Stephen T. Trickey

Bragg grating-based fibre optic sensors in structural health monitoring

This work first considers a review of the dominant current methods for fibre Bragg grating wavelength interrogation. These methods include WDM interferometry, tunable filter (both Fabry–Perot and acousto-optic) demultiplexing, CCD/prism technique and a newer hybrid method utilizing Fabry–Perot and interferometric techniques. Two applications using these techniques are described: hull loads monitoring on an all-composite fast patrol boat and bolt pre-load loss monitoring in a composite beam in conjunction with a state-space modelling data analysis technique.

Bottlenecking phenomenon near a saddle-node remnant in a Duffing oscillator

Abstract The bottlenecking phenomenon near a saddle-node remnant or ghost is discussed for an electronic circuit modeling Duffings equation. Numerical simulation and a useful experimental perturbation method, stochastic interrogation, are used to confirm the analytic inverse square root scaling law associated with saddle-node bifurcations.

Quantitative detection of low energy impact damage in a sandwich composite wing

This work describes damage detection in a foam core composite wing (1320 mm × 152.4 mm × 13.4 mm) following a series of low energy impacts. Thirteen impacts (6—8 J deposited energy) were applied at adjacent locations approximately 1/4 of the way out from the wing center. Following every one or two impacts, the wing was tested using static tip deflection and dynamic vibrational excitation. Static and dynamic strains were measured using eight fiber Bragg grating sensors. Dynamic acceleration was also monitored using three conventional accelerometers. The estimated bicoherence was used to detect the presence of damage-induced non-linearity in time-series data recorded from each sensor. Receiver operating characteristic (ROC) curves were constructed for each sensor based on 15 or more dynamic measurements made for each damage case. The ROC curves provide a quantitative, statistical approach to evaluating the damage detection capabilities of the various sensors.

Geometric time-domain methods of vibration-based damage detection

We present a new methodology for vibration based damage detection derived from the characterization of changes in the geometric properties of the time domain response of a structure. Many new features present themselves when the geometry of attracting objects in phase space are considered. The most promising avenue of study are metrics that describe changes to the attractor shape or dimension. In particular, the utility of a feature consisting of the ratio of average local variance (or spatial dispersion) of the input to the average local variance of the response is assessed. Presenting the results of the geometric time domain method in a statistical framework highlights the methods increased sensitivity to subtle damage-inflicted changes to the structure when compared to more traditional modal based methods. In addition the geometric method demonstrates a more robust handling of changes due to ambient environmental fluctuation. Results are presented from a finite element model of a thin plate with weld line damage implemented through a relaxation of a boundary condition.

Analysis of accuracy error and distortion in an operationally passive interferometric demodulation technique

Many architectures of fiber Bragg grating (FBG) interrogation systems used for mechanical motion (strain, acceleration, etc.) detection utilize interferometry for some part of the demodulation process. Using a hybrid Mach-Zehnder/tunable filter/3-by-3 coupler system architecture as a testbed, this paper examines error sources in the demodulation process giving rise to both/either accuracy and/or resolution degradation in the demodulated output. In particular, realizations of degradation metrics such as noise rise and harmonic distortion are reported due to inaccuracy in demodulation parameters, such as coupler parameters or photodetector voltages. Error models are developed where appropriate for comparison between prediction and measurement.

Excitation considerations for attractor property analysis in vibration-based damage detection

In past work, we have presented a methodology for vibration based damage detection derived from the characterization of changes in the geometric properties of the time domain response of a structure. In brief, input forcing signals and output response signals can be transformed into state space geometrical representations. When allowed to evolve to a steady state, the geometric object is called an attractor. Certain properties of the attractor, such as the local variance of neighborhoods of points or prediction errors between attractors, have been shown to correlate directly with damage. While most inputs will generate some type of attracting geometric object, prescribing a low dimensional input forcing signal helps to maintain a low dimensional output signal which in turn simplifies the calculation of attractor properties. Work to date has incorporated the use of a chaotic input forcing signal based on its low dimensionality yet useful frequency content. In this work we evaluate various forms of shaped noise as alternative effectively low dimensional inputs. We assess whether the intrinsic properties of the chaotic input leads to better damage detection capabilities than various shaped noise inputs. The experimental structure considered is a thin plate with weld line damage.

Structural Damage Assessment Using Chaotic Dynamic Interrogation

This work considers a new technique for damage detection in vibration-based structural health monitoring. The technique involves exciting a structure with a low-dimensional chaotic input, reconstructing the attractor, and analyzing certain properties of the attractor for the subtle changes which may occur due to damage. Recourse is made to the Kaplan-Yorke conjecture for motivating the technique. The technique is then demonstrated in the laboratory on a beam subject to boundary clamp damage controlled by means of a special elastic clamp. The measurements are made with a special fiber optic strain measurement system whose key performance features are described.Copyright

Quantification of Damage Detection Schemes using Receiver Operating Characteristic (ROC) Curves.

In this work we detect damage in a composite to metal bolted joint subject to ambient vibrations and strong temperature fluctuations. Damage to the joint is considered to be a degradation of the connection strength implemented by loosening the bolts. The system is excited with a signal that conforms to the Pierson-Moskowitz distribution for wave height and represents a possible loading this component would be subject to in situ. We show that as the bolts are loosened, increasing amounts of nonlinearity are introduced in the form of impact discontinuities and stick-slip behavior. The presence of the nonlinearity, hence the damage, is detected by drawing comparisons between the response data and surrogate data conforming to the null hypothesis of an undamaged, linear system. Two metrics are used for comparison purposes: nonlinear prediction error and the bicoherence. Results are displayed using Receiver Operating Characteristic (ROC) curves. The ROC curve quantifies the trade-off between false positives (type I errors) and false negatives (type II errors). Type I errors can be expressed as the probability of false alarm and 1 - type II error is the probability of detection. We demonstrate that ROC curves provide a unified quantifiable approach for directly comparing the merits of different detection schemes.

Structural monitoring using a novel high-performance fiber optic measurement system

This work considers the dominant current methods for fiber Bragg grating wavelength interrogation. In addition, a new interrogation method, based on hybridizing a scanning Fabry-Perot filter for selecting individual reflection wavelengths, and an unbalanced Mach-Zehnder interferometer for high-resolution conversion of wavelength shifts to phase changes, is presented. The method utilizes a 3x3 fiber optic coupler in such a way that a completely passive demodulation algorithm is implemented. The current version interrogates multiple FBGs at frequencies from DC to near 10 kHz with nanostrain resolution across the full band. Low-frequency resolution is maintained with an interferometer drift compensation technique. We describe the system design and operation in detail and present key performance metrics with comparison to other primary FBG interrogation architectures.

Transient Global Behavior in Nonlinear Experimental Oscillators

This papers is concerned with the determination of basin boundaries in experimental nonlinear oscillators. Global transient behaviour has received considerable attention in numerical studies but is relatively unexplored from an experimental perspective, despite the fact that a global view of transient behaviour provides a much more complete description of the dynamics of a system than a traditional concentration on steady-state behaviour. Three differnt physical systems are studied: two mechanical and one electrical.