ISO 14125:1998

INTERNATIONAL ISO
STANDARD 14125
First edition
1998-03-01
Fibre-reinforced plastic composites —
Determination of flexural properties
Composites plastiques renforcés de fibres — Détermination des propriétés
de flexion
A
Reference number
Contents Page
.............................................................................
1 Scope 1
2 Normative references. 2
3 Principle. 2
4 Definitions . 3
5Apparatus . 4
............................................................
6 Test specimens 6
7 Number of test specimens . 8
8 Conditioning . 8
9 Procedure . 8
10 Calculation and expression of results . 9
.......................................................................
11 Precision 12
12 Test report . 13
Annex A (normative) Other test specimens . 14
(normative)
Annex B Large-deflection corrections —
Calculation and expression of results.
©  ISO 1998
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ii
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ISO ISO 14125:1998(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.
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.
International Standard ISO 14125 was prepared by Technical Committee
ISO/TC 61, Plastics, Subcommittee SC 13, Composites and reinforcement
fibres.
Annexes A and B form an integral part of this International Standard.
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Introduction
This standard is based on ISO 178 but deals with fibre-reinforced plastic
composites. As such it retains the test conditions relevant for glass-fibre-
reinforced systems. The test conditions are extended from ISO 178 to
include both three-point (Method A) and four-point (Method B) loading
geometries, and to include conditions for composites based on newer
fibres such as carbon and aramid fibres.
Other source documents consulted include ASTM D 790 (four-point
loading), prEN 2562 (test conditions), CRAG 200 and JIS K 7074 (use of
shims for four-point loading, figure 6). The overall specimen length for four-
point loading is the same as for three-point loading.
The scope of ISO 178 will be revised and limited to unreinforced and filled
plastics.
EN 63:1977, Glass-reinforced plastics — Determination of flexural
properties — Three-point test, will be withdrawn.
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INTERNATIONAL STANDARD  ISO ISO 14125:1998(E)
Fibre-reinforced plastic composites — Determination of flexural
properties
1 Scope
1.1 This International Standard specifies a method for determining the flexural properties of fibre-
reinforced plastic composites under three-point (Method A) and four-point (Method B) loading. Standard
test specimens are defined but parameters included for alternative specimen sizes for use where
appropriate. A range of test speeds is included.
1.2 The method is not suitable for the determination of design parameters, but may be used for
screening materials, or as a quality-control test.
NOTE – For example, the flexural modulus is only an appropriate value of the tensile Young's
modulus of elasticity as the test is not for the additional deflection due to the shear stress which
leads to a lower value of the flexural modulus but uses test span/specimen thickness ratios that
minimise this effect. Differences between tensile and flexural properties are also caused by the
material structure/lay-up.
1.3 The method is suitable for fibre-reinforced thermoplastic and thermosetting plastic composites.
Unreinforced and particle-filled plastics and plastics reinforced with short (i.e. less than 1 mm length)
fibres are covered by ISO 178.
1.4 The method is performed using specimens which may be moulded to the chosen dimensions,
machined from the central portion of the standard multi-purpose test specimen (see ISO 3167) or
machined from semi-finished or finished products such as mouldings or laminates.
1.5 The method specifies preferred dimensions for the specimen. Tests which are carried out on
specimens of other dimensions, or on specimens which are prepared under different conditions, may
produce results which are not comparable. Other factors, such as the speed of testing and the
conditioning of the specimens can influence the results. For materials which are not homogeneous
through the section, or above the linear-elastic response region, the result applies only to the thickness
and structure tested. Consequently, when comparative data are required, these factors must be
carefully controlled and recorded.
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2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of
this International Standard. At the time of publication, the editions indicated were valid. All standards are
subject to revision, and parties to agreements based on this International Standard are encouraged to
investigate the possiblitiy of applying the most recent editions of the standards indicated below.
Members of IEC and ISO maintain registers of currently valid International Standards.
Plastics - Determination of flexural properties.
ISO 178 1993
ISO 291 1997 Plastics - Standard atmospheres for conditioning and testing.
ISO 293 1986 Plastics - Compression moulding test specimens of thermoplastic materials.
ISO 294-1 1996 Plastics - Injection moulding of test specimens of thermoplastic materials -
Part 1: General principles, and moulding of multipurpose and bar test
specimens.
ISO 295 1991 Plastics - Compression moulding of test specimens of thermosetting
materials.
ISO 1268 1974 Plastics - Preparation of glass fibre reinforced, resin bonded, low-pressure
laminated plates or panels for test purposes (under revision).
Statistical interpretation of test results - Estimation of the mean - Confidence
ISO 2602 1980
interval.
Plastics - Preparation of test specimens by machining.
ISO 2818 1994
ISO 3167 1993 Plastics - Multipurpose test specimens.
ISO 5893 1993 Rubber and plastics test equipment - Tensile, flexural and compression
types (constant rate of traverse) - Description.
3 Principle
The test specimen, supported as a beam, is deflected at a constant rate until the specimen fractures or
until the deformation reaches some pre-determined value. During this procedure, the force applied to
the specimen and the deflection are measured.
The method is used to investigate the flexural behaviour of the test specimens and for determining the
flexural strength, flexural modulus and other aspects of the flexural stress/strain relationship under the
conditions defined. It applies to a freely supported beam, loaded in three- or four-point flexure. The test
geometry is chosen to limit shear deformation and to avoid an interlaminar shear failure.
