Determination of dynamic velocity and strain using wide field holographic interferometry - Verification

High power and high capacity turbo-compressor systems frequently sustain acoustically induced vibrations. Higher order acoustic modes generated by turbo-compressors often couple selectively with structural pipe resonances producing significant increase in pipe wall vibration. In some instances, these coincidences generate high local stress levels that fatigue pipe shell or pipe attachments. In order to judge the level of dynamic strain and stress in piping systems, elaborate theories are employed. However, these are frequently not practical and relatively difficult to use in industrial applications, for example, in troubleshooting process. First, the accuracy of predicted results depends on assumed boundary conditions. The boundary conditions for on-site cases are rarely known and always difficult to estimate. Second, strains and stresses are complex and often difficult to determine, since they vary in space and time and may be caused by a multimode frequency excitation. Therefore, the strain and stress can only be predicted in reasonable bounds through laborious sensitivity and error analyses, which add further complexity to the already convoluted mathematical predictions. The correct stress level prediction in a structure, by means of directly measured vibrational velocity levels, is very desirable. Therefore, accurate mapping of the vibrational field is necessary. Since the mapping or evaluating of complex vibrational fields is veiy tedious and expensive using conventional technology (ample number of strain gauges or accelerometers), an alternative technique has been developed: Wide field pulsed holographic interferometry. This method provides three dimensional field images of vibrating structures allowing extraction of the actual vibrational responses (displacement and velocity), and calculation of dynamic strain and stress information. These are described by their gradient, peak and phase values obtained from the holograms documenting vibrational fields. This paper describes empirical verification of the wide field pulsed holographic technology which is used to predict a service life of the complex piping structure subjected to multimode frequency excitation. The experimental work was carried out on a sample thin wall vessel, which was either empty or partially filled with water and excited by the hammer or shaker. Through the conversion of vibrational response levels into strain (and stress level), and verification of the conversion against strain gauge measurement results, the technology is proven as a diagnostic tool. It is concluded that there are many advantages of using holography to evaluate complex vibrational fields. They include: i) 'instant' results, ii) nonintrusive nature (i.e. the machinery subject to testing can operate without interruptions), iii) satisfactory accuracy, iv) complete and permanent records, and v) significant savings of time and money due to reducing the analysis effort and implementation of suitable recommendations.