Composite Bridge Decks
The application of fiberglass composite bridge decks has been under development for some time. The technology has been applied at several sites around the country and engineers and designers are getting comfortable with the technology. It basically combines resin-infused decking panels that are placed over fiberglass composite or steel bridge beams. Sometimes the attachment is mechanical, and sometimes the use of high-strength adhesives carries the load.
Replacing steel and concrete, composite bridge decks claim several advantages. They can be built in a factory as opposed to onsite (better quality control) transported and lifted rather easily, and should be much more resistant to the natural elements of freeze/thaw. One of the big savings is on installation time. The on-site construction time is much less, which allows for cost savings from road closures, detours, and site crews.
There has been TONS of Research and Development money spent by several federal, state, and private entities in developing the technology, testing, and specification criteria for this application of composites to the real world. Hurdles that need to be overcome include the acceptance by engineering professionals, infrastructure managers, and the public.
As a Composites Engineer, it sure is neat to see new applications of composites. And as a taxpayer, if it can save some money, I am very excited. As a driver, it sure would be nice to see some of these road projects shortened up!
Trimming composite parts
I read an article on waterjet cutting of laminates recently, which demonstrated several advantages of using computer-controlled equipment for trimming and adding features to composites structures. Robotic-guided CNC routers have also been used with much success for large 3D shaped parts with much success. While technology is great, it is expensive. With the rest of composites processing being relatively inexpensive, the trimming operation usually follows suit. Most bimetal saws that cut steel can handle fiberglass, but for large production runs where they will wear down, diamond coated tools offer longer life.
Making a cutting guide for a production composite part is relatively easy. I always called it a “splash”, and it can be done similarly to building a part. Just wax up the gelcoated surface of the finished part, lay down a coat of gelcoat, then build a 3/16 laminate. Then mark out the desired cutout shape and allow for the tool setback for the router collar or saw guide. An aluminum channel edge should be added for wear resistance. Drill bushings can be located on this splash to mark where holesaw work needs to be done as well.
Using a guide collar on a router is essential. I was in a shop where they had a 1/4 inch steel plate guide that was the exact size of the hole, and required running the router bit against it to make the cut. There was TONS of wear on the guide and they were going though router bits like crazy. I set them up with a router collar and compensated for the offset on the jig, and it was a whole new world for the trim operators. The router bits lasted several times longer and the cuts were much faster to make.
Damaging the laminate is one of the dangers when cutting or machining composites. Delamination is the biggest risk, ans is diagnosed by a separation of the layers of fiberglass. The resin bond becomes too weak to hold them together. Delamination can be caused by several other things too, but very often it can happen around machined areas from dull tools or operators that are pushing tools too quickly along a cut.
Technicals on Fiberglass Mat
One of the most confusing parts of the glass reinforcement is the identification of the specific materials. They have various nomenclature depending upon the manufacturer of the matting, and sometimes depending upon the customer and application as well. These glass products all come on rolls of various widths and lengths, again depending upon manufacturer and customer, but pricing may be affected for specialized sizes. Wholesale fiberglass is sold by weight as opposed to yardage or square footage or anything else. Retail fiberglass is sometimes sold by the package or area depending upon the seller.
The nonwoven mats, such as Continuous Filament Mat and Chopped Strand Mat, are measured in ounces per square foot. The common weights are 3/4, 1, 1.5, 2, and 3 ounces per square foot.
The woven mats are measured in ounces per square yard. A traditional DBM1708 is composed as a double bias woven in the +/-45 direction weighing 17 oz/sq yard and stitched to a 3/4 oz Chopped Strand Mat, which would come to a real total weight of around 24 oz per sq. yard.
There are also different types of glass. E glass is the base, general purpose, high volume workhorse glass fiber formulation. The S glass is a high tensile strength glass that is more expensive but has greater physical properties. C glass exists to be used in applications where high chemical resistance is important. These are the main classification types of glass fibers.
Fiberglass Reinforcement
Fiberglass reinforcement materials are a pretty complicated subject and hopefully we can keep it clear. There has been all sorts of nomenclature developed over the years to describe the different weights and styles. When we talk about fiberglass we are really talking about fibers of glass. For the reinforcements we are discussing, they are bundled together to form larger, more workable bundles.
Roving is a ball of continuous filaments wound into a 40 to 5o pound ball. This is commonly used in the fabrication shop with chopper guns and filament winding equipment.
Chopped Strand Mat is the most basic reinforcing materials that comes as a sheet good on a roll. It is fiberglass that is chopped into small pieces and mixed with a binding material to hold it together, before being randomly oriented into a continuous mat.
