Product Reviews

In this week's feature, we'll take a look at some tools and materials I've been working with recently. The tools are some scissors I recently purchased, and the materials are some braided cords and a unidirectional graphite mat. I've also included a mini book review at the end of the feature.

Scissors

Last week in my Tools of the Trade column, I wrote about cutting tools (among other items). I recently had a chance to try the Featherweight Scissors from Fibre Glast. They only cost $14.95, so they are an inexpensive option if they work well.

The body of the scissors is black nylon, with stainless steel blades bonded to the body. The first thing you will notice is the light weight: each pair weighs only 2.5 ounces. The blades slide against each other smoothly with no out-of-plane looseness.

These scissors cut very easily through graphite and glass. They cut through a 3K, plain weave graphite fabric almost like paper. Resistance is more noticeable, but still small on a 12K unidirectional mat (described below). The performance is even better on glass: they cut easily through both an epoxy resin and a thermoplastic glass prepreg. Thermoplastics can be especially tough to cut; with these scissors, it felt like cutting thin cardboard.

Unfortunately, the only Kevlar I had to test was the braided cord (described below). The scissors cut right through the cord, whereas I literally had to saw the cord with a carpet knife or my kitchen shears.

I've only had these scissors about a week, so I don't know how they will hold up in the long run. As I use them more, I'll provide occasional updates. If you've used these scissors for a longer time, I'd be interested in hearing about how they worked for you. Send me your impressions at composite@aboutguide.com.

Reinforcements

One of my current projects is the manufacture of composite rods by rolling prepreg. It is difficult to roll the prepreg tight enough to get a solid core, so I have been wrapping the prepreg around different materials.

The first material is braided cord from Aerospace Composite Products. Both carbon and Kevlar cards are available. The carbon is available in .025 in and .034 in diameters; the Kevlar in .025 in diameter. The carbon cords are a fairly loose braid and somewhat wavy (they feel bumpy). The Kevlar braid, however, is a tight braid and feels smooth--it looks like a very thin rope.

ACP recommends using these materials to wrap joints and wing leading edges. Because of its tight weave, the Kevlar probably won't absorb epoxy very well, but the carbon should without much difficulty. These cords probably wouldn't make a good unidirectional reinforcement, primarily because they are so thin--you would need a lot of cord to build up a reasonable thickness.

The other material I have used on this project is a nonwoven mat of unidirectional graphite called UniWeb, also from ACP. The fibers are held together by a thin web of thermoplastic on one side (picture taking a bottle of Elmer's glue and spreading it on the surface in a random motion).

The mat is made from 12K tows of carbon, and you can easily see small gaps between the tows. The mat pulls apart easily at these seams, but it is difficult to pull apart between the seams (where the thermoplastic holds the individual fibers together). The fibers themselves are very straight, and I could find no obvious flaws in the one yard piece I ordered.

Book Review

The fiber-matrix interface has a significant effect on the properties of a composite. This topic is covered in Fundamentals of Interfacial Engineering by Robert J. Stokes and D. Fennell Evans. They start at the atomic level, move to the general properties of liquid-liquid, liquid-solid, and solid-solid interfaces, and finish with some practical applications.

The chapter on polymer composites will be of most interest to composite engineers. Although this chapter draws heavily on the earlier theoretical chapters, it is also worthwhile reading on its own. The discussion covers not only the interfaces in the cured composite, but also the interfaces during the impregnation process (fiber wetting, resin flow, etc.).

The content is both quantitative and qualitative. One section shows how to calculate the resin flow rate through a dry fiber bundle. Another discusses the general effect of the fiber matrix interface on composite properties: a stong interface makes the composite stiffer and stronger, a weak interface improves its fracture toughness (the fibers are able to act as crack arrestors).

The book is written as a textbook with fairly large bibliographies and excercises for each chapter. Each chapter also begins with a list of symbols and a roadmap of key concepts for quick reference (and easy skimming).

Buy this book from Amazon.com.

Details: Fundamentals of Interfacial Engineering, by Robert J. Stokes and D. Fennell Evans, published by Wiley-VCH, 1997, ISBN 0-471-18647-3.
1. Defining Interfacial Engineering; 2. Interaction Forces in Interfacial Systems; 3. General Properties of Systems Containing Fluid Interfaces; 4. Colloids; 5. Amphiphilic Systems--Liquid-Liquid Interfaces; 6. Polymers; 7. Polymer Composites; 8. Liquid Coating Processes; 9. General Properties of Crystalline Solid Surfaces; 10. Thin Films--Solid-Solid Interfaces Processed from the Vapor Phase; 11. Grain Boundary Surfaces and Interfaces in Crystalline Solids

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