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… Read more
Skyscraper design was once governed almost entirely by strength — could the frame carry gravity, wind and seismic loads with adequate margin? For the new generation of ultra-slender “pencil towers,” the governing requirement has shifted to something more subtle: occupant comfort. A building can be… Read more
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
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
Structural dynamics systems can be represented in terms of mass, damping and stiffness matrices. Each of these matrices may be coupled depending on the model complexity, degrees-of-freedom, etc. The mass and stiffness matrices in the assembled equation of motion may be uncoupled using the normal… Read more
Honeycomb sandwich structures are designed to have a high stiffness-to-mass ratio. The stiff, strong face sheets carry the bending loads, while the core resists shear loads. The face sheets are typically made from aluminum or carbon fiber with epoxy resin. The honeycomb core material is… Read more
The punch file method may be used to extract the mass & stiffness matrices from Nastran models. The format is awkward since zero terms are not stored. Also the matrices are assumed to be symmetric, and the upper triangular portion above the diagonal is not… Read more
Aircraft and launch vehicles behave as unconstrained systems in flight, with six rigid-body modes. These vehicles may be “trimmed” using aerodynamic control surfaces and thrust vector control to prevent rotation about the vehicle center-of-gravity (CG). There is a need to calculate the vehicle’s displacement response… Read more
Ideally, a modal test on a structure would be performed with completely free boundary conditions. This configuration can be approximated by mounting the structure on compliant air cushions, or by suspending it with elastic cords, so that the mounted natural frequency is much smaller that… Read more
A function for calculating the natural frequencies and mode shapes for an elastic beam undergoing rotation is given in: Matlab script: Vibrationdata Signal Analysis Package The function can be accessed via: >> vibrationdata > Miscellaneous > Structural Dynamics > Beam Bending FEA > Rotating Beam,… Read more