Gregg Johnson

Ship hull structure monitoring using fibre optic sensors

We discuss the need for ship hull monitoring and the roles such a system may fill during the different stages of a ships lifetime. We have found that fibre optic sensors are well suited for this application and present the fibre Bragg grating technology that has been employed in the composite hull embedded sensor system project (CHESS). Signal processing is the key to real-time structure monitoring, and we comment on the modular signal processing system that is being developed at FFI. Finally we present the CHESS installation on a new Norwegian naval vessel and show some results obtained with this system in systematic sea-keeping tests. The CHESS measurements on the new Norwegian fast patrol boat have been instrumental in the design verification of the vessel.

Fiber Bragg grating interrogation and multiplexing with a 3/spl times/3 coupler and a scanning filter

We present a new technique for fiber Bragg grating (FBG) sensor interrogation and multiplexing. The technique combines a scanning bandpass filter used to multiplex by wavelength multiple gratings in a single fiber, and an unbalanced Mach-Zehnder fiber interferometer made with a 3/spl times/3 coupler to detect strain-induced wavelength shifts. A demonstration system interrogates four gratings in a single fiber at a sampling rate up to 20 kHz, with a noise floor measured at less than 10 n/spl epsiv///spl radic/(Hz) above 0.1 Hz.

Deployment of a fiber Bragg grating-based measurement system in a structural health monitoring application

The development and maturation of fiber optic sensor technology has been an increasingly important component of the structural health monitoring field. A strain sensor system has been developed to provide high-resolution, low-noise sets of useful data which can be analyzed and processed with a number of existing damage detection techniques. Recent research at the Naval Research Laboratory has also begun in combining vibration-based damage detection with statistical methods, and some preliminary results are included.

Rayleigh scattering optical frequency correlation in a single-mode optical fiber

The bichromatic optical frequency correlation function for Rayleigh backscattering from a pulse of laser light propagating along a single-mode optical fiber has been calculated and measured. It is shown that the optical correlation frequency, Dnu(c) , is equal to the reciprocal of pulse width T(w) . These results are important for the development of wavelength diversity techniques for the reduction of coherent Rayleigh noise in distributed Rayleigh backscattering single-mode optical fiber sensors.

Instrumentation of a high-speed surface effect ship for structural response characterization during sea trials

We report on the instrumentation of a high-speed air-cushion catamaran (Surface Effect Ship) with more than 50 fiber optic Bragg grating strain gauges, as well as conventional resistive strain gauges, accelerometers, a Motion Reference Unit and Global Positioning System. A bow mounted wave radar was used to characterize the sea-state in order to estimate the wave loads on the hull. The relatively large number of strain gauges enabled us to determine the global deformation modes of the hull as well as local stress concentrations. This instrumentation was installed on a new Norwegian naval vessel and employed during sea-keeping tests in smooth and rough seas off the Norwegian coast. The measurements enable a detailed characterization of the vessels dynamic response to wave loading and comparison with Finite Element Analysis modeling of the ship. The experimental results provide invaluable information for the subsequent development of a system for health monitoring of the structure. We present the instrumentation layout and selected results.

Experimental investigation of high-quality synchronization of coupled oscillators

We describe two experiments in which we investigate the synchronization of coupled periodic oscillators. Each experimental system consists of two identical coupled electronic periodic oscillators that display bursts of desynchronization events similar to those observed previously in coupled chaotic systems. We measure the degree of synchronization as a function of coupling strength. In the first experiment, high-quality synchronization is achieved for all coupling strengths above a critical value. In the second experiment, no high-quality synchronization is observed. We compare our results to the predictions of the several proposed criteria for synchronization. We find that none of the criteria accurately predict the range of coupling strengths over which high-quality synchronization is observed. (c) 2000 American Institute of Physics.

Quasi-static strain monitoring during the "push" phase of a box-girder bridge using fiber Bragg grating sensors

Introduction: An interesting and increasingly popular method of building steel and concrete composite bridges involves the use of a large steel based structure called a box-girder. These massive structures, which have dimensions on the order of 5m x 5m x 150m with wall thickness sometimes exceeding 5 cm, are assembled by welding together many shorter (e.g. 5m x 5m x 5 m) box-girder pieces near a bridge building site. The massive and long box-girder structure thus created, is then ‘pushed’ from one end of the bridge pier to the other. A concrete deck is eventually built on top of the box-girder to complete the bridge structure. This bridge building approach apparently holds advantages in the sense that it requires far fewer on-site workers and also allows for rapid building of large scale bridges over difficult terrain. One of the more interesting (and potentially dangerous) phases during the construction process of a box-girder bridges is the so-called ‘push’ phase. During this phase, the long box-girder structure, which prior to the push rests on a pier closest to land, is literally pushed from one bridge pile-on to the next, requiring it to travel a distance of over 100 meters. The far pile-on typically is built in a ravine or a deep valley over which the bridge is being built. During the push, the steel box-girder structure is subjected to static strains at various locations, depending upon the position of the box-girder at any given time. For instance, the far end of the box girder would be subject to cantilever motion, which may be driven into oscillations due to winds in the ravine. Monitoring the strain at various locations in the box-girder during the push phase of the bridge construction has potential benefits in future infra-structure modeling efforts as well as in determining construction safety factors. In addition, utilizing enabling technologies (e.g. fiber Bragg grating strain sensors) for strain monitoring during an actual construction phase of a real world structure allows determination of the maturity of: (i) the instrumentation system, (ii) sensor packaging, and (iii) installation approaches [1]. This work reports the results of a 32 element fiber Bragg grating (FBG) array used to monitor strain at various locations in box-girder vaux viaduct being constructed near Lausanne, Switzerland. The entire push phase of the box-girder bridge, which took about a day, was monitored using several FBGs whose responses were interrogated using the scanning FabryPerot (FP) method [2]. Preliminary quasi-static strain sensing data shows fine as well as gross details of the push phase. The sensors are planned to operate during many phases of the boxgirder bridge construction including the push, the concrete deck pouring, and eventually traffic monitoring. We report here on the push phase only.

Development of fiber Bragg-grating-based soil pressure transducer for measuring pavement response

A soil pressure transducer by using fiber Bragg grating (FBG) sensors associated with a circular diaphragm is developed. The FBG based transducers can be used for pavement performance study and weigh-in-motion measurement. We consider three methods of bonding the FBG to the diaphragm: (1) radially, (2) radially, inside a glass capillary, and (3) circumferentially. The investigation of strain-gradient induced spectral broadening in FBG-based transducers is conducted since spectral broadening can have adverse effects on the sensor interrogations. We derive analytical closed form results for describing measurand-induced strain gradients in circular geometry transducers, which allow us to experimentally demonstrate novel FBG bonding approaches that eliminate spectral broadening. In addition, Bragg spectral broadening analysis using T-matrix calculation is also conducted to validate some of the experimental results. Two prototypes of soil pressure transducers are field tested at the Cold Region Research Engineering Laboratory (CRREL). The buried pressure transducers are impact-tested by use of a Falling-Weight- Deflectometer (FWD), and detected by NRL-developed FBG interrogation device. Lastly, we use the Boussinesq equation to verify the soil stress measured by the buried transducers.