Absolute Measurement of Elastic Waves Excited by Hertzian Contacts in Boundary Restricted Systems

Tribology Letters, Dec 2016

In most applied monitoring investigations using acoustic emission, measurements are carried out relatively, even though that limits the use of the extracted information. The authors believe acoustic emission monitoring can be improved by instead using absolute measurements. However, knowledge about absolute measurement in boundary restricted systems is limited. This article evaluates a method for absolute calibration of acoustic emission transducers and evaluates its performance in a boundary restricted system. Absolute measured signals of Hertzian contact excited elastic waves in boundary restricted systems were studied with respect to contact time and excitation energy. Good agreement is shown between measured and calculated signals. For contact times short enough to avoid interaction between elastic waves and initiating forces, the signals contain both resonances and zero frequencies, whereas for longer contact times the signals exclusively contained resonances. For both cases, a Green’s function model and measured signals showed good agreement.

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Absolute Measurement of Elastic Waves Excited by Hertzian Contacts in Boundary Restricted Systems

Tribol Lett Absolute Measurement of Elastic Waves Excited by Hertzian Contacts in Boundary Restricted Systems S. Schnabel 0 1 2 S. Golling 0 1 2 P. Marklund 0 1 2 R. Larsson 0 1 2 0 Division of solid mechanics, Lulea ̊ University of Technology , 97187 Lulea ̊ , Sweden 1 Division of machine elements, SKF-University technology center, Lulea ̊ University of Technology , 97187 Lulea ̊ , Sweden 2 Division of machine elements, Lulea ̊ University of Technology , 97187 Lulea ̊ , Sweden In most applied monitoring investigations using acoustic emission, measurements are carried out relatively, even though that limits the use of the extracted information. The authors believe acoustic emission monitoring can be improved by instead using absolute measurements. However, knowledge about absolute measurement in boundary restricted systems is limited. This article evaluates a method for absolute calibration of acoustic emission transducers and evaluates its performance in a boundary restricted system. Absolute measured signals of Hertzian contact excited elastic waves in boundary restricted systems were studied with respect to contact time and excitation energy. Good agreement is shown between measured and calculated signals. For contact times short enough to avoid interaction between elastic waves and initiating forces, the signals contain both resonances and zero frequencies, whereas for longer contact times the signals exclusively contained resonances. For both cases, a Green's function model and measured signals showed good agreement. Hertz contact; Elastic waves; Acoustic emission; Green's function; Boundary restricted system; Condition monitoring 1 Introduction Acoustic emissions (AE), or as well called high frequency elastic wave emissions, have over the past decade become increasingly popular in the application of nondestructive testing and condition monitoring. This technique has been proven in the separation of failure modes [2], the monitoring of wear [4, 21], the specification of contaminated systems [16, 20] and to differentiate lubricants [17] and lubrication regimes [7]. However, most of the investigations use simple signal processing methods such as root mean square (RMS) [7, 20] and activation counts (AC) [2, 16, 17, 21]. All these investigations use relative measurement methods, and signals are acquired using piezoelectric transducers, which limits investigations without further calibration to measure relatively. This use of relative measurement methods also limits the extraction of information of the acoustic wave. The authors hypothesize that condition monitoring techniques could be improved by improving processing of the signal. A fuller understanding of the relation between the source of the wave and the actual measured signal would improve the processing of the signal. Being able to calculate the force function of the initial source based on sensor signals would increase the possibility to distinguish between different failure types and failure sizes. However, absolute measurement would therefore be required so that the relation between the signal and the wave source could be ascertained. Both McLaskey and Glaser [15] and Jacobs and Woolsey [10] have presented methods for absolute calibration of piezoelectric transducers. However, there are no evaluations of the validity of the methods which are independent of the system. McLaskey’s and Glaser’s method for absolute calibration of piezoelectric transducers is used. The method is evaluated for boundary restricted systems using a Laser–Doppler vibrometer (LDV) and Green’s function based on an FEM simulation as a comparison. The term ‘‘boundary restricted systems’’ is used for systems where reflections of all dimensions are taken into account (in this investigation disc samples), whereas systems which are boundary free in one or two dimension are not included in this definition (calibration plate—reflection is only considered in one dimension). An absolute measurement is required to improve condition monitoring capability of high frequency emissions. However, a better understanding of the relation between source and signal is as well an essential knowledge for improving condition monitoring by acoustic emission. Several researchers have successfully connected source and signal for high frequency emissions. McLaskey and Glaser [14] have, for example, related a signal of a piezoelectric transducer to the actual force function by using a Green’s function approach. Kundu et al. [12] have presented a mathematical method to locate Hertzian impacts by minimizing error functions. The impact of cracks on wave propagation in plates was studied by Liu and Datta [13] with FEM based on a Green’s function for transfer of the initial source. Glaser et al. [5] were able to calculate the wave propagation in an isotropic half space with a viscoelastic propagator and compared it to actual measurements. All these investigations either have used thin plates in order t (...truncated)


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S. Schnabel, S. Golling, P. Marklund, R. Larsson. Absolute Measurement of Elastic Waves Excited by Hertzian Contacts in Boundary Restricted Systems, Tribology Letters, 2017, pp. 7, Volume 65, Issue 1, DOI: 10.1007/s11249-016-0790-8