An Indirect Method for Converting a Shock Response Spectrum Specification to a New Q Value

Posted on January 23, 2018 by tomirvine999

Aerospace pyrotechnic shock response spectrum (SRS) specifications are almost always given with an amplification factor Q=10
Corresponding time history waveforms for the base input acceleration are almost never given with the specifications
Users are allowed to synthesize their own waveforms to satisfy the SRS for analysis & test purposes
Some shock analysis methods use the SRS directly without time history synthesis, such as modal combination methods
The following method enables an SRS specification to be converted to a new Q value for engineering purposes

Slides: 150_SRS_specification_new_Q.pptx

– Tom Irvine

Matlab Batch Process via Vibrationdata GUI

Posted on August 30, 2016 by tomirvine999

batch

I have added batch processing as an option for time histories as shown above. I will add more options in upcoming revisions. The Vibrationdata Matlab GUI package is given at: Vibrationdata Matlab Signal Analysis Package

Here are some sample input files for practicing batch processing: batch_sample.mat

Tom Irvine

SDOF Response to Sine or Sine Sweep Base Input, Rainflow

Posted on August 5, 2014 by tomirvine999

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 rainflow cycles.

Here are Matlab scripts for performing the rainflow and damage calculations. rainflow_sine.zip

rainflow_sine.m is for the case where the natural frequency is known.

rainflow_sine_fds.m gives the fatigue damage spectrum for a family of natural frequencies.

The remaining scripts are supporting functions.

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See also:

Rainflow Cycle Counting

ramp_invariant_base.pdf

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– Tom Irvine

Webinar 22 – Integration and Differentiation of Time Histories & Spectral Functions

Posted on July 10, 2014 by tomirvine999

PowerPoint File:

webinar_22_integration_part_2.ppt

Audio/Visual File:

NESC Academy Integration and Differentiation – Recommend viewing in Firefox with Sliverlight Plugin

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ASCII Text Data Files:

navmat_spec.psd

two_wheeled_trailer_vertical.txt

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Matlab script: Vibrationdata Signal Analysis Package

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Python version:
Unit_22_integration_part_2_python.ppt

Python Signal Analysis Package GUI

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See also:

Vibrationdata Webinars

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– Tom Irvine

Pierson-Moskowitz Spectrum

Posted on July 10, 2014 by tomirvine999

The Pierson–Moskowitz (PM) spectra is an empirical relationship that defines the distribution of energy with frequency within the ocean. The result is given as a wave height power spectra density.

Developed in 1964, the PM spectrum is one of the simplest descriptions for the energy distribution. It assumes that if the wind blows steadily for a long time over a large area, then the waves will eventually reach a point of equilibrium with the wind.

This is known as a fully developed sea. Pierson and Moskowitz developed their spectrum from measurements in the North Atlantic during 1964.

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Here is a paper: wave_height.pdf

Here is a Matlab script: ocean_wave_PSD.m

– Tom Irvine

Fatigue Damage for a Stress Response PSD

Posted on July 3, 2014 by tomirvine999

Here is a paper.

Estimating Fatigue Damage from Stress Power Spectral Density Functions: estimate_fatigue_psd.pdf

Note that the duration for the example in the paper was 600 sec.

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This following Matlab program calculates the cumulative rainflow fatigue damage for an input stress PSD using the following wideband methods:

1. Wirsching & Light
2. Ortiz & Chen
3. Lutes & Larsen, Single-Moment
4. Benasciutti & Tovo, alpha 0.75
5. Dirlik
6. Zhao & Baker

Reference:

Random Vibrations: Theory and Practice (Dover Books on Physics)

The stress PSD and the fatigue strength coefficient must have consistent stress units.

The input PSD must have two columns: freq(Hz) & stress(unit^2/Hz)

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Main scripts:

stress_psd_fatigue.zip

Vibrationdata Signal Analysis Package

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The following values are “For Reference Only.”

m = fatigue exponent
A = fatigue strength coefficient

Aluminum 6061-T6 with zero mean stress

m=9.25
A=9.7724e+17 (ksi^9.25)
A=5.5757e+25 (MPa^9.25)

Butt-welded Steel Joints

m=3.5
A=1.255e+11 (ksi^3.5)
A=1.080e+14 (MPa^3.5)

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See also:

Rainflow Fatigue

Mrsnik, Janko Slavic, Boltezar, Frequency-domain methods for a vibration-fatigue-life estimation – Application to real data: mrsnik_article_vib_fatigue.pdf

Spectral Moments Notes

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– Tom Irvine