ASTM E1942-98(2010)
(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. oeability, which are performed on the top coat only.
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 E 4, or static extensometer verification, for which the user is referred to the current revision of Practice E 83. The user is also referred to Practice E 467.
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.
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Standards Content (Sample)
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Designation:E1942–98 (Reapproved 2010)
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.
1. Scope E467 Practice for Verification of Constant Amplitude Dy-
namic Forces in an Axial Fatigue Testing System
1.1 This guide covers how to understand and minimize the
E1823 Terminology Relating to Fatigue and Fracture Test-
errors associated with data acquisition in fatigue and fracture
ing
mechanics testing equipment. This guide is not intended to be
used instead of certified traceable calibration or verification of
3. Terminology
data acquisition systems when such certification is required. It
3.1 Definitions:
does not cover static load verification, for which the user is
3.1.1 bandwidth [T ]—the frequency at which the ampli-
referred to the current revision of Practices E4, or static
tude response of the channel has fallen to 1/ 2 of its value
=
extensometer verification, for which the user is referred to the
at low frequency.
current revision of Practice E83. The user is also referred to
3.1.1.1 Discussion—This definition assumes the sensor
Practice E467.
channel response is low-pass, as in most materials testing. An
1.2 The output of the fatigue and fracture mechanics data
illustration of bandwidth is shown in Fig. 1.
acquisition systems described in this guide is essentially a
3.1.2 Basic Data sample—the sampled value of a sensor
stream of digital data. Such digital data may be considered to
waveform taken at fixed time intervals. Each sample represents
be divided into two types– Basic Data, which are a sequence of
the actual sensor value at that instant of time.
digital samples of an equivalent analog waveform representing
3.1.2.1 Discussion—Fig. 2 shows examples of Basic Data
the output of transducers connected to the specimen under test,
samples.
and Derived Data, which are digital values obtained from the
3.1.3 data rate [T ]—the date rate is ⁄td Hertz where the
Basic Data by application of appropriate computational algo-
time intervals between samples is t in seconds.
d
rithms. The purpose of this guide is to provide methods that
3.1.3.1 Discussion—The data rate is the number of data
give confidence that such Basic and Derived Data describe the
samples per second made available to the user, assuming the
properties of the material adequately. It does this by setting
rate is constant.
minimum or maximum targets for key system parameters,
3.1.4 Derived Data—any waveform parameter which is
suggesting how to measure these parameters if their actual
derived from one or several of the Basic Data samples.
values are not known.
3.1.4.1 Discussion—Fig. 2 illustrates examples of Derived
2. Referenced Documents Data.
2 3.1.5 noise level—the standard deviation of the data
2.1 ASTM Standards:
samples of noise in the transducer channel, expressed in the
E4 Practices for Force Verification of Testing Machines
units appropriate to that channel.
E83 Practice for Verification and Classification of Exten-
3.1.6 peak—the point of maximum load in constant ampli-
someter Systems
tude loading (see Terminology E1823).
3.1.7 phase difference [°]—the angle in degrees separating
This guide is under the jurisdiction of ASTM Committee E08 on Fatigue and corresponding parts of two waveforms (such as peaks), where
Fracture and is the direct responsibility of SubcommitteeE08.03 on Advanced
one complete cycle represents 360°.
Apparatus and Techniques.
3.1.7.1 Discussion—The phase difference of a cyclic wave-
Current edition approved Nov. 1, 2010. Published January 2011. Originally
form only has meaning in reference to a second cyclic
approved in 1998. Last previous edition approved in 2004 as E1942 - 98(2004).
DOI: 10.1520/E1942-98R10.
waveform of the same frequency.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
3.1.8 sampling rate [T ]—the rate at which the analog-to-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
digital converter samples a waveform. This rate may not be
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. visible to the user of the data acquisition system.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1942–98 (2010)
FIG. 3 Sources of Error in Data Acquisition Systems
FIG. 1 3-dB Bandwidth of Sensor Channel
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
FIG. 2 Basic and Derived Data
performance for the measurements required for Practice E467.
