Analytical evaluation of the acoustic insulation provided by double infinite walls

The acoustic insulation provided by infinite double panel walls, when subjected to spatially sinusoidal line pressure loads, is computed analytically. The methodology used extends earlier work by the authors on the definition of the acoustic insulation conferred by a single panel wall. It does not entail any simplification other than the assumption that the panels are of infinite extent. The full interaction between the fluid (air) and the solid layers is thus taken into account and the calculation does not involve limiting the thickness of any layer, as the Kirchhoff or Mindlin theories require. The problem is first formulated in the frequency domain. Time domain solutions are then obtained by means of inverse Fourier transforms using complex frequencies.http://www.sciencedirect.com/science/article/B6WM3-48BKTGF-C/1/7dd5d637ad258a65e80a1097e6ee96a

Crack initiation and propagation during high-temperature fatigue of oxide dispersion-strengthened superalloys

The mechanisms of crack initiation and propagation have been investigated in two oxide dispersion-strengthened (ODS) Ni-base superalloys under conditions of symmetric low-cycle fatigue (LCF) and creep-fatigue. The behavior of both ODS alloys is compared with that of conventional alloys of otherwise similar composition. While the improvement in fatigue resistance previously reported for ODS metals and alloys is confirmed by the present study for temperatures below about 0.6T m, the potential advantage of dispersion strengthening is not being exploited by the current generation of ODS superalloys at higher temperatures; crack initiation is found to occur prematurely due to the presence of recrystallization defects in the form of fine grains. The mechanism of crack initiation at fine grains is creep-type cavitation on boundaries transverse to the applied stress. Experimental results indicating the influence of temperature, loading frequency, and waveshape on the crack initiation rate are presented and discussed in detail. A qualitative correlation between waveshape and creep-fatigue life is suggested based on the macroscopic inelastic strain rate which is determined by the waveform and limits in turn the rate at which cavity growth can be accommodated