Transfer Coefficient
Mathematical strain transfer models quantify the shear stress distribution across adhesive layers bonding strain gauges to printed circuit substrates. The shear lag parameter defines the rate at which mechanical strain attenuates through the finite thickness of an adhesive interface. Application of the metric determines the minimum required bond length for accurate substrate strain transmission.
Strain Distribution
Mechanical force applied to a circuit board substrate transfers into the sensing foil through interfacial shear stress concentrated at gauge edges. Thin adhesive layers with high shear moduli yield rapid strain buildup near gauge ends, achieving full strain transfer across the central grid length. Thick or compliant bond lines spread shear distribution across a wider area, reducing effective strain recorded at the center of the grid.
Differential thermal expansion between substrate epoxy and sensor backing material alters interfacial shear profiles during thermal profiling tests. High strain gradients near board cutouts require short gauge lengths and ultra-thin adhesive lines to minimize strain averaging errors. Empirical testing confirms that shear lag effects become dominant when bond line thickness exceeds twenty-five micrometers.
Layer Attenuation
Experimental verification uses optical strain mapping or finite element analysis to calibrate analytical transfer models. High strain rate testing reveals dynamic shear lag variations caused by viscoelastic response in polymeric adhesives. Insufficient gauge length relative to the calculated shear transfer length causes systematic under-measurement of true substrate strain.
