When talking about materials we have to define a few properties: stiffness, hardness, toughness and density.
- Stiffness is the measure of the force necessary to bend (or displace) the material. The Young's modulus is usually used to define stiffness as it is the measure of stress over strain with units of gigaPascals.
- Hardness is the measure of a material's resistance to permanent deformation. There are various ways of obtaining this data but the most famous one is the Mohs scale which measures the resistance to a material being scratched.
- Toughness is the measure of a material's ability to absorb energy before failure. This property is defined as the energy per volume of material absorbed before failure and is measured in Joules per cubic metre.
- Density is the measure of mass over volume. It describes how the weight of an object is different depending on the materials (think a ball of foam vs a ball of lead). Density is measured in kilograms per cubic metre.
Now you may think, why are these properties important?
- In the case of stiffness we want a material that has a stiffness low enough so we can bend it and high enough so we can get a wicked slapshot off. If we made the stick as stiff as possible it would not bend enough to be able to get a decent shot, if it bends too much we lose alot of energy and speed in our shots. There is therefore a range of stiffness that our hockey stick must fall into (and the range will vary per player).
- For hardness, we want a material that is hard enough to resist external damage but not too hard as to remove player feeling. Hardness is not really an issue except when dealing with woodens sticks. This is why they are now reinforced with other materials.
- Toughness will dictate how durable our material is. We want the maximum toughness in our hockey stick so we never have to change it!
- Lower density means a lighter stick. The lighter the stick, the faster we can get a shot off and stickhandle.
Knowing the properties of our materials can help us decide which material is best suited for hockey sticks however there is another problem. Unlike aluminium which is an isotropic material (properties in all directions are the same) composites such as carbon fibre, fiberglass and kevlar are anisotropic (properties vary depending on orientation). This is due to the fact that composites are not just a single material but are a combination of fibres and a polymer. The fibres give the material stiffness while the polymer keeps the fibres from moving with respect to one another.
Building a fibre-reinforced composite material is not a simple process. There are various techniques that can be used to make composite parts but they mostly all use the same basic principle. Fibre strands are weaved into a cloth (much like cotton or other fabrics), various weave patterns exist and will yield different properties. The fibre cloth is then layed in a mold and a polymer is spread over the cloth. The mold is then put under pressure, heated and excess polymer removed though the use of pumping systems.