Metallic materials — Method of test for the determination of resistance to stable crack extension using specimens of low constraint

ISO 22889:2007 specifies methods for determining the resistance to stable crack extension in terms of crack opening displacement and critical crack tip opening angle for homogeneous metallic materials by the quasistatic loading of cracked specimens that exhibit low constraint to plastic deformation. Compact and middle-cracked tension specimens are notched, precracked by fatigue, and tested under slowly increasing displacement. ISO 22889:2007 describes methods covering tests on specimens not satisfying requirements for size-insensitive fracture properties; namely, compact specimens and middle-cracked tension specimens in relatively thin gauges. Methods are given for determining the crack extension resistance curve. Methods for determining point values of fracture toughness for the middle-cracked tension specimen are also given.

Matériaux métalliques — Méthode d'essai pour la détermination de la résistance à la propagation stable de fissures au moyen d'éprouvettes à faible taux de triaxialité des contraintes

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Status
Withdrawn
Publication Date
13-Dec-2007
Withdrawal Date
13-Dec-2007
Current Stage
9599 - Withdrawal of International Standard
Start Date
19-Sep-2013
Completion Date
19-Sep-2013
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INTERNATIONAL ISO
STANDARD 22889
First edition
2007-12-15
Metallic materials — Method of test for
the determination of resistance to stable
crack extension using specimens of low
constraint
Matériaux métalliques — Méthode d'essai pour la détermination de la
résistance à la propagation stable de fissures au moyen d'éprouvettes à
faible taux de triaxialité des contraintes
Reference number
ISO 22889:2007(E)
ISO 2007
---------------------- Page: 1 ----------------------
ISO 22889:2007(E)
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© ISO 2007

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ii © ISO 2007 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 22889:2007(E)
Contents Page

Foreword............................................................................................................................................................ iv

Introduction ........................................................................................................................................................ v

1 Scope ..................................................................................................................................................... 1

2 Normative references ........................................................................................................................... 2

3 Terms and definitions........................................................................................................................... 2

4 Symbols ................................................................................................................................................. 3

5 General requirements........................................................................................................................... 4

5.1 Introduction ........................................................................................................................................... 4

5.2 Test specimens ..................................................................................................................................... 4

5.3 Pre-test requirements........................................................................................................................... 6

5.4 Test apparatus ...................................................................................................................................... 7

5.5 Test requirements................................................................................................................................. 8

5.6 Post-test crack measurements............................................................................................................ 9

6 Determination of δ − ∆a resistance curve and CTOA .................................................................... 11

6.1 General................................................................................................................................................. 11

6.2 Test procedure .................................................................................................................................... 11

6.3 R-curve plot ......................................................................................................................................... 12

6.4 Critical CTOA determination.............................................................................................................. 13

7 Test report ........................................................................................................................................... 13

7.1 General................................................................................................................................................. 13

7.2 Specimen, material and test environment ....................................................................................... 14

7.3 Test data qualification........................................................................................................................ 14

7.4 Qualification of the δ R-Curve ......................................................................................................... 16

7.5 Qualification of ψ .............................................................................................................................. 16

Annex A (informative) Examples of test reports ........................................................................................... 28

Annex B (informative) Apparatus for measurement of crack opening displacement, δ ......................... 33

Annex C (informative) Determination of the crack tip opening angle, ψ .................................................... 35

Annex D (informative) Determination of point values of fracture toughness ............................................ 45

Bibliography ..................................................................................................................................................... 48

© ISO 2007 – All rights reserved iii
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ISO 22889:2007(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.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards

adopted by the technical committees are circulated to the member bodies for voting. Publication as an

International Standard requires approval by at least 75 % of the member bodies casting a vote.

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.

ISO 22889 was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals, Subcommittee

SC 4, Toughness testing — Fracture (F), Pendulum (P), Tear (T).
iv © ISO 2007 – All rights reserved
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ISO 22889:2007(E)
Introduction

ISO 12135 uses compact and bend specimens to determine specific (point) values of fracture toughness at

the onset of either stable or unstable crack extension, and to quantify resistance to stable crack extension.

These specimen types have near-square remaining ligaments to provide conditions of high constraint. If

certain size requirements are met, then the values of the quantities K , δ and J determined from

Ic 0,2BL 0,2BL

these specimens are considered size insensitive, and regarded as lower-bound fracture toughness values.

Although not explicitly stated, size insensitivity holds also for the crack extension resistance curve (R-curve).

