SIST EN ISO 16808:2014
(Main)Metallic materials - Sheet and strip - Determination of biaxial stress-strain curve by means of bulge test with optical measuring systems (ISO 16808:2014)
Metallic materials - Sheet and strip - Determination of biaxial stress-strain curve by means of bulge test with optical measuring systems (ISO 16808:2014)
EN ISO 16808 specifies a method for determination of the biaxial stress-strain curve of metallic sheets having a thickness below 3 mm in pure stretch forming without significant friction influence. In comparison with tensile test results, higher strain values can be achieved.
Metallische Werkstoffe - Bleche und Bänder - Bestimmung der biaxialen Spannung/Dehnung-Kurve durch einen hydraulischen Tiefungsversuch mit optischen Messsystemen (ISO 16808:2014)
Diese Internationale Norm legt ein Verfahren zur Ermittlung der biaxialen Spannung/Dehnung-Kurve von Metallblechen mit einer Dicke unter 3 mm bei reinem Streckziehen ohne signifikanten Reibungseinfluss fest. Im Vergleich zu den Ergebnissen der Zugprüfung können höhere Formänderungswerte erreicht werden.
Matériaux métalliques - Tôles et bandes - Détermination de la courbe contrainte-déformation biaxiale par la méthode du renflement avec système de mesure optique (ISO 16808:2014)
L'ISO 16808:2014 spécifie une méthode pour la détermination de la courbe contrainte-déformation biaxiale sur tôles métalliques d'épaisseur inférieure à 3 mm en formage en expansion pure sans influence significative des frottements. En comparaison à des résultats d'essais de traction, des valeurs plus élevées de la déformation peuvent être obtenues.
Kovinski materiali - Pločevina in trak - Ugotavljanje krivulje dvoosnega diagrama z izboklinskim preskusom optičnih merilnih sistemov (ISO 16808:2014)
Standard EN ISO 16808 določa metodo za ugotavljanje krivulje dvoosnega diagrama kovinskih pločevin z debelino, manjšo od 3 mm, pri oblikovanju popolnega raztezanja brez pomembnih vplivov trenja. V primerjavi z rezultati nateznega preskusa je ob tem mogoče doseči večje natezne vrednosti.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN ISO 16808:2014
01-oktober-2014
.RYLQVNLPDWHULDOL3ORþHYLQDLQWUDN8JRWDYOMDQMHNULYXOMHGYRRVQHJDGLDJUDPD]
L]ERNOLQVNLPSUHVNXVRPRSWLþQLKPHULOQLKVLVWHPRY ,62
Metallic materials - Sheet and strip - Determination of biaxial stress-strain curve by
means of bulge test with optical measuring systems (ISO 16808:2014)
Metallische Werkstoffe - Bleche und Bänder - Bestimmung der biaxialen
Spannung/Dehnung-Kurve durch einen hydraulischen Tiefungsversuch mit optischen
Messsystemen (ISO 16808:2014)
Matériaux métalliques - Tôles et bandes - Détermination de la courbe contrainte-
déformation biaxiale par la méthode du renflement avec système de mesure optique
(ISO 16808:2014)
Ta slovenski standard je istoveten z: EN ISO 16808:2014
ICS:
77.040.10 Mehansko preskušanje kovin Mechanical testing of metals
77.140.50 3ORãþDWLMHNOHQLL]GHONLLQ Flat steel products and semi-
SROL]GHONL products
SIST EN ISO 16808:2014 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN ISO 16808:2014
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SIST EN ISO 16808:2014
EUROPEAN STANDARD
EN ISO 16808
NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2014
ICS 77.040.10
English Version
Metallic materials - Sheet and strip - Determination of biaxial
stress-strain curve by means of bulge test with optical measuring
systems (ISO 16808:2014)
Matériaux métalliques - Tôles et bandes - Détermination de Metallische Werkstoffe - Bleche und Bänder - Bestimmung
la courbe contrainte-déformation biaxiale au moyen de der biaxialen Spannung/Dehnung-Kurve durch einen
l'essai de gonflement hydraulique avec systèmes de hydraulischen Tiefungsversuch mit optischen
mesure optiques (ISO 16808:2014) Messsystemen (ISO 16808:2014)
This European Standard was approved by CEN on 4 July 2014.
