ISO 1288-1:2016

INTERNATIONAL ISO
STANDARD 1288-1
First edition
Glass in building — Determination of
the bending strength of glass —
Part 1:
Fundamentals of testing glass
Verre dans la construction — Détermination de la résistance du verre
à la flexion —
Partie 1: Principes fondamentaux des essais sur le verre
PROOF/ÉPREUVE
Reference number
ISO 1288-1:2015(E)
©
ISO 2015

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ISO 1288-1:2015(E)

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ISO 1288-1:2015(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols . 2
5 Factors to be taken into account when testing glass . 3
5.1 Glass as a material . 3
5.1.1 General. 3
5.1.2 Effect of surface condition . 4
5.1.3 Effect of rate of loading . 4
5.1.4 Effect of test surface area . 4
5.1.5 Effect of ambient medium . 5
5.1.6 Effect of aging . 5
5.1.7 Effect of temperature . 5
5.2 Bending stress and bending strength . 5
5.2.1 General. 5
5.2.2 Effective stress . 5
5.2.3 Equivalent bending strength . 6
5.2.4 Profile bending strength . 6
5.3 Types of glass . 6
5.3.1 General. 6
5.3.2 Patterned glass . 6
5.3.3 Laminated glass . 6
5.4 Orientation of the specimens . 7
5.5 Number of specimens in a sample . 7
6 Explanations of the test methods . 7
6.1 Coaxial double ring test for large test surface areas. 7
6.1.1 Elimination of edge effects . 7
6.1.2 Analysis of the stress development . 7
6.1.3 Testing of patterned glass .11
6.2 Test with specimen supported at two points (four point bending) .11
6.1.4 Limitations .11
6.1.5 Inclusion of edge effects .11
6.1.6 Analysis of the stress development .11
6.3 Coaxial double ring test for small test surface areas .13
6.1.7 Elimination of edge effects .13
6.1.8 Analysis of the stress development .13
7 Range of application of the test methods .14
7.1 General limitations .14
7.2 Limitations to ISO 1288-2 .14
7.3 Limitations to ISO 1288-3 .14
7.4 Limitations to ISO 1288-4 .15
7.5 Limitations to ISO 1288-5 .15
8 Calibration of the testing machines .15
9 Recommendations for safe use of test equipment .15
Bibliography .17
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ISO 1288-1:2015(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 160, Glass in building, Subcommittee SC 2, Use
considerations.
ISO 1288 consists of the following parts, under the general title Glass in building — Determination of the
bending strength of glass:
— Part 1: Fundamentals of testing glass
— Part 2: Coaxial double ring test on flat specimens with large test surface areas
— Part 3: Test with specimen supported at two points (four point bending)
— Part 4: Testing of channel shaped glass
— Part 5: Coaxial double ring test on flat specimens with small test surface areas
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INTERNATIONAL STANDARD ISO 1288-1:2015(E)
Glass in building — Determination of the bending
strength of glass —
Part 1:
Fundamentals of testing glass
1 Scope
This part of ISO 1288 specifies the determination of the bending strength of monolithic glass for use in
buildings. The testing of insulating units or laminated glass is excluded from this part of ISO 1288.
This part of ISO 1288 describes
— considerations to be taken into account when testing glass,
— explanations of the reasons for designing different test methods,
— limitations of the test methods, and
— gives pointers to safety requirements for the personnel operating the test equipment.
ISO 1288-2, ISO 1288-3, ISO 1288-4 and ISO 1288-5 specify test methods in detail.
The test methods specified in this part of ISO 1288 are intended to provide large numbers of bending
strength values that can be used as the basis for statistical evaluation of glass strength.
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.
1)
ISO 1288-2, Glass in building — Determination of the bending strength of glass — Part 2: Coaxial double
ring test on flat specimens with large test surface areas
1)
ISO 1288-3, Glass in building — Determination of the bending strength of glass — Part 3: Test with
specimen supported at two points (four point bending)
1)
ISO 1288-4, Glass in building — Determination of the bending strength of glass — Part 4: Testing of
channel shaped glass
1)
ISO 1288-5, Glass in building — Determination of the bending strength of glass — Part 5: Coaxial double
ring test on flat specimens with small test surface areas
ISO 16293-1, Glass in building — Basic soda lime silicate glass products — Part 1: Definitions and general
physical and mechanical properties
NOTE ISO TC 160/SC 1 is commencing work on standards for “thermally tempered soda lime silicate safety
glass”, “heat strengthened soda lime silicate glass” and “chemically strengthened glass.”
1) To be published.
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ISO 1288-1:2015(E)