In engineering practice, however, there are cases which are not covered by the method of test in ISO 12135,

for example where

⎯ the component thickness is much less than that required for size-insensitive properties as determined

using ISO 12135,

⎯ the thickness of the available material does not enable fabrication of specimens meeting the criteria for

size insensitivity, and

⎯ the loading conditions in the structural component are characterized by tension rather than bending.

In these cases, constraint in the structural component may be lower than that of the specimens specified by

ISO 12135, thus leading to higher resistance to crack extension and higher load-carrying capability in the

structural component than would have been forecast based on the test in ISO 12135.

© ISO 2007 – All rights reserved v
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INTERNATIONAL STANDARD ISO 22889:2007(E)
Metallic materials — Method of test for the determination of
resistance to stable crack extension using specimens of low
constraint
1 Scope

This International Standard specifies methods for determining the resistance to stable crack extension in

terms of crack opening displacement, δ , and critical crack tip opening angle, ψ , for homogeneous metallic

5 c

materials by the quasistatic loading of cracked specimens that exhibit low constraint to plastic deformation.

Compact and middle-cracked tension specimens are notched, precracked by fatigue, and tested under slowly

increasing displacement.

This International Standard describes methods covering tests on specimens not satisfying requirements for

size-insensitive fracture properties; namely, compact specimens and middle-cracked tension specimens in

relatively thin gauges.

Methods are given for determining the crack extension resistance curve (R-curve). Point values of fracture

toughness for compact specimens are determined according to ISO 12135. Methods for determining point

values of fracture toughness for the middle-cracked tension specimen are given in Annex D.

Crack extension resistance is determined using either the multiple-specimen or single-specimen method. The

multiple-specimen method requires that each of several nominally identical specimens be loaded to a

specified level of displacement. The extent of ductile crack extension is marked and the specimens are then

broken open to allow measurement of crack extension. Single-specimen methods based on either unloading

compliance or potential drop techniques can be used to measure crack extension, provided they meet

specified accuracy requirements. Recommendations for single-specimen techniques are described in

ISO 12135. Using either technique, the objective is to determine a sufficient number of data points to

adequately describe the crack extension resistance behaviour of a material.

The measurement of δ is relatively simple and well established. The δ results are expressed in terms of a

5 5

resistance curve, which has been shown to be unique within specified limits of crack extension. Beyond those

limits, δ R-curves for compact specimens show a strong specimen dependency on specimen width, whereas

the δ R-curves for middle-cracked tension specimens show a weak dependency.

CTOA is more difficult to determine experimentally. The critical CTOA is expressed in terms of a constant

value achieved after a certain amount of crack extension. The CTOA concept has been shown to apply to very

large amounts of crack extension and can be applied beyond the current limits of δ applications.

Both measures of crack extension resistance are suitable for structural assessment. The δ concept is well

established and can be applied to structural integrity problems by means of simple crack driving force

formulae from existing assessment procedures.

The CTOA concept is generally more accurate. Its structural application requires numerical methods, i.e. finite

element analysis.

Investigations have shown a very close relation between the concept of constant CTOA and a unique R-curve

for both compact and middle-cracked tension specimens up to maximum load. Further study is required to

establish analytical or numerical relationships between the δ R-curve and the critical CTOA values.

© ISO 2007 – All rights reserved 1
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ISO 22889:2007(E)
2 Normative references

The following referenced documents are indispensable for the application of this document. For dated

references, only the edition cited applies. For undated references, the latest edition of the referenced

document (including any amendments) applies.

ISO 3785, Metallic materials — Designation of test specimen axes in relation to product texture

ISO 7500-1, Metallic materials — Verification of static uniaxial testing machines — Part 1:

Tension/compression testing machines — Verification and calibration of the force-measuring system

ISO 9513, Metallic materials — Calibration of extensometers used in uniaxial testing

ISO 12135:2002, Metallic materials — Unified method of test for the determination of quasistatic fracture

toughness
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
crack opening displacement
COD

relative displacement of the crack surfaces normal to the original (undeformed) crack plane at the tip of the

fatigue precrack, as measured on the specimen’s side surface over an initial gauge length of 5 mm

3.2
crack tip opening angle
CTOA

relative angle of the crack surfaces measured (or calculated) at 1 mm from the current crack tip

3.3
stable crack extension

crack extension that, in displacement control, occurs only when the applied displacement is increased

3.4
crack extension resistance curve
R-curve
variation in δ with stable crack extension ∆a
3.5
critical crack tip opening angle

steady-state value of crack tip opening angle ψ at 1 mm from the current crack tip

NOTE This value is insensitive to the in-plane dimensions specified in this method; however, it may be thickness

dependent.
2 © ISO 2007 – All rights reserved
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ISO 22889:2007(E)
4 Symbols

