Elongation Measurement

Materials can deform under thermal and mechanical effects. An example of such a change in shape is elongation. This is the term used to describe the relative change in length of a component or material under mechanical stress (force) or through heat and cold. If force is exerted on a component from the outside, it is elongated (positive elongation, elongation). Elongations that occur as a reaction to the application of force cause the material to deform. If the component is exposed to pressure, it is compressed (shortened, negative elongation). If the material experiences a change in temperature that increases its dimensions, it is known as thermal elongation. High temperatures cause positive thermal elongation, cold a negative one. In addition, there is stretching due to internal stress. These deformations occur when components are forged and welded. In addition, there are elongations that are caused by a magnetic field or an electric field.

To calculate the elongation of a material, the change in length is divided by the original length and given in micrometers per meter (μm / m). With many materials, the elongation is proportional to the force acting. Stretching can occur in the longitudinal direction or as a result of a transverse contraction across the direction of force. If tensile, compressive and shear forces work together, the result is elongation in all directions. These complex deformations can also be simulated on the computer.

Materials differ in terms of their elongation: Steel deforms less when force is applied than rubber. Titanium does not elongate as much under the influence of heat as aluminum. The cause of the elongation of components is indicated by material coefficients or modules. When it comes to mechanical stress, the elongation is represented by the modulus of elasticity. The coefficient of thermal elongation describes elongation due to the action of heat. A large number of materials expand evenly in all directions. In contrast to this, the elongation caused by mechanical stress usually takes place in the direction of this force. Elongation can be calculated and also measured experimentally.

The methods of elongation measurement that are predominantly used today are electrical and optical measurement with the help of elongation measuring strips (EMS). If the EMS is made of metal foil, it can be used to measure elongations of 1/100 to 1/10 μm / m. Semiconductor EMS enable precise detection of the change in length in the range between 1/1000 and 1/100 μm / m. The elongation measuring strip always indicates the mean elongation of the material to which it is attached with a special adhesive. Depending on the environmental conditions, EMS of different sizes are used.

Electrical elongation measuring strips are also known as film elongation strips. They have been available for more than 80 years and consist of two thin polyamide foils with an integrated measuring grid made of constantan. Bridge circuits are usually used for the measurement. Instead of a metal measuring grid, a silicon grid can also be used (semiconductor EMS). These elongation measuring strips are much more sensitive than metallic EMS. Electrical measuring strips are between 0.2 and 150 mm in size. With conventional measurement, deviations between 0.1 and 1% from the full scale value are possible.

When stretching, the resistance in the measuring grid increases, so that it deforms. The sensitivity in detecting the deformation varies with semiconductor EMS according to the crystal orientation and the silicon (n or p). These EMS enable error-free measurement results in the frequency range from 5 to 8 MHz. The maximum operating voltage depends on the size of the measuring strip and the material. Common sizes of EMS that adhere to good heat conductors can withstand 5 to 10 V. The optical elongation measurement takes place with the help of fiber optic sensors (FOS), which are glued or welded to the respective material.

These optical elongation measuring strips are also known as fiber Bragg grating sensors. They are insensitive to electromagnetic interference and other unfavorable conditions. They are therefore used when the electrical EMS cannot be used, for example at temperatures between -270 and 300 °C. The optical EMS has a plastic-coated quartz glass fiber as a core, which is surrounded by a denser jacket and a protective plastic coating. The fiber contains several fiber Bragg gratings. If the laser light introduced from outside via an interrogator hits this grid, some of the light beams are reflected and sent back to the interrogator. From this, the stresses within the material and the deformations can be determined.

If the fiber is stretched during the elongation measurement, the distances between the grid parts increase. At the same time, the wavelength of the reflected light changes. Because each fiber can contain innumerable Bragg gratings, this elongation measurement is suitable for monitoring pipelines and tunnels. In contrast to electrical elongation measurement, in which each EMS has to be connected to a separate connection cable, a single glass fiber is sufficient for optical EMS. This saves installation effort and costs.

Elongation measurements have the advantage that even extremely small changes in shape and stress can be determined extremely precisely and with pinpoint accuracy and that they can be used almost universally with the help of different elongation measuring strips. Components can be monitored for years. Elongation measurements can also be carried out on complex components (housing made of die-cast aluminum, turbine runners) and under water with the help of the EMS.

The elongation measurement can also be carried out using optical measuring systems with high-resolution cameras such as the ARAMIS system from ZEISS. The material to be tested only needs to be marked beforehand with a measuring grid by a marking laser. While the material is being deformed, the two cameras take photos of it. The 3D elongation measurement can also be carried out on components with complex geometry. Based on the measured pixel coordinates of the applied pattern, the elongations can be precisely calculated using special ZEISS software. The optical measuring system can be permanently mounted on the respective testing machine.

Elongation measurements are carried out using different methods that are restricted to specific areas of application. The measurements themselves are versatile and can be used in various industries.