ISO 17281:2018
(Main)Plastics — Determination of fracture toughness (GIC and KIC) at moderately high loading rates (1 m/s)
Plastics — Determination of fracture toughness (GIC and KIC) at moderately high loading rates (1 m/s)
This document specifies the principles and provides guidelines for determining the fracture toughness of plastics in the crack-opening mode (Mode I) by a linear elastic fracture mechanics (LEMF) approach, at load-point displacement rates of up to 1 m/s. It supplements ISO 13586 so as to extend its applicability to loading rates somewhat higher than is the case in the scope of the latter document. Fracture testing at high loading rates presents special problems because of the presence of dynamic effects: vibrations in the test system producing oscillations in the recorded quantities, and inertial loads producing forces on the test specimen different from the forces sensed by the test fixture. These effects need either to be controlled and, if possible, reduced by appropriate action, or else to be taken into account through proper analysis of the measured data. The relative importance of such effects increases with increasing testing rate (decreasing test duration). At speeds of less than 0,1 m/s (loading times of greater than 10 ms) the dynamic effects may be negligible and the testing procedure given in ISO 13586 can be applied as it stands. At speeds approaching 1 m/s (loading times of the order of 1 ms) the dynamic effects may become significant but still controllable. The procedure given in ISO 13586 can still be used though with some provisos and these are contemplated in this document. At speeds of several meters per second and higher (loading times markedly shorter than 1 ms) the dynamic effects become dominant, and different approaches to fracture toughness determination are required, which are outside the scope of this document. The general principles, methods and rules given in ISO 13586 for fracture testing at low loading rates remain valid except where expressly stated otherwise in this document. The methods are suitable for use with the same range of materials as covered by ISO 13586, i.e. — rigid and semi-rigid thermoplastic moulding, extrusion and casting materials; — rigid and semi-rigid thermosetting moulding and casting materials; and their compounds containing fibres ≤ 7,5mm in length. In general, fibres 0,1 mm to 7,5 mm in length are known to cause heterogeneity and anisotropy, especially significant in the fracture processes. Therefore, in parallel with Annex B of ISO 13586:2018, where relevant Annex C of this document offers some guidelines to extend the application of the same testing procedure, with some reservations, to rigid and semi-rigid thermoplastic or thermosetting plastics containing such short fibres. Although the dynamic effects occurring at high loading rates are largely dependent on the material tested as well as on the test equipment and test geometry used, the guidelines given here are valid in general, irrespective of test equipment, test geometry and material tested. The same restrictions as to linearity of the load-displacement diagram, specimen size and notch tip sharpness apply as for ISO 13586.
Plastiques — Détermination de la ténacité à la rupture (GIC et KIC) à vitesses de charge modérément élevées (1 m/s)
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 17281
Second edition
2018-08
Plastics — Determination of fracture
toughness (G and K ) at moderately
IC IC
high loading rates (1 m/s)
Plastiques — Détermination de la ténacité à la rupture (G et K ) à
IC IC
vitesses de charge modérément élevées (1 m/s)
Reference number
©
ISO 2018
© ISO 2018
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ii © ISO 2018 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Test specimens. 2
4.1 Specimen geometry and preparation . 2
4.2 Crack length and number of test replicates . 2
4.2.1 Determination of K . 2
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4.2.2 Determination of G . 2
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4.3 Measurement of test specimen dimensions . 3
5 Test conditions . 4
5.1 Loading mode. 4
5.2 Test speed . 4
5.3 Test atmosphere and temperature . 4
6 Test equipment. 4
6.1 Loading machine . 4
6.2 Loading rigs . 4
6.3 Instrumentation . 5
7 Control of dynamic effects . 5
7.1 Electronic filtering . 5
7.2 Mechanical damping . 6
7.3 Damping level . 7
7.4 Check on speed . 7
8 Data handling . 8
8.1 Analysis of the test records and identification of fracture initiation. 8
8.2 Energy correction .10
8.2.1 General.10
8.2.2 Test piece indentation, machine compliance and damper compression .11
8.2.3 Kinetic energy and inertial loads .14
9 Expression of results .14
9.1 Determination of K .14
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9.2 Determination of σ .14
y
9.3 Determination of G .15
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10 Precision .16
11 Test report .17
Annex A (informative) Estimation of curve fit parameters .19
Annex B (informative) Recommended test report forms .21
Annex C (informative) Testing of plastics containing short fibres .26
Bibliography .29
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
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on the ISO list of patent declarations received (see www .iso .org/patents).
