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.

Necky Composites

A very basic and well-done video is from Necky Composites.

It demonstrates the toughness of epoxy laminates when he starts the video by beating the kayak with a hammer.  He also does a good job of discussing the construction techniques of his kayaks and gives some good detail on materials and processes.

I have used the Soric and Divinycell, and the adhesive he discusses.  They all have their places and special techniques for application.  Many of these materials have been around a long time, and sometimes they have been misused in applications that they didn’t belong.  I have personally witnessed some of these horror stories concerning delamination and improper adhesion.

The video quality and production is very rudimentary, but the content is very good.  I am not a kayaker, but it makes me want to get one of these!

Composite RFQ

An interesting website that I recently discovered is called CompositeRFQ.com, which is a website dedicated specifically to the composites industry, and filling Requests for Quotes.  This website works to match composites fabrication shops with those businesses and individuals needing composites fabrication work completed.

A project is posted by the person with work needing to be performed.  The proper industry is selected, choosing from aerospace, architecture, automotive, boating, military, and sporting goods.  Additional information documents can be attached for further scope identification.  Bidding details can then be specified by the person posting the project.

Composites fabricators with related expertise and experience can bid on these projects in an effort to identify work opportunities for their business.  If a bid is accepted, the details are worked out directly between the bidder and the buyer, eliminating any third party costs and delays.  Bidders and Buyers can go back and leave feedback after the job is complete.

Composite RFQ has some free accounts to go in and look around.  There is a very interesting page about using Composite RFQ that very easily explains how it work s graphically.

This site is a great tool to bring the composites world together between buyers and sellers.  The old word of mouth method of finding fabricators and customers is great, but does not always lead down the best path.  Small fabrication shops have difficulty advertising their services to the appropriate customers that might be looking for the type of work that they do, and now they can go and bid directly on the types of projects they can handle.

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.