ISO 15114:2014
(Main)Fibre-reinforced plastic composites — Determination of the mode II fracture resistance for unidirectionally reinforced materials using the calibrated end-loaded split (C-ELS) test and an effective crack length approach
Fibre-reinforced plastic composites — Determination of the mode II fracture resistance for unidirectionally reinforced materials using the calibrated end-loaded split (C-ELS) test and an effective crack length approach
ISO 15114:2014 specifies a method for the determination of mode II shear load delamination resistance. GIIC, (critical energy release rate), of unidirectional fibre-reinforced plastic composites using the calibrated end-loaded split (C-ELS) test. It is applicable to carbon-fibre and glass-fibre reinforced thermosets and thermoplastics. The scope is not necessarily limited to these fibres and lay-ups, but for laminates with other types of fibres or lay-ups, no recommendations for specimen dimensions and fibre volume content are currently available.
Composites plastiques renforcés de fibres — Détermination de la résistance à la rupture en mode II de matériaux renforcés de fibres unidirectionelles en utilisant l'essai de délaminage (C-ELS) et une approche de la longueur de fissure réelle
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INTERNATIONAL ISO
STANDARD 15114
First edition
2014-05-15
Fibre-reinforced plastic composites —
Determination of the mode II fracture
resistance for unidirectionally
reinforced materials using the
calibrated end-loaded split (C-ELS)
test and an effective crack length
approach
Composites plastiques renforcés de fibres — Détermination de la
résistance à la rupture en mode II de matériaux renforcés de fibres
unidirectionelles en utilisant l’essai de délaminage (C-ELS) et une
approche de la longueur de fissure réelle
Reference number
ISO 15114:2014(E)
©
ISO 2014
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ISO 15114:2014(E)
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© ISO 2014
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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ii © ISO 2014 – All rights reserved
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ISO 15114:2014(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Symbols and abbreviated terms . 1
4 Principle . 2
5 Apparatus . 2
6 Specimens . 4
6.1 Preparation of specimens . 4
6.2 The initial defect . 4
6.3 Attaching the load-block to the specimen . 4
6.4 Moisture conditioning . 5
6.5 Final specimen preparation and measuring dimensions . 5
6.6 Number of specimens . 6
7 Procedure. 6
7.1 Performing the calibration of the ELS fixture. 6
7.2 Pre-cracking the specimens . 7
7.3 Testing the samples in mode II from the precrack formed in 7.2 . 8
8 Data analysis . 8
8.1 The points for data analysis . 8
8.2 Determination of the ELS clamp correction .10
8.3 Determination of G values .11
IIC
9 Precision .13
10 Test report .14
Annex A (informative) Large displacement and load-block correction factors .16
Bibliography .18
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ISO 15114:2014(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 13, Composites
and reinforcement fibres.
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ISO 15114:2014(E)
Introduction
Previous attempts to determine mode II delamination resistance curves (R-curves) for composites
have been hampered by the experimental difficulty of determining crack length in the absence of any
applied beam opening displacement and when a complex damage zone develops ahead of the crack front.
The effects of friction in the different mode II test specimens have also been widely debated and have
typically been determined to introduce errors of between 1 % and 3 % in G determination for ELS
IIC
specimens (n.b. friction effects would appear to be more significant in 3 point loaded end notch flexure
(3ENF) (to be standardized by ASTM) and, particularly, in the 4 point loaded (4ENF) test specimen.
Stabilized ENF was not popular in round-robin trials).
The procedure presented here uses the end-loaded split test apparatus and specifies an experimental
procedure to calibrate the clamping fixture and simultaneously determine the flexural modulus of
the specimen. This serves two purposes. Firstly, the clamp calibration has been found to significantly
reduce scatter in the results between different test laboratories and secondly, it provides an accurate
means by which crack lengths can be calculated and thus their measurement can be avoided. Although
this procedure still includes an experimental determination of crack length, the use of calculated (or
effective crack lengths) means that values of G can be determined without experimentally measured
IIC
crack length values. The procedure is a development of that published by ESIS (the European Structural
[1]
Integrity Society), Technical Committee 4, Polymers and Composites , who carried out the preliminary
enabling research through a series of round-robin exercises conducted in 2004 and 2007.
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INTERNATIONAL STANDARD ISO 15114:2014(E)
Fibre-reinforced plastic composites — Determination
of the mode II fracture resistance for unidirectionally
reinforced materials using the calibrated end-loaded split
(C-ELS) test and an effective crack length approach
1 Scope
This International Standard specifies a method for the determination of mode II shear load delamination
resistance. G , (critical energy release rate), of unidirectional fibre-reinforced plastic composites using
IIC
the calibrated end-loaded split (C-ELS) test.
It is applicable to carbon-fibre and glass-fibre reinforced thermosets and thermoplastics.
The scope is not necessarily limited to these fibres and lay-ups, but for laminates with other types of
fibres or lay-ups, no recommendations for specimen dimensions and fibre volume content are currently
available.
