ASTM E1942-98(2018)e1
(Guide)Standard Guide for Evaluating Data Acquisition Systems Used in Cyclic Fatigue and Fracture Mechanics Testing
Standard Guide for Evaluating Data Acquisition Systems Used in Cyclic Fatigue and Fracture Mechanics Testing
ABSTRACT
This guide covers how to understand and minimize the errors associated with data acquisition in fatigue and fracture mechanics testing equipment. This guide is not intended to be used instead of certified traceable calibration or verification of data acquisition systems when such certification is required. The output of the fatigue and fracture mechanics data acquisition systems described is essentially a stream of digital data. Such digital data may be considered to be divided into two types– Basic Data, which are a sequence of digital samples of an equivalent analog waveform representing the output of transducers connected to the specimen under test, and Derived Data, which are digital values obtained from the Basic Data by application of appropriate computational algorithms. In its most basic form, a mechanical testing system consists of a test frame with grips which attach to a test specimen, a method of applying forces to the specimen, and a number of transducers which measure the forces and displacements applied to the specimen. The output from these transducers may be in digital or analog form, but if they are analog, they are first amplified and filtered and then converted to digital form using analog-to-digital converters (ADCs). The resulting stream of digital data may be digitally filtered and manipulated to result in a stream of output Basic Data which is presented to the user in the form of a displayed or printed output, or as a data file in a computer. Various algorithms may be applied to the Basic Data to derive parameters representing, for example, the peaks and valleys of the forces and displacements applied to the specimen, or the stresses and strains applied to the specimen and so forth. Such parameters are the Derived Data. The whole measurement system may be divided into three sections for the purpose of verification: the mechanical test frame and its components, the electrical measurement system, and the computer processing of data.
SCOPE
1.1 This guide covers how to understand and minimize the errors associated with data acquisition in fatigue and fracture mechanics testing equipment. This guide is not intended to be used instead of certified traceable calibration or verification of data acquisition systems when such certification is required. It does not cover static load verification, for which the user is referred to the current revision of Practices E4, or static extensometer verification, for which the user is referred to the current revision of Practice E83. The user is also referred to Practice E467.
1.2 The output of the fatigue and fracture mechanics data acquisition systems described in this guide is essentially a stream of digital data. Such digital data may be considered to be divided into two types– Basic Data, which are a sequence of digital samples of an equivalent analog waveform representing the output of transducers connected to the specimen under test, and Derived Data, which are digital values obtained from the Basic Data by application of appropriate computational algorithms. The purpose of this guide is to provide methods that give confidence that such Basic and Derived Data describe the properties of the material adequately. It does this by setting minimum or maximum targets for key system parameters, suggesting how to measure these parameters if their actual values are not known.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Relations
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
ϵ1
Designation: E1942 − 98 (Reapproved 2018)
Standard Guide for
Evaluating Data Acquisition Systems Used in Cyclic Fatigue
and Fracture Mechanics Testing
This standard is issued under the fixed designation E1942; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Sections 3.1.3, A1.2.2.1, A1.2.3, and A1.2.4 were editorially corrected in August 2018.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 This guide covers how to understand and minimize the
E4 Practices for Force Verification of Testing Machines
errors associated with data acquisition in fatigue and fracture
E83 Practice for Verification and Classification of Exten-
mechanics testing equipment. This guide is not intended to be
someter Systems
used instead of certified traceable calibration or verification of
E467 Practice for Verification of Constant Amplitude Dy-
data acquisition systems when such certification is required. It
namic Forces in an Axial Fatigue Testing System
does not cover static load verification, for which the user is
E1823 TerminologyRelatingtoFatigueandFractureTesting
referred to the current revision of Practices E4, or static
extensometer verification, for which the user is referred to the
3. Terminology
current revision of Practice E83. The user is also referred to
3.1 Definitions:
Practice E467.
3.1.1 bandwidth[T ]—thefrequencyatwhichtheamplitude
1.2 The output of the fatigue and fracture mechanics data response of the channel has fallen to 1/=2 of its value at low
frequency.
acquisition systems described in this guide is essentially a
stream of digital data. Such digital data may be considered to
3.1.1.1 Discussion—This definition assumes the sensor
be divided into two types– Basic Data, which are a sequence of
channel response is low-pass, as in most materials testing. An
digital samples of an equivalent analog waveform representing
illustration of bandwidth is shown in Fig. 1.
the output of transducers connected to the specimen under test,
3.1.2 Basic Data sample—the sampled value of a sensor
and Derived Data, which are digital values obtained from the
waveform taken at fixed time intervals. Each sample represents
Basic Data by application of appropriate computational algo-
the actual sensor value at that instant of time.
rithms. The purpose of this guide is to provide methods that
3.1.2.1 Discussion—Fig. 2 shows examples of Basic Data
give confidence that such Basic and Derived Data describe the
samples.
properties of the material adequately. It does this by setting
3.1.3 data rate [T ]—the data rate is ⁄td Hertz where the
minimum or maximum targets for key system parameters,
time intervals between samples is ⁄td in seconds.
suggesting how to measure these parameters if their actual
3.1.3.1 Discussion—The data rate is the number of data
values are not known.
samples per second made available to the user, assuming the
rate is constant.
