Aerospace series - Metrological assessment procedure for kinematic fields measured by digital image correlation

This document specifies the monitoring of mechanical tests and inspections performed both at the material (coupon) and at the structural scale by the implementation of kinematic field measurements by digital image correlation. This document describes an in situ method for evaluating the metrological performance of an extensometer system using image correlation for the delivery of displacement fields, and by extrapolation, of deformation fields. It can be implemented prior to the actual start of the test (or inspection). It will inform of the metrological performance in testing conditions.
This document allows the metrological performance of the measuring technology to be quantified. The methodology described herein is not to be considered as a calibration step. This reference document does not exhaustively specify the constitutive elements of a generic system of Digital Image Correlation measurement. This reference does not address the measurement of 3D shapes via stereocorrelation systems.

Luft- und Raumfahrt - Metrologisches Messverfahren für kinematische Felder durch digitale Bildkorrelation

Dieses Dokument legt die Überwachung von mechanischen Prüfungen und Inspektionen fest, die sowohl am Material (Coupon) als auch auf der strukturellen Ebene durch den Einsatz von Messungen kinematischer Felder durch digitale Bildkorrelation durchgeführt werden. Dieses Dokument beschreibt ein In-situ-Verfahren zur Beurteilung der messtechnischen Leistung eines Extensometer-Systems, das Bildkorrelation für die Ermittlung von Verschiebungsfeldern nutzt und Verformungsfelder mittels Extrapolation berechnet. Es kann vor dem eigentlichen Beginn der Prüfung (oder Inspektion) implementiert werden. Es informiert über die messtechnische Leistung bei Prüfbedingungen.
Mit diesem Dokument kann die messtechnische Leistung der Messtechnik quantifiziert werden. Die hier beschriebene Methodik ist nicht als Kalibrierschritt zu betrachten. Dieses Bezugsdokument enthält keine vollständige Festlegung der konstitutiven Elemente eines generischen Systems der Messung mithilfe digitaler Bildkorrelation (en: Digital Image Correlation, DIC). Dieses Dokument bezieht sich nicht auf die Messung von 3D Formen mithilfe von Stereokorrelationssystemen.

Série aérospatiale - Procédure d'évaluation métrologique applicable aux mesures de champs cinématiques par corrélation d'images numériques

Le présent document spécifie la surveillance des essais mécaniques et des inspections réalisés à la fois au niveau du matériau (éprouvette) et à l'échelle structurelle par la mise en oeuvre de mesures de champs cinématiques par corrélation d'images numériques. Ce document décrit une méthode sur site pour l'évaluation des performances métrologiques d'un système d'extensomètre en utilisant la corrélation d'images pour l'obtention de champs de déplacements, et par extrapolation, de champs de contrainte. Il peut être mis en oeuvre avant le démarrage réel de l'essai (ou de l'inspection). Il fournira des informations concernant les performances métrologiques en conditions d'essai.
Le présent document permet de quantifier les performances métrologiques de la technologie de mesure. La méthodologie décrite ici ne doit pas être considérée comme une étape d'étalonnage. Ce document de référence ne spécifie pas de manière exhaustive les éléments constitutifs d'un système générique de mesure par corrélation d'images numériques. La présente référence ne traite pas des mesures de formes 3D via des systèmes de stéréocorrélation.

Aeronavtika - Postopek meteorološkega ocenjevanja kinematičnih polj, ki se merijo z digitalno slikovno korelacijo

General Information

Status
Published
Publication Date
08-Dec-2020
Withdrawal Date
29-Jun-2021
Technical Committee
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Start Date
09-Dec-2020
Completion Date
09-Dec-2020

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SLOVENSKI STANDARD
SIST EN 4861:2021
01-februar-2021
Aeronavtika - Postopek meteorološkega ocenjevanja kinematičnih polj, ki se
merijo z digitalno slikovno korelacijo
Aerospace series - Metrological assessment procedure for kinematic fields measured by
digital image correlation
Luft- und Raumfahrt - Metrologisches Messverfahren für kinematische Felder durch
digitale Bildkorrelation
Série aérospatiale - Procédure d’évaluation métrologique applicable aux mesures de
champs cinématiques par corrélation d’images numériques
Ta slovenski standard je istoveten z: EN 4861:2020
ICS:
49.020 Letala in vesoljska vozila na Aircraft and space vehicles in
splošno general
SIST EN 4861:2021 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 4861:2021

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SIST EN 4861:2021


EN 4861
EUROPEAN STANDARD

NORME EUROPÉENNE

December 2020
EUROPÄISCHE NORM
ICS 17.180.01; 49.025.01
English Version

Aerospace series - Metrological assessment procedure for
kinematic fields measured by digital image correlation
Série aérospatiale - Procédure d'évaluation Luft- und Raumfahrt - Metrologisches Messverfahren
métrologique applicable aux mesures de champs für kinematische Felder durch digitale Bildkorrelation
cinématiques par corrélation d'images numériques
This European Standard was approved by CEN on 26 August 2019.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.





EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 4861:2020 E
worldwide for CEN national Members.

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EN 4861:2020 (E)
Contents Page
European foreword . 3
1 Scope . 4
2 Normative references . 4
3 Terms and definitions . 4
4 Symbols and abbreviations . 5
5 Principle . 5
6 System for the assessment of the metrological performance . 6
7 Pre-assessment inspection . 6
8 Measurement of physical pixel size . 7
9 Metrological assessment process . 7
10 Classification of the extensometer system . 11
11 Uncertainty determination . 13
12 Metrological performance assessment intervals for extensometer systems . 13
13 Metrological performance assessment certificates . 13
Annex A (informative) Uncertainty of measurement . 15
Annex B (informative) Classification of the system for the assessment of the metrological
performance . 21
Annex C (normative) Covariance and covariance matrix . 22
Annex D (informative) Template for metrological assessment report for kinematic fields
measured by digital image correlation . 28
Annex E (informative) Alternative approach for optical model identification in the case of
monovision measurements . 35
Bibliography . 37

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European foreword
This document (EN 4861:2020) has been prepared by the Aerospace and Defence Industries Association
of Europe — Standardization (ASD-STAN).
After enquiries and votes carried out in accordance with the rules of this Association, this Standard has
received the approval of the National Associations and the Official Services of the member countries of
ASD-STAN, prior to its presentation to CEN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by June 2021, and conflicting national standards shall be
withdrawn at the latest by June 2021.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland,
Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North
Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United
Kingdom.

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1 Scope
This document specifies the monitoring of mechanical tests and inspections performed both at the
material (coupon) and at the structural scale by the implementation of kinematic field measurements by
digital image correlation. This document describes an in situ method for evaluating the metrological
performance of an extensometer system using image correlation for the delivery of displacement fields,
and by extrapolation, of deformation fields. It can be implemented prior to the actual start of the test
(or inspection). It will inform of the metrological performance in testing conditions.
This document allows the metrological performance of the measuring technology to be quantified.
The methodology described herein is not to be considered as a calibration step. This reference document
does not exhaustively specify the constitutive elements of a generic system of Digital Image Correlation
measurement. This reference does not address the measurement of 3D shapes via stereocorrelation
systems.
2 Normative references
There are no normative references in this document.
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:
• ISO Online browsing platform: available at http://www.iso.org/obp
• IEC Electropedia: available at http://www.electropedia.org/
3.1
extensometer system
equipment used to measure displacement or strain fields on the surface of a tested piece
Note 1 to entry: The equipment consists of an image acquisition device and a computer system for calculating the
displacement and / or strain fields from the recorded images.
Note 2 to entry: For the purposes of this document, the term "Extensometer system" applies in particular to
kinematic field measurements by digital image correlation.
3.2
user
person in charge of the extensometer system implementation
3.3
2D measurement – monovision
extensometer system consisting of a single imager is a monovision system
Note 1 to entry: This system can provide full-field measurements in two (2) dimensions. The relevant plane is
perpendicular to the optical axis of the imaging system.

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3.4
3D surface measurement – stereocorrelation
extensometer system consisting of (at least) 2 (two) imagers is a stereovision system
Note 1 to entry: Through prior calibration following the supplier’s instructions, the system provides measured
displacement fields in three (3) dimensions of the monitored surface. This practice applied to image correlation is
defined by the term stereocorrelation.
4 Symbols and abbreviations
Symbols used throughout this document are given in Table 1 together with their designation.
Table 1 — Symbols and designations
Symbol Designation Unit
l Maximum limit of measured displacement mm
max
l Minimum limit of measured displacement mm
min
l Displacement indicated by extensometer system µm
i
l Displacement given by apparatus for assessment of metrological performance µm
t
q Relative bias error of extensometer system %
rb
q Absolute bias error of extensometer system µm
b
r Resolution of extensometer system µm
5 Principle
The assessment of the metrological performance of an extensometer system involves a comparison of the
readings given by the extensometer system with known variations in length provided and prescribed by
a system for the assessment of the metrological performance.
NOTE 1 The user can define the displacement range(s) over which the metrological performance assessment is
to be performed. In this way, the performance of the extensometer system can be optimized. The user should take
special care to distinguish real displacements induced in the structure of interest motions from the experimental
displacement commonly called “rigid body motions”. Hence, it would be appropriate in this case to concentrate the
performance assessment to the centre of the operating range.
The assessment process compares the known displacement from the calibration device with the output
of the extensometer system. This output is provided in the form of data from computer files generated by
the software performing the kinematic field analysis based on the acquired images. These files should
contain the displacement fields that will be evaluated and the coordinate at which they are evaluated.
NOTE 2 For certain types of extensometer systems, the calibration and classification will also be dependent upon
the ability of the system for the assessment of the metrological performance.

