IEC 60068-2-85:2019
(Main)Environmental testing - Part 2-85: Tests - Test Fj: Vibration - Long time history replication
Environmental testing - Part 2-85: Tests - Test Fj: Vibration - Long time history replication
IEC 60068-2-85:2019 demonstrates the adequacy of specimens to resist dynamic loads without unacceptable degradation of its functional and/or structural integrity when subjected to the specified vibration test requirements as defined by a time history (long time history replication). These can either be recorded in measurement exercises or generated artificially. In both cases, this method allows for generating a test tailored to very specific applications.
Essais d'environnement - Partie 2-85 : Essais - Essai Fj: Vibrations - Reproduction dans le temps par accélérogrammes
L'IEC 60068-2-85:2019 a pour objet de démontrer la capacité des spécimens à résister aux charges dynamiques sans dégradation inacceptable de leur intégrité de fonctionnement et/ou de structure, lorsqu'ils sont soumis aux exigences d'essais de vibrations spécifiées, définies par un accélérogramme (reproduction des vibrations dans le temps). Ces vibrations peuvent être enregistrées lors de sessions de mesure ou être générées artificiellement. Dans les deux cas, cette méthode permet de réaliser un essai adapté à des applications bien spécifiques.
General Information
Standards Content (Sample)
IEC 60068-2-85 ®
Edition 1.0 2019-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Environmental testing –
Part 2-85: Tests – Test Fj: Vibration – Long time history replication
Essais d'environnement –
Partie 2-85: Essais – Essai Fj: Vibrations – Reproduction dans le temps par
accélérogrammes
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IEC 60068-2-85 ®
Edition 1.0 2019-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Environmental testing –
Part 2-85: Tests – Test Fj: Vibration – Long time history replication
Essais d'environnement –
Partie 2-85: Essais – Essai Fj: Vibrations – Reproduction dans le temps par
accélérogrammes
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 19.040 ISBN 978-2-8322-7079-0
– 2 – IEC 60068-2-85:2019 © IEC 2019
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
4 Requirements for test apparatus . 9
4.1 General . 9
4.2 Basic motion . 9
4.3 Cross-axis motion . 9
4.4 Mounting . 10
4.5 Measuring systems . 10
5 Severities . 10
6 Preconditioning . 10
7 Initial measurements and functional performance test. 10
8 Testing . 10
8.1 General . 10
8.2 Initial vibration response investigation . 11
8.3 Low-level excitation for equalization prior to testing . 12
8.4 Testing with specimen functioning . 12
8.5 Final vibration response investigation . 12
9 Recovery . 12
10 Final measurements and functional performance . 13
11 Information to be given in the relevant specification . 13
12 Information to be given in the test report. 13
Annex A (informative) Guidance . 15
A.1 General . 15
A.2 Requirements for testing . 15
A.2.1 Single-point control . 15
A.2.2 Controlled input testing . 16
A.2.3 Controlled response testing . 16
A.2.4 Verification parameters . 16
A.3 Testing procedures . 18
A.4 Equipment normally used with vibration isolators . 18
A.4.1 Transmissibility factors for isolators . 18
A.4.2 Temperature effect . 18
A.5 Test severities . 18
A.6 Equipment performance . 18
A.7 Initial and final measurements . 19
A.8 Frequency range . 19
Bibliography . 21
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ENVIRONMENTAL TESTING –
Part 2-85: Tests – Test Fj: Vibration –
Long time history replication
FOREWORD
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rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60068-2-85 has been prepared by IEC technical committee 104:
Environmental conditions, classification and methods of test.
The text of this International Standard is based on the following documents:
FDIS Report on voting
104/833/FDIS 104/840/RVD
Full information on the voting for the approval of this International Standard can be found in the
report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60068 series, published under the general title Environmental testing,
can be found on the IEC website.
– 4 – IEC 60068-2-85:2019 © IEC 2019
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
INTRODUCTION
This first edition of IEC 60068-2-85 was initiated in 2008 and a first proposal was prepared in
2011. It was developed out of the existing standard IEC 60068-2-64:2008 which contains a
technically similar test method and modified accordingly. This facilitates usage and
maintenance of both standards.
Differences are that this document relies on a specified time history. A test spectrum is not
specified.
