Do's and Don'ts for Becoming a Stress Analyst
- Do select the right strain gage for the application in terms of type and environmental compatibility.
- Do choose the right elongation capability.
- Do consider the required accuracy and precision.
- Do consider all environmental aspects before making the final choice (whether they are always or occasionally present at the place(s) the strain gage will operate).
- Do provide for any additional environmental protection at the design stage.
- Do design-in adequate over/under load protection as well as protection from other mechanical damage (e.g., physical abuse, rodent problems).
- Do remove soldering flux, which causes resistance drift and insulation resistance to ground problems.
- Do follow bonding adhesive instructions since Failure to carefully follow these procedures result in the gage de-bonding, zero drift or even worse, poor strain transmission into the grid, which results in inaccurate strain readings in stress analysis, or poor creep performance in transducers.
- Do input the gage factor value and transverse sensitivity values in your DAQ software, otherwise it causes an erroneous reading due to incorrect span setting transverse sensitivity errors. This is especially important where rosettes are concerned
- Do use the correct strain gage protective coatings , otherwise it can results in gage resistance drift over time due to corrosive attack on the gage foil.
- Do shunt calibrate the proper arm of the bridge (dummy Vishay Bulk Metal Foil resistor) to remove leadwire desensitization errors in your quarter bridge measurements.
- Don't make the choice based only on price — cost of ownership is more important.
- Don't allow strain gages to operate above their rated strain limit and optimum excitation voltage.
- Don't over specify — look at overall strain measurement system and limitations on accuracy (e.g., mechanical pipe work, vibration, etc.).
- Don't ignore that hurricane or flood that comes once every 2 or 3 years. Protect the strain gage in the field.
- Don't assume “it” will never happen, and never use the strain gage as a heater.
- Don't forget to provide adequate protection for the wires, near the strain gage. Water ingression is a big problem.
- Don’t make the wrong wire connections to the DAQ or strain indicator instrument, which will prevent bridge balancing or reverse the sign.
- Don’t use the wrong shunt calibration resistor value, which causes improper incorrect span setting.
- Don’t apply excessive bridge excitation, which causes drift due to gage self-heating.
- Don’t apply strain levels that produce yield in the structure, which produces a zero shift in the strain gages (and a weakened structure).
- Don’t apply strain levels that are beyond the capability of the strain gage or bonding adhesive, which causes unintended failure.
- Don’t apply strain gages to bolted assemblies, which can produce poor return to zero after loading. If it is necessary, watch for assembly loads + service loads.
- Don’t assume that non zero return is yielding. Are there bolted connections in your structure that might be the cause of your poor zero return. Is it a first cycle event only?
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