One of the most widely measured properties used to characterize rubber is hardness. Two scales are generally in use throughout the world: the Shore scale and the IRHD (International Rubber Hardness Degree) scale.
These two test methods use completely different indentor geometries, test times, procedures and applied forces. For most of these scales, instruments exist both as tabletop instruments and as handheld versions. However, there is little knowledge about the differences between them. For most of these instruments, national and international standards exist although there are subtle differences between these standards.
This article examines these instruments, the differences between the tests and, where possible, their relationship scales. It also highlights the merits of each test type and instrument. The test method and sample dimensions may be highly affected where measurements need to be made on awkward or small production samples. Therefore, it is important to know the limitations of each test method. In conclusion, this article aims to provide a clear understanding of hardness testing methods and the results that they give.
One of the most widely measured properties used to characterize rubber is hardness. The Shore Scale and the IRHD (International Rubber Hardness Degree) are both used widely. Various instrument types exist for both, but the Shore A and the IRHD Micro/Dead Load scales are the most commonly used for rubber. International standards describe both methods.1, 2
The two test methods utilize completely different indentor geometries, test times, procedures and applied forces. The IRHD, which takes 35 seconds, is the preferred method for final product inspection because it is usually non-destructive. Conversely, the Shore method only takes 1 or 3 seconds but is often destructive and leaves behind a permanent indentation. This article starts with a historical look at the instruments and the degree of correlation between them.
Shore and IRHD scales instruments come in hand-held and tabletop versions. The IRHD Dead Load has a Micro counterpart and a standard for this has been established for more than 30 years (ISO 481 & ASTM 14152). As yet, the proposed Shore Micro does not have a released standard.
In the 1950s, the Micro IRHD instrument was introduced as a smaller version of the IRHD Dead Load and for use with smaller, thinner products and samples. The Micro IRHD results are generally comparable to those produced by the standard IRHD Dead Load instrument. Conversely, there are various Shore M scale instruments on the market and some of these differ markedly in their construction. In general, the Shore M results are not comparable to those produced by the Shore A scale.
There is often a degree of confusion between users of the two test methods. This article highlights the advantages of each test type and instrument. The test methods and sample dimensions may be highly significant in cases where measurements are made on awkward, curved or small production samples. Therefore, it is important to recognize the limitations of each test method.
In conclusion, this article aims to provide a clear understanding of common hardness testing methods and the results they produce.
According to Bassi et al3, historically the Shore instruments had priority over the IRHD instruments by more than 30 years. Gurney4 reports that, by the early 1920s, both instruments were in use, as well as other dead load (weight) and spring variants.
Results produced by the spring-type varied according to the user (Gurney4, The Rubber Age5). As a result, the dead load instrumentation was adopted, where the indentation depth largely depended on the user. Scott6 stressed the need for a standard in order to give results some common meaning in 1935. Following this, the first British Standard (BS) was introduced in 1940.
At this time, Scott and Newton7 reported a reliable pocket-type hardness gauge, which conforms to these standards. They concluded that the advantage was always with the BS Hardness Meter when compared to the Shore A Durometer. Further work examined different instrument types (Daynes and Scott8) and the new standard (Scott9). It was agreed that the Shore A and BS hardness scales were correlated in some way.
A range of hardness testers were examined for accuracy (Newton10), drawing the conclusion that the main limitations were associated with the operator. The smallest errors came from instruments with a spherical indentor and foot, while the Shore durometer was associated with the largest errors. Lack of agreement between laboratories was the largest source of variation according to Scott11.
In the 1950s, the Micro Hardness Tester was introduced. This was a 1/6th scaled-down version of the IRHD Dead Load Hardness Tester for testing small, thinner production samples. Comparable results between the Micro and Dead Load tests were reported by Scott and Soden12, with only a few degrees of difference seen for rubbers with a hardness of more than 65o.
Even though the IRHD method produced more repeatable results between operators as well as higher precision, accuracy and reproducibility, several papers3, 13, 14, 15 published in the 1960s, 70s and 80s stated that the most widely used instrument was then the Shore A type.
