3. Principal Physical Test Methods
3.4 Some Special Features of General Physical Tests
3.4.4 Hardness
a known mathematical law, it is inadvisable to extrapolate the results; therefore, long-term creep tests are essential.
frequency corresponding to a probit value of (5.897 + 1.428logE). The value of E is derived from its relation to the depth of indentation P (mm/100) by a ball of radius R (mm) under a load F (kgf), namely:
E = 263 F/(P1.35 R0.65) (3.5)
where:
E = Young’s modulus, kgf/mm2 F = load, kgf
P = depth of indentation, mm/100 R = the radius of the ball indenter, mm
This relation applies for perfectly elastic isotropic materials; usually there is some depar- ture from this ideal behavior.
Except on very soft materials, IRHD and Shore A durometer values are approximately the same.
3.4.4.1.2 Other Features of Indentation Hardness Tests
With relatively soft elastic materials (e.g., vulcanized rubbers), the result is influenced by the dimensions (especially thickness) of the test piece; the indentation usually increased somewhat with the time of loading and may be influenced by temperature. Hence stan- dard procedures define test piece dimensions, loading time, and temperature.
A micro-test, a scaled-down version of the normal IRHD test, has been developed for use on very small test pieces or finished articles.
3.4.4.2 Hardness Test Apparatus
Dead-load hardness gauge: Designed primarily for testing rubber, this is the best-known apparatus working according to ISO recommendation R48 and giving readings in IRHD. For the usual hardness range (30 to 95 IRHD) it uses a 2.5 mm diameter indenting ball, with dead loads of 30.5 gf (contact) and 580 gf (total). For very soft (10 to 35 IRHD) or very hard (85 to 100 IRHD) rubbers, the ball diameters are, respectively, 5 mm or 1 mm, but with the same loads.
Micro-hardness gauge: This scaled-down version of the above gauge has the ball diameter reduced 1/6, i.e., 0.395 mm and loads to (1/6)2, i.e., 0.85 gf (contact) and 15.7 gf (total).
It is for the range 30 to 95 IRHD.
Pocket hardness meter: This pocket-size instrument, with the hemispherical-ended plunger and approximately constant spring loading, reads in IRHD and covers the range 30 to 100 IRHD.
Shore durometers: Several models including pocket size are available for different ranges. Those mostly used are Type A (or A2), with small frustoconical indentor and variable spring loading (i.e., decreasing as indention increases), and the read- ing is in shore-A degrees (approximately equal to IRHD over the range 30 to 100);
Type D, with the sharp conical indentor and heavier (variable) spring loading, giv- ing the more open scale (30 to 100 corresponding to 80 to 100 Shore-A) and hence being suitable for semi-rigid materials.
Plastometer S: This uses a 1/8″ (usually) or 1/4″ diameter ball with dead loads that are 85 gf (contact) and 1085 gf (total); it reads directly as depth of indentation (mm/100). Initially designed to test rubber-covered rollers, it has ball-joined feet to rest on curved surfaces.
3.4.4.3 Compression of Hardness Measurements
The relation between rubber hardness scales (Table 3.3) shows, on the same horizontal line, equivalent readings on the scales most used for vulcanized rubbers and rubber-like materials. The equivalence must be regarded as approximate, especially with very hard materials that show more or less plastic deformation.
3.4.4.4 Interpretation
Table 3.3 shows the different hardness values in different hardness scales. Hardness is one of the most useful and often quoted properties of rubber, but in fact, the figures can be quite misleading. First, the measured values, especially by durometers, are often unreli- able because of the mechanical limitations of the equipment and because of operator error.
Hardness degrees should therefore never be quoted to better than 5°. Second, the charac- teristic that is measured, surface indentation, rarely bears any relation to the ability of the rubber product to function properly.
The lack of significance can best be understood by considering three products: a hose cover, a gasket to be used between rough flanges, and an automobile mounting. It is fairly obvious that the ease or difficulty of indenting the surface of the hose cover has nothing to do with utility.
The important properties in a hose cover are abrasion resistance and resistance to oil, weather, and other conditions relating to its service. The case of the gasket is unusual because surface indentation has some significance. Indentation by the point of the test instrument is similar, to some extent, to the indentation the gasket will receive from protrusions on the sealing surface.
There is much danger in attributing false significance to hardness in the above cases, but the danger is very real in the case of motor mounting. Motor mounts are typical of many rubber products which are required to carry load and in which the relationship between the load and deformation, called stiffness, is a critical design factor. Hardness
TABLE 3.3
Rubber Hardness Scales Type of
Material
IRHD and Shore-A Durometer
Shore-D Durometer
Pusey and Jones Plastometer 1/5-Inch Ball 1/4-Inch Ball
Hard 100 100 0 0
Hard 98 60 — —
Hard 95 50 14 10
Hard 90 40 27 20
Hard 80 30 48 35
Soft 70 22 68 50
Soft 60 16 92 67
Soft 50 12 125 90
Soft 40 9 170 125
Soft 30 7 260 185
cannot be assumed to be a close measure of stiffness. Hardness and stiffness are both stress/strain relationships, but the relationships are established for two entirely different kinds of deformations. Hardness measurements are derived from small deformations at the surface. Stiffness measurements are derived from gross deformations of the entire mass. Because of this difference, hardness is not a reliable measure of stiffness. Even if hardness and stiffness had a better correlation, the irreducible five-point variation in durometer readings would be equivalent to a 15 to 20% variation in stiffness as measured by a compression-deflection test. Hardness measurement would not, therefore, be suffi- ciently accurate for design purposes.
The misuse of hardness to measure stiffness is common and causes much confusion.
Wherever possible, simulated service tests should be used rather than hardness testers.