Effects of Altitude on Active Structural Health Monitoring

As the field of Structural Health Monitoring (SHM) expands to spacecraft applications, the understanding of environmental effects on various SHM techniques becomes paramount. In January of 2013, an SHM payload produced by New Mexico Tech was sent on a high altitude balloon flight to a full altitude of 102,000 ft. The payload contained various SHM experiments including impedance measurements, passive detection (acoustic emission), active interrogation (guided waves), and wireless strain/temperature sensing. The focus of this paper is the effect of altitude on the active SHM experiments. The active experiment utilized a commercial SHM product for generation and reception of elastic waves that enabled wavespeed measurements, loose bolt detection, and crack detection through the full profile of the flight. Definite deviations were observed in the data through the stages of the flight which included a ground, ascent, float, and descent phases. Several elements of the high altitude environment can have an effect on the measurement such as temperature and pressure. The flight data was compared against a ground altitude baseline and heavy emphasis is placed on comparing changes in the data with the temperature profile of the flight. Conclusions are drawn on the effect of altitude on wavespeed of elastic waves, crack detection, and the sensing of a loose bolt.

Experimental Study of Impact Modulation for Quantifying Loose Bolt Torque in On-Demand Satellites

The development of on-demand satellites has created the need for methods that quickly assess the structural integrity of the system. In particular, because of rapid testing and assembly, workmanship errors such as loose bolts within the satellite structure are a concern. Current methods of loose bolt detection, which often rely on baseline data or analytical models, are not practical due to the variable geometry of the satellites and the time constraints on testing. The method of impact modulation is a nonlinear vibration method which is quick to perform and is robust to changes in the geometry of the structure. This paper presents the results of applying impact modulation to a simplified structure used to model a single bolted joint within a satellite. Results show that impact modulation can distinguish changes in torque within the bolted joint. In addition, several examples of how the underlying linear characteristics of the system affect impact modulation results are given.