ISO 15024:2023

Fibre-reinforced plastic composites - Determination of mode I interlaminar fracture toughness, GIC, for unidirectionally reinforced materials

This document specifies a method for the determination of mode I interlaminar fracture toughness (critical energy release rate), GIC, of unidirectional fibre-reinforced plastic composites using a double cantilever beam (DCB) specimen.

Composites plastiques renforcés de fibres — Détermination de la ténacité à la rupture interlaminaire en mode I, GIC, de matériaux composites à matrice polymère renforcés de fibres unidirectionnelles

1.1 La présente Norme internationale spécifie une méthode de détermination de la ténacité à la rupture interlaminaire en mode I (taux de restitution d'énergie critique), GIC, des matériaux composites à matrice polymère renforcés de fibres unidirectionnelles, en utilisant une éprouvette double poutre encastrée (DCB). 1.2 Elle s'applique aux thermoplastiques et thermodurcissables renforcés de fibres de carbone et de fibres de verre.

General Information

• Status: Published
• Publication Date: 26-Feb-2023
• ICS: 83.120 - Reinforced plastics
• Technical Committee: ISO/TC 61/SC 13 - Composites and reinforcement fibres
• Drafting Committee: ISO/TC 61/SC 13 - Composites and reinforcement fibres
• Current Stage: 6060 - International Standard published
• Start Date: 27-Feb-2023
• Due Date: 05-Oct-2023
• Completion Date: 27-Feb-2023

Relations

• Revises: ISO 15024:2001 - Fibre-reinforced plastic composites - Determination of mode I interlaminar fracture toughness, GIC, for unidirectionally reinforced materials
• Effective Date: 06-Jun-2022

Overview

ISO 15024:2023 specifies a laboratory test method to determine mode I interlaminar fracture toughness (critical energy release rate, G_IC) of unidirectional fibre‑reinforced plastic composites using a double cantilever beam (DCB) specimen. The second edition updates the DCB configurations (including an insert‑hinge option) and provides procedures to measure both initiation and propagation fracture toughness, generate R‑curves, and report statistically valid results. The method is limited to zero‑degree unidirectional lay‑ups.

Keywords: ISO 15024:2023, G_IC, mode I interlaminar fracture toughness, DCB test, fibre‑reinforced plastic composites, unidirectional.

Key topics and technical requirements

• Test principle: Apply a mode I opening load to a DCB specimen with a non‑adhesive starter film to simulate an initial delamination and record load–displacement behaviour to obtain G_IC (initiation and propagation).
• Specimen and preparation: Guidance on test plate production, preferred and alternative specimen geometries, starter film insertion and precracking (precrack by unloading after initial delamination growth).
• Apparatus: Requirements for test machine, load blocks or piano hinges (and insert hinges), measuring devices, optional travelling microscope, and non‑adhesive insert film.
• Data acquisition and reduction: Procedures for measuring delamination length, defining key points on the load–displacement trace (VIS, NL, MAX, 5%/MAX, PROP), and calculating G_IC using specified methods:
  • Method A: Corrected beam theory (CBT)
  • Method B: Modified compliance calibration (MCC)
• R‑curve generation: Plotting G_IC versus delamination length to show initiation and propagation behaviour; discussion of fibre bridging and its effect on R‑curve shape.
• Quality and reporting: Number of specimens, conditioning (ISO 291 referenced), precision, and required test report content. Normative references include ISO 291, ISO 527‑1, ISO 1268, ISO 7500‑1 and ISO 9513.

Applications and practical value

• Provides a standardized, repeatable method to quantify delamination resistance in unidirectional polymer‑matrix composites.
• Useful for:
  • Material development and selection (resins, fibres, interleaves)
  • Comparative evaluation of composite layups and manufacturing processes
  • Failure analysis and durability studies (delamination susceptibility)
  • Qualification and quality control in aerospace, automotive, wind energy, marine and sports equipment industries
• Helps engineers and test laboratories produce defensible fracture toughness data (G_IC) for design allowables and simulation inputs.

