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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.

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Comparison of testing systems for the performance of impact tests

Comparison of testing systems for the performance of impact tests

Instrumented drop weight testers
High-speed testing machines
Classic drop weight testers
Classic pendulum impact testers
Instrumented impact testers
Instrumented drop weight tester HIT230F for 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 machine HTM5020

High-speed testing machines

  • High-speed testing machines generate testing speeds up to 20 m/s with a hydraulic drive.
  • They are always instrumented and are used flexibly for high-speed tensile tests, 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.
HIT pendulum impact testers for determination of impact strength and notched impact strength

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 pendulum impact tester – impact tensile test arrangement on the Charpy fixture

Instrumented impact testers

  • 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 mechanic 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.

Comparison of test methods with pendulum impact testers

Charpy impact test (ISO 179-1, ASTM D6110)

In the context of the standard for single-point data, ISO 10350-1, Charpy to ISO 179-1 is the preferred test method. The test is preferably performed on unnotched specimens with edgewise impact (1eU). If the specimen does not break in this configuration, the test is performed using notched specimens. The test results are therefore not directly comparable. If specimen break is still not achieved with notched specimens, the impact tensile method is used.

Advantages of the Charpy impact test:

  • In contrast to the Izod test, the Charpy test has a broader range of application and is better suited to test materials displaying interlaminar shear fractures or surface effects.
  • Furthermore, the Charpy method offers advantages for testing at low temperatures. Since the specimen support is further from the notch, quick heat transfer to the critical areas of the specimen is prevented.

Instrumented Charpy impact test (ISO 179-2)

By plotting the force time sequence, a force time diagram with excellent accuracy can be achieved through double integration using high-quality measurement technology. The acquired data can be used in a variety of ways.

  • Additional characteristic values that enhance the understanding of material behavior
  • Fracture mechanical characteristic values
  • Automatic, operator-independent determination of the type of break using the curve progression in the force travel diagram

The measured value curves always show characteristic fluctuations. These are specimen fluctuations with frequencies that are correlated in a defined, functional way to the specimen geometry, the dimensions, and the modulus value of the polymer.

Another great advantage of instrumentation is the large measurement range. In contrast to conventional pendulum impact testers, forces are measured instead of energy. Since the measurement electronics permit precise measurements as low as 1/1000 of the nominal force, the lower end of the measurable impact energy is usually determined by the duration of the test and by the natural frequency of the measuring elements. For this reason it is possible to cover the entire measurement range described in ISO 179-2 with two instrumented pendulum hammers: An instrumented 5-joule pendulum for impact velocities of 2.9 m/s and a 50-joule pendulum for an impact velocity of 3.5 m/s. With this method, Izod tests and impact tensile tests are also instrumented.

Izod impact test (ISO 180, ASTM D256, ASTM D4508, ASTM D4812)

  • For ASTM standards, the Izod test method to ASTM D256 is the method that is commonly used. For this notched specimens are always used for testing.
  • A method used less frequently is the unnotched cantilever beam impact method described in ASTM D4812, which is similar to the Izod method, but is performed with unnotched specimens.
  • If it is only possible to produce small specimens, the chip impact method to ASTM D4508 can be used. It is a counterpart to the Dynstat impact test.

Dynstat impact test (DIN 53435)

  • Some German automobile manufacturers use the Dynstat impact method to test small specimens. The advantage of this method is that very small specimens are used, which are taken from relatively small parts.
  • This method is only described in the DIN standard.

Puncture tests on test plates

Puncture tests are of particular interest for molding materials. This type of test imposes a multi-axis stress condition on a thin plate, which is induced at 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.

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.

Impact tests with high speed testing machines

The HTM high-speed testing machine can be used widely in plastics testing. It has a very high test speed and large force ranges, and can be used flexibly in tensile and compression tests. A temperature chamber can be integrated to perform tests in a wide range of temperatures.

Comparison of test methods with high-speed testing machines

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