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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
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Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2014 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 16808:2014 E
worldwide for CEN national Members.
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SIST EN ISO 16808:2014
EN ISO 16808:2014 (E)
Contents Page
Foreword .3
2
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SIST EN ISO 16808:2014
EN ISO 16808:2014 (E)
Foreword
This document (EN ISO 16808:2014) has been prepared by Technical Committee ISO/TC 164 “Mechanical
testing of metals” in collaboration with Technical Committee ECISS/TC 101 “Test methods for steel (other
than chemical analysis)” the secretariat of which is held by AFNOR.
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 January 2015, and conflicting national standards shall be withdrawn at
the latest by January 2015.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech
Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.
Endorsement notice
The text of ISO 16808:2014 has been approved by CEN as EN ISO 16808:2014 without any modification.
3
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SIST EN ISO 16808:2014
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SIST EN ISO 16808:2014
INTERNATIONAL ISO
STANDARD 16808
First edition
2014-08-01
Metallic materials — Sheet and strip
— Determination of biaxial stress-
strain curve by means of bulge test
with optical measuring systems
Matériaux métalliques — Tôles et bandes — Détermination de
la courbe contrainte-déformation biaxiale au moyen de l’essai de
gonflement hydraulique avec systèmes de mesure optiques
Reference number
ISO 16808:2014(E)
©
ISO 2014
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2014
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
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Published in Switzerland
ii © ISO 2014 – All rights reserved
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Symbols and abbreviated terms . 1
3 Principle . 2
4 Test equipment. 2
5 Optical measurement system . 6
6 Test piece . 6
6.1 General . 6
6.2 Application of grid . 6
7 Procedure. 7
8 Evaluation methods for the determination of the curvature and strains at the pole .7
9 Calculation of biaxial stress-strain curves . 8
10 Test report . 9
Annex A (informative) International comparison of symbols used in the determination of the
bulge test flow curve .11
Annex B (normative) Test procedure for a quality check of the optical measurement system .13
Annex C (informative) Computation of the curvature on the basis of a response surface
.16
Annex D (informative) Determination of the equi-biaxial stress point of the yield locus and the
hardening curve .18
Bibliography .26
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 164, Mechanical testing of metals, Subcommittee
SC 2, Ductility testing.
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SIST EN ISO 16808:2014
INTERNATIONAL STANDARD ISO 16808:2014(E)
Metallic materials — Sheet and strip — Determination
of biaxial stress-strain curve by means of bulge test with
optical measuring systems
1 Scope
This International Standard specifies a method for determination of the biaxial stress-strain curve
of metallic sheets having a thickness below 3 mm in pure stretch forming without significant friction
influence. In comparison with tensile test results, higher strain values can be achieved.
NOTE In this document, the term “biaxial stress-strain curve” is used for simplification. In principle, in the
test the “biaxial true stress-true strain curve” is determined.
2 Symbols and abbreviated terms
The symbols and designations used are given in Table 1.
Table 1
Symbol Designation Unit
d Diameter of the die (inner) mm
die
d Diameter of the blank holder (inner) mm
BH
R Radius of the die (inner) mm
1
h Height of the drawn blank (outer surface) mm
t Initial thickness of the sheet (blank) mm
0
t Actual thickness of the sheet mm
p Pressure in the chamber MPa
rms Standard deviation (root mean square) -
ρ Radius of curvature mm
r Surface radius for determining curvature mm
1
r Surface radius for determining strain mm
2
r Surface radius to determine curvature with a die diam- mm
1_100
eter of 100 mm
a , b Coefficients for response surface -
i i
σ Biaxial stress MPa
Β
e Engineering strain -
ε Major true strain -
1
ε Minor true strain -
2
ε True thickness strain -
3
ε Equivalent true strain -
Ε
l Coordinate and length of a section mm
s
dz Displacement in the z-direction mm
dz Displacement after movement correction mm
mv
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
3 Principle
A circular blank is completely clamped at the edge in a tool between die and blank holder. A bulge is
formed by pressing a fluid against the blank until final fracture occurs (Figure 1). During the test,
the pressure of the fluid is measured and the evolution of the deformation of the blank is recorded by
[1],[2],[3]
an optical measuring system. Based on the recorded deformation of the blank, the following
quantities near the centre of the blank are determined: the local curvature, the true strains at the
surface, and, by assuming incompressible deformation of the material, the actual thickness of the blank.
