IEC 60068 2 57 Edition 3 0 2013 04 INTERNATIONAL STANDARD NORME INTERNATIONALE Environmental testing – Part 2 57 Tests – Test Ff Vibration – Time history and sine beat method Essais d''''environnement –[.]
General
This test aims to assess mechanical weakness and performance degradation, using the findings alongside relevant specifications to determine the acceptability of a specimen Additionally, it can be utilized to demonstrate the mechanical robustness of specimens and to analyze their dynamic behavior.
The extent to which a specimen has to function during vibration or merely to survive conditions of vibration shall be stated in the relevant specification
Procedures are described for conducting the test and for the measurement of the vibration at given points The requirements for the vibration motion and for the choice of severities
(including frequency range, required response spectrum, number of high-stress cycles and number of time-histories, sine-beat cycles and number of sine beats) are also detailed
Vibration testing requires careful engineering judgment, and both suppliers and purchasers must recognize this necessity The author of the specification is responsible for choosing the appropriate testing procedures and severity levels that align with the specimen and its intended application.
For the purpose of this test, the specimen is always fastened directly to the vibration table with its real mounting or a fixture
To enhance the usability of this standard, the main body includes references directing readers to Annex A, which also cites clause numbers from the main text These clauses offer detailed insights into the relationship between sine beats of displacement, velocity, and acceleration.
Requirements for testing
Section 4.3 outlines the requirements for vibration response investigations, while Section 4.4 specifies the criteria for time-history testing Sine-beat testing requirements are detailed in Section 4.5, and Section 4.6 addresses the mounting procedures for testing Additionally, Table 1 provides a comparison of the tolerances relevant to vibration response investigations, time-history testing, and sine-beat testing.
Vibration response investigation Sine-beat and time-history testing Signal tolerance 5 % (see 4.3.5.3) of basic motion Not applicable
Vibration at reference point ±15 % (see 4.3.6 a)) of basic motion
Vibration at check points ±25 % up to 500 Hz (see 4.3.6 b)) of acceleration ±50 % above 500 Hz (see 4.3.6 b)) of acceleration
(for special cases see 4.3.3) 25 % (see 4.4.2)
The test frequency specifications are as follows: for frequencies up to 0.5 Hz, the tolerance is ±0.05 Hz; from 0.5 Hz to 5 Hz, it is ±10%; from 5 Hz to 100 Hz, the tolerance is ±0.5 Hz; and for frequencies above 100 Hz, it is ±0.5% Additionally, for predetermined tests (refer to section 4.5.3.2), the same tolerances apply For investigated tests (see section 4.5.3.3), the tolerance is ±2%.
Vibration response investigation
General
When prescribed by the relevant specification, the vibration response investigation shall (see
The vibration response investigation should be conducted with single axis excitation, following the guidelines of IEC 60068-2-6, particularly sections 4.3.2 to 4.3.9, to identify critical frequencies and, if necessary, the damping ratio Additionally, if deemed appropriate or specified by relevant standards, random vibration testing may be performed in accordance with IEC 60068-2-64.
Basic motion
The fundamental motion will be a sinusoidal function of time, ensuring that the specimen's fixing points on the vibration table, as specified, move in phase and along straight parallel lines, while adhering to the constraints outlined in section 4.3.3.
Transverse motion
The maximum vibration amplitude at check points in any axis perpendicular to the specified axis must not exceed 50% of the basic motion amplitude In specific instances, such as with small specimens, the allowable peak value of transverse motion may be restricted to 25% if dictated by the relevant specifications.
At certain frequencies or with large or heavy specimens, achieving specified values may be challenging In these instances, the relevant specification must indicate whether: a) any transverse motion exceeding the specified limits should be reported in the test report, or b) monitoring of transverse motion is not required.
Rotational motion
In cases where the spurious rotational motion of the vibration table is significant, a permissible level should be established and documented in the test report according to the relevant specifications.
Measuring points
The relevant specification shall state whether single point or multipoint control is to be used
When multipoint control is required by the relevant specification, it must clarify whether the average signal value at the checkpoints or the signal value at a specific point should be maintained at the designated level.
Achieving the required tolerances in section 4.3.6 b) can be challenging at certain frequencies or with large or heavy specimens In such instances, the relevant specification may allow for wider tolerances, or the test report should indicate the use of an alternative assessment method.
