All fasteners stretch when under tensile load. Just as the natural tendency of a stressed spring is to return to its original length, so is the case with a fastener. As per Hooke's Law, the greater the stretch, the greater the force. It is this stress which applies the clamp load to a joint. When tightening a fastener within the elastic region of its material's stress/strain curve defined by Young's Modulus, the relationship of stretch to applied force is proportional. Because of this, any particular bolt stress will have an associated elongation value.

By measuring the relative elongation of a tightened fastener, the stress value of that fastener can be determined. This means that one can accurately control how "tight" a fastener is rather than having to resort to assumptions, guesses and hopes.

In practical terms, accurate control is achieved by monitoring elongation as a tightening force is applied. The force is only stopped once the required bolt elongation has been attained. In so doing, the applied force (torque, tensioner pressure, nut arc etc) may be different than the tightening specification had called for if the designer incorrectly predicted the conditions actually experienced at the time of assembly. Furthermore, the proper balance and distribution of clamp load across a flange can be ensured by measuring and 'load tuning' all fasteners after the final tightening pass.

Ultrasonic Elongation Measurement

A transducer is temporarily seated onto one end of a fastener. An ultrasonic pulse is sent down the length of this fastener. The wave reflects at the bottom and then returns to the transducer. In consideration of certain parameters, the total transit time is interpreted either as length, load, stress, or % strain. 

At least two measurements are taken; one with the fastener in its relaxed state and another in its tightened (stretched) condition. The delta of the two provides an accurate indication of a tightened fastener's elongation.

Fastener condition can be quickly determined at any time in the future without having to apply tightening tools by measuring the fastener's current elongation.