Metallic materials — Tensile testing — Part 4: Method of test in liquid helium

ISO 6892-4:2015 specifies the method of tensile testing of metallic materials in liquid helium (the boiling point is ?269 °C or 4,2 K, designated as 4 K) and defines the mechanical properties that can be determined. This part of ISO 6892 may apply also to tensile testing at cryogenic temperatures (less than ?196 °C or 77 K), which requires special apparatus, smaller test pieces, and concern for serrated yielding, adiabatic heating, and strain-rate effects. To conduct a tensile test according to this part of ISO 6892 at 4 K, the test piece installed in a cryostat is fully submerged in liquid helium (He) and tested using displacement control at a nominal strain rate of 10−3 s−1 or less. NOTE The boiling point of the rare 3He isotope is 3,2 K. Usually, the tests are performed in 4He or a mixture of 3He and 4He with a high concentration of 4He. Therefore, the temperature is, as designated before, 4 K.

Matériaux métalliques — Essai de traction — Partie 4: Méthode d'essai dans l'hélium liquide

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Status
Published
Publication Date
28-Sep-2015
Current Stage
9093 - International Standard confirmed
Completion Date
09-Feb-2021
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INTERNATIONAL ISO
STANDARD 6892-4
First edition
2015-10-01
Metallic materials — Tensile testing —
Part 4:
Method of test in liquid helium
Matériaux métalliques — Essai de traction —
Partie 4: Méthode d’essai dans l’hélium liquide
Reference number
ISO 6892-4:2015(E)
©
ISO 2015

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ISO 6892-4:2015(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2015, Published in Switzerland
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
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
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CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2015 – All rights reserved

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ISO 6892-4:2015(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and designations . 4
5 Principle . 4
6 Apparatus . 4
7 Test piece . 7
7.1 General . 7
7.2 Standard round bar test piece . 7
7.3 Alternatives . 7
7.4 Sub-size test pieces . 7
7.5 Sampling . 7
8 Testing conditions . 8
8.1 Test piece installation . 8
8.2 Cooling procedure . 8
8.3 Rate of testing . 8
8.3.1 Rate limit. 8
8.3.2 Rate selection . 8
9 Procedure. 9
9.1 Determination of original cross-sectional area (S ) . 9
o
9.2 Marking of the original gauge length (L ) . 9
o
9.3 Determination of percentage elongation after fracture (A) . 9
9.4 Determination of the 0,2 % proof-strength, plastic extension (R ) . 9
p0,2
9.5 Discontinuous yielding strength (R ) .10
i
9.6 Tensile strength (R ) .10
m
9.7 Reduction of area (Z) .10
10 Test report .10
11 Measurement uncertainty .10
Annex A (informative) Examples of test pieces for tensile testing in liquid helium .11
Bibliography .13
© ISO 2015 – All rights reserved iii

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ISO 6892-4:2015(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
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
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 1, Uniaxial testing.
This first edition of ISO 6892-4 cancels and replaces ISO 19819:2004, which has been technical revised.
ISO 6892 consists of the following parts, under the general title Metallic materials — Tensile testing:
— Part 1: Method of test at room temperature
— Part 2: Method of test at elevated temperature
— Part 3: Method of test at low temperature
— Part 4: Method of test in liquid helium
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ISO 6892-4:2015(E)

Introduction
The force-time and force-extension records for alloys tested in liquid helium using displacement
control are serrated. Serrations are formed by repeated bursts of unstable plastic flow and arrests. The
unstable plastic flow (discontinuous yielding) is a free-running process occurring in localized regions of
the parallel length at higher rates than nominal strain rates with internal test piece heating. Examples
of serrated stress-strain curves for a typical austenitic stainless steel with discontinuous yielding are
shown in Figure 1.
Key
2
1 stress, N/mm
2 strain
3 temperature, K
Figure 1 — Example of typical stress-strain curves and test piece temperature histories at four
different nominal strain rates, for AISI 304L stainless steel tested in liquid helium
A constant test piece temperature cannot be maintained at all times during testing in liquid helium.
Due to adiabatic heating, the test piece temperature at local regions in the parallel length rises
temporarily above 4 K during each discontinuous yielding event (see Figure 1). The number of events
and the magnitude of the associated force drops are a function of the material composition and other
factors such as test piece size and test speed. Typically, altering the mechanical test variables can
change the type of serration but not eliminate the discontinuous yielding. Therefore, tensile property
measurements of alloys in liquid helium (especially tensile strength, elongation, and reduction of area)
may lack the usual significance of property measurements at room temperature where deformation is
more nearly isothermal and discontinuous yielding typically does not occur.
Strain control is the preferred control mode (Method A, 6892-1) and displacement control is the
secondary method, according to Method B 6892-1.
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INTERNATIONAL STANDARD ISO 6892-4:2015(E)
Metallic materials — Tensile testing —
Part 4:
Method of test in liquid helium
1 Scope
This part of ISO 6892 specifies the method of tensile testing of metallic materials in liquid helium
(the boiling point is –269 °C or 4,2 K, designated as 4 K) and defines the mechanical properties that
can be determined.
This part of ISO 6892 may apply also to tensile testing at cryogenic temperatures (less than –196 °C or
77 K), which requires special apparatus, smaller test pieces, and concern for serrated yielding, adiabatic
heating, and strain-rate effects.
To conduct a tensile test according to this part of ISO 6892 at 4 K, the test piece installed in a cryostat is
fully submerged in liquid helium (He) and tested using displacement control at a nominal strain rate of
−3 −1
10 s or less.
3 4
NOTE The boiling point of the rare He isotope is 3,2 K. Usually, the tests are performed in He or a mixture
3 4 4
of He and He with a high concentration of He. Therefore, the temperature is, as designated before, 4 K.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
1)
ISO 6892-1:— , Metallic materials — Tensile testing — Part 1: Method of test at room temperature
ISO 6892-3, Metallic materials — Tensile testing — Part 3: Method of test at low temperature
1
ISO 7500-1 , Metallic materials — Calibration and verification of static uniaxial testing machines — Part
1: Tension/compression testing machines — Calibration and verification of the force-measuring system
ISO 9513, Metallic materials — Calibration of extensometer systems used in uniaxial testing
3 Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 6892-1 and in ISO 6892-3 apply.
3.1
adiabatic heating
internal heating of a test piece resulting from deformation under conditions such that the heat generated
by plastic work cannot be quickly dissipated to the surrounding cryogen
1) To be published.
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ISO 6892-4:2015(E)