Related standards

• ISO 291 - Conditioning and testing atmospheres
• ISO 527‑1 - Tensile properties of plastics (general principles)
• ISO 1268 - Production of composite test plates
• ISO 7500‑1 - Calibration of force measuring systems
• ISO 9513 - Calibration of extensometer systems

For practitioners performing DCB tests, ISO 15024:2023 is the authoritative reference for apparatus, specimen preparation, test execution, data reduction (CBT and MCC), and reporting of mode I interlaminar fracture toughness for unidirectional fibre‑reinforced plastics.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 15024
Second edition
2023-02
Fibre-reinforced plastic composites —
Determination of mode I interlaminar
fracture toughness, G , for
IC
unidirectionally reinforced materials
Composites plastiques renforcés de fibres — Détermination de la
ténacité à la rupture interlaminaire en mode I, G , de matériaux
IC
composites à matrice polymère renforcés de fibres unidirectionnelles
Reference number
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
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Phone: [phone]
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Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Apparatus . 6
5.1 Test machine . 6
5.1.1 General . 6
5.1.2 Speed of testing . 7
5.1.3 Fixture . 7
5.1.4 Load and displacement measurements . 7
5.1.5 Recorder . 7
5.2 Load blocks or piano hinges . 7
5.3 Measuring apparatus . 7
5.4 Travelling microscope (optional) . 7
5.5 Non-adhesive insert film . 7
5.6 Ancillary equipment . 8
6 Test specimen . 8
6.1 Test plate preparation . 8
6.2 Specimen preparation . 8
6.2.1 Preferred specimens . 8
6.2.2 Alternative specimens . 9
6.3 Checking and measurement of the test specimens . 9
6.4 Attachment of loading points . 9
6.5 Measurement of delamination length . 9
7 Number of specimens . .9
8 Conditioning .10
9 Test procedure .10
9.1 Test set-up . 10
9.2 Initial loading . 10
9.3 Re-loading . 11
10 Calculation of G .11
IC
10.1 Interpretation of test results . 11
10.2 Data reduction .12
10.2.1 General .12
10.2.2 Method A: Corrected beam theory (CBT) .12
10.2.3 Method B: Modified compliance calibration (MCC) . 14
10.3 Data sheets, data plots and statistical calculation . 14
11 Precision .18
12 Test report .19
Annex A (normative) Preparation and bonding of the load blocks or piano hinges.21
Annex B (informative) Recommendations for testing .22
Annex C (informative) Recommended test result sheet .25
Annex D (informative) DCB test with flat insert hinge .28
Bibliography .32
iii
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
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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
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 13,
Composites and reinforcement fibres.
This second edition cancels and replaces the first edition (ISO 15024:2001), which has been technically
revised.
The main changes are as follows:
— a new double cantilever beam (DCB) has been added [Figure 1 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
INTERNATIONAL STANDARD ISO 15024:2023(E)
Fibre-reinforced plastic composites — Determination
of mode I interlaminar fracture toughness, G , for
IC
unidirectionally reinforced materials
1 Scope
This document specifies a method for the determination of mode I interlaminar fracture toughness
(critical energy release rate), G , of unidirectional fibre-reinforced plastic composites using a double
IC
cantilever beam (DCB) specimen.
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 291, Plastics — Standard atmospheres for conditioning and testing
ISO 527-1, Plastics — Determination of tensile properties — Part 1: General principles
ISO 1268 (all parts), Fibre-reinforced plastics — Methods of producing test plates
ISO 7500-1, Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1:
Tension/compression testing machines — Calibration and verification of the force-measuring system
ISO 9513, Metallic materials — Calibration of extensometer systems used in uniaxial testing
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
mode I interlaminar fracture toughness
critical energy release rate
G
IC
resistance to the initiation and propagation of a delamination crack in unidirectional fibre-reinforced
polymer matrix composite laminates under mode I opening load
Note 1 to entry: It is measured in joules per square metre.
3.2
mode I crack opening
crack-opening mode due to a load applied perpendicular to the plane of delamination using the double
