Material tests are important for companies to be able to assess whether the newly developed component will withstand future stresses. For certain applications and industries, they also provide the required proof of quality.

Grasse Zur Composite Testing operates an accredited materials testing laboratory and specializes in testing fibre-reinforced plastics, particularly for the automotive, aerospace, chemical and rotor blade industries. Materials testing is the core business of the Berlin-based company. In addition, it develops testing equipment - on the one hand for its own processes, but on the other hand companies that operate their own laboratories can also purchase these testing devices.

Fiber-reinforced plastics are particularly suitable for lightweight construction, as they are characterized by high specific stiffness and strength. They are generally used for flat structures. The majority of tests at Grasse Zur are carried out destructively, i.e. the customer company supplies the material to be tested in the form of a cured laminate to Grasse Zur, where a test specimen is produced from it. This is destructively tested in a universal testing machine using tensile, shear, compression or bending tests to determine material characteristics.

Strain gages record changes in the specimen

Strain gauges from Althen are a key tool for recording changes in the specimen under load. They determine the strain directly on the surface of the test specimen and are prescribed by various DIN standards for determining the material characteristics of fiber-reinforced plastics.

A strain gauge is a meandering metal conductor - usually made of constantan or karma - that is etched onto a thin carrier such as polyimide. It is primarily attached to the test specimen using specific adhesives. A strain gauge acts as an electrical conductor which, when compressed or stretched, shows a change in electrical resistance, which in turn indicates a change in voltage and is thus converted into an electrical signal. The corresponding measuring device can output the voltage change in mV/V as well as directly display the actual strain of the strain gauge in µm/m.

As fiber composites are poor heat conductors, a strain gauge should always be selected from the higher impedance range, especially for long-term measurements - with at least 350 Ω, as the heat development could lead to errors in the measurement result. Grasse Zur, on the other hand, involves very short tests until the specimen tears and the strain gauge is destroyed. These small measurement errors caused by the heat development are negligible, so that strain gages with 120 Ω are also sufficient.

Larger strain gages for a good measurement result

For the best possible measurement result, Grasse Zur selects strain gages that are as large as possible, whereby large here means a measuring grid length of 10 mm; small strain gages have a measuring grid length of approx. 2 mm. The strain gages should be as large as possible in order to achieve good averaging over the tensile specimen. In the case of a fiber composite, it is unfavorable if the strain gauge only covers one fiber bundle instead of measuring across several fiber bundles. This falsifies the measurement result. The dimensions of the respective test specimen, which is usually 100 to 250 mm long and 10 to 25 mm wide, have a limiting effect on the size of the strain gage.

The strain gages are then bonded to the specimen in a longitudinal or transverse direction, depending on the test requirements: linear strain gages are mainly used to measure a uniaxial stress state. A 0/90° strain gauge (T-rosette) is used for a biaxial stress state - when forces act both in the tensile direction of the tensile specimen and offset by 90° to the right and left. If the main direction of strain is unknown, i.e. it is unclear where the forces are applied, a triaxial 0/45/90° rosette with three grids is used. Theoretically, all measurements can be carried out with individual linear strips, which the user arranges in the correct position. To make measuring easier for users, Althen offers the 0/90° and 0/45/90° variants ready-made. This means that only a single strain gauge with the corresponding number of strain gauge grids needs to be glued on.

Accurate bonding process