Continuous Filament Mat uses a continuous fiber nonwoven to itself and contains some sort of binder material. I have also worked with some continuous filament mat that is stitched together to omit the binder.
Surfacing Veils are a non stitched mat product that use very fine glass fibers in random orientation to assist with achieving an improved surface finish. They are intended to keep the coarse fibers away from the gelcoated surface to prevent print-thru.
Woven Roving (Knytex) is a very diverse product that is widely used throughout many applications and processes. It consists of continuous rovings woven in specific directions to each other and then sewn to a Chopped Strand Mat with thread. They come in hundreds of different combinations of directions; including unidirectional, bidirectional, triaxial, and quadriaxial. The Chopped Strand Mat also comes in different weights and sometimes may be substituted with Continuous Filament Mat.
The Importance of Testing
The testing of properties for Composites laminates and structures can be tricky. Much of this is due to the complex base of ingredients and makeup of the laminate. When we are putting together a combination of resins, reinforcements, coring, fasteners, curing agents, fasteners, etc. we can radically affect the properties of the FRP structure. This can be beneficial and disadvantageous.
ASTM test specifications can be used to determine properties such as stiffness, chemical resistance, impact strength, etc. There are several designed specifically for FRP laminates. These can be very informative, especially when developing and comparing potential laminate designs.
House tests can also be developed to test for more specific properties on test panels and the overall structure. A flat and square test speciment may test out great for certain properties. When it is put into actual application, it may have differenet features which make it stronger or weaker than the flat test panel. Things such as holes, notches, square corners, curves, and other geometric shapes have a wide range of effects. They may increase localized thickness from overlap joints, may cause stress concentrators, or may have thin areas because of an area being difficult to reach.
One of the projects I worked on recently involved the conversion of the processing from hand layup to resin infusion processing. For product liability we wanted to prove that we tested the laminates for strength, and doing an overall structure test was out of the question. But testing flat and square infusion panels would just give us a number. So we setup a baseline with the same size flat and square test panels made with the old laminate schedule using hand layup. This old laminate schedule had been very suitable in its application. We just needed to ensure that the new infused laminate schedule exceeded the strength of the original hand laid laminate schedule.
We setup this direct comparison between laminates by removing the variables of the part shape itself. We also added some safety factor to ensure for any other variables that may be involved. Our supplier tested the panels for us, giving us official ASTM results. We worked through a few different scenarios with the new laminate schedule, received test results, and made our decision. Future inquiries into our engineering work can be backed up with technical testing data.
Plastic Skin
One of the other synergies of FRP composites is its use as a reinforcement behind thermoplastic decorative skin layers. A thermoformed ABS or ABS blend plastic is made as the surface of the part. Then it can be reinforced on the backside with fiberglass construction. Vinylester and ABS resins adhere very well with each other and create a strong bond. The FRP will then provide all of the strength and support that it is designed for, while the thermoplastic skin is more damage and impact resistant than gelcoat.
My experience leads me to think that the largest use of this technology is in bathtub and hottub applications. High volume manuacturers can setup to make large numbers of thin thermoplastic skins. These are then setup on holding jigs to keep their shape while fiberglass resin and reinforcement is used to permanently lock the shape together. This method allows for nicer surface finishes, can save cost depending upon volume, allows for fewer cracking issues, and can offer different coloring effects.
Role of Gelcoat
Gelcoat is the decorative surface found on fiberglass parts such as boats, bathtubs, and restaurant seats. This outer layer is needed for aesthetics and protection of the underlying laminate structure. Chemically it is unsaturated polyester/vinylester resin that is unreinforced but heavily filled with a complex variety of additives. These additives are used to determine its color, UV stability, and chemical resistance.
The manufacture of fiberglass parts typically requires a gelcoat layer to aid in the release of the parts from the mold. The parts that will be painted also are manufactured with gelcoat but it is a “sandable” variety. These parts are often automotive-related, and may include pickuptruck toppers, fiberglass kitcar bodies, and other aftermarket add-ons such as running boards or hood scoops.
The unsaturated polyester/vinylester gelcoat is directly compatible with resins and laminate of the same chemistry. Gelcoat can be used with epoxy resins but requires a tiecoat for adhesion because of the chemistry difference.
Gelcoat is applied at 18 to 25 mils (thousandths) and will lose thickness as it cures. Typically when it is dry on the part, it can be down to 12 to 18 mils. This is a lot thicker than a painted surface. One of the disadvantages of gelcoat is that it can be more difficult to repair, especially with color matching. The thickness typically allows for sanding out scratches and blemishes, but going through can be painful and will require a respray. The outer layers of gelcoat will likely discolor over time due to UV degradation, and sanding and buffing into the underlying gelcoat may cause discoloration as the layers are different colors depending upon the depth.
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