Mechanical testing

When measuring the dimensions of rubber samples and products, most common equipment can be used, e.g., measuring tapes and rulers, calipers, thickness gauges, and profile projectors. Since rubber is a soft material, it is important not to deform the rubber during measurement. For accurate thickness measurement, thickness gauges with a specified measuring load are therefore used, and the diameter of the measuring foot can range from 2 mm to 10 mm, with the load adjusted so that a measuring pressure of 22 kPa ±5 kPa is obtained.

Another good way to, for example, measure the cross-section of rubber profiles is to use a profile projector, which allows for non-contact measurement.

Standard for dimensional measurement of rubber is ISO 4648.

The hardness or stiffness (modulus) of rubber is determined by measuring how far a blunt measuring tip can be pressed into the rubber. Originally, there were several methods for hardness measurement, but nowadays mainly two methods are used: Shore and IRHD.

Shore
The oldest method for hardness measurement is the Shore method from 1915, where a truncated cone is pressed into rubber with spring force and the reading is taken after 3 seconds. The Shore method has several scales (A, B, C, D, AO, DO, O, OO, and AM). Of these, the four most common are specified in the ISO standard. Shore A is used for normal rubber hardness, Shore D for hard rubber, and Shore AO for soft rubber. Shore AM is a Shore A micrometer method for thin samples.

The Shore hardness scales have end points of 0 and 100, where 0 is "infinitely" soft and 100 is "infinitely" hard. The difference between the scales lies in different measuring tips and spring force. Shore meters can be used as both handheld meters and as stand-mounted units and are available in analog and digital versions.

Measurement accuracy is significantly improved when mounted on a stand by having the meter parallel to the sample and by using the correct load. Accuracy is further improved if a timer
used.

International Rubber Hardness Degree
The IRHD method was developed within ISO to improve the accuracy of hardness measurements. This method is called "International Rubber Hardness Degrees" or IRHD. The IRHD apparatus scale is designed to correspond to Shore A. The differences between the methods are that the IRHD apparatus is always bench-mounted, the load is applied by deadweight, the indentor is a ball, and the reading is taken after 30 seconds.

The standard for hardness measurement of rubber is ISO 48.

During tensile testing, the following properties of a rubber material are typically determined:
• Stress at a given strain, e.g., 100 MPa at 1% strain or 300 MPa at 3% strain; this is sometimes also referred to as the "rubber modulus.".
• Fracture limit in MPa, which is the strength at fracture.
• Elongation at break in %, which is the elongation at the point of fracture.

The test is normally performed on dumbbell-shaped specimens punched from 2 mm test plates. The test is carried out in a tensile testing machine at a tensile speed of 500 mm/min. To determine the elongation at break and the tensile stress at different elongations, a strain gauge or extensometer is also required. The strain gauge can be mechanical with balanced clamps that measure the strain or optical with, for example, a laser that measures against two reflective measurement marks on the specimen.

Standard for tensile testing of rubber is ISO 37.

Compression tests to measure the stiffness, spring constant, or modulus of rubber can be performed using modern tensile testers, which are equally capable of performing compression and tensile tests. A common method involves deforming the rubber 25 % and measuring the force. Mechanical conditioning is often performed by first compressing the rubber three times and then taking the measurement during the fourth compression. The result can be plotted as a force-deformation curve from which the force at, for example, 10 and 20% deformation can be read.

The standard for pressure testing of rubber is ISO 7743.

With a tear test, the tear strength of the material is determined. Two methods are most common in Sweden, and they are as follows:

Crescent specimen: A crescent-shaped specimen with a 1 mm deep notch is pulled to fracture at a tensile speed of 500 mm/min. The specimen can be considered a tensile test with a fracture initiation point.

Byxprovkropp: The trouser test specimen consists of a rectangular strip 15 x 75 mm with a notch 25 mm in from one short side. The strip is pulled in a tensile tester at 100 mm/min so that the notch propagates. The result is obtained as a tear diagram with a number of peaks, and the median peak is stated as the result.

The standard for tear strength testing of rubber is ISO 34.

There are several methods for testing adhesion, and it can be performed in tension, shear, or peel. In some cases, special test specimens are manufactured, while in others, test specimens are taken from products such as tires, conveyor belts, and more.

The most interesting aspect of adhesion testing is observing where the fracture occurs. If the fracture occurs in the rubber, it's good, but if it occurs at the surface of the reinforcing material, it's worse, as the adhesion system is then the weak link. For peel tests, a curve similar to that of a tear test is obtained and evaluated in the same way, but with a different unit.

Standards for adhesion testing of rubber are ISO 813, 814, and 1827.

Shear modulus measurements are most commonly performed on a so-called "quadruple shear test piece." The test piece consists of four rubber pieces that are glued or vulcanized between steel plates. After mechanical conditioning, the test piece is stretched to 30% deformation at a rate of 5 mm/min, and the shear modulus is calculated at 25% deformation.

The standard for measuring the shear modulus of rubber is ISO 1827.

Many rubber products are used dynamically, and therefore it is interesting to be able to measure dynamic properties. The properties most often measured are damping, stiffness (modulus) as a function of frequency, deformation, and temperature.

Material testing is most often performed with small instruments under compression, tension, bending, or shear and is called Dynamic Mechanical Analysis, DMA. The instruments consist of a part that can vibrate the samples from 0 to a few tens of Hz, as well as a load cell that measures the force. The sample is enclosed in a temperature chamber that can be both cooled and heated between approximately -70 to 200°C. This allows for temperature sweeps where the modulus is measured as a function of temperature.

Since rubber is a viscoelastic material, the modulus and damping vary with frequency, deformation, and temperature.

The standard for dynamic testing of rubber is ISO 4664.

If a rubber sample is held stretched for a period of time and then released, it does not return completely to its original length. This is called permanent set. In this test, rubber test bars are loaded with weights at room temperature. After a predetermined time, the samples are released and the permanent set is determined after a 10-minute recovery period.

The standard for testing compression set in rubber is ISO 2285.

Elasticity is a fundamental property of rubber and is also one of the oldest and simplest tests. The test is usually performed as a bounce test, where a weight is allowed to bounce against the rubber, either in the form of a free-falling ball or a pendulum. By comparing the height of the rebound to the original height, one obtains a measure of the energy absorbed by the rubber as damping.

The standard for elasticity of rubber is ISO 4662.