In-depth Use of Ultrasonic Bolt Tension Meter

The fundamentals of using ultrasonic bolt tension meter for bolt strain measurement, detecting change in the time required for a pulse of energy to travel the length of the fastener after elongation, can be performed with very good resolution and repeatability. The challenge is utilizing this core capability in a manner such that the resulting calculation of boll tension is also accurate and repeatable. Traditionally, the primary barriers to this have been in three areas maintaining a stable bolt-sensor interface where the pulse passes into and out of the fastener, accounting for the influence of temperature on the measurement and providing an accurate means of converting a given change in the pulse’s time-of-flight (TOF) to usable units of tension or load.

The last is generally refined to as calibration. Recent developments that affix the sensor to the fastener, leaving the bolt-sensor interface a fixed and permanent condition, have largely solved the interface instability problems associated with separate sensors and liquid coupling. Temperature management and compensation remains a challenge to a varying degree for both ultrasonic and other measurement techniques. As issues and solutions to temperature management are very application-dependent they will not be covered in this article. In general the desire is to minimize temperature differentials and gradients. Conversely, the means of effectively converting TOF to tension using ultrasonicbolt tension meter can be discussed in a manner that is widely applicable and will be the subject of this article. An attempt will be made to address both experienced and novice practitioners.

OBJECTIVE

In these four methods a fastener under calibration is increasingly loaded, both the TOF of the ultrasonic pulse and the load required to produce the elongation (and increase the TOF) are monitored. A series of load/TOF data pairs are produced for each sample and once all samples have been calibrated regression analysis is run to predict the bolt tension using ultrasonic bolttension meter for a given TOF Figure I illustrates the relationship between bolt elongation and the resulting bolt tension. Line A-B-C represents the behavior of a bolt loaded into yield, with permanent elongation occurring after point B. While this trace is often produced by pulling the bolt in a tensile tester, compressing a load cell by rotating the bolt into a nut member produces the same effect.

TEST RESULTS

The results of comparing load cell tension to tension calculated from the four forms of ultrasonic calibrations are summarized. Each covers a different measurement scenario. There is a compilation of intermediate measurements, all within the elastic range, taken at the most common design targets the bolt tension. The three subsequent tables are the final tension readings for bolts elongated into yield. The difference between them is how far into the plastic range the bolt was elongated. There is unique as in all other cases the bolt, nut and washer used for the comparative tests were the same used for calibration. The results shown reflect use of a plain finish nut rather than the zinc-plated nut used in all other cases. In all tables variation is defined as the absolute value of the difference in ultrasonic tension relative to load cell tension.