Why no indurance limit observed in the s n plot of aluminium?
Answers
Answer:
Fatigue failure results from accumulation of fatigue damage. Assuming a perfect electropolished sample without notches, fatigue damage starts by localization of plastic strain which may lead to micro crack initiation within grains (transgranular) or in grain boundaries (intergranular). The growth of the individual micro cracks are impeded by grain boundaries and/or grain junctions. The ability of the micro cracks to pass these obstacles depends e.g. on the stacking fault energy (SFE) of the material. In materials with high SFE the dislocations can easily cross slip and pass obstacles. Same goes for micro cracks. I.e. in materials with high SFE (like pure metals) the micro cracks can easily propagate (an accumulate) to form macro cracks. Eventually this lead to fatigue failure.
If a material has low SFE (like e.g. steel and Titanium alloys) and the imposed load/strain amplitude is low, the micro cracks may not be able to overcome the obstacles (grain boundaries or junctions) and no macro cracks will form. Thus, the accumulated fatigue damage in insufficient to form fatigue failure - i.e. the material will show a break on the SN curve interpreted as a fatigue limit.
It is still being discussed, if a true fatigue limit actually exists. Maybe it's just a matter of imposing enough load cycles to induce sufficient localized plastic strain so that the micro cracks can overcome the obstacles and start propagation to form macro cracks and eventually fatigue failure