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strain gauges

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strain gauges

  1. 1. Applications of Resistance Strain Gauges in Measurements Deepak Garg 13209010 ME 1st Year
  2. 2. STRAIN GAUGES • A Strain Gauge is a device used to measure the strain of an object • The gauge is attached to the object by a suitable adhesive • As the object is deformed, the foil is deformed, causing its electrical resistance to change • The resistance change is commonly measured using a Wheatstone bridge • The most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern
  3. 3. What’s the Wheatstone Bridge? • Wheatstone bridge is an electric circuit suitable for detection of minute resistance changes., therefore used to measure resistance changes of a strain gage • The bridge is configured by combining four resistors as shown in Fig. • Initially R1=R2=R3=R4, in this condition no output voltage is there, e=0 • When one of the Resistances is replaced by strain Gauge attached to the object whose strain is to be measured and load is applied, then there is small change in the resistance of gauge, hence some output voltage is there which can be related to strain as From this, strain can be easily determined using the relation
  4. 4. Half Bridge Configuration To increase the sensitivity of measurement, two strain gauges are connected in the bridge, this type of configuration is called as Half bridge as shown in fig. and the output voltage and strain can be related as When gauges are connected to adjacent arms and When gauges are connected to opposite arms
  5. 5. Full Bridge Configuration To further enhance the sensitivity, all 4 resistances are replaced by strain gauges. While this system is rarely used for strain measurement, it is frequently applied to strain-gage transducers. When the gages at the four sides have their resistance changed to R1 + ΔR1, R2 + ΔR2, R3 + ΔR3 and R4 + ΔR4, respectively, the bridge output voltage, e, is Or
  6. 6. Applications of 2-gage system (Strain Cantilever) The 2-gage system is mostly used for the following Case •To measure the bending Strain •To measure the tensile strain To measure the Bending Strain, Configuration 1 is used as shown in fig I because output voltage from the circuit would become if fig II is used. To measure the Tensile strain, Configuration shown in Fig II is used and not fig I as output voltage from circuit become zero in the case of tensile loading. Fig I Fig II
  7. 7. Temperature Effects and Need for Temperature Compensation Measurements are performed with strain gauges in mechanical stress analysis to examine loading and fatigue. In addition to the desired measurement signal indicating mechanical strain, each strain gauge also produces a temperature-dependent measurement signal. This signal, called the apparent strain, is superimposed on the actual measured value. Various effects contribute to the apparent strain: • Thermal expansion of the measurement object (i.e. strain due entirely to temperature with no mechanical loading as the cause) • Temperature-dependent change in the strain gauge resistance • Thermal contraction of the strain gauge measuring grid foil • Temperature response of the connection wires
  8. 8. Methods For Temperature Compensation •Active Dummy method •Temperature Response matching or self compensation method •by connecting several strain gauges together to form a half or full bridge Active Dummy Method •The active-dummy method uses the 2-gage system where an active gage, A, is bonded to the measuring object and a dummy gage, D, is bonded to a dummy block which is free from the stress of the measuring object but under the same temperature condition as that affecting the measuring object. The dummy block should be made of the same material as the measuring object. •As shown in Fig, the two gages are connected to adjacent sides of the bridge. Since the measuring object and the dummy block are under the same temperature condition, thermally-induced elongation or contraction is the same on both of them. Thus, gages A and B bear the same thermally-induced strain, which is compensated to let the output, e, be zero because these gages are connected to adjacent sides.
  9. 9.  Self-Temperature-Compensation Method • Theoretically, the active-dummy method described above is an ideal temperature compensation method. But the method involves problems in the form of an extra task to bond two gages and install the dummy block. To solve these problems, the self- temperature-compensation gage was developed as the method of compensating temperature with a single gage. • With the self-temperature-compensation gage, the temperature coefficient of resistance of the sensing element is controlled based on the linear expansion coefficient of the measuring object. Thus, the gage enables strain measurement without receiving any thermal effect if it is matched with the measuring object. • The apparent strain that comes into play as the temperature changes can be represented in a simplified manner as follows
  10. 10. Where: εs = apparent strain of the strain gauge ∝r = temperature coefficient of the electrical resistance of the measuring grid foil ∝b = thermal expansion coefficient of the measurement object ∝m = thermal expansion coefficient of the measuring grid material k = gauge factor (sometimes called k factor) of the strain gauge ∧ϑ = temperature difference that triggers the apparent strain •The temperature coefficient of the electrical resistance of the measuring grid foil is adapted by technical production measures so that the terms of the equation cancel each other out; thus r = ( m - b) • k.∝ ∝ ∝ •Accordingly, there are different types of strain gauges that are identical in terms of geometry and resistance values, but differ in temperature response matching for the material on which the strain gauge is installed. Temperature response matching to a wide range of thermal expansion coefficients is available (for example, to ferritic steel with a thermal expansion coefficient of 10.8 • 10-6/K, or aluminum with 23 • 10-6/K).
