Sec. 1-2] MECHANICAL INPUT TRANSDUCERS 35 



The relationship between the fractional resistance variation and 

 the applied strain e is linear. 



The gauge factor G is determined from measurements under standard 

 conditions, i.e., by cementing the bonded strain gauge to a specimen 

 with a Poisson ratio of 0.285 and subjecting it to a strain in the speci- 

 men along the gauge axis. The deviation from linearity is in the 

 order of 1 per cent. 



The maximum range of displacement is usually of the order of 

 3 X 10 _3 in. (~70 /*); the forces needed to produce maximum output 

 are in the order of 10 to 100 oz. 



The gauge resistance is, in general, between 50 and several 

 thousand ohms. The practically obtained resistance variations are in 

 the order of 1 per cent for a maximum range of displacement. The 

 output-voltage level depends, of course, upon the input voltage used 

 to measure the resistance variation; for an input voltage of 10 volts 

 one can expect an output voltage between 10 and 100 raV. An 

 accuracy as high as 0.1 per cent can be achieved. 



The frequency-response characteristic extends from zero to many 

 thousands of cycles per second, although most practical gauges have 

 an upper frequency limit around several hundred cycles. Under 

 sustained stress there can be a plastic deformation which corresponds 

 to a stress up to 3.5 x 10 -5 . Of this creep, 75 per cent will arise 

 within the first 24 hr. 



Since a certain length of wire is arranged in a direction y, Figs. 

 (1-2)15 and (1-2)16, perpendicular to that of principal stress x, the 

 wire strain gauge exhibits an effect of cross sensitivity. The cross- 

 sensitivity factor, i.e., the ratio k = transverse sensitivity/longitudi- 

 nal sensitivity, is in general less than 2 per cent. 1 Gauges with- 

 out transverse sensitivity have been described by Gustafsson and 

 Huggenberger. 2 The gauges consist of parallel stress-sensitive wires 

 imbedded in a flat strip of transparent plastic and jointed alterna- 

 tively at the top and the bottom with heavier (not strain-sensitive) 

 connectors. 



The largest single source of error in using a resistance strain gauge 

 is the effect of temperature fluctuations. Temperature variation has 

 a twofold effect on resistance strain gauges; first, it will alter the 

 electrical resistance of the gauge, and second, it will cause a difference 



1 R. Baumberger and F. Hines, Proc. Soc. Exptl. Stress Anal., 2 ( 1 ), 1 13 (1944). 



2 G. V. A. Gustafsson and A. H. Huggenberger, G-H, Tepic, or Sweden 

 Gauges, Natl. Bur. Standards Circ. 528, p. 79, 1954. 



