G-max Impact Testing
G-max Impact Testing
Firefly Sports Testing is the world's largest provider of G-max testing and your resource for all things G-max.
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Turf Gmax Overview / Background
We will start by discussing G-max as it relates to artifical turf fields. ASTM F355 Procedure A and ASTM F1936 are test methods used to measure the impact attenuation properties of synthetic turf playing systems. ASTM F355 is offically titled "Standard Test Method for Impact Attenuation of Playing Surface Systems and Materials" and ASTM F1936 is offically titled "Standard Specification for Impact Attenuation of Turf Playing Systems as Measured in the Field". ASTM F355 Procedure A covers the overall test method and ASTM 1936 specifies the method for measurement in the field on an installed synthetic turf playing surface.
The test procedure involves dropping a 20 pound impacting missile three times at each location from a consistent height of 24 inches. The test is typically performed at 10 locations. The locations are based on the primary sport and the discretion of the tester. The first drop conditions / compacts the loose infill. This value is recorded but not included in the location average. The second and third drops are recorded and averaged for the location average. The location averages are used to determine the field average.
Examples of ASTM F1936 G-max location figures per Standard Specification for Impact Attenuation of Turf Playing Systems as Measured in the Field
The impacting missile contains an accelerometer sensor that measures the magnitude of deceleration (measured in units of gravity or g’s) for the duration of impact. The deceleration measured during impact creates a curve. The peak of that curve is referred to as the “Gmax”. This is the primary value measured with this test. The maximum allowable Gmax as specified in the current ASTM F1936 test specification is 200 g’s.
Turf Gmax History / Rationale
Where and how did artifical turf gmax testing originate? How were the impact forces determined? According the ASTM, "Procedure A of Test Method F355 has been used to test the impact attenuation of athletic fields for over 30 years. The development of the 2-ft fall-height method can be traced back to the Ford and GM crash dummy tests of the 1960’s, medical research papers from the 1960’s and 1970’s, and a Northwestern University study in which an accelerometer was fixed to the helmet of a middle line backer to measure impacts received during actual play.
This study found the typical head-impact to be 40 ft/lb, which is equivalent to the impact generated by dropping a 20 lb missile from a height of 2 ft—the requirement specified in Procedure A of Test Method F355. For comparability and consistency, a set of standard test points were developed based on the experience of the task group in the areas of field testing and systems development. The maximum impact attenuation of <200 average g-max (as accepted by the U.S. Consumer Product Safety Commission) was adopted foruse herein."
When do ASTM F355A & F1936 apply?
Natural turf and artificial turf athletic fields for impact safety ONLY.
NOT for.... natural turf and artificial turf athletic field performance
NOT for.... determining turf underfoot stability or surface "speed"
NOT for.... turf in playground areas
Types of impact forms
In the test method F355, the "procedure" (A,D, or E) refers to different impact forms. Each impact form is used to represent different types of impacts that occur, requiring unique and specific test methods for each form. The image below shows three of the impacting forms listed in F355.
Gmax & HIC values from different impact forms do NOT relate to each other. Impact forms and devices can NOT be used interchangeably
Gmax F355A vs Clegg
Unfortunately, there have been numerous claims that results from the ASTM F355A Gmax device and the clegg can be compared via a conversion factor. This is simply not the case. We have performed extensive comparison studies to prove there is no correlation. The "ATSM F1936 Standard Specification for Impact Attenuation of Turf Playing Systems as Measured in the Field" explicitly states that the clegg hammer can NOT be used to perform this test. Beware of anyone who claims otherwise.
Gmax and Underfoot Performance
Another common misconception is the ASTM F355A Gmax can be used to indicate underfoot performance and field speed of play. Unfortunately this is simply not the case. The pressures applied to the surface and the way the data is analyzed and expressed by the ASTM F355A Gmax device is not sufficient for looking at underfoot performance. Each type (underfoot, head/body, etc) of impact interacts with different system layers making the resulting impact characteristics closely dependent on the "force" of impact. Please see the figure below.
Fortunately, there is a device intended to quantify underfoot performance, the Advanced Artificial Athlete (AAA). This device provides the important underfoot pliability values: force reduction, vertical deformation, and energy restitution. Below is a classic example of how ASTM F355A does not correlate with underfoot performance .
A minimum G-max should NEVER be specificed
Example of how G-max and Underfoot performance do not correlate
Although the underfoot performance (force reduction, vertical deformation & energy restitution) is similar for Turf A both with and without underlayment, the g-max value is dramatically larger for Turf A without underlayment. Therefore, the underfoot performance and Gmax performance do not correlate.
Example of the results from Gmax testing versus the results of vertical deformation testing on six different surfaces. The results of the Gmax testing do not correlate to the results from the vertical deformation testing,which can be seen by comparing Turf C over stone and Natural Turf. These samples have similar vertical deformations but completly different Gmax results.
Activities that use impact testing
There are a number of different activities that use impact testing to verify that the impact safety measures are sufficient. Below are just some of the ones we perform at Firefly Sports Testing.
Natural & Artificial Turf
Wrestling / Martial Arts Mats
Indoor Sports Flooring
Wresting & Martial Arts Mats
Climbing & Gymnastic Mats
Synthetic & Natural Turf
Sports Equipment Pads
Choosing a testing company
When it comes to testing, the results are only as credible as the laboratory producing them. Why pay for testing that you cannot rely on, especially when the safety and wealth-fare of children and athletes depend on it? At Firefly Sports Testing we are committed to producing reliable, unbiased results that you can count on when it matters.