STRAIN-MEASUREMENT WITH MICROSENSORS

A model is developed for determining the farfield strains in laminated composites from the strain values indicated by a microsensing device embedded inside the material. The microsensing device considered here consists of three elements: a sensor, a circuit chip, and an antenna. The sensor detects changes in the strains in the material surrounding the sensor. The circuit chip processes the sensor signals which are th en transmitted, via the antenna, to a receiver located outside of the composite. The sensor and the circuit are powered by electromagnetic w aves generated outside the material and transmitted to the antenna. Th e model proposed provides a means by which the signal emitted by the m icrosensing device, and picked up by a receiver, is converted to the e ngineering value of the farfield strain, i.e., to the strain which wou ld exist in the material at the location of the sensor if the sensor w ere not present. On the basis of the model, two computer codes were wr itten. The first code calculates the influence coefficient matrix, whi ch is the parameter required to convert the sensor strain to the farfi eld strain; the second gives the farfield strain directly from the mea sured sensor strain. Tests were performed to verify the model and the computer codes. In these tests, a strain gauge, a rectangular steel pl ate, and a circular steel disk (simulating the components of a microse nsing device) were embedded in a glass/epoxy beam. The sensor strains were measured by the embedded sensor; the farfield strains were measur ed by two surface mounted sensors. The measured farfield strains were compared to the farfield strains calculated by the present model, and good agreements were found between the measured and calculated farfiel d strains.