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Testing Plastics Physical Strength & Performance |
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| 21st August, 2008 | |||||||||
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Differentiating the tests
What is an impact test? It is applying a load to a specimen at high speed, then measuring the response of the specimen. Breaking the sample is a two-step process: Energy is needed to create a crack, and more energy is needed to enlarge the crack to failure, explains Harry Yohn, product specialist at Tinius Olsen Testing Machines.
While these traditional tests are adequate for quality control, they do not provide good information on the mechanism of failure or the cause of a fracture in an end-use application. Whereas the non-instrumented impact tests just measure the energy necessary to break a specimen, instrumented impact tests provide curves of high-speed stress/strain data that distinguish ductile from brittle failure and crack-initiation from crack-propagation energy. The latter give a more nuanced picture of the "toughness" of a specimen, explains Yohn. In addition to the limitations of traditional tests, there are other factors driving the trend to instrumented impact testing. A key one is concern about product liability for an increasing range of products, from medical and automotive components to toys or pipe. A case in point involves a plastic ladder company that learned the hard way about testing its product for the ductile-to-brittle-failure transition-the temperature at which the material is no longer flexible enough to be resilient. The firm supplied ladders to a city for use in sewer systems. The ladders worked well until winter came, at which point they started shattering when they were being pounded into their foundations. As a result, workers were falling off rungs that could not support them. While the company had run tensile tests, it had not performed impact tests at realistic temperature conditions. It was an expensive lesson: The city sued, won, and forced the processor to pay damages and replace all the ladders. Yohn notes that impact results are sensitive to numerous factors besides temperature:
Several other factors contribute to the growing dissatisfaction with the Izod and Charpy tests-particularly among materials suppliers including inaccurate or improper notching techniques, as well as subjective judgments of brittle versus ductile failure. The different specimen sizes, impact velocities, and hammer energies for the ISO and ASTM standards only make things worse. Most impact testers in use today are still not instrumented. Probably the biggest reason is historical. The industry holds a vast amount of data based on notched-Izod testing without instrumentation. The most commonly used impact-test standards do not call for instrumentation. Also, an instrumented system costs significantly more and requires additional technical expertise. | |||||||||
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Of the two basic types of plastics impact testers, pendulum types for Izod, Charpy, and tensile impact measure the energy absorbed by the specimen to cause failure. The other category is falling-weight tests—Gardner falling weight for rigid materials and dart drop for film. These are typically pass/fail tests: They give the average impact energy that breaks the sample 50% of the time.
For this reason, auto companies (each of which has its own impact-test specifications) prefer that the specimen supports be cooled. As a result, many test labs are installing a "cryobox" that encloses the Izod vise or Charpy supports.
