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The following is based on a description from Dennis Kern, NASA/JPL, which Isam Yunis, NASA Langley, referenced in a shock presentation.

Shocks travel through structure as a combination of propagating waves and vibration response, with the vibration response concentrated more in lower frequencies and the waves more in higher frequencies.

The propagating waves will not couple dynamically with the structure. However, they are strongly affected by changes in the impedance of the structure, such as at joints.

Impedance mismatch can significantly attenuate the shock wave, but the wave will bounce off the impedance boundary, causing both amplification and cancellations as a function of phasing.

Launch vehicle and spacecraft systems may be subject to ground tests where their pyrotechnic separation devices are activated in order to measure the resulting shock response. Mass simulators may be used in place of electronic flight components. The mass simulators tend to have a higher impedance than the flight components.

For a mass simulator to properly represent a flight component in the high frequency domain, the impedance at the interface must match with the flight configuration impedance. This includes the number of fasteners, interface bolt patterns, and the structural materials at the interface.

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See also: Pyrotechnic Shock Joint Attenuation

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- Tom Irvine

I will present two tutorials sessions at the Shock & Vibration Symposium.

Here are the slides:

Shock Response Spectra & Time History Synthesis
SAVE_conference_2014_Irvine_shock.pptx

Rainflow Cycle Counting for Random Vibration Fatigue Analysis
SAVE_conference_2014_Irvine_fatigue_revA.pptx

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Here is the Matlab GUI multi-function signal analysis package: vibrationdata.zip

The main script is: vibrationdata.m

The remaining scripts are supporting functions.

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- Tom Irvine

PowerPoint Slides:

webinar_36_fatigue_nonGaussian_random.pptx

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References:

Rainflow Fatigue Posts

Generating Random Time Histories with User-specified Skewness & Kurtosis

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Matlab script: Vibrationdata Signal Analysis Package

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See also: Vibrationdata Webinars

Thank you,

Tom Irvine

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I have added a function for generating random time histories with user-specified skewness & kurtosis to the Vibrationdata Matlab Signal Package posted at:

Vibrationdata Matlab Signal Analysis Package

The function includes an option for band-limiting the signal via lowpass filtering.

Tom Irvine

PowerPoint Slides:

webinar_35_fatigue_sine_random.pptx

Audio/Visual File:

Using_Fatigue_to_Compare_Sine_and_Random_Environments.wmv

YouTube version

* * *

References:

Steinberg’s Vibration Analysis for Electronic Equipment

Rainflow Fatigue Posts

* * *

Matlab script: Vibrationdata Signal Analysis Package

* * *

See also: Vibrationdata Webinars

Thank you,

Tom Irvine

PowerPoint Slides:

webinar_34_rainflow_fatigue_beam.pptx

Audio/Visual File:

Rainflow_Cycle_Counting_for_Continuous_Beams.wmv

YouTube version

* * *

References:

Steinberg’s Vibration Analysis for Electronic Equipment

Rainflow Fatigue Posts

* * *

Matlab script: Vibrationdata Signal Analysis Package

* * *

See also: Vibrationdata Webinars

Thank you,

Tom Irvine

NASA-STD-5019, Fracture Control Requirement for Spaceflight Hardware (excerpt)

4.1.1.12 Low-Risk Part

This section addresses parts that can be classified non-fracture critical because of large structural margins and other considerations that make failure from a pre-existing flaw extremely unlikely.

a. For a part to be classified low risk, it shall be constructed from a commercially available material procured to an aerospace standard or equivalent.

b. Aluminum parts shall not be loaded in the short transverse direction if this dimension is greater than 7.62 cm (3 in).

c. A part whose failure directly results in a catastrophic hazard shall be excluded from being classified low risk, except when the total (unconcentrated) stresses in the part at limit load are less than 30 percent of the ultimate strength for the material used and requirements (1) through (3) and either (4) or (5) are met.

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Thus fatigue and fracture analyses are not required for parts with peak stress less than 0.3*ultimate.

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- Tom Irvine

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