The portable kit can be repaired with UV-curable fiberglass/vinyl ester or carbon fiber/epoxy prepreg stored at room temperature and battery-powered curing equipment. #insidemanufacturing #infrastructure
UV-curable prepreg patch repair Although the carbon fiber/epoxy prepreg repair developed by Custom Technologies LLC for the infield composite bridge proved to be simple and quick, the use of glass fiber reinforced UV-curable vinyl ester resin Prepreg has developed a more convenient system. Image source: Custom Technologies LLC
Modular deployable bridges are critical assets for military tactical operations and logistics, as well as the restoration of transportation infrastructure during natural disasters. Composite structures are being studied to reduce the weight of such bridges, thereby reducing the burden on transport vehicles and launch-recovery mechanisms. Compared with metal bridges, composite materials also have the potential to increase load-bearing capacity and extend service life.
The Advanced Modular Composite Bridge (AMCB) is an example. Seemann Composites LLC (Gulfport, Mississippi, U.S.) and Materials Sciences LLC (Horsham, PA, U.S.) use carbon fiber-reinforced epoxy laminates (Figure 1). ) Design and construction). However, the ability to repair such structures in the field has been an issue that hinders the adoption of composite materials.
Figure 1 Composite bridge, key infield asset Advanced Modular Composite Bridge (AMCB) was designed and constructed by Seemann Composites LLC and Materials Sciences LLC using carbon fiber reinforced epoxy resin composites. Image source: Seeman Composites LLC (left) and the U.S. Army (right).
In 2016, Custom Technologies LLC (Millersville, MD, U.S.) received a US Army-funded Small Business Innovation Research (SBIR) Phase 1 grant to develop a repair method that can be successfully performed on-site by soldiers. Based on this approach, the second phase of the SBIR grant was awarded in 2018 to showcase new materials and battery-powered equipment, even if the patch is performed by a novice without prior training, 90% or more of the structure can be restored Raw strength. The feasibility of the technology is determined by performing a series of analysis, material selection, specimen manufacturing and mechanical testing tasks, as well as small-scale and full-scale repairs.
The main researcher in the two SBIR phases is Michael Bergen, the founder and president of Custom Technologies LLC. Bergen retired from Carderock of the Naval Surface Warfare Center (NSWC) and served in the Structures and Materials Department for 27 years, where he managed the development and application of composite technologies in the US Navy’s fleet. Dr. Roger Crane joined Custom Technologies in 2015 after retiring from the US Navy in 2011 and has served for 32 years. His composite materials expertise includes technical publications and patents, covering topics such as new composite materials, prototype manufacturing, connection methods, multifunctional composite materials, structural health monitoring, and composite material restoration.
The two experts have developed a unique process that uses composite materials to repair the cracks in the aluminum superstructure of the Ticonderoga CG-47 class guided missile cruiser 5456. “The process was developed to reduce the growth of cracks and to serve as an economical alternative to the replacement of a platform board of 2 to 4 million dollars,” Bergen said. “So we proved that we know how to perform repairs outside the laboratory and in a real service environment. But the challenge is that current military asset methods are not very successful. The option is bonded duplex repair [basically in damaged areas Glue a board to the top] or remove the asset from service for warehouse-level (D-level) repairs. Because D-level repairs are required, many assets are put aside.”
He went on to say that what is needed is a method that can be performed by soldiers with no experience in composite materials, using only kits and maintenance manuals. Our goal is to make the process simple: read the manual, evaluate the damage and perform repairs. We do not want to mix liquid resins, as this requires precise measurement to ensure complete cure. We also need a system with no hazardous waste after repairs are completed. And it must be packaged as a kit that can be deployed by the existing network. ”
One solution that Custom Technologies successfully demonstrated is a portable kit that uses a toughened epoxy adhesive to customize the adhesive composite patch according to the size of the damage (up to 12 square inches). The demonstration was completed on a composite material representing a 3-inch thick AMCB deck. The composite material has a 3-inch thick balsa wood core (15 pounds per cubic foot density) and two layers of Vectorply (Phoenix, Arizona, US) C -LT 1100 carbon fiber 0°/90° biaxial stitched fabric, one layer of C-TLX 1900 carbon fiber 0°/+45°/-45° three shafts and two layers of C-LT 1100, a total of five layers. “We decided that the kit will use prefabricated patches in a quasi-isotropic laminate similar to a multi-axis so that the fabric direction will not be an issue,” Crane said.