For the purposes of this International Standard, the following symbols and units apply. For all parameters, the

temperature is assumed to be the test temperature unless otherwise noted.
Symbol Unit Designation
a mm crack length
a mm final crack length (a + ∆a )
f 0 f
a mm length of machined crack starter notch
a mm initial crack length
mm stable crack extension
∆a mm crack extension beyond which ψ is nearly constant
min c
∆a crack extension limit for δ or ψ controlled crack extension
max 5 c
mm final stable crack extension
B mm specimen thickness
MPa Young’s modulus of elasticity
F kN applied force
F kN maximum fatigue precracking force

R MPa 0,2 % offset yield strength perpendicular to crack plane at the test temperature

p0,2
MPa tensile strength perpendicular to crack plane at the test temperature
α degrees crack path deviation
W mm width of compact specimen, half width of middle-cracked tension specimen
mm uncracked ligament length
W − a
W − a mm initial uncracked ligament length
mm final uncracked ligament length
W − a
degrees crack tip opening angle (CTOA)
degrees critical crack tip opening angle (critical CTOA)
Poisson’s ratio

mm crack opening displacement over a 5 mm gauge length at tip of fatigue precrack

NOTE This is not a complete list of parameters. Only the main parameters are given here; other parameters are

referred to and defined in the text.
© ISO 2007 – All rights reserved 3
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ISO 22889:2007(E)
5 General requirements
5.1 Introduction

The resistance to stable crack extension of metallic materials can be characterized in terms of either specific

(single point) values (see Annex D) or a continuous curve relating fracture resistance to crack extension over

a limited range of crack extension (see Clause 6). Any one of the fatigue-cracked test specimen configurations

specified in this method may be used to measure or calculate any of these fracture resistance parameters.

Tests are performed by applying slowly increasing displacement to the test specimen and measuring the

resulting force and corresponding crack opening displacement and angle. The measured forces,

displacements and angles are then used in conjunction with certain pre-test and post-test specimen

measurements to determine the material’s resistance to crack extension. Details of test specimens and

general information relevant to the determination of all fracture parameters are given in this method. A flow-

chart illustrating the way this International Standard can be used is presented in Figure 1.

Fracture resistance symbols identified for use in this International Standard method of test are given in

Table 1:
Table 1 — Fracture resistance symbols
Size-sensitive quantities
Size-insensitive
Parameter Qualifying limits
(specific to thickness B
quantities
tested)
See Annex D Not applicable
δ , point value of fracture
toughness
Not applicable a , (W − a ) W 4B ∆a < ∆a = 0,25(W − a )
δ R-curve
0 0 max 0
for compact specimens;
∆a < ∆a = W − a − 4B
max 0
for middle-cracked
tensile specimens
Not applicable a , (W − a ) W 4B ∆a > ∆a = 50/(5 + B)
0 0 min
∆a < ∆a = W − a − 4B
max 0
(see Figure 11)

NOTE The qualifying limit for ψ , ∆a > ∆a = 50/(5 + B) was established using surface measurements of crack extension for

c min
aluminium alloys and steels in sheet thicknesses ranging from 1 mm to 25 mm.
5.2 Test specimens
5.2.1 Specimen configuration and size

Specimen dimensions and tolerances shall conform to those shown in Figures 2 and 3.

The choice of specimen design shall take into consideration the likely outcome of the test (see Figure 1),

which fracture resistance value (δ or ψ) is to be determined, the crack plane orientation of interest, and the

amount and condition of test material available.

NOTE Both specimen configurations (Figures 2 and 3) are suitable for determination of δ and ψ values.

5 c

For both specimen configurations, the conditions [a , (W − a )] W 4B shall be satisfied.

0 0
4 © ISO 2007 – All rights reserved
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ISO 22889:2007(E)
5.2.2 Specimen preparation
5.2.2.1 Material condition

Specimens shall be machined from stock in the final heat-treated and mechanically worked conditions.

In exceptional circumstances where material cannot be machined in the final condition, final heat treatment

may be carried out after machining, provided that the required dimensions and tolerances for the specimen, its

shape, and its surface finish are met. Where dimensions of the machined specimen are substantially different

from the pre-machined stock, a size effect on the heat-treated microstructure and mechanical properties shall

be taken into account in the service application.
5.2.2.2 Crack plane orientation

Orientation of the crack plane shall be decided before machining, identified in accordance with ISO 3785, and

recorded in accordance with Table A.1.