Two books, Rubber and Plastics Testing16 and Physical Testing of Rubbers17 also described Shore A as the most commonly used. Shore A depends less critically on sample thickness than IRHD (Bassi et al3). Brown and Soekarnein18 compared the Shore A, IRHD Dead Load and IRHD Micro instruments and showed that inter-laboratory repeatability was likely to be best for the IRHD Micro and Dead Load instruments.
In 1993, the durometer indentation was analyzed by Briscoe and Sebastian19. This analysis provided an approximate relationship between Shore A and IRDH of (IRH ≈ HA + 4). However, this relationship is highly dependent on the sample compound.
Contemporary hardness testers are automated by nature meaning they require minimal operator intervention. This has improved the accuracy of testing. The most reliable and repeatable results are produced by bench mounted instruments (Shore A and IRHD Dead Load an Micro scales). Although pocket meters have been much improved, they rely on the hand pressure of the operator and reliable angular application in order to produce repeatable results (variations can be extreme).
The Shore M has recently been the subject of increased interest, although there is no published standard. There are a number of these instruments available on the market but these vary in their construction. Wallace has recently manufactured Shore M instruments to the best information available. It is not possible to compare the results to those produced from a Shore A instrument.
Differences Between IRHD and Shore Instruments and Relationships Between Scales
Four IRHD methods are in use: Micro-test, Low-hardness test, Normal-hardness test and the High-hardness test.
The Normal test is used for samples with a thickness greater than or equal to 4 mm and is preferably used for rubbers between 35 and 85 IRHD (but with reservation, can be used for rubbers between 30 and 95 IRHD).
The High-hardness test is used for samples with the same dimensions but between 85 and 100 IRHD. The Low-hardness test is used for samples that are 6 mm or thicker and are between 10 and 35 IRHD. For samples less than 4 mm thick, the Micro test is used. The Micro test is preferably used on rubber between 35 and 85 IRHD but may be used between 30 to 95 IRHD with reservation.
All these methods use a spherically tipped indentor. The diameters of the foot and the ball indentor vary between methods. The Low, Normal and High tests all have the same applied force, with only the Micro test requiring the application of smaller forces. It is important to note that the IRHD scale is non-linear.
The Shore range of hardness testers has eight scale types: A, B, C, D, DO, O, OO and M, and these are used for testing a broader range of materials. Soft rubbers and elastomers are measured using the A scale, while the C scale measures medium and hard rubbers and plastics. Both these scales use a truncated cone-shaped indentor.
The most commonly used rubber scale is type A. Type B is used to test rubbers that are moderately hard and type D for testing hard rubbers and plastics. Both type B and D use a 30o indentor. For very dense textile windings, the DO is used and type O is used for softer rubbers and medium-density textiles. Type OO is used for sponge and low-density windings. The indentor used in these three types is a 3/32 inch spherically ended indentor.
All types need samples more than 6 mm thick, unless it can be shown that equivalent results can be achieved with smaller samples. Type M has no published standard as yet. It is used for testing irregular and thin rubbers with a hardness between 20 and 90 using a very small, round-tipped indentor. Although thinner samples may be used, the support table begins to affect the value as thickness falls as the indentor penetrates the sample.
The spring forces vary between instruments. Type A, B and O use the same spring force and, to ensure that the spring force is repeatedly overcome, a force equivalent to 1 kg is applied to the durometer. It is important to note that the DIN standard used 1.27 kg and tighter interdimensional limits.
Type C, D and DO use the same spring that requires a force of 5 kg. Type OO uses a different spring and needs a force of 400 g to overcome it. Type M currently suggests a force that is suitable to overcome the calibrated spring force. All of the Shore scales are linear.
The IRHD method uses dead loads (weights) as a basis. The sample is held in place by a foot with a force of 8.3 N (Dead Load) or, in the case of the Micro hardness, 235 mN. A datum position is then achieved by applying a primary load of 0.3 N (Dead Load) or 8.3 mN (Micro hardness tester) for 5 seconds.
A secondary load of 5.4 N (Dead Load) or 145 mN (Micro) is subsequently applied for 30 seconds. The incremental displacement from the datum position is measured and can then be converted into an IRHD value (a non-linear scale defined in the standard). The full-range displacement of the (Normal) Dead Load is 1.8 mm and 0.3 mm for the Micro.