  11. 11. Applications of strain Gauges Strain gauges are basically strain transducers which converts the mechanical signals into electrical signals and hence measure the strain produced. This strain can be utilized further to measure the following quantities as given: •Force •Torque •Pressure •Flow Rate •Residual Stresses
  12. 12. Measurement of Force •Force can be measured using strain gauge load cells •A load cell is a transducer that is used to convert a force into electrical signal •A load cell is made by bonding strain gauges to a spring material. To efficiently detect the strain, strain gauges are bonded to the position on the spring material where the strain will be the largest •Two gauges are along the direction of applied load and other two are at right angle to these. •When there is no load, all gauges have same resistance and bridge is balanced •When load is applied, there is change in resistance and hence some output voltage is there which is the measure of applied load. e= V/2*(1+µ)*(K*P)/(A*E) P= Load to be Measured Tension-compression resistance strain-gage load
  13. 13. Pressure Measurement • Use elastic diaphragm as primary pressure transducer • Apply strain gage directly to a diaphragm surface and calibrate the measured strain in terms of pressure • Pressure is measured through force that is exerted on the diaphragm where the force will be detected by the strain gauge and resistance change will be produced Location of strain gages on flat diaphragm The central gage is subjected to tension while the outer gage senses compression
  14. 14. Flow Measurement
  15. 15. Torque Measurement • Four bonded-wire strain gauges are mounted on a 450 helix with axis of rotation and place in pairs diametrically opposite as shown in figure • If gauges are accurately placed and have matched characteristics, the system is temperature compensated and insensitive to bending, thrust or pulls • Any change in resistance is purely due to torsion of shaft, hence the torque can be determined by measuring change in voltage which can be written as T=e/(V*K)[J*E/r(1-µ)] Where e= Change in Voltage V=Applied Voltage K=Gauge Factor J=Polar Moment of Inertia E=Young’s Modulus r= Radius of Member
  16. 16. Amplification and Digitization of Output
  17. 17. Electronic Circuitry for Gain and Digitization
  18. 18. Measurement of Cutting Force and Torque in Drilling By Drill Tool Dynamometer
  19. 19. Measurement of Residual Stresses by Hole-Drilling Strain Gage Method The most widely used modern technique for measuring residual stress is the hole-drilling strain-gage method of stress relaxation, Shown in fig. Briefly summarized, the measurement procedure involves six basic steps: •A special three element strain gage rosette is installed on the test part at the point where residual stresses are to be determined •The gage grids are wired and connected to a multi channel static strain indicator •After zero-balancing the gage circuits, a small, shallow hole is drilled through the geometric center of the Rosette •Readings are made of the relaxed strains, corresponding to the initial residual stress •Using special data-reduction relationships, the principal residual stresses and their angular orientation are calculated from the measured strains Three-Element Rosettes
  20. 20. Through-Hole Analysis Depicted in Figure (a) is a local area within a thin plate which is subject to a uniform residual stress, σx. The initial stress state at any point P (R, α) can be expressed in polar coordinates by: Figure (b) represents the same area of the plate after a small hole has been drilled through it. The stresses in the vicinity of the hole are now quite different which can be given as:
  21. 21. Subtracting the initial stresses from the final (after drilling) stresses gives the change in stress, or stress relaxation at point P (R, α) due to drilling the hole. That is:
  22. 22. Selection and Installation Factors for Bonded Metallic Strain Gages •Grid material and configuration •Backing material •Bonding material and method •Gage protection •Associated electrical circuitry
  23. 23. Desirable Properties of Grid Material •High gage factor, F •High sensitivity •Low temperature sensitivity •High electrical stability •High yield strength •High endurance limit •Good solderability or weldability •Low hysteresis •Low thermal emf when joined to other materials •Good corrosion resistance
  24. 24. Properties of Common Grid Materials
  25. 25. Common Backing Materials •Thin paper •Phenolic-impregnated paper •Epoxy-type plastic films •Epoxy-impregnated fiberglass •Most foil gages use an epoxy film backing
  26. 26. Bonding Procedure • Select Strain Gauge The two primary criteria for selecting the right type of strain gauge are sensitivity and precision. So Select the strain gauge model and gage length which meet the requirements of the measuring object and purpose • Remove Dust and Paint Using a sand cloth polish the strain-gage bonding site over a wider area than the strain-gage size. Wipe off paint, rust and plating, if any, with a grinder or sand blast before polishing • Decide Bonding Position Using a pencil or a marking-off pin, mark the measuring site in the strain direction. When using a marking off pin, take care not to deeply scratch the