3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
flat glass
any glass product conforming to ISO 16293-2, ISO 16293-3, ISO 16293-4 and ISO 16293-5, or any
transformed glass made from these products without deliberately inducing profile or curvature
3.2
bending stress
tensile bending stress induced in the surface of a specimen
Note 1 to entry: For testing purposes, the bending stress should be uniform over a specified part of the surface.
3.3
effective bending stress
weighted average of the tensile bending stresses, calculated by applying a factor to take into account
non-uniformity of the stress field
3.4
bending strength
bending stress (3.2) or effective bending stress (3.3) which leads to breakage of the specimen
3.5
equivalent bending strength
apparent bending strength (3.4) of patterned glass, for which the irregularities in the thickness do not
allow precise calculation of the bending stress (3.2)
3.6
profile bending strength
quotient of the maximum bending moment and the section modulus of a channel shaped glass
3.7
stress intensity factor
measure of the stress at a crack tip
3.8
prestressed glass
any glass product that has a surface prestress, i.e. thermally tempered soda lime silicate safety glass,
heat strengthened soda lime silicate glass and chemically strengthened glass
4 Symbols
F applied load N
h specimen thickness M
L length of side of square test sample M
k constant for calculation of bending stress in ISO 1288-3 M
K , K constants for calculation of bending stress in ISO 1288-5
1 2
M maximum bending moment Nm
bB
p gas pressure applied within loading ring in ISO 1288-2 Pa
P profile bending strength (of channel shaped glass) = M /Z Pa
bB bB
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ISO 1288-1:2015(E)

r radius of loading ring M
1
r radius of supporting ring M
2
r radius of circular specimen M
3
r average specimen radius (for evaluation) M
3m
y central deflection of specimen M
0
3
Z section modulus (of channel shaped glass) m
μ poisson number of specimen
NOTE For soda lime silicate glass (see ISO 16293-1), a value of 0,23 is used.
σ bending stress Pa
b
σ effective bending stress Pa
beff
σ bending strength Pa
bB
σ equivalent bending strength Pa
beqB
σ radial stress Pa
rad
σ tangential stress Pa
T
σ stress in a direction along the length of the specimen Pa
L
5 Factors to be taken into account when testing glass
5.1 Glass as a material
5.1.1 General
Glass is a homogeneous isotropic material having almost perfect linear-elastic behaviour over its tensile
strength range.
Glass has a very high compressive strength and theoretically, a very high tensile strength, but the
surface of the glass has many irregularities which act as weaknesses when glass is subjected to tensile
stress. These irregularities are caused by attack from moisture and by contact with hard materials (e.g.
grit) and are continually modified by moisture which is always present in the air.
Tensile strengths of around 10 000 MPa can be predicted from the molecular structure, but bulk glass
normally fails at stresses considerably below 100 MPa.
The presence of the irregularities and their modification by moisture contributes to the properties of
glass which need consideration when performing tests of strength.
Because of the very high compressive strength, glass always fails under tensile stress. Since glass in
buildings is very rarely used in direct tension, the most important property for load resistance is the
tensile bending strength. All the tests described in this part of ISO 1288 are intended to evaluate the
tensile bending strength of glass.
The bending strength is influenced by the following factors:
a) surface condition (see 5.1.2);
b) rate and duration of loading (see 5.1.3);
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ISO 1288-1:2015(E)

