Showing papers by "Charles R. Farrar published in 2006"

Performance assessment and validation of piezoelectric active-sensors in structural health monitoring

Abstract: A sensor diagnostics and validation process that performs in situ monitoring of the operational status of piezoelectric (PZT) active-sensors in structural health monitoring (SHM) applications is presented. Both degradation of the mechanical/electrical properties of a PZT transducer and the bonding defects between a PZT patch and a host structure could be identified by the proposed process. This study also includes the investigation into the effects of the sensor/structure bonding defects on high-frequency SHM techniques, including Lamb wave propagations and impedance methods. It has been found that the effects are significant, modifying the phase and amplitude of propagated waves and changing the measured impedance spectrum. These changes could lead to false indications on the structural conditions without an efficient sensor-diagnostic process. The feasibility of the proposed sensor diagnostics procedure is then demonstrated by analytical studies and experimental examples, where the functionality of the surface-mounted piezoelectric sensors was continuously deteriorated. The proposed process can provide a metric that can be used to determine the sensor functionality over a long period of service time or after an extreme loading event. Further, the proposed method can be useful if one needs to check the operational status of a sensing network right after its installation.

Piezoelectric Active Sensor Self-Diagnostics Using Electrical Admittance Measurements

Abstract: This paper presents a piezoelectric sensor self-diagnostic procedure that performs in situ monitoring of the operational status of piezoelectric materials used for sensors and actuators in structural health monitoring (SHM) applications. The sensor/actuator self-diagnostic procedure, where the sensors/actuators are confirmed to be functioning properly during operation, is a critical component to successfully complete the SHM process with large numbers of active sensors typically installed in a structure. The premise of this procedure is to track the changes in the capacitive value of piezoelectric materials resulting from the degradation of the mechanical/electrical properties and its attachment to a host structure, which is manifested in the imaginary part of the measured electrical admittances. This paper concludes with an experimental example to demonstrate the feasibility of the proposed procedure.

Coupling sensing hardware with data interrogation software for structural health monitoring

Abstract: The process of implementing a damage detection strategy for aerospace, civil and mechanical engineering infrastructure is referred to as structural health monitoring (SHM). The authors' approach is to address the SHM problem in the context of a statistical pattern recognition paradigm. In this paradigm, the process can be broken down into four parts: (1) Operational Evaluation, (2) Data Acquisition and Cleansing, (3) Feature Extraction and Data Compression, and (4) Statistical Model Development for Feature Discrimination. These processes must be implemented through hardware or software and, in general, some combination of these two approaches will be used. This paper will discuss each portion of the SHM process with particular emphasis on the coupling of a general purpose data interrogation software package for structural health monitoring with a modular wireless sensing and processing platform. More specifically, this paper will address the need to take an integrated hardware/software approach to developing SHM solutions.

A miniaturized electromechanical impedance-based node for the wireless interrogation of structural health

Abstract: This paper presents the development and applications of a miniaturized impedance sensor node for structural health monitoring. The principle behind the impedance-based structural health monitoring technique is to apply high frequency structural excitations (typically higher than 30 kHz) through the surface-bonded piezoelectric transducers, and measure the impedance of structures by monitoring the current and voltage applied to the piezoelectric transducers. Changes in impedance indicate changes in the structure, which in turn can indicate that damage has occurred. Although many proof-of-concept experiments have been performed using the impedance methods, the impedance-measuring device is bulky and impractical for field-use. Therefore, a recently developed, miniaturized, low-cost impedance measurement chip was used to measure and record the electric impedance of a piezoelectric transducer. The performance of this miniaturized and portable device has been compared to our previous results and its effectiveness has been demonstrated in detecting bolt preload changes in a bolted frame structure. Furthermore, the possibility of wireless communication and local signal processing at the sensor node has been investigated by integrating the device with a microprocessor and telemetry.

Hardware design of hierarchal active-sensing networks for structural health monitoring

Abstract: This paper presents the use of relay-based hardware in conjunction with piezoelectric active-sensing techniques for structural health monitoring in large-scale structures. In many areas of active sensing technology, hundreds, even thousands, of sensors/actuators are needed to truly make health monitoring feasible in a real-world environment. Because interrogating such a large number of sensors is both time and cost prohibitive, it becomes necessary to develop a hardware system that can quickly and efficiently interrogate large numbers of the active sensors. In this work, we have developed a relay-based hardware that can serve as both a multiplexer and general-purpose signal router with special consideration given to piezoelectric active-sensing health monitoring approaches. We have also implemented this device as an expandable design that allows for easy scalability depending upon the size of the structure. Therefore, by using this hardware in conjunction with a centralized monitoring station, any number of sensors can be monitored effectively. Preliminary testing of this hardware on a test structure has experimentally proven the feasibility and advantages. This paper summarizes the hardware design, scalability, and useful advantages given today's structural health monitoring techniques.

