Installing Fiber Strain Sensors
Eric E. Sanborn, P.E.
Director, Engineering and Production
In this first part of a two part post we will describe how a fiber optic strain sensor compares physically to a conventional foil gage sensor and how it can be bonded to a simple coupon. In the second part we will explore instrumenting complex parts and structures.
Luna’s ODiSI sensing solution can replace hundreds or even thousands of foil gage strain sensors with a single optical fiber. Foil gages are the industry standard method for measuring strain in a structure and are available in many different configurations and sizes. In a foil gage the strain measurement is converted into a resistance change in a pattern of thin electrical conductors. The strain measured by the foil gage is an average of the strain over the length of the gage. This length is called the gage length. The gage length is one of the parameters used to select the foil gage and is fixed by its geometry.
Our strain sensing fiber measures strain axially along the fiber, but the gage length is a system configurable parameter. While this parameter can be adjusted differently on different ODiSI systems, for the purposes of this discussion a gage length of 5 mm [.197 inch] will be used. This is very close to a common gage length size of ¼ inch or 6.35 mm. In the ODiSI this strain measurement is continuous down the fiber, so over a given meter of fiber there will be 200 strain readings; each representing the average strain over a 5 mm increment of the fiber.
Fiber optic sensors can be bonded using the same type of adhesives common for foil gages. Like with foil gages, the decision on the bonding agent depends on the purpose of the test and the environment to which the sensor will be subjected. For coupon testing or for a short-term test, a Cyanoacrylate based adhesive like Micro Measurements’ M-Bond 200 product allows for rapid installation and curing. For high humidity, wider temperature, high strain, or long term stability, an epoxy is a better choice. We often use AE-10 and GA-2, both products of Micro Measurements. GA-2 has the added benefit of being black, which helps the path of the small fiber stand out visibly on a structure.
To instrument a coupon the fiber must be run parallel to the desired measurement axis. The surface should be prepared per the instructions of the adhesive manufacturer, just like with bonding a foil gage. For most structures this involves sanding with a fine sand paper and cleaning with an acid and neutralizer. For something like a cast part, a different adhesive system may be required or more surface preparation in order to provide the correct surface finish.
Once the surface is prepared the fiber can be laid across the coupon and held in place temporarily with tape. We recommend Kapton tape dots. They hold the fiber, are small, and can be removed easily with tweezers leaving no residue. Once the fiber is routed, tap the fiber to ensure it is in contact with the surface. If the fiber is not in contact, the Kapton dots should be repositioned until the entire length of the fiber desired for strain measurement is in contact with the surface. For a coupon one would typically use a Cyanoacrylate based adhesive. A cotton swab can be used to apply the adhesive. Place a drop of adhesive on the swab and then lightly drag down the length of the fiber. You will only cover a few centimeters [an inch] at a time, but you should still be able to bond about 1 meter per minute. You want to use as little adhesive as possible; it should be nearly invisible once cured. Within a few minutes the adhesive should be cured. At this time the Kapton dots can be removed and adhesive can be applied where the dots were. Once these spots have cured the coupon can be used.
A “Getting Started Kit” is shipped along with every ODiSI instrument. This kit includes everything required to bond your first strain sensor including the Cyanoacrylate adhesive, Kapton dots, tweezers, sand paper, alcohol pads for cleaning, and a section of fiber to practice with. The kit also includes a 1 meter ODiSI strain sensor and a 1 meter ODiSI temperature sensor. The video below shows some of the contents of the getting started kit along with process of bonding a fiber to a simple aluminum coupon.
In a future post we will cover the steps for planning and installing a fiber strain sensor on a large or complex structure.