This part of IEC 60068 deals with long time history replication vibration testing intended for
general application to components, equipment and other products, hereinafter referred to as
”specimens”, that may be subjected to vibrations of an arbitrary nature not covered by the other
existing methods for vibration testing. The methods and techniques in this document are based
on digital control of vibration in the time domain which allow a more flexible definition of the
vibration input signal to suit individual cases that are specified in the relevant specification.
Compared with most other tests, test Fj is based on deterministic techniques, and the time
history is supposed to have a long duration. There are nearly no restrictions to the vibration
characteristics besides the technical limitations of the test apparatus.
As the vibration input signal in this test is specified by a digital time history stored in a file, there
are no general methods for comparing two different test severities. The vibration tolerances
cannot be given in a single measure, as this depends on the purpose of the test. Therefore, it
is emphasized that long time history replication testing always demands a high degree of
engineering judgement by the user and specifier. The writer of the relevant specification is
expected to select the testing procedure, test time history and its severity, tolerances and
analysis methods, appropriate to the specimen and its use.
The test method is based primarily on the use of an electrodynamic or a servo-hydraulic
vibration generator with an associated computer-based control system used as a vibration
testing system.
Long time history replication vibration testing can be used to identify accumulated stress effects
and the resulting mechanical weakness and degradation in the specified performance. This
information, in conjunction with the relevant specification, can be used to assess the
acceptability of specimens.
If the specimens are subjected to vibration of a deterministic transient or periodic nature
resulting from transportation or real life environments that are covered by other test methods,
these are generally preferred. See IEC 60068-3-8 [1] for estimating the dynamic vibration
environment of the specimen and based on that, selecting the appropriate test method.
Annex A provides guidance and a list of details that can be considered for inclusion in
specifications.
___________
Numbers in square brackets refer to the bibliography.
– 6 – IEC 60068-2-85:2019 © IEC 2019
ENVIRONMENTAL TESTING –
Part 2-85: Tests – Test Fj: Vibration –
Long time history replication
1 Scope
This part of IEC 60068 demonstrates the adequacy of specimens to resist dynamic loads
without unacceptable degradation of its functional and/or structural integrity when subjected to
the specified vibration test requirements as defined by a time history (long time history
replication). These can either be recorded in measurement exercises or generated artificially.
In both cases, this method allows for generating a test tailored to very specific applications.
Typical applications are tests in which very specific deterministic transient, periodical or random
excitation is necessary and the characteristics of the motion are not covered by other test
standards. This includes time histories not sufficiently represented by the standard shock tests
of IEC 60068-2-27 [2] or a general description by a shock response spectrum as in
IEC 60068-2-81 [3], periodical vibration that is not covered by a sinusoidal waveform as in
IEC 60068-2-6, and random vibration that is not covered by the description of Gaussian or non-
Gaussian (high kurtosis) broad-band random vibration of IEC 60068-2-64. However, the user is
made aware that long time history replication uses a deterministic time history. Simulation of
random vibration of any kind is approximated by quasi-random.
In addition, additional mixed mode tests are possible with this test method by generating time
histories that are representations of the required test signals. This includes tests of high
complexity.
The purpose of this test is different from IEC 60068-2-57 [4]. The purpose of IEC 60068-2-57
is an evaluation for a transient vibration using mainly a synthesized time history. A long time
history test is mainly used for a durability and functionality test using an actual time history
measured in a real field environment. It can also be used as a method to apply a simulated non-
gaussian time history.
This document is applicable to specimens which can be subjected to vibration of a very specific
nature resulting from transportation or operational environments, for example in aircraft, space
vehicles and land vehicles. It is primarily intended for unpackaged specimens, and for items in
their transportation container when the latter can be considered as part of the specimen itself.
However, if the item is packaged, then the item itself is referred to as a product and the item
and its packaging together are referred to as a test specimen. This document can be used in
conjunction with IEC 60068-2-47, for testing packaged products.
Although primarily intended for electrotechnical specimens, this document is not restricted to
them and can be used in other fields where desired (see Annex A).
