FRIEND OR FOE? MR STRAIN SENSOR

July 21, 2016 Craig Nyssen News, Stress Strain Comments Off
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StrainBlogTM – the online community offering expert advice on all high – precision strain and stress measurement. Their most recent article “don’t blame the strain sensor” StrainBlogTM explains the most common mistakes made during strain measurements.

 

 

10 most common mistakes according to the StrainBlogTM:

1.     Failure to remove the soldering flux causes resistance drift

  • When the solder flux (helps in the wetting process) is left on the grid, it etches the metal foil and shifts the gage electrical resistance because it contains an acid.
  • As such, ensure that no flux gets on the grid of the strain gage.
  • Flux must be removed by cleaning the oxide layer from the tab.

2.     Failure to follow gage bonding adhesive instructions causes gage un-bonding

  • Strain gage adhesives are designed to bond the specimen strain to the gage grid.
  • Specific procedures must be followed in order to bond the specimen strain to the gage grid.
  • Failure to do so may lead to gage un-bonding or even worse – poor strain transmission into the grid!
  • Failure to follow the wrong procedures results in inaccurate strain readings during stress analysis measurements or poor creep performance in transducers.

3.     Making the wrong wire connections to the instrument leads to an imbalance

  • Connecting the wires to the instrument incorrectly causes incorrect strain values to be read off the instruments.
  • Follow the instrument manufacturer’s instructions so that the lead wires are connected properly to the gage and to the instrument.

4.     Inputting the incorrect gage factor value results in an incorrect reading due to transverse sensitivity

  • Gage factors correlate between the change in electrical resistance of the strain and the strain which caused the said electrical resistance.
  • Gage factor is determined by a statistical procedure.
  • The error in displayed strain is one-to-one with the error in gage factor value.
  • Error is due to:
  • calibration procedure of the original manufacturer 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!

5.     Using the wrong shunt calibration resistor value causes improper registration of the strain signal

  • Shunt calibration is used to scale the strain indicator instrument.
  • Accounts for the desensitization of the gage factor caused by long lead wires.
  • Depending on the gage electrical resistance, certain values of shunt-cal resistors produce specific strain readings on the instrument.
  • Make sure the shunt-cal resistor used corresponds to the gage resistance and desired strain level.

6.     Applying excessive bridge excitation causes drift

  • Strain gages are electrical resistors.
  • Applying excess excitation does not damage the strain gage, however, reduces the performance.
  • Remember that the voltage applied to the Wheatstone bridge is twice what an individual strain gage in the bridge will see.

7.     Applying strain levels that produce yield in the structure produces a zero shift in the strain gages

  • Zero shifts can occur:
  • yield/fatigue the gage
  • yield/fatigue the specimen to which it is bonded
  • Generally, strain gages have an excellent fatigue life, however, when attached to a very high fatigue material (composite or plastic), the strain gage can experience fatigue due to the additional loading.
  • 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.

8.     Applying strain levels that are beyond the capability of the strain gage or bonding adhesive causes unintended failure

  • Foil strain gages are very versatile, however, using the wrong gage and adhesive system for a specific deformation leads to failure.
  • Always choose the correct gage and adhesive system for the required deformation.

9.     Applying strain gages to bolted assemblies can produce poor return to zero after loading

  • For high precision transducers, always avoid putting a fastener between the gage-section and the applied load.
  • This leads to slippage and poor zero return in the transducer.
  • The point: Stress – relieved welded joints are okay – however don’t play with fastener joints.

10.   Not applying or incorrectly applying strain gage protective coatings results in gage resistance drift over time due to corrosive attack on the gage foil

  • Remembering that the metal foil used to manufacture the strain gage is delicate – after a certain period of time and exposure to different environments, the metal foil can undergo corrosion.
  • Protective coats must be applied correctly using the correct procedure.

 

For a detailed explanation, refer to the link or for a further discussion on strain gages, contact our Melbourne office by email or phone: +61 3 9874 5777