Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718
Notches were machined in specimens of INCONEL 718 by a broaching process, where differing broaching runs led to differing extents of subsurface deformation and surface roughness. Fatigue tests were carried out at 600 °C with a trapezoidal loading waveform at 0.25 Hz. The broaching process that led to the more severe subsurface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elastoplastic finite-element (FE) model showed that the work hardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behavior were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work-hardened layer.
On the effects of particle strengthening and temperature on the VHCF behavior at high frequency
The fatigue properties in the high-temperature range at very high number of cycles of the precipitation-hardened nickel-base alloy Nimonic 80A were investigated. With the applied global strain amplitude being well below the elastic limit, cyclic life is dominated by heterogeneously distributed and localized plastic deformation in the microstructure. The material studied was fatigued in the peak-aged and overaged condition applying high frequency testing methods. Microstructural analysis by means of scanning and transmission electron microscopy was performed to determine the dominating damage mechanism. At room temperature planar single slip and at elevated temperatures partially wavy dislocation arrangements were found, with the wavy arrangement being accompanied by dislocation climbing processes as well as Orowan loops. At room temperature the overaged Nimonic 80A showed a slightly superior fatigue behavior compared to the peak-aged condition in the VHCF range, whereas at elevated temperatures the difference in fatigue behavior at high number of cycles seemed to be of less significance. The overaged condition showed a slight increase in fatigue life at 600 °C accompanied by a tendency towards a more pronounced homogeneous dislocation movement. Isothermal cyclic deformation at 800 °C led to a pronounced decrease of cyclic life with both heat treatments exhibiting a very similar fatigue behavior. Early failure was primarily ascribed to the formation of microcracks in the emerging oxide layers.
The Effectiveness of Two Approaches to the Teaching of High School American History
The purpose of this study was to test the effectiveness of studies in depth utilizing selected materials which had been developed for teaching high school American history.