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 291, Plastics — Standard atmospheres for conditioning and testing
ISO 5893, Rubber and plastics test equipment — Tensile, flexural and compression types (constant rate of
traverse) — Specification
ISO 15024, Fibre-reinforced plastic composites — Determination of mode I interlaminar fracture toughness,
GIC, for unidirectionally reinforced materials
3 Symbols and abbreviated terms
For the purposes of this document, the following terms and definitions apply.
a measured delamination length, distance between the load-line (intersection of the plane
through the pin-hole centre of the load-block normal to the specimen width and the
plane of delamination) and the tip of the delamination on the edge of the specimen (see
Figure 1)
a insert film length, measured from the load-line to the tip of the insert film (seeFigure 1)
o
a precrack length, the length between the load-line and the tip of the precrack formed in
p
during the precracking step
b width of the specimen
C compliance δ/P of the specimen
C compliance of the specimen at maximum load
max
C initial compliance of the specimen neglecting start-up effects, e.g. due to play in the
0
specimen fixture
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ISO 15114:2014(E)
C initial compliance, C of the specimen increased by 5 %
5 % 0,
δ displacement of the cross-head of the testing machine
E elastic modulus determined from “three-point bending” flexural test or from the clamp
1
calibration test
G critical energy release rate for mode II shear loading
IIC
H height of the load-block
l total length of the specimen
l distance between the centre of the loading pin and the mid-plane of the specimen beam
1
to which the load-block is attached (see Figure 5), i.e. equal to (H + h)/2 if the pin hole is
through the centre of the block
l distance between the centre of the loading pin and the edge of the load block, measured
2
towards the tip of the insert (starter film) or the tip of the mode I or mode II precrack
(see Figure 5), i.e. equal to l /2 if the pin hole is through the centre of the block
3
l length of the load-block (see Figure 5)
3
L free length of the specimen between load-line and clamp (see Figure 1)
MAX maximum load on the load-displacement trace (see Figure 7)
3
m slope of C versus a plot
NL onset of nonlinearity on the load-displacement trace (see Figure 7)
P load measured by the load-cell of the testing machine
PROP increments of the delamination length during stable delamination growth (propagation)
that are marked on the load-displacement curve (see Figure 7)
2
r correlation coefficient of linear fit
VIS onset of visually recognizable delamination growth on the edge of the specimen that is
marked on the load-displacement trace (see Figure 7)
2h total thickness of the specimen (thickness of each specimen arm is h)
5 % point of intersection of a straight line with the load-displacement trace, with the slope of
the straight line corresponding to C
5 %
4 Principle
This procedure specifies a method for the determination of the delamination resistance of unidirectional
fibre-reinforced polymer laminates under mode II shear load using the calibrated end-loaded split
(C-ELS) test. The resistance to the initiation and propagation of a delamination is determined from a
non-adhesive insert and from a mode I (opening) or a mode II (shear) precrack. The critical energy
release rate for mode II loading can be calculated and a resistance-curve (R-curve, i.e. a plot of the
critical energy release rate versus delamination length) determined.
5 Apparatus
A tensile testing machine in compliance with ISO 5893, capable of producing a constant load-rate
between 1 mm/min and 5 mm/min in displacement control should be used. The load-cell should be
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ISO 15114:2014(E)
calibrated and accurate within ±1 % for the chosen load-range (loads are typically expected to be in the
range of 100 N to 1 000 N). The testing machine shall be equipped with a fixture to introduce the load to
the pin inserted into the load-block that allows rotation of the specimen end.
The recommended loading jig requires a clamping arrangement to freely slide in bearings in the
horizontal direction (side-ways) with a fixed load point. This is shown schematically in Figure 1. Two
test fixtures used in the round-robin programmes (see Clause 9) are shown in Figure 2.
Figure 1 — ELS test specimen showing the clamping fixture and loading
Figure 2 — Two alternative ELS test fixtures
A calibrated lever arm (torque wrench) is required to apply a consistent pressure while fixing the
specimens into the sliding fixture. It is recommended that this device can apply closing torsion in the
range of 0 Nm to 30 Nm to a precision of ±1 Nm. During the test, the load shall be applied vertically
on the load-block by pulling upward provided the clamp is symmetrical with respect to the specimen.
The testing machine shall be equipped with means for recording the complete load-displacement traces
(loading and unloading) that allow a determination of the loads and the corresponding displacements
with an accuracy of ±1 %. Vernier callipers or a micrometer should be used to measure the specimen
thickness (2h) to an accuracy of ±0.02 mm and the specimen width, b, to an accuracy of ±0.02 mm. A
travelling microscope (or video camera) shall be used to monitor the length of the delamination along
one edge of the specimen with a magnification of between ×10 and ×25.
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ISO 15114:2014(E)
6 Specimens
6.1 Preparation of specimens
The recommended specimen width, b, and length, l, are 20 mm and 190 mm, respectively. The specimen
length shall not be less than the active length of the insert, a , plus 110 mm; thus, l ≥ a + 110 mm. For
o o
recommendations on the length of a , see 6.2. The free length, L, is typically 100 mm. The recommended
o
specimen thickness (2h) is 3 mm for 60 % by volume carbon fibre-reinforced and 5 mm for 60 % by
volume glass fibre-reinforced composites.
