Finite Element Analysis and Artificial Turf


One artificial turf design firm in The Netherlands calls upon finite element analysis software to ensure that all four elements that make up the turf are working properly together.

Artificial turf has come a long way since the 1960s. Back then, many people knew it as AstroTurf, so named because it first became known to the public in 1966, when it was laid down in the Houston Astrodome.

At the time, the playing field was enclosed under a glass roof painted white to cut glare in the players’ eyes. But the paint kept sunlight from the natural grass, which of course died. When the owners rolled down artificial turf at the Astrodome, the event made national news.

Although it is a product whose job is to lie still, there’s a lot of engineering that goes into artificial turf. Designers of the product often call upon engineering software and finite element modeling to get it right.

Today, more than 3,500 United States playing fields are covered in artificial turf, according to estimates by the Synthetic Turf Council in Atlanta. And 800 more are installed each year.

“Artificial turf can take a lot of wear at a high frequency. So you can play game after game on it without having to wait for it to rejuvenate,” said Tim Pope, senior development specialist at The Dow Chemical Co., which makes chemicals in artificial turf.

An entire square of turf, with fibers, infill, and backing characteristics built into the model, can be evaluated for compression by a virtual foot or a bouncing ball.

Although the turf is meant to look like grass, it’s actually closer to wall-to-wall carpeting in its makeup, Pope said. The turf is made up of individual blades known in industry parlance as yarn. The blades are affixed to a backing. Depending on use, the whole thing lies atop a shock-absorbing layer, Pope said.

Unless you’ve played a sport on artificial turf, you may not realize it resembles real grass in a way that goes beyond the color. Bits of artificial dirt dot the turf. The pieces of dirt, actually called infill, are small particles of elastic material that helps with shock absorbency. It also helps add weight to the artificial turf to better attach it to the ground, Pope said.

“If you don’t have loose material in the grass blades, when an athlete plants a foot and turns and twists, he can pop a knee out,” Pope said. “Infill moves with the athlete and diminishes injury.”

Some infill used on artificial turf is even smaller than dirt particles. These smaller pieces, or sub-bases, resemble bits of sand. The smaller particle size means the infill sits lower in the yarn, making it less visible.

All of these elements must be fine-tuned to work together, said Martin Olde Weghuis, the international manager of research and development at the TenCate grass division of Nijverdal, The Netherlands. His company makes a line of the four major components of synthetic turf: yard, infill, backing, and subbases.

To make sure those parts work together, turf design undergoes finite element analysis, said Marco Ezendam, director of Reden BV of Hengelo, The Netherlands, which serves as engineering consultant to TenCate.

“A playing field is an entire system, not just individual components,” Ezendam said. “If you want better performance from the field, you have to know how the entire system functions and what the interactions are within it. That’s the reason we started modeling turf design with finite element analysis.” 

The Fédération Internationale de Football Association (FIFA), founded in 1904 to promote the sport of soccer, has deemed artificial turf “an acceptable playing surface for football” and cites numerous advantages over natural grass. But FIFA has also spelled out detailed regulations about the materials, substructure, installation, testing, and certification of artificial turf for playing fields—which means that turf manufacturers have to be on top of their game when designing their products for performance and safety.

Reden uses Abaqus FEA software from Simulia, a Dassault Systèmes company. A grass-fiber model in Abaqus can be subjected to virtual bending tests, and its mass, shape, height, etc. modified and retested, until the desired characteristics are achieved. Infill models can be adjusted for morphology, size, material, distribution, friction, and layer thickness, and then run through triaxial compression tests. An entire square of turf, with fibers, infill, and backing characteristics built into the model, can be evaluated for compression by a virtual foot or a bouncing ball.

“We use FEA to model the properties of a single fiber, translate those into the properties of a group of fibers, and then predict the characteristics of the mass, spring, and damping of the field interacting with a ball or player,” Ezendam said.

Next time you’re running across a field of fake grass, take a second to think about all the chemistry and technology under your feet.

Jean Thilmany is Associate Editor, Mechanical Engineering.

It’s easy to generate a neat-looking model with really pretty colors and get totally bogus results.

Steve Remy, PE principal, Concinity Product Design and Engineering


October 2011

by Jean Thilmany, Associate Editor