ISO 22838:2020

INTERNATIONAL ISO
STANDARD 22838
First edition
2020-12
Composites and reinforcements
fibres — Determination of the fracture
energy of bonded plates of carbon
fibre reinforced plastics (CFRPs) and
metal using double cantilever beam
specimens
Reference number
ISO 22838:2020(E)
©
ISO 2020

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ISO 22838:2020(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
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ISO 22838:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2  Normative references . 1
3  Terms, definitions and symbols . 1
3.1 Terms and definitions . 1
3.2 Symbols . 2
4  Principle . 3
5  Apparatus . 3
6 Specimens . 4
6.1 Number of specimens . 4
6.2 Conditioning . 4
6.3 Manufacture of adhesive joint specimens . 4
6.3.1 General. 4
6.3.2 DCB specimen measurements . 5
6.4 Preparation of specimens . 5
7  Procedure. 6
7.1 Test set-up and data recording . 6
7.2 Initial loading (precracking stage) . 6
7.3 Re-loading: Testing from the precrack . 6
7.4 Determination of the thickness ratio of the CFRP and metal beams . 9
7.4.1 Theoretical prediction of thickness ratios . 9
7.4.2 Procedure to detect the occurrence of plastic deformation during a DCB
adhesive joint test . 9
7.5 Measurement of machine compliance .10
7.6 Measurement of curvature induced by coefficient of thermal expansion difference
between metal and composite beams .10
8  Data analysis .12
8.1 Determination of the raw data from the load-displacement trace .12
8.1.1 General.12
8.1.2 Initiation values .12
8.1.3 Propagation values . .13
8.2 Determination of adhesive fracture energy .13
8.2.1 General.13
8.2.2 DCB test with identical thickness: Corrected beam theory (CBT) .13
8.2.3 DCB test with dissimilar thicknesses: modified beam theory for DCB
specimens with dissimilar thicknesses .14
8.2.4 DCB test with identical and dissimilar thicknesses: Area method .15
9  Precision .15
10  Test report .16
Annex A (informative) Work flow chart as brief guideline .18
Annex B (informative) Formula for calculating G value using (EI) or E I E I .19
eq 1 1 2 2
Annex C (normative) Measurement of test system compliance .20
Bibliography .22
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ISO 22838:2020(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 of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 13,
Composites and reinforcement fibres.
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.
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ISO 22838:2020(E)

Introduction
The potential benefits to the users of CFRP/metal assemblies of implementing the adhesive fracture
energy of DCB specimen based on this document are:
a) expanding CFRP applications to the fields where it could be used in combination with metallic
components;
b) the detection or the prevention of physical properties loss — such as ion migration and time-related
degradation in sealant film, injected calking layer and glass fibre reinforced plastics (GFRPs) layer;
c) demonstrating the conformity to specified conditions for type certification requirements in the
engineering such as aircraft developments;
d) evaluating the procedures for maintenance, repair and overhaul (MRO) in the engineering
operations such as CFRP in aerospace, or in constructions such as steel bridges and industrial
applications (e.g. pipework repair, etc.)
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INTERNATIONAL STANDARD  ISO 22838:2020(E)
Composites and reinforcements fibres — Determination
of the fracture energy of bonded plates of carbon fibre
reinforced plastics (CFRPs) and metal using double
cantilever beam specimens
SAFETY STATEMENT — Persons using this document should be familiar with normal laboratory
practice, if applicable. This document does not purport to address all of the safety problems, if
any, associated with its use. It is the responsibility of the user to establish appropriate safety
and health practices. It is recognized that some of the materials permitted in this document
might have a negative environmental impact. As technological advances lead to more acceptable
alternatives for such materials, they will be eliminated to the greatest extent possible. At the
end of the test, care should be taken to dispose of all waste in an appropriate manner.
1 Scope
This document specifies the test method for the determination of adhesive fracture energy of adhesively
bonded plates of carbon fibre reinforced plastic (CFRP) and metal using a double cantilever beam (DCB)
specimen. The test method is also applicable to bonded joints between metals and other composite
materials, such as glass fibre reinforced plastics.
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 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 10365, Adhesives — Designation of main failure patterns
ISO 25217, Adhesives — Determination of the mode 1 adhesive fracture energy of structural adhesive joints
using double cantilever beam and tapered double cantilever beam specimens
3  Terms, definitions and symbols
3.1  Terms and definitions
No terms and definitions are listed in this document.
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 .electropedia .org/
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ISO 22838:2020(E)

