Installing Fiber Strain Sensors – Part II
Eric E. Sanborn, P.E.
Director, Engineering and Production
In the first part of our story on installing fiber strain sensors we covered the basics of applying Luna’s ODiSI sensing fiber to simple parts for measuring strain. In this second part we will discuss the planning and execution of instrumenting large or complex structures. The planning stage tends to be a sizable chunk of the total time required for instrumenting a complex structure. In most applications there are key areas of the structure that we want to learn more about. Key measurement points should be marked on the part keeping in mind that the actual path of the fiber will need to be sanded and cleaned prior to bonding. It might be helpful to sketch the full path of the fiber either on the structure or on a drawing or photograph of the structure. If you need to traverse across an area of the structure between areas of interest it is recommended to run along an axis of interest. For a pressure vessel this might be parallel to the hoop stress, or for a simply loaded structure it might be parallel or perpendicular to the applied load. This will make it easier to evaluate the results later. Once a path is identified a tape measure can be used to estimate how much fiber will be required.
When planning the routing on a structure there are a couple items that need to be considered. The first is that fiber can’t make a sharp turn. In most cases a radius of about 25 mm [1 inch] is desired; however for tight geometry this can be reduced. The long term strength limit is a radius of 15 mm [5/8 inch], but for shorter term installations radii of down to 5 mm [3/16 inch] can be used. When your structure has a sharp step, epoxy or something similar can be used to smooth the step to within the radii limitations of the fiber. It is wise to assume that any place the fiber is bonded to the structure will eventually be stepped on by someone so the fiber should always be in contact with the surface. If you have to span across a groove, or transition up a step, be sure to provide support under the fiber.
You should also assume that where the fiber transitions from a bonded region to an unbonded region that it will get tugged on. Our sensors ship with a Teflon outer sleeve over the last 300 mm [1 ft] of the fiber before the connector. The Teflon is etched on the outside diameter so it can be bonded. It is recommended to bond the Teflon to the test article to provide strain relief to the fiber.
Once the path is decided the surface must be prepared per the instructions of the adhesive to be used. Starting at the connector end using the same Kapton tape dots described in part one, attach the fiber to the structure. The spacing of the Kapton dots will depend on the structure. As with the coupon in part one, be sure that the entire length of the fiber is in contact with the structure now before you mix up your epoxy.
Once the fiber is placed and any adjustments are made it is time to bond the fiber down. If a cyanoacrylate solution is to be used it can be applied as described in part one. Luna uses room temperature air cure epoxies for almost all applications. These products cure based on an exothermic reaction. We need to use very little adhesive, too much and the fiber will float off the part and not measure the strain of the structure. The epoxy will likely have a minimum mix quantity that is many times more volume than will be needed. We want to slow down the reaction time so we can apply as much as possible before it starts to gel. To do this, after the epoxy is mixed together, pour it out on to an aluminum plate and spread it around. This will minimize the buildup of heat in the epoxy and can extend the working time dramatically. For long fibers 10+ meters it is helpful to work in pairs on different sections of fiber to decrease the time it takes to bond the fiber.
Applying the epoxy can be done a number of ways. A fine point artist’s brush works very well and usually allows you to work with the epoxy even once it starts to get a little thicker. A small squeegee can be used to apply the adhesive as well as insure the fiber is in tight contact with the surface. This approach requires the epoxy to be in a very low viscosity state or the fiber may shift from the drag. Fortunately if small quantities of epoxy are used, the fiber maintains good contact with the surface regardless of method.
After the fiber is bonded you need to be able to correlate data given by the ODiSI system to points in real space. The video below shows how this can be done what we call “Touch to Locate.”
Instrumenting a large or complex structure takes time and care, but compared to conventional techniques it can be done very quickly. Planning, bonding, and performing Touch to Locate on a 10 meter sensor can be completed by 2 people in less than 2 days. Bonding an equivalent 2000 foil gages would take months.
Luna has Engineers experienced in bonding fiber to a whole host of complex and large structures. Give us a call and we can help you with yours.