  27. 27. Bonding Procedure • Remove grease from bonding surface and clean Using an industrial tissue paper (SILBON paper) dipped in acetone, clean the strain-gage bonding site. Strongly wipe the surface in a single direction to collect dust and then remove by wiping in the same direction. Reciprocal wiping causes dust to move back and forth and does not ensure cleaning • Apply adhesive Ascertain the back and front of the strain gage. Apply a drop of adhesive to the back of the strain gage. Do not spread the adhesive. If spreading occurs, curing is adversely accelerated, thereby lowering the adhesive strength
  28. 28. Bonding Procedure • Bond strain gage to measuring site After applying a drop of the adhesive, put the strain gage on the measuring site while lining up the center marks with the marking off lines • Press strain gage Cover the strain gage with the accessory polyethylene sheet and press it over the sheet with a thumb. Once the strain gage is placed on the bonding site, do not lift it to adjust the position • Complete bonding work After pressing the strain gage with a thumb for one minute or so, remove the polyethylene sheet and make sure the strain gage is securely bonded. The above steps complete the bonding work. However, good measurement results are available after 60 minutes of complete curing of the
  29. 29. Some Adhesives and Their Preferred Curing Time
  30. 30. Protecting the Strain Gage • The strain gages must be protected from ambient conditions e.g. moisture, oil, dust and dirt •Protective materials used are Petroleum waxes, silicone resins, epoxy preparations, rubberized brushing compounds
  31. 31. Many materials can be used to protect strain gage installations. Perhaps none is more versatile for short-term applications than room-temperature vulcanizing (RTV) silicone rubber. The list of this material's capabilities is indeed impressive: • Available as an easy-to-apply single-component coating with uncured consistencies ranging from a low-viscosity brush-on material for thin coats, to a medium viscosity self-levelling form for use on level surfaces, to a high-viscosity no-run paste for vertical and overhead applications. • Cures at room temperature, yet is usable over a temperature range of -75° to +550°F (-60° to +290°C). • Has a low modulus of elasticity that is ideal for thin or flexible structures for which coating reinforcement effects may become significant. • Provides good short-term protection from water; resists many RTV Silicone Rubber Coatings
  32. 32. Some Gage Orientation and Interpretation of Results Bar with Axial Loading
  33. 33. Bar with Transverse Loading
  34. 34. Torsion
  35. 35. Possible sources of error in strain gauge signals 1- Cross-sensitivity Because a strain gauge has width as well as length, a small proportion of the resistance element lies at right angles to the major axis of the gauge, at the points where the conductor reverses direction at the ends of the gauge. So as well as responding to strain in the direction of its major axis, the gauge will also be somewhat responsive to any strain there may be at right angles to major axis. 2- Bonding faults For perfect bonding, the suitable adhesives and procedures for bonding gauges to the strain surface should be complied. If the bonding is unsatisfactory, creep may occur. Creep is a gradual relaxing of the strain on the strain gauge, and it has the effect of decreasing the gauge factor, so that the output of the bridge becomes less than it should be. Creep may also occur where gauges have been used to measure dynamic strain, and have been subjected to many thousands of cycles of strain.
  36. 36. 3- Hysteresis If a strain gauge installation is loaded to a high value of strain and then unloaded, it may be found that the gauge element appears to have acquired a permanent set, so that resistance values are slightly higher when unloading. The same effect continues when the direction of loading is reversed. To manipulate this problem, repeating cycles of loading/unloading should cause the hysteresis loop to narrow to negligible 4- Effects of moisture The gauges or the bonding adhesive may absorb water. This can cause dimensional changes which appear as false strain values. Another effect when moisture connections forms high resistance connected in parallel with the gauge. To prevent this, gauges should be bonded in dry condition or a suitable electrically insulating water repellent, such as a silicone rubber compound. 5- Temperature change One possible source of temperature difference is the heat produced by the current through a strain gauge. When the bridge is first switched on, the gauges may warm up, so the bridge should not be used for measurement until sufficient time for temperature to stabilize.

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