Exercise Improves Performance
Athletes the world over know the value of “working up a sweat” to enhance performance. In the world of precision strain measurement, “sweating the details” also enhances performance. One such detail is working to relieve residual stress in a bonded strain gage installation.
High-performance strain gage installations require the use of epoxy or epoxy-phenolic adhesives. These adhesives require a clamped cure at elevated temperature. The elevated temperature may be the result of an exothermic reaction from mixing a resin and catalyst together or external heating from an oven, heat lamp, or other heat source. The thermal excursion results in expansion of the gage, adhesive system, test specimen and clamping tooling, all at dissimilar rates. The adhesive cross-links (locks up) at elevated temperature, and as the assembly cools stresses are locked into the adhesive.
These stresses will be released as a result of thermal and mechanical cycling. When making a precision strain measurement such as those needed for high-accuracy load cells and transducers, and some critical stress analysis measurements, the release of these stresses in service has a negative effect upon repeatability, hysteresis, creep, and zero return. It is therefore imperative to release these stresses prior to delivery of the transducer to the end user or prior to attempting to make a critical stress analysis measurement.
For best performance, two steps are necessary to fully relieve these stresses.
- Post curing the strain gaged specimen above the clamped cure temperature or the maximum operating temperature. After the clamped cure cycle is complete and the specimen has cooled to room temperature, the clamps are removed and the specimen is again heated for 1-2 hours at 15-30°C above the clamped cure temperature or above the maximum operating temperature, whichever is greater. See specific data sheet for recommended time/temperature.
- Mechanically load cycling. Typically, load cells and transducers should be load cycled at least three times to approximately 150% of rated capacity.
Thermally and mechanically cycling the assembly above the max rating ensures that under normal operation the device will never see such extreme conditions again and, therefore, no additional stresses will be relieved, thus producing a more reliable, stable and repeatable measurement.