c) area of surface stressed in tension (see 5.1.4);
d) ambient medium, through stress corrosion cracking, as well as healing of surface damage in the
glass (see 5.1.5 and Reference [1]);
e) age, i.e. time elapsing since the last mechanical surface treatment or modification to simulate
damage (see 5.1.6);
f) temperature (see 5.1.7).
The influence exerted by factors b) to f) on bending strength has been taken into account in this
part of ISO 1288.
5.1.2 Effect of surface condition
For the purpose of bending strength tests according to this part of ISO 1288, glass behaves as an almost
ideally linear-elastic material that fails in a brittle manner. This brittleness means that contact with
any hard object can lead to surface damage in the form of ultra-fine, partly submicroscopic cracks and
chips. Surface damage of this kind, which is practically unavoidable during normal handling of glass,
exerts a notch action which is a major factor in reducing mechanical strength, whereas the chemical
composition of the glass has only a minor, and in some cases, entirely negligible, significance.
Hence, it follows that the bending strength determined by the methods referred to in this part of
ISO 1288 is related largely to the surface condition of the specimen to be tested.
This surface condition is characterized by the following main features:
a) the surface condition imparted by a particular method of treatment, which produces a specific
damage spectrum and thus, results in a strength which is specific to the finished surface condition;
b) residual stress, e.g. in the form of thermal or chemical prestress intentionally imparted, as well as
unintended residual stresses.
5.1.3 Effect of rate of loading
For the interpretation of the bending strength values determined as described in this part of ISO 1288,
the rate of loading is of special importance.
Cracks propagate in glass over a wide range of values of tensile stress (see Reference [2]). There is
a lower limit to the stress intensity factor below which cracks do not propagate (see Reference [1]).
There is then some subcritical crack propagation at higher levels of stress intensity factor, which is
influenced by humidity, temperature and chemical agents. Above a critical stress intensity factor,
crack propagation is very rapid and leads to (almost) instantaneous failure. The consequence of the
subcritical crack propagation is, for example, that the rate of load increase and/or the duration of static
loading influences the bending strength.
For prestressed glass, this time dependence does not manifest itself until the tensile stress induced in
the surface exceeds the compressive stress permanently present there (see Reference [3]).
5.1.4 Effect of test surface area
The decrease in bending strength of glass with increasing size of the test area exposed to high stress is
also of importance (see Reference [4]). This area effect is accounted for by the statistical distribution
of surface defects varying in effectiveness; the larger the test area, the greater is the probability of
its containing a large surface defect. Consequently, the influence of the area effect increases with
decreasing incidence of defects in the surface, so that this influence is more pronounced in the case of
undamaged, e.g. fire-finished glass surfaces (see Reference [5]).
Differences are likely between the mean values of the bending strength as measured in accordance
2
with ISO 1288-2 (maximally stressed area: 240 000 mm ), or by using devices R105, R60, R45 and
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ISO 1288-1:2015(E)

