Induction welding is used in a whole host of industries and applications where safety is of paramount importance and as a result, weld integrity is crucial. However, vital as it is, the process of validating a compositive weld zone is time-consuming and expensive.
Zeus was recently involved in extensive research seeking to replace traditional electrical thermocouple test methods with fiber optic sensors in a bid to simplify this lengthy, costly process. Unlike thermocouples, which affect the mechanical properties of the bond, fiber optic sensors have the potential to accurately monitor temperature and pressure during the weld process without threatening a component’s structural integrity.
Enlisting industry expertise
A proposal was initially put forward by the SmartState Center for Multifunctional Materials and Structures at the University of South Carolina (USC), which sought to modify the standard coating and cladding applied to the core of a fused silica optical fiber.
With funding from the South Carolina Research Authority (SCRA), and support from GKN Aerospace, the research looked to implement a polymer coated optical fiber sensor that would effectively melt into the weld line. By selecting a polymer that could blend with the composite part, the investigation hoped to facilitate in-situ monitoring. To be successful, the polymer coating needed to fully incorporate, leaving a functional sensor safely embedded in the joint.
USC McNair Aerospace Center enlisted Zeus to develop a custom polymer coated optical fiber that would be entirely compatible with the composite weld of the part. Zeus coated the sensors with a specific grade of PPS resin designed to match the melt and flow rate of the chosen composite material; a carbon fabric reinforced polyphenylene sulfide (CF/PPS) from TenCate Advanced Composites. The sensors were subsequently provided to Luna Innovations, for keying and integration with the company’s ODiSI-B 5.0 interrogator system, which would perform in-process and in-situ monitoring during testing.
Findings transform in-process monitoring
Results have been incredibly positive. Not only did the PPS polymer coating melt away from the fiber optic sensor, reducing its final diameter and creating a tight, secure bond to the part, it also left the sensor in place for real-time monitoring of in-process temperatures throughout the length of the bond line.
With data such as this at their disposal, composite manufacturers can determine optimal process control for bonding stronger cohesive joints. A more precise picture of weld quality reduces uncertainty in joint strength. And by proving the feasibility of a non-destructive validation method, scrappage costs can be significantly and immediately reduced. GKN Aerospace, which has supported this research, could ultimately see savings of up to $500,000 per part validation run.
For the wider engineering and scientific community, the findings also open up exciting opportunities for innovative real-time monitoring applications – even after components are fitted and deployed in the field. “Smart structure” monitoring could revolutionize the aerospace industry by providing real-time data highlighting structural issues before flight, providing a cue for preventative maintenance on composite parts without the need for external secondary monitoring equipment – just one of the many potential applications we’re excited about.
You’ll find the full research paper here.
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