Assume that input and response acceleration PSDs have been measured. The corresponding displacement PSDs can be readily calculated. But the relative displacement cannot be calculated accurately because the phase angles are discarded in the PSD calculation. The best method is to perform the relative displacement… Read more
Rainflow fatigue cycles can be easily calculated for a single-degree-of-freedom subjected to a sine or sine sweep base input. The reason is that each pair of consecutive positive and negative response peaks forms a half-cycle. The relative fatigue damage can then be calculated from the… Read more
Method 1 Secondary & Component loads can be specified in the form of a Mass-Acceleration curve as described in: NASA-STD-5002, Paragraphs 4.2.1.2, 5.3.2, 5.4. The Mass-Acceleration curve can be derived from measured flight or test data, or analytical loads. Care must be taken to avoid… Read more
There is a need to derive a PSD envelope for nonstationary acceleration time histories, including launch vehicle data, which may be similar to that shown in the above figure. A PSD can be derived using rainflow fatigue cycle counting along with a Miners-type relative fatigue… Read more
Engineers prepare the MESSENGER spacecraft for a vibration test at The Johns Hopkins University Applied Physics Laboratory, Laurel, Md., where the Mercury-bound NASA spacecraft was designed and built. * * * Introduction Flight configured spacecraft are subjected to base input vibration tests for certain programs.… Read more
The scripts in integrate_th.zip integrate an acceleration time history to a velocity time history and/or a velocity time history to a displacement time history. The main script is: integrate_th.m The remaining scripts are supporting functions. Integrating acceleration to velocity typically causes a spurious offset in… Read more
These Matlab scripts calculate the steady-state response of a single-degree-of-freedom (SDOF) system to a sinusoidal force or base excitation: steady.zip – Tom Irvine
The Iwate-Miyagi Nairiku earthquake struck northeast Honshu, Japan, on 14 June 2008. This earthquake had a moment magnitude Mw 6.9 according to the USGS. The peak ground acceleration (PGA) had a maximum vector sum (3 component) value of 4278 cm/sec^2 (4.36 G). This is the highest ever recorded… Read more
Shock and vibration test specifications for avionics and military equipment have almost always been specified in terms of acceleration. The main reason is that acceleration can easily be measured by accelerometers. Velocity sensors are also available but are less common. Gaberson, Chamblers et al, claim… Read more
Introduction Neither shock nor vibration response should be used to cover rigid-body acceleration because material limits depend on the strain rate and on the duration of the load. Furthermore, static deflection shapes differ from dynamic mode shapes. Material Stress Limits The following is an excerpt… Read more