Furthermore, assuming the stress state of a thin-walled spherical pressure vessel at the centre of the
blank, the true stress is calculated from the fluid pressure, the thickness and the curvature radius.
NOTE In addition to the bulge test procedures with optical measurement systems introduced in Reference [1]
[4] [5] [6] [7] [8] [9]
and described in the following, there are also laser systems , , or tactile systems , , valid for bulge test
investigation, which are not covered in this International Standard.
Key
h height of the drawn blank (outer surface) ρ radius of curvature
p pressure in the chamber t initial thickness of the sheet (blank)
0
ε true thickness strain (at the apex of the dome) t actual thickness of the sheet
3
d diameter of the die (inner)
die
Figure 1 — Principle of the bulge test
The coordinate origin shall be in the centre of the blank holder. The XY-plane should be parallel to the
surface of the blank holder (parallel to the clamped metal sheet before forming). Herein, the x-direction
corresponds to the rolling direction. The z-direction shall be normal to the clamped metal sheet before
forming, with the positive direction towards the optical sensor.
4 Test equipment
4.1 The bulge test shall be carried out on a machine equipped with a die, a blank holder and a fluid
chamber. The proposed equipment is illustrated in Figure 2.
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
Key
1 deformation measurement system 3 chamber with fluid
2 lock bead 4 pressure measurement system
Figure 2 — Proposal of a testing equipment (principle drawing)
4.2 The lay out of the test equipment shall be such that it is possible to continuously measure the
outside surface of the test piece continuously during the test, i.e. to be able to determine the deformation
of the geometry by recording the evolution of X, Y, Z coordinates of a grid of points on the bulging blank
surface, in order to calculate the shape and the true strains in the central area of interest until failure
occurs.
4.3 During the test, the system shall be able to measure optically (without contact) the X, Y, Z coordinates
of a grid of points on the bulging surface of the specimen. Out of these coordinates, the true strains ε and
1
ε for each point of the selected area, the thickness strain ε and the curvature radius ρ for the apex of the
2 3
dome are calculated.
4.4 The system should be equipped with a chamber fluid pressure measurement system. An indirect
measurement system is also possible. Starting from 20 % of the maximum measured pressure value, the
precision should be 1 % of the actual measured value.
4.5 The die, the blank holder and the fluid chamber shall be sufficiently rigid to minimize deformation
during testing. The blank-holder force shall be high enough to keep the blank holder closed. Any movement
of the test piece between the blank holder and die should be prevented. Typically during the test, the
bulge pressure is acting on parts of the blank holder reducing the effective blank-holder force. This shall
be taken in consideration when defining the necessary blank-holder force.
4.6 The fluid shall be in contact with the blank surface (without any air bubbles) to prevent energy
storage during the test through compressed air bubbles which would lead to higher energy release and
greater oil splashing at failure. No fluid shall be lost through the blank holder, die and sheet or elsewhere
during the test until failure occurs.
4.7 A lock bead (or comparable geometry in the circular surface), designed to suppress any material
flow, is recommended. The lock bead shall not initiate cracks in the material. The lock bead shall be located
between blank holder and die. A location close to the die radius is recommended. The lock bead geometry
should avoid a curvature and a wrinkling of the blank when closing the tool and prevent the sliding of the
blank during the test.
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
4.8 It is recommended to place glass plates in front of the lenses and the illumination in order to protect
[7],[12]
the optical measuring system from oil splashing due to blank failure at the end of the test. The
plates can be fixed on the blank holder (thick glass) or near the camera lenses and illumination (thinner
glass); see Figure 3. The inserted protection shall not to disturb the optical measurement quality (see
Clause 5). After each test, the glass plates shall be well cleaned without damaging or scratching them and
precisely repositioned to not alter calibration. Typically, a calibration of the optical system including the
protection increases the measurement quality.
