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I NT ERN AT1 O NA L 0 RG A N IZAT I O N FOR
STA N DA RD I ZAT 1 O N
IS0 RECOMMENDATION
R 178
PLASTICS
DETERMINATION OF FLEXURAL PROPERTIES
OF RIGID PLASTICS
1st E D IT1 ON
February 1961
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The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in aoy country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standaxds organizations.
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BRIEF HISTORY
The IS0 Recommendation R 178, Determination of Flexural Properties of Rigid
Plastics, was drawn up by Technical Committee ISO/TC 61, Plastics, the Secretariat of
which is held by the American Standards Association (ASA).
Work on this matter which the Technical Committee had begun since 1951, came to
an end in 1956, with the adoption of a proposal as a Draft IS0 Recommendation.
On 28 November 1958, the Draft IS0 Recommendation (No. 193) was distributed to
all the IS0 Member Bodies and was approved, subject to some editorial amendments,
by the following Member Bodies:
Hungary Romania
Australia
Austria India Spain
Belgium Israel Sweden
Italy Switzerland
Bulgaria
Japan Turkey
Burma
Czechoslovakia Netherlands U.S.A.
France Poland U.S.S.R.
Germany Portugal
No Member Body opposed the approval of the Draft.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0
Council, which decided, in February 1961, to accept it as an IS0 RECOMMENDATION.
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ISO/ R 178-1981 (E)
IS0 Recommendation R 178 February 1961
PLA S TI C S
DETERMINATIqN OF FLEXURAL PROPERTIES
OF RIGID PLASTICS
I
1. SCOPE
This method covers the determination of the following flexural properties in order to get a measure
of the resistance in flexure of test specimens having standard dimensions and of test specimens
having other than standard dimepsions.
1.1 The flexural stress (in kilogrdmmes-force per square centimetre) and the deflection at break
of materials that break before or at a definite deflection.
1.2 The flexural stress (in kilogrpmmes-force per square centimetre) at a definite deflection of
materials that do not break before or at a definite deflection.
1.3 The flexural stress (in kilogrammes-force per square centimetre) at maximum load.
1.4 Optional: the modulus of elasticity in flexure.*
2. DEFINITIONS
2.1 Deflection is the distance over which the loaded surface of the test specimen has deviated
during flexure from its posit5on in the original plane of this surface.
2.2 Flexural stress (kgf/cm2) at a given moment of the test is the maximum fibre stress of the
material in the section of the rtest specimen directly under the load. It is calculated according
to the relationship given in section 9.
2.3 Flexural stress (kgf/cm2) at conventional deflection is the flexural stress at a deflection
conventionally fixed at 1.5 times the height of the test specimen.
Flexural stress (kfg/cm2) at imaximum load and flexural stress at rupture are, respectively,
2.4
that at the moment when the load reaches a maximum value and that at the moment of
rupture.
2.5 Rigid plastics are plastics that show a modulus of elasticity in shear which in any direction
is larger than 5 x los kgf/cm2.
* The modulus of elasticity in flexure is to be considered only as an approximate value of Young's modulus of elasticity.
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IS0 / R 178 - I901 (E)
I
3. TEST SPECIMENS
Bars of rectangular cross-section are prepared according to IS0 Recommendation on preparing
test specimens. *
3.1 Standard specimens. The standard dimensions are in millimetres :
length
I = 80 or more
breadtb b = 10f0.5
height
h = 4 3: 0.2
3.2
Other specimens. When for any reason it is not possible or desirable to use the standard
specimen, the following rules should be observed:
(a) The length and the $eight of the test specimen should be in the same ratio as in the
standard specimen; that is:
1 minimum = 20 h
(b) The breadth should be chosen either in the same ratio to the height as in the standard
:
specimen
b = 2.5 h f 0.5 mm
or, in ordzr to simplify the preparation of the specimens, in conformance with Table 1,
because the breadth (width) has only a very slight influence on the flexural properties.
TABLE 1
Dimension in millimetres
Height h Breadth b
from 1 to 3 included 25 f 0.5
from 3 (excluded) to 10 ,, 10 f 0.5
from 10 ,, to 20 ,, 20 f 0.5
from 20 ,, to 35 ,, 35 f 0.5
from 35 ,, to 50 ,, 50 0.5
3.3 In the case of anisotropic materials, the specimens should be chosen so that the flexural
loading to which they are subjected will be applied in the same direction as that to which
the products (articles, sheets, tubes, etc.) from which the specimens will be taken, will be
subjected in service. It is this consideration that will determine, in particular, the possibility
or impossibility of using standard specimens and, in the latter case, will govern the choice
of dimensions of specimens in accordance with clause 3.2.
It should be noted that the bearing for position and the dimensions of the test specimens
have sometimes a very significant importance with respect to the final result. This is par-
ticularly applicable to laminates (see Fig. 1).
* To be drafted.
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MO/ R 178-1861 (E)
FIGURE 1. Bearing of test specimens
4. -ER OF TEST SPECIMENS
4.1 The number of test specimens should be a minimum of five and such that the results of
5 of them do not show a larger deviation from the arithmetical mean than twice t
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