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6 System for the assessment of the metrological performance
6.1 Principle
The system for the assessment of the metrological performance, which allows a known displacement l to
t
be applied with respect to the object of interest, may consist of a rigid frame to which the image
acquisition device is attached. The system for the assessment of the metrological performance shall
comprise a mechanism for moving along the 3 (three) axes in space by translation, optionally from
1 (one) to 3 (three) rotations and a measuring device allowing to allow these displacements to be known
accurately. These variations in length can be measured by, for example, by an interferometer, a linear
incremental encoder or gauge blocks and a comparator, or a micrometre.
The calibration apparatus should be calibrated and should meet the performance requirements given in
Table B.1.
6.2 Traceability of metrological performance assessment
The calibration apparatus and the supporting equipment (such as micrometres, callipers, and optical
projection microscopes) shall be calibrated using standards that are traceable to the International System
of Units (SI).
The uncertainty associated with any measurement made by the supporting equipment shall not exceed
one third of the allowable error of the extensometer system being calibrated, see Table 2.
The temperature measurement instrument shall have a resolution of 0,1 °C.
7 Pre-assessment inspection
7.1 Aim
Prior to the assessment, the extensometer system shall be inspected. The quality of mechanical, optical,
electronic components and devices have been validated in terms of equipment such as the free motion of
tables, lenses, wiring and connections, hard drive space.
The extensometer system shall be assessed in the as-found condition if at all possible. The results shall
be analyzed and, if necessary, the system shall be adjusted and re-assessed. In this case, both data sets
shall be reported.
7.2 Records of the inspection
Records of the pre-assessment inspection shall be kept, identifying the “as-found” condition of the
extensometer system, when the inspection was performed and who performed it. These pre-assessment
inspection records can take the form of either a written report or a completed “pro-forma” checklist.
7.3 Identification of extensometer system elements
The extensometer system shall be uniquely identified. Parts that may be changed by the user during
normal use of the extensometer system that affects the metrological assessment of the extensometer
system shall also be uniquely identified whenever possible (e.g. camera, lens, lighting). These unique
identifiers are part of the records for the extensometer system. It will enable each component of the
system and the adjustment parameters to be referenced (e.g. lens settings, calculation parameters of
digital image correlation).
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8 Measurement of physical pixel size
8.1 Case of monovision measurements – software procedure
The measurement of the physical size of the pixel is performed according to the measuring system
manufacturer specifications.
8.2 Case od monovision measurements – manual procedure
The measurement of the physical size of the pixel is performed according to the internal procedure of the
user. The procedure and the result of this measurement shall be documented.
8.3 Case of monovision measurements – identification during the performance
assessment procedure
An alternative is to consider an identification procedure proposed in Annex E. This procedure requires
the user to be able to export the displacement field measurements in pixels with the extensometer system
and metric measurements for the evaluation of the metrological performance.
8.4 Case of stereovision measurements
This measurement is not applicable because it is treated during the calibration step of stereocorrelation
codes.
9 Metrological assessment process
9.1 Environmental considerations
9.1.1 General
The ambient temperature during the metrological performance assessment of the extensometer system
shall be recorded.
In general, the calibration of the extensometer system should be carried out at a temperature stable to
within ± 2 °C, the target temperature being within the range of 18 °C to 28 °C. Temperature changes
during the metrological performance assessment process may add to the uncertainty of the calibration
and in some cases may affect the ability to properly assess the metrological performance of the
extensometer system.
For extensometer systems used at temperatures outside the recommended range of 10 °C to 35 °C, the
metrological assessment should be carried out at or near the test temperature, if facilities exist.
The extensometer system shall be placed near the system for the assessment of the metrological
performance, or be mounted on it, for a sufficient duration prior to its assessment so that the parts of the
extensometer system and of the system for the assessment of the metrological performance that are in
contact stabilize at the metrological assessment temperature.
9.1.2 Lightning conditions
Lighting conditions for the metrological performance assessment process should be identical and
consistent with those of the operational use of the Extensometer system.
NOTE A histogram of gray level distribution could prove the quality of the lighting conditions between the
2 (two) uses.
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9.1.3 Case of artificial random speckle pattern
For experimental coupons or components requiring an artificial application of a random speckle pattern
(e.g. paint), it should be stable and unchanged during the metrological performance evaluation process
and during operational use of the extensometer system.
NOTE The measurement is performed with the same artificial random speckle pattern as for the metrological
performance assessment.
9.2 Calibration increments
9.2.1 The user shall establish the range of displacements over which the extensometer system shall be
assessed.
9.2.2 The number of calibration points, and the number of ranges over which the assessment is
performed, shall be based upon the relationship between the minimum displacement at which a property
is determined, l , and the maximum displacement at which a property is determined, l This approach
min max
shall be performed for each measurement axis, 2 (two) for monovision, and 3 (three) in
stereocorrelation.
9.2.3 The following series of readings shall be made for 1 (one) measurement axis:
a) If (l /l ) is less than or equal to 10, 1 (one) range of at least 5 (five) increments shall be recorded.
max min
b) If (l /l ) is greater than 10 but less than or equal to 100, 2 (two) ranges (l to 10 l and 10 l
max min min min min
to l ), or (l to 0,1l and 0,1 l to l ), each of at least 5 (five) increments, shall be recorded.
max min max max max
c) If (l /l ) is greater than 100, 3 (three) ranges (l to 10 l , 10 l to 100 l , 100 l to l ), or
max min min min min min min max
(l to 0,01 l , 0,01 l to 0,1 l , 0,1 l to l ), each of at least 5 (five) increments, shall be recorded.
min max max max max max
For each of the 3 (three) categories [a), b), c), see above], the increment between any 2 (two) adjacent
points shall not exceed one third of the range. Examples of these increments are shown in Figure 1.