cantilever beam specimen
Note 1 to entry: The double cantilever beam specimen shown in Figure 1 is shown in Figure 1.
3.3
NL point
point of deviation from linearity on the load versus displacement trace
Note 1 to entry: As shown in Figure 2.
3.4
VIS point
point of the onset of delamination, as determined by visual observation, at the edge of the specimen,
marked on the load-displacement trace
Note 1 to entry: As shown in Figure 2.
3.5
5 % / MAX point
point which occurs first on loading the specimen between:
a) the point of 5 % increase in compliance (C ) from its initial value (C ); and
5 % 0
b) the maximum load point.
Note 1 to entry: See Figure 2.
3.6
PROP points
points of discrete delamination length increments beyond the tip of the insert or starter crack tip
marked on the load-displacement trace, points where the crack has been arrested being excluded
Note 1 to entry: See Figure 2.
3.7
delamination-resistance curve
R curve
cross-plot of G for initiation and subsequent propagation values for mode I crack opening (3.2) as a
IC
function of delamination length
Note 1 to entry: See Clause 10.
4 Principle
A mode I double cantilever beam (DCB) specimen, as shown in Figure 1, is used to determine G , the
IC
critical energy release rate, or interlaminar fracture toughness, of fibre-reinforced plastic composites.
Figure 1 represents three different loading arrangement for the specimen as following, a) Specimen
loading using load blocks, b) Specimen loading using piano hinges, c) Specimen loading using insert
hinges (see Annex D). The test method is limited to zero-degree unidirectional lay-ups only (see B.1).
Data reduction yields initiation and subsequent propagation values of G for mode I opening fracture
IC
toughness. A delamination-resistance curve, or R curve, is generated by plotting G on the ordinate as a
IC
function of delamination length plotted on the abscissa.
The aim of the test method is to determine initiation values for the composite material tested. Fibre
bridging is observed in a DCB test and it might not be representative of the composite material tested.
Fibre bridging is considered to be the main cause for the observed shape of the R curve, which typically
rises before reaching a roughly constant value of G for long delamination lengths. A crack-opening
IC
load is applied to the DCB specimen, perpendicular to the plane of delamination, through load blocks
or piano hinges under displacement control at a constant rate. The DCB specimen contains a thin, non-
adhesive starter film embedded at the midplane as shown in Figure 3, which is used to simulate an
initial delamination. The specimen is precracked by unloading the DCB specimen immediately after
the first increment of delamination growth from the insert, followed by re-loading. The onset of stable
delamination growth is monitored, and the delamination initiation and propagation readings are
recorded. The R curve is plotted with the initiation values from both the insert and the mode I precrack,
and with the propagation from the precrack. Under certain prescribed circumstances (see 9.2.7), an
alternative wedge precracking procedure can be used but is not recommended.
a) Specimen loading using load blocks
b) Specimen loading using piano hinges
c) Specimen loading using insert hinges
Key
b specimen width
2h specimen thickness
a initial delamination length
a total delamination length
A insert length
l specimen length
l distance from the loading pin (or piano hinge axis) to the midplane of the half-beam to which the load block (or
piano hinge) is attached
l distance from centre of loading pin (or piano hinge axis) to edge of load block (or piano hinge)
l block length
H block thickness
1 centres of hinge axis
NOTE 1 Alternative loading arrangements are (a) load blocks and (b) piano hinges.
NOTE 2 The fibre orientation is parallel to the length l.
NOTE 3 Details of DCB test with insert hinges are described in Annex D.
Figure 1 — Geometry for the double cantilever beam (DCB) specimen with a starter
delamination
Key
X displacement, in millimetres
Y load, in newtons
1 crack initiation followed by unloading
2 crack propagation by re-initiation from the resulting mode I precrack
3 crack propagation markers
4 NL point
5 VIS point
6 MAX point
7 C
8 C
5 %
NOTE Figure shows case where 5 % values follow maximum load, and reload curve has been offset 5 mm for
clarity
Figure 2 — Load-displacement curve for a DCB test
Dimensions in millimetres
Key
1 film insert