NOTE Crack extension resistance depends on the orientation and direction of crack extension in relation to the

principal directions of mechanical working, grain flow and other forms of anisotropy.

5.2.2.3 Machining

The specimen notch profile shall not exceed the envelope shown in Figure 4. The root radius of a milled notch

shall be not greater than 0,10 mm. Sawn, disk ground, or spark-eroded notches shall not have a width greater

than 0,15 mm.
5.2.2.4 Fatigue precracking
5.2.2.4.1 General

Fatigue precracking shall be performed with the material in the final heat-treated, mechanically worked or

environmentally conditioned state. Intermediate treatments between fatigue precracking and testing are

acceptable only when such treatments are necessary to simulate the conditions of a specific structural

application; such departure from recommended practice shall be (explicitly) reported.

Maximum fatigue precracking force during any stage of the fatigue precracking process shall be accurate to

±2,5 %.

Measured values of specimen thickness, B, and width, W, determined in accordance with 5.3.1, shall be

recorded and used to determine the maximum fatigue precracking force F in accordance with 5.2.2.4.3 and

5.2.2.4.4.

The ratio of minimum-to-maximum force in the fatigue cycle shall be in the range 0 to 0,1 except that, in order

to expedite crack initiation, one or more cycles of −1,0 may be applied first.
5.2.2.4.2 Equipment and fixtures

Fixtures for fatigue precracking shall be carefully aligned and arranged so that loading is uniform through the

specimen thickness B and symmetrical about the plane of the prospective crack.
© ISO 2007 – All rights reserved 5
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ISO 22889:2007(E)
5.2.2.4.3 Compact specimens

For compact specimens, the maximum fatigue precracking force during the final 1,3 mm or 50 % of precrack

extension, whichever is less, shall be the lowest value of
F = ξΕ (1)
ga(/W)
−40,5
where ξ=×1, 6 10 m , and
−1,5 2 3 4
aa a a a a
⎡ ⎤ ⎡ ⎤ ⎛⎞ ⎛⎞ ⎛⎞
00 0 0 0 0
ga( /W )= 1− 2+ 0,886+ 4,64−+−13,32 14,72 5,6 (2)
10 ⎢⎥ ⎢ ⎥ ⎜⎟ ⎜⎟ ⎜⎟
WW W W W W
⎣ ⎦ ⎣ ⎦ ⎝⎠ ⎝⎠ ⎝⎠
5.2.2.4.4 Middle-cracked tension specimens

For middle-cracked tension specimens, the maximum fatigue precracking force during the final 1,3 mm or

50 % of precrack extension, whichever is less, shall be the lowest value of
−0,5
⎡⎤πa
FE=πξB2sW aec (3)
−40,5
where ξ=×1, 6 10 m
5.3 Pre-test requirements
5.3.1 Pre-test measurements

The dimensions of specimens shall conform to those shown in Figures 2 and 3. Measurement of the thickness

B and width W shall be within 0,02 mm or to ±0,2 %, whichever is the larger.

Specimen thickness B shall be measured, before testing, at a minimum of three equally spaced positions

along the intended crack extension path. The average of these measurements shall be taken as the thickness

Specimen width W of the middle-cracked tension specimen shall be measured at a minimum of three equally

spaced positions within ±0,1 W of the crack plane. The average of these measurements shall be taken as the

width W.

The compact specimen width W shall be measured with reference to the loading-hole centreline. Customarily,

the loading-hole centreline is established first, and then the dimension W is measured to the specimen edge

ahead of the crack tip in the plane of the crack. This measurement shall be made at a minimum of three

equally spaced positions across the specimen thickness. The dimension 1,25 W (between the specimen

edges ahead and behind the crack tip) shall be measured in addition, at the same equally spaced positions

across the thickness in a plane as close as possible to the plane of the crack.
5.3.2 Crack front shape and length requirements

A fatigue crack shall be developed from the root of the machined notch of the specimen as follows:

⎯ for compact specimens (see Figure 2), the ratio a /W shall be in the range 0,45 to 0,65;

⎯ for middle-cracked tension specimens, the ratio a /W shall be in the range 0,25 to 0,50.

6 © ISO 2007 – All rights reserved
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ISO 22889:2007(E)

The minimum fatigue crack extension shall be the larger of 1,3 mm or 2,5 % of the specimen width W. The

notch plus fatigue crack shall be within the limiting envelope shown in Figure 4.

5.4 Test apparatus
5.4.1 Calibration

Calibration of all measuring apparatus shall be traceable either directly or indirectly via a hierarchical chain to

an accredited calibration laboratory.
5.4.2 Force application
The combined force sensing and recording device shall conform to ISO 7500-1.
The test machine shall operate at a constant displacement rate.

