NASTRAN FRF & Modal Transient Analysis
SOL 111 or SOL 112? Why a Synthesized Time History Earns Its Keep A base input power spectral density arrives on a specification sheet, and the reflex is automatic. Build the model, run SOL 111, read the RMS stress, apply a three-sigma factor, and compare… Read more
Spring Surge
Every introductory vibration course begins by drawing a mass on a spring and declaring that the spring has stiffness but no mass. That assumption is the reason the single-degree-of-freedom system has exactly one natural frequency instead of infinitely many. It is an excellent assumption almost… Read more
The Round House on Highway 101
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,… Read more
The Barrett Vibroacoustics Scaling Method
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… Read more
Rossiter Tones in Aircraft Cavities
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… Read more
Aircraft External Store Buffet Random Vibration Modeling
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… Read more
Random Vibration Response Peak Distribution
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… Read more
Quadratic Failure Metrics in Random Vibration
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.… Read more
Seismic Mass under Large Shaker
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… Read more