Stopping a Crack
Composites can be very impact-resistant compared to other materials. Based upon their makeup, different composites materials will offer different degrees of resistance to impact. Once the threshold to impact resistance is passed, cracking will occur. Localized impact can form a crack in the weakened material, and vibration and additional loading can keep the cracks spreading. There is a relatively simple solution to stopping this.
I am currently working on repairing an SMC composite truck hood, which has various forms of damage including cracks. I want to repair the cracks, but also must keep them from spreading. So the solution is …
Drill a Hole!
A hole that is drilled at the end of a crack in the material will stop it from spreading. There is no place for it to restart. It cannot travel any farther because there is not any high-stress area that is weaker than the surrounding surface.
In my example, I am working on a complete repair, not just stopping the crack from spreading. I drilled the hole, reinforced the back side, and sanded the surface to accept filler. The filler will fill this low spot over the crack as well as the hole that was drilled. It can be filled right back in but will still retain its crack resistance!
Another way for cracks to start is from high-stress areas such as square corners. When making a hole in composites, it is important to always avoid sharp corners. Round holes are the best, but if the opening must be rectangular, the corners should have some radii incorporated into the corners.
A cracked laminate is a sign of failure, and it must be addressed before it gets worse.
SMC Truck Fender
SMC (Sheet Molding Compound) works well for heavy duty truck parts, especially ones requiring complexity, strength, and impact resistance. One of the projects I am working on is a heavy truck with these SMC parts, including a hood and a driver’s and passenger’s truck fender extensions. Each of these SMC parts is very complex in geometry because of its complex application. The following is a picture of one of the SMC fender extensions.
SMC Truck Fender Extension
This SMC part is all one single molded piece, having very complicated geometry. It can also be noted that there are not any undercuts, i.e. the mold can open and close without having to move around the part. It has a mostly constant cross section, is not supported by any metal struts, and is attached to the cab by three bolts. The geometry and details to match the cab and hood styling are molded-in so that the part can be painted and bolted on the truck.
If a part needing this much complexity was manufactured with sheetmetal, it would have many more pieces of the assembly and much more bracing. Metal would also not have the resistance to corrosion and impact that is enjoyed by this composite part.
This fender extension is from a truck that has been on the road since 1993, with over 270,000 miles on it. This part has been in the wild for over 15 years, and has been exposed to tons of road salt, debris from the tires, and lots of other environmental exposures. It has survived well, only needing an update in paint to refresh its look.
Another great application of composites!
The Green Aspects of SMC
Sheet Molding Compound (SMC) is used to create many composite parts especially for the transportation industry, and contributes heavily to a positive environmental impact. SMC has been developed over the last 25 years to replace steel/sheet metal mostly in transportation applications. It is widely used in many heavy duty semi truck hoods, agricultural equipment, and pickup trucks, SUV’s and muscle cars.
The main goal of this substitution is to reduce weight, which improves fuel efficiency. Other positive side effects include fewer assembly operations, additional design freedom, dent and impact resistance, and the elimination of corrosion. Several “green” resin formulations have been introduced that make use of bio resins, which use much more renewable resources such as soy products. The fillers and reinforcements in this material can also be made from recycled and renewable materials.
SMC has overcome several hurdles in order to get to its present use and application. General acceptance and education had to be proven to the OEM manufacturers and consumers. Paint application and adhesion was one large consideration that had to be proven out. There were issues with popping and blistering from the SMC surface. Making sure the SMC parts held dimensions and aesthetics was also an important milestone.
SMC has become widely used today for many applications, and will find its way into many more. The weight saving aspects are paramount for reducing fuel consumption. The anti-corrosion and dent resistance are loved by consumers.
Polypropylene Fiber Reinforcement
One of the press releases I recently came across discussed the commercial release of Polypropylene fiber for use as a reinforcement in composites. The one I saw is sold under the trade name Innegra S Fiber by Innegrity LLC.
Polypropylene’s low density is a huge weight advantage, especially as compared to glass. Measured at 0.84 grams per cubic centimeter, it can be compared to glass which is at 2.55 grams per cubic centimeter, Kevlar (aramid) which is 1.44 grams per cubic centimeter, Carbon which is 1.76 grams per cubic centimeter, and UHMWPE which is 0.97 grams per cubic centimeter.
This material exhibits high toughness, which will make it great as a potential replacement for aramid reinforcements in applications such as bulletproof vests and mass transit. The low cost of polypropylene and its huge cost benefits because it is more of a commodity material will bring it into many new applications. It will be exciting to see new materials like this find commonplace usage and application.
Fiberglass Truck Toppers
One of the composite applications that has been around for many years is fiberglass truck toppers. Styles and trends have changed, but there is still a level of usefulness and utility associated with this application. Besides being dent resistant, lightweight, and strong; the fiberglass will not rust or rot. It does, however require maintenance and upkeep for maximum aesthetics.
An example of a truck topper that has been left to the Michigan weather for at least ten years.
Fiberglass Truck Topper extremely weathered
This truck topper could very easily be returned to good working order. Some sandpaper and paint could fix this topper up to as good as new condition. It is still very structurally sound, and the only aging affect has been on the surface which got a little funky with mold and mildew.
Composite Utility Poles
Composite Utility Poles have been under development for a number of years. Replacing the existing wooden poles with fiberglass composites have many long-term advantages and yet have many obstacles to implementation. These poles are pultruded and use polyester resin and E-glass reinforcement. UV additives are employed to minimize one of the Achilles heels of composites resin.
Advantages of composite poles over wood are numerous. Composite poles have a lifespan of roughly 80 years versus the 25-30 years for wood, due to rotting issues. These rotting issues with wooden poles are combatted with chemical additives that are sometimes corrosive and toxic. Composite poles have a weight advantage, as they weigh about two-thirds less than a wooden pole, which allows for easier transportation to the jobsite and reduced equipment requirements for installation. Due to their controlled and known construction, composite poles have physical properties that are more stable and reliable over the duration. Composites are intrinsically non-conductive, which is ideal in this application.
Replacing an existing material in a current application always brings hurdles and challenges for acceptance. The wooden poles are known by purchasing, engineering, installation, and line utworker crews. Composite poles have a higher upfront cost, though long-term savings are significant. As the educational resources reach those affected, the transition will take place and composite utility poles will become widespread.
Weathered Filament Wound Pipe
I recently had the opportunity to examine some weathered composites in the form of fiberglass pipe. There are some very important observations to be made from visual clues. One of the difficulties with too much analysis is the unknown history with the original manufacturing process, materials, and intended application. We also do not know much about the actual use and environmental exposure history. These factors would be great to have, and could probably give us additional data regarding the degradation of materials.
Here is the picture showing the pipe in an outdoor setting where it has rested for at least twenty years.
weathered filament wound pipe
The exposed pattern is showing that the outer layer of resin has weathered away, leaving the white strands of fiberglass exposed. It is pretty minimal loss, likely just a few thousandths of resin is missing. The weathering has nicely exposed the crosshatch pattern of the filament winding process to see the angles and directions that were used. An old pipe joint can also be seen at the top of the picture.
Many physical properties could be tested on this fiberglass today, but would be pretty useless without a baseline for comparison. Armed with data as to the original strength, materials, and processing characteristics, it would be interesting to test the loss of properties such as impact resistance, shear strength, load fatigue, etc. Many products undergo aging and weathering testing before final design approval, but do so in an accelerated fashion.
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