Don’t Blame the Strain Sensor
Before blaming the strain gage, make sure these ten incredibly common strain measurement mistakes haven’t been made:
- Failure to remove soldering flux, which causes resistance drift
- Solder flux aids the solder wetting process when attaching lead wires to strain gage tabs by cleaning the oxide layer from the tab. To do so, the flux, which typically contains an acid, must be aggressive to the metal tab. Left on the solder tab, the flux will etch the metal foil and shift gage electrical resistance. Always ensure no flux gets on the grid of a strain gage, as the resistance shift will be accelerated.
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- Solder flux aids the solder wetting process when attaching lead wires to strain gage tabs by cleaning the oxide layer from the tab. To do so, the flux, which typically contains an acid, must be aggressive to the metal tab. Left on the solder tab, the flux will etch the metal foil and shift gage electrical resistance. Always ensure no flux gets on the grid of a strain gage, as the resistance shift will be accelerated.
- Failure to follow gage bonding adhesive instructions, which may cause gage un-bonding
- Strain gage adhesives are specially formulated to provide the high shear modulus needed to faithfully transmit the specimen strain to the gage grid. These high-performance adhesives require strict adherence to prescribed preparation and cure procedures. Failure to carefully follow these procedures can result in the gage un-bonding, or even worse, poor strain transmission into the grid, which results in inaccurate strain readings in stress analysis, or poor creep performance in transducers.
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- Strain gage adhesives are specially formulated to provide the high shear modulus needed to faithfully transmit the specimen strain to the gage grid. These high-performance adhesives require strict adherence to prescribed preparation and cure procedures. Failure to carefully follow these procedures can result in the gage un-bonding, or even worse, poor strain transmission into the grid, which results in inaccurate strain readings in stress analysis, or poor creep performance in transducers.
- Making the wrong wire connections to the instrument, which will not allow balancing
- A rookie mistake, or just in a hurry, but unless the strain gage(s) is properly wired to the instrument, the instrument cannot display the correct strain values. Follow the instrument manufacturer’s instructions and ensure the lead wires are connected properly to the gage and to the instrument.
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- A rookie mistake, or just in a hurry, but unless the strain gage(s) is properly wired to the instrument, the instrument cannot display the correct strain values. Follow the instrument manufacturer’s instructions and ensure the lead wires are connected properly to the gage and to the instrument.
- Inputting the incorrect gage factor value, which causes an erroneous reading due to transverse sensitivity
- Gage factor serves as the correlation between electrical resistance change of the strain gage, and the strain which caused the resistance change. It is determined in a statistical calibration procedure, because the strain gage must be bonded and, therefore, no longer available. The error in displayed strain is one-to-one with the error in gage factor value, either caused by the original manufacturer’s calibration procedure, or by setting the wrong value on the instrument. Double-check that the value entered into the instrument matches the value provided with the gage, and don’t mix gage factor values between similar gage patterns. Even similar looking gage patterns can have different gage factors.
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- Gage factor serves as the correlation between electrical resistance change of the strain gage, and the strain which caused the resistance change. It is determined in a statistical calibration procedure, because the strain gage must be bonded and, therefore, no longer available. The error in displayed strain is one-to-one with the error in gage factor value, either caused by the original manufacturer’s calibration procedure, or by setting the wrong value on the instrument. Double-check that the value entered into the instrument matches the value provided with the gage, and don’t mix gage factor values between similar gage patterns. Even similar looking gage patterns can have different gage factors.
- Using the wrong shunt calibration resistor value, which causes improper registration of the strain signal
- Shunt calibration is used to scale the strain indicator instrument, primarily to account for the gage factor desensitization caused by long lead wires (resistance attenuation). Dependent on the gage electrical resistance, then special values of shunt-cal resistors produce specified strain readings on the instrument. Make sure the shunt-cal resistor being used is correct for the gage resistance and desired strain level.
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- Shunt calibration is used to scale the strain indicator instrument, primarily to account for the gage factor desensitization caused by long lead wires (resistance attenuation). Dependent on the gage electrical resistance, then special values of shunt-cal resistors produce specified strain readings on the instrument. Make sure the shunt-cal resistor being used is correct for the gage resistance and desired strain level.
- Applying excessive bridge excitation, which causes drift
- Strain gages are electrical resistors in as much as an applied voltage is necessary so the change in resistance can be measured. Many gages have small grid areas, over which to dissipate the I²R heating from the applied voltage. For very small grids (<2.5 mm2) and low electrical resistance (<350 ohm) installed on a poor heat sink, then the applied voltage must remain very low (<5 volt). Remember that the voltage applied to the Wheatstone bridge is twice what an individual strain gage in the bridge will see.
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- Strain gages are electrical resistors in as much as an applied voltage is necessary so the change in resistance can be measured. Many gages have small grid areas, over which to dissipate the I²R heating from the applied voltage. For very small grids (<2.5 mm2) and low electrical resistance (<350 ohm) installed on a poor heat sink, then the applied voltage must remain very low (<5 volt). Remember that the voltage applied to the Wheatstone bridge is twice what an individual strain gage in the bridge will see.