1.3 This international standard was developed in accor-
dance with internationally recognized principles on standard-
3.1.4 derived data—data obtained through processing of the
ization established in the Decision on Principles for the
raw data.
Development of International Standards, Guides and Recom-
3.1.4.1 Discussion—Fig. 2 illustrates examples of Derived
mendations issued by the World Trade Organization Technical
Data.
Barriers to Trade (TBT) Committee.
3.1.5 noise level—thestandarddeviationofthedatasamples
of noise in the transducer channel, expressed in the units
appropriate to that channel.
This guide is under the jurisdiction of ASTM Committee E08 on Fatigue and
Fracture and is the direct responsibility of SubcommitteeE08.03 on Advanced
Apparatus and Techniques. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved June 1, 2018. Published August 2018. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ɛ1
approved in 1998. Last previous edition approved in 2010 as E1942 - 98(2010) . Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/E1942-98R18E01 the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
ϵ1
E1942 − 98 (2018)
4. Description of a Basic Data Acquisition System
4.1 In its most basic form, a mechanical testing system
consists of a test frame with grips which attach to a test
specimen, a method of applying forces to the specimen, and a
number of transducers which measure the forces and displace-
ments applied to the specimen (see Fig. 3). The output from
these transducers may be in digital or analog form, but if they
are analog, they are first amplified and filtered and then
converted to digital form using analog-to-digital converters
(ADCs). The resulting stream of digital data may be digitally
filtered and manipulated to result in a stream of output Basic
FIG. 1 3-dB Bandwidth of Sensor Channel
Data which is presented to the user in the form of a displayed
or printed output, or as a data file in a computer. Various
algorithms may be applied to the Basic Data to derive
parameters representing, for example, the peaks and valleys of
the forces and displacements applied to the specimen, or the
stresses and strains applied to the specimen and so forth. Such
parameters are the Derived Data.
4.1.1 The whole measurement system may be divided into
three sections for the purpose of verification: the mechanical
test frame and its components, the electrical measurement
system, and the computer processing of data. This guide is
specifically concerned only with the electrical measurement
system commencing at the output of the transducers. Before
themechanicalsystemisinvestigatedfordynamicerrorsbythe
methods given in Practice E467, this guide can be used to
ascertain that the electrical measurement system has adequate
performance for the measurements required for Practice E467.
FIG. 2 Basic and Derived Data
If the requirements of Practice E467 for the mechanical system
and the recommendations of this guide are met, then the user
has confidence that the Basic Data produced by the testing
system are adequate for processing by subsequent computer
3.1.6 peak—the point of maximum load in constant ampli-
algorithms to produce further Derived Data.
tude loading (see Terminology E1823).
4.1.2 At each stage of the flow of data in the electrical
3.1.7 phase difference [°]—the angle in degrees separating
measurement system, errors can be introduced. These should
corresponding parts of two waveforms (such as peaks), where
be considered in the sequence in which these are dealt with in
one complete cycle represents 360°.
this guide. The sequence includes:
3.1.7.1 Discussion—The phase difference of a cyclic wave-
4.2 Errors Due to Bandwidth Limitations in the Signal
form only has meaning in reference to a second cyclic
Conditioning—Where there is analog signal conditioning prior
waveform of the same frequency.
to analog-to-digital conversion, there will usually be restric-
3.1.8 sampling rate [T ]—the rate at which the analog-to-
tions on the analog bandwidth in order to minimize noise and,
digital converter samples a waveform. This rate may not be
in some cases, to eliminate products of demodulation. After
visible to the user of the data acquisition system.
digital conversion, additional digital filtering may be applied to
3.1.8.1 Discussion—A distinction is made here between
reduce noise components. These bandwidth restrictions result
sampling rate and data rate, because in some data acquisition
in cyclic signals at higher frequencies having an apparent
systems, the analog waveform may be sampled at a much
higher rate than the rate at which data are made available to the
user.(Suchatechniqueiscommonlyknownas over-sampling).
3.1.9 word size—the number of significant bits in a single
data sample.
3.1.9.1 Discussion—The word size is one parameter which
determines the system resolution. Usually it will be determined
by the analog-digital converter used, and typically may be 12
or 16 bits. If the word size is w, then the smallest step change
w
in the data that can be seen is 1 part in 2 , that is the
–w
quantization step is d=2 .
