Multi-axis Shock & Vibration Testing

Equipment must be designed and tested to withstand shock and vibration.  Ideally, all equipment would be tested on a shaker table with six-degree-of-freedom control (three translations and three rotations).  Such tables and control systems exist but are very expensive.  Furthermore, any multi-axis testing requires careful consideration of phase angles between the six degrees.

Another option is to test equipment on a triaxial table where the three translations are controlled, and the three rotational degrees are constrained to zero motion.  Testing on a biaxial table is yet another choice.

The most common test method, however, remains testing in each of three orthogonal axes, one axis at a time, on a single-axis shaker.  This is simplest and least expensive method.

The question arises “Should the acceleration level be increased for the case of single-axis testing?”

There is a tacit understanding that aerospace and military equipment test levels already have a sufficient uncertainty margin or safety factor so that the levels can be used without further increase.  In other words, the specifications are already intended for single-axis testing.  In many cases, a uniform level is used in each axis which is the maximum envelope of the maximum expected levels in the three axes plus some margin.

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The standards which address testing equipment for earthquakes take a different approach. The following descriptions are taken from five common standards.

Only KTA 2201.4 gives a scaling formula.  This is also the only standard from the five samples which may be freely downloaded.

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IEEE 344-2013  Standard for Seismic Qualification of Equipment for Nuclear Power Generating Stations

8.6.6 Multiaxis tests

Seismic ground motion occurs simultaneously in all directions in a random fashion. However, for test purposes, single-axis, biaxial, and triaxial tests are allowed. If single-axis or biaxial tests are used to simulate the 3D environment, they should be applied in a conservative manner to account for the absence of input motion in the other orthogonal direction(s). One factor to be considered is the 3D characteristics of the input motion. Other factors are the dynamic characteristics of the equipment, flexible or rigid, and the
degree of spatial cross-coupling response. Single and biaxial tests should be applied to produce adequate levels of excitation to equipment where cross coupling is significant and yet minimize the level of overtesting where the cross coupling is not significant.

* * *

KTA 2201.4   Design of Nuclear Power Plants against Seismic Events, Part 4: Components

This document may be freely downloaded: link

See paragraphs

5.3.3 Excitation Axes

5.5.2.5 Simultaneity of excitation directions

Simultaneous three-axis testing is preferred. But single-axis testing can be substituted by testing in each of three axes sequentially.

The standard shows, for example, that the uniform single-axis level should be the “square root of the sum of the squares” of the three orthogonal installation site levels.

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IEC 980 Recommended practices for seismic qualification of electrical equipment of the safety system for nuclear generating stations

6.2.9 Qualification test method

6.2.9.1 General

As is well known, seismic excitation occurs simultaneously in all directions in a random way. According to this point of view, the test input motion should consist of three mutually independent waveforms applied simultaneously along the three orthogonal axes of the equipment.

However, taking into account that three axial testing installations are rare and that triaxial testing is desirable when significant coupling exists simultaneously between the two preferred horizontal axis of the specimens, biaxial testing with multifrequency independent input motion in the horizontal and vertical direction is an acceptable test.

Tests shall be performed according to 6.3.2 and, in terms of total duration and fatigue induced, are intended to become conservative.

In some cases, single axis tests with multiple, or single frequency excitation are also acceptable methods of test if properly justified considering the effect of coupling between axes.

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Telcordia GR-63-CORE

Assumes single-axis testing.  The base input time history is specified in the standard.

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IEEE 693-2005 – IEEE Recommended Practice for Seismic Design of Substations

paragraph 4.9

The shaker table shall be biaxial with triaxial preferred.

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See also:

Seismic Test & Analysis Webinars

Hypersphere SRS

 

– Tom Irvine

Webinar Index

Here is a listing of the webinars and related materials.

