Wind Blades
The new composites application that everybody is discussing is composites wind blades. The large, three-bladed wind generators have been around for a few decades, mostly in Europe. The U.S. has been catching on in the last couple of years as a way to make cleaner electricity. These windmills are very tall, and have blades that are 100 to 400 feet long, depending upon output rating and location.
The wind blades use glass carbon fiber, resin, and coring to make a long, stiff and lightweight blade that will attach to the hub of the windmill. These blades are very long, requiring huge manufacturing facilities to make them. The transportation of these blades is important as well, as they require specialized trucks and trailers to handle such large pieces. Large cranse are required to lift them into place at the job site. They are relatively heavy, and must be lifted fairly high, requiring a significant lift capacity.
Resin infusion with epoxy resins is the normal manufacturing technique of which I am aware. They use compsite molds that have a constantly changing surface shape due to the complex geometry of the blade. The holy grail for these blades is to make longer blades at lower weight.
This application again demonstrates the advantages of composites. Complex geometry, high strength to weight ratio, and impact resistance are important aspects of wind blades.
There are several manufacturers of the wind blades in the U.S. MFG is a specialty composites molder that has been around for ages and is in the wind blade market. Vestas is another company with operations in the U.S., along with LM Glasfiber, as well as others.
Repairing the Inner Fender
One of my recent projects involved the repair of a 1993 International Medium-Duty truck hood made from SMC. There were several areas needing attention, and one of them was the driver’s side inner fender. This piece had formerly been attached with button-head pop rivets. This design is common to composites, and allows for easy replacement of the separate fiberglass pieces. The pop rivets had come loose over time, allowed to move around, and cause severe damage to the extent that the riveting flange was broken off. My only solution was to bond the two pieces together.
Material Fatigue in the corner
The loose panel flexed so much and for so long that it fatigued the material and failed in the corner of the inner fender next to the attachment to the rest of the hood. To repair this, I removed the area with the rivets, ground down the surfaces of both pieces on both sides, and reattached them with fiberglass and epoxy resin.
Prepared glass and resin
I wanted to place epoxy and fiberglass on both sides of the repair area to ensure a good, solid bond that would hold very well.
Epoxy Resin and Fiberglass applied
After the area was prepared, I applied epoxy resin to the surface to ensure good adhesion. I had a low spot that was a gap, so I mixed some microfiber and epoxy to make a paste and fill this gap. A stronger bond is produced when the fiberglass is not spanning an open gap between the two pieces. I placed two layers of 3oz Chopped Strand Mat over the paste and worked the air out to make a nice consistent repair. I then ground down the surface to make a nice-looking, consistent repair.
Back end of the inner fender
The rear of the inner fender had similar problems. A hole had emerged in the black SMC piece. I ground down both surfaces and placed some fiberglass across the area to bond it together.
Grinding Tools
During my recent International Delivery truck hood project, I did some grinding on the composite SMC truck hood prior to performing extensive repairs. It is important during any composite repair exercise to have good surface preparation. There are many ways to accomplish this depending upon the work area, tools available, and the work to be performed.
The following is how I did it.
Composite Grinding Tools
Beginning with personal safety, I used a dust mask to prevent inhaling the dust into my lungs. I used OSHA Z87 approved safety glasses to protect my eyes. A pair of earplugs protected my ears. Abrasion/dust resistant gloves for my hands, and a light jacket to keep the dust from my skin.
For removing the layers of SMC to feather edge the surfaces, I used several abrasive tools. A five-inch pneumatic grinder with 80 grit sandpaper worked well on the large areas to quickly remove large amounts of material. A right-angle die grinder with Roloc 3″ or 1.5″ sanding disks worked well for the concave areas and other difficult access areas. A straight die grinder with a fluted burr worked to get into the detail areas of the front grill and other cracks that needed material removed.
All of the tools I used were air powered, so I had a good air supply that would keep up with me. The large amount of dust that is produced from this sort of work presents explosion hazards when using electric tools. Large dust piles can also spontaneously combust, so care must be taken during dust storage and disposal.
A air blow gun was also used in this process to clean the dust from the part and from my clothes. It is important to keep personal safety equipment such as safety glasses and earplugs in place when using the blowgun. A small vacuum can also be substituted with care taken to prevent dust ignition/explosion.
After all of the material is removed from the SMC hood, all of the dust is blown off to leave a part with a bonding surface that is clean, dust-free and ready to be repaired with epoxy and fiberglass.
IH Hood Repair– Passenger Corner
One of the repairs on my International Hood was for the passenger corner. This portion of the hood was damaged before I got it, and was COMPLETELY MISSING!
Time, effort, epoxy, and fiberglass, allowed me to successfully completed the repair. This SMC hood will be repaired as good as new!
Damaged area to be repaired
The hood was placed upside to allow for work access. The repair area had several cracks and breakage areas. The repair began with surface preparation.
Surface Preparation on the repair area
I used a grinding disc to remove material on the front and back side of the repair to scarf the repair into the large area. Good surface preparation gives us a clean area that can hold a bond and create a transition area. I also drilled some holes in the end of the cracks to stop their propagation.
