Ultrasonic pulsed phase locked loop interferometer for bolt load measurements

The pulsed phase-locked-loop bolt monitor (P2L2) that uses ultrasonic waves to measure bolt preload with accuracies ranging from 1 to 3 percent (depending on the specific bolt) is described. To remeasure bolt load after installation, a thermal calibration factor compensates for bolt temperature changes, and a standard reference block allows correction for acoustic phase errors due to measurement equipment configuration such as utilization of a different transducer, couplant, or cable. Some examples of critical applications including Space Shuttle landing-gear wheels and NASA wind-tunnel fan blades are discussed.