SPORT EQUIPMENT TESTING
Baseball/Softball bats
Bat performance compliance testing has evolved and improved since SSI first introduced it in 1993. Most recently, it was observed that the performance of composite bats improved with usage. These bats could therefore be found compliant with the existing standard when they were tested before being used in the field, but could become non-compliant (too powerful) if tested after sufficient usage. To address this issue, an “automated break in” (ABI) procedure was proposed in which a bat is broken-in by rolling it between cylinders. In this procedure, performance measurements on a tested bat are alternated with the “breaking-in” process until either the bat’s performance exceeds a specified limit or the bat exhibits visible damage that would make it no longer allowed in a ball game. If the damage occurs first, the bat is compliant, but if the performance limit is exceeded first, the bat is not compliant.
There are a number of serious problems with this rolling ABI protocol. It is complicated and time-consuming to use, and it is not precise, accurate, controllable, or repeatable. A bat could show damage when rolled to a sufficient distance, but not when impacted by balls. Such a bat could become too powerful because of impacts with balls, but would not be removed from play because it would not display visible damage arising from these impacts. Also, the elasticity measurements are not executed in the way a struck ball experiences the elasticity of a bat.
We have invented an alternative ABI procedure that overcomes the above difficulties. The elasticity of the bat at each step is determined with the bat held in a cradle, so that the bat is compressed on only one side, as it is when the bat is impacted by a ball. The compressing solid is a spherical section so that the compression is similar to the compression arising from an impact with a ball. This same compression mechanism is used to execute the break-in of the bat. These break-in compressions, to increasing distances, are therefore also similar to compressions arising from an impact with a ball. And since the bat is compressed in the ABI procedure the same way that it is compressed during hits in ball games, a bat that shows damage in the ABI procedure before it becomes too powerful will also show damage when impacted by balls before it becomes too powerful.
Tennis racquets
It is desirable for a chosen tennis racquet to help provide three important quantities to players: power, control, and consistency. Each of these is highly correlated with the tension in the racquet’s strings, and so it is important for a player to choose tension values that are appropriate for his game. This is problematic because, within minutes after a racket is strung to a requested tension T, the actual string tension within the string bed almost always becomes less than T, and subsequently the tension continues to decrease as a consequence of usage, climate changes, and from the intrinsic weakening that occurs as the internal chemical bonds within the strings break. The effect can be substantial – for many string brands in common use, a 10% reduction in string tension within a week is not uncommon.
When a racquet is strung, a stringing machine is used to pull each string to the desired tension, and it is assumed that, because of slippage through the grommets and around the outer frame sides, all of the strings end up with the same tension. But the fact is that the actual tensions in a racket given to a player by a stringer are almost always different from the requested tensions. Because of this, and because of tension degradation, it is important for a player to be able to determine the actual tensions in his racquet at all times. To address this, a variety of tension-measuring devices are available, but none of these devices are practical because they are either too destructive, too complicated to use, or too inaccurate. We have therefore invented a tension-measuring device that does not have these problems. (This product was developed in collaboration with our partner Division, LLC.)
We also perform dynamic measurements on racquets by impacting them with spherical metal elements and with tennis balls. These devices are used to measure and compare the performance and sweet spot sizes of racquets. We have also performed extensive (finite element) modeling of the impacts between a ball and a racquet.
Football shoulder pads
SSI has designed equipment and software to evaluate the protective performance of football shoulder pads. Original testing equipment and protocols were designed and assembled, and relevant data analysis methods and performance metrics were introduced. This testing involved measurements of large (500 lb) impact forces exerted on top of, and under, various pads attached to a manikin.
It involved comparisons of the impact-force-reduction capabilities of the various pads, and comparisons with the forces exerted (by identical impacts) when no protective pad is present. The lab also compared the relative protective contributions of pad interior foams and exterior shells, and directly compared the effectiveness of different foam and shell combinations
To test a pad, we impact it elastically with a falling load and record two types of force measurements for each impact. The force exerted onto the outside of each pad during the impact is measured using an accelerometer attached to the falling load. In addition, we measure the actual forces transmitted through the pads onto the manikin. These forces are recorded at separate locations on the manikin, so that we can measure the degree to which the pads are effective in spreading out the applied impact force.
Football helmets
It has been widely realized during the past 20 years that the collisions experienced by participants in contact sports and other potentially injurious endeavors are often strong enough to give rise to mild traumatic brain injuries (MTBI) such as concussions. Although more protective helmets have been introduced and used by participants in sports and other endeavors, the frequency of MTBI has remained alarmingly high.
In response to the MTBI epidemic, we have designed, constructed, and evaluated a new type of football helmet that provides increased protection to users. (This product was developed in collaboration with our partner Division, LLC.) This helmet dissipates a significant amount of the incident kinetic energy arising from an impact, thus reducing the force and torque applied on the helmet, and spreading out the force transmitted through the helmet onto a user’s head. We have constructed a partial helmet prototype that incorporates our energy-dissipating elements, and have used conventional and original testing equipment to demonstrate the significantly increased user-safety that this prototype provides. Compared to the highest rated available helmet, our prototype helmet is seen to significantly reduce the applied force and extend the impact time.