- Applying strain levels that produce yield in the structure, which produces a zero shift in the strain gages
- Two mechanical ways to cause zero shifts in a strain gage are to yield/fatigue the gage, or yield/fatigue the specimen to which it is bonded. Strain gages have good fatigue life, but when attached to a very high fatigue material like well-engineered composites and plastics, then the specimen can endure more loading than the strain gage before the gage takes a permanent zero shift. Strain gages specifically manufactured to survive high elongations (post-yield gages) are especially susceptible to permanent zero shift when experiencing higher strain levels (>5000 µm/m) and/or repetitive strain cycling. This is why it is important to carefully monitor the strain levels and number of cycles experienced by strain gages, so specimen yielding can be separated from gage yielding.
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- Two mechanical ways to cause zero shifts in a strain gage are to yield/fatigue the gage, or yield/fatigue the specimen to which it is bonded. Strain gages have good fatigue life, but when attached to a very high fatigue material like well-engineered composites and plastics, then the specimen can endure more loading than the strain gage before the gage takes a permanent zero shift. Strain gages specifically manufactured to survive high elongations (post-yield gages) are especially susceptible to permanent zero shift when experiencing higher strain levels (>5000 µm/m) and/or repetitive strain cycling. This is why it is important to carefully monitor the strain levels and number of cycles experienced by strain gages, so specimen yielding can be separated from gage yielding.
- Applying strain levels that are beyond the capability of the strain gage or bonding adhesive, which causes unintended failure
- Foil strain gages are quite versatile. They can be used for everything from precision transducers (10 000D) to hostile environments (jet engine). But strain gages do have limits, especially strain limits. Common sized gages (>3 mm gage length) work well up to about 3-5% strain level. For a 3 mm gage length, this is equivalent to about a 0.12 mm elongation of the grid. Specially manufactured high-elongation strain gages (post-yield) can easily measure strains greater than 15%. But, the gage and adhesive system must match the required deformation. The same strain gages and adhesive systems that achieve 10 000D transducer performances, cannot survive 15% strain levels. http://www.vishaypg.com/docs/11085/tt605.pdf
- Foil strain gages are quite versatile. They can be used for everything from precision transducers (10 000D) to hostile environments (jet engine). But strain gages do have limits, especially strain limits. Common sized gages (>3 mm gage length) work well up to about 3-5% strain level. For a 3 mm gage length, this is equivalent to about a 0.12 mm elongation of the grid. Specially manufactured high-elongation strain gages (post-yield) can easily measure strains greater than 15%. But, the gage and adhesive system must match the required deformation. The same strain gages and adhesive systems that achieve 10 000D transducer performances, cannot survive 15% strain levels. http://www.vishaypg.com/docs/11085/tt605.pdf
- Applying strain gages to bolted assemblies, which can produce poor return to zero after loading
- For high-precision transducers, it is always best to avoid putting a fastener between the gage-section and the applied load. Invariably, this results in slippage in the fastener and poor zero return in the transducer. Stress-relieved welded joints are okay, but don’t tempt fastener joints. (see Strain Gage Based Transducers, available from Micro-Measurements)
- For high-precision transducers, it is always best to avoid putting a fastener between the gage-section and the applied load. Invariably, this results in slippage in the fastener and poor zero return in the transducer. Stress-relieved welded joints are okay, but don’t tempt fastener joints. (see Strain Gage Based Transducers, available from Micro-Measurements)
- Not applying or incorrectly applying strain gage protective coatings, which results in gage resistance drift over time due to corrosive attack on the gage foil
- The highly sensitive metal foil used to manufacture strain gages is somewhat delicate. Given sufficient time, and especially temperature, many common substances encountered in the environment can cause the foil to corrode, for example, plain water. Removal of solder flux has already been discussed. Now it’s time to provide appropriate protection over the gage installation, so nothing can get to the foil and cause undesirable zero shifts. But don’t forget that whatever is placed on top of the grid must also not cause corrosion of the foil. Many excellent protective coatings are also highly corrosive and intermediate layers must be placed between them and the foil to ensure stable gage operation over extended periods (years).
Read more here...
- The highly sensitive metal foil used to manufacture strain gages is somewhat delicate. Given sufficient time, and especially temperature, many common substances encountered in the environment can cause the foil to corrode, for example, plain water. Removal of solder flux has already been discussed. Now it’s time to provide appropriate protection over the gage installation, so nothing can get to the foil and cause undesirable zero shifts. But don’t forget that whatever is placed on top of the grid must also not cause corrosion of the foil. Many excellent protective coatings are also highly corrosive and intermediate layers must be placed between them and the foil to ensure stable gage operation over extended periods (years).
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应变片应用之十条准则