Dr. Waruna Seneviratne, NIAR Technical Director
Dr. Waruna Seneviratne
NIAR Technical Director

MTS Strain Gage Measurement
Fig. 1: Embedded spiral DFS configuration prior to installing repair patch

MTS Strain Gage Measurement
Fig 2: Repair with embedded fiber in progress

MTS Strain Gage Measurement
Fig 3: Comparison of strain measurements from foil strain gages, digital image correlation, and distributed fiber sensors

Entering a New Era of Strain Measurement

Dr. Waruna Seneviratne is a technical director of Composites and Structures Laboratories at the National Institute for Aviation Research (NIAR). NIAR at Wichita State University is the largest university aviation R&D institution in the U.S. In this Q&A, Dr. Seneviratne discusses an innovative approach to strain measurement in bonded aircraft repairs, which the NIAR team presented recently for the 2015 MTS Aerospace Users’ Group Meeting.

Q: What problem is your research team trying to solve?
Our work is focused on finding better ways to measure strain in patch repairs and bonded joints in composite structures. Bolted joints are the most common repair technique. But in composite structures, bolted repairs require drilling holes that break the fibers that carry the load. Bonded joints are superior, but they are extremely process dependent. If the surface preparation is done incorrectly, the bond will not hold or the strength of the repair will be compromised. Obviously, that is a serious risk. The problem is there’s no reliable way to evaluate the integrity of a bonded repair outside of a test lab. In the lab, we can load the repair and measure strain, but that is not practical in the field.

Q: What are the conventional methods for measuring strain?
We typically use foil strain gages to measure strain at a certain point or digital image correlation (DIC) for full-field strain measurement. Each of those methods has its tradeoffs. In a full-scale structural test, in which there is not a complete understanding of strain distributions, hot spots and other anomalies, it is difficult to place the gages exactly where you need them. Sometimes, failures occur at points where there is no strain gage. Digital image correlation works very well, but only if both cameras have an unobstructed view of the specimen. In structural tests, the test rig and whiffletree load mechanisms obscures many portions of the areas of interest.

Q: Can you describe the new measurement technique your research is evaluating?
Seneviratne: It is called distributed fiber sensing (DFS). It’s not a single point or full-field, but somewhere in between, so to speak. DFS generates strain measurements at the sub-millimeter level along a 10-50 meter cable. Most critically for our research, the fiber optic sensor is small enough to be embedded within the bond line of the composite patch repair. The motivation for the study was to investigate whether it is feasible to use DFS in large-scale durability and damage-tolerance test programs, as well as in the field as a quality control measure for these kinds of repairs or even as a potential structural health monitoring technique.

Q: What are some of the challenges of using DFS to measure strain in this way?
Seneviratne: Creating a patch repair involves preparing the damaged area and then applying the adhesive and repair plies in layers, which then need to be cured. Because we are embedding the fiber optic sensor underneath these layers, we first needed to demonstrate that the technology could survive the temperature and pressure of the curing process, which we did. It survived and functioned normally. Then we needed to show that it could deliver accurate strain measurement after curing, and that the presence of the fiber optic sensor did not affect the integrity of the repair.

Q: Can you tell us about the test process and results?
Seneviratne: Because DFS is relatively new, we wanted to compare it to established techniques that we have a great deal of confidence in, namely foil strain gages and DIC. We overlaid strain data from all three techniques and saw that they were very closely aligned. We also compared strain data from embedded fiber optic sensors to fiber optic sensors on the surface of the repair, which also aligned, indicating that the embedded sensor was just as accurate as one that was not. These were the results we were expecting to find and they demonstrate that DFS is a viable technique.

Q: When should test teams use DFS in the lab?
Seneviratne: It is ideal for test specimens with hot spots and large strain gradients, as well as areas of the structure where accessibility is limited for foil strain gage installation or visibility is limited for DIC cameras. The sensors can be embedded in both bond lines and composite plies, which offers a great deal of flexibility.

Q: Can you describe the role of MTS technology in your research?
Seneviratne: We used MTS FlexTest® digital controllers to test the smaller “picture frame” patch repairs with embedded fiber optic sensors. We also used MTS AeroProTM Control and Data Acquisition Software to test DFS on a full-scale wing test. Right now we are collaborating with MTS to figure out how best to integrate the fiber optic strain data into MTS DAQ to create a single data file.

Q: What are the implications of this research for the aerospace industry?
Seneviratne: The big advantage of DFS is greater confidence in the quality of the finished bonded joint. It is relatively simple to embed the sensor in the bond line and then measure the strain after the repair has been made to perform a quality control check. In the long term, it may be possible to use DFS as a health monitoring technique. In other words, the aircraft would be manufactured with sensors embedded in critical parts of its structure. If a certain part of the structure is damaged, sensor readings could help determine the extent of the damage and help visualize it. To do this, we first need to demonstrate the durability of the sensors and the hardware such as optical connectors for operational environments and the ability of the system to reliably produce accurate data.

Q: What comes next in your research?
Seneviratne: Right now we are taking the same patch repair specimens and performing fatigue tests to understand the durability of the DFS technology. We know it survives curing, but how long will it provide accurate strain measurements? We are interested in learning more about how to protect the cable from physical damage. But at the moment, DFS is a great tool for the test community.



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