2 fibre direction
d margin to allow for initial trimming
Figure 3 — Example of test plate preparation showing the laminate structure, the dimensions
and the position of the film insert
5 Apparatus
5.1 Test machine
5.1.1 General
The testing machine shall be in accordance with the following requirements.
5.1.2 Speed of testing
The testing machine shall be capable of maintaining the constant displacement rate required in 9.2.1
and 9.3.1 within a tolerance of ±20 %, as specified in ISO 527-1.
5.1.3 Fixture
The test machine shall be equipped with a fixture to introduce the load to the pins inserted into the
load blocks or with grips to hold the piano hinges. In each case, rotation of the specimen end shall be
allowed. The axis of the load-introduction fixtures shall be aligned with the loading axis of the test
machine.
5.1.4 Load and displacement measurements
The force measurement system shall be in accordance with class 1 as defined in ISO 7500-1. The
displacement measurement shall be in accordance with class 2 of ISO 9513 within the relevant range
used for results determination. Apply machine compliance compensation if the crosshead monitor is
used, to ensure that the required accuracy level is as well achieved under loading conditions.
5.1.5 Recorder
The test machine shall allow the displacement and corresponding load to be measured and recorded,
preferably on a continuous basis.
5.2 Load blocks or piano hinges
Load blocks or piano hinges, as shown in Figure 1, may be used for introducing the load into the
specimen. They shall be at least as wide as the specimen. For the load blocks in Figure 1 a), the maximum
value of l shall be 15 mm. The hole to inset the loading pin shall be at the centre of l .
3 3
5.3 Measuring apparatus
5.3.1 Micrometer, or equivalent, capable of reading to 0,02 mm or less, suitable for measuring the
thickness of the specimen. The micrometer shall have contact faces appropriate to the surface being
measured (i.e. flat faces for flat, polished surfaces and hemispherical faces for irregular surfaces).
5.3.2 Vernier calipers, or equivalent, capable of reading to 0,05 mm or less, for measuring the width
of the specimen.
5.3.3 Linear scale (ruler), with 1 mm divisions, for measuring the specimen length and marking the
edges of the specimen to monitor the delamination crack growth.
5.4 Travelling microscope (optional)
A travelling microscope may be used to measure the delamination length. If used, it shall have a travel
range of 0 mm to 200 mm, have a magnification no greater than × 70 and be readable to 0,05 mm.
5.5 Non-adhesive insert film
A polymer film of thickness not exceeding 13 µm shall be used as a non-adhesive insert. For epoxy
resin matrix composites cured at temperatures below 180 °C, a film of polytetrafluoroethylene (PTFE)
is recommended. For composites cured at temperatures above 180 °C (for example those including
polyimide or bismaleimide thermoplastics), a film of polyimide is recommended (see B.2).
5.6 Ancillary equipment
5.6.1 Desiccator, for storing the test specimens after conditioning, including a suitable desiccant
such as silica gel or anhydrous calcium chloride.
5.6.2 Mould release agent. When a polyimide film is used as the non-adhesive insert film, a mould
release agent of the polytetrafluoroethylene (PTFE) type is recommended (see B.2).
5.6.3 Adhesive. A cyanoacrylate adhesive or epoxy adhesive of the two-component room-
temperature-cure type to bond the load blocks or piano hinges to the test specimen (see Annex A).
5.6.4 Solvent. Organic solvent such as acetone or ethanol (see Annex A).
5.6.5 Sandpaper (abrasive paper), with 500 grade grit or finer (see Annex A).
5.6.6 White ink. Water-soluble typewriter correction fluid.
6 Test specimen
6.1 Test plate preparation
A test plate shall first be prepared in accordance with the part of ISO 1268 appropriate to the production
process used. The recommended plate thickness is 3 mm for 60 % by volume carbon-fibre-reinforced
composites and 5 mm for 60 % by volume glass-fibre-reinforced composites.
An even number of unidirectionally aligned layers shall be used (seeB.1). The non-adhesive film insert
shall be placed at laminate mid-thickness during lay-up. The insert shall not exceed 13 µm, in order
to simulate a sharp crack and cause minimum disturbance of the individual plies of the laminate.
Guidelines for the insert material and its preparation are given in B.2.
If a polyimide film is used, the film shall be painted or sprayed with a mould release agent before
