Volume 5, Issue 9 (October 2008)
Investigation of Material Fatigue Behavior Through Cyclic Ball Indentation Testing
An experimental investigation of cyclic indentation testing to characterize the fatigue response of three different metallic materials: two aluminum alloys (2014-T651 and 7175-T7351) and a duplex stainless steel (2205), was carried out. The force-displacement response during cyclic loading was logged continuously throughout the entire duration of the test and the data were analyzed to identify parameters such as change in total depth of penetration, change in loading and unloading slopes, change in unloading intercept as a function of number of cycles, and change in displacement range as a function of number of cycles. From the results, one could identify the transient response of the material during cyclic loading, and identify some specific points relating to fatigue failure life, such as knee-point response in depth of penetration as a function of number of cycles of loading. It was observed that data on unloading slope plotted as a function of number of cycles also provide an indication of specimen failure in compression-compression fatigue. The responses were found to be similar for all the three materials tested at different maximum compressive force levels. Failure life data in the low cycle fatigue (LCF) region was evaluated for Al-Cu-Mg alloy 7175-T7351 and the data compared with the failure indicators (knee point) during cyclic indentation testing. A reasonable correlation was established between failure life, as indicated by LCF testing and knee point indicated by cyclic indentation. Experiments were also carried out on virgin material of 7175-T7351 alloy and plastically deformed material of the same alloy. Both static and cyclic indentation tests show a difference in material behavior before and after residual plastic deformation. Further work is required to correlate failure life data as obtained from cyclic indentation with specimens having controlled damage levels, before this technique can be used for residual life estimation purposes.