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.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 2,
Mechanical behaviour.
This second edition cancels and replaces the first edition (ISO 17281:2002), which has been technically
revised with the addition of Annex C.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2018 – All rights reserved
Introduction
This document is based on a testing protocol developed by the European Structural Integrity Society
(ESIS), Technical Committee 4, Polymers, Polymer Composites and Adhesives, who carried out the
preliminary enabling research through a series of round-robin exercises which covered a range of
material samples, specimen geometries, test instruments and operational conditions (see References [3]
to [6]). This activity involved about 30 laboratories from 12 countries.
INTERNATIONAL STANDARD ISO 17281:2018(E)
Plastics — Determination of fracture toughness (G and
IC
K ) at moderately high loading rates (1 m/s)
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1 Scope
This document specifies the principles and provides guidelines for determining the fracture toughness
of plastics in the crack-opening mode (Mode I) by a linear elastic fracture mechanics (LEMF) approach,
at load-point displacement rates of up to 1 m/s. It supplements ISO 13586 so as to extend its applicability
to loading rates somewhat higher than is the case in the scope of the latter document.
Fracture testing at high loading rates presents special problems because of the presence of dynamic
effects: vibrations in the test system producing oscillations in the recorded quantities, and inertial
loads producing forces on the test specimen different from the forces sensed by the test fixture. These
effects need either to be controlled and, if possible, reduced by appropriate action, or else to be taken
into account through proper analysis of the measured data.
The relative importance of such effects increases with increasing testing rate (decreasing test
duration). At speeds of less than 0,1 m/s (loading times of greater than 10 ms) the dynamic effects
may be negligible and the testing procedure given in ISO 13586 can be applied as it stands. At speeds
approaching 1 m/s (loading times of the order of 1 ms) the dynamic effects may become significant but
still controllable. The procedure given in ISO 13586 can still be used though with some provisos and
these are contemplated in this document. At speeds of several meters per second and higher (loading
times markedly shorter than 1 ms) the dynamic effects become dominant, and different approaches to
fracture toughness determination are required, which are outside the scope of this document.
The general principles, methods and rules given in ISO 13586 for fracture testing at low loading rates
remain valid except where expressly stated otherwise in this document.
The methods are suitable for use with the same range of materials as covered by ISO 13586, i.e.
— rigid and semi-rigid thermoplastic moulding, extrusion and casting materials;
— rigid and semi-rigid thermosetting moulding and casting materials;
and their compounds containing fibres ≤ 7,5mm in length.
In general, fibres 0,1 mm to 7,5 mm in length are known to cause heterogeneity and anisotropy,
especially significant in the fracture processes. Therefore, in parallel with Annex B of ISO 13586:2018,
where relevant Annex C of this document offers some guidelines to extend the application of the same
testing procedure, with some reservations, to rigid and semi-rigid thermoplastic or thermosetting
plastics containing such short fibres.
Although the dynamic effects occurring at high loading rates are largely dependent on the material
tested as well as on the test equipment and test geometry used, the guidelines given here are valid in
general, irrespective of test equipment, test geometry and material tested.
The same restrictions as to linearity of the load-displacement diagram, specimen size and notch tip
sharpness apply as for ISO 13586.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 13586:2018, Plastics — Determination of fracture toughness (G and K ) — Linear elastic fracture
IC IC
mechanics (LEFM) approach
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13586 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:/ /www. iso. org/obp
— IEC Electropedia: available at http:/ /www.e lectropedia. org/
4 Test specimens
4.1 Specimen geometry and preparation
As for the low-rate testing case covered by ISO 13586, two test configurations are recommended,
namely the three-point bending [also called single edge notch bend (SENB)] and the compact tension
(denoted CT), see Figure 1.
