Panel Stiffness
Composites structures have requirements for stiffness to provide support and stability. Tests can be completed to identify the stiffness of a given area on a composites structure, i.e. how much it will bend for a given force.
The required stiffness of a section of a composites part depends upon the overall design and service expectations. Several factors, including the life expectancy of the object, the load rating of the composites surface, the inter-laminar bond strength, will help determine the threshold requirements. Testing is very important to determine the life expectancy of the part and whether it meets the requirements of its’ job.
Panel stiffness can be modified to meet these requirements using two basic methods. One way to increase the stiffness of an unsupported composites panel is to reduce the size of the panel through additional support structures. The other way is to increase the panel thickness across the same area. Choosing which method to use depends upon the engineering of the part and determining which method is acceptable with the surrounding part layout. If there is room to add supports, this is likely a quick and easy option. If there is room to increase the thickness, adding new or additional coring materials may be a simple solution.
Building a strong and stiff composites structure can be accomplished with the extremes of building a robust “skeleton” with small open areas and a thin skin, or having a basic, limited “skeleton” with a heavy duty cored laminate that supports itself.
A combination of the two usually works out best.
Making Holes
If you are working with fiberglass parts, you may need to attach other parts, pieces, and features mechanically with fasteners. Bolts and rivets are the most common mechanical fasteners used to accomplish this.
Composites with a nice, decorative gelcoat finish such as boats and RV’s require special care to make holes in them for placing bolts and rivets. Disturbing the area around your hole in a gelcoated surface can lead to very expensive repairs by a fiberglass expert.
You can make holes yourself, but it requires extra care and attention. I found a great Youtube video that demonstrates this from user CenturionCrew.
Of course the biggest mistake that can be made is improper placement of the hole.
Following the instructions in the video and drilling a nice slow speed hole is the best way to be successful. He also mentions the caution that must be noted to stop the drill chuck from contacting the gelcoated surface. One tip that I have is to place a small piece of rubber hose over the drill bit to contact the gelcoat before the drill chuck.
One other note with holes (all shapes and sizes) in cored composites fiberglass pieces. If there is a layer of balsa or foam core in the cross section, extra precautions should be exercised. One is to coat the inside surface of the hole with gelcoat, resin, or silicone to keep moisture and UV out of the core.
Another concern is compression of the core with mechanical fasteners. Balsa and foam cores typically are low in density, and are not meant to be highly compressed. If you are going to bolt something on, and it is going to be really tight, it is best to use a metal sleeve in the hole that is the same thickness of the fiberglass part. Large washers or backer plates should also be used to distribute the load across a larger surface.
5 Axis Filament Winder
Filament winding is a process that can be used to create round fiberglass shapes with exceptional strength characteristics. Used for piping, tubing, and tanks, filament winding is normally an automated process that has computer-controlled equipment to place glass and resin around a mandrel- the piece that functions as the mold.
There are many variables that can be modified for filament winding, and these will affect the strength characteristics of the finished piece. The angle of the glass, number of passes (thickness), use of glass mat, and type of resin will affect the finished product strength characteristics.
I found a short video that demonstrates the equipment and the process.
As you can see, the glass is applied in a consistent manner across the part, allowing for uniform strength characteristics. As you can see, this is yet another process that the composites industry uses to create useful products with advantages over those of competitors.
Vacuum Bagging Video
Vacuum bagging is a process that requires unique materials and processes, but can be simple to operation in an ongoing basis.
There are many advantages to vacuum bag molding, a few of which include:
- Improved resin/glass ratio
- More consistency across the laminate and part -to -part as compared to open layup
- Containment of air emissions from the resins
As compared to hand layup and chop layup, there are a few disadvantages, including
- Higher consumable material cost
- Higher capital equipment cost
- Difficulty with superior surface finish
Some parts are more suitable for vacuum bag molding than others. It also depends upon which process it is being compared with.
Vacuum bag molding requires an extremely tight seal between the mold and the bag. Molds with multiple pieces or holes for inserts can be difficult to complete a seal.
Parts that are overly large and complex can present challenges with placing resin and reinforcement before the cure cycle starts. The bag must be completely sealed and under full vacuum before the curing cycle of the resin begins.
The basic premise of vacuum bag molding is that the air is removed from the bag, allowing the atmosphere (air on the outside of the bag) to push the bag onto the part on the mold, compressing the layers of resin and reinforcement. Many misinterpret the process as “sucking the extra resin out.” We are merely allowing the laminate to be compressed by the weight of the air above us in the atmosphere to consolidate it before cure. The excess resin is usually absorbed by extra layers of sacrificial material inside the bag.
Composite Dock
Building outdoor structures around water require special considerations. Wood rots, steel rusts, and dirt erodes. Fiberglass composites have been making strides in uses for marine-related activities. Round fiberglass pilings can replace wood pilings, and sheet pilings made of steel can be replaced with fiberglass retention walls.
The fiberglass composites have several advantages. They have a much longer service life because they do not rot or corrode. Compared to wood structures, they are more uniform in size. They are typically lower in weight to allow for lower lifting, handling, and transportation load requirements.
All of these advantages are used by a company called Green Heron Docks that builds docks in a “green” manner. Their docks are build from the previous dock sections, allowing for minimal interruption of the surrounding environment. Their installed docks will have a long life that will not require replacement related disturbance for time to come.
Their video on Youtube
As you can see, their equipment is relatively lightweight and inexpensive compared to bringing in cranes and barges. Their environmental impact is minimal, and the resulting product has a very long service life that will endure for years to come.
Liquid Bulk Transport Tanks
Lincoln Composites has introduced their Titan™ Liquid Bulk Transport tank. This unit is comprised of four composite tanks that meet the specifications of a universal shipping container, allowing for transport via ship, rail, or semi on existing infrastructure.
TITAN Gas Transport
The tanks are require three basic parts to complete the engineering challenge. An inner liner made of High Density Polyethylene provides an impermeable layer to hold the gas. Next, a filament-wound composite shell made with epoxy and carbon fiber contains the pressure of the gas. A Polyurethane coating on the outside of the tank protects from moisture and abrasion.
The system is designed for Natural Gas, Hydrogen, Argon, Helium, Nitrogen, etc. Existing transport via semi truck is accomplished with large steel-tanked semi trucks that are heavy and prone to corrosion. The TITAN composite solution allows for multiple transport options (rail, ship, semi) as well as lower tank weight. The gas stored versus the tank weight is a huge advantage over steel tanks.
Lincoln Composites claims that traditional steel tanks hold 4000 SCM less CNG than the TITAN™ and weigh 16,000 kg more than the TITAN™ composite tanks.
For more information, check out the Titan page at Lincoln Composites.
Machining Composites CNC Video
Much like other materials can be machined, so can composites. Everything from basic fiberglass fabrications to advanced composites materials can be machined to add details and features.
This can be accomplished using hand-held tools guided by fixtures and measurements. Hand tools such as air routers, drills, and saws. A disadvantage is that the operator can commit errors and must be protected from safety hazards. These hazards include airborne dust, bending strain, lifting strain, and physical cuts.
For high volume or high precision applications, there are CNC routers that can be employed. These are fast, efficient, and safe. They may be expensive though.
Several manufacturers make CNC routers, and Thermwood is one as shown below.
This video shows several different applications and parts that can be routed with this large Computer Numerically Controlled machine.
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