Standard optical-fibre crack-monitoring sensors can detect the initiation and growth of cracks with widths in the range 20 to 40 µm. They can be desensitized of required to detect wider cracks. The sensors are easy to apply to most structural surface and can withstand a wide range of hostile environments. Instrumentation is fully developed to allow continuous remote on-line surveillance of both fatigue test specimens or actual structural components.

INTRODUCTION

In-house development of the optical-fibre crack-monitoring sensor has now reached a highly satisfactory stage and the system is starting to find widespread use. The concept is very simple. An optical fibre is bonded to the surface of a structure by means of a high-modulus adhesive. If the structural surface crack immediately below the fibre, then the fibre will crack also and thus light transmitted down the fibre will be attenuated. This attenuation can be detected remotely, so the system is a real-time crack-detection device. The principle of the crack-monitoring sensor is shown schematically in Fig 1. The fibres are encapsulated in translucent reinforced epoxy packages which can be made with a large range of dimensions, which both protect the fibres and also facilitate their accurate placement.

Fig 1 Conceptional presentation of the method of detecting cracks by the use of optical-fibre crack-monitoring sensors

The advantages of this system over conventional witness devices, electrical crack-propagation gauged or other crack-detection systems are that it is :

  • non-electrical and free from electromagnetic interference.

  • non-conductive to electrical sources;

  • safe in inflammable or explosive environments;

  • capable of operation in more severe chemical and aqueous environments;

  • capable of faithful signal transmission over long distances;

  • usable with equal facility with the simplest light source or light source/detector monitoring instrumentation of the required degree of sophistication, depending upon the application;

  • a precise crack locator by virtue of scattered light, even after a crack has closed up after removal of the strain that caused the crack to initiate

The level of crack0detection sensitivity and the range of hostility of the working environment will be discussed.

Description

The sensors are designed to enable bare fibres of the required fracture strength to be bonded close to the surface of the structure to be monitored. The fibres must be quality-assured within tight optical and mechanical specifications in order to have a repeatable crack-width detection sensitivity with a reasonably narrow range of scatter and to be compatible with attenuation detection instrumentation.

Development of the sensors has benefitted from the large quantities of high-quality optical fibre, produced for the very demanding tele-communications industry, that are now readily available at low cost. This fibre is required to have closely defined optical properties for low-attenuation long-distance transmission. It is also designed to withstand abuse whilst it is pulled along narrow ducts and around small radius bends, and therefore has specified mechanical strength.

Fig 2 Scanning electron micrograph of part of the sensing surface of an optical-fibre crack-monitoring sensor (magnification 50 x)

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