The signal tolerance measurement shall be carried out at the reference point at frequencies up to five times the test frequency
The signal tolerance as defined in 3.7 shall not exceed 5 % of the basic motion
In certain cases, achieving the desired signal tolerance may not be feasible; however, a tolerance level exceeding 5% is permissible if the control signal's test amplitude at the fundamental frequency is adjusted back to the specified value, potentially through the implementation of a tracking filter.
For large or complex specimens where signal tolerance values cannot be met across certain frequency ranges and using a tracking filter is impractical, it is unnecessary to restore the acceleration amplitude In such cases, the signal tolerance should be documented in the test report (refer to A.1.1).
The test report must specify the signal tolerance and the affected frequency range, as outlined in Clause 13, along with information on the use of a tracking filter.
Vibration amplitude tolerances
The motion along the designated axis at the check and reference points must match the specified values within defined tolerances, which account for instrument errors.
Tolerance on the control signal at the reference point shall be ±15 % of basic motion b) Check points
Tolerance at each check point: up to 500 Hz shall be ±25 % of acceleration; above 500 Hz: ±50 % of acceleration (see also 4.3.5.2).
Frequency tolerances
The tolerances on the critical frequencies shall be as follows:
− from 5 Hz to 50 Hz: ±1 Hz;
To compare critical frequencies before and after testing, the relevant specification must outline the comparison criteria The determination of critical frequencies is subject to specific frequency tolerances.
− from 5 Hz to 100 Hz: ±0,5 Hz;
Sweeping
Sweeping shall be continuous with the frequency changing exponentially with time at a rate not exceeding one octave per minute (see 3.20)
NOTE With a digital control system, it is not strictly correct to refer to the sweeping as being “continuous” but the difference is of no practical significance.
Damping ratio
The damping ratio of a specimen is typically assessed through vibration response analysis, which relies on the specific test apparatus and requires engineering judgment Alternative methods may be utilized if adequately justified in the test report.
Time-history testing
Basic motion
Time histories can be derived from two sources: a natural time history or a synthesized time history created by combining frequencies within a specified range When generating a synthesized time history, it is essential to ensure that it is produced with the appropriate resolution.
– not more than 1/12 octave bands when the specimen damping (= damping ratio) is lower than or equal to 2 %;
– not more than 1/6 octave bands when the specimen damping lies between 2 % and
– not more than 1/3 octave bands when the specimen damping is higher than or equal to
The value of the damping ratio (see 3.6) may be defined by the relevant specification or otherwise obtained (see 4.3.9) A value of 5 % is normally taken.
Transverse motion
The maximum peak value of acceleration or displacement at any check points on axes perpendicular to the specified axis must not exceed 25% of the designated peak value in the time history or in a sine beat, unless otherwise specified in the relevant documentation.
The recorded measurements need only cover the specified frequency range
At certain frequencies or with specimens that are large or have high mass, achieving the specified values may be challenging In these instances, the relevant specification must clarify whether: a) any transverse motion exceeding the stated limits should be included in the test report, or b) monitoring of transverse motion is not required.
Rotational motion
Tolerance zone for the required response spectrum
The tolerance zone to be applied to the required response spectrum shall be in a range between 0 % and +50 % as shown in Figure 4
A small percentage of individual points on the test response spectrum can fall outside the acceptable zone while still allowing the test to be deemed acceptable It is important to report the values of these points in the test report, as outlined in Clauses 13 and A.1.
In the range above 1/3 f2 (Figure A.1) a tolerance zone > 50 % is permitted
The test response spectrum shall be checked at least:
– in 1/12 octave bands if the specimen damping is lower than or equal to 2 %;
– in 1/6 octave bands if the specimen damping lies between 2 % and 10 % (general case);
– in 1/3 octave bands if the specimen damping is higher than or equal to 10 %
In some cases, the required response spectrum may have been so artificially shaped or broadened that the test response spectrum cannot be generated within this tolerance zone
The tolerances in the test specification may then require to be revised.
Frequency range
The signal from the reference point must not include frequencies higher than the test frequency range, except for those generated by the test facilities and specimen Additionally, any signal induced by the test facilities without the specimen should not surpass 20% of the maximum specified signal from the reference point If these criteria cannot be met, the resulting values must be documented in the test report.
Frequencies outside the frequency range shall not be taken into account when evaluating the test response spectrum.