2 2 2
R30 in accordance with ISO 1288-5 (maximally stressed areas: 3 850 mm , 1 260 mm , 254 mm and
2
113 mm ), due to the size of the stressed area. Depending on surface damage, the results obtained
from testing smaller surface areas may be significantly higher than those obtained from testing larger
surface areas, as shown in Table 1.
Table 1 — Approximate effects of test surface area on the mean measured bending strength
Test method Device Relative bending strength
ISO 1288-2 — 100 %
ISO 1288-5 R105 120 % to 180 %
ISO 1288-5 R65 125 % to 210 %
ISO 1288-5 R45 140 % to 270 %
ISO 1288-5 R30 145 % to 300 %
Since glass for use in buildings is often in large sizes, the test methods specified in ISO 1288-2 and
ISO 1288-3 give values which are more appropriate as the basis for designing flat glass for use in
buildings. The test method specified in ISO 1288-5 can be useful as a method of evaluating the
comparative bending strength of flat glass.
5.1.5 Effect of ambient medium
The surrounding medium in which the glass is tested has an influence on the strength of the glass,
particularly if the moisture level is very low. When glass is used in buildings, the relative humidity
typically ranges from 30 % to 100 %. Within this range, the effect on the bending strength, as tested
according to this part of ISO 1288, is not great. However, tests on glass for use in buildings shall be
undertaken in test conditions with relative humidity levels in the range of 40 % to 70 %, in order to
eliminate this effect when comparing bending strength results.
5.1.6 Effect of aging
If the glass surface is modified (by abrasion, etching, edge working, etc.) before the testing, it is
necessary to allow the fresh damage to heal before the test is undertaken. The continual surface
modification by moisture affects the damage in a way that can reduce any weakening effect (see
Reference [1]). In practice, glass is highly unlikely to be stressed directly after it has been treated, so it
shall be conditioned for at least 24 h before testing.
5.1.7 Effect of temperature
The bending strength of glass is affected by changes in temperature. Within the normal range of
temperatures experienced by glass in buildings, this effect is not very significant, but to avoid possible
complications in the comparison of bending strength values, testing shall be undertaken in a restricted
range of temperatures.
5.2 Bending stress and bending strength
5.2.1 General
The test methods described in ISO 1288-2, ISO 1288-3, ISO 1288-4 and ISO 1288-5 are designed to
induce a uniform bending stress over an area (the test area) of the specimen. However, the stresses
induced by the applied loads depend on the nature of the material tested as well as the load distribution.
5.2.2 Effective stress
Where the stress varies significantly over the test area, as is the case in ISO 1288-3 (see 6.1.6), it can
be represented by a weighted average stress called the effective bending stress, σ . The weighting is
beff
obtained by statistically evaluating the probability of fracture at any point in the stressed area.
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5.2.3 Equivalent bending strength
Variations in homogeneity or thickness of the specimen affect the stress distribution. Hence, the
bending strength, σ , is never entirely an accurate value and, in some instances, it is better termed the
bB
equivalent bending strength, σ .
beqB
For some of the glass types tested (for example, float glass), such variations are very small and the
bending strength determined by the tests is sufficiently close to the actual bending strength for the
difference to be unimportant.
In the case of patterned glass, however, only the equivalent bending strength can be determined.
5.2.4 Profile bending strength
When channel shaped glass is tested according to ISO 1288-4, most of the specimens fail from fractures
originating at the corner of the profile, where the web and flange meet, and not at the extreme of the
flange or surface of the web. This is due to secondary stresses generated by the spreading of the flanges
when the channel section is bent. In this test, the bending strength is better expressed as the profile
bending strength, P .
bB
5.3 Types of glass
5.3.1 General
The tests specified in ISO 1288-2, ISO 1288-3 and ISO 1288-5 are for testing flat glass. This includes float
glass, drawn sheet glass, patterned glass, patterned wired glass, polished wired glass and prestressed
glass, provided there has been no deliberately induced curvature or profile (other than the patterned
surface of patterned glass).
5.3.2 Patterned glass
The coaxial double ring test for large test surface areas (see ISO 1288-2) can be used to determine the
equivalent bending strength of patterned glass, provided the maximum and minimum thicknesses do
not deviate by more than 30 % or 2 mm, whichever is the lower, from the average thickness. This is
because of difficulties in sealing the pressure ring to a patterned surface.
There is no limitation on the depth of pattern if the four point bending test (ISO 1288-3) is used.
5.3.3 Laminated glass
The testing of the bending strength of laminated glass (see ISO 12543-1) is excluded from this part of
ISO 1288.
In a bending test, additional shear deformation arises in the elastic or plastic interlayers (sliding of the
hard glass plies on the interlayer). This effect means that measuring the bending strength of laminated
glass is likely to give a strength value less than the actual bending strength of a monolithic glass of
the same thickness. This shear deformation is particularly sensitive to the effects of temperature and
loading rate.
Laminated glass is manufactured from monolithic glass products that can be tested individually by the
test methods described in ISO 1288-2, ISO 1288-3 and ISO 1288-5.
The process of manufacture is unlikely to cause significant changes in the bending strength of the
component glasses, so it is unnecessary to test laminated glass which can be assumed to have bending
strengths appropriate to their individual components.
The load resistance of laminated glass depends on the interactions between the component parts of the
composite structure, which is beyond the scope of this part of ISO 1288.
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5.4 Orientation of the specimens
Many glass products lack symmetry in their production. This may be immediately obvious, such as a
patterned glass, which is likely to have one surface much more deeply patterned than the other and
possibly in which the pattern is directional, or it may be less obvious, such as the side on which the
wheel cut was made (see ISO 1288-3:—, Figure 1).
Where such asymmetry is present, it may be necessary to test the glass in several different orientations
in order to determine the bending strength. Samples of glass to be tested shall have all the specimens
nominally identical.
5.5 Number of specimens in a sample
The bending strength of glass displays a large variation between nominally identical specimens.
Very little information can be obtained by testing only a few specimens, since there is considerable
uncertainty about whether the results are representative.
In statistical terms, this uncertainty can be expressed as confidence limits, values between which there
is a given probability that the target being sought will lie.
Where the target being sought is in the central part of the bending strength distribution (for insta
...