Impact Tests on Plastics

Impact testing is used to determine material behavior at higher deformation speeds. Pendulum impact testers, drop weight testers, and high-speed testing machines are used to characterize the behavior of plastics subjected to impact loading.
Pendulum impact tester HIT 5.5 impact tests on plastics

Instrumented drop weight testers

Instrumented drop weight testers have a force sensor and a light barrier for precise determination of speed and fast measured value acquisition. They allow you to plot the entire force travel diagram in a puncture test. These diagrams help to calculate characteristic force points and absorbed impact energy. 

High-speed testing machines

HTM5020_without background
High-speed testing machines generate testing speeds up to 20 m/s with a hydraulic drive. They are instrumented and are used flexibly for tensile/puncture and flexure tests under high strain rate. They test methods relevant to impact testers and instrumented drop weight testers. This type of machine offers a major advantage in terms of surplus energy, which, together with a special control, provides almost constant speeds during the test. 

Classic drop weight testers

Classic drop weight testers function on the principle of a linear fall movement. A mass with an impact object is released from a height that is usually defined.  Since no other measurement follows specimen penetration, these simple drop weight testers are used for go/no go analysis with the staircase method or in around-the-clock method.

Classic pendulum impact testers

Classic pendulum impact testers determine the impact energy absorbed by a standardized specimen up to break by measuring the height of rise of the pendulum hammer after impact. The result is impact strength or notched impact strength, which is indicated in relation to area, for example in kJ/m². When testing pipes, pendulum impact testers are also used for go/no go analysis of the specimen break, without a quantitative depiction of the test result.

Instrumented impact testers

Instrumented pendulum impact tester – impact tensile test arrangement on the Charpy fixture
Instrumented impact testers are equipped with a force sensor and fast measured value acquisition, which records up to 4 million force and time values per second. Along with the impact energy value, it is possible to plot other data, for example the force and flexure curve or fracture mechanical characteristics.  

Impact Testing with Impact Testers

There are 4 standardized pendulum impact test methods:

  • Charpy test (ISO 179-1, ASTM D6110)
  • Instrumented Charpy test (ISO 179-2)
  • Izod test (ISO 180, ASTM D256, ASTM D4508) and "Unnotched cantilever beam impact" (ASTM D4812)
  • Impact tests (ISO 8256 and ASTM E1822)
  • Dynstat impact test (DIN 53435)

Differences between ISO and ASTM impact tests

In ISO tests, each pendulum hammer may be used in a range of 10% to 80% of its nominal initial potential energy. ASTM permits use up to 85%. A fundamental difference between ISO and ASTM is the choice of pendulum size. According to ISO, the largest possible pendulum hammer must be used, although the overlaps between pendulum sizes is often very small. This requirement is based on the consideration that loss of speed during specimen penetration should be kept as low as possible. With ASTM, the standard pendulum hammer has a rated initial potential energy of 2.7 joules and all further sizes are arrived at by doubling. Here, the smallest possible hammer in the range should be used for the test.

Puncture Tests on Test Plates

Puncture tests are of particular interest for molding materials. This test type places a multi-axis stress state on a thin plate, which is induced by a high strain rate. The result is a force time or force travel diagram and single-point data that describe both the deflection of characteristic points of the diagram and the maximum force.

Puncture tests on plates are defined in the following standards: ISO 6603-2 and ASTM D3763. ISO 7765-2 is a variation of the standard and is used for testing films.

Force travel diagram puncture test on plastics ISO 6603-2
Puncture test on plastics ISO 6603-2

The tests are performed using a drop height of 1 m, which corresponds to an impact velocity of 4.43 m/s. The potential energy of the drop weight must be at least 2.73 times greater than the puncture energy absorbed by the specimen. This ensures compliance with the standard after limiting the loss of speed by a maximum of 20% of the impact velocity.

In the case of viscous polymers, such as polycarbonate, in particular, friction occurs at the tip of the indenter, which would lead to a significant falsification of the test results. For this reason, the standards state that the indenter must be lightly lubricated.

To test at low temperatures, the test plates must be conditioned to the test temperature for a sufficient amount of time. Depending on the test temperature, commercial coolers can be used. They should be located close to the test instrument. For the test, the specimens are removed from the cooler and placed in the drop weight tester. They are then tested within a few seconds.

The HIT 230F drop weight tester is designed to allow easy access to the specimen table. Activating the two-hand release closes the clamp, covering all movable masses. This means there is no risk to the operator and the test can be performed in seconds. In contrast to instruments with integrated temperature chambers, HIT 230F offers puncture tests with high specimen throughput and ease of use.