Mechanical testing

Almost all testing today is carried out statically, i.e. you pull, push or shear at a constant speed in one direction and usually until the sample breaks or gives way in some other way.

The majority of products are exposed to dynamic stresses in their use. There may be regular or random variations in deformation rate and direction, often linked to other changes in the use environment. In principle, you should always test a product dynamically if it will be subjected to dynamic stresses.

The most common method for determining the density of a rubber material is to weigh a sample in air and water. When weighing in air, you get the weight, and when weighing in water, you get the volume. The density is then calculated by dividing the weight by the volume.

The rubber's hardness or stiffness (modulus) is determined by measuring how far a blunt measuring tip can be pushed 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 of hardness measurement is the Shore method from 1915, where a truncated cone is pressed into the 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 micromethod for thin samples.

All Shore measurement scales have end positions 0 and 100, where 0 is "infinitely" soft and 100 "infinitely" hard. The difference between the scales consists of different measuring tips and spring force. The shore meters can be used both as hand-held meters and as stand-mounted meters and are available in analogue and digital versions.

The measurement accuracy is significantly improved when mounted on a stand, as the meter sits parallel to the sample and the correct load can be used. Accuracy is further improved if timer
is used.

IRHD: The IRHD method was developed within ISO to improve the accuracy of hardness measurement. This method is called "International Rubber Hardness Degrees" or IRHD. The scale of the IRHD device is designed to correspond to Shore A. What differentiates the methods is that the IRHD device is always mounted on a stand, the load is dead weight, the measuring tip is a ball and the reading takes place after 30 seconds .

Vid dragprov bestäms normalt följande egenskaper på ett gummimaterial:
• Spänning vid viss töjning, t ex 100 % eller 300 %, uttryck i MPa, detta kallas ibland även "gummimodul".
• Brottgräns i MPa, vilket är styrkan vid brott.
• Brottöjning i %, vilket är töjningen vid brott.

Provningen utförs normalt på hantelformade provkroppar som stansas ur 2 mm provplattor. Provet görs i en dragprovare med en draghastighet på 500 mm/min. För att kunna bestämma brotttöjningen och dragspänningen vid olika töjning behövs även en töjningsmätare eller extensometer. Töjningsmätaren kan vara mekanisk med utbalanserade klämmor som mäter töjningen eller optisk med till exempel en laser som mäter mot två reflekterande mätmärken på provkroppen.

Pressure tests to measure the rubber's stiffness, spring constant or modulus can be carried out in modern tensile testers which can be run in pressure as well as in tension. A common method is to deform the rubber 25% and measure the force. Often a mechanical conditioning is done by first compressing the rubber three times and then performing the measurement during the fourth compression. The result can be recorded as a force/deformation curve from which the force can be read at, for example, 10 and 20% deformation.

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 provided with a 1 mm deep notch is drawn at a tensile speed of 500 mm/min to failure. The test can be considered a tensile test with breaking instructions.
 
Pants test body: The pants test body 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 runs further. The result is obtained as a burst diagram with a number of peaks and the median peak is given as the result.

There are several methods for testing the adhesion and it can be done in tension, shear or splitting. In some cases, special samples are manufactured, in other cases, samples are taken from products such as tires, conveyor belts, etc.

The most interesting thing about adhesion testing is to see where the break occurs. If the break occurs in the rubber, it is good, but if the break occurs at the surface of the reinforcement material, it is worse. This is because the adhesion system is then the weak link. In the tear test, a curve similar to that in the trouser tear test is obtained and it is evaluated in the same way fixed in a different unit.

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

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

Material testing usually takes place with small instruments in compression, elongation, 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 one to run temperature sweeps where the modulus is measured as a function of temperature.

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

If a rubber sample is kept stretched for a time and then relieved, it does not return completely to its original length. This is called residual strain. In this test, rubber test rods are loaded by means of weights at room temperature. After a predetermined time, the samples are unloaded and the residual strain is determined after 10 minutes of recovery.

Rebound elasticity is a fundamental property of rubber and it is also one of the oldest and simplest tests. The test is usually performed as a bounce test where a weight is bounced against the rubber, either in the form of a free-falling ball or a pendulum. By comparing the height of the rebound with the original height, you get a measure of the energy absorbed as damping by the rubber.

If a rubber sample is kept stretched for a time and then relieved, it does not return completely to its original length. This is called residual strain. In this test, rubber test rods are loaded by means of weights at room temperature. After a predetermined time, the samples are unloaded and the residual strain is determined after 10 minutes of recovery.