Strain gauging is an indispensable part of the engineering toolbox. Matt Rudas from Aspec Engineering covers the principles, applications and significance of strain gauging in engineering practice.
The structural assessment of machines is based on strength of material calculations, where the quantity of interest is typically stress. Determination of structural stresses requires knowledge of the loads to which the structure is subjected, and this information is not always readily available.
One practical method for the determination of stresses is based on the relationship between material stress and the accompanying deformation, called “strain.”
What is a strain gauge?
Strain gauges function because the electrical resistance of metal changes proportionally to the amount of deformation that is caused in it by an external force. Strain gauges consist of a thin metal grid that is etched onto an electrical insulator backing. The strain gauge is bonded to a structure and deforms at the same rate as the structure surface. The measured resistance change in the strain gauge can be used to calculate the surface strain of the structure.
The Wheatstone bridge
The resistance change that a strain gauge undergoes during deformation is very small. In order to accurately measure it, an electrical circuit called a Wheatstone bridge converts the small resistance change into a voltage change which can more easily be measured.
Strain gauge amplifiers can be set up for various Wheatstone bridge configurations, depending on the strain gauge arrangement and temperature compensating requirements for the structure.
Strain gauge arrangements
Strain gauges are almost always installed onto the surface of structures, and it is typically necessary to measure three strain directions at a single point to completely define the strain field. Measuring strains in three directions is done by installing a stacked strain gauge rosette. There do however exist situations where fewer gauges are required:
Components with uniaxial, isotropic, and pure torsional stress states require a single strain gauge.
Components with biaxial stress states only require two strain gauges, however they need to be aligned with the principal stress directions.
Strain gauges can also be installed and wired to either amplify or cancel out particular strain components.
Case study: Guide roller lateral load measurement
A screenhouse tripper had experienced premature failure of wheel and cam follower bearings due to higher-than-expected long travel skewing loads. ASPEC installed strain gauges to measure the lateral loads during tripper movements.
A wireless telemetry strain gauging system was installed with a 5G network connection that allowed remote and live viewing, processing, and collection of strain data. The system had been calibrated prior to being put into service using a rotable spare bogie set and hydraulic jack for application of the calibration load.
Case study: Measurement of bucket wheel reclaimer long travel loads
A strain gauging system was installed onto the long travel arrangement of a bucket wheel reclaimer. There was concern that misalignment of the long travel rails was inducing high lateral loads in the equalisers, causing cracking of bogie pillow blocks.
The aim of the strain gauging was to determine whether there was a correlation between measured strains and the position of the reclaimer along the rail length, to use the correlation to identify areas where the rails required improved alignment and to generate estimates of the lateral and skewing loads on the equalisers.
Conclusions
Strain gauging is a measurement technique that gives deep insights into structural behaviour. Understanding this behaviour allows engineers to detect weaknesses and address vulnerabilities before they escalate into costly failures and extended downtime.