Impact tests are short-term tests which provide information onthe failure behavior of materials or components subjected torapid loading and at varying temperatures.The testing systems used for these tests are either pendulumimpact testers or drop-weight testers.

All materials are used on a daily basis with fluctuating temperatures.Since the fracture behavior depends on the temperature,materials are often tested in the entire operating temperaturerange, indicating at which temperature and to whatextent a material becomes brittle under temperature.

The typical diagram below shows that the decrease instrength of mild steel at -40°C is 25% compared with thestrength at 0°C. Plastics display similar behavior, generallyconsiderably more pronounced. Impact tests are frequentlycarried out on plastics at differing temperatures also.

The most common impact test is Charpy or IZOD. Charpy tests are implemented according to ISO 179-1 and ASTM D 6110. Instrumented Charpy tests according to ISO 179-2. Izod tests are carried out according to ISO 180, ASTM D 256, ASTM D 4508 and "Unnotched cantilever beam impact" according to ASTM D 4812.

 

Charpy impact test

Charpy according to ISO 179-1 is the preferred test method within the conext of the standard for single-point parameters ISO 10350-1. The test is preferably performed on unnotched specimenswith edgewise impact (1eU). If the test specimen does not break during the test, the subsequent tests are carried out with notched samples. It is not possible to compare the test results. If the notched sample still does not break, the impact method is used. 

Instrumented Charpy impact test 

By plotting the force time sequence, a force time diagram with excellent accuracy can be achieved through double integration using qualitative high-quality measurement technology. Theobtained data can be used in different 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.

The large measuring range is another great advantage of instrumentation. Forces and no energies are measured, as with conventional pendulum impact testers. 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. This makes it possible to cover the entire measuring range described in ISO 179-2 with two instrumented pendulum hammers.

 

Izod impact test

Within the American standard ASTM, the Izod test method, which is specified in ASTM D 256.testing is mainly used. All impact tests with notched specimens are implemented within this standard. If only small test specimens can be produced, the "chip-impact" method according to ASTM D 4508 can be used, which is a counterpart to the Dynstat impact test.

 

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.

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.