RDS crack propagation gauges for determining crack propagation in components
HBM crack propagation gauges are used to determine crack propagation in a component. HBM offers three different types.
Types RDS20 and RDS40 consist of electrically isolated resistors. In this case individual circuits are interrupted as the crack propagates.
Type RDS22 consists of conductor tracks connected in parallel which will tear if the crack extends under the crack propagation gauge. This will gradually increase the electrical resistance of the strain gauge as the crack continues to extend.
This change in resistance can be measured using a resistance meter or strain gauge amplifier.
Connecting a crack propagation gauge
There are two different types of crack propagation gauges: Type RDS22 consists of conductor tracks connected in parallel which will tear if the crack extends under the crack propagation gauge.
This will gradually increase the electrical resistance of the strain gauge as the crack continues to extend. Types RDS20 and RDS40 consist of electrically isolated conductor tracks, i.e. as the crack extends, individual circuits will be interrupted. If the circuits are contacted individually, the direction in which the gap is extending can be detected.
The easiest way to detect the signals of crack propagation gauges (RDS) is with a resistance measurement. Direct resistance measurements of this type can be taken with many amplifiers from HBM (e.g. MGCplus or Spider8 with the appropriate modules).
The resulting resistance (R) of the RDS is dependent on the number of torn links and can easily be calculated. In this case n identical resistors (Ri) are connected in parallel:
If a grid line is interrupted, this is described by
However, the measurement can be performed just as well with a measuring amplifier for SG measurements. The circuit diagram sketch shows how the RDS has to be complemented to obtain a resistance change that is inside the measuring range of an amplifier for strain gauge quarter bridges.
To minimize the effects of temperature, temperature-stable fixed resistors or SGs should be used as completion resistors. Greater sensitivity can be obtained by selecting a parallel resistor with a higher resistance value.
Tips for selecting strain gauges
The right measuring grid length: The measuring grid length depends on the aim of measurement, since the result of a measurement with strain gauges (SGs) will be determined as the average of strains. In general, measuring grid lengths of 3 to 6 mm represent a good solution.
Long measuring grids are recommended where there is an inhomogeneous material such as e.g. concrete or wood. A long strain gauge will bridge the inhomogeneities of the workpiece and return the strain underneath the measuring grid as the measurement result.
Short measuring grids are suitable for detecting a local strain state. They are therefore suitable for determining strain gradients (strain gauge chains), the maximum point of notch stresses and similar stresses.
The right resistance: The selection of the resistance depends on the constraints of the measurement task. 120-ohm strain gauges are relatively insensitive to fluctuations in the insulation resistance, for example due to the effects of moisture.
The advantage of higher impedance strain gauges is that they produce less specific heat due to the lower measuring current. In addition they are less sensitive to resistances in the connecting cables to the measuring amplifier. There is also a disadvantage that higher impedance strain gauges may be more sensitive if noise pulses are received.
Customer-specific strain gauges
Do you have special requirements that are not adequately covered by any of the strain gauges in our standard program? Are you looking for an equivalent strain gauge for the product you are current using? Have you designed your own strain gauge? Contact us, we will produce customized strain gauges according to your requirements! From quantities of 20 packages and up.
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