Plastics — Determination of mode I plane-strain crack-arrest toughness

ISO 29221:2014 specifies a method for the determination of the plane-strain crack-arrest fracture toughness, Kla, of polymeric materials by using a side-grooved, crack-line-wedge-loaded compact tension specimen to obtain a rapid crack run-arrest segment of flat-tensile separation with a satisfactory crack front. ISO 29221:2014 employs a static fracture analysis determination of the stress intensity factor at a short time after crack arrest. The estimate is denoted as Ka and when certain size requirements are met, the test result provides an estimate, termed as Kla, of the plane-strain crack-arrest toughness of the polymer. The specimen size requirements provide for in-plane dimensions large enough to allow the specimen to be modelled by linear elastic analysis.

Plastiques — Détermination de la ténacité d'arrêt de fissure en déformation plane

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Published
Publication Date
15-Jan-2014
Current Stage
9093 - International Standard confirmed
Start Date
25-Sep-2019
Completion Date
25-Sep-2019
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INTERNATIONAL ISO
STANDARD 29221
First edition
2014-01-15
Plastics — Determination of mode I
plane-strain crack-arrest toughness
Plastiques — Détermination de la ténacité d’arrêt de fissure en
déformation plane
Reference number
ISO 29221:2014(E)
ISO 2014
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ISO 29221:2014(E)
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© ISO 2014

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ii © ISO 2014 – All rights reserved
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ISO 29221:2014(E)
Contents Page

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

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

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

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

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle ........................................................................................................................................................................................................................ 2

5 Apparatus ..................................................................................................................................................................................................................... 2

5.1 General ........................................................................................................................................................................................................... 2

5.2 Loading arrangement ........................................................................................................................................................................ 2

5.3 Displacement gauge ............................................................................................................................................................................ 4

6 Test specimen .......................................................................................................................................................................................................... 5

6.1 General ........................................................................................................................................................................................................... 5

6.2 Dimensions ................................................................................................................................................................................................. 5

6.3 Starter notch ............................................................................................................................................................................................. 6

7 Procedure..................................................................................................................................................................................................................... 7

7.1 Measurements of specimen dimensions........................................................................................................................... 7

7.2 Conditioning .............................................................................................................................................................................................. 7

7.3 Loading .......................................................................................................................................................................................................... 7

7.4 Displacement measurement........................................................................................................................................................ 8

7.5 Arrested crack length (a ) measurement ........................................................................................................................ 8

7.6 Number of tests ...................................................................................................................................................................................... 9

8 Calculation and validation of results ............................................................................................................................................10

8.1 Calculation of K and K .............................................................................................................................................................

ο Qa 10

8.2 Validity requirement .......................................................................................................................................................................10

9 Precision ....................................................................................................................................................................................................................11

10 Test report ................................................................................................................................................................................................................11

10.1 Test details ...............................................................................................................................................................................................11

10.2 Calculations .............................................................................................................................................................................................12

10.3 Validity requirements (see Table 1) ..................................................................................................................................12

10.4 Photographic record of fracture surface and descriptive comments (optional) .......................12

Annex A (informative) Determination of the initial crack length, a , when using the

chevron notch .......................................................................................................................................................................................................13

Annex B (informative) Procedure for measuring the arrested crack length (a ) ...............................................15

Annex C (informative) Fracture surface acceptability.....................................................................................................................16

Annex D (informative) Comment on precision statement ...........................................................................................................17

Bibliography .............................................................................................................................................................................................................................18

© ISO 2014 – All rights reserved iii
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ISO 29221:2014(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

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 61, Plastics, Subcommittee SC 2, Mechanical

properties.
iv © ISO 2014 – All rights reserved
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ISO 29221:2014(E)
Introduction

There has been much interest in a better understanding of the fracture behaviour of polymeric materials

and, as a consequence, several International Standard methods for evaluating the fracture properties

have been developed. In the light of the fact that these standard methods provide critical information

on fracture prevention of structures and products made from polymeric materials, as well as give

directions for the research and development of materials, any additional test methods of importance

to fracture need to be added to the list. In line with such importance, in particular, a test method for

evaluating the resistance to rapid crack propagation in terms of a material’s ability to arrest the fast-

[1]-[4][10]-[12][14]
running crack would be of interest for polymers.