Shore instruments on the other hand, use calibrated springs. For example, the Shore M scale spring force varies from 0.3 N to 0.8 N over the full displacement and the Shore A scale varies from 0.5 N to 8.1 N. A force sufficient to overcome the spring force is applied by the presser foot. When the presser foot contacts the sample, the indentation depth is recorded after a pre-set dwell time. The standard ASTM dwell times are 1 and 3 seconds. Since the reading is usually still changing after 1 second, the DIN standard uses 3 seconds. The force increases in a linear trend with indentor displacement (full range is 1.25 mm for the M scale and 2.5 mm for the A scale).
In 1948, the IRHD scale was set in order to correspond to the Shore scale. A low number indicates a soft rubber and a high number indicates a harder rubber. The Micro hardness test was originally designed to be a smaller version of the Normal Dead Load test (with displacements in the ratio 6 to 1). The forces were applied in a ratio of 36 to 1. Consequently, 1/6th of the result will be obtained if the limited thickness sample tested in the case of a Micro instrument is 1/6th of the thickness of the Dead Load piece. Scaling is applied so that the results from both instruments should be the same. Results from the Low, Normal and High Dead Loads also show correlation.
The Shore M test was designed as an instrument capable of testing smaller samples, not as a scaled-down version of the Shore A test. Shore M uses an unrelated spring and indentor and there is no easy relationship between the two instruments.
As previously discussed, results obtained from hand-held durometers are not reliable due to operator variability. As a result, the experimental results were obtained using only bench-mounted instruments. However, any conclusions drawn will also be relevant to hand-held instruments. The majority of rubber and elastomers use either the Shore A or M scales, other Shore scales will not be examined. The Shore A and M are also the main counterparts of the Micro and Normal IRHD instruments.
Before starting, all instruments were calibrated and the calibration was rechecked at the end. Except where otherwise noted, a standard temperature of 23 ± 2 °C was used. Since the results from 1 and 3 second dwell times differ, the Shore instruments were set to both. Test times, defined by the standard for the IRHD instruments, 5 and 30 seconds, were used. Flat samples were each tested in five different places and the procedure for testing curved samples is outlined below.
The Micro IRHD and Micro Shore instruments have been compared previously20 regarding lateral dimensions, temperature, sample thickness, bent samples, the effect of foot force and retesting a previous spot on the Shore M instrument. This article aims to elaborate on this work by including the Dead Load and Shore A instruments as well as incorporating results from samples with curved surfaces.
To provide comparative results for both instruments, standard Wallace test blocks (varying compounds of natural rubber, supplied by MRPRA) were used for both the micro and dead load instruments.
Further investigations into sample thickness were also conducted because previous work20 on this area is limited. Although samples that are 1 mm thick can be used as per the ISO standard1, the preferred thickness is 2 ± 0.5 mm.
The Shore M is known to have similar requirements. Tests were undertaken on a range of thinner materials. In order to increase effective thickness, the Shore standard2 suggests that samples be plied. This was done in order to work out the effect of varying sample thickness. This was extended to similar work on the Shore A and IRHD dead load instruments. The Shore A standard thickness is 6 mm whilst the IRHD is 8-10 mm. To determine the effect of varying sample thickness, a selection of thinner samples were tested and plied.
Previous work20 with the Micro instruments included an examination into the effect of increasing the ambient temperature. Therefore, tests were carried out on the Shore A and Dead Load instruments at a raised temperature in order to determine if there was any effect.
Curved samples, for example ‘O’ rings, are commonly tested. The effect of testing these on different instruments was also investigated. So that they could be accurately displaced laterally, ‘O’ rings (of varying core and outer diameters) were placed on a specially designed table. This meant that the effect of testing away from the top dead center could be examined.
Standard Test Blocks
Using Micro and Normal Dead Load instruments, the standard test blocks gave repeatable results. Equivalent and repeatable results were produced from both the 1 and 3 second dwell times (Shore A and M). Over the range tested (40 to 90 IRHD), the Dead Load readings were consistently higher than the Shore A readings by a few units. However, with increasing hardness values, there was an increasing tendency for the Shore M results to diverge from the Micro IRHD result.
Effects of Thickness
The Shore A and IRHD Dead Load instruments were used to test the standard Wallace Micro samples (2 mm thick). Results differed as expected from those produced when using the instrument specified for that particular sample thickness i.e. the Shore M and Micro IRHD types.
