In-situ composite sensors for productivity, smart processes, and life cycle monitoring | World of Composites

2021-11-24 04:55:01 By : Mr. Will Shen

Com&Sens FBG sensors are embedded in pressure vessels, rudders and bridges, and adopt new edge and surface connection technologies to achieve cost-effective mass production and monitoring of composite structures. #hydrogen #SHM #composites4_0

Com&Sens (Exeke, Belgium) claims that it has a non-invasive in-situ sensor for measuring temperature, strain or pressure changes in composite materials, but it can also provide resin flow and curing monitoring during processing and the entire structure manufacturing process Strain diagrams in, assembly and service life. application? In addition to structural health monitoring (SHM) for bridges, wind turbines, and ship rudders (all they do so far), it is also possible to assess the strain level in the composite pressure vessel layer during filament winding and curing, and then use these same Sensors to monitor the integrity of fuel tanks over thousands of refueling cycles to predict required maintenance and enable re-certification.

"What we provide is a digital solution that provides a digital twin data pool for each composite pressure vessel produced," said Grégoire Beauduin, Com&Sens business development director. "Using a set of sensors, data can be obtained from the parameters in the composite processing, from quality testing, certification and service life to recertification." The same digital solution is being developed for many other composite applications, from satellite components to Bridge deck.

Image source: FBG sensor principle video of FBGS (Belgium/Germany)

These optical fibers are like thin glass filaments of small size (100-200 microns in diameter) that can transmit large amounts of data compared to wires. Optical fibers have replaced copper cables used to transmit information, for example to provide broadband Internet and medical imaging. Because optical fibers can transmit data from multiple sensors in a single strand (ie, multiplexing capability), they are replacing resistance strain gauges (RSG).

In order to be used as a sensor, the optical fiber uses a fiber Bragg grating (FBG), which is a small part of the optical fiber that is engraved with a pattern of reflected light and acts as a sensor. The distance between the reflection points constituting the pattern is always equal. Only the wavelength of light that matches the distance between the two reflection points will be reflected; all other wavelengths pass through unaffected. When the FBG is strained, the distance between the reflection points—and its refractive index—will change, so it will reflect different wavelengths. FBG is sensitive to temperature and strain, and in the case of changes in both, well-known compensation methods can be used to isolate each measurement.

Fiber optic sensors (FOS) using FBG have been studied for decades, including successful demonstrations using various composite material structures and processing methods, as well as long-term structural health monitoring (SHM), which is suitable for both "smart structures" and Adhesive bonding layer. FOS is also used for design verification and finite element analysis (FEA), and as a tool for digital testing and certification (see "Fiber Optic Sensors Revealing the Toughest Composite Design Challenges").

If this technology is applicable to composite materials and has been well proven, why not use it more widely? "It is used for composite materials, but mainly for test environments and laboratory scales," said Eli Voet, co-founder of Com&Sens. "There is a perception that fiber optic sensors have great advantages, but they are very fragile and expensive. Our goal is to make this technology simple and cost-effective so that even small companies can use it on an industrial scale. Monitor the process parameters and also monitor the finished product structure during service."

Com&Sens was spun off from Ghent University (Ghent, Belgium) in 2012 by Voet and co-founder Geert Luyckx. Both have completed PhD studies using FOS and FBG to monitor complex aerospace composites made using thermoset and thermoplastic matrices. Com&Sens-which stands for composite materials and sensing-then through their technical cooperation to embed these sensors in a cost-effective and practical way. 

Voet pointed out: "We make the fiber stronger by embedding the fiber in a single flat or round composite tape." "This allows for versatile and cost-effective installation on almost any building material that can withstand harsh environments. We use an internal pultrusion production line that can produce strips with lengths ranging from one meter to hundreds of meters. These usually include 20 to 40 sensing points. We have experience in embedding sensors in almost all composite material production processes, including Filament winding, pultrusion, weaving, prepreg layup, AFP and ATL [automatic fiber placement and tape placement], as well as each version of RTM [resin transfer molding] and infusion."