The next issue is the resin matrix used for laminate repair. In order to avoid mixing liquid resin, the patch will use prepreg. “However, these challenges are storage,” Bergen explained. To develop a storable patch solution, Custom Technologies has partnered with Sunrez Corp. (El Cajon, California, USA) to develop a glass fiber/vinyl ester prepreg that can use ultraviolet light (UV) in six minutes Light curing. It also collaborated with Gougeon Brothers (Bay City, Michigan, USA), which suggested the use of a new flexible epoxy film.
Early studies have shown that epoxy resin is the most suitable resin for carbon fiber prepregs-UV-curable vinyl ester and translucent glass fiber work well, but do not cure under light-blocking carbon fiber. Based on Gougeon Brothers’ new film, the final epoxy prepreg is cured for 1 hour at 210°F/99°C and has a long shelf life at room temperature-no need for low-temperature storage. Bergen said that if a higher glass transition temperature (Tg) is required, the resin will also be cured at a higher temperature, such as 350°F/177°C. Both prepregs are provided in a portable repair kit as a stack of prepreg patches sealed in a plastic film envelope.
Since the repair kit may be stored for a long time, Custom Technologies is required to conduct a shelf life study. “We purchased four hard plastic enclosures—a typical military type used in transportation equipment—and put samples of epoxy adhesive and vinyl ester prepreg into each enclosure,” Bergen said. The boxes were then placed in four different locations for testing: the roof of the Gougeon Brothers factory in Michigan, the roof of the Maryland airport, the outdoor facility in Yucca Valley (California desert), and the outdoor corrosion testing laboratory in southern Florida. All cases have data loggers, Bergen points out, “We take data and material samples for evaluation every three months. The maximum temperature recorded in the boxes in Florida and California is 140°F, which is good for most restoration resins. It’s a real challenge.” In addition, Gougeon Brothers internally tested the newly developed pure epoxy resin. “Samples that have been placed in an oven at 120°F for several months start to polymerize,” Bergen said. “However, for the corresponding samples kept at 110°F, the resin chemistry only improved by a small amount.”
The repair was verified on the test board and this scale model of AMCB, which used the same laminate and core material as the original bridge built by Seemann Composites. Image source: Custom Technologies LLC
In order to demonstrate the repair technique, a representative laminate must be manufactured, damaged and repaired. “In the first phase of the project, we initially used small-scale 4 x 48-inch beams and four-point bending tests to evaluate the feasibility of our repair process,” Klein said. “Then, we transitioned to 12 x 48 inch panels in the second phase of the project, applied loads to generate a biaxial stress state to cause failure, and then evaluated the repair performance. In the second phase, we also completed the AMCB model we built Maintenance.”
Bergen said that the test panel used to prove the repair performance was manufactured using the same lineage of laminates and core materials as AMCB manufactured by Seemann Composites, “but we reduced the panel thickness from 0.375 inches to 0.175 inches, based on the parallel axis theorem. This is the case. The method, together with the additional elements of beam theory and classical laminate theory [CLT], was used to link the moment of inertia and effective stiffness of the full-scale AMCB with a smaller-size demo product that is easier to handle and more cost-effective. Then, we The finite element analysis [FEA] model developed by XCraft Inc. (Boston, Massachusetts, USA) was used to improve the design of structural repairs.” The carbon fiber fabric used for the test panels and the AMCB model was purchased from Vectorply, and the balsa core was made by Core Composites (Bristol, RI, US) provided.
Step 1. This test panel displays a 3 inch hole diameter to simulate damage marked in the center and repair the circumference. Photo source for all steps: Custom Technologies LLC.
Step 2. Use a battery-powered manual grinder to remove the damaged material and enclose the repair patch with a 12:1 taper.
“We want to simulate a higher degree of damage on the test board than might be seen on the bridge deck in the field,” Bergen explained. “So our method is to use a hole saw to make a 3-inch diameter hole. Then, we pull out the plug of the damaged material and use a hand-held pneumatic grinder to process a 12:1 scarf.”
Crane explained that for carbon fiber/epoxy repair, once the “damaged” panel material is removed and an appropriate scarf is applied, the prepreg will be cut to width and length to match the taper of the damaged area. “For our test panel, this requires four layers of prepreg to keep the repair material consistent with the top of the original undamaged carbon panel. After that, the three covering layers of carbon/epoxy prepreg are concentrated on this On the repaired part. Each successive layer extends 1 inch on all sides of the lower layer, which provides a gradual load transfer from the “good” surrounding material to the repaired area.” The total time to perform this repair-including repair area preparation, Cutting and placing the restoration material and applying the curing procedure-approximately 2.5 hours.
