Tuesday, 24 April 2012 17:06

Hockey Stick Materials - Materials - part 2

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The wooden stick was the traditional weapon of the hockey player since the sport was invented. Quite durable, inexpensive and simple, it was the only choice available for years. The wooden stick has slowly but continuously improved over the years as manufacturers tried making sticks out different types of wood. However, as wood is an organic material, its mechanical properties vary from tree to tree and therefore from stick to stick. The stick durability, stiffness and overall feel lacked consistancy. To overcome these issues, laminated sticks were eventually developped. These sticks are manufactured by glueing together different types of wood and yielded sticks with increased performance compared to the single piece wooden sticks.

Eventually, manufacturers started using plastics and fibreglass layers to further enhance and protect the wooden sticks as the hardness of plastics and glass is much higher than that of wood. These layers were effective in increasing the durability of the sticks and this design is still used today.

 

In the 1980s, Easton, a manufacturer of aluminium baseball bats and archery arrows, developped and marketed the first accepted non-wooden stick, the easton aluminium. This stick was composed of a hollow aluminium shaft in which a wooden blade was inserted. Since aluminium is much stiffer than wood, a hollow stick with thin walls can offer the same stiffness as a full wooden stick. The Easton aluminium had the advantage of being lighter than conventional hockey sticks as well as being much more durable. As the properties of the aluminium and the manufacturing process can be controlled much more easily than with wooden sticks, there was also a consistency in feeling from stick to stick. Another upside to this design was that in the event of a break the shaft would be intact in most cases with the failure occuring in the wooden blade. The player only had to replace the wooden blade and not the entire stick.


 

The stick took some time to gain acceptance but its popularity skyrocketed after Wayne Gretzky started using them. The sticks became widespread throughout the NHL and minor leagues. However some players complained the stick was too light or that it did not have a good "feeling" compared to wooden sticks and preferred using the latter.

In an effort to increase the "feeling" of the stick and to further reduce weight, manufacturers developped composite shafts. These shafts were similar to the aluminium sticks but employed carbon fibre, aramid fibres (kevlar) and glass fibres instead of aluminium. This made them "feel" much more like conventional wooden sticks and increased their popularity.  The sticks were still constructed in two-pieces however and many players still did not like the feel of the stick. Manufacturers then developped the one-piece sticks. This enabled a further reduction in stick weight and improved the feeling of the sticks to match that offerd by wooden ones. 

Hockey sticks made of carbon fibre

Currently, the most popular sticks today are the one-piece composite sticks followed two-piece composite sticks far ahead of the aluminium and wooden sticks. But even as the norm is to use a composite stick, there is a high variety of quality and cost in this kind of stick. At your local pro shop you may find two one-piece sticks that look almost identical where one is priced at 350$ and the other at 60$. The difference is in the materials as will be explained in the following pages.


 

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.


 

In order to get the most out of the material, the volume fraction of fibres must be maximised (more fibres, less polymer). This can be done by reducing the space between the fibres while weaving (with an increase in cost) however this also means it is harder to get the polymer to flow between the fibres. If there is no polymer in some areas, the fibres will move out of alignment and the material will lose its stiffness and be prone to failure. Composites are composed of multiple layers of fibre cloth stacked onto one another in order to produce a part as a single layer of composite is not strong enough for most applications. Therefore, to avoid large areas of polymer buildup between layers, significant pressure must be applied to bring the layers as close as possible to eachother. Finally, the component must be heated to allow for the polymer to flow but not so hot as to allow for thermal degradation of the polymer. Careful engineering of molds must be undertaken to allow the polymer to flow in the right areas while avoiding any "hot" or cold spots. 

In short, the quality of your component depends on the quality of your manufacturing. For composites used in airplanes there are extremely high standards and parts can cost millions of dollars. However in the case of hockey sticks, higher costs could mean better quality manufacturing and better performance.

Composites get more complicated however. As mentionned previously, composites are an anisotropic material, the properties vary depending on the orientation of the fibres. Typically a fibre weave will have fibres running at 0o and 90o which means the best properties are located in the direction of these fibres - 0o and 90o. However if the loads aren't applied in these directions, we have a serious problem! The fibres won't be in the appropriate direction to handle the load and the part will likely fail. But there is a rather simple solution to this problem: stacking sequences. 