Shape and size, preparation, notching and conditioning of test specimens shall comply with the
requirements set out in Clause 4 of ISO 13586:2018.
4.2 Crack length and number of test replicates
4.2.1 Determination of K
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As in the low-rate testing case covered by ISO 13586, measuring test specimens having the same crack
length is adequate for determining K . The initial crack length a should be in t he range 0,45 ≤ a/w ≤ 0,55.
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However, in view of the lower degree of accuracy to be expected with measurements at high rates of
loading as compared with low-rate testing, it is recommended that at least five replicates, with crack
lengths in the range specified above, be used to determine K , and the results averaged.
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4.2.2 Determination of G
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At variance with the low-rate testing case covered by ISO 13586, a multi-specimen procedure, using a
series of test specimens with identical dimensions but varying crack-length as specified below, shall be
applied for determining G .
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At least 15 valid determinations shall be made, with initial crack length varying over the range
0,20 ≤ a/w ≤ 0,70 for the SENB configuration and 0,40 ≤ a/w ≤ 0,75 for the CT configuration. They
may include the five determinations made on test specimens having initial crack lengths in the range
0,45 ≤ a/w ≤ 0,55 to obtain K . It is then suggested that, of the remaining ten test specimens to be used,
IC
six have initial crack lengths in the range 0,20 ≤ a/w ≤ 0,45 and four in the range 0,55 ≤ a/w ≤ 0,70 in
the case of the SENB configuration and three have initial crack length in the range 0,40 ≤ a/w ≤ 0,45 and
seven in the range 0,55 ≤ a/w ≤ 0,70 in the case of the CT configuration.
2 © ISO 2018 – All rights reserved
a) SENB
b) CT
Key
L span between supports
w specimen width
h specimen thickness
a crack length
Figure 1 — Test configurations as specified in 4.1 and 6.2
4.3 Measurement of test specimen dimensions
Measurement is carried out as described in 5.7 of ISO 13586:2018.
5 Test conditions
5.1 Loading mode
The test shall be performed at constant load-point displacement rate. A maximum variation of 10 % in
the load-point displacement rate during the test is allowed (see 6.1).
5.2 Test speed
As a basic test condition, it is recommended that a load-point displacement rate of 1 m/s be used. If a
different rate is applied, it shall be quoted in the test report.
With rate-sensitive materials such as plastics, a more significant measure of the rate of the experiment
is probably its duration, i.e. the time required to bring the test specimen to fracture. The time to
fracture, t , is understood here as the time interval between the moment when the load starts acting on
f
the test specimen and the point of fracture initiation as defined in 8.1.
With a fixed load-point displacement rate the time to fracture varies with material and specimen
geometry. If results at a given time to fracture (e.g. 1 ms) are desired, it is necessary to adapt the load-
point displacement rate of the test to each material and specimen geometry (type and dimensions).
For this purpose it is expedient to run some preliminary trial tests at different testing speeds (i.e.
load-point displacement rates) to determine the testing speed required to obtain the assigned time to
fracture under the given test conditions.
In any case, the time to fracture, t , shall also be quoted in the test report.
f
5.3 Test atmosphere and temperature
These are determined as described in 5.6 and 5.8 of ISO 13586:2018.
6 Test equipment
6.1 Loading machine
Any type of loading machine (impact pendulums, falling-weight towers, servo-hydraulic universal
testing machines, etc.) is permitted, provided it is capable of applying an adequate load to bring the
test piece to fracture at the required load-point displacement rate and of maintaining this rate constant
throughout the test up to fracture initiation. With testing machines of limited capacity, this requirement
may need to be verified by preliminary tests, especially when new materials are tested or when new
test conditions (e.g. change in specimen size) are used.
Any variation in the load-point displacement rate during the test shall be determined and quoted if it
exceeds 10 % of the rate at fracture initiation.
6.2 Loading rigs
Unlike for low-rate testing, the use of fixed anvils rather than moving rollers is preferred for conducting
three-point bend (SENB) fracture tests under high rate conditions, as is normally the case with standard
impact pendulums. The span between the supports shall be adjustable however, so that specimens of
different size can be accommodated, as specified in Clause 4 of ISO 13586:2018.
