A clear difference exist between surface mounted and embedded optical fibre sensors. Surface mounted sensors are installed on the surface of parts after production and perhaps after assembly of the complete structure while embedded sensors are installed in between the layers of the different parts during production. Both techniques have advantages over the other and can be used for different sensing strategies.

Surface mounted sensing:

Surface mounted DTG®s can be attached using a methodology developed by FOS&S. With regard to the specimen preparation, the method is similar to the fixation of electrical strain gauges. Grinding paper and alcohol are used to prepare the specimen and degrease the specimen and sensor, respectively. Next, a fixation method is applied based on a UV-curable glue and a sensor pad (Figure 1, left), to position the optical fibre (i.e. sensor). First, the UV curable glue is spread along the sensor zone (approximately 30 mm x 8mm) and the sensor is positioned manually using the UV transparent sensor pad. Then, the optical fibre is gently pushed against the specimen by using the glass plate and in the same time a first UV  pre-cure is applied, by illuminating the location of the sensor zone, through the UV transparent pad, for approximately 30 seconds. After the pre-cure, the glue has become stiff, and the sensor pad can be removed to apply a post-cure by illuminating the sensor zone for approximately 5 minutes. The sensor is now fixed to the specimen. Finally, to protect the optical fibre from being ripped off, it can be finished at both sides of the sensor zone using an instant adhesive (e.g. X60 from HBM) or simply an adhesive tape (Figure 1 , right).

Figure 1: Preparation of surface mounted DTG® on the Cetex® test-coupons, applying the methodology of FOS&S [8]with a UV-curing system and sensor pad (left), result of a fixated DTG® (right).

 Note that, to ensure accurate surface strain measurements, it is important that the position of the FBG in the optical fibre is well known, to ensure that the glue zone covers the whole sensor length. The position of the DTG®s used in this work are marked in the draw tower process with two black dots on the coating surface, with the DTG®s positioned in the centre of the marking. The DTG®s have a grating length of 1 to 10 mm and they are positioned exactly in the middle of the sensor zone.

Embedded sensing:

Embedded DTG®s are installed during the production phase of the composite parts. This is feasible thanks to the compatibility of the optical fibre with the composite material. They are light-weight with small diameters and therefore do not distort the composite material. A 100% strain transfer is reached because of the surrounding of the optical fibre. They have the advantage that they can be used to measure strain during the production of the composite part. Spring back and residual strains can as such be monitored. Afterwards they can be used to judge on the structural integrity of a composite component.

In Figure 2 an overview is given of optical fibres embedded in different kinds of composite material. Clear is that the optical fibre do not distort the composite material.

Figure 2: Overview of optical fibre sensors embedded in different kinds of composite materials (thermoset, thermoplastic, UD, cross-ply and fabrics)