This document is applicable for single axis excitation.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60068-2-6, Environmental testing – Part 2-6: Tests – Test Fc: Vibration (sinusoidal)
IEC 60068-2-47, Environmental testing – Part 2-47: Tests – Mounting of specimens for vibration,
impact and similar dynamic tests
IEC 60068-2-64:2008, Environmental testing – Part 2-64: Tests – Test Fh: Vibration, broadband
random and guidance
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
NOTE The terms used are generally defined in IEC 60050-300 [5], IEC 60068-1 [6], IEC 60068-2-6, and
IEC 60068-5-2 [7] and ISO 2041 [8].
3.1
cross-axis motion
motion not in the direction of the stimulus, generally specified in the two axes orthogonal to the
direction of the stimulus
Note 1 to entry: The cross-axis motion should be measured close to the fixing points.
[SOURCE: IEC 60068-2-64:2008, 3.1]
3.2
fixing point
part of the specimen in contact with the fixture or vibration table at a point where the specimen
is normally fastened in service
Note 1 to entry: If a part of the real mounting structure is used as the fixture, the fixing points are taken as those of
the mounting structure and not of the specimen.
[SOURCE: IEC 60068-2-64:2008, 3.3]
3.3
measuring points
specific points at which data are gathered for conducting the test
Note 1 to entry: These points are of three types, as defined in 3.4 to 3.6.
[SOURCE: IEC 60068-2-64:2008, 3.6]
3.4
checkpoint
point located on the fixture, on the vibration table or on the specimen as close as possible to
one of its fixing points, and in any case, rigidly connected to it
[SOURCE: IEC 60068-2-64:2008, 3.7, modified – The notes to entry have been deleted.]
3.5
control point
point, chosen from amongst the checkpoints, whose signal is used to control the test, such that
the requirements of IEC 60068-2-85 are satisfied
– 8 – IEC 60068-2-85:2019 © IEC 2019
[SOURCE: IEC 60068-2-64:2008, 3.8, modified – The term “reference point (single-point
control)” has been replaced with "control point" and in the definition "this standard" has been
replaced with "IEC 60068-2-85".]
3.6
response points
specific points on the specimen from which data is gathered for the purpose of the vibration
response investigation
Note 1 to entry: These points are not the same as checkpoints or control points.
[SOURCE: IEC 60068-2-64:2008, 3.10]
3.7
preferred testing axes
three orthogonal axes that correspond to the most vulnerable axes of the specimen
[SOURCE: IEC 60068-2-64:2008, 3.11]
3.8
specified time history
data file containing digital acceleration values varying with time to be replicated during the test
Note 1 to entry: The specified time history is usually based on recorded ‘real life’ digitized data, properly modified
(e.g. filtered) for reproducibility on a shaker.
3.9
control time history
time history measured at the control point simulating the specified time history
3.10
error time history
difference between the specified time history and the control time history
3.11
equalization
minimization of the RMS of error time history
3.12
RMS value
root-mean-square value
square root of the average of the squared values of all functions over the total frequency interval
f and f
1 2
[SOURCE: IEC 60068-2-64:2008, 3.33, modified – The definition has been simplified.]
3.13
test frequency range
frequency range to use for processing control, as given in the relevant specification
3.14
probability density function
at a specified amplitude, ratio of the probability that the amplitude will be within a given
incremental range, to the size of the incremental range
3.15
maximum response spectrum
curve giving the value of the highest peak of the response of a linear single degree of freedom
system (SDOF system) to vibration, according to its natural frequency, for a given damping ratio
Note 1 to entry: The response is described by the relative movement of the mass of this system in relation to its
support.
3.16
fatigue damage spectrum
FDS
spectrum obtained by tracing the fatigue damage experienced by a linear single degree of
freedom system (SDOF) according to its natural frequency, for a given damping ratio and for a
given value of parameter b
Note 1 to entry: Parameter b comes from the Basquin law representing the Wöhler curve of the material constituting
the structure.
Note 2 to entry: This note applies to the French language only.
4 Requirements for test apparatus
4.1 General
The required characteristics apply to the complete vibration system, which includes the power
amplifier, vibrator, test fixture, specimen and control system when loaded for testing.
The standardized test method consists of the following test sequence normally applied in each
of the mutually perpendicular axes of the test specimen:
1) an initial vibration response investigation, with low level sinusoidal excitation, or low level
random excitation, (see 8.2 and Clause A.1);
2) the long time history replication as the mechanical load or stress test;
3) a final vibration response investigation to compare the results with the initial one and to
detect possible mechanical failures due to a change of the dynamic behaviour
(see 8.2 and 8.5).