3.2
axial strain
longitudinal strains measured at opposite or equally spaced surface locations on the sides of the
longitudinal axis of symmetry of the test piece
Note 1 to entry: The longitudinal strains are measured using two or more strain-sensing transducers located at
the mid-length of the parallel length.
3.3
bending strain
difference between the strain at the surface of the test piece and the axial strain
Note 1 to entry: The bending strain varies around the circumference and along the parallel length of the test piece.
3.4
dewar
vacuum-insulated container for cryogenic fluids
3.5
discontinuous yielding strength
R
i
peak stress at the initiation of the first measurable serration on the stress-strain curves
3.6
tensile cryostat
test apparatus for applying tensile forces to test pieces in cryogenic environments
Note 1 to entry: See Figure 2.
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ISO 6892-4:2015(E)

1
12
11
2
3
10
9
4
8
5
7
6
Key
1 force 7 extensometer
2 room temperature load frame 8 vacuum-insulated dewar
3 vent 9 dewar seal
4 vacuum-insulated transfer tube 10 electrical feed-through
5 cryogenic load frame 11 load cell
6 test piece 12 pull rod
Figure 2 — Schematic illustration of typical cryostat for tensile testing at 4 K
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ISO 6892-4:2015(E)

4 Symbols and designations
Symbols and corresponding designations are given in Table 1.
Table 1 — Symbols and designations
Symbol Unit Designation
d mm diameter of the parallel length of a cylindrical test piece or diameter of a circular wire
o
L mm original gauge length
o
L mm final gauge length after fracture
u
L mm parallel length
c
L mm extensometer gauge length
e
2
S mm original cross-sectional area of the parallel length
o
2
S mm minimum cross-sectional area after fracture (final cross-sectional area)
u
Z % percentage reduction of area:
SS−
ou
Z= ×100
S
o
A % percentage elongation after fracture:
LL−
uo
A= ×100
L
o
F N maximum force
m
2
R N/mm tensile strength
m
2
R N/mm 0,2 % proof strength, plastic extension
p0,2
2
R N/mm discontinuous yielding strength
i
5 Principle
The test consists of straining a test piece in liquid helium by a tensile force, generally to fracture, for the
purpose of determining one or more of the mechanical properties defined in Clause 4.
6 Apparatus
6.1 Testing machine
The testing machine shall be verified and calibrated in accordance with ISO 7500-1 and shall be of at
least class 1, unless otherwise specified in the product standard.
6.1.1 Testing machine compliance
Compliance (displacement per unit of applied force of the apparatus itself) of the test facility (tensile
machine and the cryogenic load frame) should be known. Measure the compliance by coupling
the load train with a rigid test piece or by using a special calibration test piece. Then, measure the
compliance at a low force and at the highest force used to qualify the machine, as indicated in 6.1.4. A
practical procedure for the determination of the compliance respective of the stiffness is described in
ISO 6892-1:—, Annex F.
NOTE Different system compliances may result in different stress-extension curves and material properties
(e.g. elongation after fracture, tensile strength) of the material because a larger discontinuous deformation
occurs in a lower compliance test facility.
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ISO 6892-4:2015(E)

6.1.2 System design
Typically, alloys in liquid helium exhibit double or triple their ambient strengths at ambient
temperature. For the same test piece geometry, higher forces shall be applied to the cryostat, test piece,
load train members, and grips at cryogenic temperatures. Since many conventional test machines have
a maximum force of 100 kN or less, it is recommended that the apparatus be designed to accommodate
one of the small test pieces cited in 7.2.
6.1.3 Construction materials
Many construction materials, including the vast majority of ferritic steels, are brittle at 4 K. To prevent
service failures, fabricate the grips and other load train members using strong, tough, cryogenic alloys.
Materials that have low thermal conductivity are desirable to reduce heat flow. Austenitic stainless
steels (AISI 304LN), maraging steels (200, 250, or 300 grades, with nickel plating to prevent rust),
wrought nickel-base superalloys, and titanium alloys (Ti-6Al-4V and Ti-5Al-2,5Sn) have been used with
proper design, for grips, pull rods, and cryostat frames. Non-metallic materials (for example, glass-
epoxy composites) are excellent insulators and are sometimes used for compression members.
6.1.4 Alignment
Proper system alignment is essential to minimize bending strains in the tensile tests. The machine
and grips should be capable of applying force to a precisely machined calibration test piece so that
the maximum
...

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