Embedded Module for 3-D Mechanical Strain Measurement

This paper presents the development and evaluation of a miniature (1 cm
³
) embedded electronic module that can resolve temperature-compensated mechanical strain in three dimensions. The module is designed to be embedded into materials and to measure, in-situ, all nine components of strain and to relay the information back to a PC via wired communications. The module can also be assembled as a component in a “parent” encapsulated electronic system and interface with the system's digital bus. The module comprises three miniature, off-the-shelf, three-gauge, rectangular rosettes that are assembled in a 3-D array so that all components of strain can be resolved. The module format is a cube, with the gauges on the outer faces of the cube, and the electronics on the inside. The electronics consists of signal conditioning circuitry, a 24-bit sigma-delta ADC, a microcontroller which sends the digital data directly to a master device such as PC, and an onboard temperature sensor for thermal compensation of the gauges. The module was evaluated by encapsulating it in epoxy and subjecting it to compressive and tensile testing in a mechanical tester. The results were compared to finite element simulations with an average of 7% difference between magnitudes and a standard deviation of 4%. Thermal evaluation of the module was also carried out by subjecting the test pieces to thermal loads and monitoring the response of the system. The module is believed to be the smallest yet reported self-contained, embedded system capable of resolving all components of 3-D strain, it is small enough to be embedded in a wide range of materials and structures without significant impact on the mechanical properties of its host. It can, therefore, be used in applications such as structural health monitoring and reliability prognostics and for calibration of mechanical/thermo-mechanical simulations.