The value of the stress intensity factor, K, during the short time interval in which a fast-running crack

arrests is a measure of the ability of materials to arrest such a crack. The values of the stress intensity

factor of this kind, which are determined using the dynamic methods of analysis, provide a value for the

crack-arrest fracture toughness, K . To ease complexity arising from the dynamic effects, static methods

of analysis, which are much less complex, can often be used to determine the stress intensity factor at a

short time (1 ms to 2 ms) after crack arrest. The estimate of the crack-arrest fracture toughness obtained

in this fashion is termed K and the difference between K and K can be made small by minimizing the

a A a
[5]-[8]

macroscopic dynamic effects during the test. For cracks propagating under the conditions of crack-

front plane-strain, in situations where the dynamic effects are also known to be small, K determinations

using laboratory-sized specimens have been used successfully to estimate whether, and at what point, a

[9]-[11]

crack arrests in a structure. Depending upon the component design, the loading compliance, and

the crack-jump length, a dynamic analysis of a fast-running crack propagation event can be necessary

in order to predict whether crack arrest will occur and the arrest position. In such cases, values of K

la,

determined by this International Standard can be used to identify those values of K below which the

crack speed is zero. More details on the use of dynamic analyses can be found in Reference [8].

This International Standard describes a method for mode I plane-strain crack-arrest toughness

measurement for polymers.
© ISO 2014 – All rights reserved v
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INTERNATIONAL STANDARD ISO 29221:2014(E)
Plastics — Determination of mode I plane-strain crack-
arrest toughness
1 Scope

This International Standard specifies a method for the determination of the plane-strain crack-arrest

fracture toughness, K , of polymeric materials by using a side-grooved, crack-line-wedge-loaded

compact tension specimen to obtain a rapid crack run-arrest segment of flat-tensile separation with a

satisfactory crack front. This International Standard employs a static fracture analysis determination

of the stress intensity factor at a short time after crack arrest. The estimate is denoted as K and when

certain size requirements are met, the test result provides an estimate, termed as K , of the plane-strain

crack-arrest toughness of the polymer. The specimen size requirements provide for in-plane dimensions

large enough to allow the specimen to be modelled by linear elastic analysis. If the specimen does not

exhibit rapid crack propagation and arrest, K cannot be determined.
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 527-1, Plastics — Determination of tensile properties — Part 1: General principles

ISO 16012, Plastics — Determination of linear dimensions of test specimens
ISO 18872, Plastics — Determination of tensile properties at high strain rates
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
conditional value of the plane-strain crack-arrest fracture toughness

conditional value of K , calculated from the test result and subject to the validity criteria specified for

the side-grooved, crack-line-wedge-loaded specimen used

Note 1 to entry: The calculation of K is based upon the measurements of both the arrested crack length and of

the crack-mouth opening displacement prior to the initiation of a fast-running crack and shortly after crack arrest.

−3/2
Note 2 to entry: It is expressed as N·m .
3.2
crack-arrest fracture toughness
value of the stress intensity factor, K, shortly after crack arrest

Note 1 to entry: The in-plane specimen dimensions shall be large enough for adequate enclosure of the crack-tip

plastic zone by a linear-elastic stress field.
−3/2
Note 2 to entry: It is expressed as N·m .
© ISO 2014 – All rights reserved 1
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ISO 29221:2014(E)
3.3
plane-strain crack-arrest fracture toughness

value of the crack-arrest fracture toughness, K , for a crack that arrests under the conditions of crack-

front plane-strain

Note 1 to entry: The requirements for attaining the conditions of crack-front plane-strain are specified in the

procedures of this International Standard.
−3/2
Note 2 to entry: It is expressed as N·m .
3.4
stress intensity factor at crack initiation
value of K at the onset of rapid fracturing