For carbon fiber/epoxy prepreg, the repair area is vacuum packed and cured at 210°F/99°C for one hour using a battery-powered thermal bonder.
Although carbon/epoxy repair is simple and quick, the team recognized the need for a more convenient solution to restore performance. This led to the exploration of ultraviolet (UV) curing prepregs. “The interest in Sunrez vinyl ester resins is based on previous naval experience with the company’s founder Mark Livesay,” Bergen explained. “We first provided Sunrez with a quasi-isotropic glass fabric, using their vinyl ester prepreg, and evaluated the curing curve under different conditions. In addition, because we know that vinyl ester resin is not like epoxy resin That provides suitable secondary adhesion performance, so additional efforts are required to evaluate various adhesive layer coupling agents and determine which one is suitable for the application.”
Another problem is that glass fibers cannot provide the same mechanical properties as carbon fibers. “Compared with carbon/epoxy patch, this problem is solved by using an extra layer of glass/vinyl ester,” Crane said. “The reason why only one additional layer is needed is that the glass material is a heavier fabric.” This produces a suitable patch that can be applied and combined within six minutes even at very cold/freezing infield temperatures. Curing without providing heat. Crane pointed out that this repair work can be completed within an hour.
Both patch systems have been demonstrated and tested. For each repair, the area to be damaged is marked (step 1), created with a hole saw, and then removed using a battery-powered manual grinder (step 2). Then cut the repaired area into a 12:1 taper. Clean the surface of the scarf with an alcohol pad (step 3). Next, cut the repair patch to a certain size, place it on the cleaned surface (step 4) and consolidate it with a roller to remove air bubbles. For glass fiber/UV-curing vinyl ester prepreg, then place the release layer on the repaired area and cure the patch with a cordless UV lamp for six minutes (step 5). For carbon fiber/epoxy prepreg, use a pre-programmed, one-button, battery-powered thermal bonder to vacuum pack and cure the repaired area at 210°F/99°C for one hour.
Step 5. After placing the peeling layer on the repaired area, use a cordless UV lamp to cure the patch for 6 minutes.
“Then we conducted tests to evaluate the adhesiveness of the patch and its ability to restore the load-bearing capacity of the structure,” Bergen said. “In the first stage, we need to prove the ease of application and the ability to recover at least 75% of the strength. This is done by four-point bending on a 4 x 48 inch carbon fiber/epoxy resin and balsa core beam after repairing the simulated damage. Yes. The second phase of the project used a 12 x 48 inch panel, and must exhibit more than 90% strength requirements under complex strain loads. We met all these requirements, and then photographed the repair methods on the AMCB model. How to use infield technology and equipment to provide a visual reference.”
A key aspect of the project is to prove that novices can easily complete the repair. For this reason, Bergen had an idea: “I have promised to demonstrate to our two technical contacts in the Army: Dr. Bernard Sia and Ashley Genna. In the final review of the first phase of the project, I asked for no repairs. Experienced Ashley performed the repair. Using the kit and manual we provided, she applied the patch and completed the repair without any problems.”
Figure 2 The battery-powered curing pre-programmed, battery-powered thermal bonding machine can cure the carbon fiber/epoxy repair patch at the push of a button, without the need for repair knowledge or curing cycle programming. Image source: Custom Technologies, LLC
Another key development is the battery-powered curing system (Figure 2). “Through infield maintenance, you only have battery power,” Bergen pointed out. “All the process equipment in the repair kit we developed is wireless.” This includes battery-powered thermal bonding developed jointly by Custom Technologies and thermal bonding machine supplier WichiTech Industries Inc. (Randallstown, Maryland, USA) machine. “This battery-powered thermal bonder is pre-programmed to complete curing, so novices don’t need to program the curing cycle,” Crane said. “They just need to press a button to complete the proper ramp and soak.” The batteries currently in use can last for a year before they need to be recharged.
With the completion of the second phase of the project, Custom Technologies is preparing follow-up improvement proposals and collecting letters of interest and support. “Our goal is to mature this technology to TRL 8 and bring it to the field,” Bergen said. “We also see the potential for non-military applications.”
Explains the old art behind the industry’s first fiber reinforcement, and has an in-depth understanding of new fiber science and future development.
Coming soon and flying for the first time, the 787 relies on innovations in composite materials and processes to achieve its goals
Post time: Sep-02-2021