Biaxial Measurement
Arrangement of multiple strain gauges placed on a single plane to determine the principal stresses and their orientation within a material. The planar rosette configuration allows for the calculation of biaxial strain states when the direction of the applied load is unknown. It is commonly used on the flat surfaces of printed circuit boards and structural metal plates.
Geometric Layout
Three independent sensing elements are typically oriented at 0, 45, and 90 degrees or 0, 60, and 120 degrees relative to each other. This specific geometry of the planar rosette enables the derivation of the Mohr circle of strain. Mathematical transformation of the three measured values yields the maximum and minimum principal strains.
Surface Footprint
Width and length of the sensing area determine the spatial resolution of the measurement. Because a planar rosette occupies a larger surface area than a single-element gauge, it averages the strain over a broader region. This averaging effect can lead to underreporting of peak stresses if the strain gradient is steep near a solder joint or a mounting hole.
Larger footprints are also more susceptible to local variations in the thickness of the circuit board or the epoxy layer used for bonding.
Installation Precision
Alignment of the primary axis with a known reference edge on the PCB is required for accurate data interpretation. A small angular error during the bonding of the planar rosette introduces large errors in the calculated direction of the principal stress. Qualified technicians use magnifying optics to verify the orientation before the adhesive cures.
