By Karl Hoffmann

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Extra resources for An Introduction to Measurements using Strain Gages

Example text

The residual stress has the opposite sign. It has been found that it is unnecessary to completely mill out a core from the object under investigation, particularly if compact structural parts are involved and where access from both sides, which is only available with plates, is not possible. It is sufficient to establish a relatively shallow, ring-shaped groove that adequately releases the stress on the core's surface, see Fig. 2-11. With the “ring-core method”, as this is known, the restrictions of the method regarding the part's thickness no longer apply.

4 3. 7 4. 3 5. g. 0-1: Chart for the analysis of conditions which must be fulfilled by a strain gage measurement point. 36 3 Selection criteria If as much as possible is known about the conditions affecting the measurement, then the required cost and effort can be more easily assessed and the chance of success is also greater. Unpredictable results can never be completely eliminated, but they can be reduced to a minimum. Knowledge of strain gage characteristics is needed to fulfill the 2nd requirement for the selection of the optimum gage.

Example: The significance of percentage figures in conjunction with strain values. 5% however means that if percent is used as a measure of the strain: 27200 µm/m ±5000 µm/m! Pseudo-units, such as “microstrain”, “µD” or “µİ” have no mathematical basis and do not conform to dimensional calculations. In particular the designation µİ, which originates in America, gives misleading results, as shown by the example “İ = 250 µİ”. It is poor mathematical practice to use formula symbols (İ for strain) as dimensional units.