insertion into the laminate. The film shall be cut to the proper size for insertion into the laminate
before applying the mould release agent. Mould release agents containing silicone may contaminate
the laminate by migration through the individual layers. Baking of the film will help to prevent silicone
migration within the composite. The film shall be coated and baked twice for 30 min at 130 °C. Care
shall be exercised in handling the film so that the coated layer of release agent is not damaged or
removed from the film.
Figure 3 shows an example of how the test plate can be configured. The positioning of the insert shall
allow for the initial trimming of the test plate.
6.2 Specimen preparation
6.2.1 Preferred specimens
Machine the test specimens from the trimmed test plate, with their longitudinal axes parallel to the
fibre direction in the test plate. Specimens shall be identified to indicate their original position in the
test plate. The specimen configuration and dimensions are illustrated in Figure 1. The dimensions and
tolerances for the preferred specimens are shown in Table 1. Specimen surfaces shall not be machined
to meet the thickness requirement.
The thickness and width of individual specimens shall not vary by more than ±1 % of the mean value
for that type of specimen.
Table 1 — Recommended specimen dimensions and tolerances
Unit Carbon fibre Glass fibre Tolerance
Width, b mm 20 20 ±0,5
Minimum length, l mm 125 125 —
Thickness, 2h mm 3 5 ±0,1
6.2.2 Alternative specimens
Other specimen thicknesses may be used, depending on the tensile modulus of elasticity and the
anticipated interlaminar fracture toughness of the specimen. Guidelines for choosing a specimen
thickness that will yield negligible displacement corrections based on the anticipated interlaminar
fracture toughness are given in B.3.
Other specimen widths between 15 mm and 30 mm may be used. Increasing the length of the
specimen is not critical. However, shortening is not recommended because it will reduce the maximum
delamination length that can be investigated and thus yield too few data points for the analysis.
6.3 Checking and measurement of the test specimens
After machining the specimens, check that they are free from twist and warpage, and free from
machining damage. Check that the cut edges are suitably smooth to allow preparation for monitoring
the crack length in accordance with B.4 and B.5.
Measure and record the length, l, of each specimen to the nearest millimetre. 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 these three points along the centreline of the specimen, and at two additional points
near the edge at the middle measurement point, to check for tapering of the specimen.
Record the mean thickness and width of each specimen and check that the values are within the range
given in Table 1. Check also that the variations along the specimen are within the range given in Table 1.
Discard specimens not meeting these requirements.
Measure the length of the insert at both side edges of the specimen. Record the average value, but if
the insert length measurements differ by more than 1 mm on the two edges this shall be noted in the
report. The minimum distance of the tip of each insert edge from the near ends of the load blocks or
piano hinges shall be 45 mm.
6.4 Attachment of loading points
Bond the load blocks or piano hinges for load introduction on the surfaces at the end of the specimen
where the insert has been placed, as shown in Figure 1. The load-introduction fixtures shall be well
aligned with the specimen, and with each other, and held in position with clamps while the adhesive
sets. The requirements for bonding the load blocks or piano hinges given in Annex A shall be followed.
6.5 Measurement of delamination length
For the measurement of the delamination lengths, marks shall be drawn at 5 mm intervals along the
edge of the specimen, extending at least 55 mm beyond of the tip of the insert. Additionally, the first
10 mm and last 5 mm shall be marked at 1 mm intervals.
7 Number of specimens
A minimum number of five specimens shall be tested. Specimens found to be invalid (see 9.3.6) shall be
discarded and new specimens tested in their place.
8 Conditioning
The specimens shall be dried using the drying temperature and duration recommended by the resin
supplier. This conditioning shall be performed after bonding of the load blocks or piano hinges. After
conditioning, the specimens may be stored in a desiccator for not more than 24 h before testing.
