Experimental evaluation of MEMS strain sensors embedded in composites

Micromechanical in plane strain sensors were fabricated and embedded in fib er-reinforced laminated composite plates. Three different strain sensor des igns mere evaluated: a piezoresistive filament fabricated directly on the w afer; a rectangular cantilever beam; and a curved cantilever beam. The cant ilever beam designs were off surface structures, attached to the wafer at t he root of the beam. The composite plate with embedded sensor was loaded in uniaxial tension and bending. Sensor designs mere compared for repeatabili ty, sensitivity and reliability. The effects of wafer geometry and composit e plate stiffness mere also studied, Typical sensor sensitivity to a uniaxi al tensile strain of 0.001 (1000 mu epsilon) ranged from 1.2 to 1.5% of the nominal resistance (dR/R). All sensors responded repeatably to uniaxial te nsion loading. However, for compressive bending loads imposed on a 2-3-mm-t hick composite plate, sensor response varied significantly for all sensor d esigns. This additional sensitivity can be attributed to local buckling and subsequent out of plane motion in compressive loading. The curved cantilev er design, constructed with a hoop geometry, showed the least variation in response to compressive bending loads. All devices survived and yielded rep eatable responses to uniaxial tension loads applied over 10 000 cycles.