Evaluation of Strain Measurement Devices for Inflatable Structures

Abstract

Inflatable structures provide a significant volume savings for future NASA deep space missions. The complexity of these structures, however, provides difficulty for engineers in designing, analyzing, and testing. Common strain measurement systems for metallic parts cannot be used directly on fabrics. New technologies must be developed and tested to accuractly measure the strain of inflatable structures. This paper documents the testing of six candidate strain measurement devices for use on fabrics. The resistance devices tested showed significant hysteresis during creep and cyclic testing. The capacitive device, however, showed excellent results and little-to-no hysteresis. Because of this issue, only two out of the six proposed devices will continue in development. The resulting data and lessons learned from this effort provides direction for continued work to produce a structural health monitoring system for inflatable habitats.

Aerosol jet printed capacitive strain gauge for soft structural materials

“…Interdigitated electrodes preserve the advantages associated with CSGs vs RSGs in harsh environments, while also providing a vehicle by which CSGs can integrate directly onto structural components. Previous testing has been conducted on Kevlar webbing with flexible strain gauges and it was concluded that CSGs performed better than RSGs in dynamic and long term loading conditions similar to those that were investigated within this work 3 . Finally, using capacitance as the sensing mechanism makes the sensor more suitable for applications where wireless strain sensor measurements are required 10,14 .…”

“…SHM systems for inflatable habitats, parachutes and decelerating systems are used to ensure the safety of crewmembers, aim to establish an emergency notification system, and enable smart entry, descent, and landing (EDL) operations. Ideally, SHM systems for these applications would provide continuous monitoring of strain/load to detect, diagnose and locate damage in real time for continuous monitoring and also after incident 3 .…”

“…Traditionally, strain gauges are intended for use with metals, they employ relatively stiff substrates, operate most effectively at room temperature before corrections must be incorporated, experience strains under 5%, display hysteresis in long-term testing, and undergo mechanical failure at higher strains [2][3][4][5] . When considering soft structural materials strain gauges must be capable of withstanding high rates of strain (5-50%) associated with materials being under high load, but the inherent stiffness of traditional strain gauges results in device failure, and classifies them as unsuitable for the inspection of soft structural materials 3,5 . Additional limitations of the more popular RSGs are found in the dependence of resistivity upon operating temperature and applied strain 4,6,7 .…”

Closing the Wearable Gap—Part III: Use of Stretch Sensors in Detecting Ankle Joint Kinematics During Unexpected and Expected Slip and Trip Perturbations

“…Finally, the material properties of the SRS have not yet been analyzed, which may further have more limitations. However, a previous study has reported "excellent linear trend with little noise" and no hysteresis [38]. Finally, the SRS device is still a prototype and would require further refining, ruggedization, testing (especially beyond a laboratory), and incorporating real-life falls and environments to test the effectiveness of the wearable SRS fall detection solution.…”

Flexible Sensors—From Materials to Applications

“…Capacitive type strain sensors are mostly assembled by sandwiching elastomer dielectric layers in between elastomer electrodes that are filled with conducting particles. Capacitive-type strain sensors in general exhibit a better hysteresis performance when compared to the resistive types [101,362,411].…”