This is the first in a series of articles on materials selection authored by Eric Larson, a mechanical engineer with 30 years of experience in plastics design, for PlasticsToday. His most recent book is Plastics Materials Selection: A Practical Guide.

We often think of summer as an easy time of year. Things slow down, the days get longer, we take vacations, we kick back and relax. We also often watch sports, like baseball, golf, tennis and soccer. During the recent FIFA Women's World Cup Canada 2015, I was struck by how often I heard the word tough. A tough draw. A tough match. A tough player.

In common use, the term toughness often describes the behavior of something, or of someone. When something can withstand harsh treatment we describe it as being tough. It is also sometimes used to describe a person's character, even to the point of challenging someone on his or her ability to survive in the real world. Are you tough enough?

In the world of engineering, the term toughness is used to describe the ability of a material to withstand sudden impact without failure. It is a simple concept, but the toughness of a material is often difficult to accurately measure. What exactly is a "sudden" impact? What is, and is not, failure?

I like to think of toughness as the ability of a material to absorb energy without breaking. (To paraphrase a former colleague: "Toughness is whatever property is lacking in that part that just broke.") Still, toughness can be difficult to quantify. Just as there multiple ways to evaluate strength and stiffness, there are multiple ways to evaluate toughness, and there are a number of standard tests that are used to quantify the toughness of thermoplastic materials. Some of these tests are simple, and can be easily performed on a bench top with minimal equipment. Some tests are quite sophisticated and involve advanced equipment and extensive instrumentation.

Some of the more simple tests include low-speed impact tests (like the Izod test) and various kinds of drop tests (like the Gardner falling dart impact test).

In an Izod test, a test specimen is fixed in a vise, and a swinging pendulum hits it. The device measures the amount of energy absorbed as the test specimen breaks. This test is normally done with a notch in the test specimen. A modified version of the test is sometimes when the test specimen has no notch (this is called an Unnotched Izod test).

A Gardner Falling Dart Impact test utilizes a test plaque suspended over an opening. A dart is then dropped from a specified height. However, instead of measuring the amount of energy absorbed, a pass/fail criteria is often developed, where a threshold of weight and height for failure is determined. Below that threshold the test plaque will typically withstand the impact; above it, the plaque will fail.

Izod test values (both standard and un-notched) are readily available for most materials at standard environmental conditions (room temperature, 50% relative humidity, etc.). This allows for a simple, basic comparison. However, property data sheets rarely provide any data at lower temperatures (or higher temperatures), nor do they provide any curves of impact test value versus temperature (this is typical, since most resin suppliers do not provide this kind of data). Also, most engineers rarely consider the impact velocity of the Izod test in comparison with the impact velocity of their end-use application. (The impact velocity of an Izod test is just under eight miles per hour, which, in the grand scheme of things, is not that fast, let alone "sudden." Impact velocities of some other tests can be found in a table in my new book, Plastics Materials Selection: A Practical Guide).

Regardless of the test data that is being evaluated, it is helpful to understand which aspect of toughness that test is measuring.

First, what is the stress state of the material as it is being impacted? Is it a pure tensile impact? A shear impact? Or does it involve a combination of stress states?

Second, are we concerned about a single impact? Or multiple impacts? Failure caused by multiple impacts--what we call impact fatigue--can be difficult to quantify. Also, what is the mode of failure of the test specimen? Is it a ductile failure, or a brittle fracture?

Oftentimes, the failure mode is the result of crack propagation. So if cracks can be prevented—or somehow handled on a molecular level—the structure can withstand a higher input of energy without failure. However, crack propagation is a complex issue. It involves stress, strain, elasticity and plasticity, and countless other phenomena involving mechanics and material science. Crack propagation is also rate sensitive, so the speed of impact is very important. And the issue becomes even more complex when you add in temperature variations.   

Many thermoplastic materials can be modified for improved toughness. This usually involves an additive that is dispersed throughout the material. The additive then acts on a local level to absorb the energy that is being transmitted along a crack surface. (In ABS, the additive is butadiene, aka synthetic rubber). Additives like glass fiber and/or structural reinforcements can also affect toughness, sometimes for the better, sometimes for the worse.

In summary, while we can often make some educated guesses about toughness, to thoroughly evaluate the impact performance of a given material in a given application, you will often need to do your own testing. That testing should account for the impact velocity of the application and the end-use environmental conditions (temperature, humidity, chemical exposure, etc.).

Selecting a material for optimal toughness can be, for lack of a better description, a tough challenge.  

It isn't a picnic.