Showing papers by "Charles R. Farrar published in 2015"

A New Transmissibility Analysis Method for Detection and Location of Damage via Nonlinear Features in MDOF Structural Systems

Abstract: In this paper, a new transmissibility analysis method is proposed for the detection and location of damage via nonlinear features in multidegree-of-freedom (MDOF) structural systems. The method is derived based on the transmissibility of nonlinear output frequency response functions (NOFRFs), a concept recently proposed to extend the traditional transmissibility concept to the nonlinear case. The implementation of the method is only based on measured system output responses and by evaluating and analyzing the transmissibility of these system responses at super-harmonics. This overcomes the problems with available techniques, which assume there is one damaged component with nonlinear features in the system and/or require loading on inspected structural systems is measurable. Both numerical simulation studies and experimental data analysis have been conducted to verify the effectiveness and demonstrate the potential practical applications of the new method.

Comparative Study of Laser Doppler Vibrometer and Capacitive Air‐coupled Transducer for Ultrasonic Propagation Imager and the New Development of an Efficient Ultrasonic Wavenumber Imaging Algorithm

Abstract: Damage detection techniques using guided waves have been studied for decades with very few successful real-world applications. The recent development with the full wavefield technique using the Ultrasonic Propagation Imager (UPI) is one of those few exceptions. In this paper, we study two non-contact sensors: the laser Doppler vibrometer and the capacitive air-coupled transducer in the context as the sensing modules for the UPI. The aim of this paper is to provide a comprehensive study for optimisation of the two sensors, as well as a comparison between them for use in the UPI. First, the parameters for laser ultrasonic measurement of each sensor were studied: surface treatment, measurement angle and stand-off distance in the case of the laser Doppler vibrometer and measurement angle, lift-off distance and bias voltage in the case of the capacitive air-coupled transducer. Two optimised sensors were then compared in terms of their ability to detect damages using the UPI. Also, in this paper, we presented the ultrasonic wavenumber imaging (UWI) algorithm with the new development towards an efficient implementation. The uniqueness of the UWI algorithm with the capability of damage size estimation makes this algorithm very attractive for the future study with full wavefield signal processing.

Structural health monitoring for bolt loosening via a non-invasive vibro-haptics human–machine cooperative interface

Abstract: For the last two decades, developments in damage detection algorithms have greatly increased the potential for autonomous decisions about structural health. However, we are still struggling to build autonomous tools that can match the ability of a human to detect and localize the quantity of damage in structures. Therefore, there is a growing interest in merging the computational and cognitive concepts to improve the solution of structural health monitoring (SHM). The main object of this research is to apply the human–machine cooperative approach on a tower structure to detect damage. The cooperation approach includes haptic tools to create an appropriate collaboration between SHM sensor networks, statistical compression techniques and humans. Damage simulation in the structure is conducted by releasing some of the bolt loads. Accelerometers are bonded to various locations of the tower members to acquire the dynamic response of the structure. The obtained accelerometer results are encoded in three different ways to represent them as a haptic stimulus for the human subjects. Then, the participants are subjected to each of these stimuli to detect the bolt loosened damage in the tower. Results obtained from the human–machine cooperation demonstrate that the human subjects were able to recognize the damage with an accuracy of 88 ± 20.21% and response time of 5.87 ± 2.33 s. As a result, it is concluded that the currently developed human–machine cooperation SHM may provide a useful framework to interact with abstract entities such as data from a sensor network.

A graphite oxide (GO)-based remote readable tamper evident seal

Abstract: This paper presents a prototype of a remotely readable graphite oxide (GO) paper-based tamper evident seal. The proposed device combines the tunable electrical properties offered by reduced graphite oxide (RGO) with a compressive sampling scheme. The benefit of using RGO as a tamper evident seal material is the sensitivity of its electrical properties to the common mechanisms adopted to defeat tamper-evident seals. RGO's electrical properties vary upon local stress or cracks induced by mechanical action (e.g., produced by shimming or lifting attacks). Further, modification of the seal's electrical properties can result from the incidence of other defeat mechanisms, such as temperature changes, solvent treatment and steam application. The electrical tunability of RGO enables the engraving of a circuit on the area of the tamper evident seal intended to be exposed to malicious attacks. The operation of the tamper evident seal, as well as its remote communication functionality, is supervised by a microcontroller unit (MCU). The MCU uses the RGO-engraved circuitry to physically implement a compressive sampling acquisition procedure. The compressive sampling scheme provides the seal with self-authentication and self-state-of-health awareness capabilities. The prototype shows potential for use in low-power, embedded, remote-operation non-proliferation security related applications.