The IRHD Dead Load instrument and the softer rubbers showed the biggest differences between the micro and macro instrument results. The Dead Load instrument gave a very close value for the hardest rubber (76 to 79 IRHD) (see figure 1). As expected, the Shore A instrument read a few units lower, but the readings were closer between the Shore M and Shore A instruments. The Shore A value of the hardest rubber only differed by 1 unit to the Shore M value (figure 1).
The results were within the specified tolerances of the test pieces once the 2 mm thick samples were plied to form 8 mm thick samples (the standard thickness required for the IRHD Dead Load tester). Further increasing the thickness made little difference to the result. When using the Shore A instrument with material plied to a thickness of 6 mm (standard thickness for Shore A), the results were not equivalent to the values obtained using the Shore M (see figure 2).
The standard Dead Load blocks of 8 mm generally gave approximately the same results as the Dead Load IRHD and Shore M instruments when tested on the Micro IRHD and Shore M instruments.
With the micro instruments, various thinner samples were used. Up to five pieces of neoprene (0.6 mm thick) were plied, which created samples that were within the standard tolerance region (and slightly beyond).
With increasing thickness, there was a continual decrease in hardness and this was true for both the Shore Micro and IRHD Micro (see figure 3). As previously, the IRHD hardness values were higher than the Shore M values by a significant amount. For a nitrile sample, the readings at the initial thickness of 1.5 mm (within the tolerance specified in the standard) were similar for both instruments.
During the thickness increase, the Shore M results remained constant but the IRHD Micro instrument showed a decrease in hardness with increasing thickness up to 4.5 mm. For a silicone sample, both instruments exhibited a decrease in hardness of 1 unit, when doubling the thickness of the sample from 0.9 mm to 1.8 mm (within the standard). In this example, the Shore M results were consistently lower than the IRHD Micro values.
Effect of Temperature
A temperature raise of 10 oC did not appear to make much difference to the results from the IRHD Dead Load on the standard test blocks (natural rubber compound). However, when the harder samples were tested on the Shore A instrument, slightly lower values were observed.
Effect of Curved Surfaces
The ‘O’ rings with a smaller diameter were laterally displaced in increments of 0.25 mm and the larger curved surfaces in increments of 0.5 mm. Graphs, which showed the change in apparent hardness as the sample, was displaced across the indentor. The resulting curves on the graphs were generally flatter when using the Micro IRHD instrument to test the ‘O’ rings than when the Shore M instrument was used. The Shore M instrument gave curves which were more peaked and the hardness values decreased rapidly on either side of top dead center (figure 4).
Both the IRHD Dead Load and Shore A instruments were used to test a piece of pipe which was 8 mm in diameter. Both instruments produced fairly flat curves. When tested on the Shore A instrument, an EPDM ‘O’ ring (core diameter 7.8 mm) produced a gentle curve, but this was inverted when tested using the IRHD Dead Load.
It is clear from the results that there is a correlation between the Micro IRHD and Dead Load instruments. This is evident when the IRHD Dead Load result of a plied micro sample corresponds to the standard result from IRHD Micro instrument. However, the same is not true for the Shore A and M scales.
These results agree with Bassi et al3 in that the thickness of the sample used on the IRHD Dead Load affects the result more than on the Shore A.
A trend of decreasing hardness with increasing thickness was observed when the neoprene, silicone and nitrile samples were plied. Some differences were observed and it is apparent that different rubber types influence results in different ways. This is an area that requires more extensive work.
In general, the macro instruments (Dead Load and Shore A) are better for macro samples but the micro instruments can be used for testing both micro and macro samples. In fact, many people now use the Micro IRHD in place of the Dead Load instrument.
When testing curved samples, flatter curves are produced with the Shore A, Micro and IRHD Dead Load. This implies that these instruments depend less critically on accurate sample positioning. The graphs produced using the Shore M are more peaked, suggesting that it is important to accurately place the sample (to within approximately 0.1 mm). Although this is controlled when using an instrument attachment to centralize the ‘O’ rings, it is still important when testing curved shapes that cannot accurately be held in such an attachment.
A temperature increase of approximately 10 oC appears to make a greater difference to harder natural rubber samples only on the Shore A. It does not seem to make a difference when using the IRHD Dead Load instrument. This is in comparison to a slight effect described by previous work20 for both the IRHD Micro and the Shore M instruments.