3.2  Symbols
A insert film length (mm), i.e. the distance between the end of the specimen and the tip of the
insert film (see Figure 1)
a crack length (mm), i.e. the distance between the load-line (intersection of plane through
pin-hole centres and plane of crack) and the tip of the precrack or crack on the edge of the
specimen (see Figure 1)
a pre-crack length (mm), measured from the load-line to the tip of the precrack (see Figure 1)
p
a insert film length (mm) between the load-line and the tip of the insert film (see Figure 1)
0
Δa crack growth at i-th load step in the stick-slip behaviour of the crack propagation
i
(see Figure 1)
b width of the specimen (mm) (see Figure 1)
C compliance δ/P of the specimen (mm/N)
C initial compliance of the specimen, neglecting start-up effects, e.g. due to play in the speci-
0
men fixture (mm/N) (see Figure 2)
C initial compliance of the specimen, C raised by a factor 1,05 (mm/N) (see Figure 2)
0+5 % 0,
E flexural modulus of the arms of the substrate beam, calculated from the crack propagation
f
using DCB test (GPa)
E flexural modulus of the carbon fibre reinforces plastic (CFRP) beam in DCB specimen (GPa)
1
E flexural modulus of the metal beam in DCB specimen (GPa)
2
2
(EI) equivalent stiffness (N·m ) (see Figure 7)
eq
F large-displacement correction
2
G adhesive fracture energy for the applied opening mode (J/m )
C
H height of the load-block (mm) (see Figure 1)
h thickness of the carbon fibre reinforces plastic (CFRP) beam (mm) in DCB specimen
1
(see Figure 1)
h thickness of the metal beam (mm) in DCB specimen (see Figure 1)
2
h thickness of the adhesive layer (mm) (see Figure 1)
a
4
I moment of inertia of area in CFRP (m )
1
4
I moment of inertia of area in metal (m )
2
l total length of the specimen (mm) (see Figure 1)
l distance from the centre of the loading pin to the mid-plane of the arm of the substrate
1
beam to which the load-block is attached (mm) (see Figure 1)
l distance between the centre of the pin-hole in the load-block and the edge of the load-
2
block, measured towards the tip of the insert (starter film) or the tip of the precrack (mm)
(see Figure 1)
l total length of the load-block (mm) (see Figure 1)
3
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ISO 22838:2020(E)

MAX/5 % either the maximum load on the load-displacement curve or the point of intersection of
a straight line with the load-displacement curve with the slope of the straight line corre-
sponding to C (see Figure 2)
0+5 %
N load-block correction
NL onset of nonlinearity on the load-displacement curve (see Figure 2)
P load measured by the load-cell of the test machine (N)
P(δ) is the experimentally obtained load value indicating the maximum extent of the portion of
load-displacement curve, from the origin O to the point A in Figure 3”
P (δ) is a linearly increasing load value indicated by the dashed line a in Figure 3, and it is calcu-
I
lated by Formula (8):
PROP increments of the crack length during stable crack growth (propagation) that are marked
on the load-displacement curve (see Figure 2)
VIS onset of visually recognizable crack growth at the edge of the specimen that is marked on
the load-displacement curve (see Figure 2)
Δ crack-length correction for a beam that is not perfectly built-in (mm) (see Figure 7)
δ displacement of the cross-head of the test machine (mm)
ρ radius of curvature of the bonded plate specimen (m) (see Figure 6)
4  Principle
A double cantilever beam (DCB) specimen is used to determine the adhesive fracture energy of
structural bonded joint between a metal and CFRP components
Resistance to both crack initiation and propagation is determined. The resistance to crack initiation
is determined from both a non-adhesive insert placed in the adhesive layer and from a precrack. The
resistance to crack propagation is determined from the precrack. The adhesive fracture energy versus
applied opening load is estimated and a resistance-curve (R-curve), i.e. a plot of the value of the adhesive
fracture energy versus crack length, is determined.
In the case where the CFRP/metal interlaminar toughness and interface are significantly tough, the
plastic deformation of the metal beam preferentially occurs during the crack propagation. This causes
the overestimation of fracture energy values. In such cases, double cantilever beam specimens with
dissimilar thicknesses shall be used. An appropriate ratio of the two beams thickness (h /h ) shall
1 2
be determined such that the plastic deformation of the metal beams during the crack propagation is
avoided.
5  Apparatus
5.1  Tensile-testing machine, capable of maintaining a crosshead displacement speed between
0,125 mm/min and 10 mm/min accurate to ±20 % and higher speeds accurate to ±10 %. The test tensile
testing machine shall be equipped with a fixture to introduce the load to the pins inserted into the
loading-blocks. Measurement of test system compliance is described in Annex C and Figure C.1.
The tensile testing machine shall comply with ISO 7500-1 and the force measurement system shall
comply with ISO 7500-1:2018, class 1.
The opening displacement of the test specimen shall be deduced from the position of the test machine
cross-head. The test machine shall be equipped with means for recording the complete load versus
displacement curves (loading and unloading) during the test.
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ISO 22838:2020(E)

