Philipp M. Nellen

Application and reliability of a fiber optical surveillance system for a stay cable bridge

In this paper we report on the application of a fiber optical system for the surveillance of a new stay cable bridge. The novelty of the bridge is the use of cables made of carbon-fiber-reinforced polymer wires. This new concept in bridge construction requires monitoring of the cables to ensure their reliability. The results of an observation period of more than six months during the construction are presented. Tests of the reliability of the fiber optical monitoring system are discussed.

Milling micro-structures using focused ion beams and its application to photonic components

Investigations in micro-structuring optical fibre tips with direct focused ion beam writing will be discussed. The first part considers fundamental aspects and shows relevant experiments to understand and control ion patterning. The second part exemplifies structuring of photonic elements by the fabrication of apertures on an optical fibre tip and the corresponding optical characterizations.

Optical fiber Bragg gratings for tunnel surveillance

We report on application tests of novel sensor elements for long term surveillance of tunnels. The sensors are made of glass fiber reinforced polymers (GFRP) with embedded optical fiber Bragg gratings. The tests were made in a tunnel near Sargans in Switzerland and we will present strain and temperature data of more than one year of operation of the sensor elements. Two sensor types were tested. First, GFRP rockbolts with a diameter of 22 mm were produced. They have a load-bearing function as anchors for tunnel or mine roofs and in addition measure distributed strain fields and temperature with embedded optical fiber Bragg grating arrays. Rockbolts are key elements during construction and operation of tunnels. Data about strain inside the rockbolts can support decision about precautions to be taken and reveal information about the long term movement of the rock. Second, thin and flexible GFRP wires of 3 mm in diameter were found to be robust and versatile sensors not only for tunnel surveillance but for many civil engineering applications where they can be attached or embedded (e.g., in concrete). The fabrication of both sensor types and solutions for the connection of the embedded fiber sensors to a fiber cable will be presented. Moreover, laboratory and tunnel data of functionality and long term stability tests will be discussed and compared.

Application of fiber optical and resistance strain gauges for long-term surveillance of civil engineering structures

We report on applications of surveillance and test systems for civil engineering structures. The system key elements are optical-fiber Bragg grating sensors and conventional resistance strain gauges. A recently built stay cable bridge with a world novelty of two carbon-fiber-reinforced-polymer cables was equipped with both types of sensors. The sensor system on the bridge is now operational for ten months and the bridge is open for traffic for 4 months. Results of the bridge surveillance are presented. To monitor a large concrete structure, the electrical power dam of Luzzone in the Swiss Alps, a prototype sensor rod was designed. First measurements with a sensor rod embedded in a concrete test prism are discussed. Several redundant measurements are made to compensate for temperature drift and to monitor the reliability of the measurement chain.

Strain measurements on concrete beam and carbon fiber cable with distributed optical fiber Bragg grating sensors

We report on civil engineering applications of wavelength multiplexed optical fiber Bragg grating arrays directly produced on the draw tower for testing and surveying advanced structures and materials such as carbon fiber reinforced concrete elements and prestressing cables. We equipped a 630.930.5 m concrete beam, which was rein- forced with carbon fiber reinforced epoxy laminates, and a 7-m long prestressing carbon fiber cable made of seven twisted strands, with op- tical fiber Bragg grating sensors. Static strains up to 8000 mm/m and dynamic strains up to 1200 mm/m were measured with a Michelson in- terferometer used as Fourier spectrometer with a resolution of about 10 mm/m for all sensors. Comparative measurements with electrical resis- tance strain gauges were in good agreement with the fiber optic results. We installed the fiber sensors in two different arrangements: some Bragg grating array elements measured local strain while others were applied in an extensometric configuration to measure moderate strain over a base length of 0.1 to 1 m.