Other specimen dimensions can be used, but the specimen width should be between 15 mm and
30 mm. Increasing the length of the specimen is not critical, shortening will reduce the maximum
delamination length that can be investigated, and thus, yield too few data points for the analysis (see
8.1). If specimens are too thin or not sufficiently stiff, delamination growth might not be induced or
occur at large displacements only, or permanent deformation of the specimen might occur, invalidating
the assumptions of linear elastic fracture mechanics.
6.2 The initial defect
A crack starter film should be placed at the laminate mid-thickness during the lay-up of the composite
panel prior to moulding. The film should be PTFE or another fluoro-polymer with excellent non-stick
properties. The film should be thin (between 10 microns and 13 microns) to minimize the disturbance
of the laminate. The upper service temperature of the film should be greater than the cure temperature
of the laminate. When the composite panel is trimmed, the active starter film length should satisfy the
requirement that a > 50 mm, so that the influence of the load-block can be neglected. This initial defect
o
will be extended in mode I or mode II loading prior to testing (see 7.2).
6.3 Attaching the load-block to the specimen
One load-block should be bonded to each specimen for the purposes of load-introduction [see Figure 3 b)].
The block should be of the same width as the specimen. Prior to bonding, the load-block and the specimen
(in the position where the block will adhere) should firstly be lightly abraded using an abrasive paper or
grit blasting. Both the load-block and the specimen should then be cleaned with a solvent.
A tough, room-temperature cure adhesive (e.g. two part epoxy) is recommended. If bond failure occurs it
might be necessary to consult ISO 4588 for a more sophisticated surface treatment procedure. Bonding
of the load-block should be done immediately after the surface preparation.
The load-block should be well aligned with the specimen and held in position with a clamp while the
adhesive sets. Specimen edges should be smoothed prior to determining the dimensions. For the clamp
calibration procedure (as described in 7.1), one specimen should be prepared with the load-block bonded
to the end not containing the insert film as shown in Figure 3 a). After the clamp calibration, this load-
block can be removed and one should be bonded at the insert end [see Figure 3 b)] to allow the specimen
to be tested in mode II.
NOTE If the specimen is sufficiently long, that the end-block attached for the clamp calibration measurement
does not interfere with the clamping of the specimen in the subsequent mode II test, then this load-block can be
left in place on the beam.
a) (1 specimen per sample)
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ISO 15114:2014(E)
b) (5 specimens per sample)
Figure 3 — Position of the load-blocks for
a) inverse ELS specimen for clamp calibration and b) for the ELS fracture specimen
6.4 Moisture conditioning
Moisture conditioning is required for obtaining baseline data in order to test specimens with uniform
moisture content. The drying conditions (temperature and duration) shall be chosen according to the
recommendations of the resin supplier. Conditioning should be performed after bonding of the load-block.
Before testing, the specimens can be stored in a desiccator for at most three days after conditioning.
NOTE Other conditioning procedures can be applied for the investigation of specific conditioning effects.
6.5 Final specimen preparation and measuring dimensions
In preparation for the visual measurement of crack length, applying a thin layer of typewriter correction
fluid (“white ink”) on the edges of the specimen after conditioning will facilitate the measurement. The
following procedure is recommended:
a) Apply a thin coat of white type-writer correction fluid to the edge of one side of each specimen.
(Ensure the use of a new bottle and try to apply the coating in a single brush stroke, avoiding re-
brushing if possible. Practicing on another specimen or the reverse side of the test specimen is
recommended).
b) When the layer is dry, locate the position of the end of the insert film and mark this with a black pen.
(A nib of 0,1 mm is recommended).
c) Mark the specimen edge at regular increments, starting one division before the end of the film
insert and extending to a = 100 mm. (Drawing straight vertical lines across the beam edge at the
crack length increments is helpful). The mark increment should be chosen to be either 2,0 mm or
2,5 mm, depending upon the specimen length available for crack propagation. For shorter lengths,
the narrower increment should be selected.
d) With the specimen to be used for the clamp calibration, draw lines at 50 mm, 60 mm, 70 mm, 80 mm,
90 mm, 100 mm, and 110mm from the load-block pin hole centre (load-line). (These will be the
positions at which the specimen will be clamped in the clamp calibration test; see 7.1.)
Some typewriter correction fluids contain solvents that might be harmful to the laminate matrix
material. A water-based paint is thus recommended.
NOTE The vertical lines will shear when the crack front passes and might kink due to the shear strain ahead
of the crack tip. The crack position is determined from the lines shearing.
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ISO 15114:2014(E)
Figure 4 — A specimen during loading in an ELS test, showing the crack length markers at
2,5 mm increments along the specimen edge (L = 100 mm for this test)
Measure and record the length, l, of each specimen to the nearest mm. Measure the width, b, to the
nearest 0,02 mm at three evenly spaced points along the length. Measure the thickness, 2h, to the nearest
0,02 mm at three points along the centre-line of the specimen. The variation in thickness and average
values of the width and the thickness should be recorded for each specimen. The variation in thickness,
i.e. the maximum difference between the thickness measurements, should not exceed 0,1 mm for each
specimen. The length, l , and height, H, of the end block should be measured to the nearest 0,1 mm and
3
be recorded. The load-block details are shown in Figure 5.
