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ELECTRICAL RESISTANCE STAIN GAUGE Recap ELECTRICAL RESISTANCE STAIN GAUGE

Characteristics of Electrical Resistance Strain Gauge The gauge should be extremely small in size. The gauge should be significant enough to record dynamic strain. Easy to attach and easy to handle. Strain Sensitivity and Accuracy should be high. It should be unaffected by temperature, vibration, humidity.

-contn.. The gauge should be capable to measure static and dynamic strain It should be possible to read the gauge either on location or remotely. The gauge should exhibit linear response to strain. The gauge and associated equipment should be available at reasonable cost. The gauge should be suitable enough to use as a sensing element or transducer system.

Types of Resistance Strain Gauge Unbonded Gauges: a) Non-Metallic b) Metallic 2. Bonded Gauges: i) Wire type ii) Foil Type 3.Weldable Gauges 4. Piezoresistive Gauges

Bond or Unbond ?

Unbonded Non-Metallic Gauge It is a mechanically actuated gauge contains resistance element. Arranged in such a way that when one part of the gauge displaced with respect to another devloping a change in pressure on the measuring gauge. This change in pressure changes the resistance of the element and the recorded electrically.

-Contn.... The gauge is composed of series of plates arranged in a stack. Change is displacement creats pressure difference in the stack and finally the resistance changes.

Applications To determine displacement, loads, strain in flexible cables, airplanes, bridges, vibrating members, dynamometers, pressure gauges. The disadvantages are Mechanical Friction Hyserisis

Unbonded Metallic Gauge The principle is based on the change in electrical resistance of a metallic wire is due to change in tension of the wire Designed by Carlson and Eaton in 1930. It is constructed by winding wire in three coils which are placed under initial tension into four arm wheatstone Bridge.

-Contn. As the compressive strain is applied the prestrain would simply be relieved and the unbonded element would remain taut. The types of the gauges are rarely used for experimental stress analysis. The mechanism is incorporated in Accelerometers and pressure pick ups.

Bonded Non Metallic Gauge A strain gauge of non- metallic resistor element is bonded (Using common glue) directly the material in which strain is to be measured. For metallic structures the surface is first coated with non-conducting material. In this type carbon coating is applied directly to the surface of the structure in which strain is to be measured.

When the Carbon Coating Stretched, the carbon particles move apart. It the carbon coating compressed the particles would move closer creates change in resistance. This change in resistance is measured as strain.

Carbon sheets are cut into strips of 6mm wide and 25mm long On each end silver band is plated to attach lead wire The sensitivity and resistance of the gauge are affected by temperature and humidity.

Bonded Metallic Gauge This type of strain gauge consist of a strain sensitive conductor mounted on a backing paper. The gauge is cemented on the surface to be tested. This gauge can be wire type or foil type.

-Contn.. The wire grid may be wither a flat type or wrap around type. Attach the lead wires to the two ends of the grid. Second layer of paper is cemented over the wire grid. In wrap type of wire gauges the wire is wound around the cylindrical core in form of helix

Disadvantages Comparatively costly Time consuming Complicated Bonding In order to overcome the limitations led to the development of weldable wire resistance strain gauge

Weld able strain gauges

Description The weldable strain gauge consist of a strain sensitive element Nickel Chrome or Platinum Tungsten housed with a small diameter stainless steel tube. The strain element is insulated with highly compacted ceramic insulation or metallic oxide powder.

-Contn… The basic construction of quarter bridge or half bridge includes integral flexible lead wire configurations. When the gauge is welded to the specimen and the test specimen put into tension or compression, the stress is transmitted through the weld to the mounting flange into the strain tube through the MgO powder

Piezo resistive strain gauge Crystals of Silicon, Germanium, quartz and Rochelle salt shows the change in resistance when deformed by applying pressure