Conditioning is required to obtain baseline data on test specimens with a uniform moisture content,
because the interlaminar fracture toughness of polymer-matrix composites is sensitive to the amount
of moisture present in the resin. Hence, a dry condition is recommended for this document. Guidelines
for conditioning are given in B.6.
9 Test procedure
9.1 Test set-up
9.1.1 The test shall be performed under standard conditions in accordance with ISO 291 [i.e.
23 °C ± 2 °C, (50 ± 5) % relative humidity].
9.1.2 Mount the specimen in the fixture of the test machine. Support the end of the specimen, if
required, in order to keep the beam perpendicular to the direction of the applied load.
9.1.3 For recommendations on further aspects of test preparation, see B.4.
9.2 Initial loading
9.2.1 Load the specimen at a constant cross-head rate between 1 mm/min and 5 mm/min.
9.2.2 Record the load and the displacement values, continuously if possible. Record the position of the
delamination with an accuracy of at least ±0,5 mm (seeB.5).
9.2.3 During loading, record the point on the load-displacement curve, or record the load-
displacement data values, at which the onset of delamination movement is visually observed on the
edge of the specimen (VIS, see Figure 2).
If the start of delamination growth is difficult to observe, a change of illumination conditions or the use
of a cross-head speed from the lower end of the range is recommended.
9.2.4 Stop the loading after 3 mm to 5 mm of delamination crack growth. If unstable delamination
growth from the insert is observed (see B.7), this shall be noted in the report and loading shall be
continued until the delamination length has increased by 3 mm to 5 mm beyond the arrest point. Note
in the test report if the delamination length is outside the range of 3 mm to 5 mm.
9.2.5 Unload the specimen at a constant cross-head rate of up to 25 mm/min.
9.2.6 After unloading, mark the position of the tip of the precrack on both edges of the specimen.
Note in the test report if the position on the two edges differs by more than 2 mm and if the specimen is
removed from the fixture for this procedure.
NOTE Mismatch of greater than 2 mm between the two positions can be an indication of asymmetrical
loading.
9.2.7 In the atypical case that the R-curve shows a decrease in apparent toughness with delamination
length (as indicated in Figure 8), the initial-loading process may be replaced by wedge precracking.
Use of wedge precracking (see B.8) is not recommended, since it may be difficult to produce a suitable
precrack by wedge opening. Its use shall be reported. In addition, the precrack may not always lie in the
midplane of the specimen. Deviations of the precrack from the midplane will invalidate the test results
and shall also be reported.
9.3 Re-loading
9.3.1 The specimen shall be re-loaded at the same constant cross-head speed of 1 mm/min to 5 mm/
min as the initial loading, without stopping or unloading, until the final delamination length increment
has been reached (see 9.3.3). The load and displacement values shall be recorded, including those for
the unloading cycle. The position of the delamination shall be pinpointed with an accuracy of at least
±0,5 mm on the edge of the specimen.
9.3.2 Record the load and displacement values at which the onset of delamination movement from
the precrack is observed on the edge of the specimen (VIS, Figure 2).
9.3.3 On continuation of the loading, record the load and displacement values at as many delamination
length increments as possible in the first 5 mm, ideally every 1 mm. Subsequently, record the load and
displacement data at every 5 mm, until the delamination crack has propagated at least 45 mm from the
tip of the precrack, and again at every 1 mm increment of crack growth for the last 5 mm of delamination
propagation, up to a total delamination length of 50 mm beyond the tip of the precrack (see Figure 2).
9.3.4 Finally, unload the specimen at a constant cross-head rate of up to 25 mm/min.
9.3.5 Mark the positions of the tip of the delamination crack after unloading on both edges of the
specimen. Note in the report if these positions differ by more than 2 mm.
NOTE Mismatch of greater than 2 mm between the two positions can be an indication of asymmetrical
loading.
9.3.6 Any permanent deformation of the specimen after unloading shall be noted in the report.
Deviations of the delamination from the midplane of the laminate will invalidate the test results and
