A proposed family of devices for inducing fatigue in bolts in order to break the bolts would incorporate piezoelectric actuators into resonant fixtures as in ultrasonic/ sonic drills/corers and similar devices described in numerous prior NASA Tech Briefs articles. These devices were originally intended primarily for use as safer, more-reliable, more-versatile alternatives to explosive bolts heretofore used to fasten spacecraft structures that must subsequently be separated from each other quickly on command during flight. On Earth, these devices could be used for accelerated fatigue testing of bolts.
Fatigue theory suggests that a bolt subjected to both a constant-amplitude dynamic (that is, oscillatory) stress and a static tensile stress below the ultimate strength of the bolt material will fail faster than will a bolt subjected to only the dynamic stress. This suggestion would be applied in a device of the proposed type. The device would be designed so that the device and the bolt to be fatigue-tested or broken would be integral parts of an assembly (see figure).
The static tension in the tightened bolt would apply not only the clamping force to hold the joined structures (if any) together but also the compression necessary for proper operation of the piezoelectric actuators as parts of a resonant structural assembly. The constantamplitude dynamic stress would be applied to the bolt by driving the piezoelectric actuators with a sinusoidal voltage at the resonance frequency of longitudinal vibration of the assembly. The amplitude of the excitation would be made large enough so that the vibration would induce fatigue in the bolt within an acceptably short time.
In the spacecraft applications or in similar terrestrial structural-separation applications, devices of the proposed type would offer several advantages over explosive bolts: Unlike explosive bolts, the proposed devices would be reusable, could be tested before final use, and would not be subject to catastrophic misfire. In fatigue-testing applications, devices of the proposed type would offer advantages of compactness and low cost, relative to conventional fatigue-testing apparatuses. In both structural-separation and fatigue-testing applications, bolts to be broken or tested could be instrumented with additional ultrasonic transducers for monitoring of pertinent physical properties and of fatigue failure processes.
This work was done by Stewart Sherrit, Mircea Badescu, Yoseph Bar-Cohen, Jack Barengoltz, and Vanessa Heckman of Caltech for NASA’s Jet Propulsion Laboratory.
This Brief includes a Technical Support Package (TSP).
Piezoelectric Bolt Breakers and Bolt Fatigue Testers
(reference NPO-43977) is currently available for download from the TSP library.
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Overview
The document presents a novel invention related to piezoelectric bolt/pin shearing devices, aimed at providing a controlled and reusable alternative to traditional explosive bolts used in aerospace applications. The invention addresses the challenges associated with explosive mechanisms, which are one-time use and can cause detrimental shock to spacecraft structures, potentially affecting sensitive instruments and communication systems.
The disclosed device utilizes a piezoelectric stack to generate fatigue in fasteners (bolts or pins) that are held in tension, allowing for the controlled breaking of these fasteners. This method is based on fatigue theory, which suggests that a bolt subjected to both dynamic constant amplitude stress and static tensile stress will fail faster than one subjected to only dynamic stress. The device operates at resonance frequencies, specifically noted at 20 kHz, and can induce failure in bolts after a short duration of voltage application, with the potential for failure occurring after approximately 1 second under optimal conditions.
The document includes technical details about the stress levels involved, indicating that the device can exceed the AC stress limit of 100,000 psi after a significant number of cycles, thus providing a reliable means of testing and breaking bolts. The design allows for increased margins by adjusting the number of piezoelectric elements and the area ratio of the device.
Additionally, the invention features a monitoring capability through the use of ultrasonic sensors, which can detect changes in the physical properties of the bolt during the separation process. This monitoring ensures the integrity of the mechanism and allows for pre-launch testing, enhancing safety and reliability.
The document emphasizes the potential applications of this technology in various space missions that require the separation of structures, highlighting its advantages over explosive bolts, such as reduced design constraints and the ability to conduct multiple tests prior to launch. Overall, the piezoelectric bolt/pin shearing device represents a significant advancement in aerospace technology, offering a low-cost, compact, and efficient solution for bolt fatigue testing and controlled separation in space environments.
