We have developed and tested a strain gage that surpasses conventional foil technology, which is limited to less than 20 percent strains. This was a significant shortcoming given that new structural components on aerospace vehicles include highly elastic, low Young's modulus materials. For example, Kevlar-reinforced rubber and elastomers have a nonlinear stress-strain relationship - some with extreme rupture strains greater than 500 percent. Results from sensor tests indicate potential to use this new gage for elastic strain greater than 100 percent, with negligible localized stiffening. These tests indicate that, when used with specifically designed constant current signal conditioning, accurate static strain measurements are achievable for ground testing. Also, a conceptual temperature-compensation method has been conceived to greatly reduce measurement error for atmospheric flight applications in environments of -30 degrees Fahrenheit.

MEETING ADVANCED AEROSPACE NEEDS:
The technology was developed to meet the needs of various NASA projects. For example, the Adaptive Compliant Trailing Edge (ACTE) and Hypersonic Inflatable Aerodynamic Decelerator (HIAD) projects needed high elastic strain measurements. Due to time constraints, fabricating a sensor from scratch was infeasible, so a similar gage used by the medical field was modified, prototyped, and tested to meet aerospace requirements.

BENEFITS AND APPLICATIONS:
The technology can benefit many applications, including:
-- Elastomer skins for highly flexed wing and control surfaces
-- Rubberized fabric skin
-- Cargo-carrying airships
-- High-cycle, high-strain fatigue testing
-- Inflatable wing-morphing aircraft
-- Aeroservoelastic control
-- Flexible wind turbine blades
-- Myriad aerospace/aircraft components

The strain gage is:
-- Robust: Provides high strain measurements with negligible localized stiffening of the substrate
-- Accurate: Offers a real-time temperature-cancellation method to minimize the effects of varying thermal conditions in aerospace applications, and provides constant current signal conditioning to eliminate post-test leadwire resistance corrections
-- Streamlined: Includes a data-acquisition system that eliminates the need for two-point tensile calibrations

HOW IT WORKS:
The technology is based on a medical sensor modified in a single looped strain gage configuration. A simple tool was developed to reduce initial resistance scatter between gages and provide consistency in end-loop radii for conformity of transverse sensitivity. A new circuit design incorporated the excitation current required for a range of 1 million microstrain (i.e., 100 percent strain), with a step resolution of less than 10 microstrain, and was designed to compensate for changing temperatures in varying thermal environments. Taking advantage of constant current makes it possible to derive strain using accurate initial resistance measurements (Kelvin) and plugging them into the strain equation.

The sensors were tested under both bending and single-axial tensile modes. Aluminum, Plexiglas, and fiberglass materials were used for bending, and tensile testing was conducted on graphite-epoxy tensile coupons to 10,000 microstrain using conventional foil strain gages as reference. Further testing used photogrammetry technology for higher strains on elastomers (greater than 100 percent) with excellent repeatability and accuracy.