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Sonic waves detect loaded-bolt stress.

A computer-based workstation analyzes ultrasonic waves to measure in-place stresses or fatigue in bolts of unknown length. Prior systems use a single shock wave applied to the stressed bolt and to the same or identical bolt unstressed, and both the bolt material and length must be accurately known to compute the stress load. With the system developed by Zendex Corp, Dublin, CA, only the bolt's composition is required.

According to Keith Riley of Zendex, the system uses simultaneous longitudinal and shear waves, separately induced and received by a single composite transducer/sensor. Explains Riley, "We recommend you measure a bolt from the lot at no load, but that is not absolutely necessary. You need to know its density, and that can be determined by weighing a representative bolt. That density measurement is then transformed into an acoustic-conduction constant, based on our testing of standard bolt materials.

With only a general idea of bolt composition (knowing whether the bolt metal is Grade 5 or Grade 8 steel, Inconel, etc.), the system can measure stress within 10% to 20%, which in terms of conventional bolt-stress measurement is considered fair. Weighing a sample from the lot the bolt came from brings measurement accuracy into the 3% or 4% range.

"We don't even consider bolt length. From its density or specific gravity, we can infer much more about the chemical and physical properties of the bolt, based on knowing two acousto-elastic constants for each material. The longitudinal wave can be visualized as hitting the bolt head with a hammer; it's a shock wave that travels the length of the bolt and yields a length indication. The shear wave is a lateral wave that ripples along the intersticial stress line, transverse to the bolt axis. It takes longer to return, and the angle that it moves laterally is a function of the crystal structure of the material."

Thus, the returning waves are directly related to the bolt's basic metallography and present state of stress. The system consists of the sensing head and the minicomputer workstation. The heavy computer requirement is because the the analog waves must be conditioned and converted to digital data before analysis. Once in hexadecimal notation, the data can be modemed or faxed.

System cost ranges between $25,000 to $30,000 depending on graphics. Unfortunately at the moment, it's a lab instrument weighing 60 lb and not portable. The company is working on a PC-based version to make it readily portable and less expensive.
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Publication:Tooling & Production
Date:Sep 1, 1991
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