H
Figure 5 — Load-block dimensions
6.6 Number of specimens
A minimum number of five specimens shall be tested. A minimum of one calibration specimen per
sample of nominally identical specimens should be used.
7 Procedure
7.1 Performing the calibration of the ELS fixture
The following procedure should be followed:
a) Position the specimen in the clamping fixture as shown in Figure 6, loosely tighten the clamp, attach
the load-block to the test machine, e.g. via the loading pin, then clamp the specimen with a free
length, L = 110 mm and record the clamping pressure applied.
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ISO 15114:2014(E)
b) Apply load to the load-block at a cross-head displacement rate of 1 mm/min such that the beam
deforms elastically and record the load-displacement trace.
Loading to a maximum load of 250 N is recommended for CFRP specimens and 150 N for GFRP
specimens.
NOTE It is important that the insert film is held fully within the clamp, so that it does not influence the
measured beam compliance. Adjust the length, L, if necessary to ensure this.
c) Stop the loading, and then, fully unload the specimen and repeat the procedure with the beam
clamped at free lengths of 100 mm, 90 mm, 80 mm, 70 mm, 60 mm, and 50 mm. The unloading can
be performed at up to 10 mm/min.
NOTE It is important that the clamping of the specimen in the ELS fixture can be performed in a
reproducible manner. This is conveniently achieved using a torque wrench, such that the retaining nuts can
be tightened to the same level of torque in each test. (A torque of 8 Nm has been found to be suitable for the
clamping of carbon-fibre epoxy specimens, see Figure 2, upper test fixture.)
Figure 6 — The clamp calibration set up with the delamination fully within the clamp
When the clamp calibration has been performed, the specimen used can then have the load-block
removed and another should be bonded to the end containing the crack (see 6.3) so that a fracture test
can be performed on this specimen [see Figure 3 b)].
7.2 Pre-cracking the specimens
Experience has shown that initiation values of G will depend on the precracking method used.
IIC
Specimens should always be precracked and either mode I or mode II precracking can be used. The
procedure used should be clearly stated in the report.
7.2.1 Mode I precracking
The specimens should be loaded in mode I (tensile opening) until the crack has grown at least 2 mm (but
no greater than 5 mm) ahead of the insert. The procedures of ISO 15024 should be consulted for mode I
loading using the double cantilever beam (DCB) test specimen.
NOTE If mode I rather than mode II precracking is chosen (see 7.2), two load blocks shall be bonded to each
specimen rather than one. After precracking, one of the two load blocks shall be removed before mode II testing.
7.2.2 Mode II precracking
The procedures for precracking in mode II are somewhat more complex, as it is intended that unstable
crack initiation from the insert be avoided. Thus, the following procedure is recommended. A load-scale
of up to 500 N is usually sufficient for composite laminates.
a) Determine the initial length of the crack, a .
o
b) Determine an initial free length, L, via L = a × (4/3) and clamp the specimen at this value of L.
o
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ISO 15114:2014(E)
c) Load the specimen at a displacement rate of 1 mm/min until the delamination has grown at least
2 mm (but no more than 5 mm) ahead of the insert. The load-displacement trace should be recorded
during this procedure.
d) Immediately stop loading and fully unload the specimen (displacement rates of up to 5 mm/min can
be used for unloading).
e) Record the length of the newly formed precrack, a .
p
f) Release the clamp, remove the specimen, and repeat for the remaining specimens in the sample.
NOTE The initial free length is chosen such that the initial ratio of (a/L) = 0,75. This is chosen to promote
stability. An energy analysis predicts that the ELS test is stable provided approximately that (a/L) > 0,55. Thus,
the factor of 0,75 provides a margin for safety, promoting the conditions for stability.
7.3 Testing the samples in mode II from the precrack formed in 7.2
A travelling microscope and specimen illumination should be used to observe delamination growth. A
magnification of between ×10 to ×25 should be employed. The following procedure should be used for
each specimen in the sample.
a) Determine the free length, L, such that (a /L) > 0,55 and clamp the specimen.
p
NOTE Stability can be improved by using larger values of a /L, but using larger values will reduce the
p
length available for crack propagation.
b) Load the specimen at a displacement rate of 0,5 mm/min, until the crack has grown to within 10 mm
from the clamp, recording the observed crack lengths on the load-displacement trace. Crack lengths
should be recorded at 2,0 mm (or 2,5 mm) intervals. The first point should correspond to the instant
when the crack is seen to grow ahead of the precrack.
NOTE The slow loading rate will improve the accuracy of the crack length measurements.
c) Immediately stop the loading and fully unload the specimen (displacement rates of up to 5 mm/min
can be used for unloading).
d) Continue to record the load and displacement during the unloading.
e) Remove the specimen and inspect for any damage.
NOTE Some materials have been observed to microcrack severely ahe
...
