Change in Work Function of Fatigue Damaged Surface and Its Relevance to Fatigue Failure(Student Poster Session)

Abstract

As the initiation of fatigue cracks on smooth surfaces is predominantly a surface phenomenon, extensive efforts have been focused on characterizing a surface damage such as extrusion and intrusion during fatigue. Therefore careful surface analysis techniques such as measurement of the work function (have after denoted as WF) using a Kelvin Probe would have a potential to predict an initiation of fatigue cracks. The WF is very sensitive to changes on the surface and hence it is believed that the formation of extrusions and intrusions during fatigue would affect the work function of the sample and this can be easily detected by the Kelvin probe technique (vibrating capacitor method). Generally, the bare surface of a metal has an active work function following the general e.m.f series. In this present investigation, the Kelvin probe is employed to study the change in work function of a specimen surface after predetermined increments in the number of fatigue cycles. During the initial fatigue cycles, the work function becomes more active than that prior to fatigue loading. This can be attributed to the formation of new surfaces due to the generation of intrusions and extrusions during the fatigue test. When exposed to ambient conditions at room temperature, the work function increases towards a more noble direction with time as the new surface oxidised. Through the continuous measurement of the WF during fatigue, it was observed that the formation ratio of the new surface area rapidly decreased after N/N_f=0.02. On the other hand, the logarithmic rate law constant of oxidation of the new surface gradually decreased with the degree of damage. Furthermore, during the final stages of fatigue, a greater number of intrusions without oxides develop compared to that of extrusions. Oxides formed preferentially on extrusions as observed by the elemental mapping obtained by Time-Of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) analysis. More active development of intrusions can result in the initiation of a crack.