Self-diagnosis and validation of active sensors used for structural health monitoring

Abstract: This paper presents a self-diagnostic and sensor validation procedure that performs in-situ monitoring of the operational status of piezoelectric (PZT) sensors and actuators used in structural health monitoring (SHM) applications. The sensor/actuator self-diagnostic procedure, where the sensors/actuators are confirmed to be functioning properly during operation, is a critical component to successfully complete the SHM process with large numbers of active sensors typically deployed in a structure. Both degradation of the mechanical/electrical properties of a PZT transducer and the bonding defects between a PZT patch and a host structure could be identified using the proposed procedure. First, the effects of bonding defects between a PZT patch and a host structure on high frequency SHM techniques, including Lamb wave propagations and impedance methods, have been experimentally investigated. It has been found that the effects are remarkable, modifying wave phase and amplitude, creating new wave modes, and changing measured impedance spectrum. These changes can lead to the false indications on structural conditions without an efficient sensor-diagnostic procedure. The feasibility of the proposed sensor diagnostics procedure was then demonstrated by analytical studies and experimental examples, where the functionality of surface-mounted piezoelectric sensors was continuously deteriorated. The proposed procedure can provide a metric that can be used to determine the sensor functionality over a long period of service time or after an extreme loading event. Further, the proposed procedure can be useful if one needs to check the operational status of a sensing network right after its installation.

Ultrasonic Guided Wave Monitoring of Composite Bonded Joints Using Macro Fiber Composite Transducers

Abstract: The monitoring of adhesively-bonded joints through the use of ultrasonic guided waves is the general topic of this paper. Specifically, composite-to-composite joints representative of the wing skin-to-spar bonds of Unmanned Aerial Vehicles (UAVs) are examined. This research is the first step towards the development of an on-board structural health monitoring system for UAV wings based on integrated ultrasonic sensors. The study investigates two different lay-ups for the wing skin and two different types of bond defects, namely poorly-cured adhesive and disbonded interfaces. The guided wave propagation problem is studied numerically by a semi-analytical finite element method that accounts for viscoelastic damping, and experimentally by utilizing macro fiber composite (MFC) transducers which are inexpensive, flexible, highly robust, and viable candidates for application in on-board monitoring systems. Based upon change in energy transmission, the presence of damage is successfully identified through features extracted in both the time domain and discrete wavelet transform domain. A unique "passive" version of the diagnostic system is also demonstrated experimentally, whereby MFC sensors are utilized for detecting and locating simulated active damage in an aluminum plate. By exploiting the directivity behavior of MFC sensors, a damage location algorithm which is independent of wave speed is developed. Application of this approach in CFRP components may alleviate difficulties associated with damage location in highly anisotropic systems.

The use of time reversal methods with Lamb waves to identify structural damage in a pipeline system

Abstract: Harsh environmental and operating conditions often leave pipeline systems prone to cracks, corrosion, and other aging defects. If left undetected, these forms of damage can lead to the failure of the pipeline system, which may have catastrophic consequences. Most current forms of health monitoring for pipeline systems involve nondestructive evaluation (NDE) techniques. These techniques often require a pipeline system to be taken out of operation at regularly scheduled intervals so that a technician can perform a prescribed NDE measurement. Such a measurement also requires direct access to the pipe's exterior or interior surface. This access may require excavation if the pipe is underground and the removal of insulating layers when present. This research proposes the use of Macro-fiber composite (MFC) actuators for damage detection in pipeline structures. Because MFC actuators are durable and relatively inexpensive, they can be permanently bonded to the surface of a pipe during installation. Therefore, measurements for damage detection can be performed at any time, even while the system is still in operation. The time reversal methods use the propagation of Lamb waves to evaluate the structural health of a pipeline system. A burst waveform is used to excite Lamb waves in a pipe at an initial location using an array of MFC patches. The measured response at the actuation location is reversed in time and used as the excitation signal at the second location. The initial excitation signal is then compared to the final response signal. The performance of the time reversal methods was compared to the traditional methods of Lamb wave propagations using standard tone burst waveforms.

Validating finite element models of composite aerospace structures for damage detection applications

Abstract: Carbon-fiber-reinforced-polymer (CFRP) composites represent the future for advanced lightweight aerospace structures. However, reliable and cost-effective techniques for structural health monitoring (SHM) are needed. Modal and vibration-based analysis, when combined with validated finite element (FE) models, can provide a key tool for SHM. Finite element models, however, can easily give spurious and misleading results if not finely tuned and validated. These problems are amplified in complex structures with numerous joints and interfaces. A small series of all-composite test pieces emulating wings from a lightweight all-composite Unmanned Aerial Vehicle (UAV) have been developed to support damage detection and SHM research. Each wing comprises two CFRP prepreg and Nomex honeycomb co-cured skins and two CFRP prepreg spars bonded together in a secondary process using a structural adhesive to form the complete wings. The first of the set is fully healthy while the rest have damage in the form of disbonds built into the main spar-skin bondline. Detailed FE models were created of the four structural components and the assembled structure. Each wing component piece was subjected to modal characterization via vibration testing using a shaker and scanning laser Doppler vibrometer before assembly. These results were then used to correlate the FE model on a component-basis, through fitting and optimization of polynomial meta-models. Assembling and testing the full wing provided subsequent data that was used to validate the numerical model of the entire structure, assembled from the correlated component models. The correlation process led to the following average percent improvement between experimental and FE frequencies of the first 20 modes for each piece: top skin 10.98%, bottom skin 45.62%, main spar 25.56%, aft spar 10.79%. The assembled wing model with no further correlation showed an improvement of 32.60%.