Tightening bolts is relatively easy to conduct in practice but attaining a specific preload and assessment of the preload achieved is a lot more difficult than what is typically believed. In previous tech tips we have discussed the need for using a device that can measure preload (such as Skidmore-Wilhelm machine) to calibrate the pretensioning technique employed and improve the accuracy of the tightening technique. In practice this calibration should be conducted daily for each specific joint configuration, lubricant used, set of tooling and when operators are changed. However, this sort of calibration still only improves the accuracy of the preload achieved, and does not give surety as to the actual preload of any specific fastener in a connection, which may vary by ±30% or more depending on the tightening method employed.
One method for checking the preload achieved is to assess the elongation of the fastener by accurately measuring its length before and after preloading. Together with the geometry of the fastener/joint, the change in length associated with preloading can be used to give a relatively accurate indication of preload.
Although this sounds like a simple exercise, measuring the change in length accurately is more difficult than it would seem. For a grade 12.9 M42x200mm bolt you can expect somewhere around 1.0 mm (1000 µm) extension at the full 945kN (~96 tonne) preload. Obviously, for smaller (and shorter) bolts this would be much less (a grade 8.8 M20x100 bolt, will extend approximately 260µm at preload of 125 kN (~13 tonnes)) and errors in measurement would have a greater effect. For example, a 10µm (0.01mm) measurement error in the latter example will have a 4% error in preload while in the former it would only have a 1% error. One can use a micrometer to measure the length of the bolt, but to ensure accuracy the faces would need to be machined, with a small protrusion on either end to facilitate consistent measurement. In addition, one would require access to both sides of the bolt to facilitate measurement, which is often not possible especially if there are large flanges in the way, the bolts are long, or if the bolts are fastened into blind holes.
Another potential option for measuring change in length would be by utilising Ultrasonic Testing (UT) techniques. These techniques utilise the time taken for ultrasonic sound waves to propagate through the material (the so called ‘time of flight’) to calculate the distance travelled, using the speed of the wave and the material parameters. In addition, when materials are stressed there is a predicable decrease in the velocity of the sound wave as the load is increased. Depending on the sophistication of the UT device, these two effects can be used to measure the ‘ultrasonic length’ of the bolt and changes in this length during preloading are used to calculate the strain and hence preload in the bolt. Practically this is simply achieved by placing the ultrasonic probe of the ultrasonic device onto one end of the bolt (with an appropriate couplant), and recording a ‘time of flight’ for the reflected signal from the other end of the bolt. These measurements are made both prior to the application of load and during/after loading so the change in length can be evaluated. The UT device then utilises these time of flight measurements, together with user entered material properties, temperature, and probe details to calculate i) the bolt length, ii) change in length (if data can be recorded, otherwise this would have to be done manually), and iii) if sufficiently sophisticated, the fastener preload. Obviously, the preload in an already tightened bolt can also be evaluated if measurements are made before and after loosening of the fastener.
In house testing at Origen and in-service trials by some of our clients has shown the technique can be implemented into bolt pretensioning strategies with some success. However, it is important to note that i) the instrumentation is sensitive and must be handled with care, and ii) the technique is not ‘fool proof’ and errors can be easily introduced by poorly trained personnel.
One of the most significant benefits of using UT techniques to measure preload is that, if space is available (i.e. if one of the faces of the bolt is free), you can measure preload while tightening the bolt, which allows any potential issues (e.g. those associated with seizure of the contact faces/thread or deformation) to be addressed during the tightening process. Furthermore, if the data for each bolt is stored it is in theory possible to evaluate any loss in pretension over time (when subsequent measurements are made) but this requires careful calibration of the device against a reference standard/fastener.
Undoubtedly, UT measurement can improve both the precision and accuracy of the fastener preloading process, however there are some key issues that need to be born in mind. These include i) temperature variations in the fastener may increase or decrease the ultrasonic properties of the material and hence the ‘apparent bolt length’ due (in some devices can be addressed to some extent by measuring temperature using a separate probe), ii) sensor position is very important and can make replicating results, even in laboratory settings, difficult (tests at Origen have highlighted changes over 300+ micron just by slightly shifting the position of the sensor, which is about 1/3rd the extension of an M42 bolt at full preload), iii) to limit the effects of sensor location and ensure proper coupling, both ends of the fastener need be machined to be smooth, perpendicular to the axis of the bolt and free of paint/rust, iv) the equipment is sensitive which can result in practical issues when employed in harsh environments such as the offshore/mining industries (judicious ‘ruggedizing’ of the units/sensors is recommended), and v) for best results calibration on samples of the actual bolts needs to be undertaken especially when measurement of in-service relaxation are required.
The message is clear, when undertaken with due diligence by skilled operators, UT measurement of preload is a valuable tool that can reduce the risks associated with inadequate preload. However, like many things in life, poor equipment, the lack proper technique and inadequate care can result in errors and increase uncertainty.