DRAFT INTERNATIONAL STANDARD ISO/DIS 15114
ISO/TC 61/SC 13 Secretariat: JISC
Voting begins on Voting terminates on
2012-10-08 2013-01-08
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION МЕЖДУНАРОДНАЯ ОРГАНИЗАЦИЯ ПО СТАНДАРТИЗАЦИИ ORGANISATION INTERNATIONALE DE NORMALISATION
Fibre-reinforced plastic composites — Determination of the
mode II fracture resistance for unidirectionally reinforced
materials using the calibrated end-loaded split (C-ELS) test and
an effective crack length approach
Composites plastiques renforcés de fibres — Détermination de la résistance à la rupture en mode II de
matériaux renforcés de fibres unidirectionelles en utilisant l'essai de délaminage (C-ELS) et une approche de
la longueur de rupture effective
ICS 83.120
To expedite distribution, this document is circulated as received from the committee
secretariat. ISO Central Secretariat work of editing and text composition will be undertaken at
publication stage.
Pour accélérer la distribution, le présent document est distribué tel qu'il est parvenu du
secrétariat du comité. Le travail de rédaction et de composition de texte sera effectué au
Secrétariat central de l'ISO au stade de publication.
THIS DOCUMENT IS A DRAFT CIRCULATED FOR COMMENT AND APPROVAL. IT IS THEREFORE SUBJECT TO CHANGE AND MAY NOT BE
REFERRED TO AS AN INTERNATIONAL STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS BEING ACCEPTABLE FOR INDUSTRIAL, TECHNOLOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON OCCASION HAVE TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL TO BECOME
STANDARDS TO WHICH REFERENCE MAY BE MADE IN NATIONAL REGULATIONS.
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT, WITH THEIR COMMENTS, NOTIFICATION OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPORTING DOCUMENTATION.
© International Organization for Standardization, 2012
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ISO/DIS 15114
Copyright notice
This ISO document is a Draft International Standard and is copyright-protected by ISO. Except as permitted
under the applicable laws of the user’s country, neither this ISO draft nor any extract from it may be
reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic,
photocopying, recording or otherwise, without prior written permission being secured.
Requests for permission to reproduce should be addressed to either ISO at the address below or ISO’s
member body in the country of the requester.
ISO copyright office
Case postale 56 CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Reproduction may be subject to royalty payments or a licensing agreement.
Violators may be prosecuted.
ii © ISO 2012 – All rights reserved
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ISO/DIS 15114
Contents Page
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references. 1
3 Symbols and abbreviated terms. 1
4 Principle . 2
5 Apparatus . 3
6 Specimens . 4
6.1 Preparation of specimens . 4
6.2 The initial defect . 4
6.3 Attaching the load-block to the specimen . 4
6.4 Moisture conditioning . 5
6.5 Final specimen preparation and measuring dimensions . 5
6.6 Number of specimens . 6
7 Procedure . 6
7.1 Performing the calibration of the ELS fixture . 6
7.2 Pre-cracking the specimens . 7
7.2.1 Mode I precracking . 7
7.2.2 Mode II precracking . 7
7.3 Testing the samples in mode II from the precrack formed in 7.2 . 8
8 Data analysis . 8
8.1 The points for data analysis . 8
8.1.1 Initiation points . 8
8.1.2 Propagation points . 9
8.2 Determination of the ELS clamp correction . 9
8.3 Determination of G values . 10
IIC
8.3.1 Method 1: Experimental Compliance Method (ECM) . 11
8.3.2 Method 2: Simple Beam Theory (SBT) . 11
8.3.3 Method 3: Corrected Beam Theory using Effective Crack Length (CBTE) . 11
9 Precision . 12
10 Test report . 13
Bibliography . 14
Annex A (informative) Large displacement and load-block correction factors . 15
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ISO/DIS 15114
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 15114 was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 13,
Composites and reinforcement fibres.
This second/third/. edition cancels and replaces the first/second/. edition (), [clause(s) /
subclause(s) / table(s) / figure(s) / annex(es)] of which [has / have] been technically revised.
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ISO/DIS 15114
Introduction
Previous attempts to determine Mode II delamination resistance curves (R-curves) for composites
have been hampered by the experimental difficulty of determining crack length in the absence of any
applied beam opening displacement and when a complex damage zone develops ahead of the crack
front. The effects of friction in the different Mode II test specimens have also been widely debated and
have typically been determined to introduce errors of between 1-3 % in G determination for ELS
IIC
specimens (n.b. friction effects would appear to be more significant in 3 point loaded end notch
flexure (3ENF) [to be standardised by ASTM] and, particularly, in the 4 point loaded (4ENF) test
specimen. Stabilised ENF was not popular in round-robin trials).
The test protocol presented here uses the end loaded split test apparatus and specifies an
experimental procedure to calibrate the clamping fixture and simultaneously determine the flexural
modulus of the specimen. This serves two purposes. Firstly, the clamp calibration has been found to
significantly reduce scatter in the results between different test labs and secondly, it provides an
accurate means by which crack lengths may be calculated and thus their measurement can be
avoided. Although this protocol currently still requires an experimental determination of crack length
to be attempted, the use of calculated (or effective crack lengths) is intended to make this requirement
redundant when sufficient data have been collected in round-robin programmes to validate the newly
proposed scheme. The protocol is a development of that published by ESIS (the European Structural
Integrity Society), Technical Committee 4, Polymers and Composites,[1] who carried out the
preliminary enabling research through a series of round-robin exercises conducted in 2004 and 2007.