But can FOS be embedded in composite parts without reducing mechanical properties? "There are a lot of documents documenting this," Voet said. "We are talking about optical fibers with diameters between 100 and 200 microns, including fiber coatings. A human hair is approximately 75 microns. FRP yarns are in the same order of magnitude as optical fibers, depending on the material used. At the microscopic level At the level, it is influential, but in terms of structure, the impact is negligible. For example, the impact of process variability and woven fabric patterns in laminates is usually much higher. We also tried to place the fiber line in the reinforcement direction Place or place between two UD fibers so that it does not really affect the mechanical performance."

Com&Sens makes sensor detection easy through its patented edge and surface connection technology (bottom). The prototype is shown at the top. Image source: Com&Sens

Another issue is interrogation. In order to access the information provided by the FBG, a physical connection must be established between the FOS and the data acquisition unit. This is another area where Com&Sens develops key technologies. "In the composite production process, the challenge of embedding the required connectors to interrogate the sensor has always been a problem," Beauduin said. "We have developed a unique edge connection technology that allows us to reconnect to the embedded fiber that is cured and processed as part of the composite part. The fiber is just another element in the stacking sequence-you don't need to modify the mold or Curing cycle. After the part is processed, we can align and connect the embedded fiber to the edge of the part using a standard non-invasive connection."

Use a non-intrusive connection method at the edge of the component to align and connect the power supply to the embedded fiber at the edge of the component to any standard fiber pigtail. "

This patented technology (US 10466419) is being further developed for surface connectors. "This will allow the embedded fiber optic sensor to be interrogated from the surface of the part," Voet said. "We see this technology as a game changer," Beauduin added, "it will truly open the door to the automation and mass production of smart composite materials."

"As a pioneer in fiber optic technology," Voet said, "Com&Sens pays close attention to and evaluates the latest developments in the market, and cooperates with the most advanced readers that meet our requirements. These FBG interrogators are getting smaller and smaller in size. When there are smart composite panels with embedded sensor networks, they can provide continuous monitoring up to 5 kHz or higher, depending on the manufacturer."

"There is no need to recalibrate after installation," Voet continues, "because we use absolute measurement. The interrogator-through the patented Com&Sens connection-then transfers the FOS data to the PC for instant monitoring, data visualization, archiving and further Post-processing analysis."

Voet and Beauduin cited a series of advantages that make FOS and FBG more cost-effective than other sensors because they do not require special protection against lightning strikes, electrostatic discharge, electromagnetic radiation (such as induction heating and welding), chemicals, seawater, oil , Vibration, pressure or temperature, the maximum value of the latter is 10 bar and 300°C long-term. Another key point is that each FOS line can contain up to 40 sensors with a minimum distance of 1 cm. The exact number depends on the application, including the optical bandwidth of the interrogator and the expected strain range.

Beauduin said that the average cost of FBG sensing points is 50-125 Euros, "For non-automotive composite pressure vessels, the volume is small, or we deal with unique products and designs (for example, space). This will decrease with the volume. In our work using composite pressure vessels, we usually need a small number of sensors, so only a few fiber optic lines, or even one.” He added that if it is applied to 100,000 ships, each sensor’s The cost will drop to approximately 25-35 Euros. "Considering the added value of the entire life cycle," Beauduin said, "this is a negligible cost."

He said that this kind of life cycle monitoring for multiple designs, production and in-service benefits is another cost advantage. "We can provide different benefits through a set of sensors: we optimize and digitize the process, and make every composite structure leaving the factory "smart" and ready to be monitored during its lifetime. We are achieving complete digital data From understanding and verifying part design, to optimizing the production process, to digital ID cards supporting digital twins."

This can facilitate the re-certification of composite pressure vessels within the currently required 10-year inspection interval. However, in fact, it can achieve the same function at the end of the service and/or design-specified service life of all composite structures. "Our sensor-supported digital twin captures all life cycle information, from design to production to service period data, and enters it into a larger repository," Voet added. "For example, this may come from every composite container manufactured by the manufacturer. Then apply AI [artificial intelligence] and/or machine learning data analysis methods to gain more understanding and optimization."

The image below is from an RTM application where Com&Sens provides resin flow monitoring. "We used a network of four channels, each with a maximum of 10 sensor points," Voet explained. "We show both strain and resin position as a function of time. (What does the strain during RTM mean?) We can also see this in the resin infusion of very large structures, including at the resin inlet during the entire curing cycle. And follow up at the exit."

