Tom Irvine The Modal Truncation Problem Any mode-based dynamic analysis—whether a frequency response sweep, random vibration PSD solve, response spectrum analysis, or transient modal superposition—retains only a finite number of modes. In a finite element model with $N$ degrees of freedom (DOFs), there are technically $N$ natural… Read more
A recurring question in random vibration analysis is: How do we determine the relative displacement between two points when the structure is subjected to a Power Spectral Density (PSD) excitation? This quantity is often far more critical than the absolute response. Connector pin engagement, cable… Read more
VibrationData Blog | Structural Materials If you have ever looked at a stress-strain curve for mild (low-carbon) steel and wondered why it has that distinctive “flat shelf” between the upper and lower yield points, you are not alone. That plateau is one of the most… Read more
Estimating structural strain in a rubber vibration isolator from acceleration measurements is a challenging problem because elastomers behave very differently from metals. Their stiffness is strongly frequency dependent, highly temperature sensitive, and inherently viscoelastic. Nevertheless, useful engineering estimates can often be obtained from vibration measurements… Read more
PORON® is a commercial polyurethane foam, Rogers Corporation Introduction Microcellular Urethane (MCU) mounts are a highly capable candidate for this application and are widely utilized for military electronics isolation. Several aerospace and defense manufacturers offer products with extensive MIL-STD-810 qualification data and field history in tactical… Read more
Introduction Many engineers are proficient at measuring strain and converting strain-gage output into stress using the elastic modulus and gage factor. Fewer have worked through the next step: reconstructing the applied forces and moments that produced those strains. This is the load identification problem, and… Read more
Dynamic Versus Static Modulus: Why ASTM E1876 and ASTM E111 Don’t Always Agree Engineers frequently encounter situations where material properties reported in handbooks don’t quite match experimental data. A classic example occurs when comparing elastic modulus values obtained from different test standards. When evaluating stiffness,… Read more
San Mateo Bridge, Highrise Section, San Francisco Bay Area UC Berkeley Professor Ed Wilson raised a pointed concern about the Response Spectrum Method (RSM) that structural engineers should take seriously. In his 2015 note, he described the San Mateo Bridge retrofit as a revealing case… Read more
At John Wayne Airport (SNA) in Orange County, California, noise abatement on departures is managed through a combination of specialized Noise Abatement Departure Procedures (NADPs), strict regulatory limits, and operational constraints. Because SNA is located in a densely populated, noise-sensitive residential area, the airport enforces… Read more
NASA’s Nancy Grace Roman Space Telescope has passed its final major prelaunch environmental tests at Goddard Space Flight Center. The test sequence included electromagnetic compatibility, vibration, and acoustic testing. These are the kinds of tests that determine whether a large precision observatory can survive launch… Read more