Where the dynamic behaviour is known, and it is not considered relevant, or sufficient data can
be gathered during the test at full level, the relevant specification may not require pre and post
test vibration response investigations.
4.2 Basic motion
The basic motion of the fixing points of the specimen shall be specified by the relevant
specification. The fixing points shall have substantially identical motions in phase and amplitude
and shall be rectilinear relative to the direction of excitation.
4.3 Cross-axis motion
Cross-axis motion shall be checked, if required by the relevant specification, either before the
test is applied by conducting a sine or random investigation at a level specified by the relevant
specification, or during testing by utilizing additional monitoring channels in the two
perpendicular axes.
The maximum cross axis amplitude at the control point shall not exceed 50 % of the value of
the specified axis. With large-size or high mass specimens it may be difficult to achieve this
requirement. In such cases, the relevant specification shall state which of the following
requirements applies:
a) any cross-axis motion in excess of that given above shall be stated in the test report;
– 10 – IEC 60068-2-85:2019 © IEC 2019
b) a cross-axis motion which is known to offer no hazard to the specimen need not be
monitored.
4.4 Mounting
The specimen shall be mounted in accordance with IEC 60068-2-47.
4.5 Measuring systems
The characteristics of the measuring system shall be such that it can be determined whether
the value of the vibration as measured in the intended axis at the control point is within the
tolerance required for the test.
The frequency range for long time history replication needs to extend to higher frequencies than
the conventional vibration test.
The frequency response of the overall measuring system, which includes the transducer, the
signal conditioner and the data acquisition and processing device, has a significant effect on
the accuracy of the measurements. The frequency response of the measuring system shall be
flat within ±5 % over the test frequency range. Outside of this range any further deviation shall
be stated in the test report. See Clause A.8.
5 Severities
The parameters for this test shall be based on the purpose for which it is being conducted and
on the conditions the equipment is likely to experience in-service. Test time history is normally
based on measured data derived from in-service conditions and usage. The time history may
be edited to make it suitable for the system to handle. The test parameters may contain the
number of repetitions of the time history, as long as fulfilling the intended purpose of the test is
not endangered by the modifications.
6 Preconditioning
If the relevant specification calls for preconditioning it shall then specify the conditions.
7 Initial measurements and functional performance test
The specimen shall be submitted to visual, dimensional and functional and any other checks as
specified by the relevant specification.
8 Testing
8.1 General
Testing follows the sequence specified by the relevant specification. The different steps are as
follows:
– initial vibration response investigation, if specified;
– low-level excitation for equalization before proceeding to the full level test in one continuous
mode;
– long time history replication;
– final vibration response investigation, if specified.
The specimen shall be excited in each of the preferred testing axes in turn, unless otherwise
specified by the relevant specification. The order of the testing along these axes is not important,
unless specified by the relevant specification. If the specimen is sensitive to gravity, for example
a mercury tilt switch, then vibration may only be applied in its normal service position and shall
be specified by the relevant specification.
Special action is necessary when a specimen normally intended for use with vibration isolators
needs to be tested without them. See Clause A.3 and Clause A.4. See also IEC 60068-2-47.
8.2 Initial vibration response investigation
The relevant specification may specify a vibration response investigation in each axis either
before, or both before and after long time history replication testing.
When specified in the relevant specification, the dynamic response for at least one point on the
specimen in the defined frequency range shall be investigated. The number and position of the
response points shall be clearly defined in the relevant specification. The vibration response
investigation may be performed with sinusoidal or random vibration in a test frequency range
and with a test level as specified by the relevant specification. Reference shall be made to
IEC 60068-2-6 for sinusoidal vibration and to IEC 60068-2-64 for random vibration excitation.
Also see IEC 60068-3-8 for more information and the advantages and disadvantages of each
method.
The response investigation shall be carried out with a test level selected so that the response
of the specimen remains less than during long time history replication but at a sufficiently high
level to detect critical frequencies.