8

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Key
1 assessment points
a) series according to 9.2.3 a)
b) series according to 9.2.3 b)
c) series according to 9.2.3 c)
Figure 1 — Schematic diagram showing assessment point distribution
NOTE 1 A tensile test measuring, from the extensometer system, the modulus and ultimate stresses only, would
fall into category a). A tensile test, establishing ultimate stresses and elongation to failure from the extensometer
system, or a creep to rupture test, would fall into category b) or category c).
NOTE 2 For fatigue tests, a range of at least 5 (five) increments (with the increment ranging between any 2 (two)
adjacent points not exceeding one third of the range between l and ) is used.
min lmax
NOTE 3 The values derived from the above calculations can be adjusted to the nearest convenient increments to
match those of the calibration apparatus.
9.2.4 When establishing lmax and lmin, operational factors such as thermal expansion of elevated
temperature tests and additional displacement contingencies to cover matters such as test to test set-up
variability shall be taken into account.
The implementation must mix the different measurement axes or spatial degrees of freedom satisfying
3 (three) mixing ratios: 0,5, 1 and 2, see Figure 2.
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Key
a mix ratio
assessment point
Figure 2 — Mapping of an assessment point distribution for a 2D monitoring
9.3 Process for metrological performance assessment
The evaluation consists of a series of measurements, as defined in 9.2. Depending on the intended use of
the extensometer system, the series is made for increasing lengths or decreasing lengths.
9.4 Determination of the extensometer system characteristics
9.4.1 General
The characteristics of the extensometer system defined below should be evaluated for each measurement
axis and independently even in case of mixing of points.
9.4.2 Resolution
9.4.2.1 The resolution r is the smallest quantity which can be read on the extensometer system.
9.4.2.2 The resolution of the extensometer system is the result of a measurement performed on
11 images in a “reference” state of the measured structure. No loading, no motion shall be applied to the
experimental support.
9.4.2.3 The result is the covariance matrix built from ten measured displacement fields. The resolution
field is deduced from the so-called covariance matrix. Taking the square root of the diagonal terms of the
covariance matrix, which corresponds to the variance of each random variable taken independently, to
set the resolution field.
NOTE The use of the covariance matrix is presented in Annex C.

