Impact tests are short-term tests, which provide information on the failure behavior of materials or components subjected to rapid loads and at varying temperatures. The testing systems used for these tests are either pendulum impact testers or drop-weight testers.
All materials are used on a daily basis with fluctuating temperatures. Since the fracture behavior also depends on the temperature, materials are often tested in the entire operating temperature range, indicating at which temperature and to what extent a material becomes brittle.
The example diagram shows that the decrease in strength of mild steel at -40 °C is 25 % in relation to the strength at 0 °C. Plastics display similar behavior, generally considerably more pronounced. Impact tests are also frequently carried out on plastics at various temperatures.
Charpy to ISO 179-1 is the preferred test method within the standard for single-point data ISO 10350-1. The test is preferably performed on unnotched specimens with edgewise impact (1eU). Should the specimen not break during the test, any subsequent tests are performed with notched specimens. In this case, a comparison of the test results is not possible. If a specimen break still does not occur using the notched specimen, the impact tensile method is used.
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 resulting 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
For one, the measured value curves always display characteristic oscillations. These oscillations originate from the specimen, whose frequency is in a defined functional relationship with the specimen geometry, the dimensions and the modulus value of the polymer. The wide measuring range is another major advantage of the instrumentation. Here forces and not energies, as with conventional pendulum impact testers, are measured. Since the measurement electronics allow for precise measurements as low as 1/100 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 measurement range described in ISO 179-2 with two instrumented pendulum hammers.
In the American ASTM standard testing is mainly performed according to the Izod test method, which is described in ASTM D256. Within this standard all impact tests are performed using notched 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.
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 ISO 6603-2 and ASTM D3763. ISO 7765-2 is a variation of the standard and is used for testing films.