Rubber softness and flexibility testing measures how readily rubber materials deform under load and bend at service temperatures, providing essential data for material qualification and quality control. This guide explains the principal rubber softness test methods, including Shore durometer hardness, IRHD, Gehman and Clash-Berg low-temperature flexibility, and De Mattia and Ross flex testing, together with the relevant ISO, ASTM, GB and JIS standards. Whether you are specifying seals, footwear soles, or low-temperature components, understanding these methods ensures reliable and comparable test results.
Technical Overview
Softness, in rubber technology, is the inverse expression of hardness and stiffness: the softer the rubber, the greater its deformation under a defined force and the more easily it conforms to mating surfaces or flexes without damage. Because softness governs sealing performance, tactile feel, damping behavior, and low-temperature serviceability, virtually every rubber material specification contains at least one softness- or flexibility-related requirement.
No single instrument measures "softness" as an absolute quantity. Instead, standardized method families quantify it indirectly through indentation hardness, torsional stiffness at low temperature, flex-crack resistance, and compression stress-strain behavior. The most widely applied methods include:
- Shore durometer hardness — ISO 7619-1, ASTM D2240, GB/T 531.1, JIS K 6253
- International Rubber Hardness Degrees (IRHD) — ISO 48, ASTM D1415, GB/T 6031
- Gehman low-temperature flexibility — ASTM D1053, ISO 1432, GB/T 6036
- Clash-Berg torsional stiffness — ASTM D1043
- Temperature-retraction (TR) test — ASTM D1329, ISO 2921
- De Mattia flex cracking and crack growth — ASTM D430, ISO 132
- Ross flex testing for footwear — ASTM D1052
- Compression stress-strain (softness under load) — ISO 7743, ASTM D575
- Plasticity of unvulcanized rubber — ASTM D926, ISO 2007
All testing should be performed in accredited third-party laboratories or under controlled laboratory conditions, with conditioning and testing at 23 °C ± 2 °C and 50 % ± 5 % relative humidity in accordance with ISO 23529, because hardness and stiffness values are sensitive to temperature, dwell time, and specimen geometry.
Testing Samples
Vulcanized Standard Test Pieces
The preferred specimen is a flat sheet with smooth, parallel surfaces, typically at least 6 mm thick for Shore hardness and preferably 8 mm to 10 mm thick for normal-scale IRHD. Thinner products may be tested by stacking a maximum of three plies, provided no visible gaps remain, or by micro-hardness techniques suitable for specimens down to about 2 mm thickness.
Finished Rubber Components
O-rings, gaskets, oil seals, hoses, vibration isolators, bushings, tire sections, and footwear soles can be tested directly on defined flat regions. Measurement points must be located on sufficiently flat areas away from edges, ribs, and fabric reinforcement. Small or thin parts, such as O-ring cord, are best suited to micro IRHD testing.
Cellular and Latex Rubber
Soft sponge, foam, and latex materials fall below the practical range of the Shore A scale and are measured using the Shore 00 or Shore 000 durometer scales in accordance with ASTM D2240. Specimens should be thick enough to avoid full collapse of the cell structure during indentation, which would overstate the measured hardness.
Unvulcanized Rubber
For process control of raw or masticated compounds, small cylindrical pellets or slabs are prepared for Williams plastimeter testing (ASTM D926) or rapid plastimeter testing (ISO 2007), both conducted at elevated temperature to assess the plasticity and recovery of the compound before vulcanization.