Note 1 to entry: Only a nominal estimate of the initial driving force is needed. For this reason, K is calculated

based on the initial crack (or notch) length and the crack-mouth opening displacement at the initiation of a fast-

running crack.
−3/2
Note 2 to entry: It is expressed as N·m .
4 Principle

This International Standard estimates the value of the stress intensity factor, K, at which a fast-running

crack arrests. In this test method, a wedge is forced into a split pin, which applies an opening force across

the crack starter notch in a modified compact specimen, causing a crack run-arrest segment of crack

extension. The rapid run-arrest event suggests the need for a dynamic analysis of test results. However,

experimental observations indicate that, for this test method, an adjusted static analysis of test results

[1]-[2]

provides a useful estimate of the value of the stress intensity factor at the time of crack arrest.

The calculation of nominal stress intensity at initiation, K is based on the measurements of the initial

notch length and the crack-mouth opening displacement at initiation. The value of K is based on

the measurements of the arrested crack length and the crack-mouth opening displacements prior to

initiation and shortly after crack arrest.
5 Apparatus
5.1 General

The procedure involves testing the modified compact specimens that have been notched by machining.

To minimize the introduction of additional energy into the specimen during the crack run-arrest event,

the loading system shall have a low compliance compared with the test specimen. For this reason, a

wedge and split-pin assembly is used to apply a load on the crack line. This loading arrangement does not

permit easy measurement of the opening loads. Consequently, the opening displacement measurement,

in conjunction with crack size and compliance calibrations, is used for calculating K and K .

ο a
5.2 Loading arrangement

A typical loading arrangement is shown in Figure 1. The specimen is placed on a support block whose

thickness should be adequate to allow the completion of the test without interference between the wedge

and the lower crosshead of the testing machine. The support block should contain a hole that is aligned

with the specimen hole, and whose diameter should be between 1,05 and 1,15 times the diameter of the

hole in the specimen. The load that forces the wedge into the split pin is transmitted through a load cell.

The surfaces of the wedge, split pin, support block, and specimen hole should be lubricated if necessary.

The lubricant used shall not affect the polymer being tested. It can be also helpful to have the sliding

surfaces of the wedge, the split pin, and the support block matte-finished (grit-blasted), so as to prevent

2 © ISO 2014 – All rights reserved
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ISO 29221:2014(E)

possible galling. A low-taper-angle wedge and split-pin arrangement is used. The split pin shall be long

enough to contact the full specimen thickness, and the radius should be large enough to avoid plastic

indentations of the test specimen. In all cases, it is recommended that the diameter of the split pin shall

be 0,10 mm less than the diameter of the specimen hole. The wedge shall be long enough to develop the

maximum expected opening displacement and any air or oil-hardening tool steel is suitable for making

the wedge and split pins. Hardness in the range from Rc45 to Rc55 has been used successfully. The

dimensions of a wedge and split-pin assembly suitable for use with a 25 mm diameter loading hole are

shown in Figure 2. The dimensions should be scaled when other hole diameters are used.

Key
P load
1 wedge
2 split pin or bushing
3 test specimen
4 support block

Figure 1 — Schematic illustration of the wedge-loading system — Specimen assembly: a)

pictorial view, b) standard arrangement, and c) arrangement in case high friction between the

support block and specimen exists
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ISO 29221:2014(E)
Dimensions in millimetres
Figure 2 — Suggested geometry and dimensions of wedge and split-pin assembly