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9.4.3 Bias error
9.4.3.1 Relative bias error and relative bias error field
The relative bias error of accurary, q , for a given displacement, l , is calculated from equation (1):
rb t
𝑙𝑙 − 𝑙𝑙
i t
𝑞𝑞 = × 100 (1)
rb
𝑙𝑙
t
This calculation is applied at all points of the displacement field provided by the extensometer system for
computing a relative bias error field.
9.4.3.2 Absolute bias error and absolute bias error field
The absolute accurary error, q , for a given displacement, l , is calculated from equation (2):
b t
q = (l − l ) (2)
b i t
This calculation is applied at all points of the displacement field provided by the extensometer system for
computing an absolute bias error field.
10 Classification of the extensometer system
10.1 Input data
The required input data for the classification of the extensometer system are:
a) the pixel/mm ratio for 2D monitoring;
b) the resolution (absolute and/or relative) of the extensometer system (see 9.4.1);
c) for each point, the bias error field (absolute and/or relative);
d) the confirmation that the metrological assessment system fulfilled the requirements of this document
for each evaluation data point.
10.2 Analysis of the data
The collected data are assessed as follows:
e) the resolution field of the extensometer system for each point of metrological assessment is
compared with the limits in Table 2 and a classification is obtained;
f) for each evaluation data point, the bias error is compared to the limits in Table 2 and a classification
is obtained.

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10.3 Classification criteria
Table 2 gives the maximum allowable values for the resolution field and the bias error field.
Table 2 — Classification of the extensometer system
a a
Class of Resolution  Bias error
extensometer
b
Percentage of reading Absolute value Relative value
system
(r/l )·100 r q
i rb
% px %
0,2 0,1 0,02 ± 0,2
0,5 0,25 0,05 ± 0,5
1 0,5 0,1 ± 1,0
2 1,0 0,2 ± 2,0
a
Whichever is greatest.
b
Only for monovision monitoring.
10.4 Assessment of the results
10.4.1 The data specified in 9.2 are collected and the maximum classification value for each of the
following is determined:
g) for the data points to the reference state of the metrological performance assessment, the resolution
field of the extensometer system;
h) for the data points of the metrological performance assessment, the bias error field;
i) for the data points of the metrological performance assessment, the classification of the system for
the assessment of the metrological performance.
This maximum value of these three (3) parameters is defined as the classification for the extensometer
system.
10.4.2 Whenever adjustments are needed for the extensometer system to comply with class
requirements for its intended use, the calibration provider can, with laboratory approval, make such
adjustments to enhance the extensometer system performance. The records from the initial metrological
performance assessment shall be stored and supplied as part of the metrological performance
assessment documentation. The post-adjustment results shall be reported on the metrological
performance assessment certificate.

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11 Uncertainty determination
11.1 Uncertainty of the metrological performance assessment
Many elements contribute to the uncertainty of the assessment process. The following items shall be
assessed and incorporated into the uncertainty budget calculation:
a) calibration uncertainty of the system for the assessment of the metrological performance;
b) ambient temperature fluctuations during the metrological performance assessment;
c) inter-operator variability where more than 1 (one) person performs metrological performance
assessments within a laboratory;
For further information, refer to Annex A (informative).
11.2 Uncertainty budget determination
The uncertainty shall be determined. An example of calculation, which shows how to perform an
uncertainty evaluation for an extensometer system, is presented as Annex A (informative).
NOTE The requirements of this document limit the major components of uncertainty when assessing the
metrological performance of digital image correlation measuring systems. By complying with this metrological
document, the uncertainty is explicitly taken into account as required by some accreditation standards. Reducing
the allowable bias by the amount of the uncertainty would result in double counting of the uncertainty.
The classification of an extensometer system certified to meet a specific class does not ensure that the accuracy
including uncertainty will be less than a specific value. For example, an extensometer system meeting class 0,5 does
not necessarily have a bias including uncertainty of less than 0,5 %.
12 Metrological performance assessment intervals for extensometer systems
12.1 The time between 2 (two) metrological performance assessments depends on the type of
extensometric system, the maintenance standard and the number of times the extensometer system has
been used. Under normal conditions, it is recommended that metrological performance assessment be
carried out at each new use of the digital image correlation monitoring system.
12.2 The extensometer system shall be assessed after each repair or adjustment that affects the
accuracy of measurements.
13 Metrological performance assessment certificates
13.1 Mandatory information
The metrological performance assessment certificate shall contain at least the following information:
a) reference to this document;
b) name and address of the owner of the extensometer system;
c) identification of the extensometer system (type, make, serial number and mounting position);
d) type and reference number of the system for metrological performance assessment;
e) temperature during the metrological performance assessment process;
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f) nature of the variations of length for which the metrological performance assessment was carried
out, i.e. either for increases and/or for decreases in length;
g) computation parameters used for DIC Measurements (e.g., facet or zone of interest size, facet step,
element size, number of scales)
h) date of metrological performance assessment;
i) name of the person who performed the metrological performance assessment, plus the name or
brand of the calibrating organization;
j) all results from the metrological performance assessment (as-found condition and, if adjusted, after
adjustment measurements);
k) a statement of uncertainty;
l) classification for each range of the Exte
...

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