Testing Items
Depending on the selected method and the end-use requirement, the following quantitative results are typically reported:
- Shore hardness value on the Shore A, Shore D, Shore 00 or Shore 000 scale (dimensionless, 0 to 100)
- IRHD value on the normal, micro, or high-hardness scale (dimensionless, 0 to 100)
- Apparent torsional modulus as a function of temperature, together with the characteristic temperatures T2, T5, T10 and T100 (Gehman method)
- Clash-Berg stiffness-temperature curve and the temperature at a defined apparent modulus
- TR10, TR30, TR50 and TR70 retraction temperatures (temperature-retraction test)
- Number of flexing cycles to crack initiation and crack growth length (De Mattia method)
- Crack growth rate in millimeters per kilocycle (Ross method)
- Compression stress-strain curve, deflection under a defined load, or stress at a defined deflection
- Plasticity number and recovery percentage for unvulcanized compounds
Testing Methods
Shore Durometer Hardness Testing
Standards: ISO 7619-1, ASTM D2240, GB/T 531.1, JIS K 6253. A spring-loaded indenter — truncated cone for Type A, sharp cone for Type D, and spherical or blunt geometry for Types 00 and 000 — is pressed against the specimen under the force of a calibrated spring. The penetration depth after a defined dwell time (commonly 3 s per ISO 7619-1, or instantaneous readings as permitted by ASTM D2240) is read directly on the 0 to 100 scale. The specimen rests on a rigid flat surface, and at least five readings are taken at points spaced apart and away from the specimen edge; the median value is reported. Shore A covers conventional soft rubbers, Shore D is used for hard rubbers and rigid compounds, and Shore 00 or 000 is used for very soft foams and gels. As a general guideline, readings above about 90 indicate that the next harder scale should be used, and readings below about 20 indicate that the next softer scale should be used.
International Rubber Hardness Degrees (IRHD) Testing
Standards: ISO 48, ASTM D1415, GB/T 6031. A polished steel ball — 2.50 mm diameter for the normal scale and 0.395 mm for the micro scale — is pressed into the specimen first under a small preliminary force and then under a larger total force. The difference in penetration depth between the two forces is converted mathematically to an IRHD value, a scale defined to closely approximate Shore A on well-vulcanized compounds. Because the loading is applied by dead weights rather than a spring, IRHD instruments deliver superior reproducibility and are the preferred method for referee testing. The micro scale permits reliable hardness measurement of small parts, thin-walled goods, and O-ring sections that cannot be tested accurately with a standard durometer.
Gehman Low-Temperature Flexibility Testing
Standards: ASTM D1053, ISO 1432, GB/T 6036. A strip specimen, typically about 40 mm long with a small rectangular cross-section, is clamped at one end and twisted through a fixed angle at the free end by a calibrated torsion wire inside a temperature-controlled bath. As the temperature is lowered stepwise, the apparent torsional modulus increases. The results are expressed as T2, T5, T10 and T100 — the temperatures at which the apparent modulus reaches 2, 5, 10 and 100 times its value at 23 °C. Lower characteristic temperatures indicate better retention of flexibility in cold service; T100 approximates the region of glassy, near-brittle behavior.
Clash-Berg Stiffness Testing
Standard: ASTM D1043, widely applied to elastomeric materials. The specimen is twisted in a torsion apparatus while the temperature is reduced, and the apparent torsional modulus is plotted against temperature. This method produces a complete stiffness-temperature profile, which is particularly useful for ranking compounds for cold-weather flexibility and for defining the lowest service temperature at which a component still deforms elastically under torque.
Flex Fatigue Testing — De Mattia and Ross
Standards: ASTM D430 and ISO 132 (De Mattia), ASTM D1052 (Ross). In the De Mattia test, a rectangular specimen — plain, grooved, or pierced — is repeatedly flexed to a fixed arc at a typical rate of about 5 Hz until cracks initiate at the stressed zone. The result is reported as the number of cycles to visible crack initiation or as the growth length of a crack after a specified number of cycles. The Ross flex test bends a notched specimen through a defined arc around a mandrel and is the industry-standard method for footwear soling materials; crack growth is reported in millimeters per kilocycle. Together, these methods describe dynamic flexibility — the ability of rubber to bend repeatedly without failing.