If the requirements of Practice E467 for the mechanical system
and the recommendations of this guide are met, then the user
3.1.8.1 Discussion—A distinction is made here between
has confidence that the Basic Data produced by the testing
sampling rate and data rate, because in some data acquisition
system are adequate for processing by subsequent computer
systems, the analog waveform may be sampled at a much
algorithms to produce further Derived Data.
higher rate than the rate at which data are made available to the
4.1.2 At each stage of the flow of data in the electrical
user.(Suchatechniqueiscommonlyknownas over-sampling).
measurement system, errors can be introduced. These should
3.1.9 word size—the number of significant bits in a single
be considered in the sequence in which these are dealt with in
data sample.
this guide. The sequence includes:
3.1.9.1 Discussion—The word size is one parameter which
4.2 Errors Due to Bandwidth Limitations in the Signal
determinesthesystem resolution. Usually it will bedetermined
Conditioning—Where there is analog signal conditioning prior
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 to analog-to-digital conversion, there will usually be restric-
w
tions on the analog bandwidth in order to minimize noise and,
in the data that can be seen is 1 part in 2 , that is the
–w
quantization step is d=2 . in some cases, to eliminate products of demodulation. After
digitalconversion,additionaldigitalfilteringmaybeappliedto
3.1.10 valley—The point of minimum load in constant
amplitude loading (see Terminology E1823). reduce noise components. These bandwidth restrictions result
in cyclic signals at higher frequencies having an apparent
4. Description of a Basic Data Acquisition System
amplitude which is lower than the true value, and if the
waveform is not sinusoidal, also having waveform distortion.
4.1 In its most basic form, a mechanical testing system
consists of a test frame with grips which attach to a test The bandwidth restrictions also cause phase shifts which result
in phase measurement errors when comparing phase in two
specimen, a method of applying forces to the specimen, and a
channels with different bandwidths.
number of transducers which measure the forces and displace-
ments applied to the specimen (see Fig. 3). The output from 4.3 Errors Due to Incorrect Data Rate—Errors can result
these transducers may be in digital or analog form, but if they fromaninsufficient data rate,wheretheintervalsbetweendata
are analog, they are first amplified and filtered and then samples are too large and intervening events are not recorded
converted to digital form using analog-to-digital converters in the Basic Data. These result also in errors in the Derived
(ADCs). The resulting stream of digital data may be digitally Data, for example, when the peak value of a waveform is
filtered and manipulated to result in a stream of output Basic missed during sampling. Data skew, where the Basic Data are
Data which is presented to the user in the form of a displayed not acquired at the same instant in time, can produce similar
or printed output, or as a data file in a computer. Various errors to phase shifts between channels.
E1942–98 (2010)
4.4 Errors Due to Noise and Drift—Noise added to the Basic Data rate used should equal or exceed the appropriate
signal being measured causes measurement uncertainty. Short– minimumdatarate(dependingonwaveformtype).Thisshould
term noise causes variability or random error, and includes be verified even if the external rate at which samples are
analog noise at the transducer output due to electrical or presented is less than this minimum value. For a discussion of
mechanical pick up, and analog noise added in the amplifier, data rate, see A1.3.1.
together with digital noise, or quantization, due to the finite 5.6 Actual Data Rate—The actual data rate must equal or
digital word length of the ADC system.
exceed the minimum data rate. If the actual data rate is not
4.4.1 Long-term effects, such as drifts in the transducer known, then it must be ascertained using a method such as that
output or its analog signal conditioning due to temperature or
in A1.3.2.
aging effects, are indistinguishable from slow changes in the
5.7 Maximum Permitted Noise Level—Thenoiselevelisthe
forces and displacements seen by the specimen, and cause a
standard deviation of the noise in the transducer channel,
more systematic error.
expressed in the units appropriate to the channel. The maxi-
4.4.2 Further details of these sources of error are given in
mum permitted noise level is 0.2 % of the expected peak value
Annex A1.
of the waveform being measured. For example, if the expected
peak value in a load channel is 100 kN, then the standard
5. System Requirements
deviation of the noise in that channel must not exceed 0.2 kN.
