Gunnar Wang

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.

An investigation of the pressure-to-acceleration responsivity ratio of fiber-optic mandrel hydrophones

The acceleration responsivity of the fiber-optic air-backed mandrel hydrophone is studied. This responsivity may be a significant noise source in high vibration environments. The pressure-to-acceleration responsivity ratio is therefore a figure of merit for the hydrophone. It is shown that an ideal hydrophone with a sufficiently rigid support cylinder should not show any first-order acceleration responsivity. The dominant contribution to any first-order acceleration responsivity is the deformation of the support cylinder due to acceleration. The responses are evaluated both theoretically and experimentally. This treatment gives a set of guidelines which should be followed in order to maximize the pressure-to-acceleration responsivity ratio. It is also theoretically and experimentally demonstrated that the axial acceleration responsivity can be significantly reduced by symmetrical elicitation of the hydrophone.

Transient load monitoring on a composite hull ship using distributed fiber optic Bragg grating sensors

We describe strain data recorded using fiber optic Bragg grating sensors mounted on the hull of a GRP composite ship. Twelve gratings were attached to the structure, in three arrays of four elements. The electro-optic system used was able to monitor a single set of four elements at a time. The preliminary results indicate the usefulness of distributed fiber Bragg grating sensor systems for monitoring transient loading events on such structures.

Wavelength multiplexed fibre Bragg grating system for high-strain health monitoring applications

We have developed and demonstrated a fibre Bragg grating strain gauge system for high strain applications using spatial and wavelength multiplexing. The system takes advantage of a priori knowledge about the strain levels and relative phases between the different sensor signals to accommodate many sensors per fibre in an application where we expect to measure several thousand microstrains. In combination with scanning Fabry-Perot filter interrogation, this principle has been utilized in the design of a system for health monitoring of a naval surface effect ship and the system has been used in long-term load monitoring on the vessel.

Wet deck slamming experiments with a FRP sandwich panel using a network of 16 fibre optic Bragg grating strain sensors

Drop test experiments have been performed with a FRP sandwich panel instrumented with a network of 16 fibre optic (FO) Bragg strain sensors, together with conventional electrical strain gauges for control and verification. The drop tests simulate slamming loads on the wet deck of a surface effect ship (SES). The objectives were to show the possibility of using a network of FO sensors to monitor strain during a slamming impact, and to test out a technique for signal processing. The strain measurements provided both peak strain data and served as a base for frequency analysis. The results showed that the FO strain sensors performed satisfactorily and were in general agreement with the conventional strain gauges used. The FO interrogation system was, however, not designed with sufficiently large dynamic range for the most extreme drop sequences. The peak strain in the panel was found to increase almost proportionally with the drop velocity, or drop height, and the wet fundamental frequency increased with increasing drop angle. Furthermore, the frequency decreased with increasing drop velocity.

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.

Fiber optic system for ship hull monitoring

The authors describe a system which will, at about 10 installations in continuous operation after 2002, be the first fiber optic sensor system for ship hull monitoring with a wide user base. The experiences made with these systems in the coming years will provide valuable information on the design, reliability and possible use of such systems, and provide a basis for further development.

Composite plate drop test using a 16-channel fibre Bragg grating strain sensor system

Under the Composite Hull Embedded Sensor System (CHESS) program, which is a co-operative program run jointly by FFI and NRL, a drop test with a composite sandwich plate has been performed [1]. The plate was designed to have similar mechanical properties to a bottom panel in the Norwegian Navy’s new fast patrol boat (MTB).

Monitoring vibration of sandwich plates and delamination detection with fibre optic sensors

The experimental feasibility study using fiber optic sensors, strain gauges and speckleinterferometry (ESPI), indicates that delamination in FRP-sandwich structures can be detected by monitoring changes in the vibrational resonance frequencies. The frequencies are also determined analytically.

Measurement of higher harmonics in periodic vibrations using phase-modulated TV holography with digital image processing.

We separately measure the higher harmonics vibration patterns of a periodic vibrating object by using time-average TV holography and phase modulation. During measurements the frequency of the phase modulation is adjusted to each harmonic component while the excitation of the object is set low enough to record all components on the linear part of the fringe function. Using acoustical phase stepping and calibration of the fringe function, we compute the amplitude and phase distributions of the frequency component. We measure components up to the 65th harmonic by using square-wave excitation.