Compression Softness Testing
Standards: ISO 7743 and ASTM D575. A cylindrical specimen, typically about 29 mm in diameter and 12.5 mm high, is compressed between parallel plates at a defined rate, with or without a preliminary cycling step. The result is a compression stress-strain curve, from which the deflection at a given stress or the stress at a given deflection is extracted. This directly quantifies "softness under load" for applications such as mounting pads, gaskets, diaphragms, and cushioning products, where the indentation hardness alone does not fully describe the load-bearing behavior.
Plasticity Testing of Unvulcanized Rubber
Standards: ASTM D926 and ISO 2007. A small preformed pellet of unvulcanized compound is compressed between parallel plates at 100 °C under a defined load for a specified time. The compressed thickness, expressed as a plasticity number, indicates the softness and processability of the raw compound, while the recovery measured after load removal characterizes its elastic memory. These values are essential for mixing control, mastication verification, and the prediction of molding behavior.
Applications
Rubber softness and flexibility test data support material selection, product design, incoming inspection, and batch release across a broad range of industries:
- Automotive — weatherstrips, door and window seals, engine mounts, bushings, brake and cooling hoses, and battery sealing pads for electric vehicles
- Footwear — outsole and midsole flexibility, crack growth resistance, and cold-weather performance
- Industrial sealing — hardness and softness verification of O-rings, gaskets, and diaphragms matched to compression and sealing requirements
- Medical and pharmaceutical — softness and low-temperature flexibility of gloves, catheters, seals, and tubing
- Wire and cable — flexibility and cold-bend performance of insulation and jacketing compounds
- Aerospace and defense — qualification of seals and elastomeric components for low-temperature and high-altitude service
- Research and quality control — compound development, comparative ranking of formulations, supplier verification, and certification testing in accredited third-party laboratories
FAQ
What is the difference between Shore A hardness and IRHD?
Both are 0 to 100 scales and produce broadly comparable values on well-prepared vulcanized rubbers. Shore hardness uses a portable spring-loaded durometer, making it convenient for field and production use but more sensitive to operator technique. IRHD uses dead-weight loading of a ball indenter and delivers higher reproducibility, which is why it is preferred for laboratory referee testing. Micro IRHD additionally enables testing of small or thin articles that a standard durometer cannot measure reliably.
Which hardness scale should be used for very soft foam rubber?
Below approximately 20 Shore A, the resolution of the Type A durometer becomes poor. Very soft sponge, foam, and gel materials should be tested on the Shore 00 or Shore 000 scale in accordance with ASTM D2240, using specimens thick enough to prevent bottoming-out of the indenter.
How thick does a test specimen need to be?
For Shore hardness, specimens should be at least 6 mm thick; thinner sheets may be stacked up to a maximum of three plies without gaps. For normal-scale IRHD, a thickness of 8 mm to 10 mm is preferred, while micro IRHD is suitable for specimens from about 2 mm thick. Testing on curved or textured surfaces should be avoided unless a flat measurement area can be prepared.
What do T5 and T10 mean in the Gehman test?
T5 and T10 are the temperatures at which the apparent torsional modulus of the rubber reaches 5 times and 10 times its value at 23 °C. Lower T-values indicate that the compound remains more flexible at low temperature. T100, where the modulus is 100 times the room-temperature value, approximates the transition to glassy, brittle behavior.
What conditioning is required before testing?
Vulcanized test pieces should be conditioned at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a minimum of 16 hours, in accordance with ISO 23529, and tested at the same temperature unless the method itself specifies sub-ambient or elevated conditions. The elapsed time between vulcanization and testing should also comply with the relevant standard.
Can softness results be compared between different laboratories?
Yes, provided that specimen preparation, conditioning, instrument calibration, dwell times, and the standard edition are all identical. Using accredited third-party laboratories operating under ISO/IEC 17025 management systems, with traceably calibrated instruments, is the most reliable way to obtain intercomparable softness and flexibility data.