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DRAFT INTERNATIONAL STANDARD ISO/DIS 15114
Fibre-reinforced plastic composites — �Determination
of the Mode II fracture resistance for unidirectionally
reinforced materials using the calibrated end loaded split
(C-ELS) test and an effective crack length approach
1 Scope
1.1 This International Standard specifies a method for the determination of Mode II shear load
delamination resistance. G , (critical energy release rate), of unidirectional fibre-reinforced plastic
IIc
composites using the Calibrated End Loaded Split (C-ELS) test.
1.2 It is applicable to carbon-fibre and glass-fibre reinforced thermosets and thermoplastics.
1.3 The scope is not necessarily limited to these fibres and lay-ups, but for laminates with other types
of fibres or lay-ups, no recommendations for specimen dimensions and fibre volume content are
currently available.
Annex A is informative.
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 291, Plastics; standard atmospheres for conditioning and testing
ISO 4588, Adhesives; preparation of metal surfaces for adhesive bonding
ISO 5893, Rubber and plastics test equipment; tensile, flexural and compression types (constant rate
of traverse); description
ISO 14125, Fibre-reinforced plastic composites — Determination of flexural properties
ISO 15024, Plastics — Determination of the mode I energy release rate for fibre-composites using the
double cantilever beam (DCB) test specimen
3 Symbols and abbreviated terms
For the purpose of this protocol the following symbols and conventions apply
a measured delamination length, distance between the load-line (intersection of the plane
through the pin-hole centre of the load-block normal to the specimen width and the plane of
delamination) and the tip of the pre-crack or delamination on the edge of the specimen
(Figure 1)
A starter delamination (insert) length, distance between end of specimen on which the load-
block is mounted and tip of the insert
b width of the specimen
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ISO/DIS 15114
C compliance /P of the specimen
C initial compliance of the specimen neglecting start-up effects, e.g. due to play in the specimen
0
fixture
C compliance of the specimen at maximum load
max
C initial compliance C of the specimen increased by 5 %
5% 0
displacement of the cross-head of the testing machine
E elastic modulus determined from "three-point bending" flexural test or from the clamp
1
calibration test
G critical energy release rate for Mode II shear loading
IIC
2h total thickness of the specimen (thickness of each specimen arm is h)
H height of the load-block
l total length of the specimen
l distance from the centre of the loading pin to the mid-plane of the specimen beam to which
1
the load-block is attached (Figure 5), i.e. equal to (H+h)/2 if the hole is through the centre of
the block.
l distance between the centre of the pin-hole of the load-block and its edge, measured towards
2
the tip of the insert (starter film) or the tip of the Mode I or Mode II precrack (Figure 5),
i.e. equal to l /2 if the hole is through the centre of the block.
3
l length of the load-block (Figure 5)
3
L free length of the specimen between load-line and clamp
3
m slope of a plot of C versus a
MAX maximum load on the load-displacement curve (Figure 7)
NL onset of non-linearity on the load-displacement curve (Figure 7)
P load measured by the load-cell of the testing machine
PROP increments of the delamination length during stable delamination growth (propagation) that
are marked on the load-displacement curve (Figure 7)
2
r correlation coefficient of linear fit
VIS onset of visually recognisable delamination growth on the edge of the specimen that is
marked on the load-displacement curve (Figure 7)
5% point of intersection of a straight line with the load-displacement curve, with the slope of the
straight line corresponding to C
5%
4 Principle
This procedure specifies a method for the determination of the delamination resistance of
unidirectional fibre-reinforced polymer laminates under Mode II shear load using the Calibrated End
Loaded Split (C-ELS) test. The resistance to the initiation and propagation of a delamination is
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ISO/DIS 15114
determined from a non-adhesive insert and from a Mode I (opening) or a Mode II (shear) precrack.
The critical energy release rate for Mode II loading can be calculated and a resistance-curve (R-curve,
i.e. a plot of the critical energy release rate versus delamination length) determined.
5 Apparatus
A tensile testing machine in compliance with ISO 5893, capable of producing a constant load-rate
between 1 and 5 mm/min in displacement control should be used. The load-cell should be calibrated
and accurate within ± 1 % for the chosen load-range (loads are typically expected to be in the range
of 100 – 1000 N). The testing machine shall be equipped with a fixture to introduce the load to the pin
inserted into the load-block that allows rotation of the specimen end.
The recommended loading jig requires a clamping arrangement to freely slide in bearings in the
horizontal direction (side-ways) with a fixed load point. This is shown schematically in Figure 1. Two
test fixtures used in the round-robin programmes(see clause 9) are shown in Figure 2.
2h
a
a a
o p
L
Load, P
Figure 1 — ELS test specimen showing the clamping fixture and loading
Figure 2 — Two alternative ELS test fixtures
A calibrated lever arm (torque wrench) is required to apply a consistent pressure whilst fixing the
specimens into the sliding fixture. During the test, the load shall be applied vertically on the load-block
by pulling upward provided the clamp is symmetrical with respect to the specimen. The testing
machine shall be equipped with means for recording the complete load-displacement curves (loading
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ISO/DIS 15114
and unloading) that allow a determination of the loads and the corresponding displacements with an
accuracy of ± 1 %. Vernier callipers or a micrometer should be used to measure the specimen
thickness (2h) to an accuracy of ± 0.02 mm and the specimen width, b, to an accuracy of ± 0.02 mm.