When sinusoidal excitation is used, at least one sweep cycle over the test frequency range
specified by the relevant specification shall be performed with an acceleration amplitude
≤10 m/s or a displacement amplitude of ±1 mm, whichever is less. The vibration amplitude
shall be adapted in order to prevent a higher stress on the specimen than during long time
history replication. A sweep rate of one octave per minute shall be applied to determine the
frequencies and amplitudes of the resonances. If there is concern about exciting the structure
to a full resonance then a faster sweep rate may be applied as an indication of frequency and
relative amplitude of the resonance within the frequency band of interest. Investigations at
slower sweep rates or sweeping back and forth around a known resonance may be required but
shall be limited to the minimum time to obtain the results required. Undue dwell time is to be
avoided. The vibration amplitude may be varied as required.
In the case of sinusoidal excitation, it should be remembered that, in the case of non-linear
behaviour, the frequency corresponding to the maximum response will change depending on
the direction of the frequency variation during the sweep. For random excitation non-linearities
can influence the resonance behaviour. For sinusoidal and random excitation, the amplification
at resonances may be dependent on the magnitude of the input vibration.
The response investigation with random vibration shall be carried out taking into account that
the time of the test shall be long enough to minimize stochastical variations in the response. A
random vibration response test shall be carried out over the specified test frequency range. At
the lowest resonance frequency there shall be a minimum of five spectral lines within the
frequency band at –3 dB of the resonance peak.
When random excitation is used, the RMS value of acceleration shall be selected in order to
prevent a higher stress on the specimen of not more than 25 % of that during long time history
replication. The duration shall be as short as possible, but at least long enough to make an
analysis with degrees of freedom (DOF) = 120 possible. If the resonance response is observed
and documented periodically during the full level test, special resonance investigations are not
necessary.
The specimen shall be in functioning mode during this investigation if required by the relevant
specification. Where the mechanical vibration characteristics cannot be assessed because the
specimen is functioning, an additional vibration response investigation with the specimen not
– 12 – IEC 60068-2-85:2019 © IEC 2019
functioning shall be carried out. During this stage, the specimen shall be examined in order to
determine the critical frequencies which shall then be stated in the test report.
For the vibration response investigations of an ‘undefined type’ specimen or package, it may
be necessary to measure different signals such as driving force or velocity. If specified by the
relevant specification, for example, the mechanical impedance of the specimen shall be
calculated before and after the test.
NOTE Mechanical impedance and other similar terms are defined in ISO 2041.
8.3 Low-level excitation for equalization prior to testing
Prior to long time history replication at the specified level, a preliminary excitation at lower
levels with the real specimen may be necessary to equalize the signal and for preliminary
analysis. It is important that at this stage the level of the acceleration applied is kept to a
minimum.
The permitted durations for preliminary excitation are the following:
– below –12 dB of the specified level: no time limit;
– from –12 dB to –6 dB of the specified level: not more than 1,5 times the specified
test duration;
– between –6 dB and 0 dB of the specified level: not more than 50 % of the specified
test duration.
– The duration of preliminary excitation above −6 dB shall be kept to an absolute minimum.
However, the possible complex nature of the time history may require longer times of
preliminary excitation. The parameters of this process shall be recorded and reported in the
test report.
– The duration of the preliminary excitation shall not be subtracted from the specified test
duration, unless permitted by the relevant specification.
8.4 Testing with specimen functioning
When specified by the relevant specification, the specimen shall be functioning during a
specified time interval during the testing, and its performance shall be checked.
8.5 Final vibration response investigation
If the relevant specification has specified an initial response investigation, it may also require
an additional vibration response investigation on completion of the long time history testing, in
order to determine whether changes or failures have occurred since the initial vibration
response investigation. The final response investigation shall then be performed in the same
manner at the same response points and with the same parameters as used for the initial
vibration response investigation. Guidelines for the use of changes in vibration response, for
example change of critical frequencies, is given in IEC 60068-3-8. The relevant specification
shall state what action is to be taken if different results are obtained in the two investigations.
9 Recovery
It is sometimes necessary to provide a period of time after testing and before final
measurements in order to allow the specimen to attain the same conditions, for example of
temperature, as existed for the initial measurements. The relevant specification shall then
specify the conditions for recovery.
10 Final measurements and functional performance
The specimen shall be submitted to visual, dimensional and functional checks and any others
as specified by the relevant specification.
The relevant specification shall provide the criteria upon which the acceptance or rejection of
the specimen shall be based.