shall be noted in the report. In such cases, a replacement specimen shall be tested.
10 Calculation of G
IC
10.1 Interpretation of test results
Several initiation fracture toughness G values are determined from the corresponding points on the
IC
load-displacement curve. G values corresponding to the points listed below shall be determined for
IC
testing from the starter film and from the mode I precrack for each specimen. These initiation values
are indicated on the typical R-curve shown in Figure 7. These values are determined as follows.
— The NL point is determined by drawing a straight line through the linear portion of the load versus
displacement trace to obtain the point of deviation from linearity, or onset of non-linearity (NL in
Figure 2). Recommendations for obtaining this point are given in B.9.
— The VIS point is determined from the first visual observation that the delamination has moved
from the tip of the insert, or from the mode I precrack, on the edge of the specimen (VIS in Figure 2).
The corresponding load and displacement data at this point are used for the calculation. A travelling
microscope (5.4) may be used to determine the VIS point.
— The 5 % / MAX point corresponds to the values of the load and displacement which occur first on
loading the specimen. For the 5 % value, a straight line is drawn to determine the initial compliance
C , ignoring any initial deviation due to take-up of play in the loading system. A new line is then drawn
with a compliance equal to 1,05 C . The intersection of the new line with the load-displacement
curve yields the load and displacement to be used for the calculation of G , unless the intersection
IC
is at a larger displacement than the maximum load point. In the latter case, the maximum load and
the corresponding displacement shall be used for the calculation of G .
IC
— PROP points are determined for each delamination length measured during propagation (PROP in
Figures 7 and 8). Data taken at points where the crack has been arrested are excluded. The minimum
number of PROP points shall be 15. If fewer points are used, this shall be noted in the report because
the values determined from the linear fit may be influenced by statistical effects.
10.2 Data reduction
10.2.1 General
Either Method A (see 10.2.2) or Method B (see 10.2.3) shall be used for the data reduction. Both methods
will give equivalent results. The data required for the analysis are:
— a  initial delamination length;
— a   total delamination length (a = a + measured delamination length increments);
— P   load;
— δ   load line displacement;
— C   load line compliance δ /P;
— b   width of specimen;
— 2h  thickness of specimen.
10.2.2 Method A: Corrected beam theory (CBT)
Establish the relation between the delamination length and the compliance by plotting the cube-root
1/3 1/3
of the compliance, C [or (C/N) if load blocks are being used, where N is the load block correction
described below], as a function of delamination length a for the reloading data (see Figure 4). The
extrapolation of a linear fit through the data in the plot yields Δ as the x-intercept.
If the Δ-value from the fit is positive, a value of Δ = 0 shall be used and this shall be noted in the report.
The VIS and the PROP points are used for the linear fit, but not for the NL or 5 % / MAX points. If the
VIS point has not been determined or clearly lies outside the range defined by the PROP points when
plotted as in Figure 4, it shall be excluded from the linear fit and this shall be noted in the report.
The critical energy release rate G is given by Formula (1):
IC
3Pδ 3Pδ F
G = ×F or G = × (1)
IC IC
2ba+ Δ 2ba+ Δ N
() ()
where
F is the large-displacement correction (described in this subclause);
N is the load block correction.
The G values corresponding to all initiation and propagation points shall be calculated. The
IC
delamination length for the initiation value from the insert shall be taken as the distance between the
load line and the tip of the insert (a in Figure 1) whereas the delamination length for the initiation
value from the precrack shall be taken as the distance between the load line and the tip of the precrack
(a in Figure 1).
The large-displacement correction F shall be applied for all specimens (see Reference [14]). This
correction factor will contribute significantly if δ /a > 0,4. The large-displacement correction F and the
load block correction N are calculated as follows (for piano hinges, N = 1):
δl
3 δ 3 
 
