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

ISO 22889:2013 specifies methods for determining the resistance to stable crack extension in terms of crack opening displacement, δ5, and critical crack tip opening angle, ψc, 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 22889:2013 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. 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 δ5 is relatively simple and well established. The δ5 results are expressed in terms of a resistance curve, which has been shown to be unique within specified limits of crack extension. Beyond those limits, δ5 R-curves for compact specimens show a strong specimen dependency on specimen width, whereas the δ5 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 δ5 applications. Both measures of crack extension resistance are suitable for structural assessment. The δ5 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 δ5 R-curve and the critical CTOA values.

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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Published
Publication Date
18-Sep-2013
Current Stage
9093 - International Standard confirmed
Start Date
11-Oct-2021
Completion Date
11-Oct-2021
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INTERNATIONAL ISO
STANDARD 22889
Second edition
2013-10-01
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 22889:2013(E)
ISO 2013
---------------------- Page: 1 ----------------------
ISO 22889:2013(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2013

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Published in Switzerland
ii © ISO 2013 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 22889:2013(E)
Contents Page

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

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

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

2 Normative references ...................................................................................................................................................................................... 1

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

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

5 General requirements ..................................................................................................................................................................................... 3

5.1 Introduction .............................................................................................................................................................................................. 3

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

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

5.4 Test apparatus ......................................................................................................................................................................................... 6

5.5 Test requirements ................................................................................................................................................................................ 7

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

6 Determination of δ − Δa resistance curve and CTOA ..................................................................................................11

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

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

6.3 R-curve plot ......... ....................................................................................................................................................................................11

6.4 Critical CTOA determination ....................................................................................................................................................12

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

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

7.2 Specimen, material and test environment ...................................................................................................................13

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 ...................................................................................................................................27

Annex B (informative) Apparatus for measurement of crack opening displacement, δ .........................32

Annex C (informative) Determination of the crack tip opening angle, ψ ...................................................................34

Annex D (informative) Determination of point values of fracture toughness ........................................................44

Bibliography .............................................................................................................................................................................................................................47

© ISO 2013 – All rights reserved iii
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ISO 22889:2013(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. 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. 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.

The committee responsible for this document is ISO/TC 164, Mechanical testing of metals, Subcommittee

SC 4, Toughness testing — Fracture (F), Pendulum (P), Tear (T).

This second edition cancels and replaces the first edition (ISO 22889:2007), of which it constitutes a

minor revision.
The following changes have been made:
— in 5.2.2.3, the notch width is now limited to W/30;

— in 5.2.2.4.3, revised the zone of application for the limited precracking force and corrected the logic

of the subclause for the compact specimen;

— in 5.2.2.4.4, revised the zone of application for the limited precracking force and corrected the logic

of the subclause for the middle crack tension specimen;

— in 5.5.5, replaced the stress intensification rate with the stress intensity factor rate;

— in 5.6.1.3 a), revised the ratio a /W from between “0,45 to 0,65” to “from 0,4 to 0,7” for compact

specimens;

— in 5.6.1.3 d), revised the allowed distance between the fatigue crack and the notch at the start of the

test from 0,025W to 1,3 mm or 0,013W, whichever is the larger;

— in 5.6.2, added a provision: “However, measuring positions in the thickness direction shall be based

on the contracted thickness at the final crack tip location.”;

— in 6.4, added Formula (12) to define the minimum amount of crack extension, Δa ;

min

— in 6.4, added the statement: “Formulae (11) and (12) apply to both the compact and middle crack

tensile specimen geometries.”;
— corrected notes 2 and 3 on Figure 2;
— corrected notes 2 and 3 on Figure 3;
— designated a new notch width as W/30 on Figure 4;
— revised the report in 7.3.5 to be consistent with the above changes.
iv © ISO 2013 – All rights reserved
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ISO 22889:2013(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

Ic 0,2BL 0,2BL

determined from 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 2013 – All rights reserved v
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INTERNATIONAL STANDARD ISO 22889:2013(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

5 c

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.

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

5 5

a 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.

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.

© ISO 2013 – All rights reserved 1
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ISO 22889:2013(E)

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 extensometer systems 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 Δa
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 1 to entry: This value is insensitive to the in-plane dimensions specified in this method; however, it may be

thickness dependent.
2 © ISO 2013 – All rights reserved
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ISO 22889:2013(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 + Δa )
f 0 f
a mm length of machined crack starter notch
a mm initial crack length
Δa mm stable crack extension
Δa mm crack extension beyond which ψ is nearly constant
min c
Δa mm crack extension limit for δ or ψ controlled crack extension
max 5 c
Δa mm final stable crack extension
B mm specimen thickness
E MPa Young’s modulus of elasticity
F kN applied force
F kN maximum fatigue precracking force

MPa 0,2 % offset yield strength perpendicular to crack plane at the test tempera-

p0,2
ture
R 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
W − a mm uncracked ligament length
W − a mm initial uncracked ligament length
W − a mm final uncracked ligament length
ψ degrees crack tip opening angle (CTOA)
ψ degrees critical crack tip opening angle (critical CTOA)
ν Poisson’s ratio
mm crack opening displacement over a 5 mm gauge length at tip of fatigue pre-
crack

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.
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

© ISO 2013 – All rights reserved 3
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ISO 22889:2013(E)

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
quantities
(specific to thickness B
tested)
See Annex D Not applicable
δ , point value of fracture
toughness
Not applicable a , (W − a ) ≥ 4B Δa < Δa = 0,25(W − a
0 0 max
δ R-curve
) for compact speci-
mens;
Δa < Δa = W − a − 4
max 0
B for middle-cracked
tensile specimens
ψ Not applicable a , (W − a ) ≥ 4B Δa > Δa = 50/(5 + B)
0 0 min
Δa < Δa = W − a − 4B
max 0
(see Figure 11)

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

c min
for 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 )] ≥ 4B shall be satisfied.

0 0
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.

4 © ISO 2013 – All rights reserved
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ISO 22889:2013(E)
5.2.2.2 Crack plane orientation

The orientation of the crack plane shall be decided before machining, identified in accordance with

ISO 3785, and recorded. An example test report, including this information, is shown in 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

machined notch shall be not greater than 0,10 mm and the maximum allowed notch width is W/30.

Sawn, disk ground, or spark-eroded notches shall not have a width greater than 0,15 mm; see Key item

“c” in Figure 4.
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.

5.2.2.4.3 Compact specimens

For compact specimens, the maximum fatigue precracking force during the final 1,3 mm or 0,4N of

precrack extension, whichever is larger, shall be equal to or less than
 
F =ξΕ (1)
 
ga(/W)
 
 
−40,5
where ξ =×16, 10 m , and
−15, 2 3 4
 
a a a a a a
         
0 00 0 0 0
ga(/W),=−12+ 0 886+−46,,41332 +14,,72 −56  (2)
         
W W W W W W
 
         
 
© ISO 2013 – All rights reserved 5
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ISO 22889:2013(E)
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 0,4N of precrack extension, whichever is larger, shall be equal to or less than

−05,
 πa 
FE=ξ BW2 πa sec (3)
f 0
 
 
−40,5
where ξ =×16, 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 B.

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 0,4 to 0,7;

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

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
6 © ISO 2013 – All rights reserved
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ISO 22889:2013(E)
BW()−a
F = R (4)
()2Wa+
— or for middle-cracked tension specimens
FB=−2 ()Wa R (5)
Lm0
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 3 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 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 show
...

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