Series of articles: Measurement accuracy in experimental stress analysis - Part 1

Strain gauge technology with its ample opportunities for error compensation has been optimized for decades. And yet there are influences that may affect strain gauge measurements. The aim of this article is to point out the many (often avoidable) sources of error when strain gauges are used in experimental stress analysis and to provide assistance so that measurement uncertainty can be assessed already in the design stage.

Fundamental questions

The following observations that may be useful prior to taking strain gauge measurements in experimental stress analysis are to summarize the authors' experiences. The following questions are essential to the required measures (e.g. measuring point protection) and the measurement uncertainty that can be obtained:

  • When will the measuring point reach the end of its useful life?
  • How high will the strain values be?
  • Will there be any temperature variation? If yes, how great and how fast?
  • Will special environmental influences (water, humidity, etc.) affect the measuring point?
  • What material is the strain gauge being installed on (inhomogeneous, anisotropic, highly hygroscopic, etc.)?
  • Is there any possibility to readjust the zero point, if necessary?

The experienced test engineer will be looking for the answers already when analyzing the measurement task (long before the first strain gauge is being installed). The answer to the last question decides whether the measurement is zero-point related or non zero-point related.

Zero-point related measurements

Zero-point related measurements are generally understood as measurements involving comparison of current measured values with measured values obtained at the start of measurement over several weeks, months or even years. No "zero balancing" of the measurement chain is performed in the meantime. Zero-point related measurements are far more critical than non zero-point related measurements, because zero drifts (resulting from temperature and other environmental influences) are fully incorporated into the result of measurement.

Zero errors are particularly dangerous with small strain values, because this results in very large relative deviations related to the measured value. Strains occurring in machine components and structures often do not even amount to 100 µm/m, because a high safety factor is "built in". 100 µm/m zero drift, in this case, results in 100 % measurement error.

Due to the fact that a continuous measurement for structural monitoring is almost always a zero-point related measurement, special attention needs to be paid to protecting the strain gauges from environmental influences. It is essential that the measuring point offers sufficient long-term stability. Since large temperature variations have to be expected, the temperature coefficients need to be small. Low measurement signal amplitudes at generously dimensioned components are likely to be superimposed by effects resulting from deficient strain gauge installation. The measurement electronics responds to every change in resistance with a change on its display.

This may be due to the change in the quantity to be measured or, also, the ingress of water molecules. The actual measured value, as the aggregate signal of all strain proportions at the strain gauge, does not allow a distinction to be made between wanted and unwanted strain proportions.



Non zero-point related measurements

Non zero-point related measurements are understood as measurement tasks that allow zero balancing without any information loss at specific points in time. Only the variation of the measured quantity after "zero balancing" is relevant.  (Modern bathroom scales are automatically tared every time they are switched on, without any loss of information.) "Zero balancing" is often possible with one-off load tests (often in the form of short-term measurements), hence zero drifts are totally insignificant.

Very high strains occur in destructive tests, which means that strain gauges with adequate measuring ranges are required. It is embarrassing and costly when after weeks of preparatory work it becomes obvious that the strain gauges installed at the component have failed.

Measurements in laboratories and test halls are considered rather uncritical, because the ambient conditions (temperature, humidity) are moderate.

Measurements in the field and in environmental chambers with high humidity and large temperature gradients, however, are critical.



Experimental stress analysis

Experimental stress analysis enables mechanical stresses in components to be measured. Experimental stress analysis can be performed to measure stress due to three types of causes: external forces, residual stresses, and thermal stresses.

Loading stress is due to forces applied from outside that cause material loading. Residual stress is due to internal forces in the material, without any external forces being involved. Residual stress arises from non-uniform cooling of cast components, forging, or welding. Thermal stresses occur in systems in which parts with different thermal expansion coefficients are used. They can arise if free thermal expansion of the components is prevented, or as a result of non-uniform heating in the same way as loading stress.

Depending on their absolute value and sign, residual and thermal stress can reduce a component’s loading capacity with respect to external loads.

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Read on ...

Read more about this topic in Part 2 of our series of articles on "Measurement accuracy in experimental stress analysis".

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