In order to characterise dynamically stressed plastics with regard to their fatigue behaviour, standardised Wöhler tests are available according to DIN. However, these tests are still only intended for metallic materials and mean a long testing time for plastics due to their strong temperature dependence. Alternatively, the load-increase method offers a significant reduction in the number of test specimens required as well as a considerable time saving. However, this method is not a standardised one. The two methods are presented in more detail below based on an example.

The aim of both tests is to determine the dynamic load limit at which the plastic can be loaded infinitely often without showing failure.

Method: Single-stage and multi-stage fatigue tests

  • Characterisation of a PA 12 by means of a fatigue test on servo-hydraulic operated hydropulsers (Fig. 1).
  • In the fatigue test according to Wöhler (DIN 50100), test specimens are loaded at appropriately staggered load levels (compare Fig. 2, Part 1), the number of cycles to failure is noted for 5 to 10 test specimens each and plotted in the Wöhler diagram.
  • In the multi-stage fatigue test, also known as the load increase method, a specimen is subjected to a successive increase in the middle and upper load after a defined number of load cycles, so that the load limit is indicated in the form of a critical stress/strain in the response signal, compare Fig. 2, Part 2.

Result: Determination of critical load limits in the multi-stage fatigue test

Figure 3 shows the result of a load increase test for PA 12. The critical strain and the corresponding critical stress are derived from the strain/cycle diagram by means of regression analysis. The determined value pairs for the five tested specimens are shown in Table 1.

From the regression analysis, an average of 12 N/mm² is determined as the critical load limit. The load level before this with 11.5 N/mm² is considered subcritical. This corresponds to about 24 % of the quasi-static tensile strength (48 N/mm²). Evaluation tests at 11.5 N/mm² in the fatigue test showed no early specimen failure up to 10 million load cycles. Depending on the material, a load limit can thus be reliably determined in a shorter test time by means of the load-increase method.

Please do not hesitate to contact us if you have any questions about the methodology.


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Dipl.-Ing. (FH) Christiane Wintgens

Mechanical testing Project engineer +49 241 80-28342

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