NOTE – The four-point loading geometry provides a constant bending moment between the
central loading members. The compressive contact stresses due to the two central loading
members are lower in comparison with the stresses induced under the single loading member of
the three-point test. The four-point geometry is chosen so that the centre span equals one-third of
the outer span. The distance between the outer support points is the same as in the equivalent
three-point loading case, therefore the same specimen can be used.
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ISO ISO 14125:1998(E)
4 Definitions
For the purpose of this International Standard, the following definitions apply:
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4.1 speed of testing,
The rate of relative movement between the supports and the loading member(s), expressed in
millimetres per minute (mm/min).
4.2 flexural stress, σ
f
The nominal stress in the outer surface of the test specimen at mid-span. It is calculated according to
the relationship given in clause 10, equation (3) or (8), and is expressed in megapascals (MPa).
4.3 flexural stress at break (rupture), σ
fB
The flexural stress at break (or rupture) of the test specimen (see figure 1, curves A and B). It is
expressed in megapascals (MPa).
4.4 flexural strength, σ
fM
The flexural stress sustained by the test specimen at the maximum load (see figure 1) for acceptable
failure modes (see subclause 9.9 and figure 6). It is expressed in megapascals (MPa).
s
4.5 deflection,
The distance through which the top or bottom surface of the test specimen at mid-span has deflected
during flexure from its original position. It is expressed in millimetres (mm).
s
4.6 deflection at break,
B
The deflection at break of the test specimen (see figure 1, curves A and B). It is expressed in millimetres
(mm).
4.7 deflection at flexural strength, s
M
The deflection at the load equal to the flexural strength (4.4) (see figure 1, curves A and B). It is
expressed in millimetres (mm).
ε
4.8 flexural strain,
f
The nominal fractional change in length of an element in the outer surface of the test specimen at mid-
span. It is used for calculating the flexural modulus (4.9) and is expressed as a dimensionless ratio.
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E
4.9 modulus of elasticity in flexure; flexural modulus; chord modulus,
r
The ratio of the stress difference (σ '' – σ ') divided by the corresponding strain difference (ε '' = 0,0025 –
f f f
ε ' = 0,0005) (see 10.1.2 and 10.2.2). It is expressed in megaspascals (MPa).
f
NOTE – With computer-assisted equipment, the determination of the modulus using two distinct
stress/strain points can be replaced by a linear regression procedure applied to the part of the
curve between the two points.
4.10 interlaminar shear modulus, G
The shear modulus in the through-thickness direction for laminated materials. It is expressed in
megapascals (MPa).
NOTE – For materials with mainly in-plane reinforcement, the shear modulus G is of the order of
3 000 MPa to 6 000 MPa.
4.11 specimen coordinate axes (aligned materials)
The coordinate axes for an aligned material are defined in figure 2. The direction parallel to the fibre
axes is defined as the "1" direction and the direction perpendicular to it the "2" direction.
x y z
For other materials, the 1, 2 and 3 directions are generally described by the , , system of
coordinates.
NOTES
1 The "1" direction is also referred to as the 0 degree (0°) or longitudinal direction, and the "2"
direction as the 90 degree (90°) or transverse direction.
2 A similar definition can be used for material with a preferred fibre lay-up or in cases where a
direction (e.g. the lengthwise direction) can be related to the production process.
For materials with anisotropy as defined above, the designations include an additional subscript "1" or
"2" to indicate the direction tested.
5 Apparatus
5.1 Test machine
5.1.1 General
The test machine shall comply with ISO 5893 as appropriate to the requirements given in 5.1.2 to 5.1.4,
as follows:
5.1.2 Speed of testing
The test machine shall be capable of maintaining the speed of testing (4.1), as specified in table 1.
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ISO ISO 14125:1998(E)
Table 1 – Recommended values for the speed of testing
Speed Tolerance
(mm/min) (%)
0,5
± 20
±
± 20
± 20
±
± 10
± 10
100 ±
± 10
± 10
The speed 0,5 mm/min is not indicated in ISO 5893. The tolerances on the speeds 1 mm/min and
2 mm/min are lower than those indicated in ISO 5893.
5.1.3 Loading member(s) and supports
Supports and central loading member(s) are arranged according to figure 3 (3-point) or figure 4
R R
(4-point). The radius and the radius shall be as given in table 2. The axes of the supports and the
1 2
loading member(s) shall be parallel.
L
The span (distance between the supports) shall be adjustable.
Table 2 – Loading and support member dimensions
Dimension Value
(mm)
R
1 5 ± 0,2
R h ≤ ±
for  3 mm 2 0,2
R for h > 3 mm
2 5 ± 0,2
5.1.4 Load and deflection indicators
The error in the indicated force shall not exceed ± 1 % and that in the indicated deflection shall not
exceed ± 1 % of full scale (see ISO 5893).
Deflection obtained from movement of the test machine crosshead shall be corrected for loading train
deflection and indentation at the loading points.
5.2 Micrometers and gauges
5.2.1 Micrometer, or equivalent, capable of reading to 0,01 mm or less, and suitable for measuring the
width b and thickness h of the test 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).
5.2.2 Vernier callipers, or equivalent, accurate to within 0,1 % of the span L, for determining the span
(see 9.2).
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6 Test specimens
6.1 Shape and dimensions
6.1.1 General
Unless otherwise agreed, the dimensions of the specimen shall comply with those given in the standard
for the material under test or those given in 6.1.3.
6.1.2 Test direction
The test specimen axis shall be in one of the principal directions (see 4.11 and figure 5).
NOTE – When the material under test shows a significant dif
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