Since multiple layers of cloth must be stacked onto one another in order to form a part, we can vary the orientation of each layer and direct the fibres in the direction of the loads. In terms of stacking, we refer to the first fibre orientation as we know the fibre orientation will be identical in all quadrants. ie 0o = 90o = 180o = 270o, similarly, 45o = 135o = 225o = 315o. For example, if we have a part which is composed of 5 layers of carbon fibre, we could choose from the following stacking sequences depending on where we need the best properties : 0o 0o 0o 0o 0-- 0o 45o 0o 45o 0o -- 0o 20o 40o 60o 80o. Our first stacking sequence would be heavily oriented in the 0o direction, while all other directions would have minimal load bearing capabilities. The second sequence would be better suited to handle loads in different directions while maintaining higher properties in the 0o and 45o directions. The third sequence would have approximately equal properties in all directions. While the third option may seem like the best choice it is quite the contrary. It is effectively reducing your maximum properties in the directions required. One of my professors called a stacking sequence of this type when used with carbon fibre "Black aluminium" as carbon fibre stacked in this way would have essentially the same properties as aluminium but cost much much more to produce. In short, composites need to be engineered to work to the best of their ability by determining where the loads will be and how to orient your fibres to handle these loads. If this is not followed, you have essentially a part that could have been made much cheaper using aluminium.

 


Now assuming your part is well engineered and is well manufactured there is one final thing that must be considered -- what material should I use as my fibres : glass, aramid (Kevlar) or carbon?

Glass has been used for years in various applications including boats, cars and certain parts of airplanes. Glass is one of the cheapest fibre materials while having decent properties. The stiffness of glass fibres is similar to that of aluminium at 68 GPa (vs 69 GPa for aluminium). It has a higher hardness than aluminium (6 vs 2.75 on Moh's scale) and a lower density at 2400 kg/m3 (vs 2700 kg/m3). Glass also has a decent toughness for a composite although metals such as aluminium are much tougher. When weaved glass is white in appearance.

Glass seems to be a good choice on first observation but due to the nature of composites, the composite material does not retain 100% of the properties of the fibre material (as the volume fraction is not 100%). It heavy, not very stiff but has good hardness and a decent toughness. 

Closeup of protective fibreglass on hockey stick

                    Image of Glass-fibre protected stick.

Aramid (Kevlar) is best known for its use in bullet-proof vests but is also used in firefighter's uniforms for its good heat resistance and flexibility. It has a relatively high Young's modulus (70-110 GPa) but it is very weak if compressed. Kevlar also has the lowest density of the materials considered at 1440 kg/m3. Aramid also has high toughness (which is why it is used for ballistics protection) but it absorbs moisture which can lead to degradation. It also has some issues with bonding to the polymer which can reduce its performance. When weaved Aramid is golden/yellow in appearance.

Carbon is the fibre most often associated with composites. It has the highest stiffness of the materials (depending on the grade it can reach 450GPa). Carbon has relatively low density of 1750 kg/m3 but has the lowest toughness of the materials. It is black in appearance.

Closeup of carbon fibre weave on hockey stick

                                      Close-up of carbon fibre hockey stick


 

Current high-end hockey sticks are mostly composed of carbon fibre due to its high stiffness and low density. It allows sticks to be extremely light while having enough stiffness to be effective. These sticks however are not good against impacts which is why we see so many sticks breaking in the NHL. When hit with another stick or taking a slapshot, the carbon cannot absorb the energy and shatters. 

To reduce the shattering of sticks, some companies are including kevlar with the carbon fibre. This requires more layers to offset the loss of stiffness which makes for a heavier but more durable stick.

Going down the price range, we can find sticks made of lower quality carbon fibre, these sticks are heavier as they require either more material or more polymer to offset the lower quality. 

Lower still are the composite sticks which are composed of carbon fibre and fibreglass. These sticks are heavier due to the inclusion of glass but are also tougher. 

carbon fibre and fibreglass weave hockey stick

                         Image of a stick composed of fibreglass and carbon fibre

Aluminium sticks are still around but in limited numbers, they are extremely durable but don't have the same feeling due to the material's high hardness. 

Wood sticks are available reinforced with either glass, carbon or both. However most players have left these aside for the lighter composite sticks.

Closeup of carbon fibre and fibreglass protective layer on hockey stick

                   Image of a wooden stick protected by glass and carbon fibres

So which material should you look for in your next hockey stick? The answer depends on you:

  • If you are a competitive player playing in a high-end league, you probably want an all-carbon stick. The lightest possible stick you can find to give you that edge in the game. The increased price of the stick should not be a problem!
  • If you play recreationally, a "low-end" carbon stick or a fibreglass/carbon stick should work nicely. The slight increase in weight should not negatively impact your game and your stick should last longer. 

 

 

(Page 4 of 6)
Read 13242 times Last modified on Sunday, 19 August 2012 17:42
Jean-Michel Roy

15+ Years experience as a goaltender

20+ Years experience playing hockey

Currently a goalie coach for Phil Martin's Goalie Academy.

E-mail: jm@thehockeyresource.ca

Connect with me on Google +

Website: www.thehockeyresource.ca
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