Analysis Of Rotating Component Strains Using Electronic Speckle Pattern Interferometry

Abstract

Analysis of rotating component strains usingelectronic speckle pattern interferometryR.W.T. PreaterDepartment of Mechanical Engineering, The City University,Northampton Square, London EC1V OHBAbstractThe measurement of in -plane strain rather than out -of -plane displacement is a distinctadvantage to engineers. Electronic speckle pattern interferometry (ESPI) rather thanholographic interferometry can provide this type of information. The use of a pulsedlaser in place of a c/w laser removes the otherwise rigorous stability requirements ofconventional holographic techniques and the component movement. Pulsed laser ESPIis a non -contact method which may be used under service environmental conditions. Initialtests using a tv- camera sensitive to low light levels have shown that in conjunction withprecision laser triggering interference subtraction fringes can be recorded for componenttangential velocities up to 5 ms -l. 1 Digital storage and the recent incorporation of ahigh resolution tv- camera system has increased the tangential velocity range to 15 ms -1.The combination of this system with image derotation may enable components with muchhigher velocities to be studied.IntroductionConventional experimental techniques such as photoelasticity or electrical resistancestrain gauges are more easily carried out in the laboratory. The latter method can seldombe used on the prototype without lengthy shut -down periods for surface preparation andgauge installation. A non - contact method of approach which requires a minimum of surfacepreparation and which may be used under normal operating service conditions is thereforedoubly attractive. The ESPI technique, pioneered at Loughborough University by Buttersand Leendertz 2 for static in -plane displacement measurement, is now being developed atThe City University for use on rotating components. Replacement of the c/w laser with apulsed ruby laser with pulse width of 50 ns removes the rigorous stability requirementsof conventional holography and freezes the component movement. Use of a closed circuittv- system and electronic processing provides good high contrast speckle images of thecomponent surface and immediate subtraction of the no- load /live load images to giveinterference fringes for in -plane displacement. Use of a video tape recorder allows asequence of events to be recorded as well as a permanent record. Both steady state andtransient load conditions may be studied provided initial no- loadimages can be recorded.Holographic techniques possess the required accuracy and sensitivity to allowmeasurement of small displacements, so that ESPI would be suitable for measurement ofsmall in -plane displacements on components subjected to low stress levels under fatigueconditions in service.Electronic equipmentThe subtraction of two speckle images of the same part of the component surfaceseveral revolutions apart requires precision triggering of the laser Q- switch to achieveimage correlation and satisfactory interference fringe patterns. The need to avoidrelative rotational movement of the images of less than 1 speckle requires a triggeringaccuracy to within less than 1 us.Standard television equipment was used initially but with minor modifications toprovide camera blanking so that complete tv- frames are recorded even though laserQ- switching may not coincide with the start of a tv- frame. Subsequent use of a specialhigh resolution speckle camera has shown a considerable improvement in fringe contrastat all component speeds, as well as the possibility of covering a wider speed range ofcomponent.The video equipment used is shown schematically in Figure 1 where the disc store isused to record the initial no -load speckle image, which is then continuously replayed onthe monitor. The live load speckle image is then subtracted from the first, the resultingdisplacement pattern of interference fringes is recorded on the video tape recorder.This recorder has a slow motion replay facility to allow a single frame search for theframe of interest.The upper limit of the component tangential velocity which produces acceptablesubtraction fringes has yet to be established. Earlier tests have shown that results maybe achieved at tangential velocities in excess of 5 ms-1 1 which is rather better