5.2  Travelling microscope or video camera, with suitable magnification, capable of measuring the
crack length along the edge of the specimen to an accuracy of at least ±0,5 mm.
5.3  Micrometer or vernier callipers, capable of measuring the thickness of the substrate arms and
bonded plates with an accuracy of at least ±0,05 mm.
5.4  Micrometer or vernier callipers, capable of measuring the width of the specimens with an
accuracy of at least ±0,05 mm.
5.5  White spray-paint or typewriter correction fluid (“white ink”).
6 Specimens
6.1  Number of specimens
A minimum of five specimens shall be tested.
6.2  Conditioning
Most adhesives absorb small quantities of water from the atmosphere which can have a significant
influence on the measured properties. Following specimen preparation, the adhesive will generally
be dry. If testing is carried out within a few days of specimen manufacture, then it is not necessary
to condition the specimen under controlled humidity since negligible absorption of water takes place
in the thin adhesive layer. However, if the specimen is tested after longer times or if the influence of
absorbed water on the properties is of interest, then the humidity shall be controlled by conditioning
and the properties will depend on the conditioning time (see ISO 291).
In addition, if composite substrates are used, it can be important to dry these prior to manufacture of
the specimen. The properties of some adhesives are very sensitive to the presence of small amounts of
moisture in a substrate prior to curing. The drying out of the substrates prior to cure will ensure that
the integrity of the adhesive joint is not influenced by pre-bonding moisture effects.
6.3  Manufacture of adhesive joint specimens
6.3.1  General
The DCB specimen shall be as shown in Figure 1. The thickness of the film to be inserted in the adhesive
layer during manufacture shall be less than 13 µm. The film shall be non-stick. For joint specimens
bonded at temperatures below 180 °C, a thin polytetrafluoroethylene (PTFE) film is recommended. For
specimens bonded at temperatures above 180 °C, a thin polyimide film is recommended. Appropriate
[5]
surface treatments for metallic substrates can be found in ISO 17212 .
The thickness of the adhesive layer shall be carefully controlled and shall be less than 1 mm in
accordance with ISO 25217. The thickness of the layer shall not vary by more than 20 % within a plate,
nor shall the average thickness of the layer in one joint differ by more than 20 % from that in another
joint. When fully cured, remove any excess adhesive by mechanical means that do not weaken the bond,
to leave the joint with smooth sides.
It should be recognized that the value of G measured from these tests depends upon the thickness of the
adhesive layer in the joint. The value of the layer thickness shall be determined by the user, based upon
the adhesive manufacturer's recommendations or upon consideration of the intended application.
It is not within the scope of this document to specify full manufacturing details of the specimens to
be tested. Such information should be sought from the adhesive manufacturer and/or the substrate
manufacturers.
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ISO 22838:2020(E)

Repeat the measurements of the total beam thickness after bonding. Determine the adhesive layer
thickness, h , by subtracting the substrate thicknesses from the total thickness of the joint at each of
a
the three locations.
6.3.2  DCB specimen measurements
Remove any excess adhesive from the sides of the beam. After bonding, measure the width of the DCB
specimen with vernier calipers or a micrometer at three points along the length of the beam, at 30 mm
from either end and at the mid-length. Measurement tolerance to be ±0,5 mm. Calculate the mean value, b.
Key
1 composite (CFRP) beam
2 metal beam
3 adhesive layer
Figure 1 — Geometry of DCB bonded plate specimen with load-blocks
6.4  Preparation of specimens
Apply a thin layer of white spray-paint, or typewriter correction fluid (“white ink”), on the edges of the
specimen after conditioning to facilitate the detection of crack growth.
NOTE Some typewriter correction fluids and paints contain solvents which can harm the adhesive or the
laminate matrix material of a composite substrate. A material with an aqueous solvent is usually safe to use.
Apply marks every 1 mm from the tip of the insert or the precrack for at least the first 10 mm, then
apply marks every 5 mm. Apply marks for every 1 mm for the final 5 mm.
Measure the radius of curvature of the warped specimen following the method illustrated in 7.6.
If the specimen radius of the curvature, ρ, is smaller than 1,5 m, such specimens shall not be tested in
order to avoid the effect of thermally induced residual stress. The value of ρ shall be calculated using
Formula (2) and as shown in Figure 6.
For the DCB test specimen, the extent of crack propagation should be approximately 50 mm. If early
breakage happens at shorter propagation, the data should be recorded with information of failure mode
as defined by ISO 10365.
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ISO 22838:2020(E)

