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 AE311/EXPERIMENTAL TECHNIQUES/Unit 2
Unbonded Non-Metallic Gauge It is a mechanically actuated gauge contains resistance element. The gauge is composed of series of plates arranged in a stack such a way that when one part of the gauge displaced with respect to another developing a change in pressure on the measuring gauge. This change in pressure changes the resistance of the element and the recorded electrically. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 -Contn.... When the strain is applied in the structure, change in length is communicated to carbon plate stack. Change is displacement creates pressure difference in the stack and finally the resistance changes. Designed by Mccollum & Peter in 1923 & 1924. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 Applications To determine displacement, loads, strain in flexible cables, airplanes, bridges, vibrating members, dynamometers, pressure gauges. The disadvantages are Mechanical Friction Hyserisis AE311/EXPERIMENTAL TECHNIQUES/Unit 2
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. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 -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. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2
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. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 When the Carbon Coating Stretched, the carbon particles move apart. If the carbon coating compressed the particles would move closer creates change in resistance. This change in resistance is measured as strain. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 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. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 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. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 -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 AE311/EXPERIMENTAL TECHNIQUES/Unit 2
Advantages of Foil type strain gauge The width of the foil at the end of each loop increased to reduce sensitivity of the gauge to transverse strains. The cross-section of the gauge conductor is rectangular resulting in high ratio of surface area to cross section area. Gauge factor is higher by 4-10 % than other gauges. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 -Contn.. Easier to manufacture. Easy measure strain in curved surface. Suitable for static & dynamic strain. Very good fatigue properties. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
Weld able strain gauges AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 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. AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2 -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 AE311/EXPERIMENTAL TECHNIQUES/Unit 2
Piezo resistive strain gauge Crystals of Silicon, Germanium, quartz and Rochelle salt shows the change in resistance when deformed by applying pressure AE311/EXPERIMENTAL TECHNIQUES/Unit 2
AE311/EXPERIMENTAL TECHNIQUES/Unit 2