Plastic materials selection: Summertime, and the living is . . . tough

July 21, 2015

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.

American Gothic via Eric Larson
What, exactly, is a sudden impact? Image courtesy Eric Larson.  

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

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