Fidlin, Alexander

Sah, Si Mohamed

Tcherniak, Dmitri

Thomsen, Jon Juel

In recent pilot studies we have started investigating how to possibly use measured flexural (i.e. transverse/bending) vibrations, induced by bolt-tapping, to estimate bolt tightness. Some of the vibration features we investigated showed strong correlation with bolt tightness. For example, the lowest natural frequency, as estimated from the vibration signal, is close to being proportional to bolt tension, except at very low tensions (Fig. 1(d)). To obtain an estimate of bolt tightness this way only requires a period of time of the order of a second where the user taps the blot with a light hammer, whichtriggers measurement and data processing. However, experimental results revealed that this technique encounters twoproblems, for it to be presently useful in real applications: First the variability in results is too large, about twice that for a torque wrench. Figure 2(a) shows that the slope of the curves varies for separate experiments with similar increasing bolt tension, which suggests that the quantity ω/β is an unreliable feature for estimating bolt tension. A second fundamental problem is that the relative change in natural frequency is approximately proportional not only to bolt tension, but also to slenderness ratio. Thus, if only the natural frequency feature were to be used for estimating bolt tension, accuracy will drop off for the short and thick bolts that are often used in critical joints

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Downloaded from orbit.dtu.dk on: Dec 16, 2017Vibration-based testing of bolted jointsThomsen, Jon Juel; Sah, Si Mohamed; Fidlin, Alexander; Tcherniak, DmitriPublished in:Proceedings of the 9th European Nonlinear Dynamics ConferencePublication date:2017Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Thomsen, J. J., Sah, S. M., Fidlin, A., & Tcherniak, D. (2017). Vibration-based testing of bolted joints. In G.Stépán, & G. Csernák (Eds.), Proceedings of the 9th European Nonlinear Dynamics Conference (pp. 2).CongressLine Ltd..ENOC 2017, June 25-30, 2017, Budapest, HungaryVibration-based testing of bolted jointsJon Juel Thomsen⇤, Si Mohamed Sah⇤, Alexander Fidlin⇤⇤ and Dmitri Tcherniak⇤⇤⇤⇤Department of Mechanical Engineering, Technical University of Denmark⇤⇤ Department of Engineering Mechanics, Karlsruhe Institute of Technology, Germany⇤⇤⇤ Brüel & Kjær Sound & Vibration Measurement, DenmarkSummary. For many technical installations that are held together by bolted joints (Fig. 1(a,b)), the regular checking and documentationof proper bolt tightness is essential for certification and safe operation. However, with large structures and heavy bolts, the workrequired for the inspectors can be physically demanding and time consuming, e.g. with heavy hydraulic torque wrenches in demandingenvironments. In this work we attempt to develop reliable vibration-based techniques for bolt tightness estimation, using advancedmathematical modeling/analysis, and suitable measurement equipment and signal processing.IntroductionIn recent pilot studies we have started investigating how to possibly use measured flexural (i.e. transverse/bending) vibra-tions, induced by bolt-tapping, to estimate bolt tightness. Some of the vibration features we investigated showed strongcorrelation with bolt tightness. For example, the lowest natural frequency, as estimated from the vibration signal, is closeto being proportional to bolt tension, except at very low tensions (Fig. 1(d)). To obtain an estimate of bolt tightness thisway only requires a period of time of the order of a second where the user taps the blot with a light hammer, whichtriggers measurement and data processing. However, experimental results revealed that this technique encounters twoproblems, for it to be presently useful in real applications: First the variability in results is too large, about twice that for atorque wrench. Figure 2(a) shows that the slope of the curves varies for separate experiments with similar increasing bolttension, which suggests that the quantity !/  is an unreliable feature for estimating bolt tension. A second fundamentalproblem is that the relative change in natural frequency is approximately proportional not only to bolt tension, but also toslenderness ratio. Thus, if only the natural frequency feature were to be used for estimating bolt tension, accuracy willdrop off for the short and thick bolts that are often used in critical joints.Traditional bolt tightening proceduresTraditional bolt tightening procedures [1] include torque wrenches, elongation measurements (e.g. by LVDT or ultrasonictransducers), and sensor force washers (typically strain gauge based). Various ultrasonic methods and the problems as-sociated with them are overviewed in [2], while the use of changing magnetic permeability for estimating bolt tension isdescribed in [3], and dynamic problems associated with bolts (e.g. loosening) in [4]. Most articles on ultrasonic meth-ods are practically oriented