Showing papers on "Structural health monitoring published in 1995"

Structural health monitoring using active members and neural networks

Abstract: A system for monitoring the structural integrity of a mechanical structure. The system utilizes a trainable adaptive interpreter such as a neural network to analyze data from the structure to characterize the structure's health. An actuator is attached to the mechanical structure for generating vibrations in response to an input signal. A sensor, also attached to the mechanical structure, senses the vibrations and generates an output signal in response thereto. The sensor output signal is then coupled to a pre-trained adaptive interpreter for generating an output which characterizes the structural integrity of the mechanical structure. The system can provide continual health monitoring of a structural system to detect structural damage and pinpoint probable location of the damage. The system can operate while the structural system is in service there by significantly reducing structural inspection costs.

Modal parameter extraction from large operating structures using ambient excitation

Abstract: A technique called the Natural Excitation Technique or has been developed to response extract response parameters from large operational structure when subjected to random and unmeasured forces such as wind, road noise, aerodynamics, or waves. Six applications of NExT to ambient excitation testing and NExT analysis are surveyed in this paper with a minimum of technical detail. In the first application, NExT was applied to a controlled-yaw Horizontal-Axis Wind Turbine (HAWT). By controlling the yaw degree of freedom an important class of rotating coordinate system effects are reduced. A new shape extraction procedure was applied to this data set with good results. The second application was to a free-yaw HAWT. The complexity of the response has prompted further analytical studies and the development of a specialized visualization package. The third application of NExT was to a parked three-bladed Vertical-Axis Wind Turbine (VAWT) in which traditional modal testing could not excite all modes of interest. The shape extraction process used cross-correlation functions directly in a time-domain shape-fitting routine. The fourth application was to ground transportation systems. Ongoing work to improve driver and passenger comfort in tractor-trailer vehicles and to refine automobile body and tire models will use NExT. NExT has been used to process ambient vibration data for Finite Element Model correlation and is being used to study Structural Health Monitoring with ambient excitation. Shape fitting was performed using amplitude and phase information taken directly from the cross-spectra. The final application is to an offshore structure. This work is on-going, however initial studies have found a high-modal density, high noise content, and sparse data set.

Aircraft structural health monitoring system development: overview of the Air Force/Navy smart metallic structures program

Abstract: Significant progress in fulfilling the current joint Air Force/Navy `Smart Metallic Structures (SMS)' program primary objective, to demonstrate a viable structural health monitoring system (SHMS) for a large structural aircraft component, is presented. Structural health monitoring and its relation to current Force Management (FM) and Aircraft Structural Integrity Program (ASIP) procedures are first reviewed together with a brief status overview of the relevant sensor technologies (e.g. AE, fiber-optic, corrosion, etc.). Key features of the SHMS architecture are described for the selected F/A-18 bulkhead and T-38 wing spar structural demonstration articles, highlighting sensors, processors, data busses, hardware, and software. Results from acoustic monitoring of the program sub-element structural tests are presented in some detail along with a status review of the SHMS multiplex bus component hardware and software. Finally, structural requirements for an SHMS meeting minimum ASIP guidelines for damage detection are discussed along with foals for future testing and development of the SHMS under the SMS program.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Design and performance of a fiber Bragg grating distributed strain sensor system

Abstract: We describe the design and performance of a prototype fiber Bragg grating demodulation system based on the use of a scanning fiber Fabry-Perot filter. The computer driven system is capable of demodulating several arrays of wavelength division multiplexed gratings at various scanning rates for real-time strain display and data logging. The instrument represents a new measurement tool which should be useful in a variety of structural health monitoring applications. Results obtained by the system in several applications are presented and system performance limitations are discussed.

Design and evaluation of passive and active structural health monitoring systems for bridges and buildings

Abstract: Structural health monitoring systems have been designed and evaluated in the laboratory for installation in several bridges and commercial buildings. The systems employ solid-state sensor elements which experience a strain-dependent phase transformation from a metastable, nonmagnetic, austenitic phase to the stable, ferromagnetic, martensitic phase. The irreversible phase transformation is useful for indicating the level of peak structural strain experienced in a particular monitored location. Some of the sensor material characteristics and details related to the phase transformation are discussed as applied to structural health monitoring. The design of representative systems for bridges and commercial buildings is included. Important system(s) features and design capabilities are discussed. Finally, the evaluation of passive and active systems in the laboratory is discussed. The results of experiments detailing the behavior of these systems under uniaxial tension and cyclic loading conditions are presented.