For the evaluation of vibration response results see IEC 60068-3-8.
11 Information to be given in the relevant specification
When this test is included in a relevant specification, the following details shall be given in so
far as they are applicable, paying particular attention to the items marked with an asterisk (*)
as this information is always required.
Clause/subclause
a) Control point*, A.2.2, A.2.3
b) Measuring points*, 3.3
c) Basic motion*, 4.2
d) Fixing points*, 3.2
e) Cross-axis motion, 4.3
f) Mounting of the specimen*, 4.4
g) Vibration tolerances for testing large-size or high-mass specimens, 4.3
h) Test frequency range*,
i) The specified time history including any factors to achieve the required severities,
j) Time history duration and number of replications or the total duration of the test, A.3
k) Any applicable rest periods,
l) Preconditioning, 6
m) Initial measurements*, 7
n) Preferred testing axes and order of testing*, 8.1
o) Initial and final vibration response investigation, 8.2, 8.5
p) Intermediate measurements,
q) Recovery, 9
r) Final measurements and acceptance or rejection criteria*, 10
s) Uncertainty of measuring system,
t) Performance and functional check. 10
12 Information to be given in the test report
As a minimum the test report shall show the following information:
1) Customer (name and address)
2) Test laboratory (name and address)
3) Test report identification (date of issue, unique number)
4) Test dates
5) Purpose of the test (development test, qualification, etc.)
– 14 – IEC 60068-2-85:2019 © IEC 2019
6) Test standard, edition (relevant test procedure)
7) Test specimen description (initial status, unique ID, quantity, photo,
drawing, etc.)
8) Mounting of test specimen (fixture ID, drawing, photo, etc.)
9) Performance of test apparatus (cross motion, etc.)
10) Measuring system, sensor location (description, drawing, photo, etc. Also
see 4.5)
11) Uncertainties of measuring system, (overall uncertainty, calibration data,
if required by relevant specification last/next date of calibration)
12) Initial, intermediate and/or final measurements
13) Test specification with documentation (measuring points, specified time history
file name or ID, control time history file
name or ID, test duration, number of
repetitions)
14) Test results (final status of test specimen)
15) Observations during testing and actions taken
16) Summary of test
17) Test manager (name and signature)
18) Distribution (list of those receiving the report)
NOTE 1 A test log is written for the testing, where the test is documented by, for example, a chronological list of
test runs with test parameters, observations during testing and actions taken and data sheets on measurements
made. The test log can be attached to the test report.
NOTE 2 See also ISO/IEC 17025 [9].
Annex A
(informative)
Guidance
A.1 General
The long time history replication test method utilizes measured time history response data in a
laboratory test on a vibration exciter under closed loop control. The long time history replication
test method is preferable over random vibration or shock SRS approaches when specific details
of a complex time history are required to be retained in the test, which can only be achieved by
directly reproducing the actual service loading. Positive benefits are that the damage potential
of the long time history replication test most closely matches those of the in-service environment
viz. specimen internal stress intensities and other internal loading effects. A further advantage
is that it is relatively simple to include factors on the time history to take into account the effects
of unknowns and to build in an appropriate margin of confidence.
In order to ensure that the long time history replication test is within the capabilities of the
vibration exciter some manipulation of the measured time history will invariably be required.
Typically, it may be necessary to adopt a two-level test or to use data manipulation tools such
as filters.
The use of any vibration inducing equipment capable of satisfying the test requirements stated
in this test method is acceptable.
The performance of most digital vibration control equipment likely to be employed for long time
history replication testing can be expected to be similar. Using some selectable parameters of
the vibration control equipment, an estimate can be made to establish the difference between
the specified time history and the control time history. This does not take into account other
sources of uncertainty as defined in ISO/IEC 17025 [9]. These parameters, which are dependent
on each other, can therefore be chosen so that an optimum similarity between the two time
histories is achieved.
Equalization of the specified time history may require several repetitions of the control loop, the
duration depending on several factors, such as hardware configuration, total system transfer
function, control algorithm and test parameters, which can be adjusted prior to the test.
A vibration response investigation gives essential information about the specimen/vibrator
interaction. For example, this investigation could reveal excessive test fixture vibration
amplification or coincident resonance between fixture and specimen. It is therefore
recommended
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