3.1.10 valley—The point of minimum load in constant
amplitude loading (see Terminology E1823). FIG. 3 Sources of Error in Data Acquisition Systems
ϵ1
E1942 − 98 (2018)
amplitude which is lower than the true value, and if the bandwidth is 100f Hz. For example, for a 10–Hz sinusoidal
waveform is not sinusoidal, also having waveform distortion. waveform,theminimumbandwidthis100Hz.Foradiscussion
The bandwidth restrictions also cause phase shifts which result of minimum bandwidth, see A1.2.1 and A1.2.2.
in phase measurement errors when comparing phase in two
5.4 Actual Bandwidth—The actual bandwidth must be equal
channels with different bandwidths.
to or greater than the minimum bandwidth. If this condition
cannot be met, then the errors will increase as shown in A1.2.1
4.3 Errors Due to Incorrect Data Rate—Errors can result
from an insufficient data rate, where the intervals between data andA1.2.2.Iftheactualbandwidthisnotknown,thenitcanbe
ascertained using one of the suggested methods in A1.2.3,or
samples are too large and intervening events are not recorded
in the Basic Data. These result also in errors in the Derived otherwise.
Data, for example, when the peak value of a waveform is
5.5 Minimum Data Rate—For measurement of the peak
missed during sampling. Data skew, where the Basic Data are
value of sinusoidal or square waveforms, the minimum data
not acquired at the same instant in time, can produce similar
rate is 50 points/cycle, or 50f points/s. For measurement of the
errors to phase shifts between channels.
peak value of triangular waveforms, the minimum data rate is
400 points/cycle, or 400f points/s. If the data acquisition
4.4 Errors Due to Noise and Drift—Noise added to the
system produces the peak value as an output, then the internal
signal being measured causes measurement uncertainty. Short–
Basic Data rate used should equal or exceed the appropriate
term noise causes variability or random error, and includes
minimumdatarate(dependingonwaveformtype).Thisshould
analog noise at the transducer output due to electrical or
be verified even if the external rate at which samples are
mechanical pick up, and analog noise added in the amplifier,
presented is less than this minimum value. For a discussion of
together with digital noise, or quantization, due to the finite
data rate, see A1.3.1.
digital word length of the ADC system.
4.4.1 Long-term effects, such as drifts in the transducer
5.6 Actual Data Rate—The actual data rate must equal or
output or its analog signal conditioning due to temperature or
exceed the minimum data rate. If the actual data rate is not
aging effects, are indistinguishable from slow changes in the
known, then it must be ascertained using a method such as that
forces and displacements seen by the specimen, and cause a
in A1.3.2.
more systematic error.
5.7 Maximum Permitted Noise Level—The noise level is the
4.4.2 Further details of these sources of error are given in
standard deviation of the noise in the transducer channel,
Annex A1.
expressed in the units appropriate to the channel. The maxi-
mum permitted noise level is 0.2 % of the expected peak value
5. System Requirements
of the waveform being measured. For example, if the expected
5.1 How This Section is Organized—This section gives the
peak value in a load channel is 100 kN, then the standard
steps that must be taken to ensure the errors are controlled.
deviation of the noise in that channel must not exceed 0.2 kN.
There are several sources of error in the electrical system, and
5.8 Actual Noise Level—The actual noise level must be
these may add both randomly and deterministically. To give
equal to or less than the maximum permitted noise level. If the
reasonable assurance that these errors have a minor effect on
actual noise level is not known, then it must be ascertained
overall accuracy of a system with 1 % accuracy, recommenda-
using a method such as that in A1.4.6. Guidance on how to
tions are given in this guide, which result in a 0.2 % error
investigate sources of noise is given in A1.4.7.
bound for each individual source of error. However, AnnexA1
5.8.1 If the actual noise level exceeds the maximum permit-
also shows how the error varies with each parameter, so that
ted noise level, it can usually be reduced by reducing
the user may choose to use larger or smaller error bounds with
bandwidth,butthiswillrequirebeginningagainat5.3toverify
appropriate adjustments to bandwidth, data rate, and so forth.
that the bandwidth reduction is permissible.
5.1.1 In this section, which is intended to be used in the
5.9 Maximum Permissible Phase Difference and Maximum
order written, a minimum value or a maximum value is
Permissible Data Skew—These terms are discussed in A1.5.1
recommended for each parameter. If the actual value of each
and A1.5.2. No value is recommended for the maximum
parameterisknown,thenthesystemrequirementisthatineach
permissible phase difference and data skew between channels,
case either:
since this is very dependent on the testing application. If
Maximum value ≥ actual value
typical phase shifts between displacement and force due to the
or
material under test are 10 to 20°, then an acceptable value for
Minimum value ≤ actual value.
themaximumphasedifferencemightbe1°.However,iftypical
However, if the actual value is not known, then help is given
phase shifts are 2 to 3°, the acceptable value for the maximum
as to how to determine it.
phase difference might be only 0.1°.
5.2 Frequency and Waveshape—The first step is to deter-
5.10 Actual Phase Shift and Data Skew—Methods for esti-
mine the highest cyclic frequency, f Hz, at which testing will
mating the combined effect of phase shift and data skew in a
occur, and the waveshape to be employed (for example,
data acquisition system are given in A1.5.3.
sinusoidal, triangular, square).
6. Report
5.3 Minimum Bandwidth—If the waveform is sinusoidal or
square, then the minimum bandwidth is 10f Hz to measure the 6.1 The purpose of the report is to record that due consid-
peak value. If the waveform is triangular, then the minimum eration was given to essential pe
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.