Passive monitoring systems for structural damage assessment

Abstract: Engineering structures are designed to function within the elastic domain and designs are chosen based on their relative merits and capabilities to suitably address the specific requirements. The decision to monitor a structure to determine the long-term performance characteristics and operational stability involves many factors including, but not limited to, the objectives of the monitoring and the available resources to do so. One must choose between active monitoring systems, i.e., those that require real-time power supplies, and ancillary data acquisition, storage and monitoring systems and passive systems which, as the name implies, operate without any of these constraints. Passive techniques for structural health monitoring and materials damage assessment are reviewed. Data are presented to illustrate their application as related to structural health monitoring, crack detection and crack growth monitoring, and damage assessment in composite materials.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Multiplexed extrinsic Fabry-Perot interferometers and applications

Abstract: Extrinsic Fabry-Perot interferometric (EFPI) sensors have previously been demonstrated for relative strain and temperature measurements for smart structure applications. Inherent difficulties in the signal processing of these devices has created the need for absolute measurement capabilities. In this paper, we present an absolute measurement technique based upon white-light interferometric path matching. The system matches a reference gap to the sensing gap of an EFPI. When the difference of these two lengths is within the coherence length of the source, an intensity envelope is created in the system output. Determination of the corresponding path mismatch indicates the size of the sensor gap and hence strain can be determined. This measurement technique is capable of multiplexing an array of EFPI sensors and data will be presented demonstrating four multiplexed devices. Theoretical considerations for system optimization are also presented. As the only fiber-optic sensors subcontractor to Northrop Corporation on the Navy/Air Force-sponsored Smart Metallic Structures (SMS) program, Fiber & Sensor Technologies (F&S) is developing the optical fiber fatigue gage instrumentation for a multiplexed, in situ structural health monitoring system for aging aircraft. In March, 1995, F&S successfully demonstrated the system on a full-size F/A-18 wing-box spar fully instrumented with 12 of F&S' patented EFPI optical fiber strain gages. F&S is now in process of up-scaling the signal processing system in addition to the optics and intends to demonstrate a second generation multipoint sensor system capable of simultaneously monitoring strains at up to 60 different sites throughout the aircraft later in 1995 or early 1996.

Remotely queried embedded sensors in composite structures

Abstract: Embedded sensor technology in polymer-based structural composites has received much attention in recent years. Embedded sensors are critical to structural health monitoring in high performance applications, e.g. 'smart' skins and structures. To date, optical fiber sensors have been the principal sensing technique for these applications. However, some notable shortcomings of these sensors include difficulties with ingress and egress from part, and interdependence of strain and temperature measurements. This paper discusses an alternate approach for structural health monitoring: remote query of silicon microsensors embedded in composite structures. The technologies involved will first be reviewed, followed by a listing of the potential benefits of applying this technology. Next, a newly initiated program to demonstrate the feasibility of remote query techniques for structural monitoring of composites will be described.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Structural monitoring for fatigue crack detection and prediction

Abstract: The applicability of structural monitoring is discussed for crack- type damage of metal structures. Two types of monitoring methods are examined: health monitoring using sensors for the detection of existing fatigue cracks and the attaching of sacrificial specimens on members for the prediction of fatigue damage.

Composite tank with structurally integrated noninvasive fluid-level monitoring

Abstract: One of the goals of smart structures research is the integration of functions such as sensing, actuation and control into structures themselves. Smart structures research activity to date has largely focused on process control or integrated structural health monitoring. Other functions can also be integrated into structures. In this paper, results of an experimental demonstration integrating fluid level sensing with the structural support of a nonconductive tank are reported. The system utilized a composite capacitance fuel probe permanently bonded into the tank walls as a structural support. The probe was connected in series with a resistor and inductive coil forming a resonant RLC circuit. Non-contact excitation of the circuit through the tank wall and measurement of the resonance frequency allowed the fluid level to be determined. The theoretical basis of the concept is provided and compared with experimental results. Potential applications of the technology are also discussed.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.