NOTE The dimensions given are suitable for use with a 25 mm diameter loading hole in 25-mm to 50-mm thick

test specimens.
5.3 Displacement gauge

A displacement gauge is used to measure the crack-mouth opening displacement at L = 0,25 W measured

from the load-line, which is the centre line of the wedge loading hole, with distance L away from the

specimen edge (Figure 3). Accuracy to within 2 % over the working range is required. Either the gauge

described in ISO 13586 or a similar gauge is satisfactory. It is necessary to attach the gauge in a fashion

such that the seating in contact with the specimen is not altered by the jump of the crack. The methods

that have proven satisfactory for doing this are shown in Figure 3. The gauge can be affixed to the

specimen by using elastic bands, either a gauge edge flat on the specimen, or a gauge with knife edges

sliding into v-grooves in the specimen.
4 © ISO 2014 – All rights reserved
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ISO 29221:2014(E)
Key
1 v-groove
2 loading hole
3 side groove
4 displacement gauge
5 elastic band

Figure 3 — Methods of positioning and attaching the displacement gauge to the specimen

The more recommended choice is the use of v-grooves along with elastic bands to fix the gauge as this

provides better gauge-holding stability as well as consistency in initial gauge opening.

6 Test specimen
6.1 General

The shape of the compact-crack-arrest test specimen is shown in Figure 4. It is a double-cantilever-

type flat specimen having side grooves introduced along the plane containing the initial notch of the

specimen. The side grooves are known to help create the plane-strain condition at the crack front and to

keep the running crack in a straight manner throughout the crack run-arrest segment.

6.2 Dimensions

The dimensions shall be such that the extent of plastic deformation in the specimen prior to crack

initiation shall be limited. For this, certain size requirements should be met, which depend upon the

specimen yield strength and K , as well as the K needed to achieve an appropriate crack run-arrest

a ο

event. In addition, the in-plane specimen dimensions shall be large enough to allow for the linear elastic

analysis employed by this test method. These requirements are given in 8.2, in terms of allowable crack-

jump lengths. For a test result to be termed plane-strain K by this test method, the specimen thickness,

B, shall meet the requirement also given in 8.2. Side grooves of depth B/8 per side shall be used. The

specimen width, W, shall be within the range 2 B ≤ W ≤ 10 B. Other specimen dimensions of importance

are illustrated in Figure 4.
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ISO 29221:2014(E)
Key
D loading hole
R side groove notch radius
a initial crack (or notch) length
a machined notch length

H = 0,6 W ± 0,005 W; S = (B−B )/2 ± 0,01 B; N ≤ W/10; 0,15 W ≤ L ≤ 0,25 W; 0,20 W ≤ a ≤ 0,40 W;

N o
0,125 W ± 0,005 W ≤ D ≤ 0,250 W ± 0,005 W.

NOTE For more brittle polymers, a final razor notch might not be necessary, and a = a .

o m
Figure 4 — Suggested geometry and dimensions of a crack-line-wedge-loaded
compact-crack-arrest test specimen
6.3 Starter notch

The function of the starting notch is to produce a crack initiation at an opening displacement (or wedging

force) that permits an appropriate length of rapid crack extension prior to crack arrest. Different

polymers can require different starter notches and preparation procedures. Typically, however, the

final starter notch is generally made after the initial machine notching, either through the use of razor

blade or fatigue cracking, or by other means appropriate. It was shown that in case of razor notching, the

notching speed should be as such that the notch tip is not damaged.

In the case of relatively brittle polymers, such as polystyrene and polymethylmethacrylate, a straight-

machined notch (e.g. a in Figure 4) was shown to be effective enough to cause crack initiation without

yielding. For polycarbonate, a straight notch allows sufficient yielding such that on reaching the onset

opening displacement, the crack propagates through the whole specimen width without arresting. It has

been shown that this problem can be eliminated by using a chevron notch (see References [10] and [14]).

The determination of the initial crack length, a , for the chevron notch is given in Annex A. For polymers

such as polyacetal, nylon, and polyethylene, the chevron notch is not sufficient to create a running crack.

In this case, the starter notch can be cooled in the liquid nitrogen below Tg to allow yielding suppression

and cause the initiation of a self-arresting crack. To do this effectively, a technique is described (see

Reference [14]) wherein a hole is drilled just adjacent to the final razor notch through which liquid

nitrogen is circulated via copper tube placed through the drilled hole, as shown in Figure 5. It has been

shown that by using this technique, a straight razor notch is enough to effectively cause brittle crack

initiation. A thermocouple placed near the side groove at a location about 10 mm ahead of the notch-tip

would be useful for the crack-tip temperature monitoring prior to the specimen loading. A small hole

drilled to the depth of 0,25 B is appropriate for accommodating the thermocouple wire.

