Strain Measurement Validation Of Embedded Fiber Bragg Gratings

This study investigates the influence of strain state distribution on the accuracy of embedded optical fiber Bragg gratings (FBGs) used as strain sensors. An optical fiber embedded parallel to adjacent structural fibers in a graphite epoxy quasi-isotropic (90/+/- 45/0)(S) lay-up is evaluated with mechanical loading parallel to the fiber optic direction. Finite element analysis (FEA) is used to evaluate the fiber optic sensors' responses both in the far field and near field regions of the mechanical grips. Comparison between experimental fiber optic strains, strain gauges, and FEA provides good correlation in the far field with differences of less than 1%. However, in the near field region, some discrepancies are found and attributed to birefringence arising from complex strain states.

In situ strain and temperature monitoring of adaptive composite material

Hans Georg Limberger, Véronique Michaud, René Salathé

An optical fiber sensor is designed to simultaneously measure strain and temperature in an adaptive composite material. The sensor is formed by splicing two fiber Bragg gratings (FBGs) close to each other, which are written in optical fibers with different core dopants and concentrations. Their temperature sensitivities are hence different. The sensor is tested on an adaptive composite laminate made of unidirectional Kevlar-epoxy prepreg plies. Several 150 μm diameter prestrained NiTiCu shape memory alloy (SMA) wires are embedded in the composite laminate together with one fiber sensor. Simultaneous monitoring of strain and temperature during the curing process and activation in an oven is demonstrated

SAGE Publications2006

Construction material monitoring with "optical hair" hygrometers

Pascal Kronenberg

Moisture is an essential parameter in the behaviour of capillary-porous construction materials such as timber and concrete. It affects liquid and gas transport phenomena, chemical and biological degradation processes, mechanical properties and, in the case of concrete, hydration. From a scientific point of view, moisture monitoring is essential in order to improve the understanding of material behaviour. Moisture measurements may increase the predictive accuracy of established material behaviour models. In practice, the increased knowledge of material behaviour improves structural maintenance planning. The main objective of this work is to propose a measurement method for non-destructive, in-depth moisture monitoring of construction materials. In this context, an intrinsic point and an averaging fibre optic relative humidity sensor have been developed and tested. The sensors are based on optical fibres that are coated with a hygroscopically swelling transducer polymer. When wet, the swelling of the coating strains the fibre (analogy with the hair hygrometer). The induced strain is assessed with conventional fibre optic strain sensing techniques such as fibre Bragg gratings and Michelson interferometry. Theoretical and experimental studies lead to a detailed understanding of the influence of humidity and temperature on the steady and transient state sensor behaviour. The sensors have an accurate, linear, reversible and reproducible response to relative humidity between 5 and 95 %RH and between 13 and 60 °C, at least. The sensor response time is in the order of 20 minutes. However, when packaged, the sensor responds slower. The temperature cross-sensitivity of the fibre Bragg grating sensor may be compensated with an additional non-hygroscopic grating, while the Michelson interferometric sensor provides an auto-compensation of temperature effects. Tests in mortar and timber samples demonstrate that the sensors preserve their sensing ability when embedded. These tests have also clarified the multiplexing potential of fibre Bragg gratings for forming a multi-point RH sensor. Multi-point sensors are particularly useful for profile measurements.

EPFL2003

Construction material monitoring with "optical hair" hygrometers

Pascal Kronenberg

EPFL2003