Process monitoring of the car structure shows strain (top right) and resin position (bottom right) as a function of time during RTM. Image source: Com&Sens.

"We have also done this on very large composite pressure vessels," Beauduin said. "These results are confidential, but we got very in-depth information about what is happening. Therefore, instead of using strain gauges to measure the surface, we are trying to understand the effects within each layer of the composite laminate." Voet points out, The FOS line is placed inside the layer stack, "depending on the critical area of ​​the part. Customers clearly know what they want to measure and where they want to manage temperature, pressure, strain or waste."

Image source: Com&Sens

For pressure vessels, Com&Sens manually embeds the fibers during the filament winding process, Beauduin said, “but we are also working on automatic placement. However, once they are placed, we can start online measurement in the process and clearly see, For example, the change of different parameters between the hoop layer and the spiral layer."

“Even in the small-batch production of standard containers,” he continued, “we also see huge differences in the fiber tension of different layers, which have a great impact on product quality. Therefore, we can already monitor and control the process in the process. Use embedded sensors for design verification, as well as monitoring and better understanding of the curing cycle and strain effects during the curing process. Then manufacturers can use it to achieve better performance and efficiency during the winding and curing cycle."

“The same set of sensors can then look at the pressure vessel during the filling process to see if there is any degradation in tank integrity and whether maintenance is required,” Voet said. "Our goal is to extend the service life of these pressure vessels. The graph in the upper right corner of the figure below shows the evolution of strain between 10 cycles, 2,000 cycles, and 5,000 cycles. The tank performance can then be fed back to the design middle."

Com&Sens fiber optic sensors are used to monitor parameters such as strain during winding and curing of composite pressure vessels (right) and strain between different layers during refueling/filling and expansion tests (bottom right) and thousands of refill cycles (bottom ) , remain). The same sensor can help create a unique fingerprint/digital twin of each pressure vessel, including its materials, processes, tests, and in-service data. Image source: Com&Sens

One of the key issues in the design of composite pressure vessels is the need to improve storage efficiency and reduce costs. As a manufacturer of Mirai fuel cell vehicle hydrogen storage tanks, Toyota has improved the storage efficiency of its hydrogen storage tanks by reducing carbon fiber reinforced polymer (CFRP) laminates by approximately 20%. "Pressure vessel manufacturers must prove their design to reduce material usage and lower costs," Beauduin said. "And they also need to monitor as they increase production and strive to meet the output needed for the next 5 to 10 years."

“We also see that these companies need to get to market faster when developing new ship designs,” Voet said. "Today, it takes too long from the start of a new design to the launch of a new design for the needs that must be met. We have also seen that for pressure vessel and vehicle operators, these storage tanks nowadays have no traceability. There is a need for a Tools to help predict failures, track root causes when failures occur, and enable predictive maintenance. There is currently little information about ship integrity or ship usage. These ships should also be re-certified. If they are not, that will increase costs. Today The recertification process can become more cost-effective."

Com&Sens customer reference. Image source: Com&Sens

Com&Sens has been very active in deploying FOS in pipelines, monopile foundations for wind turbines, underwater turbines and rudders, and civil engineering applications including bridges and roads. The company has collaborated with FiberCore Europe and InfraCore (Rotterdam, Netherlands) on a number of projects, including:

Com&Sens installed a fiber optic sensor in one of the compound gates of the Wilhelmina Canal to measure creep and forces in service. Image source: Com&Sens, FiberCore Europe

Use Com&Sens FBG sensor for resin infusion monitoring. Image source: Com&Sens, FiberCore Europe.

The latest project was completed earlier this year. "We conducted a comprehensive test to evaluate a new infusion monitoring technology, and the results were very good," Beauduin said. “We explored the demolding problem and were able to measure the residual stress inside the composite laminate. We can also monitor key parameters during the infusion and curing process, and then emit a green light when each parameter is completed and when demolding is possible, and There is no excessive residual stress in the laminate. The goal is to make these processes as efficient as possible for different resin and fiber combinations according to the actual situation of the laminate." 

