Strain gauges

When a gauge is subjected to a positive strain , its length increases while its area of cross-section decreases.

Resistance of a conductor is directly proportional to its length and inversely proportional to its area of cross section.

The resistance of the gauge increases with positive strain.

Strain gauges

Types of strain gaugesUnbounded metal strain gauges

Bonded metal strain gauges

Bonded metal foil strain gauges.

Bonded semiconductor strain gauges

Vacuum deposited thin metal strain gaugesDiffused metal strain gauges

Strain gauges

Unbounded metal strain gaugesThis gauge consists of a wire stretched between two points in an insulating medium such as air.The wire may be made of various metal such as copper nickel, chrome nickel, nickel iron alloys etc.Dia. is about 0.003mm , gauge factor of 2 to 4, length of wire is 25 mmAt initial condition , the strains and resistances of four arms are nominally equal, with the result the o/p voltage of the bridge e0 =0

Strain gauges

Unbounded metal strain gauges

Strain gauges

Unbounded metal strain gaugesApplication of pressure increases the resistance of the gauge , which causes an unbalance of the bridge producing an output voltage .Bonded wire strain gaugesConsists of a fine wire of about 0.025 mm dia. The grid is cemented to carrier(base) which may be a thin sheet of paper or Bakelite or sheet of Teflon.The carrier is bonded with an adhesive material to the specimen under study.This permits a good transfer of strain from carrier to the grid wires.

Strain gauges

Bonded wire strain gauges

Strain gauges

For excellent and reproducible result ………..The strain gauges should have a high value of gauge factor,

The resistance of the strain gauge should be as high as possible since this minimizes the effects of undesirable variations of resistance in the measurement circuit.

The strain gauges should have a low resistance temp. co-efficient.Should have a linear characteristics.

Base materialsEpoxy,Bakellite cellulose etc.

Strain gauges

Bonded metal foil strain gauges

Strain gauges

Bonded metal foil strain gauges use identical materials to wire strain gauges and are used today for most general purpose stress analysis application

Foil type gauges have a much greater heat dissipation capacity .Used for higher operating temp.

Foil gauges are formed from sheets less than 0.005 mm thick .Foil type gauge are mounted on a flexible carrier film about 0.025 mm thick which is made of glass phenolic

Strain gauges

Semiconductor strain gaugesAre used where a very high gauge factor .The resistance of semi-conductor changes with change in applied strain.Also change in resistance due to change in resistivity also.Semi-conducting material such as silicon and germanium are used as resistive materials Gold leads are generally employed for making the contacts.Electrical resistivity is the measure of how much a material resists carrying an electrical current.

Strain gauges

Semiconductor strain gauges

Strain gauges

RosettesA combination of strain gauges.In practical situation, it is not possible to locate the

principal stresses.It is not possible to orient the strain gauges along the

direction of principle stresses.Hence there is a necessary to evolve a strain gauge

measurement system which measures the values of principle strain and stresses without actually knowing their directions.

Strain gauges

Rosettes

Strain gauges

Temperature compensationPoisson's methodThis temp. compensation utilizes two active gauges Rg1 and Rg3 which are bonded at right angles to the structure member .Rg1 is in tension , but the gauge Rg3 reduces in length depending up o the poissons ratio, in the adjacent arm of the bride.The output of the bridge circuit is increased by a factor of (1+γ)e0 =( 1+ γ)Gf ε ei /4

Strain gauges

Temperature compensationPoisson's methodThis temp. compensation utilizes two active gauges Rg1 and Rg3 which are bonded at right angles to the structure member .Rg1 is in tension , but the gauge Rg3 reduces in length depending up o the poissons ratio, in the adjacent arm of the bride.The output of the bridge circuit is increased by a factor of (1+γ)e0 =( 1+ γ)Gf ε ei /4

Strain gauges

Temperature compensationSelf temp. compensationIn many applications it may be difficult to obtain temp. compensation by adjacent arm compensating gauge acting either as a dummy gauge or a compensating gauge.This case may arise , where it is not possible to keep the gauge s at the same temp.Two type of self –compensated gauges (1)Selected melt gaugeBased up on the fact that through proper manipulation of alloy and processing , usually cold working, some control over temp. sensitivity of the grid material may be obtained.

Strain gauges

Temperature compensationTwo type of self –compensated gauges

(2) Dual element gaugeIn this case of self compensation two wire elements are connected in series to make one strain gauge assembly.The two elements have different temp. characteristics and are so chosen that the net strain produced on account of temp. changes is minimized.

Strain gauges

Load cellsDevice that can be used for force measurement .An elastic member is used as a primary transducer and

strain gauges a secondary transducer. The factors which are considered in the design of load

cells using strain gauges are stiffness of elastic member , optimum positioning of gauges on the element and provision for temp. compensation.

Tensile compressive cellThe arrangement uses four strain gauges each mounted at

90º to each other.Two strain gauges experience tensile stresses while other

two are subjected to compressive stress.

Strain gauges

Load cellsTensile compressive cell

In case all the gauges are similar temp. compensation is obtained as all gauges contribute equal to unbalance the bridge.

Torque measurement methods using strain gauges

Dynamic measurement of torque transmitted by a rotating shaft is based up on the angular displacement or twist in the shaft.

e0 =( 1+ γ)Gf ε ei /4

Strain gauges

Load cellsTorque measurement methods using strain gauges

Strain may be measured by electrical means to indicate torque.

The strain in the shaft may be measured by means of strain gauges attached to its surface

The gauges should be so mounted that they give max. sensitivity to the strain produced by torsion.

e0 =( 1+ γ)Gf ε ei /4

Strain gauges

Load cellsTorque measurement methods using strain gauges

Multiple strain gauges may be installed and connect in a bridge circuit configuration so that any deformation due to axial loads is cancelled out in the final readout.

e0 =( 1+ γ)Gf ε ei /4

• Temperature compensation