6 © ISO 2014 – All rights reserved
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ISO 29221:2014(E)
Key
1 notch tip cooling hole
2 razor notch

Figure 5 — Modified crack-arrest test specimen for crack-tip low-temperature exposure

7 Procedure
7.1 Measurements of specimen dimensions

Measure and record the dimensions, as appropriate, in accordance with ISO 16012. Specimen thickness,

B, and the crack plane thickness, B , shall be measured to an accuracy of ±1 % of B. Also, the specimen

width shall be measured to an accuracy of W to ±1 % of W.
7.2 Conditioning

The specimens can be moulded in various ways and machining and notching follow to make them into

predetermined dimensions of compact tension crack-arrest specimen. Newly prepared specimens

shall be preconditioned in accordance with the requirement of specific polymers to be evaluated so

as to prevent any anomalies that can arise from the specimen preparation. Test specimens shall be

conditioned at the test temperature for 24 h prior to carrying out the test.
7.3 Loading

Position the wedge-loading system and specimen assembly to a testing machine that is capable of

applying the load at a constant rate of traverse. Attach the displacement gauge to the specimen using

appropriate means, as described in 5.2. Apply loads to the specimen through the wedge-split pin assembly

in contact with the specimen until a fast-running crack initiates. Throughout the load application, wedge

load versus crack-mouth opening displacement should be recorded. A typical crack run-arrest record is

illustrated in Figure 6. The load should be applied with the crosshead speed of 2 mm/min to 25 mm/min.

To measure K , a segment of unstable crack extension shall occur. The occurrence of unstable crack

extension normally becomes apparent, both audibly and as an abrupt load drop on the test record. After

the event, the load on the wedge should be removed to avoid further crack propagation.

If on loading, attempts to increase the opening displacement are accompanied by either a constant,

or a decrease in the applied wedge load, then a substantial crack tip yielding or stable tearing maybe

occurring. For these cases, it is unlikely that the specimen will exhibit rapid crack run-arrest fracturing.

© ISO 2014 – All rights reserved 7
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ISO 29221:2014(E)

It is recommended that under these circumstances, the test be discontinued. A new specimen should be

used with the starter notch techniques described in 6.2.

NOTE 1 While it is expected that a values for the starting notch typically lie in the range 0,30 W ≤ a ≤ 0,40 W,

o o

it is sometimes useful to utilize values as low as 0,20 W. The lower initial value of a /W results in a greater and

quicker drop in the crack driving force as the crack extends. This can aid in arresting the running crack by a

shorter final crack length and could be useful for conditions where the crack extension is too great with larger

initial a /W values.

NOTE 2 When the notch-tip cooling method is used to initiate the self-arresting crack, K at the test temperature

is calculated, but not K .
Key
X crack-mouth opening displacement, δ (mm)
Y applied load on the wedge, P (N)
1 maximum load, P (N)
max
2 load short time after crack arrest, P (N)
arrest
3 displacement at maximum load, δ (mm)
4 displacement short time after crack arrest, δ (mm)

Figure 6 — Typical experimental wedge-load versus crack-mouth-opening-displacement curve

for crack run-arrest segment

In this method, the load is applied to the wedge until a rapid crack initiates and, hence, does not allow

direct measurement of the opening loads applied to the specimen by the wedge and split-pin assembly.

The load applied to the specimen is therefore obtained from the measurements of the crack-mouth

opening displacement. Thus, it becomes important to carefully arrange the seating of the specimen, the

load train, and the displacement gauge at the mouth opening, which can all affect the measurement of

the opening displacement.
7.4 Displacement measurement
From the wedge load versus crack-mouth-opening-displaceme
...

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