Gage Length

The gage length of a strain gage is the active or strain-sensitive length of the grid. The endloops and solder tabs are considered insensitive to strain because of their relatively large cross-sectional area and low electrical resistance. To satisfy the widely varying needs of experimental stress analysis and transducer applications, the Micro-Measurements Division offers gage lengths ranging from 0.008 in (0.2 mm) to 4 in (100 mm).

Strain Concentration

Gage length is often a very important factor in determining the gage performance under a given set of circumstances. For example, strain measurements are usually made at the most critical points on a machine part or structure - that is, at the most highly stressed points. And, very commonly, the highly stressed points are associated with stress concentrations, where the strain gradient is quite steep and the area of maximum strain is restricted to a very small region. The strain gage tends to integrate, or average, the strain over the area covered by the grid. Since the average of any nonuniform strain distribution is always less than the maximum, a strain gage which is noticeably larger than the maximum strain region will indicate a strain magnitude which is too low. The sketch illustrates a representative strain distribution in the vicinity of a stress concentration, and demonstrates the error in strain indicated by a gage which is too long with respect to the zone of peak strain.

As a rule of thumb, when practicable, the gage length should be no greater than 0.1 times the radius of a hole, fillet, or notch, or the corresponding dimension of any other stress raiser at which the strain measurement is to be made. With stress-raiser configurations having the significant dimension less than, say, 0.5 in (13 mm), this rule of thumb can lead to very small gage lengths. Because the use of a small strain gage may introduce a number of other problems, it is often necessary to compromise.

Short Gages

Strain gages of less than about 0.125 in (3 mm) gage length tend to exhibit degraded performance - particularly in terms of the maximum allowable elongation, the stability under static strain, and endurance when subjected to alternating cyclic strain. When any of these considerations outweigh the inaccuracy due to strain averaging, a larger gage may be required.

Longer Gages

When they can be employed, larger gages offer several advantages worth noting. They are usually easier to handle [in gage lengths up to, say, 0.5 in (13 mm)] in nearly every aspect of the installation and wiring procedure than miniature gages. Furthermore, large gages provide improved heat dissipation because they introduce, for the same nominal gage resistance, lower wattage per unit of grid area. This consideration can be very important when the gage is installed on a plastic or other substrate with poor heat transfer properties. Inadequate heat dissipation causes high temperatures in the grid, backing, adhesive, and test specimen surface, and may noticeably affect gage performance and accuracy (see Vishay Measurements Group Tech Note TN-502, Optimizing Strain Gage Excitation Levels ).

Strain Averaging

Still another application of large strain gages - in this case, often very large gages - is in strain measurement on nonhomogeneous materials. Consider concrete, for example, which is a mixture of aggregate (usually stone) and cement. When measuring strains in a concrete structure it is ordinarily desirable to use a strain gage of sufficient gage length to span several pieces of aggregate in order to measure the representative strain in the structure. In other words, it is usually the average strain that is sought in such instances, not the severe local fluctuations in strain occurring at the interfaces between the aggregate particles and the cement. In general, when measuring strains on structures made of composite materials of any kind, the gage length should normally be large with respect to the dimensions of the inhomogeneities in the material.

Summary

As a generally applicable guide, when the foregoing considerations do not dictate otherwise, gage lengths in the range from 0.125 to 0.25 in (3 to 6 mm) are preferable. The largest selection of gage patterns and stock gages is available in this range of lengths. Furthermore, larger or smaller sizes generally cost more, and larger gages do not noticeably improve fatigue life, stability, or elongation, while shorter gages are usually inferior in these characteristics.

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