Sine-beat testing
General description
The fundamental motion will follow a sine-beat function over time, ensuring that the specimen's fixing points on the vibration table, as specified by the relevant guidelines, move in phase and along straight parallel lines, while adhering to the constraints outlined in section 4.3.1.
4.3.2, 4.3.3 and 4.3.5 The sine-beat test applies only to single axis excitation (see Table 1 in
Vibration amplitude tolerances
The basic motion along the required axis at the check and reference points shall be equal to the specified values within the following tolerances These tolerances include instrument errors
Test frequency tolerances
The tolerances on the test frequency are as follows for the two types:
– from 5 Hz to 100 Hz: ±0,5 Hz;
The deviation of the test frequency from the critical frequency obtained by the vibration response investigation shall not exceed ±2 %.
Transverse motion
Mounting
The specimen shall be mounted in accordance with IEC 60068-2-6, wherever that standard makes reference to IEC 60068-2-47
If a specimen is normally mounted on isolators, but it is necessary to test without them, the specified excitation level shall be modified to take this into account
When testing a specimen in its actual mounting, if the specified excitation is intended for testing without real mounting, the excitation level must be adjusted accordingly, as outlined in Clause A.2 of IEC 60068-2-47:2005.
The influence of connections, cables, piping, etc., shall be taken into account when mounting the specimen
The normal “in service” mounting structure of the specimen should be included in the test
The response spectrum and time history for exciting the actual mounting structure must differ from those utilized for the fixture or specimen.
The orientation and mounting of the specimen during testing must adhere to the relevant specifications, as this is the sole condition under which the specimen meets the standard requirements Exceptions may only be made if sufficient justification is provided, such as demonstrating that gravity does not affect the specimen's behavior.
General
The test severity for time history is determined by a combination of the following parameters:
– number and duration of time histories;
– number of high stress cycles (if applicable)
The relevant specification shall state the values for each parameter on the basis of the information given in 5.2 to 5.5
The test severity for sine beat is determined by the combination of the following parameters:
– number of cycles in the sine beat;
The relevant specification shall state the values for each parameter on the basis of the information given in 5.6.
Time history
The test frequencies and the test frequency range are obtained as shown in 5.3.
Test frequency range
The test frequency range shall be given in the relevant specification by selecting a lower frequency from 0,1 Hz, 1 Hz, 5 Hz, 10 Hz, 55 Hz, 100 Hz and an upper frequency from 10 Hz,
20 Hz, 35 Hz, 55 Hz, 100 Hz, 150 Hz, 300 Hz, 500 Hz, 2 000 Hz The recommended ranges are shown in Table 2
Table 2 – Recommended test frequency ranges
Recommended test frequency ranges from f 1 to f 2
55 to 2 000 a These ranges are not in the recommended ranges of IEC 60068-2-6.
Required response spectrum
The specification must detail the required response spectrum's level and shape for testing, including the zero period acceleration value Additionally, it should specify the axes of the specimen along which the spectra are applied, particularly when these axes differ.
Guidance for the development of a required response spectrum in the situation where environmental conditions are not well known is provided in A.1.3.
Number and duration of time-histories
Number of time histories
The relevant specification shall specify the number of time histories to be applied to the specimen and the axes concerned
Unless otherwise specified, the number of time histories to be applied for each test axis and for each time-history level shall be selected from the following series: 1, 2, 5, 10, 20, 50
When more than one time-history level is used, testing shall always begin with the lowest and continue with higher levels Each time-history shall be followed by a pause.
Time-history duration
The relevant specification shall state the duration of each time history for which recommended values in seconds are given by the following series: 1, 2, 5, 10, 20, 50
If the period of the test sample is known or can be calculated, the duration of each time- history shall be not less than 3 or 5 times the period
If specified by the relevant specification 3 s duration may be used
NOTE 2 Typical duration for earthquakes is 30 s.
Duration of the strong part of the time history
In certain instances, specifications may dictate that the strong part of the time history must constitute a specific percentage of the total duration If not specified, and unless restricted by the requirements of section 5.6, the strong part should be chosen from designated percentages of the total duration.
The selected value shall be reported in the test report.
Number of high stress cycles
The relevant specification may state a specified value and the number of high stress cycles leading to values of stress greater than a specified value (see A.1.4)
The selection of high stress cycles, unless specified otherwise, should follow the series of 4, 8, 16, and 32 It is important to ensure that the alternate positive and negative cycles are distributed approximately equally, as illustrated in Figure 5.
