I have a fair amount of experience in the design and manufacture of composites from my Undergraduate Masters project, and so this week I decided to experiment with the anisotropic deflection behaviour of composties by weaving my own fibre structures and making some bespoke fibre shapes.
A QUICK INTRO TO COMPOSITES MATERIAL SCIENCE
Isotropy means uniformity in all directions. In relation to materials, it means a uniform (or as close to as possible) material structure, e.g. metals. In an isotropic composite, all the fibres or particles must be randomly oriented throughout the resin to create this same uniform material structure. It is this uniformity which makes isotropic materials easier to predict their behaviour when exposed to different loads, e.g. the simple narrowing of the width of a piece of material when under tension (Possion's ratio)
Anisotropic materials do not have a uniform material structure, and as such do not have as uniform a reaction to loads. Most composite materials react in an anisotropic way to some extent due to the very nature of their heterogeneous material structure. In fact the design of a composite material, especially the shape and interaction of the fibres contained in the resin matrix, can be tailored to certain forces and made to deflect in a specified way, which can be very different to the way an isotropic equivalent material would do, e.g opposite to the example above, a material can be made to increase in width as well as length when under tension (auxetic composites)
HOW TO MAKE WOVEN COMPOSITES
1: LASER CUT A LOOM
A quick web search for other DIY looms showed that it was very easy to make a loom. Simply make 4 sides of a box, with at least 2 with slots along the top edge, with the slots offset from each other to allow a straight weave.
Weave your warp - the vertical threads - by looping your thread continuously through and around the slots on the opposite sides of your loom
Weave your weft by weaving another thread or material through your wefts - it's handy to attach your warp thread to a solid rod (I used a pencil) as it makes it easier to thread it above and below the wefts
For a simple weave, this is how your weft thread should interact with your warp threads:
Row 1: Over, under, over, under.... until you get to the end of weft threads
Row 2: Start with the opposite 'over, under' that you finished row 1 with, e,g. if you finished row 1 with the weft going under the warp, then the first weave of row 2 should be going over the warp, and carry on from there
Repeat for as many rows as you need!
The end result should look like the picture opposite
3: WEAVING A STRUCTURE WITH VARIABLE SHEAR DEFLECTION
My first weave was using the simple weaving pattern with the jute string provided in the workshop
I also wanted to experiment with varying the deflection of the material in shear i.e. how the material reacts when you apply a load at 45 degrees to the fibre direction (roughly at 0 and 90 degrees in this weave)
I varied the shear deflection behaviour here by increasing the distance between the weft fibres in one half of the woven structure
4: WEAVING A STRUCTURE WITH VARIABLE BENDING DEFLECTION
This weave I decided to play with adding different materials for the warp threads
I varied the bending stiffness of the material along the length of the woven structure by using different stiffness materials for the weft fibres: hard to bend 1mm steel rod, slightly more flexible 0.5mm steel rod, flexible but not stretchy jute string, and finally flexible and stretchy elastic string
Here as I wanted to experiment with the deflection of the sample in tension as well as bending, I also used elastic thread as the warp
4: CAST THE WOVEN FIBRES IN RESIN
The matrix I chose for my composites was SmoothOn ReoFlex polyurethane rubber, as I wanted a resin which would stick to the fibres and also be fairly flexible in itself
I taped the fibre structures onto a piece of plastic and poured a thin layer of urethane rubber onto the weaves, ensuring that it covered all of the fibres
Leave to cure overnight
5: THE RESULT
It stayed together! The rubber soaked into and encaspulated the fibres
The flexible urethane rubber allows the fibres to bend and flex within the structure, allowing the whole material sample to demonstrate some of the anisotropic material properties I was trying to achieve
Variable shear deflection sample
Variable bending deflection sample
HOW TO MAKE ANISOTROPIC FIBRE COMPOSITES
1: CREATING FIBRE PATTERNS
I wanted to experiment even more with creating different deflection behaviours, so I created bespoke fibre patterns from wire and cast these in polyurethane rubber
The fibre structure on the left should create a bulge in the centre when the sample is in tension, as the convex shape of the fibres will straighten and come together, compressing the central section of the rubber
The fibre structure on the right should create a curved shape in tension, as the distribution of fibres means that the right edge of the sample is less supported in tension and will stretch more than the left side
