JPL Tunable Shock Beam

jpl_shock2

jpl_shock3

jpl_shock5

The NASA/JPL Environmental Test Laboratory (ETL) developed and built a tunable beam shock test bench based on a design from Sandia National Laboratory many years ago. ETL has been using this test system successfully since October 2008.

The excitation is provided by a projectile driven by gas pressure.

The beam is used to achieve shock response spectrum (SRS) specifications, typically consisting of a ramp and a plateau in log-log format. The intersection between these two lines is referred to as the “knee frequency.” The beam span can be varied to meet a given knee frequency. The high frequency shock response is controlled by damping material.

The tunable-beam system is calibrated with a center-of-gravity (CG) mass and footprint model of the test article. The mass simulator is mounted in the test axis with the appropriate accelerometers installed as they would be for the testing the test article. Then the system is tuned by performing test runs until the data plots meet the requirement.

Finally, the test article is mounted to the tuned beam for the actual test.

See also:  JPL Tunable Beam

– Tom Irvine

Embraer E190 Landing Shock

EMBRAER_E190_(8373095236).jpg

e190

I recently flew as a passenger on a E190 similar to the one in the top image. The landing shock is shown in the bottom image. The data was recorded on a Slam Stick X, sampled at 400 samples/sec. The initial set of peaks have a frequency of about 0.9 Hz.

Matlab File: E190_landing_shock.mat

See also: Landing Shock

– Tom Irvine

A330-200 Landing Shock

a330_200

Image Courtesy of Justin Kane

I recently flew as a passenger on a A330-200 similar to the one in the image.  I used a Slam Stick X Vibration Data Logger to measure the landing shock, with the sensor mounted on the cabin floor.  The acceleration time histories for two axes are shown in the following figures.

a330_200_landing_fore_aft

a330_200_landing_vertical

The vertical axis response has several spectra between 0.5 and 2.0 Hz.

A330-200 Landing Shock Matlab file

See also:  Landing Shock

– Tom Irvine

A320 Landing Shock

a320_jb

I recently flew as a passenger on a A320 similar to the one in the image.  I used a Slam Stick X Vibration Data Logger to measure the landing shock, with the sensor mounted on the cabin floor.  The resulting acceleration time history is shown in the following two figures, longer and shorter views.  The main wheels touch down at the zero second mark. The nose wheels contact the runway about 3.5 seconds later.

logan1

logan2

The higher frequency energy between zero and 0.5 seconds consists of components in the 10 to 15 Hz frequency domain, likely representing structural modes.  The sample rate was 400 samples per second.

A320 Landing Shock Matlab file

See also: Landing Shock

– Tom Irvine

SRS Synthesis – New Option

wdecay

(Click here for better image of Matlab GUI screenshot)

I have added an “Exponential Decay” option to the wavelet synthesis function in the Matlab GUI package.  The goal is to synthesize a wavelet series that has a gradual overall exponential decay, somewhat similar to an actual pyrotechnic or seismic shock event. Here is an example.

The advantage of using wavelets as a basis is that each has zero net velocity and zero net displacement, as does a complete series.  These conditions are needed for both analysis and testing.

Note that some pyrotechnic SRS specifications begin at a natural frequency of 100 Hz. A good practice is to extrapolate the specification down to 10 Hz, especially if there are any vibration modes below 100 Hz.

The Vibrationdata Matlab GUI package is given at: Vibrationdata Matlab Signal Analysis Package

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I previously came up with a method which synthesized a series of damped sines to satisfy an SRS.  The damped sines where then decomposed into a wavelet series.  The previous method is still available.   See Webinar 27 – SRS Synthesis

– Tom Irvine

Shock and Vibration Severity Thresholds for Structures and Equipment

Launch vehicle and spacecraft equipment must withstand pyrotechnic shock from stage separation and other flight events.   Civil engineering structures and equipment must survive seismic events.  There are many other examples where military, automotive, telecommunication and other equipment must be designed and tested to meet shock requirements derived from field or flight data.  The likelihood that a structure or equipment piece will fail a shock environment is often expressed in terms of a severity threshold.  The threshold may be expressed in terms of a base input or response level.  The purpose of this paper is to compile severity acceleration and velocity thresholds for both equipment and buildings into a single document for comparison purposes.

Link:  shock_vibration_severity_thresholds.pdf

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See also:
Shock Severity Limits for Electronic Components
Dr. Howard Gaberson’s Papers
Stess-Velocity Relationship

– Tom Irvine

Hypersphere SRS

hsrs

Figure 1. El Centro Earthquake 1940, Peak Pseudo Velocity, 36.4 in/sec
 
Launch vehicle and spacecraft equipment must withstand pyrotechnic shock from stage separation and other flight events. Civil engineering structures and equipment must survive seismic events. There are many other examples where military, automotive, telecommunication and other equipment must be designed and tested to meet shock requirements derived from field or flight data. The specification is commonly given as a shock response spectrum (SRS). An SRS may be calculated for each orthogonal input axis, assuming the availability of triaxial accelerometer data. Each axis may thus have a separate specification. Alternatively, a maximum envelope can be drawn over the three SRS curves and then applied as a uniform specification to each orthogonal axis.

The uniform enveloping method, however, can underestimate the maximum resultant shock when all possible orthogonal axes sets are considered. The purpose of this paper is to introduce a hypersphere method to achieve a true maximum SRS.

Paper link:  hypersphere_SRS

The Matlab script for this calculation is included in the GUI package at:
Vibrationdata Matlab Signal Analysis Package

– Tom Irvine

 

European Space Agency Shock Handbook

clampband

LV/SC Clampband Release Test

Here is an excellent reference on shock testing and analysis:

European Cooperation for Space Standardization, Space engineering, Mechanical Shock Design and Verification Handbook, ECSS-E-HB-32-25A, 14 July 2015  download link

The Shock Handbook was covered in several presentations at the ESA 2nd Workshop on Spacecraft Shock Environment and Verification, 2015   presentation link

Tom’s Summary Slides

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See also:

IXV Separation Test Video

SRS Educational Animation

Vibrationdata Matlab Signal Analysis Package

Vibration Testing & Analysis Standard References

Pyrotechnic Shock

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– Tom Irvine

Energy Response Spectrum

My colleagues at Sandia National Laboratories have presented some conference papers recently on energy response spectrum.

Here is a paper that I wrote on this topic:  energy_response_spectrum.pdf

Also, I have added the energy response spectrum as an option for acceleration time histories to the Vibrationdata GUI package.

See also:
Shock Response Spectrum
David O. Smallwood Papers
Temporal Moments

More later …

Tom Irvine

Satisfy a Shock Response Spectrum with a Classical Pulse

A typical SRS specification has a ramp and plateau.  The best way to meet this specification type is with a complex oscillating pulse.

In some cases, a classical shock pulse may be used, but this will cause an over-test at certain frequencies.  Also, classical pulses tend to have positive and negative SRS curves which diverge at some frequencies.  The classical pulse would thus need to be applied in each direction of each axis.

The terminal sawtooth pulse, however, has positive and negative curves which have smaller divergence relative to other classical pulse types.

A function to calculate a classical pulse to meet an SRS has been added to the Vibrationdata GUI package.

Matlab script: Vibrationdata Signal Analysis Package

vibrationdata > Shock Response Spectrum >  Satisfy SRS with Classical Pulse

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See also:  SRS Synthesis

– Tom Irvine