the traditional method of assembly for steel-based automotive structures, while spot-welding of aluminum is being considered for future vehicular structures. Though spot welding of steel is well researched and understood, the spot-welding of aluminum still poses a considerable challenge. The durability and health monitoring of aluminum spot-welded joint is of major importance. The present paper addresses the use of electro-mechanical (E/M) method and piezoelectric active sensors for health monitoring spot-welded structural joints. Experiments were performed on aluminum-alloy spot-welded lap shear specimens under fatigue loading to determine a stable pattern of crack propagation in correlation with applied fatigue cycles. The specimen stiffness method was used to correlate crack advancement through the specimen with specimen stiffness reduction. In this way, an independent method of assessing the degree of structural deterioration as a function of applied fatigue cycles and remaining life was established for that class of specimens. Then, representative test specimens were instrumented with piezo-electric wafer transducers, and the base E/M impedance signature was recorded over the relevant frequency range. Subsequently, the specimens were subjected to fatigue loading such that crack-type damage was initiated and propagated under controlled conditions. During tests, the specimens were continuously monitored for stiffness reduction, and E/M impedance signature was recorded at predetermined damage levels. The crack damage initiation and propagation was correlated with E/M impedance measurements. Damage index values were compared and correlated with the crack damage propagation. Sources of experimental errors were identified and discussed.

Novel autonomous structural health monitoring using piezoelectrics

Abstract: A simple, novel method of automatically monitoring the health of a structure with piezoelectric devices is presented. With this method, piezoelectric devices are attached or embedded to a structure. When damage occurs in the piezostructure, a change in stiffness typically occurs. Since the piezoelectric patch is electromechanically coupled to the structure, it will respond with a change in permittivity or capacitance. A relatively new transducer technology, the adaptive piezoelectric sensoriactuator, inherently monitors the capacitance of a piezoelectric device, providing the opportunity for real-time health monitoring. This method is simple, cheap, and easy to implement. A simple experiment demonstrates the success of the concept by detecting a change in the boundary conditions of a composite beam piezostructure.

Title: Recent Progress in the Application of E/M Impedance Method to Structural Health Monitoring, Damage Detection, and Failure Prevention

Abstract: 1 Recent progress in the application of the electro-mechanical (E/M) impedance method to structural health monitoring, damage detection, and failure prevention is presented. A brief review of the E/M impedance method principles is followed by applications to bolted and spot-welded structural joints, and to polymeric composite overlays to construction engineering structures. The modeling of the interaction between the piezoelectric active sensor and the host structure is described. The development of a meaningful structural damage index that can identify the damage presence, location, and magnitude is treated, and several recent works, including neural networks approach, are cited. Conclusions and suggestions for further work, and relevant bibliography are included.

Microsensors and MEMS for health monitoring of composite and aircraft structures in hight

Abstract: Microsensors and Microelectromechanical Systems (MEMS) are currently being applied to the structural health monitoring of critical aircraft components. The approach integrates acoustic emission, strain gauges, MEMS accelerometers and vibration monitoring devices with signal processing electronics to provide real-time indicators of incipient failure of aircraft components with a known history of catastrophic failure due to fracture. Recently a combination of the need for safety in the air and the desire to control costs is encouraging the use of in-flight monitoring of aircraft components and systems using light-weight, wireless and cost effective microsensors and MEMS. An in-situ Aircraft structural health monitoring (ASHM) system, with sensors embedded in the composite structure or surface-mounted on the structure, would permit the timely detection of damage in aircraft. In this paper we give an overview of microsensors and MEMS (of physical dimensions of the order of centimeter or less) and their associated driving electronics for health and condition monitoring of existing (and aging) and future aircraft and composites. Silicon micromachining offers the potential for fabricating a range of microsensors and MEMS for structural applications including load, vibration and acoustics characterization and monitoring. Such microsensors are extremely small; they can be embedded into structural materials, can be mass-produced and are therefore potentially cheap. Additionally a range of sensor types can be integrated onto a single chip with built-in electronics and ASIC (Application Specific Integrated Circuit), providing a low power Microsystems The smart sensors are being developed using the standard microelectronics and micromachining in conjunction with novel Penn State smart electronics or wireless communication systems suitable for condition monitoring of aircraft structures in-flight. The main application areas of this investigation include continuos monitoring of a) structural integrity of aging aircraft, b) fatigue cracking, c) corrosion, d) deflection and strain of aircraft structures, wings, and rotorblades, e) impact damage, f) delamination, g) location and propagation of cracks, h) the quality of conventional bonds and 'kissing bonds' in composite structures.

Advanced Monitoring System for Large Structural Systems

Abstract: The concept of a health monitoring system for civil structural systems is continuously evolving as a result of new requirements and the development of new sensor technologies. The area of structural health monitoring is still rather underdeveloped, and there is a lack of consensus on what needs to be monitored and how it can be implemented. The goal of a health monitoring system is to be able to provide advance notice of impending failures, and if possible provide objective data on preventive maintenance and retrofit requirements. In this article the authors have enumerated the general requirements of a successful health monitoring system for bridges, and have provided a real life example of a bridge monitoring system, that the authors are in the process of implementing.