NOTE In the case of three-point bend testing (SENB specimens), improved results can be obtained if the test
piece is held in contact with the anvils by light springs (e.g. rubber bands). These will assist in maintaining the
test piece in position during the sudden load transmission from the machine to the test specimen, and ensure
more reproducible records.
4 © ISO 2018 – All rights reserved
6.3 Instrumentation
Acquisition of a complete record of the load/time response of the material sample under test is
essential for the determination of K . In addition, a means of evaluating the displacement of the moving
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load-point during the test is necessary for the independent determination of G . Instrumentation
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of the testing machine should thus comprise, basically, a force sensing and recording system and a
displacement measuring and recording system or devices to measure and record quantities from which
the load and the load-point displacement can also be indirectly determined.
The adequacy of the response of this equipment to the dynamic events occurring in the relevant
determinations shall be checked. It can be considered satisfactory if a plain plastic specimen (without
any mechanical damping device in place) shows an inertial peak (see Figure 2) larger than 100 N at
1 m/s test speed. The response time shall be <20 % of the input signal rise time.
If a digital recording system is used, the sampling time should be less than 1/200 of the time to fracture,
i.e. at least 200 data points should be collected over the time interval from the first increase of the signal
to the point of fracture initiation in order to define the required data curve with sufficient accuracy.
7 Control of dynamic effects
7.1 Electronic filtering
The first manifestation of dynamic effects is the presence of oscillations in the load-recording signal.
They may complicate the interpretation of the test records up to the point of obscuring the basic
response of the specimen under test. It is thus desirable that these effects be contained. Reducing these
oscillations artificially, a posteriori, by electronic filtering or attenuation can be fallacious however,
since it may wipe out some real features of the specimen response. Therefore, electronic filtering or
attenuation is not permitted unless the source of the removed “noise” is known and the effect on the
data is understood.
Key
F load
t time
a
Inertial peak.
Figure 2 — Typical load/time record in the absence of signal attenuation and mechanical damping
7.2 Mechanical damping
Some control of the effects of inertial loads can be achieved by proper mechanical damping of the load
transmission. With impact testing machines the impact may be cushioned by means of a soft pad, placed
where the tup strikes the specimen. The pad should reduce the inertial effects by reducing the “contact
stiffness”. With high-speed testing machines (e.g. servo-hydraulic), initial acceleration of the specimen
can be controlled by means of a damper applied in the motion transmission unit.
With impact testing machines and SENB test specimens the damping pad can be made by spreading a
layer of a paste or a highly viscous grease over the contact surface either of the tup of the striking hammer
or of the test piece. For the sake of reproducibility it is important that the grease be homogeneous and
evenly applied, with thickness constant to ±0,05 mm. This can be obtained by delivering the grease
with a spatula through an aluminium stencil having the required thickness, normally a few tenths of a
millimetre, as shown in Figure 3.
With high-speed testing machines and CT test specimens the damping pad can be more conveniently
made of a viscoelastic rubber-like material with a low coefficient of restitution. The rubber-like
character should ensure a more or less complete recovery of the pad deformation after each test, thus
allowing the same pad to be used repeatedly.
6 © ISO 2018 – All rights reserved
7.3 Damping level
If mechanical damping is applied, it shall be kept to a minimum, sufficient to contain the fluctuations in
the force-time trace within the 10 % envelope defined in 8.1. To obtain this optimal result it is advisable
to run some preliminary trial tests to gauge the performance of the damper. This can be varied by
changing the consistency of the damping material used and/or the thickness of the pad made thereof.
Key
1 aluminium stencil
2 damping pad
Figure 3 — Deposition of damping pad on SENB test specimen
If the test specimens are in short supply, it is advisable to use an unnotched specimen to assay the
performance of the damper. The dynamic effects that are to be controlled by mechanical damping are
in fact largely independent of crack length and the use of an unnotched specimen offers the advantage
that it can stand repeated strokes without breaking.
