Strain Gage Technology

The optical Strain Gage is mounted to the structure by means of a patented methodology which makes use of a specially designed mounting tool called a ‘sensor pad’. The design of the sensor pad is fine tuned in order to allow fast, easy and reproducible installation of the sensor. The UV-transparent pad has dimensions of 45 x 8 mm. At both ends of the pad, there are slits that can be clipped onto the 900 μm buffer. This way, the fiber is secured to the pad. In between these clamping areas, there is a zone of 30 x 8 mm where the adhesive layer will be applied. In this zone, the fiber will be free (no 900 μm buffer) so that it can make direct contact with the surface to which it will be attached.

Features:

Easy fixation / removal of the pad to the fiber cable.
Transparency for UV-light (when UV-curing is required).
No bonding with the adhesives so that it can be removed after curing.
Flexible enough to bend around the fiber (in transverse direction) when put under pressure. This is needed to have the adhesive layer thin enough around the cylindrical fiber shape.
Hard enough to hold the buffered fiber ends without slippage. This is needed to keep the fiber under slight tension during installation.

There are two adhesives used for the installation of the optical gages: a UV-curable adhesive and the Z70 cyano-acrylate based rapid curable adhesive. Installation with the UV-curable adhesive is recommended as it is highly controllable. Installation with the cyano-acrylate based adhesive is only advised when a UV-lamp is not available or difficult to use on the site of installation.

Curing Z70 cyano-acrylate: The curing of the Z70 adhesive is initiated by the humidity in the air and by the mechanical pressure that is being applied. This makes that the curing process is less controllable since it depends on: humidity, temperature, Adhesive thickness, surface roughness, applied pressure, chemical composition of the surface.

Curing UV-adhesive: The curing of the UV-adhesive on the other hand only requires UV-light of the right wavelength for the curing. The ambient conditions, adhesive thickness and applied pressure play no critical role. This makes that the installation with the UV-curable adhesive is much more controllable.

Advantages and disadvantages of both adhesives:

Adhesive	Pros	Cons
UV-curable	Highly controllable and reliable	UV-lamp relatively expensive and less practical to take into the field
	More extended temperature ranges
Cyano-acrylate	Applicable when UV-lamp not available	Curing less predictable and hence installation less controllable
		Reduced temperature ranges