Materials Science and Engineering in Sports

I've previously noted that, in some cases, materials in sports equipment can possess a placebo effect.  But in other cases, the role of materials science and engineering is real to performance and critical to safety.  In this week's post I'll focus on examples of both. 

Engineering a 105.4 mph Slapshot

Zdeno Chara's slapshot - the one that recently registered a new speed record - leads to the questions How does he do it? and What is his stick made of?  The answer to both questions can be answered with materials selection and knowledge of how the material has been tailored to the user.

The choice of material for a stick can be understood by considering the mechanism by which hockey sticks deflect.  At its simplest, the stick can be considered as a beam in three-point bending (Figure 1).  The outer two points are made up by 1) the hand at the top of the stick and 2) the ice surface; the hand closest to the ice provides the third point, adding force in a direction opposite the other two points. 

Figure 1.  Zdeno Chara's record-breaking slapshot.

During a slapshot, the stick stores up energy as it flexes.  In order to maximize energy without requiring too great a deflection - the energy must be released quickly, after all - a key material property of the stick is targeted: its Young's modulus.  The modulus of the material and the shape of the beam make up the stiffness; given a standard shape, beam stiffness increases with modulus. 

A materials engineering solution, especially in sports equipment, would not be complete without considering weight.  In particular, materials with greater modulus also tend to have increased densities.  A suitable engineering material can be selected by performing design analysis and optimization (for reference, see Prof. Michael F. Ashby's book [1] as an extensive resource in this field).  Specifically for beams in bending, the performance metric to be maximized is the square root of modulus, divided by density [1].  By comparing this metric for a wide range of engineering materials (see Figure 5.13, p. 98 in [1]), two groups emerge as optimal choices: woods and carbon-fiber reinforced polymer (CFRP) composites.  On a relative scale, the latter CFRP materials provide modulus values nearly 10-fold greater than those of former wood materials.  Zdeno Chara's stick is a composite, the Easton Synergy EQ50.  But not just anyone can use it: you still need to be able to flex those stiff beams to get the necessary energy output. 

Materials to Combat Concussions

There are frequent reminders that concussions occur to athletes in a number of sports, e.g. hockey, football, and baseball.  ScienceBlogger Dr. Jeffrey H. Toney has recently centered a post around football helmets, focusing on a National Geographic report that studied the impacts sustained by a 21-year old University football player in a single season (over 500 hits).  Dr. Toney's basic message is that "currently used helmets may not offer sufficient protection, particularly in cases in which players experience hundreds of hits."  Concussions are also a key topic in hockey, which has led to much materials research and development such as the Messier Project and the M11 helmet.  Baseball players can experience personal contact with 100 mph-pitches, which in some cases have led to brain trauma.  In response, improved materials and helmets, e.g. Rawlings' S100 batting helmet, have been designed to withstand such an impact.

In a collision, energy is transmitted to the head and brain, based on the pre-collision masses and velocities of the two objects, e.g. the athlete and the opposition.  The post-collision velocity is determined through conservation of momentum.  To go from its pre-collision velocity to its post-collision velocity, the head undergoes a high rate of velocity change, i.e. a high acceleration.  There is a finite acceleration that the human head can withstand without damage to the brain.  The helmet aims to reduce the transmitted energy and thus the acceleration imparted to the head; the mechanism of energy absorption is therefore critical to head protection.  

Simplified, ideal materials for energy absorption undergo permanent deformation - this deformation expends energy, reducing that transmitted to the head.  The force required to deform such a material should be high enough to absorb a maximum amount of energy; but it can't be too high such that it requires less force to deform the head rather than deform the energy-absorbing material.  Cellular polymers, e.g. foams and expanded polymers, are well-known and useful energy-absorbing materials, and which also possess low weight critical for athletes [2].  More novel materials have also been developed which improve upon the load-bearing response: for example, in the M11 helmet, liners of tubular shapes have been designed to aid in energy absorption (dubbed Seven Technology).  Future endeavors may also incorporate shape-memory polymers to offer multiple-hit capacities.  But despite these advances, as Dr. Toney suggests above, there is still work to be done.

The Future of Materials in Sports

The two parts I've provided here represent opposing sides of the same coin: a development in one can require or enable a development in the other.  In some cases, however, increased equipment performance is seen as a threat; for example, Little League International has recently banned composite baseball bats because their Bat Performance Factor (BPF) increases with use.  This returns us to an issue I previously raised regarding the evolving perception of performance increases with material developments.  At present, this appears to be the one of the most important topics in sports, and it is likely to remain so for the foreseeable future.   

[1] M.F. Ashby.  Materials Selection in Mechanical Design, 3rd Edition.  Butterworth-Heinemann (2005). (Available On-Line.)
[2] M.F. Ashby, et al.  Metal Foams - A Design Guide.  Butterworth-Heinemann (2000).