Manufacturing and quality evaluation of component integrated fibre optical sensors for condition monitoring

Metal embedding of fibre optical sensors provides new possibilities in structural monitoring. The embedded sensors can either be used for validating of prototype design or for condition monitoring of functional components in service. In this paper two different methods for embedding of optical fibres in high melting point metallic alloys are presented. One method is based on a shape deposition manufacturing process consisting of a combination of electroplating and laser cladding. The other method is based on vacuum brazing. The base materials used are Inconel 600 and stainless steel 304 and 316L. The embedded fibres are equipped with in-fibre Bragg gratings (FBGs) in both cases. First results for temperature and strain measurement using the embedded FBGs are demonstrated. A long term (over five months) elevated temperature test up to 550ºC was also performed as well as a thermal cycling test up to 600ºC lasting a few days. The high temperature capabilities of metal embedded FBGs is thus well demonstrated. Temperature and strain cycling tests are of great importance in final evaluation of the performance of the embedded FBGs. However, during development of the embedding techniques, non-destructive methods for finding pores, cavities or cracks near the fibre are also useful because these defects may have a catastrophic effect on the sensing function and light guiding properties of the embedded fibres. The potential of two non-destructive methods was therefore investigated and found useful. These methods are scanning acoustic microscopy (SAM) and microfocus X-ray tomography.