It is clear from previous work that repeated testing at the same location makes a difference to the results. This effect is more apparent when the Shore M instrument is used. It is therefore important to make sure that the sample is displaced between tests although this can be difficult when samples are small.
It is worth noting that the results from the Shore instrument with a dwell time of 1 second differ from those obtained using a 3 second swell time. This has been demonstrated more effectively in previous work20. Consequently, for Shore instruments it is important that the timing is repeatable and accurate even though different timings are unimportant for comparative work. The IRHD takes 35 seconds (specified by the standard) and this is a disadvantage. Previous work by Lackovic et al5 (1997) demonstrates that this time can be reduced using a predictive technique that takes it into direct competition with the Shore-M timing (i.e. 3 seconds).
This article has looked historically at the Shore and IRHD Hardness measurement instruments. It has also discussed and emphasized the fundamental differences between the most common instruments used for elastomer and rubber hardness testing. The various instruments exhibit advantages and disadvantages with certain sample types, and this has been previously demonstrated20.
For non-destructive testing, IRHD instruments are preferred whereas the Micro IRHD is generally better for testing curved surfaces. For testing non-standard thickness samples and when shorter test cycle times are required, the Shore A instrument is preferred. Repeatable and accurate timing is critical to allow Shore A and M instruments to provide comparable and consistent results.
- ISO 48: 1994, Physical Testing of Rubber, Methods for the Determination of Hardness
- ASTM 1415-88 (1994), Test Method for Rubber Property – International Hardness
- A. C. Bassi, F. Casa & R. Mendici, Polymer Testing 7, 165 (1987)
- H. P. Gurney, India Rubber Journal 497 (1921)
- ‘A New Rubber Hardness Tester’, The Rubber Age 29, 242 (1939-40)
- J. R. Scott, Transactions I. R. I. 11, 224 (1935)
- J. R. Scott & R. G. Newton, Journal of Rubber Research 9, 91 (1940)
- H. A. Daynes & J. R. Scott, Journal of Rubber Research 12, 94 (1943)
- J. R. Scott, Journal of Rubber Research 17, 145 (1948)
- R. G. Newton, Journal of Rubber Research 17, 178 (1948)
- J. R. Scott, Transactions I. R. I. 27 (5), 249 (1951)
- J. R. Scott & A. L. Soden, Proceedings of the International Rubber Conference, Washington, Paper 22, pp. 170-176 (1959)
- K. Price, Progress of Rubber Technology 42, 59 (1979)
- J. C. Warner & J. A. Jerdonek, European Rubber Journal & Urethanes Today 162, 11 (1980)
- W. V. Chang & S. C. Sun, Rubber Chemistry and Technology 64, 202 (1991)
- “Rubber and Plastics Testing”, Klucklow, pp. 153-162, 1963
- “Physical Testing of Rubbers”, J. R. Scott, pp. 91-110, 1965
- R. P. Brown & A. Soekarnein, Polymer Testing 10, 117 (1991)
- B. J. Briscoe & K. S. Sebastian, Rubber Chemistry and Technology 66, 827 (1993)
- R. Morgans, S. Lackovic & P. Cobbold, The International Rubber Exhibition & Conference Book of Papers, Manchester, UK, Materials Paper 12 (1999)
- S. Lackovic, R. Morgans & B. McGarry, The International Conference on Rubbers, Calcutta, India, Paper AT-5 (1997)
Figure 1. Testing 2 mm thick samples on micro and macro instruments.
Figure 2. Decreasing hardness with increasing sample thickness.
Figure 3. Increasing the thickness of a neoprene sample.
Figure 4. Differences in testing an O Ring of core diameter 2.5 mm on IRHD micro and Shore M.
R. Morgans1* BSc, S. Lackovic2 BSc, PhD, P. Cobbold2
1. University of Greenwich, School of Engineering, Medway Campus, Chatham Maritime, Kent, ME4 4TB, England
H W Wallace & Co. Ltd, 172 St James’s Road, Croydon, CR9 2HR, England
This information has been sourced, reviewed and adapted from materials provided by Wallace Instruments.
For more information on this source, please visit Wallace Instruments.