The acceleration value during high stress cycles must be represented as a percentage of the peak required response spectrum value at critical frequencies within the strong segment of the required response spectrum (RRS) The chosen acceleration value should be selected from the specified options.
Specified value (as % of the RRS) 50 %, 70 % (preferred), 90 %
A cc el er at ion ( % )
Zone of negative peaks exceeding the specified value
Zone of positive peaks exceeding the specified value
Figure 5 – Typical response of an oscillator excited by a specific time history during a test
Sine-beat test level
General
The relevant specification shall state the value of test level (displacement, velocity or acceleration or all) for each axis (see also A.2.1)
Below the crossover frequency, peak values are defined at constant displacement, while above this frequency, they are defined at constant acceleration Recommended values can be found in Tables 3, 4, and 5, as well as Figures 6, 7, and 8, corresponding to various selected crossover frequencies.
Table 3 – Recommended test levels with a crossover frequency of 0,8 Hz (see Figure 6)
Displacement levels below the crossover frequency mm
Acceleration levels above the crossover frequency m/s 2
5 NOTE 1 All levels quoted are peak values in a sine beat
NOTE 2 For those wishing to continue giving values of acceleration in “g n ”, the value of 10 m/s 2 is conventionally ascribed to “g n ” (see 3.9) for the purpose of this standard
A cc el er at ion ( m /s 2 )
Figure 6 – Recommended test level with crossover frequency at 0,8 Hz
Table 4 – Recommended test levels with a crossover frequency of 1,6 Hz (see Figure 7)
Displacement levels below the crossover frequency mm
Acceleration levels above the crossover frequency m/s 2
20 NOTE 1 All values quoted are peak values in a sine beat
NOTE 2 For those wishing to continue giving values of acceleration in “g n ”, the value of 10 m/s 2 is conventionally ascribed to “g n ” (see 3.9) for the purpose of this standard
A cc el er at ion ( m /s 2 )
Figure 7 – Recommended test level with crossover frequency at 1,6 Hz
Table 5 – Recommended test levels with a crossover frequency of 8 Hz (see Figure 8)
Displacement levels below the crossover frequency mm
Acceleration levels above the crossover frequency m/s 2 0,4
50 NOTE 1 All values quoted are peak values in a sine beat
NOTE 2 For those wishing to continue giving values of acceleration in “g n ”, the value of 10 m/s 2 is conventionally ascribed to “g n ” (see 3.9) for the purpose of this standard
A cc el er at ion ( m /s 2 )
Figure 8 – Recommended test level with crossover frequency at 8 Hz
In cases where the crossover frequencies outlined in this subclause are unsuitable, the relevant specification may link peak values of displacement and acceleration, resulting in an alternative crossover frequency Additionally, for specialized applications, multiple crossover frequencies may be defined.
Test frequency determination
The test frequencies to be used are the critical frequencies as determined by the vibration response investigation, any predetermined frequencies or both
If no critical frequencies are found during the vibration response investigation and the specifications do not specify a method for selecting test frequencies, testing should be conducted at frequencies in increments no greater than one-half octave across the chosen test frequency range outlined in section 5.3.
Sine-beat test wave
The sine-beat test wave is determined by the test frequency and the number of cycles within the sine beat (see Figure 2) in accordance with 5.6.5 and 5.6.6.
Number of cycles in the sine beat
The number of cycles in the sine beat shall be prescribed by the relevant specification from the following values (see Figure 2):
A preferred value of five cycles is recommended, as it strikes a balance between accommodating the uncertainties of the critical frequency and achieving a high response value, based on practical experience.
A m pl ifi cat ion fa ct or
Figure 9 – Amplification factors of different sine beats, continuous sine and a typical natural time-history
Modulating frequency
The modulating frequency is derived from the test frequency and the number of cycles in the sine beat (see also A.2.2.1).
Number of sine beats
The number of sine beats shall be prescribed by the relevant specification from the following series (see Figure 1):
High-stress low-cycle fatigue effects
The relevant specification may prescribe the required number of high-stress cycles each resulting in a greater than specified value of excitation/stress (see A.2.4)
The relevant specification may call for preconditioning as, for example, conditioning the specimen with defined temperature and/or humidity
The specimen shall be submitted to the visual, dimensional and functional checks prescribed by the relevant specification
General
The specimen must be excited according to the procedures outlined in sections 8.2, 8.3, and 8.4 across the three preferred testing axes, unless specified otherwise The sequence of testing along these axes is flexible, except when dictated by the relevant specification.
