Work performed at Johnson Space Center has brought about a major improvement in the means for measuring the bending that occurs whenever stress is applied to a bolted joint. This major improvement is a measurement method based on ultrasonics. The strain gauges used heretofore in the space program in efforts to measure bending loads or deformations in bolted joints have proven inadequate, in that it has been difficult to perform accurate measurements by use of them. The art of ultrasonics offers an alternative and superior means of performing such measurements; as such, it can be expected to contribute to cost savings and increased safety, not only in the space program but also in military and commercial applications — wherever there are stressed bolted joints that could pose bending hazards. The ability to measure stresses in joints accurately can be expected to contribute to understanding the mechanisms that produce stress failures in critical joints and thereby contribute to the success of efforts to design safeguards that will lower risks to both vehicles and personnel.
In the present method, an ultrasonic transducer is bonded to one end (normally, the head end) of the bolt of interest. The other end (normally, the tip of the threaded shank) is modified to a stepped end; that is, one machines that end to form sector-of-circle facets (steps) perpendicular to the longitudinal axis, each located at a different axial position (see figure). The transducer generates an ultrasonic pulse, which travels to and is reflected by the steps. The reflection from each step becomes a separate pulse, each of which returns to the transducer at a different time, depending on the position of the step and thus the effective ultrasonic path length. The reflected pulses received by the transducer are converted to electronic signals, which are processed by associated instrumentation to obtain a time trace of the ultrasonic signal and to extract information on the timing of the reflections.
When the bolt is subjected to bending, there occur geometry- and stress-related changes in the effective speed of sound and path lengths for reflections from some or all of the steps. These changes give rise to changes in pulse travel times. In the processing of the ultrasonic signals, the changes in pulse travel times are used to compute an ultrasonic bending parameter, which can be used to calculate the bending deflection and/or load. The proportionality between the bending parameter and the bending deflection and/or load is obtained by correlating various values of the ultrasonic bending parameter with known bending loads applied in calibration measurements.
This work was done by Ajay M. Koshti of Rockwell International for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category. MSC-22706