Nondestructive inspection using continuous ultrasonic wave generation

Abstract: Methods and apparatus are disclosed for analyzing structures by applying a continuous ultrasonic excitation and measuring steady state response of the structures using laser Doppler vibrometery, or other techniques. In one example, a method comprises applying a continuous signal having one or more periodic tones to the structure, generating measurements of wave response to the signal at each of a plurality of inspection points of the structure, and, for each of the periodic tones, estimating wavenumbers for a number of the inspection points of the structure based on the wave response measurements and the frequency of the periodic tones. The estimated wavenumbers can be used to determine properties of the structure, including defects, damage, or variation in thickness.

Adaptive Reverberation Suppression Techniques for SHM in Composite Materials

Abstract: This paper introduces a new method for Structural Health Monitoring using error functions computed from guided waves reflected from damage. The approach is experimentally tested on anisotropic specimens such as composite plates. The baseline and test signals of each sensing path (between two PZT transducers) are measured and the energy of the scattered signal for each path is calculated in a given frequency range. Assuming that there is damage in the evaluated position, the wave will reflect at this point and travel to the next transducer. According to the distance between the first transducer to the evaluated point plus the distance between same point to the second transducer (pitch-catch configuration) the time-of-flight is calculated for each grid point on the structure. The wave speeds in anisotropic specimens are propagation direction dependent. The wave speed for different angles were experimentally computed and incorporated in the algorithm in order to calculate the proper time-offlight. The energy of the scattered signal is computed in a time range based on the time of flight of each analyzed position. Finally, a resultant error function for an estimation of the damage location in the monitoring area is applied. As the error function is based on the interference of the damage in the propagation of guided waves, the higher value of the error implies the less likelihood of damage in that position. An image is generated with an error value for each mesh position in the plate. This error function compares the energy in the given ranges for each pair of transducers. The experiment was performed in a 500x500x2mm carbon/epoxy composite formed by 10 plainweave layers with 9 PZT transducers in the surface. The resultant error function at each driving frequency is calculated as a sum of all error functions. In addition, several frequencies were tested and the results for each one were combined in order to get a better result. doi: 10.12783/SHM2015/223

A remote-readable graphite oxide (GO) based tamper-evident seal with self-reporting and self-authentication capabilities

Abstract: The blossoming of sensing solutions based on the use of carbon materials and the pervasive exploration of compressed sensing (CS) for developing structural health monitoring applications suggest the possibility of combining these two research areas in a novel family of smart structures. Specifically, the authors propose an architecture for security-related applications that leverages the tunable electrical properties of a graphite oxide (GO) paper-based tamper-evident seal with a compressed-sensing (CS) encryption/authentication protocol. The electrical properties of GO are sensitive to the traditional methods that are commonly used to remove and replace paper-based tamper-evident seals (mechanical lifting, solvents, heat/cold temperature changes, steam). The sensitivity of the electro-chemical properties of GO to such malicious insults is exploited in this architecture. This is accomplished by using GO paper to physically realize the measurement matrix required to implement a compressive sampling procedure. The proposed architecture allows the seal to characterize its integrity, while simultaneously providing an encrypted/authentication feature making the seal difficult to counterfeit, spoof, or remove/replace. Traditional digital encryption/authentication techniques are often bit sensitive making them difficult to implement as part of a measurement process. CS is not bit sensitive and can tolerate deviation caused by noise and allows the seal to be robust with respect to environmental changes that can affect the electrical properties of the GO paper during normal operation. Further, the reduced amount of samples that need to be stored and transmitted makes the proposed solution highly attractive for power constrained applications where the seal is interrogated by a remote reader.