7  Procedure
7.1  Test set-up and data recording
The test shall be performed at one of the temperatures specified in ISO 291 or at another temperature
agreed between the interested parties. After mounting the specimen in the fixture of the test machine,
support the end of the specimen, if necessary, to keep the test beam orthogonal [i.e. at 90 degrees (90°)]
to the direction of the applied load. Record the load and the displacement signals of the test machine
electronically throughout the test, including the unloading cycle.
If using a tensile-testing machine with a paper chart recorder, the ratios of cross-head speed to chart
speed are recommended to be about 1:10.
Measure the crack lengths, with the respective load and displacement along both edges of the specimen
to an accuracy of at least ±0,5 mm using a travelling microscope, a scale, a vernier calliper or a video
camera with suitable magnification (5.2). Calculate an average of both lengths.
7.2  Initial loading (precracking stage)
For testing from the insert (starter film), load the specimen at a constant cross-head rate of 1,0 mm/
min to 5,0 mm/min.
NOTE Lower values are more accurate for crack-length measurement.
Record the point on the load-displacement curve at which the onset of crack movement from the insert
is observed on the edge of the specimen, on the load-displacement curve or in the sequence of load-
displacement signals [see VIS in Figure 2 a)].
Stop the loading as soon as the crack is seen to move on the edge of the specimen. Completely unload
the specimen at a constant cross-head rate of up to five times the loading rate. Mark the position of the
tip of the precrack on both edges of the specimen.
7.3  Re-loading: Testing from the precrack
For testing from the precrack which has been formed as a result of the test procedure in 7.2, load the
specimen at a constant cross-head rate of 1,0 mm/min to 5,0 mm/min.
NOTE 1 Lower values are more accurate for crack-length measurement.
Record, on the load-displacement curve or in the sequence of load-displacement signals, the point at
which the onset of crack movement from the insert is observed to occur [see VIS in Figure 2 b)].
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ISO 22838:2020(E)

a)   Testing from the insert with b)   Testing from the precrack with
initiation points NL, VIS and MAX/5 % initiation points NL, VIS and MAX/5 %
and propagation points (PROP)
Key
X displacement, δ
Y load, P
1 initiation values
2 propagation values
3 MAX/5 % — 5 % value as that point on the load-displacement curve at which the compliance has increased by
5 % of its initial value
NL point of deviation from linearity determined by drawing a straight line from the origin.
VIS first point at which the crack is observed.
NOTE This figure shows an example where the MAX and the 5 % offset points coincide such that they lie on
the same point on the curve. This is not generally the case. Usually, these points will be separated. The MAX/5 %
point is assigned to the point C0+5 % or MAX, whichever occurs first.
Figure 2 — Schematic load-displacement curve for the DCB test of continuous growth case
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ISO 22838:2020(E)

Key
X displacement, δ
Y load, P
Figure 3 — Schematic load-displacement curve for the DCB test of stick-slip case
After this, the testing procedure is divided into two cases: 1. Continuous growth case and 2. Stick-
slip case.
1) Continuous growth case: Note as many crack-length increments as possible in the first 5 mm on the
corresponding load-displacement curves, ideally every 1 mm. Subsequently, note crack lengths at
every 5 mm, until the crack has propagated about 50 mm from the tip of the precrack. Note every
1 mm for the last 5 mm of crack propagation. Record a minimum number of 10 propagation points.
2) Stick-slip case: When the first load drop occurs from load level A to B as indicated in Figure 3. Stop
the loading immediately. Measure the crack lengths along the both edges and calculate an av
...