Matlab script: Vibrationdata Signal Analysis Package

1. Natural Frequencies

2. Sine Vibration

3. Sine Sweep Vibration

4. Random Vibration

5. Fourier transforms

6. Leakage Error, Hanning Window

7. FFTs

8. Waterfall FFT

9. White Noise FFT

10. Sample Rate & Aliasing

11. Power Spectral Density

12. Power Spectral Density Functions of Measured Data

13. SDOF Response to Power Spectral Density Base Input

14. Synthesizing a Time History to Satisfy a PSD Specification

15. SDOF Response to Base Input in the Frequency Domain

16. Vibration Response Spectrum

17. SDOF Response to Applied Force

18. Force Vibration Response Spectrum

19. Digital Filtering

20. Digital Filtering, Part 2

21. Integration & Differentiation of Time Histories

22. Integration and Differentiation of Time Histories & Spectral Functions

23. Classical Shock Pulse

24. Seismic Shock

25. Pyrotechnic Shock

26. Pyrotechnic Shock, part 2

27. SRS Synthesis

28. Multi-degree-of-freedom SRS

29. Stress-Velocity Relationship

30. Rectangular Plate Shock & Vibration

31. Rectangular & Circular Plate Shock & Vibration

32. Electronic Circuit Board Fatigue

33. Rainflow Fatigue

34. Rainflow Fatigue for Continuous Beams

35. Using Fatigue to Compare Sine and Random Environments

36. Non-Gaussian Random Fatigue and Peak Response

37. Acoustic Fatigue

38. Electronic Circuit Board Fatigue Part 2

39. Sine-on-Random Vibration

40. Shock Fatigue

41. PSD Special Topics

42. Shock Special Topics

43. Two-degree-of-freedom System, Two-stage Isolation

44. Sine Filtering

45. Two-degree-of-freedom System with Rotation and Translation

46. Two-degree-of-freedom System with Multi-point Enforced Motion

47. Shock Response Spectrum Synthesis, Special Topics

Seismic Test & Analysis Webinars

Structural Dynamics Webinars

Circuit Board Shock & Vibration Analysis

HALT/HASS for Product Reliability

More later. . .

– Tom Irvine

Seismic Test & Analysis Webinars

This is a work-in-progress…

I am creating a series of webinars with Matlab exercises for seismic testing.

Here are the slides.

Telcordia Technologies Generic Requirements GR-63-CORE:  Bellcore_GR_63_Core.ppt
This unit contains an alternative waveform for VERTEQII.

CEI.IEC 980, Recommended practices for seismic qualification of electrical equipment of the safety system for nuclear generating stations:  CEI/IEC 980: 1989

IEEE Std 693-2005, Recommended Practice for Seismic Design of Substations: IEEE_693_sine_beat.pptx

IEEE Standard for Seismic Qualification of Equipment for Nuclear Power Generating
Stations: IEEE_std_344.ppt

Matlab script: Vibrationdata Signal Analysis Package

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See also:

Cummins Generator Seismic Shaker Test

Earthquake Conference

Seismic Shock

Webinar 47 – Shock Response Spectrum Synthesis, Special Topics

Seismic Peak Ground Acceleration

Some Earthquake Engineering Terminology

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– Tom Irvine

Shock & Vibration Courses, Trieste, Italy & Singapore

You are welcome to participate in any of the following courses.

I will teach shock & vibration course in Trieste, Italy on January 25-27, 2017.   Course Link

* * *

I will teach a shock & vibration finite element analysis course in Singapore, February 13-15, 2017.  Brochure Link

Thank you,
Tom Irvine

Structural Dynamics Webinars

This is a work-in-progress…

I am creating a series of webinars with Matlab exercises for structural dynamics and finite element analysis.

Here are the slides:

Unit 1  Basic FEA

Unit 2  Damping

Unit 3 Modal Analysis

Unit 4 Transfer Functions

Unit 5  Transient Analysis

Unit 6  Applied Force Response Analysis

Unit 7  Response to Seismic Base Mass Excitation

Unit 8  Response to Enforced Motion

Unit 9  Beam Bending FEA

Unit 10 Beam Bending FEA with Added Mass and Stiffness

Unit 11 Beam Bending FEA with Steady-State Sinusoidal Base Excitation

Unit 12 Beam Bending FEA with Sine Sweep Base Excitation

Unit 13  Beam Bending FEA with PSD Base Excitation

Unit 14  Beam Bending FEA Fatigue

Unit 15  Beam Bending FEA Shock

Matlab script: Vibrationdata Signal Analysis Package

* * *

See also:

Transfer Functions from Normal Modes

Convert Modal Damping to a Damping Coefficient Matrix

Beam Bending, Finite Element Analysis

* * *

– Tom Irvine

A320 Takeoff Vibration

8096965_ff12ee2c26_b

Figure 1.  A320 Takeoff

I recently flew as a passenger in an A320 similar to the aircraft shown in Figure 1.