A travelling microscope (or video camera) shall be used to monitor the length of the delamination
along one edge of the specimen with a magnification of between 10 and 25.
6 Specimens
6.1 Preparation of specimens
The recommended specimen width b and length l are 20 mm and 190 mm, respectively. The
specimen length shall not be less than the length of the insert or of the starter delamination plus
110 mm. The free length L is typically 100 mm. The recommended specimen thickness (2h) is 3 mm
for 60 % by volume carbon fibre-reinforced and 5 mm for 60 % by volume glass fibre-reinforced
composites.
NOTE Other specimen dimensions may be used, but the specimen width should be between 15 and 30 mm.
Increasing the length of the specimen is not critical, shortening will reduce the maximum delamination length that
can be investigated and thus yield too few data points for the analysis (see clause 8.4). If specimens are too thin
or not sufficiently stiff, delamination growth may not be induced or occur at large displacements only, or
permanent deformation of the specimen may occur, invalidating the assumptions of Linear Elastic Fracture
Mechanics.
6.2 The initial defect
A crack starter film should be placed at the laminate mid-thickness during the lay-up of the composite
panel prior to moulding. The film should be PTFE or other an fluoro-polymer with excellent non-stick
properties. The film should be thin (between 10 and 13 microns) to minimise the disturbance of the
laminate. The upper service temperature of the film should be greater than the cure temperature of
the laminate. When the composite panel is trimmed, the starter film length should be at least 50 mm
from the load-line so that the influence of the load-block can be neglected. This initial defect will be
extended in mode I or mode II loading prior to testing (see clause 7.2).
6.3 Attaching the load-block to the specimen
One load-block should be bonded to each specimen for the purposes of load-introduction (Figure 3b).
The block should be of the same width as the specimen. Prior to bonding, the load-block and the
specimen (in the position where the block will adhere) should firstly be lightly abraded using an
abrasive paper or grit blasting. Both the load-block and the specimen should then be cleaned with a
solvent.
NOTE A tough, room-temperature cure adhesive (e.g. two part epoxy) is recommended. If bond failure
occurs it may be necessary to consult ISO 4588 for a more sophisticated surface treatment procedure. Bonding
of the load-block should be done immediately after the surface preparation
The load-block should be well aligned with the specimen and held in position with a clamp while the
adhesive sets. Specimen edges should be smoothed prior to determining the dimensions. For the
clamp calibration procedure (as described in clause 7.1), one specimen should be prepared with the
load-block bonded to the end not containing the insert film as shown in Figure 3a. After the clamp
calibration, this load-block may be removed and one may be bonded at the insert end (Figure 3b) to
allow the specimen to be tested in mode II.
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ISO/DIS 15114
(a) (1 specimen per sample)
(b) (5 specimens per sample)
Figure 3 — Position of the load-blocks for
(a) Inverse ELS specimen for clamp calibration and (b) for the ELS fracture specimen
6.4 Moisture conditioning
Moisture conditioning is required for obtaining baseline data in order to test specimens with a uniform
moisture content. The drying conditions (temperature and duration) shall be chosen according to the
recommendations of the resin supplier. Conditioning should be performed after bonding of the load-
block. Before testing, the specimens may be stored in a dessicator for at most three days after
conditioning.
NOTE other conditioning procedures may be applied for the investigation of specific conditioning effects.
6.5 Final specimen preparation and measuring dimensions
In preparation for the visual measurement of crack length, applying a thin layer of typewriter
correction fluid ("white ink") on the edges of the specimen after conditioning will facilitate the
measurement. The following procedure is recommended:
a) Apply a thin coat of white type-writer correction fluid to the edge of one side of each specimen.
(Ensure the use of a new bottle and try to apply the coating in a single brush stroke, avoiding re-
brushing if possible. Practicing on a spare specimen, or the reverse side of the test specimen
may help).
b) When the layer is dry, locate the position of the end of the insert film, and mark this with a black
pen. (A nib of 0.1 mm is recommended).
c) Mark the specimen edge at regular increments, starting one division before the end of the film
insert and extending to a = 100 mm. (Drawing straight vertical lines across the beam edge at the
crack length increments is helpful). The mark increment should be chosen to be either 2.0mm or
2.5mm, depending upon the specimen length available for crack propagation. For shorter
lengths, the narrower increment should be selected.
d) With the specimen to be used for the clamp calibration, draw lines at 50, 60, 70, 80, 90, 100 and
110mm from the load-block hole (load-line). (These will be the positions at which the specimen
will be clamped in the clamp calibration test, clause 7.1)
NOTE 1 It should be noted that some typewriter correction fluids contain solvents that may be harmful to the
laminate matrix material. A water based paint is thus recommended; 2 The vertical lines will shear when the
crack front passes and may kink due to the shear strain ahead of the crack tip. The crack position is determined
from the lines shearing.
