Impact testers are among the first instruments that plastics compounders, extruders, and molders consider when outfitting a lab. Historically, the choice between traditional falling-weight and pendulum impact testers seemed relatively simple. It was determined by the material, end-use application requirements, and the customer's preference for a particular type of test data. 

Instrumented impact tests are becoming more widespread, particularly for R&D at compounding operations or anywhere there is a need to examine in detail how the material fractures. Tinius Olsen's Yohn says the auto industry is showing new interest in instrumented impact tests because they are looking to develop impact data for plastics that better simulate real-life conditions.

Instrumented impact can be performed either on a falling-weight tester, such as those supplied by Instron and Ceast, or on a pendulum-type instrument like those offered by Tinius Olsen and Atlas. Most instrumented drop-weight testers allow users to also perform the Izod and Charpy tests by changing the impact striker and fixture on the instrument. However, Ceast's Nelson notes that even with these fixtures, the drop-weight tests do not comply with ASTM Izod or ISO Charpy standards. Furthermore, an ASTM task group has recently debated evidence that data on some materials tested this way do not correspond well to impact data derived from a traditional pendulum.

With instrumented impact, the falling dart's tip or the pendulum's hammer is fitted with a load cell. The force-time data during the actual impact are stored by a high-speed data-acquisition system. These data can be used to generate curves showing force, energy, velocity, and deformation versus time. By analyzing these curves, one can learn the force, energy, and deformation necessary to initiate a crack and then to cause total failure; the rate sensitivity of a material to impact loading; and the temperature of a material's transition from ductile to brittle failure mode.

Instrumented impact tests are performed according to ASTM D3763 and ISO 6603 and 7765 for drop-weight instruments, and ISO 179 Part 2 or ISO 180 Part 2 for pendulum testers. Prices of instrumented impact testers start at around $20,000 for a basic model to $40,000-70,000 for advanced systems with additional sensors and environmental chambers, and over $100,000 for fully automated systems.

Instrumented falling-weight tests can be performed on films, plaques, pipe sections, and finished products such as safety helmets. Instrumented pendulum (Izod and Charpy) tests can be done on standard specimens or on sections cut from injection molded or compression molded finished parts. Some falling-weight instruments have a support table on the base of the unit that allows testing of larger whole parts or assemblies.

Says Nova's Elston, "I don't see any interest in instrumented impact tests on commodity-resins. Basic falling-weight or Izod impact are sufficient for products such as CD cases. But instrumented impact is desirable for rigid structural products."

But that could be changing. The advent of piezoelectric sensors for instrumented impact testers is said to provide greatly increased sensitivity, allowing for testing of very light films, foams, and most other materials used in packaging.

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