ULTRASONIC TENSION MEASUREMENT The principle of the method explained to the uninitiated and re-explained to the experienced Measuring principle The The measuring principle is very simple. It consists in measuring the elongation of the screw or stud as a result of the tension produced by tightening. As this elongation is proportional to the tension (let’s assume for now that we are remaining within the sphere of elasticity), the tension can be deduced from it simply by applying a coefficient. The ultrasonic transducer placed on the head of the screw or stud acts: Firstly as an emitter that converts the electric pulse supplied by the mea- suring system into an ultrasonic wave that will propagate longitudinally in the material.. Then as a receiver that will receive the greatly weakened ultrasonic wave eflected off the bottom of the screw, and convert it into an electric signal that will be processed by the measuring system. This operation is repeated several hundreds or thousands of times per second. Each time a pulse is «fired», the measuring system measures the time for the echo to return after the pulse is emitted. This is the principle of sonar, but the high propagation velocity of ultrasonic waves (approximately 5,800 m.s-1 in steel) and the required resolution of the elongation measurement (a few μm) requires a time measurement resolution in the order of one nanosecond (10-9 s). The time measured by the system is known as the “ultrasonic length”. As we will see, we do not need accurate knowledge of the ultrasonic wave propagation velocity in the material. At rest, in the so-called INITIAL state, an ultrasonic length Li is measured. After tightening, in the so-called FINAL state, an ultrasonic length Lf will be measured. This ultrasonic elongation Lf-Li can be attributed to: The mechanical elongation of the screw The decrease in the propagation velocity of the ultrasonic wave due to the tension introduced in the material. Generally, the effect due to the velocity is largely predominant, to a greater or lesser extent depending on the material. The system cannot distinguish between the two causes. However, various research projects, including those at CETIM (the French Industrial and Mechanical Technical Centre), have shown for a long time (forty years) that as these two effects are proportional to the tension, so is a combination of them. And luckily so, because it’s what makes the method applicable! The proportionality ratio between the ultrasonic elongation (the quantity measured) and the tension produced (the quantity we want to find out) will be determined by PRIOR CALIBRATION of the assembly. For this, a mock-up representative of the assembly (same type of screw/stud and same tightened length) is subjected to calibrated stresses, for example on a tensile testing machine. For each tension value, measured by the tensile testing machine or by a force sensor, the measuring system records the corresponding ultrasonic elongation. The system then calculates and records the proportionality coefficient K. Tension (daN) = K x Ultrasonic elongation (ns) K is expressed in daN.ns-1 or kN.ns-1 This method automatically takes into account the ultrasonic wave propagation velocity and the changes made to it by the tension, without it being necessary to know them. This operation takes only a few minutes. It is only carried out once for a given type of assembly. Example of actual calibration The x-axis shows the ultrasonic elongation in nanoseconds measured by the system. The y-axis shows the tension in kiloNewtons measured by the tensile testing machine. The calculations and graph are produced automatically by the Traxx-M2 measuring system. Here, the near-perfect linearity gives a K of 0.151 kN per ns. The coefficient K’ has been added by the system to take account of the fact that the calibration line does not pass through the origin (due to the positioning of the assembly at the start of tensioning).