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ISO/DIS 15114
Figure 4 — A specimen during loading in an ELS test, showing the crack length markers at
2.5 mm increments along the specimen edge. L = 100 mm for this test
Measure and record the length, l, of each specimen to the nearest mm. Measure the width, b, to the
nearest 0.02 mm at three evenly spaced points along the length. Measure the thickness, 2h, to the
nearest 0.02 mm at three points along the centre-line of the specimen. The variation in thickness and
average values of the width and the thickness should be recorded for each specimen. The variation in
thickness, i.e. the maximum difference between the thickness measurements, should not exceed
0.1 mm for each specimen. The length, l and height, H of the end block should be measured to the
3
nearest 0.1 mm and be recorded. The load-block details are shown in Figure 5.
a
l
1
2h
H
l
2
l
3
Figure 5 — Load-block dimensions
6.6 Number of specimens
A minimum number of five specimens shall be tested. A minimum of one calibration specimen per
sample of nominally identical specimens should be used.
7 Procedure
7.1 Performing the calibration of the ELS fixture
The following procedure should be followed.
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ISO/DIS 15114
a) Position the specimen in the clamping fixture as shown in Figure 6, loosely tighten the clamp,
attach the load-block to the test machine, e.g. via the loading pin, then clamp the specimen with a
free length, L = 110 mm and record the clamping pressure applied.
b) Apply load to the load-block at a cross-head displacement rate of 1 mm/min such that the beam
deforms elastically and record the load-displacement trace.
NOTE 1. Loading to a maximum load of 250 N is recommended for CFRP specimens and 150 N for GFRP
specimens. 2. It is important that the insert film is held fully within the clamp so that it does not influence the
measured beam compliance. Adjust the length, L, if necessary to ensure this.
c) Stop the loading and then fully unload the specimen and repeat the procedure with the beam
clamped at free lengths of 100, 90, 80, 70, 60 and 50 mm. The Unloading may be performed at
up to 10 mm/min.
NOTE It is important that the clamping of the specimen in the ELS fixture can be performed in a
reproducible manner. This is conveniently achieved using a torque wrench, such that the retaining nuts can be
tightened to the same level of torque in each test. (A torque of 8Nm has been found to be suitable for the
clamping of carbon-fibre epoxy specimens).
L
Load, P
Figure 6 — The clamp calibration set up with the delamination fully within the clamp
When the clamp calibration has been performed, the specimen used can then have the load-block
carefully removed and another can be bonded to the end containing the crack so that a fracture test
may be performed on this specimen. (Figure 3b).
7.2 Pre-cracking the specimens
Experience has shown that initiation values of G will depend on the precracking method used.
IIC
Specimens should always be precracked and either mode I or mode II precracking may be used. The
procedure used should be clearly stated in the report.
7.2.1 Mode I precracking
The specimens should be loaded in mode I (tensile opening) until the crack has grown at least 2 mm
(but no greater than 5 mm) ahead of the insert. The procedures of ISO 15024 should be consulted for
mode I loading using the double cantilever beam (DCB) test specimen.
NOTE: If mode I rather than mode II precracking is chosen (see clause 7.2), two load blocks shall be bonded to
each specimen rather than one. After precracking, one of the two load blocks shall be removed before mode II
testing.
7.2.2 Mode II precracking
The procedures for precracking in mode II are somewhat more complex, as it is intended that
unstable crack initiation from the insert be avoided. Thus, the following procedure is recommended. A
load-scale of up to 500 N is usually sufficient for composite laminates.
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ISO/DIS 15114
a) Determine the initial length of the crack, a .
o
b) Determine an initial clamping length, L, via L = a (4/3) and clamp the specimen at this value
o
of L.
c) Load the specimen at a displacement rate of 1mm/min until the delamination has grown at least
2 mm (but no more than 5 mm) ahead of the insert. The load-displacement trace should be
recorded during this procedure.
d) Immediately stop loading and fully unload the specimen at 5 mm/min.
e) Record the length of the newly formed precrack, a
p
f) Release the clamp, remove the specimen and repeat for the remaining specimens in the sample.
NOTE The initial clamp length is chosen such that the initial ratio of (a/L) = 0.75. This is chosen to promote
stability. An energy analysis predicts that the ELS test is stable provided approximately that (a/L) > 0.55. Thus
the factor of 0.75 provides a margin for safety, promoting the conditions for stability.
7.3 Testing the samples in mode II from the precrack formed in 7.2
A travelling microscope and specimen illumination should be used to observe delamination growth. A
magnification of between 10 to 25 should be employed. The following procedure should be used
for each specimen in the sample.
a) Determine the clamping length, L, such that (a /L) > 0.55 and clamp the specimen.
p
(NOTE stability may be improved by using larger values of a /L but using larger values will
p
reduce the length available for crack propagation)
b) Load the specimen at a displacement rate of 0.5 mm/min until the crack has grown to within
10 mm from the clamp, recording the observed crack lengths on the load-displacement trace.
Crack lengths should be recorded at 2.0 mm (or 2.5mm) intervals. The first
...
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