Applications of distributed fiber Bragg grating sensors in civil engineering

We report on civil engineering applications of wavelength multiplexed optical-fiber Bragg grating arrays produced directly on the draw tower for testing and surveying advanced structures and material like carbon fiber reinforced concrete elements and prestressing tendons. We equipped a 6 m X 0.9 m X 0.5 m concrete cantilever beam reinforced with carbon fiber lamellas with fiber Bragg grating sensors. Static and dynamic strain levels up to 1500 micrometers /m were measured with a Michelson interferometer used as Fourier spectrometer with resolutions of about 10 micrometers /m for all sensors. Comparative measurements with electrical resistance strain gauges were in good agreement with the fiber optic results. We used the fiber sensors in two different arrangements: some Bragg grating array elements measured the local strain while others were configured in an extensometric way to measure moderate strain over 0.1-1 m.

Reliability of optical fiber Bragg grating sensors at elevated temperature

Mechanical and optical reliability of fibers and Bragg gratings at elevated temperature of up to 250/spl deg/C are modeled and parameters are determined in an extended test program. Stress corrosion and grating decay are investigated for two commercially available Bragg grating types.

Fiber optic Bragg grating sensors embedded in GFRP rockbolts

Rockbolt anchors for tunnel or mine roofs are key elements during construction and operation. We report on the fabrication of glass fiber reinforced polymer (GFRP) rockbolts with embedded fiber optical Bragg grating sensors and their first field application in a test tunnel. Optical fibers and in-fiber Bragg grating sensors were embedded in GFRP rockbolts during a continuously ongoing pultrusion process on an industrial production machine. Depending on their outer diameter the rods equipped with fiber sensors serve as measuring rockbolts or as extensometric sensors for the motion of boulders in the tunnel roof. The adhesion and force transfer of different fiber coatings were tested by push-out experiments. By temperature and strain cycle tests the performance of the rockbolt sensors was evaluated. We will present these results and the measurements made during a first installation of fiber optical rockbolt sensors in a tunnel.

Lifetime of fiber Bragg gratings under cyclic fatigue

We present results on optical and mechanical stability of single-layered acrylate coated fiber Bragg gratings produced on a draw tower, and exposed to high temperature annealing and to cyclic loading. Optical stability was assessed in terms of strain response and reflectivity changes with annealing temperatures up to 400 degree(s)C. Cyclic loading of Bragg gratings with mean stresses between 2.4 GPa and 3.0 GPa was compared with predictions made by using power-law based crack growth theory with parameters obtained by dynamic tensile tests. Comparison with theory confirms predicted strength decrease and lifetime reduction.

Structurally embedded fiber Bragg gratings: civil engineering applications

In civil engineering it is of interest to monitor long-term performance of structures made of new lightweight materials like glass or carbon fiber reinforced polymers (GFRP/CFRP). In contrast to surface applied optical fiber sensors, embedded sensors are expected to be better protected against rough handling and harsh environmental conditions. We report on two recently done fiber optical sensor applications in civil engineering. Both include structurally embedded fiber Bragg grating (BG) arrays but have different demands with respect to their operation. For the first application fiber BGs were embedded in GFRP rockbolts of 3 - 5 m in length either of 3, 8, or 22 mm diameter. The sensor equipped rockbolts are made for distributed measurements of boulder motion during tunnel construction and operation and should withstand strain up to 1.6%. Rockbolt sensors were field tested in a tunnel near Sargans in Switzerland. For a second application fiber BGs were embedded in CFRP wires of 5 mm diameter used for the pre- stressing cables of a 56 m long bridge near Lucerne in Switzerland. The permanent load on the cable corresponds to 0.8% strain. Due to the embedded sensors, strain decay inside the cable anchoring heads could be measured for the first time during loading and operation of the cables. For both applications mechanical and thermal loading tests were performed to assess the function of these new elements. Also, temperature and strain sensitivity were calibrated. Reliability studies with respect to stress transfer, fiber mechanical failure, and wavelength shift caused by thermal BG decay as well as monitoring results of both applications are presented.