F =−1 − (2)
 
 
10 a 2
 
 
a
 
ll9 δl 9 δ
   
 
22 1
N =−1 −−1 − (3)
 
     
aa8 35 a 
   
  a
 
where
l is the distance from the loading pin (or piano hinge axis) to the midplane of the half-beam to
which the load block (or piano hinge) is attached;
l is the distance from the loading-pin centre to its edge (see Figure 1);
If large-displacement corrections which are less than 0,9 are found, this shall be noted in the report.

Key
X delamination length, a
1/3
Y cube-root of the normalized compliance, (CIN)
1 VIS
1/3
Δ x-axis intercept Δ of the linear fit of (C/N) , versus a
NOTE The VIS point can be excluded from the linear fit (see 10.2.2).
Figure 4 — Linear fit used to determine the correction Δ in the corrected beam theory method
10.2.3 Method B: Modified compliance calibration (MCC)
Establish the relation between the delamination length and the compliance by plotting the width-
1/3 1/3
normalized cube root of the compliance (bC) [or (bC/N) if load blocks are used], as a function of
the thickness-normalized delamination length a/2h for the reloading data (see Figure 5). The slope of
this relation is defined as m.
The critical energy release rate G is given by Formula (4):
IC
2 23
3 m P bC
   
G =× × ×F (4)
   
IC
22()h B   N 
where factors F and N are given by Formulae (2) and (3), respectively. The G values corresponding to
IC
all initiation and propagation values shall be calculated.
For the case where the delamination length is measured in the horizontal direction (x in Figure 6)
using a travelling microscope (as described in B.5), the delamination length x may be used for plotting
Figure 4 or 5 and to calculate G . In this case, the large-displacement correction factor F is equal to
IC
1. However, if end blocks are used instead of piano hinges, correction factors N in accordance with
Formula (3) are required for G .
IC
10.3 Data sheets, data plots and statistical calculation
All results corresponding to NL, VIS and 5 % / MAX points from the starter film, the mode I precrack,
and PROP values are used to draw a delamination-resistance curve (R-curve), consisting of a plot of
G versus delamination length a for each specimen (see Figures 7 and 8). When quoting characteristic
IC
material values from testing, the five replicates required (see Clause 7), the arithmetic mean, the
standard deviation σ, and the coefficient of variation CV of G (CV = σ/mean in %) corresponding to
IC
each VIS, NL and 5 % / MAX point shall be calculated.
A single test result sheet shall be used to report the test data obtained using the insert (values
corresponding to NL, VIS and 5 % / MAX points) and from the mode I precrack (values corresponding to
NL, VIS, 5 % / MAX and PROP points) for each specimen. Recommended test result sheets are included
in Annex C.
Key
X thickness-normalized delamination length, a/2h
1/3
Y (bCIN)
1 VIS
NOTE The VIS point can be excluded from the linear fit (see 10.2.3).
Figure 5 — Linear fit used to determine the slope m in the modified compliance calibration
(MCC) method
Key
P load
δ load line displacement
a delamination length
x delamination length in the horizontal direction
Figure 6 — DCB specimen under load showing the delamination length measured along the
horizontal direction, x, and along the curved coordinate scale fixed to the specimen, a
Key
X delamination length (mm)
Y G (J/m )
IC
1 NL
2 VIS
3 5 %
4 PROP
NOTE Initiation G values labelled NL, VIS and 5 %, and propagation values, PROP, are defined in Clause 3.
IC
Initial-loading values only are shown.
Figure 7 — Typical delamination-resistance curve (R-curve) showing decreasing delamination
resistance with delamination length
Key
X delamination length (mm)
Y G (J/m )
IC
1 NL
2 VIS
3 5 %
4 PROP
NOTE Initiation G values labelled NL, VIS and 5 %, and propagation values, PROP, are defined in Clause 3.
IC
Initial loading values only are shown.
Figure 8 — Atypical delamination-resistance curve (R-curve) showing decreasing delamination
resistance with delamination length
11 Precision
Table 2 shows the materials used and the precision data obtained in two interlaboratory evaluations of
this test method. Initiation values quoted are NL values, which for the carbon-fibre-reinforced epoxy
(CFRE) material, were identical to VIS values. For the carbon-fibre-reinforced thermoplastic (CFRT)
material, the NL point preceded the VIS point. The test procedure was a modification of that in this
standard as these tests were conducted continuously with no unloading step following the initial
precrack. In all cases, the typical R-curve was obtained (see Figure 7).
a
Table 2 — Precision data abstracted from ASTM D 5528-94a
Average G (CV) (CV)
Tests per IC r R
Material No. of labs Insert s s
r R
lab 2
kJ/m % %
13 µm poly-
CFRE 3 3 0,085 0,015 17,6 0,014 16,5
imide
7,5 µm poly-
CFRT 9 5 1,182 0,126 10,8 0,111 9,4
imide
13 µm poly-
CFRT 9 5 1,262 0,132 10,5 0,110 8,7
imide
a [10]
These results are selected from data first published in ASTM D 5528-94a, to which reference should be made
for further information. It is noted in this ASTM standard that the data are limited to selected carbon-fibre-reinforced
materials, and variations may be greater for other materials. Further information is also given in Reference [5]. The
precision measures of “r
...