4.9 The smallest die diameter recommended should have a ratio of die diameter to initial thickness
d / t ≥ 33 (see Figure 2). The radius of the die should not lead to cracks in the blank during the test. A
die 0
recommendation is (5 × t ) to (15 × t ) (maximum 15 mm).
0 0
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
a) b)
c)
Key
1 lamp 4 test piece
2 cameras 5 fluid
3 glass plates
Figure 3 — Examples for possible positions of oil shielding plates and lamps
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
5 Optical measurement system
For the determination of the radius of curvature ρ, and the true strains ε and ε , an optical-deformation
1 2
field measurement system with the following characteristics is recommended:
— Optical sensor based on two or more cameras;
— Measurement area, whereas d ≥ 1/2 d ;
measurement area die
The used measurement area should be larger than a concentric diameter of half the diameter of the
blank holder. This area should be observable during the entire forming process for all heights of the
drawn blank.
— Local resolution (grid distance between the independent measurement points): The distance g
max
between two adjacent points on the unformed blank should follow the requirement:
d
die
g ≤
max
50
— The determination of the curvature requires an accuracy of the z-coordinates in an area with a
diameter of 1/2 d concentric to the blank holder of
die
rmsd()z ⋅100mm
rmsd()z = ≤0,015mm
n
d
die
NOTE The accuracy of the shape measurement can be checked with a test of the optical measurement system
(see Annex B).
Accuracy for strain measurement: rms (ε ) = 0,003 rms (ε ) = 0,003
1 2
For each real strain value for the mentioned rms (ε) above, the acceptable measurement values are:
ε = 0 acceptable measurement range: –0,003 … 0,003
real
ε = 0,5 acceptable measurement range: 0,497 … 0,503
real
— Missing measurement points: In order to avoid unbalanced curvature approximations, only the
absence of less than 5 % of the measurement points in the concentric area with a diameter = 1/2 d
die
is acceptable (without interpolation). If adjacent points are missing, the inscribed circle of this area
shall not be larger than 2 points.
6 Test piece
6.1 General
The test piece shall be flat and of such shape that the blank is clamped and material flow is stopped. The
use of lock beads is recommended. The edge of the blank shall be outside the lock bead.
The preparation of the blank does not influence the results as long as the surface of the test piece was
not damaged (scratches, polishing). The dimension of the outer edges can be circular (preferred) or
angular.
6.2 Application of grid
6.2.1 Type of grid
For optical full-field measurement devices, the grid shall fulfil two objectives:
a) the curvature radius determination of the specimens’ surface;
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
b) the strain calculation of the material deformation.
6.2.2 Grid application
Deterministic grids (e.g. squares, circles, dots) should have a strong contrast and have to be applied
without any notch effect and/or change in microstructure. Some common application techniques are:
— electrochemical etching, photochemical etching, offset printing and grid transfer,
— stochastic (speckle) patterns which can be applied by spraying paint on the surface of test piece
surfaces. Paint adherence to the surface after deformation should be checked. It is possible first to
spray a thin, matt, white base layer to reduce reflections from the test piece surfaces, then to spray
a cloud of randomly distributed black spots (e.g. black spray paint or graphite). The spray shall be
both elastic and tough enough not to crack or peel off during deformation. The random distribution
of the fine sprayed spots allows the determination of each point of the virtual grid on the specimen.
The pattern should have sufficient black/white density and appropriate size features in each point
position search area as required by the optical system used.
7 Procedure
7.1 The test shall be carried out at ambient temperature of (23 ± 5) °C.
7.2 Determine the initial thickness of the test piece to the nearest 0,01 mm.
7.3 Clamp the test piece between blank holder and die. Avoid air bubbles between test piece and fluid
to prevent formation of compressed air during testing, leading to stronger oil splashing at failure.
−1
7.4 A constant strain rate of 0,05 s is recommended. If a constant strain rate is not possible, a constant
forming velocity of the punch or fluid should be guaranteed. In order to avoid big influences in the biaxial
stress-strain curve of temperature or strain rate sensitive materials, the bulge test should be conducted in
(2 to 4) min. This time frame guarantees slow and acceptable strain rates and a cost-effective testing time.
The plot of the strain rate versus time is recommended.