5.1 How This Section is Organized—This section gives the
5.8 Actual Noise Level—The actual noise level must be
steps that must be taken to ensure the errors are controlled.
equal to or less than the maximum permitted noise level. If the
There are several sources of error in the electrical system, and
actual noise level is not known, then it must be ascertained
these may add both randomly and deterministically. To give
using a method such as that in A1.4.6. Guidance on how to
reasonable assurance that these errors have a minor effect on
investigate sources of noise is given in A1.4.7.
overall accuracy of a system with 1 % accuracy, recommenda-
5.8.1 If the actual noise level exceeds the maximum permit-
tions are given in this guide, which result in a 0.2 % error
ted noise level, it can usually be reduced by reducing band-
bound for each individual source of error. However,AnnexA1
width, but this will require beginning again at 5.3 to verify that
also shows how the error varies with each parameter, so that
the bandwidth reduction is permissible.
the user may choose to use larger or smaller error bounds with
5.9 Maximum Permissible Phase Difference and Maximum
appropriate adjustments to bandwidth, data rate, and so forth.
Permissible Data Skew—These terms are discussed in A1.5.1
5.1.1 In this section, which is intended to be used in the
and A1.5.2. No value is recommended for the maximum
order written, a minimum value or a maximum value is
permissible phase difference and data skew between channels,
recommended for each parameter. If the actual value of each
since this is very dependent on the testing application. If
parameterisknown,thenthesystemrequirementisthatineach
typical phase shifts between displacement and force due to the
case either:
material under test are 10 to 20°, then an acceptable value for
Maximum value$ actual value
themaximumphasedifferencemightbe1°.However,iftypical
or
phase shifts are 2 to 3°, the acceptable value for the maximum
Minimum value# actual value.
phase difference might be only 0.1°.
However, if the actual value is not known, then help is given
5.10 Actual Phase Shift and Data Skew—Methods for
as to how to determine it.
estimating the combined effect of phase shift and data skew in
5.2 Frequency and Waveshape—The first step is to deter-
a data acquisition system are given in A1.5.3.
mine the highest cyclic frequency, f Hz, at which testing will
occur, and the waveshape to be employed (for example,
6. Report
sinusoidal, triangular, square).
6.1 The purpose of the report is to record that due consid-
5.3 Minimum Bandwidth—If the waveform is sinusoidal or
eration was given to essential performance parameters of the
square, then the minimum bandwidth is 10f Hz to measure the
dataacquisitionsystemwhenperformingaparticularfatigueor
peak value. If the waveform is triangular, then the minimum
fracture mechanics test. Since the report should ideally be an
bandwidth is 100f Hz. For example, for a 10–Hz sinusoidal
attachment to each set of such test results, it should be
waveform,theminimumbandwidthis100Hz.Foradiscussion
sufficient but succinct. The report should contain the following
of minimum bandwidth, see A1.2.1 and A1.2.2.
information, preferably in a tabular format.
5.4 Actual Bandwidth—Theactualbandwidthmustbeequal
6.2 Measurement Equipment Description—This should in-
to or greater than the minimum bandwidth. If this condition
clude the manufacturer’s name, model number, and serial
cannot be met, then the errors will increase as shown inA1.2.1
number for the test hardware used.
andA1.2.2.Iftheactualbandwidthisnotknown,thenitcanbe
6.3 Waveshape and Highest Frequency Used During the
ascertained using one of the suggested methods in A1.2.3,or
Test
otherwise.
6.4 Minimum Bandwidth, Actual Bandwidth, and a Note
5.5 Minimum Data Rate—For measurement of the peak
value of sinusoidal or square waveforms, the minimum data About its Source—The source is a note describing how actual
bandwidth was ascertained, for example, from a manufactur-
rate is 50 points/cycle, or 50f points/s. For measurement of the
peak value of triangular waveforms, the minimum data rate is
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