Various sensors and sensing technologies are used throughout industry to detect and respond to external stimuli for control and monitoring purposes. These sensors are conventionally attached to the component or system externally since gained information is adequate for the application. In some cases, external mounting can confuse the data collection, limit the gained information, and create challenges for sensor mounting due to the use limits of adhesives, like high temperature or humid environments.
Therefore, it is desirable to build-in these sensors into the metal matrix directly to provide in-situ information and robustness. 3D metal printing offers the opportunity to build these fiber materials directly into the metal matrix.
Ultrasonic Additive Manufacturing and Fiber-Optic Sensors
Ultrasonic additive manufacturing (UAM) is a 3D metal printing technology that uses high-frequency ultrasonic vibrations to scrub metal foils together layer by layer as opposed to using a directed energy heat source (e.g., laser, e-beam, etc.). UAM systems are integrated into computer-numerical-control (CNC) frameworks to enable subtractive operations interchangeably with the additive ultrasonic process—a form of hybrid additive manufacturing.
Ultrasonic joining is a solid-state (no melting) process, which enables direct integration of temperature sensitive components into the 3D metal part. Fiber optic strain sensors are particularly vulnerable due to their temperature sensitive inscriptions, commercially common plastic coatings, and large thermal expansion mismatch with many metal alloys (i.e., cracking susceptibility). Melt-based 3D metal printing has been used to build some fiber types into metal, yet voids around the fiber, atmosphere requirements, dimensional accuracy, and cracking remain challenges.
There are many different types of fiber-optic strain sensors. Common traits are their small size, electromagnetic immunity (light-powered), remote interrogation, multiplexing, low noise, high-temperature operation, and long-length capability. These attributes are attractive for certain industries and applications such as aerospace, nuclear, and oil & gas. Such sensors work by measuring internal stress in the fiber via light reflections.
How it Works
To embed small fiber materials into a metal part, a channel path is cut during the CNC stage of the UAM process. The fiber is then placed into the channel and consolidated with the additive stage. Metal flow in the UAM process (which is similar to metal flow in friction stir) creates a strong mechanical joint between the matrix and sensor material, which in turn enables excellent strain transfer to the metal matrix for stress and temperature measurements.
1. Building fiber-optic strain sensors into aluminum components.
An instrumented bracket is shown in Figure 1 along with a micrograph of a consolidated fiber. The fiber is fully integrated with the metal matrix. A bracket is shown with the sensors embedded near the stress riser in the bracket, a measurement impossible to make with externally mounted strain gauges. The metal flow induced in the UAM process creates a strong mechanical joint between the fiber and matrix.
Fundamental work has shown that the joint strength between the fiber and metal matrix is stronger than the yield stress of the metal and that the joint doesn’t degrade with fatigue loading. Fabrisonic collaborated with EWI’s mechanical test lab to perform this testing. This joint robustness enables technology scale-up and application evaluation. (Click here for more info.)
The team is also collaborating with the NASA Langley Research Center for advanced mechanical testing and analysis. Fatigue data from the bracket in Fig. 1 is shown in Figure 2. Live load data can be used for monitoring and control, while high outputs can be used to identify and track damage evolution.
2. FBG strain information from fatigue test at NASA LaRC.
Building fiber-optic strain sensors into metal using UAM is still in its infancy, yet shows strong promise. To help with technology transition and adoption, the team is working to explore other key areas. For example, sensor use is made friendlier by building fiber terminations/connectors into the part directly. Metal-coated fibers are also of interest due to their high operating temperatures (Fig. 3). Lastly, other metal matrix alloys and applications are being evaluated in conjunction with the EWI additive manufacturing group.
3. The future of embedded fiber-optic materials: Built-in connectors are used for usability and robustness, and metal-coated fibers are used for higher-temperature applications.
The author would like to acknowledge financial support from NASA’s SBIR Office, NNX16CL33C. The authoris also grateful for the support of NASA’s Convergent Aeronautics Solutions (CAS) Program Digital Twin Project, and for the collaboration and support of Sheridan Solutions.
Adam Hehr is a research engineer at Fabrisonic LLC.