Shock, Vibration, Fatigue, Acoustics Structural Dynamics
The Wall Street Journal ran a piece this week on a circular house in Novato, California, listed for sale for the first time since it was built. Sam Harkleroad put it up in 1963: 1,560 square feet of floor area, roughly 50 feet in diameter, cantilevered off a single cylindrical core, and designed to rotate ... Read more
The Barrett scaling law is one of the oldest tools in the launch vehicle random vibration toolbox, and it is still one of the most used. It answers a question that comes up on every new program before a single accelerometer has been mounted: I have flight data from a previous vehicle, and I have ... Read more
Cut a rectangular hole in an aerodynamic surface and fly it at high subsonic or supersonic speed, and the hole will sing. Not quietly. Overall sound pressure levels inside a fighter weapon bay routinely exceed 160 dB, and the energy is not spread evenly across the spectrum. It is concentrated into a set of sharp ... Read more
A large external store is to be carried under an aircraft wing on a pylon. The airframe manufacturer runs its buffet analysis and hands the store vendor an acceleration power spectral density at the pylon/wing interface. The vendor applies that PSD as base excitation to a finite element model of the pylon and store, usually ... Read more
Nearly every random vibration fatigue course teaches that the response of a single-degree-of-freedom system to broadband base excitation has Rayleigh-distributed peaks. The claim is repeated so often that it has hardened into doctrine. It is a good approximation for the largest peaks, which is why it has survived. But it is quantitatively wrong for the ... Read more
A random vibration analysis in a finite element code will happily report an RMS von Mises stress, or an RMS resultant shear force in a bolt, and the analyst will happily multiply it by three and compare it against an allowable. Both quantities are useful. Both are also quietly non-Gaussian, and their spectral content is ... Read more
A question that comes up whenever a laboratory is planning its first large electrodynamic shaker installation: the specification and the quotations keep referring to a seismic mass, or a reaction mass, or an inertia block. Is that a real, physically heavy object that has to be purchased and installed? Or is it a tuned-mass arrangement ... Read more
A question that comes up whenever a laboratory is planning its first large electrodynamic shaker installation: the specification and the quotations keep referring to a seismic mass, or a reaction mass, or an inertia block. Is that a real, physically heavy object that has to be purchased and installed? Or is it a tuned-mass arrangement ... Read more
The Problem Here is a problem that comes up more often than the standards would suggest. An engineer is deriving a random vibration maximum predicted environment (MPE) for launch vehicle components mounted very close to the engines. Accelerometer data from engine test stand and stack tests show significant broadband energy above 2000 Hz, with peaks ... Read more
Does Compression Grow Cracks? — Vibrationdata Vibrationdata Fracture Fatigue Stress intensity conventions At a stress ratio of R = −1, the choice between full-range and clipped ΔK is worth a factor of 2n in life. It is usually the largest single lever in a spectrum fatigue deck, and the conservatism runs the opposite direction ... Read more