Calibration of the bolt pretension by strain gauges vs. FEA

Influence of Symmetry of Tightened Parts on the Force in a Bolted Joint

“…The conversion constant was around 60 kN/(mV/V) and differed by ±8% between the individual bolts. This confirms the appropriateness of separate calibration, recommended in [19], if more reliable measurement is required. The constant determined for a specific bolt and subsequently used in the calculation had a standard deviation of up to 1%, which corresponds to the uncertainty of the tensile testing machine used.…”

“…However, these limits only apply to measurements where the force in the bolt is calculated from the strain in gauge fitted inside the shank (i.e., using the so-called "strain gauge constant"), so without any calibration. This issue was discussed by N. Fric and co-authors in [19]. According to their research, it is possible to do this without performing strain-force calibration if the measured bolts have a longer length with a sufficient "strain plateau" for applying strain gauges and if we use adequate coefficients that include the effect of thread along their entire shank (i.e., the difference between the effective and the nominal diameter of the bolt shank with and without thread).…”

“…Furthermore, considering the nature of the problem of strain measurement, this is necessary when these items are applied inside the bolt shank without the thread, even if these threads are not "active". The manufacturers of strain gauges, mentioned here [19], also insist on these conditions. However, these limits only apply to measurements where the force in the bolt is calculated from the strain in gauge fitted inside the shank (i.e., using the so-called "strain gauge constant"), so without any calibration.…”

“…For this purpose, the information presented in [15][16][17], which describes the load and stress behavior across the contact threaded region of the bolt, was very useful. Other references were crucial in terms of the design and execution of the experiment, e.g., basic information [18] or advanced information from N. Fric [19] about the limitations of strain gauging when using internal strain gauges in bolts. Publications [20][21][22] dealing with the issue of bolt tightening accuracy (achieving bolt preload) were also examined.…”