a320_takeoff_alt

 

Figure 2.  Time History Plots

The takeoff vibration is shown in Figure 2 for the lateral and vertical axes.   The aircraft went airborne at 393 seconds.  The fore-aft axis is omitted since its level was lower.  The sensor was a Slam Stick X mounted on the cabin floor.

a320_takeoff_2

Figure 3.  PSD, Lateral Axis

a320_takeoff_3

Figure 4.  PSD, Vertical Axis

The PSD plots show some distinct spectral peaks which are most likely forcing frequencies, or possibly lightly-damped structural resonances.

Here is the time history data file:  takeoff_data

– Tom Irvine

LDS V-8900 Shaker Table

bk_shaker

Years ago, I had the opportunity to perform hands-on shaker table testing, mostly for avionics components.  That was a great learning experience.  Alas, that opportunity has gone, but I am still involved writing software and providing training for test engineers.

LDS was one of the shaker manufacturers during my testing days.  LDS has since merged with Bruel & Kjaer.

I met my colleague Joel Leifer at an engineering conference recently.  He informed me about a new LDS shaker model, which has a 4 inch displacement stroke.  This would be very useful for low frequency shock and vibration tests.  Videos and further information is given at:  Video Link

* * *

Joel wrote:

I am interested in working with the space community as they define test requirements as this could be the tool they have been looking for to do some of the harder tests. Any advice or guidance you could give will be greatly appreciated.

Thanks for your attention to this.

Regards,

Joel Leifer, PE
District Manager (Western Region) VTS
Bruel & Kjaer North America
8566 Van Ness Ct Unit 24F
Huntington Beach, CA 92646
Direct: 817 475-2329

So please contact Joel if you are interested in collaboration, etc.

I was going to ask Joel to provide a demo model for me so that I could do some science projects at my home, but my wife would never let me keep a shaker table in our garage :)

– Tom Irvine

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

Matlab Batch Process via Vibrationdata GUI

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

Hospital Vibration Environments for Medical Devices

 

vc

An engineer recently asked me to recommended a vibration test level for his device, which would typically be mounted on a hospital table, including surgical tables.

Well…  shock would almost certainly be worse than vibration, particularly if the device were somehow accidentally dropped on the floor.  Then there are transportation and shipping shock and vibration environments.

A logical approach would be to take some accelerometer measurements on hospital tables, but this is rather impractical for a number of reasons.  One is that there could be wide variation from one hospital to the next, depending on HVAC systems, vibration-inducing surgical devices, etc.

But my acquaintance insisted that he needed a vibration test level for hospital environments.  Well there are no ISO-type standards that specify hospital vibration that I am aware of.  So I made an innovation as shown in the following edited response.

* * *

I am enclosing a paper, which has levels in terms of one-third octave velocity spectra. These are intended as “not to exceed” levels for floor vibration, for both equipment and people.

So here is what I propose for hospitals… Start with the Workshop level in Figure 1 of the paper. This is the highest curve in the family of curves shown.

Assume that the Hospital level would be the same Workshop level, which is conservative for our approach. Now the device may be mounted on a table which amplifies the floor vibration at least at certain frequencies. So add a conservative 12 dB margin as a goal.

The coordinates are shown in the following table.

Freq (Hz) Nominal
Accel (G^2/Hz)
Nominal +12 dB
Accel (G^2/Hz)
4 2e-05 3.2e-04
8 1e-05 1.6e-04
80 1e-04 1.6e-03

The nominal PSD is 0.0634 GRMS overall.

The nominal plus 12 dB is 0.25 GRMS overall.

Power and monitor the device during the following test steps.

Start the test at the nominal level (after the shaker equalization) for some TBD length of time.

Then increase the level in 3 dB increments will the same dwell time as the nominal level.

The goal is the plus 12 dB level.

If the component passes the plus 12 dB, then you are successfully done. If the component fails at a lower level, then we might need to “sharpen the pencil” on the input level, or make design modifications.

The innovation is that we are using a widely-accepted, “not to exceed,” floor vibration level as a basis for deriving a component test level.

* * *

Tom Irvine