In order to determine the level of damping needed to meet the requirement stated in 8.1, reference
should be made to the worst case to be expected in the testing programme, i.e. the case of the specimen
with the deepest notch, which will present the lowest fracture resistance and thus the largest (force
oscillation)/(fracture load) ratio.
7.4 Check on speed
Because of damping, some deviations from the pre-set load-point displacement rate may ensue. Thus,
if mechanical damping is applied, the instrument shall be reset to the desired load-point displacement
rate and its constancy checked (as requested under 5.2) under the actual test conditions, i.e. with the
damping device in place.
If mechanical damping is applied, it shall be recorded in the test report.
8 Data handling
8.1 Analysis of the test records and identification of fracture initiation
These tests, as well as the low speed tests covered in ISO 13586, are designed to characterize the
toughness at fracture initiation. Once a fracture test has been performed and the load-time or load-load
point displacement curve has been obtained, the question arises of identifying the point of fracture
initiation. Several techniques are possible, but in this document it is deduced from the load diagram.
The same rules as those stated in ISO 13586, for the determination of F are used here, but in the case
Q
of high-rate testing some preliminary analysis of the load-time record is required to make sure that
dynamic effects do not obscure the basic response of the specimen under test.
Firstly, in the case of high-rate testing, a load drop before maximum load should not be assumed to be
an arrested crack extension (“pop-in”), unless borne out by examination of the fracture surface.
Secondly, the occurrence of force peaks and fluctuations in the initial part of the load-time record
is tolerated, but a limit is placed on force fluctuations in the portion of the force-time record where
the force exceeds 1/2 of its value at fracture initiation and the curve is smoothed. The linearity
requirements referred to in 6.1 of ISO 13586:2018 need to be verified here on the “smoothed” load-
displacement curve.
The procedure is as follows.
Draw a smooth mean force-time curve through the experimental load-time record, F(t), and determine
F and F /3 on that curve (see Figure 4). Then improve the determination of the mean load/
max max
time curve by a computer-aided curve-fitting procedure. The following empirical fitting formula is
suggested:
n
Ft()=−mt()tb−−()tt (1)
where t , m, b and n are (positive) fitting parameters, with n preferably ≥5.
Use the curve drawn previously to obtain a first estimate of these parameters (see Annex A) and use
this set of values at the start of the regression analysis. The regression analysis should be confined to
the portion of the experimental curve comprised in the time interval defined by F / 3 and F . The
max max
value of the initial time, t , should also be derived from the regression analysis. However, if that value
turns out to be smaller than the time when the force signal first rises, take the latter one as initial time
t and repeat the curve fitting by forcing the new curve Ft() to pass through the point t = t , F = 0.
0 0
Finally, determine F on the curve Ft() (Figure 4), as indicated in 6.1 of ISO 13586:2018 (see also the
Q
Note below). To this end, the “maximum load” — to be denoted F — is defined here as the value of
max
the fitted force, Ft(), at time t = t corresponding to the maximum of the experimental curve (see
max
Figure 4).
The curve Ft() so obtained is assumed to be a good representation of what the load-time response of
the system would be in the absence of dynamic effects, provided it meets the following requirement
(see Figure 5): the force F(t) recorded experimentally shall not deviate from the mean current value
Ft() by more than 5 % of the critical value F over the time interval defined by F /2 and F . To
Q Q Q
check this draw two lines parallel to the curve Ft() at a distance of 5 % of F on either side of it, over
Q
the time interval defined by F /2 and F . All parts of the experimental curve F(t) in that interval
Q Q
should fall within this 10 % envelope. If the experimental curve F(t) fails this requirement, then the
8 © ISO 2018 – All rights reserved
determination shall be deemed invalid. Before abandoning any determination however, action shall be
taken to try and reduce the dynamic effects further, as stated in Clause 7.
NOTE Once the parameters of the best fit have been determined, the two straight lines to be used in order to
identify F (see ISO 13586:2018, 6.1) can be simply obtained as given by the formulae Fm= ()tt− and
Q 0
11/(n − )
Fm=09,(5 tt− ) and the value of F can
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