Cyclic Hardness Test PHYBAL_CHT: A New Short-Time Procedure to Estimate Fatigue Properties of Metallic Materials

Abstract

Conventional methods to characterize the fatigue behavior of metallic materials are very time and cost consuming. That is why the new short-time procedure PHYBAL_CHT was developed at the Institute of Materials Science and Engineering at the University of Kaiserslautern. This innovative method requires only a planar material surface to perform cyclic force-controlled hardness indentation tests. To characterize the cyclic elastic-plastic behavior of the test material the change of the force-indentation-depth-hysteresis is plotted versus the number of indentation cycles. In accordance to the plastic strain amplitude the indentation-depth width of the hysteresis loop is measured at half minimum force and is called plastic indentation-depth amplitude. Its change as a function of the number of cycles of indentation can be described by power-laws. One of these power-laws contains the hardening-exponent_CHT e _II , which correlates very well with the amount of cyclic hardening in conventional constant amplitude fatigue tests.

To identify heat-treatment-parameters and SAE 52100 TRIP-alloys with high hardening rates as well as their damage tolerance the PHYBAL_CHT- and PHYBAL_LIT-method were used. One modified SAE 52100 TRIP-alloy was investigated in detail. The fatigue behavior and the cyclic hardening of the selected SAE 52100 TRIP-alloy was investigated and compared with conventional SAE 52100. Conventional stress-controlled fatigue tests were performed on a servo-hydraulic test system with a frequency of 5 Hz and a load ratio of R = −1. Different rates of cyclic hardening were identified. The size of the nonmetallic inclusions leading to crack initiation and the number of cycles to failure were used to evaluate differences in the damage tolerance capability.