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Ref. No. : ISO/R 48-1968 (E)
IS0
I N T ER N AT i ON A L O R G A N 12 AT I ON FOR S TA N DARD 1 ZATl O N
IS0 RECOMMENDATION
R 48
DETERMINATION OF HARDNESS
OF VULCANIZED RUBBERS
2nd EDITION
September 1968
This second edition supersedes the first edition
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any 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 standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 48, Determination of hardness of vulcanized natural and synthetic rubbers,
was drawn up by Technical Committee ISO/TC 45, Rubber, the Secretariat of which is held by the British
Standards Institution (BSI).
Work on this question by the Technical Committee began in 1948 and led, in 1953, to the adoption of a
Draft IS0 Recommendation.
In August 1954, this Draft IS0 Recommendation (No. SO) was circulated to all the IS0 Member Bodies for
enquiry. It was approved by 26 Member Bodies. 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 July 1957, to accept it as an IS0 RLCOMMENDATION.
BRIEF HISTORY RELATING TO THE 2nd EDITION
Work for the revision of IS0 Recommendation R 48-1957 was subsequently started and, in
1963, a draft proposal was adopted by the Members of Technical Committee ISO/TC 45 as a Draft
IS0 Revision.
In November 1964, this Draft IS0 Revision (No. 742) was circulated to all IS0 Member Bodies
for enquiry. It was approved. subject to a few modifications of an editorial nature, by the following
Member Bodies :
Argentina Hungary Poland
Australia India Spain
Austria Iran Sweden
Canada Israel Switzerland
Czechoslovakia Italy Turkey
Chile Japan U.A.R.
France Korea, Rep. of United Kingdom
Germany Netherlands U.S.A.
Greece New Zealand U.S.S.R.
No Member Body opposed the approval of the Draft.
The Draft IS0 Revision was then submitted by correspondence to the IS0 Council which
decided, in September 1968, to ,accept it.
The title of IS0 Recommendation R 48-1957 is amended as follows : Determinatiorz of
hardness of vulcanized rubbers.
The present edition (2nd edition) supersedes the first edition of IS0 Recommendation
R 48-1957.
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ISO/R 48-1968 (
I
IS0 Recommendation R 48 September 1968
DETERMINATION OF HARDNESS
OF VULCANIZED RUBBERS
INTRODUCTION
The hardness test is based on a measurement of the indentation of a rigid ball into the rubber test
piece under specified conditions. For the normal test the standard test piece is between 8 and 1 O mm
thick; test pieces less than 8 mm thick give smaller indentation than the standard. For tests on thin
pieces of rubber a scaled-down version (herein after referred to as the micro-test) of the normal test
is therefore used, in which the apparatus dimensions are reduced to one-sixth. When used on a piece
1.6 to 2 mm in thickness, the result of the micro-test will be about the same as that obtained by the
normal test.
It is considered unrealistic to fix a precise thickness above which the normal test should be used and
below which the micro-test should be used, but in general the latter test should be used for thicknesses
below about 4 mm. There will, however, be exceptions; for instance, the micro-test would be prefe-
rable even on thicknesses above 4 mm if the lateral dimensions of the test piece are much less than
those specified for the normal test (see Table 2), because the latter test would then be inaccurate.
The micro-test would also be preferable for testing some small awkwardly-shaped rubber articles.
The figure of 4 mm has been chosen for the following reasons :
at this thickness the normal test will give readings in international rubber hardness degrees
(a)
(IRHD) higher than the “standard” reading (i.e., on 8 to 10 mm thickness), and the
micro-test will give readings lower than this (because this test gives the “standard”
reading on a thickness of about 1.6 to 2.0 mm). These two errors are about equal when
the thickness tested is 4 mm.
4 mm is the greatest thickness on which the micro-test could be made without increasing
(b)
the lateral dimensions of the test piece above that now specified (i.e.2 mm minimum
between the indenter and the edge of the test piece).
In either the normal test or the micro-test, the measured indentation is converted into international
rubber hardness degrees, the scale of degrees being so chosen that O represents the hardness of a
material having an elasticity modulus of zero and 100 represents the hardness of a material of
infinite elasticity modulus, and so that the following conditions are fulfilled over most of the normal
range of hardness :
one international rubber hardness degree always represents approximately the same
(a)
proportionate difference in Young’s modulus;
for highly elastic rubbers, the scales of international rubber hardness degrees and the
(b)
Shore A durometer are comparable.
For substantially elastic isotropic materials like well-vulcanized natural rubbers, the hardness in
international rubber hardness degrees bears a known relation to Young’s modulus, although for
markedly plastic or anisotropic rubbers the relationship will be less precisely known.
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ISO/R 48-1968 (E
1. SCOPE
The method of test described in this IS0 Recommendation should preferably be used for rubbers
of a hardness range between 30 to 85 IRHD, but may be used for those rubbers with a hardness
between 30 and 95 IRHD.
Methods for very hard or very soft rubbers will be the subject of separate IS0 Recommendations and
the range of applicability of each is indicated in the following figure :
IRHD t
1
O 30 35 50 85 95 100
FIGURE - Range of applicability of hardness tests
2. PRINCIPLE OF METHOD
The hardness test consists in measuring the difference between the depths of indentation of the ball
into the rubber under a small contact force and a large total force. From this difference, multiplied
when using the micro-test by the scale factor 6, the hardness in international rubber hardness degrees
is derived by using either Table 3, or a graph based on this table, or a scale reading directly in
international rubber hardness degrees and derived from the table, fitted to the indentation-measuring
instrument.
The relation between the difference of indentation and the hardness expressed in international rubber
hardness degrees is based on :
the known relation, for a perfectly elastic isotropic material, between indentation P,
(1)
expressed in hundredths of a millimetre, and Young's modulus M, expressed in bar,
namely :
F/M = 0.00038 R0.65p'.35 *
where
F is the indenting force, expressed in newtons,
R is the radius of ball, expressed in millimetres;
the use of a probit (integrated normal error) curve to relate log, ,M and the hardness in
(2)
international rubber hardness degrees, as shown in the diagram on page 9. This curve is
defined as
value of log, ,M corresponding Co midpoint of curve
(a)
= 1.364 (M expressed in bar),
(b) maximum slope
= 57 international rubber hardness degrees per unit increase in log,,M.
This formula is approximate and is included as an indication.
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ISO/R 48-1968 (
3. APPARATUS
The essential parts of the apparatus are as follows, the appropriate dimensions and forces being
Table 1.
A vertical plunger, having a rigid ball or spherical surface on the lower end, and means
for supporting the plunger so that the spherical tip is kept slightly above the surface on
the annular foot prior to applying the contact force.
Means for applying a contact force and an indenting force to the ball, allowing for the
weight of the plunger and of any fittings attached to it, and for the force of any spring
acting on it, in order that the forces actually applied to the ball shall be as specified.
Means, which may be mechanical, optical or electrical, for measuring the increase in
depth of indentation of the plunger caused by the indenting force, either in metric units,
or reading directly in international rubber hardness degrees.
A flat annular foot, normal to the axis of the plunger and having a central hole for the
passage of the plunger. The foot rests on the test piece and exerts a pressure on it of
0.3 f 0.05 bar, provided the total force on the foot does not fall outside the values given
in Table 1. The foot is rigidly connected to the indentation-measuring device, so that a
measurement is made of the movement of the plunger relative to the foot (i.e. the top
surface of the test piece), not relative to the surface supporting the test piece.
Means, for example, an electrically operated buzzer, for gently vibiating the apparatus to
overcome any slight friction (this can be omitted in instruments where friction is
completely eliminated).
A chamber for
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