The back side was prepared too
The back side was prepared for repair by grinding the surface to allow for good bonding by the epoxy resin and glass reinforcement.
Beginning to add material
I started to add epoxy and fiberglass back onto the fender. I had to gradually move back out to where the existing fender used to be. I worked both sides bit by bit and allowed it to start curing before adding more.
Getting the shape back
I actually went a bit beyond the shape I needed. This allowed me to get the surface planes in the right spot, and later return and trim back the proper hood edge.
Back side getting rebuilt
The back side of the repair also got transitioned into the hood and out to meet the front repair area.
Grinding the repair
After most of the rebuilding was complete, some grinding got it back into shape to allow an evaluation of the repair progress.
Grinding on the back
The back side was cleaned up as well, getting the appropriate part thickness back to where it was originally intended. A nice transitioned surface was created, and all sharp edges were removed.
Bodyfiller for cosmetics
I used some bodyfiller to smooth the repair and restore the cosmetics of the hood. I could also have used epoxy, and it would have bonded better. Polyester bodyfiller is less expensive, easier to apply, and easier to sand. I smoothed the surface and feathered it back into the surrounding area to allow for a consistant surface.
Application of Primer-Surfacer
Urethane Primer-Surfacer is applied to the whole area to allow for removing the sanding/grinding scratches and preparing the surface for paint application.
The repair is complete, and the hood looks back like it was original. This repair, and many others, can be detected with some investigation of the back side of the repair area. This will be a very durable repair and will last as long as the rest of the composite body panel.
Hood Repair– A big hole!
My IH SMC truck hood project involved repairing a large hole. This was one of my largest hurdles to the whole project. This is what I initially saw.
The Big Hole
The picture is showing the hood upside-down on a work table. The hole is the result of some sort of long-ago impact the shattered the composite material and did significant damage. My first reaction was to just repair it from the backside. This hope was lost when I quickly discovered that there was not any access to the backside of the repair. At least not without cutting some support structures out of the way. Which would mean that I would need to rebuild those after I repaired the hole. Not impossible, but it seemed like a lot of work.
I began this repair like any other, with the surface preparation. The hole got larger as I removed all of the damaged material and created a transition area.
Damage is removed and transition area prepared
The edge of the hole was a razor’s edge as I transitioned out to the existing finished surface. I also used some sandpaper to sand around the inside of the hole to allow epoxy to adhere to the inside surface when I put in my patch.
My strategy was to create a thin layer of composite using epoxy resin and 3oz Chopped Strand Mat. The layup was done on a piece of plastic and allowed to cure. Once cured, I cut it to be about 1/2 inch larger than the hole. Then I mixed up some thickened epoxy and applied it around the perimeter of my patch and placed it in the hole. I had already placed a sheet metal screw in my patch, and attached a wire to a support to hold the patch in place until cure.
Patch in the hole
In the picture you can see the patch, the wire, and the support. The tension on the wire held the patch in place until the patch was cured in place. Next I worked to add material from the front to fill in the low spot. This returned strength to the laminate and restored the surface profile back to where it should be. I again used 3oz Chopped Strand Mat and started with small diameters and worked out to larger ones until the surface at the correct level.
Most of the Glass and Resin have been applied
Now most of the surface has been filled in, and it can be ground to remove any high spots, air bubbles, and allow for adhesion of the next layer.
Grinding the surface
Now I can fill the low spots with thickened epoxy or bondo and sand the surface to match the surrounding areas and get a nice finish.
This is how a repair should be made. A nice transition in the existing laminate will keep the repair from cracking or breaking in the future. After the bodywork is complete nobody will know it was repaired.
Working with Chopped Strand Mat
One of my projects is repairing a composite hood for a large International Medium-Duty truck. This hood is made of SMC, and is damaged in various locations to various degrees. In doing the repairs, I am using epoxy resin and glass reinforcement. The fiberglass reinforcement I am using is referred to as Chopped Strand Mat, and is the 3 oz. per square foot version.
Chopped Strand Mat
The chopped strand mat is made up of random glass fibers held together with a light binding material. A good pair of scissors can cut the glass into manageable sizes, though the scissors will be dull when you are finished. The chopped strand mat can also be torn by hand. This leaves a “feathered” edge to create nice transitions between the patched areas and the existing surface. Pieces that are cut will leave a silhouette that will show and may require other methods to make it disappear.
The first thing that I checked was to make sure the glass was compatible with the epoxy resin. The package told me so, but I also did a small test sample to ensure that the epoxy would mix with it, was workable, and would harden properly. Testing away from the actual part can save many potential headaches and pitfalls.
Applying the glass and resin is relatively easy. Thoroughly mix some resin and apply it to the properly prepared surface (clean, dry, and sanded). Tear off an appropriate size piece of glass and place it on a piece of cardboard. Use a cheap paintbrush to get the glass wet with resin. Flip the glass over, and wet the backside. The glass will go from white to transparent on the cardboard. Now lift the glass from the cardboard and apply it to the area with the resin recently-applied resin. Use the brush to push it down against the surface to get as much contact area as possible.
Additional layers can be applied on top to build thickness, making sure to create a nice transition and limiting air bubbles in the laminate.
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