Dynamic indentation testing: Is there an influence on mechanical properties?Tuesday (27.09.2016) 15:00 - 15:15 Part of:
Dynamic indentation techniques, particularly the continuous measurement of stiffness (CSM) by a superimposed oscillating force signal, came to the center of attention in the indentation community within the past two decades. The main asset of this method is that within a single indentation test, one can not only obtain mechanical properties at the point of unloading but continuously over the entire penetration depth. For elastic isotropic materials this enables to calculate the actual area in contact with the tip, thus allowing to perform advanced drift corrections or tip shape corrections.
However, in-depth knowledge about the influence of this oscillating sinusoidal force signal is still lacking, and from some reports in literature an influence on material properties cannot be excluded definitely. Therefore, this study will contrast dynamic with static indentation tests, where no superimposed force is applied. In order to consider the effect of lattice type and microstructure, various body-centered cubic (bcc) and face-centered cubic (fcc) metals with single crystalline and ultra-fine grained microstructures have been examined. Based on this we show that the impact of CSM is insignificant at common indentation depths of several 10 nm. However, it turns out another important experimental parameter is often neglected. Static indentation tests are commonly not performed with a constant indentation strain-rate, especially the needed hold segment before unloading results in a drop of the strain-rate dependent on the tested material. This will lead to distinct errors in obtained mechanical properties for materials with a high strain-rate sensitivity, which could by mistake be ascribed to the use of CSM.