7.5 Measure the fluid pressure during the test.
7.6 Measure the X, Y, Z coordinates of the grid on the test piece surface during the test.
7.7 The fluid pressure data and forming data shall be measured and saved at the same time scale. A
minimum of 100 values is recommended. In order to represent the whole strain and pressure development,
at least 100 images of the bulge testing are recommended.
7.8 The failure of the test piece shall be considered as obtained when a through crack, i.e. a crack which
goes through the thickness of the test piece, has occurred. The failure is detected by decreasing fluid
pressure; this defines the end of the test.
7.9 A sufficient number of test pieces should be prepared in order to achieve at least three valid tests.
8 Evaluation methods for the determination of the curvature and strains at the
pole
For the following explanation of the calculation of the curvature and strains, a spherically shaped surface
near the pole is assumed (best-fit sphere). On the last image before failure, as defined in 7.8, the area of
the dome with the highest deformation is selected and defined as the position where to determine the
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SIST EN ISO 16808:2014
ISO 16808:2014(E)
true stress and the true thickness strain ε . To obtain a stable radius of curvature of the dome, a best-
3
fit sphere can be calculated based on a selected area of points. For this selection, a radius r is defined
1
around the apex of the dome in the last image before bursting and the fit is performed for all forming
stages with the same selection of points (Figure 4).
A certain number of the first forming stages (images) are rejected, since the specimen is still too flat
for a reliable determination of the best-fit sphere, since the bending radius is very high and the fit is
not stable. For robust values of the true strain and thinning in the apex, the average value of a number
of selected points is taken. Therefore, a second area is defined by a radius r in a similar manner (see
2
Figure 4).
Based on this procedure, for every forming stage (image) the radius of curvature, the average thickness
strains, as well as the corresponding thickness and stress values at the dome apex are calculated. This
evaluation can be carried out for different r and r values (see Figure 4).
1 2
For a good convergence and robust values, the recommended range of r and r is defined:
1 2
r = (0,125 ± 0,025) × d (1)
1 die
r = (0,05 ± 0,01) × d (2)
2 die
Figure 4 — Choice of r and r for calculation of true stress and true strain for each forming
1 2
stage
An alternative proposal for the calculation of the curvature and strains is given in the Annex C.
9 Calculation of biaxial stress-strain curves
For the calculation of the biaxial stress-strain curves, a simple membrane stress state of a thin-walled
spherical pressure vessel is assumed at the centre of the blank. This implies the following simplifications:
a) equi-biaxial stress state:
σσ==σ (3)
12 B
b) representation of the curvature by the mean curvature radius:
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−1
1
ρρ=+()11 ρ (4)
12
2
Then the biaxial true stress can be calculated according to the following equation
ρp
σ = (5)
B
2t
using the fluid pressure p, the curvature radius ρ and the actual thickness t, with
tt= exp(ε ) (6)
03
Assuming plastic incompressible deformation of the material and neglecting elastic strains, the total
thickness strain for the calculation of the actual thickness can be approximated by the total major and
minor true strain:
εε≈− −ε .
31 2
(7)
Based on the plastic work principle, the biaxial stress-strain curve is a function of the plastic thickness
pl
strain:σε()− , see also Annex D. Assuming an isotropic linear elastic material behaviour and plastic
B
3
incompressibility, the plastic thickness strain is then given by:
1−ν
pl
εε=− −+ε 2 σ .
3 12 B
E
(8)
For the elasticity modulus E and the Poisson ratio ν, literature values are generally sufficient to subtract
the elastic contribution, e.g. E = 210 GPa and ν = 0,33 for steel, respectively E = 70 GPa and ν = 0,33 for
aluminium alloys.
The ratio of die diameter to thickness should be reasonably high to ensure a near membrane stress
state in the test piece, and a negligible influence of bending. For die diameter to thickness ratios lower
than 100, it is recommended to check if the bending strains are relatively small compared to the actual
thickness strain result ε using the following estimate for the bending strains:
3
t
0
ε ≈−ln 1− exp(ε ) (9)
bending 3
2ρ
NOTE The biaxial stress-strain curve is obtained without any assumption on the type of yield criterion. This
biaxial stress-strain curve can be used to identify the equi-biaxial
...
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