towards measuring techniques, and less towards fundamental theoretical issues of relationsbetween tension and measurable features such as resonance frequencies and wave pulse passage time. The theory usedtypically involves longitudinal vibrations, not including the flexural vibrations which are of lower in frequency, and atmain focus in the current project. Longitudinal resonance frequencies drop with tension due to a combined effect of in-creased length and reduced longitudinal wave speed; this is utilized in traditional ultrasonic transducer based monitoringof bolt tension. For flexural vibrations, it is well known how tension increase the flexural resonance frequencies , but onlylittle seems reported on using measured flexural resonances for bolt tension monitoring. A number of technical applica-tions based on ultrasonic transducers (pulsed or continuous wave) has been patented but seemingly not on systematic useof measured features of flexural resonances/vibrations.Figure 1: Dense arrays of bolts in (a) a wind turbine main shaft, and (b) a wind turbine tower flange [Brüel & Kjær customer tests].(c) Impact hammer, microphone, accelerometer. (d) Experimentally measured change in lowest natural frequency for two impactedM12x240 mm bolts vs. normalized bolt tension [MSc thesis (J.S. Knudsen, 2013)]; Symbol markers: results for each bolt using twodifferent ways of determining natural frequency. For higher tension, measurements approach the theoretical prediction (upper line) fora simple Bernoulli-Euler clamped-clamped beam model (lower line for a hinged-hinged beam).ENOC 2017, June 25-30, 2017, Budapest, HungaryFigure 2: (a) Measured ratio of natural frequency ! to decay rate   for the second flexural vibration mode of an impacted M12x140mm mm bolt-nut assembly vs. normalized bolt tension [MSc thesis (E. Vind and S. Jespersen, 2014)]; each curve connects averagedexperimental measurements as the tension is increased. (b) Numerically simulated response (log(|acceleration|+10 4)) vs. time for animpacted bolt joint with an advanced serial/parallel nonlinear friction model; Three markedly different decay envelopes are identified,separating times where the energy dissipation is dominated by, respectively, pure slip friction (initial phase), stick-slip friction (middlephase), and viscous-like (could also be hysteretic) linear damping (final phase).Bolt tightness indicatorsThe two fundamental problems mentioned in the introduction call for considering other vibration features as well, maybein combination. Therefore, to remedy to these shortcomings, we have started investigating additional temporal, spec-tral, and temporal-spectral features which include higher natural frequencies, attack time, equivalent linear decay rates,multiphase nonlinear decay rates (Fig. 2(b)), ratio of natural frequency to decay rate, psychoacoustic measures (e.g. loud-ness), amplitudes of nonlinear higher harmonics, and (with two-frequency excitation) amplitudes of nonlinear sidebands[5, 6, 7].Alternative bolted joint integrity measuresThe variability in measurements might be due to an inherent feature of the traditional way of using torque wrenches forquantifying bolted joint integrity: Torque relates only indirectly to joint tightness, it depends also on other quantities, mostimportantly friction. To address this fundamental issue, in this work we will consider also new ways of quantifying boltedjoint integrity, i.e. how vibrations could be used not just as a substitute for measuring torque, but maybe as a better means.ConclusionsThis is a work in progress; updated results will be orally presented at the conference.Acknowledgment This work is financially supported by the Danish Council for Independent Research, grant DFF-6111-00385.References[1] SKF Catalogue no. TSI 1101 AE, Bolt-tightening Handbook, SKF Equiements - ITE Dept., Montigny-le-Bretonneux, France, 2001.[2] Srinivasan L. (2006)Measurement of Stress in Bolts using Resonant Ultrasound Spectroscopy. MSc thesis, Durham University.[3] Gotoh Y., Teshima Y., and Takahashi N. (2011) Electromagnetic inspection method of slack of high tension bolt. IEEE Transactions on Magnetics47(10):2566-2569.[4] Ibrahim R.A. and Pettit C.L. (2005) Uncertainties and dynamic problems of bolted joints and other fasteners. Journal of Sound and Vibration279:857-936.[5] Bailey S.E., Estevez L., Murphy D. and Puckett A. (2007) Fatigue crack monitoring with nonlinear acoustics. Proceedings of Modal AnalysisConference (IMAC-XXV), Orlando, Florida.[6] Donskoy D., Sutin A., and Ekimov A. (2001) Nonlinear acoustic interaction on contact interfaces and its use for nondestructive testing. NDT&EInternational 34:231-238.[7] Meyer J.J. and Adams D.E. (2015) Theoretical and experimental evidence for using impact modulation to assess bolted joints. Nonlinear Dynamics81:103-117.

Vibration-based testing of bolted joints

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Vibration-based testing of bolted joints

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