Strain gauge | Terms releted to strain gauge | Types | materials | Characteristics

Strain gauge

Term Related to Strain gauge

1) Cross sensitivity :
In mounting if strain gauges most of the strain sensitrve filament is made aligned with the sensitive axis of the gauge, But due to some unavoidable reasons, a part of the grid gets aligned transversely.

Therefore this portion of the grid senses the strain in the transverse direction and the output on account of that is superimposed on the longitudinal output. This is called cross sensitivity.

2) Creep : When the measurement are carried out over a long duration of time     -creep is considered only in static strain gauge testing. When the gauge is subjected to stress only once and loading cycle is not,repeated or it is subjected to slow varying loading cycles. Creep exists       in gauge.
3) Fatigue : Strain gauge may fail because of fatigue loading. The fatigue sensitive area is the discontinuity      formed at the junction at the grid and the lead wire where the connections are made usually. Isoelastic is better than constantan carrier material is an important factor in determining the life at gauge under fatigue conditions.
4) Post yield gauge : Post yield gauges are those whose usable range extended to approximately 10 to 20 percent. A very ductile grid material is used. This is known as development ofelastic plastic grid.

Types of strain Gauge :

Strain gauge
A) Unbounded
     a) Metallic wire
B) Bonded
     a) Semi- Conductor
     b) Metallic
          i) Helical wrap wound on a flattened grid
          ii) Etched following gauge : Single element , Multiple element
         iii) Wire type flat grid : Single element, multiple element
        iv) Wire type woven grid.

Strain Gauge Material :

               Following are some  of the popular metal alloys used for strain gauge sensing element :
1) Constantan - Nickel 45%, Chromium 55%
2) Advance - Copper 57% ,  Nickel 43%
3) Isoelastic - Iron 52%, Nickel 36%, Chromium 8%, Molybdinum 0.5 %
4)Nichrome - Nickel 80%, Chromium 20%

 Deslrable characterIstIcs of grId materIal :
1) It should have high gauge factor which should remain constant over a wide strain and temperature rangeB.
2) It should have high specific resistance.
3) It should have low temperature sensitivity.
4) It should have low hysteresis.
5) It should have good workability, solderability and weldability.
 The grid material commonly used are :

(a)   Advance material:  

It is 55%  copper, 45%  nickel having gauge factor 2. It is most commonly used as it has reasonable gauge factor. It can be easily worked and soldered.

(b) Isoelastic :
1) It is 36%  nickel, 8%  copper, 4% Mn, Si and molybdenum and rest is iron.
2) It has gauge factor 3.5.
3) It has high gauge factor. It is usefulin dynamic measurement.

(c)  Nichrome : It is nickel, chromium alloy having gauge factor 2.

(d)   Manganin :  It  has  0.47  gauge  factor and low temperature coefficient.

(e) Monel: It has high temperature coefficient and gauge factor as 1.9.

(f) Nickel  : It has negative gauge factor (- 12). It exhibits reduced resistance through length increases and diameter       decreases

Supporting or backing material :        Desirable characteriatics of support material :  

1) Minimum thickness.
2) High mechanical strength.
3) High dielectric strength.