Characteristics of Strain Gauges

7/28/2019 Characteristics of Strain Gauges

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In the name of Allah the most beneficent the most merciful



The development of strain gages hasfollowed many different paths, and gages

have been developed based on• electrical,

• mechanical,

•

optical,• acoustical, and

• pneumatic principles.

Yet in spite of the very wide variationsin the strain-gages designs, there existfour basic characteristics which each gageexhibits.



Characteristics

•Gauge length

•Gauge sensitivity

•Range

• Accuracy or reproducibility



Gauge length• Strains cannot be measured at a point with any type of

gage, and as a consequence nonlinear strain fieldscannot be measured without some degree of error beingintroduced. In these cases, the error will definitelydepend on the gage length and may also depend on thegage width .The gage size for a mechanical strain gage

is characterized by the distance between the two knifeedges in contact with the specimen and by the width of the moveable knife edge.

The gage length of the metal film resistance straingage is determined by the length of the straight portionof the grid, and the width is determined by the width of the grid. In selecting a gage for a given application,gage length is one of the most important considerations.



Gauge sensitivity•

A second basic characteristic of a strain gage is itssensitivity .Sensitivity is the smallest value of strainwhich can be read on the scale associated with thestrain gage.

• The term sensitivity should not be mistaken for accuracyor reproducibility, since very large values of magnification can be designed into a gage to increase itssensitivity; but friction, wear, and deflection introducelarge errors which limit the accuracy.

• In certain applications gages can be employed withsensitivities of less than 1 uin/in.if proper procedures are

established. In other applications, where sensitivity isnot important, 50 to 100 uin/in is often quite sufficient.The choice of a gage is dependent upon the degree of sensitivity required, and quite often the selection of agage with a very high sensitivity ,which is not really

necessary, needlessly increase the complexity of themeasuring method



• Electrical Strain Gauge is a resistive elasticunit whose change in resistance is afunction of applied strain.

• where R is the resistance, e is the strain,

and S is the strain sensitivity factor of thegage material (gage factor).



• Consider a wire strain gage. The wire iscomposed of a uniform conductor of electricresistivity with length l and cross-section area.Its resistance R is a function of the geometrygiven by

The resistance change rate is a combination

effect of changes in length, cross-section area,and resistivity.



•The resistance change rate becomes .



• For a given material, the sensitivity of resistance versus strain can be calibrated bythe following equation.



Range

• A third basic characteristic of a strain gage is itsrange. Range represents the maximum strainwhich can be recorded without resetting orreplacing the strain gages. The range and

sensitivity are interrelated since very sensitivegage respond to small strains with appreciableindicator deflections and the range is usuallylimited to the full scale deflection of theindicator. Often it is necessary to compromisebetween the range and sensitivity characteristicsof gage to obtain reasonable performance for

both of these categories.



Accuracy or reproducibility• The final basic characteristic is the accuracy or

reproducibility. As was previously pointed out,sensitivity does not ensure accuracy. Usually thevery sensitive instruments are quite prone toerrors unless they are employed with the utmost

care. In a mechanical strain gage, inaccuraciesmay result form lost motion such as backlash ingear train, friction, temperature changes, wearin the mechanism, slippage, or flexure or

deflection of the components. On all straingages there is a readout error whether the gageis manually recorded or the output is read on adigital printer.

Characteristics of Strain Gauges

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