Com&Sens embedded FBG sensors into the CFRP yacht rudder to verify FEA, monitor the load in use and provide force feedback to the rudder operator to achieve more efficient steering. Image source: Com&Sens, Royal Huisman

Two years ago, Com&Sens began to study another large structure-a composite rudder for the 80-meter Royal Huisman (Warrenhof, Netherlands) yacht. The composite materials and engineering supplier Gurit (Wattville, Switzerland) requested an integrated sensor, Voet said, “because they have never designed such a large CFRP rudder and want to understand the load during service. We embed it in the rudder manufacturing process. The sensor was installed and some load tests were carried out on the structure in the factory. We also conducted several sea trials, and we can see the strain level of the rudder during the voyage."

He added that Royal Huisman is now working with rudder manufacturer Rondal (Vollenhove) to develop a force feedback system using sensors. For smaller rudders, there is a feeling, the thrust in the rudder, which the ship operator uses to measure the force applied when turning. "But this is such a big yacht and rudder," Beauduin explained. "You lose this feeling and driving is more challenging." The force feedback system will amplify the measurement results from the Com&Sens FBG sensor and make the operation The crew can feel the load on the rudder and react in real time to steer the ship more effectively.

The company also worked with Dutch partners Airborne (The Hague) and Tocardo (Wieringerwerf) to embed FBG sensors in glass fiber reinforced polymer (GFRP) tidal turbine blades. These sensors are used for design and FEA verification as well as for testing in certified test centers and for structural monitoring of turbines after they are installed in the ocean.

Image source: Com&Sens, Airborne, Tocardo

“We have also conducted a research and development project with Damen Shipyard (Gorins, The Netherlands) to test the adhesive hybrid joint between the GFRP superstructure and the steel substructure,” Beauduin points out. "We are monitoring the bonding layer between these two materials, with the aim of working with Bureau Veritas to develop a certification agreement." He pointed out that Com&Sens is also performing metal weld monitoring for more than 30 offshore wind turbines. "We installed four fiber optic lines on the monopile foundation and monitored the welds for cracks."  

Voet pointed out that Com&Sens can also provide monitoring of welds in thermoplastic composites. Voet pointed out in the CW June 2021 webinar on induction welding that because the FBG sensor is not affected by electromagnetic force, a single optical fiber line can be placed in the welding substrate, not only can be measured in real time during parameter research The temperature and pressure, as well as the actual welding and assembly process, are also monitored during the entire certification test and service process. If induction welding is indeed to be more widely used in aircraft structures, all these issues are the key.

Another high-growth area of ​​FBG applications is space launch structures. Com&Sens is working with Dawn Aerospace (Christchurch, New Zealand and Delft, Netherlands) to produce turnkey small satellite propulsion systems, including pressure vessels for use in space. "These usually have a long development cycle," Voet said. "Our goal is to design tools based on our FBG sensor data to help accelerate the time to market for mass production of pressure vessels for space vehicles. We have obtained EASA approval for use in space pressure vessels and estimate that we can help reduce development time by 50%. % And reduce quality, cost and process variability by 10%."

New Space is not the only industry hoping to increase productivity while reducing costs and improving sustainability. In fact, these are the main drivers of future commercial aircraft production declared by Airbus. "We have been negotiating with Airbus, Safran and many major aerospace companies," Baudouin said, "but so far, progress has been slow. So, for now, we are more focused on pressure , Not because these manufacturers see the value in their ability to shorten time to market and increase productivity while improving quality."

He continued: “In order to achieve the current rapid increase in pressure vessel output in a cost-effective manner, manufacturers need to better control the production process. With our technology, they can control the process parameters, and then we can compare them with product quality. In conjunction, it is also possible to monitor the pressure vessel during its lifetime-all of which requires only one sensor line."

Voet and Beauduin saw great potential to play a role in composite pressure vessels. "They saw the benefits we can bring to the market," Beaudin said. "So far, we have received a good response from the manufacturers." Voet pointed out that Com&Sens is a small team with a total of 9 members. "But because of our composite material expertise, we are unique in this field, and our goal is mass production. The era of using only FBG sensors for laboratory testing and R&D is over."

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