Impact tests are short-term tests which provide information on the failure behavior of materials or components subjected to rapid loading and at varying temperatures. The testing systems used for these tests are either pendulum impact testers or drop-weight testers.
Because many materials used for a number of different applications on a daily basis are subjected to fluctuating temperatures, and since the fracture or failure behavior also depends on temperature, these materials must often be tested in their entire operating temperature range. These tests are used to indicate at what temperature and to what extent the 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 a similar behavior, but 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 on plastics within the standard for single-point data ISO 10350-1. The test is ideally 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 the notched specimen still does not break, 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 measurement range is another major advantage of the instrumentation. These instruments measure forces and not energies, as is the case with conventional pendulum impact testers. 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 only small specimens can be produced, the chip impact method to ASTM D4508 can be used. It is a counterpart to the Dynstat impact test.
The HTM high-speed testing machine is of universal use in plastics testing. It covers a very high test speed and large force range, and can be used in both tensile and compression tests with a great level of flexibility. The performance of tests in a wide range of temperatures is made possible through the use of temperature chambers.