When specified, the control of the designated test level must include a maximum limit on the driving force applied to the vibration table, with the method for force limitation also outlined in the relevant specification.
Vibration response investigation
The test frequency range must be examined as specified to analyze the specimen's behavior under vibration This investigation utilizes a sinusoidal wave within the prescribed frequency range and test level Typically, a logarithmic sweep rate of no more than one octave per minute is employed, although it can be reduced for more accurate response characterization It is important to avoid excessive dwell during the testing process.
The excitation peak value must be chosen to ensure that the specimen's response is lower than that observed during time-history or sine-beat testing, while still being high enough to identify critical frequencies.
Alternatively, the investigation can be conducted with random vibration as specified in 8.2 of
For effective sine-beat testing, it is crucial to select a sine-beat frequency that closely matches the specimen's resonance frequency In cases where the specimen exhibits nonlinear behavior, such as surge arresters made of porcelain components connected by rubber pads, it is advisable to perform the analysis using high-level sine vibrations.
However, if the investigation is only to be used as a dynamic characterization of the specimen, then an investigation using random vibration is suitable
The specimen must be operational during the investigation as specified If mechanical vibration characteristics cannot be evaluated while the specimen is functioning, a separate vibration response investigation will be conducted with the specimen non-operational This phase will involve examining the specimen to identify critical frequencies, which will be documented in the test report.
In specific situations, the relevant specification may require an additional vibration response investigation after completing time-history or sine-beat testing to compare critical frequencies before and after testing The specification will outline the necessary actions if any frequency changes occur It is crucial that both vibration response investigations are conducted consistently and at the same test level For further details on vibration investigations, changes in critical frequencies (CCF), and the criteria for pass/fail based on CCF, please refer to the relevant documentation.
Time-history testing
For time-history testing, severity values are specified in accordance with Clause 5 It is essential to include a pause between consecutive time histories to prevent significant superposition of the specimen's response motion Additionally, the relevant specification will indicate whether single-axis, biaxial, or triaxial testing is necessary.
Sine-beat testing
For sine-beat testing, severity values are specified in Clause 5, and a pause must be included between consecutive sine beats to prevent significant superposition of the specimen's response motion The test report must include a recording of the actual control signal at the reference point, accounting for any filters used Additionally, the relevant specification will indicate whether single-axis or biaxial testing is necessary.
Multi-axis testing
General
The following applies to both time-history and sine beat testing.
Single axis testing
Single axis testing is generally favored unless stated otherwise, and it is conducted sequentially along each designated testing axis The sequence of testing along these axes is flexible, except when dictated by the applicable specifications.
Biaxial testing
In each series of tests, two time histories or sine beats are applied simultaneously along two preferred testing axes of the specimen If the time histories are dependent, each test is conducted twice: first with a relative phase angle of 0° and then with 180° It is important to note that biaxial testing is not advisable for the sine-beat method.
When biaxial testing is required, it can also be conducted using a single inclined axis setup; however, the movements along the two axes will be interdependent It is essential to adjust the test response spectrum for each axis to encompass the necessary response spectrum for that specific axis.
Triaxial testing
In each series of tests, time histories are applied simultaneously across all three preferred testing axes, making single-axis or biaxial installations unsuitable Additionally, triaxial testing is not appropriate for the sine-beat method.
When prescribed by the relevant specification, the specimen shall function during a prescribed number of time-history or sine-beat tests and its performance shall be checked
When specified, it is essential to allow a period after testing and before final measurements for the specimen to reach the same conditions, such as temperature, as during the initial measurements.
The specimen shall be submitted to the visual, dimensional and functional checks prescribed by the relevant specification
The relevant specification shall provide the criteria upon which the acceptance or rejection of the specimen is to be based
12 Information to be given in the relevant specification
When including these tests in a relevant specification, it is essential to provide specific details, especially for the items marked with an asterisk (*), as this information is always mandatory.