A force measuring system of nominal capacity exceeding 1,2 F shall be used, where

⎯ for compact specimens
BW()−a
F = R (4)
(2Wa+ )
⎯ or for middle-cracked tension specimens
F=−2(BW a )R (5)
L0m
5.4.3 Displacement measurement

The displacement gauge used for the determination of δ shall have an electrical output that accurately

represents the displacement between two precisely located gauge positions 5 mm apart, spanning the crack

at the fatigue crack tip. The design of the displacement gauge (or transducer where appropriate) and

specimen shall allow free rotation of the points of contact between the gauge and specimen.

NOTE 1 Guidance for determining δ is given in Annex B.

NOTE 2 The crack mouth opening displacement is not needed for the δ and ψ determinations, but a force crack

5 c

mouth opening displacement record may be suitable for evaluating the methods from finite element analyses and other

fracture analysis methods. Examples of proven displacement gauge designs are given in References [1] and [2] (see

Bibliography), and similar gauges are commercially available.

Gauges for crack mouth opening displacement measurement shall be calibrated in accordance with ISO 9513,

as interpreted in relation with this International Standard, and shall be at least of Class 1. Calibration shall be

performed at least each week when the gauges are in use.

NOTE Calibration may be carried out more frequently depending on use and agreement between contractual parties.

Verification of the displacement gauge shall be performed at the test temperature ±5 °C. The response of the

gauge shall be true to ±0,003 mm for displacements up to 0,3 mm and to ±1 % of the actual reading thereafter.

5.4.4 Test fixtures

Compact specimens shall be loaded using a clevis and pin arrangement designed to minimize friction. The

arrangement shall ensure load train alignment as the specimen is loaded under tension. Clevises for R-curve

measurements shall have flat-bottomed holes (see Figure 5) so that the loading pins are free to roll throughout

the test. Round-bottomed holes (see Figure 6) shall not be allowed for single-specimen (unloading

compliance) tests. Fixture-bearing surfaces shall have a hardness greater than 40 HRC (400 HV) or a yield

© ISO 2007 – All rights reserved 7
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ISO 22889:2007(E)

strength of at least 1 000 MPa. Middle-cracked tension specimens shall be loaded using hydraulically clamped

or bolted grips designed to carry the applied load by friction. Bolt bearing should be avoided in order to

minimize non-uniform loading. The arrangement shall ensure alignment of the specimen with minimal in-plane

and out-of-plane bending. All specimens shall be tested with anti-buckling guide plates, as shown in Figure 7.

The anti-buckling guide plates shall cover a large portion of the specimen. Support only along the crack plane

has been shown to be insufficient to prevent buckling between the grip lines and the crack plane for thin-sheet

materials. Flat plates are sufficient for small middle-cracked tension specimens (W < 600 mm); but flat plates

and I-beams, as illustrated in Figure 7a), are required for middle-cracked tension specimens with widths larger

than about 600 mm. A suitable design for compact specimens is shown in Figure 7b).

5.5 Test requirements

It is recommended that anti-buckling plates be attached to both sides of the tension specimen covering the

expected path of the crack for a distance four times the initial total crack length perpendicular to the crack.

Frictional forces between the specimen and anti-buckling plates shall be minimized by the use of an inert

lubricant such as Teflon® applied to the mating surfaces. An access hole is required in one of the plates for

mounting the δ gauge on the specimen or, if the potential method is used, for the attachment of cables.

5.5.1 Compact specimen testing
5.5.1.1 Specimen and fixture alignment

The loading clevises shall be aligned to within 0,25 mm, and the specimen shall be centred on the loading

pins within 0,75 mm with respect to the clevis opening.
5.5.1.2 Crack opening displacement δ
A method of measuring the crack opening displacement δ is described in Annex B.
5.5.1.3 Crack tip opening angle ψ

The crack tip opening angle ψ may be measured or calculated as described in Annex C.

5.5.2 Middle-cracked tension specimen testing
5.5.2.1 Specimen and fixture alignment

The fixture shall be designed to distribute the load uniformly over the cross-section of the specimen. The

fixture may be rigidly connected to the machine if uniform loading of the specimen in the machine can be

assured at all loads. Otherwise, pinloading via detachable grips is recommended.
5.5.2.2 Crack opening displacement δ
A method of measuring the crack opening displacement δ is given in Annex B.
5.5.2.3 Crack tip opening angle ψ

The crack tip opening angle ψ may be measured or calculated as described in Annex C.

5.5.3 Specimen t
...

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