The article outlines essential testing parameters, including fixing points (4.3.2 and 4.5.1), transverse motion (4.3.3 and 4.4.2), and rotational motion (4.3.4 and 4.4.3) It emphasizes the importance of measuring points (4.3.5) and signal tolerance (4.3.5.3), along with vibration amplitude tolerances (4.3.6 and 4.5.2) and damping ratio (4.3.9) The mounting of the specimen (4.6) and the required response spectrum (5.4) are also critical, as are the number and duration of time histories (5.5.1 and 5.5.2) The article discusses the duration of the strong part of the time history (5.5.3) and the number of high-stress cycles (5.5.4), alongside test levels (5.6) and sine beat parameters (5.6.4 to 5.6.6) Preconditioning (6) and initial measurements (7) are vital, as are preferred testing axes (8.1) and driving force limitations (8.1) Vibration response investigation (8.2) and performance checks (7, 9, 11) are highlighted, along with single, biaxial, or triaxial testing (8.5) and intermediate measurements (9) Finally, recovery (10) and final measurements (11) are crucial for comprehensive testing outcomes.
13 Information to be given in the test report
The test report must include essential information such as the customer's name and address, the test laboratory's details, and the report identification with the date of issue and a unique number It should specify the test dates, purpose (e.g., development or qualification), and the relevant test standard and edition A description of the test specimen, including its initial status, unique ID, quantity, and any accompanying photos or drawings, is required Additionally, the report must detail the mounting of the test specimen, the performance of the test apparatus, and the measuring system with sensor locations It should also outline the uncertainties of the measuring system, including overall uncertainty and calibration data, as well as initial, intermediate, and final measurements Required and actual test severities, along with documentation of test frequency ranges and response spectra, must be included The report should conclude with the test results, observations during testing, a summary, the test manager's name and signature, a distribution list, and the testing axes used.
A test log is essential for documenting the testing process, featuring a chronological list of test runs, parameters, observations, actions taken, and measurement data sheets This log can be included with the test report for comprehensive documentation.
NOTE See also ISO/IEC 17025
Guidance for time-history and sine-beat methods
Various established testing procedures are available to assess a specimen's ability to endure different vibrational forces These methods vary from straightforward continuous sinusoids to intricate time-history and sine-beat techniques, each tailored for specific needs or vibration environments This standard outlines a laboratory method to replicate effects similar to those encountered in real-world scenarios, although it does not aim to perfectly mimic the actual environment.
Standardized parameters and appropriate tolerances are essential for achieving consistent results across various testing locations This standardization allows for the categorization of equipment based on their capacity to endure specific levels of vibration severity.
In vibration testing, the standard method involves performing a vibration response analysis to identify the critical frequencies of the specimen within the specified frequency range This is typically succeeded by endurance testing, where the specimen is subjected to vibrations at each identified critical frequency for predetermined durations.
The investigation of vibration response typically employs single-axis sinusoidal excitation with a single sweep cycle across the desired frequency range It is crucial that the vibration amplitude remains low enough to avoid effects similar to those observed in endurance testing, while the sweep rate must be sufficiently slow to accurately identify critical frequencies.
A vibration response analysis conducted before and after an endurance test can reveal shifts in the frequency of resonance or other response changes Such frequency alterations may suggest fatigue in the specimen, potentially rendering it unsuitable for its intended operational environment, as outlined in IEC 60068-3-8.
When testing large or heavy specimens, especially those with a significantly offset center of gravity, caution is essential These specimens can induce transverse or rotational motion on the vibration table, making it challenging to meet required tolerances at checkpoints.
The time-history method is crucial for applications that require precise reproduction of the vibration environment and for scenarios where there is limited knowledge about the specimen, making it challenging to identify critical aspects such as critical frequencies.
Time-history testing minimizes the risk of over-testing compared to other methods, as it accurately simulates real environmental conditions This approach reduces the likelihood of overstressing or fatiguing materials due to overly conservative testing practices.
In reproducing the real or field environment, a response spectrum is developed by the specifier Usually a damping ratio is assigned which represents the damping of the specimen
This developed response spectrum is called the required response spectrum and is a part of the specification and represents the test criterion which has to be fulfilled
During specimen testing, the laboratory simulates a specific environment to create a test response spectrum by monitoring the vibration table's motion This spectrum is then compared to the required response spectrum to assess if the test criteria are met, which necessitates that the test response spectrum envelops the required one To develop the test response spectrum, preliminary runs are often conducted using an equivalent mass in place of the test specimen, allowing the laboratory to adjust test levels without causing unnecessary fatigue or overstress to the specimen.